C H A P T E R ➃

Servo Tuning

In a Hurry?
You should tune the 6270 before attempting to execute any
motion functions. At a minimum, complete this chapter's Tuning Setup
Procedure and Controller Tuning Procedures until you have found a
proportional feedback gain that can give a stable response for your system.
(The Drive Tuning Procedure below is for use with velocity drive systems
only, not for servo valve systems.) Then you can proceed to execute your
motion functions. To gain a full understanding of tuning, you should read
through this entire chapter and follow its procedures to ensure your system
is properly tuned.

Servo Tuning Software Available

To effectively tune the 6270 (and any velocity drives you may be using), use the interactive
tuning features in the Servo Tuner add-on module for Motion Architect. It greatly improves
your efficiency and gives you powerful graphical tools to measure the performance of the
system. Instructions for using Servo Tuner to tune the 6270 are provided in the Servo
Tuner User Guide .
The Servo Tuner option is an add-on module and does not automatically come with the basic
Motion Architect software package. To order your copy of Servo Tuner, which is provided on
a separate disk, contact your local Automation Technology Center.

➃ Servo Tuning 29
Servo System Terminology
This section gives you with an overall understanding of the principles and
the terminology used in tuning the 6270.

Servo Tuning Terminology

30 6270 Motion Controller User Guide

The 6270 uses a digital control algorithm to control and maintain the
position and velocity. The digital control algorithm consists of a set of
numerical equations used to periodically (once every servo sampling
period) calculate the value of the control signal output. The numerical
terms of the equations consist of the current commanded and actual position
values (plus a few from the past sampling period) and a set of control
parameters. Each control parameter, commonly called a gain, has a
specific function (see Servo Control Techniques later in this chapter).
Groups of gains may be saved to specified gain sets that can be invoked to
affect motion under varied conditions at different times. Tuning is the
process of selecting and adjusting these gains and gain sets to achieve
optimal servo performance.
When this control algorithm is used, the whole servo system is a closed-
loop system (see diagram below). It is called closed loop because the control
algorithm accounts for both the command (position, velocity, tension, etc.)
and the feedback data (from the LDT, encoder, or ANI input); therefore, it
forms a closed loop of information flow.
When all gains are set to zero, the digital control algorithm is essentially
disabled and the system becomes an open loop system (see diagram below).
During system setup or troubleshooting, it is desirable to run the system in
open loop so that you can independently test the drive/motor or valve
operation (further details are provided in the Open Loop Operation section
later in this chapter).
Close d Loop Sy stem
Offset
Control Drive Command = Servo Hydraulic
Command Digital Signal Control Signal + Offset Valve Cylinder,
Control or Motor, Load
Algorithm Drive etc.

Feedback Data Feedback Device

(LDT, Encoder, or AN Input)

Offset
Ope n Loop Sys tem SOFFS Servo Hydraulic
Offset Drive Command = Offset Valve Cylinder,
or Motor, Load
Drive etc.

Feedback Device
(LDT, Encoder, or AN Input)

The 6270 has the capability of providing an analog voltage output of ±10V or a
current of ±20mA, ±50mA, ±60mA, ±80mA, ±100mA, or ±150mA for
commanding the valve or drive. After the digital control algorithm has
calculated the digital control signal, this digital value is sent out from the DSP
(digital signal processor) to the Digital-to-Analog converter (DAC). The DAC
has an analog output range of -10V to +10V or maximum current. It is often
possible that the digital control signal calculated by the control algorithm can
exceed this limit. When this happens, the analog output would just stay, or
saturate, at the maximum limit until the position error changes such that the
control algorithm would calculate a control signal less than the limit. This
phenomenon of reaching the output limit is called controller output
saturation. When saturation occurs, increasing the gains does not help
improve performance since the DAC is already operating at its maximum level.

➃ Servo Tuning 31
Position Variable Terminology
In a servo system, there are two types of time-varying (value changes with
time) position information used by the controller for control purposes:
commanded position and actual position. You can use this information to
determine if the system is positioning as you expect.
Commanded The commanded position is calculated by the motion profile routine
Position based on the acceleration (A, AA), deceleration (AD, ADA), velocity (V) and
distance (D) command values and it is updated every servo sampling period.
Therefore, the commanded position is the intended position at any given
point of time. To view the commanded position, use the TPC (Transfer
Commanded Position) command; the response represents the commanded
position at the instant the command is received.
When this user guide refers to the commanded position, it means this calculated
time-varying commanded position, not the distance (D) command. Conversely,
when this user guide refers to the position setpoint, it means the final
intended distance specified with the distance (D) command. The following plot
is a typical profile of the commanded position in preset (MCØ) mode.

Setpoint

Profile
Complete
Commanded
Position
Position

Distance
(D)

Constant
Acceleration Deceleration
Velocity

Time

32 6270 Motion Controller User Guide

Actual Position The other type of time-varying position information is the actual
position; that is, the actual position of the motor (or load or cylinder, etc.)
measured with the feedback device (LDT, encoder, or ANI input). Since this is
the position achieved when the drive/valve responds to the commanded
position, we call the overall picture of the actual position over time the
position response (see further discussion under Servo Response
Terminology).
To view the actual position, use the TFB (Transfer Position of Feedback
Device) command; the response represents the actual position at the instant
the command is received. The goal of tuning the servo system is to get the
actual position to track the commanded position as closely as possible.
The difference between the commanded position and actual position is the
position error. To view the position error, use the TPER (Transfer Position
Error) command; the response represents the position error at the instant
the command is received. When the motor/valve is not moving, the position
error at that time is called the steady-state position error (see
definition of steady-state under Servo Response Terminology). If a position
error occurs when the motor/valve is moving, it is called the position
tracking error.
In some cases, even when the system is properly tuned, the position error can
still be quite significant due to a combination of factors such as the desired
profile, the servo mechanism's limitation, the dynamic characteristics of
the system, etc. For example, if the value of the velocity (V) command is
higher than the maximum velocity the hydraulic cylinder (or motor, etc.)
can physically achieve, then when it is commanded to travel at this velocity,
the actual position will always lag behind the commanded position and a
position error will accumulate, no matter how high the gains are.

➃ Servo Tuning 33
Servo Response Terminology

Stability The first objective of tuning is to stabilize the system. The formal definition
of system stability is that when a bounded input is introduced to the
system, the output of the system is also bounded. What this means to a
motion control system is that if the system is stable, then when the
position setpoint is a finite value, the final actual position of the system is
also a finite value.
On the other hand, if the system is unstable, then no matter how small the
position setpoint or how little a disturbance (motor torque variation, load
change, noise from the feedback device, etc.) the system receives, the position
error will increase continuously, and exponentially in almost all cases. In
practice, when the system experiences instability, the actual position will
oscillate in an exponentially diverging fashion as shown in the drawing
below. The definition here might contradict what some might perceive. One
common perception shared by many is that whenever there is oscillation,
the system is unstable. However, if the oscillation finally diminishes
(damps out), even if it takes a long time, the system is still considered stable.
The reason for this clarification is to avoid misinterpretation of what this
user guide describes in the following sections.

Position The following table lists, describes, and illustrates the six basic types of
Response Types position responses. The primary difference among these responses is due to
damping, which is the suppression (or cancellation) of oscillation.

Response Description Profile (position/time)

Unstable Instability causes the position to

Position
oscillate in an exponentially diverging
fashion.

Time

Over-damped A highly damped, or over-damped,

Position

system gives a smooth but slower

response.
Time

Under-damped A slightly damped, or under-damped,

Position

system gives a slightly oscillatory

response.
Time

Critically damped A critically-damped response is the

Position

most desirable because it optimizes

the trade-off between damping and
speed of response. Time
Position

Oscillatory An oscillatory response is

characterized by a sustained position
oscillation of equal amplitude.
Time

Chattering Chattering is a high-frequency, low-

Position

amplitude oscillation which is usually

audible.
Time

34 6270 Motion Controller User Guide

Performance When we investigate the plot of the position response versus time, there are a
Measurements few measurements that you can make to quantitatively assess the
performance of the servo:
• Overshoot—the measurement of the maximum magnitude that the actual position
exceeds the position setpoint. It is usually measured in terms of the percentage
of the setpoint value.
• Rise Time—the time it takes the actual position to pass the setpoint.
• Settling Time—the time between when the commanded position reaches the
setpoint and the actual position settles within a certain percentage of the position
setpoint. (Note the settling time definition here is different from that of a control
engineering text book, but the goal of the performance measurement is still
intact.)

These three measurements are made before or shortly after the hydraulic
cylinder (or motor, etc.) stops moving. When it is moving to reach and settle
to the setpoint, we call such period of time the transient. When it is not
moving, it is defined as in steady-state.

A typical stable position response plot in preset mode (MCØ) is shown below.

Settling Time
Target Zone Mode
Settling Band

Setpoint Setpoint

Commanded
Position Overshoot Steady State
Position Error
Position

Actual
Position

Rise Time

Transient Steady State

Time

➃ Servo Tuning 35
6000 Series Servo Commands
NOTE
The following list briefly describes each servo-related 6000 Series command. More
detailed information can be found in the rest of this chapter and within each command's
description in the 6000 Series Software Reference Guide .

Command Title Brief Description (detailed descriptions in 6000 Series

Software Reference Guide )
LDTUPD LDT Position Update Rate Use LDTUPD to select the rate at which the LDT position is sampled.
Decreasing the LDTUPD command value (speeding up the update rate)will
improve the quality of the dynamic response. However, if the update rate is
too fast, the LDT will not have enough time to read the position, resulting in
read errors. The occurrence of read errors can be monitored with the TER
and [ ER ] commands if you enable ERROR bit #15, and you can check the
LDT status with bit #27 of TAS and AS.
SFB Select Servo Feedback Selects the servo feedback transducer (options: LDT, encoder, or ANI
Source analog input). The SFBØ command sets all the gains to zero so that the
controller runs in open loop mode, and disables the Setpoint Window
feature (equivalent to SSWGØ)
SGAF Acceleration Feedforward Sets the acceleration feedforward gain in the PIV&Fa servo algorithm.
& SGAFN * Gain
SGENB Servo Gain Set Enable Enables a previously-saved set of PIV&F gains. A set of gains (specific to
the current feedback source selected with the SFB command) is saved
using the SGSET command.
SGI Set Integral Feedback Gain Sets the integral gain in the PIV&F servo algorithm.
& SGIN *
SGILIM Set Integral Windup Limit Sets a limit on the correctional control signal that results from the integral
gain action trying to compensate for a position error that persists too long.
SGP Proportional Feedback Gain Sets the proportional gain in the PIV&F servo algorithm.
& SGPN *
SGSET Save a Set of Servo Gains Saves the presently-defined set of PIV&F gains as a particular gain set
(specific to the current feedback source on each axis). Up to 5 gain sets
can be saved and enabled at any point in a move profile, allowing different
gains at different points in the profile.
SGV Set Velocity Feedback Gain Sets the velocity gain in the PIV&F servo algorithm.
& SGVN *
SGVF Velocity Feedforward Gain Sets the velocity feedforward gain in the PIV&Fv servo algorithm.
& SGVFN *
SMPER Maximum Allowable Position Sets the maximum allowable error between the commanded position and the
Error actual position as indicated by the feedback device. If the error exceeds
this limit, the 6270 activates the Shutdown output and sets the DAC output
to zero (plus any SOFFS offset). If there is no offset, a valve will return to
the null position and a rotary motor will freewheel to a stop. You can enable
the ERROR command to continually check for this error condition, and when
it occurs to branch to a programmed response defined in the ERRORP
program.
SOFFS Servo Control Signal Offset Sets an offset to the commanded analog output voltage, which is sent to
& SOFFSN the drive system. The SOFFSN command allows you to set an offset voltage
when the position error is negative. The SOFFSN value tracks the SOFFS
value until a separate SOFFSN value is entered. To return SOFFSN to the
default mode in which it track SOFFS, issue the SOFFSN command with a
minus sign (-) in the command field for the affected axis (e.g., SOFFSN,-
restores axis 2 to the default mode).

36 6270 Motion Controller User Guide

SSFR Servo Frequency Ratio Sets the ratio between the update rate of the move trajectory and the
update rate of the servo action. The intermediate position setpoints
calculated by the trajectory generator is updated at a slower rate then the
servo position correction. This command allows you to optimize this for
your application. The default setting (SSF4) is sufficient for most
applications.
SSWD Setpoint Window Gains You may now activate a specified set of gains to be used when with a
& SSWG defined region either side of the position setpoint (“setpoint window”). The
SSWD command is used to specify the distance on both sides of the position
setpoint (“setpoint window”) in which the gain set specified with the SSWG
command is used. Specifically, the gain set is automatically invoked by the
controller after the commanded move profile is complete and the actual
position is within the setpoint window. The setpoint window includes a
hysteresis loop equal to 25% of the value used in the SSWD command.
NOTE: The gain set to be used must first be defined with the SGSET
command.
For more information, refer to the Setpoint Window Gains section later in
this chapter.
STRGTD Target Zone Distance When using the Target Zone Mode, enabled with the STRGTE command, the
actual position and actual velocity must be within the target zone (that is,
STRGTE Target Zone Mode Enable within the distance zone defined by STRGTD and within the velocity zone
STRGTT Target Zone Timeout Period defined by STRGTV). If the motor/load does not settle into the target zone
before the timeout period set by STRGTT, the 6270 detects an error.
STRGTV Target Zone Velocity
To prevent subsequent commands/moves from being executed when this
error condition occurs, you must enable the ERROR command to continually
check for this error condition, and when it occurs to branch to a
programmed response defined in the ERRORP program. Otherwise,
subsequent commands/moves can be executed regardless of the actual
position and velocity.
This feature is explained in greater detail later in the Target Zone section.
TDAC Value of DAC Output Transfers [or assigns/compares] the output from the 6270's digital-to-
& DAC analog converter. This is the analog control signal output at the 6270's
CMD terminal.
TFB Position of Servo Feedback Transfers [or assigns/compares] the actual position of the transducers
& FB Devices selected for feedback (see SFB).
TGAIN Transfer Servo Gains Transfers the currently active set of PIV&F gains. The servo gain set
reported represents the last gain values specified with the individual servo
gain commands (SGI, SGP, SGV, SGAF, and SGVF), or the last gain set
enabled with the SGSET command.
TPC Position Commanded Transfers [or assigns/compares] the commanded position (intermediate
& PC position setpoint) to the drive or valve.
TPER Position Error Transfers [or assigns/compares] the error between the commanded
& PER position (TPC) and the actual position (TFB, or TPE, TLDT, TANI) as
measured by the feedback device.
TSGSET Transfer Servo Gain Set Transfers a previously-saved set of servo gain parameters. A gain set is
saved with the SGSET command.
TSTLT Transfer Servo Settling Time Transfers the time it took the last move to settle within the target zone (that
is, within the distance zone defined by STRGTD and within the velocity zone
defined by STRGTV). The Target Zone Mode does not need to be enabled to
use this command.

* The negative gain commands (SGAFN, SGIN, SGPN, SGVN, and SGVFN) allow you to establish gains to be used when the
position error is negative. The negative gain value will track the positive gain value until a separate negative gain value is
entered. To re-establish the default mode where the negative gain tracks the positive gain, issue the negative gain
command with a minus sign (-) in the command field for the affected axis (e.g., SGPN,- restores axis 2 to the default mode).

➃ Servo Tuning 37
Servo Control Techniques
To ensure that you are tuning your servo system properly, you should
understand the tuning techniques described in this section.
The 6270 employs a PIV&F servo control algorithm. The control techniques
available in this system are as follows:
P .....Proportional Feedback (controlled with the SGP and SGPN commands)
I ......Integral Feedback (controlled with the SGI and SGIN commands)
V.....Velocity Feedback (controlled with the SGV and SGVN commands)
F .....Velocity and Acceleration Feedforward (controlled by the SGVF and SGAF
and SGVFN and SGAFN commands, respectively)

The block diagram below shows these control techniques in relation to the
servo control algorithm configuration. The following table presents a
condensed summary of each control's effect on the servo system.

Servo Control Algorithm

Servo System
Velocity Feedforward
(SGVF & SGVFN) Current, Torque, or Velocity
Control Signal
Acceleration Feedforward
(SGAF & SGAFN) Dither Control
Variable Integral Limit Output Offset, Frequency (SDTFR)
Servo
(set with SGILIM) Direction Dependent and Amplitude Digital-to-Analog Valve or
(SOFFS) and (SOFFSN) (SDTAMP) Conversion (DAC) Motor/Drive
Position
Integral Feedback +
error (SGI & SGIN) + +10V System
+ +
+ + -10V
- Proportional Feedback - Analog
(SGP & SGPN) Control Signal Position
Velocity Feedback Feedback
(SGV & SGVN ) Device

Disturbance Steady Tracking

Gain Stability Damping Rejection State Error Error
Proportional (SGP and SGPN) Improve Improve Improve Improve Improve
Integral (SGI and SGIN) Degrade Degrade Improve Improve Improve
Velocity Feedback (SGV and SGVN) Improve Improve ------------- ------------- Degrade
Velocity Feedforward (SGVF and SGVFN) ------------- ------------- ------------- ------------- Improve
Acceleration Feedforward (SGAF and SGAFN) ------------- ------------- ------------- ------------- Improve

Proportional Feedback Control (SGP)

NOTE: The Proportional feedback is the most important feedback for
proportional feedback
gain (SGP) should stabilizing a servo system. When the 6270 uses proportional feedback,
never be set to zero, the control signal is linearly proportional to the position error (the
except when open-loop difference between the commanded position and the actual position—see
operation is desired.
TPER command). The proportional gain is set by the Servo Gain
Proportional (SGP) command. Proportional feedback can be used to make
the servo system more responsive, as well as reduce the steady state position
error.
Since the control is proportional to the position error, whenever there is any
disturbance (such as torque ripple or a spring load) forcing the load away
from its commanded position, the proportional control can immediately
output a signal to move it back toward the commanded position. This
function is called disturbance rejection.
If you tune your system using only the proportional feedback, increasing the
proportional feedback gain (SGP value) too much will cause the system
response to be oscillatory, underdamped, or in some cases unstable.

38 6270 Motion Controller User Guide

Integral Feedback Control (SGI)
Using integral feedback control, the value of the control signal is integrated
at a rate proportional to the feedback device position error. The rate of
integration is set by the Servo Gain Integral (SGI) command.
In most hydraulic servo applications, it is best to set the SGI gain to zero
(SGIØ,Ø) during the move. Then, after motion has stopped, use the proper SGI
gain to hold the position.
The primary function of the integral control is to overcome friction and/or
gravity and to reject disturbances so that steady state position error can be
minimized or eliminated. This control action is important for achieving
high system accuracy. However, if you can achieve acceptable position
accuracy by using only the proportional feedback (SGP), then there is no need
to use the integral feedback control.
In the task of reducing position error, the integral gain (SGI) works
differently than the proportional gain (SGP); this is because the magnitude of
its control signal is not dependent on the magnitude of the position error as
in the case of proportional feedback. If any position error persists, then the
output of the integral term will ramp up over time until it is high enough to
drive the error back to zero. Therefore, even a very small position error can
be eliminated by the integral feedback control. By the same principle,
integral feedback control can also reduce the tracking error when the system
is commanded to cruise at constant velocity.
Controlling If integral control (SGI) is used and an appreciable position error has
Integral Windup persisted long enough during the transient period (time taken to reach the
setpoint), the control signal generated by the integral action can end up too
high and saturate to the maximum level of the controller's analog control
signal output. This phenomenon is called integrator windup.
After windup occurs, it will take a while before the integrator output returns
to a level within the limit of the controller's output. Such a delay causes
excessive position overshoot and oscillation. Therefore, the integral windup
limit (SGILIM) command is provided for you to set the absolute limit of the
integral and, in essence, turn off the integral action as soon as it reaches the
limit; thus, position overshoot and oscillation can be reduced (see
illustration below). The application of this feature is demonstrated in Step 4
of the Tuning Procedure below.
Without SGILIM With SGILIM
Position Overshoot

Position Setpoint Position Setpoint

(D Command) (D Command)
Position

Position

Position Error at T1

Time Time

Internal Actual Output

Integral Generated
Value by the Integral Term
Integral at T1
Max. Analog Output (+10V) Max. Analog Output (+10V)

Integral
Windup wd Windup Limit wd
Duration (SGILIM)
(wd)

0V 0V
T1

wd

Min. Analog Output (-10V) Min. Analog Output (-10V)

➃ Servo Tuning 39
Velocity Feedback Control (SGV)
When velocity feedback control is used, the control signal is proportional to
the feedback device's velocity (rate of change of the actual position). The Servo
Gain Velocity (SGV) command sets the gain, which is in turn multiplied by the
feedback device's velocity to produce the control signal. Since the velocity
feedback acts upon the feedback device's velocity, its control action essentially
anticipates the position error and corrects it before it becomes too large. Such
control tends to increase damping and improve the stability of the system.
A high velocity feedback gain (SGV) can also increase the position tracking
error when traveling at constant velocity. In addition, setting the velocity
feedback gain too high tends to slow down (overdamp) the response to a
commanded position change. If a high velocity feedback gain is needed for
adequate damping, you can balance the tracking error by applying velocity
feedforward control (increasing the SGVF value—discussed below).
Since the feedback device's velocity is derived by differentiating the feedback
device's position with a finite resolution, the finite word truncation effect
and any fluctuation of the feedback device's position would be highly
magnified in the velocity value, and even more so when multiplied by a high
velocity feedback gain. When the value of the velocity feedback gain has
reached such a limit, the motor (or hydraulic cylinder, etc.) will chatter
(high-frequency, low-amplitude oscillation) at steady state.

Velocity Feedforward Control (SGVF)

The purpose of velocity feedforward control is to improve tracking
performance; that is, reduce the position error when the system is
commanded to move at constant velocity. The tracking error is mainly
attributed to three sources—friction, torque load, and velocity feedback
control (SGV).
Velocity feedforward control is directed by the Servo Gain Velocity
Feedforward (SGVF) setting, which is in turn multiplied by the rate of change
(velocity) of the commanded position to produce the control signal.
Consequently, because the control signal is now proportional to the velocity
of the commanded position, the 6270 essentially anticipates the commanded
position and initiates a control signal ahead of time to more closely follow
(track) the commanded position.
Applications requiring linear interpolation can benefit from improved
tracking performance; however, if your application only requires short,
point-to-point moves, velocity feedforward control is not necessary.
Because velocity feedforward control is not in the servo feedback loop (see
Servo Control Algorithm drawing above), it does not affect the servo system's
stability. Therefore, there is no limit on how high the velocity feedforward
gain (SGVF) can be set, except when it saturates the control output (tries to
exceed the 6270's analog control signal range).

Acceleration Feedforward Control (SGAF)

The purpose of acceleration feedforward control is to improve position
tracking performance when the system is commanded to accelerate or
decelerate.
Acceleration feedforward control is directed by the Servo Gain Acceleration
Feedforward (SGAF) setting, which is in turn multiplied by the acceleration of
the commanded position to produce the control signal. Consequently,
because the control signal is now proportional to the acceleration of the
commanded position, the 6270 essentially anticipates the velocity of the
commanded position and initiates a control signal ahead of time to more
closely follow (track) the commanded position.

40 6270 Motion Controller User Guide

 Same as velocity feedforward control, this control action can improve the
performance of linear interpolation applications. In addition, it also
reduces the time required to reach the commanded velocity. However, if your
application only requires short, point-to-point moves, acceleration
feedforward control is not necessary.
Acceleration feedforward control does not affect the servo system's stability,
nor does it have any effect at constant velocity or at steady state.

Negative Gains
The negative gain commands (see list below) allow you to establish gains to
be used when the position error is negative.
SGPN ........Proportional Gain Negative
SGVN ........Velocity Gain Negative
SGIN ........Integral Gain Negative
SGVFN ......Velocity Feedforward Gain Negative
SGAFN ......Acceleration Feedforward Gain Negative

Negative Position Error SGPN

(negative gains used) SGVN
SGIN
SGVFN
SGAFN
Commanded Steady State
Position

Position Position Error

SGP
SGV
Positive Position Error
SGI
Actual (positive gains used)
Position SGVF
SGAF
Time

The 6270 automatically switches between the positive and negative gains to
correlate with the positive or negative position error. This is particularly
useful when controlling hydraulic cylinders in which the different surface
areas on each side of the piston react differently with the same gain settings.
Each negative gain changes with (tracks) the corresponding positive gain
value until a negative gain command is executed. For example, the SGPN
value automatically tracks the SGP value until an SGPN command is executed
with a value other than the current SGP command value. After the negative
gain command is executed, separate positive and negative gain commands
must be used.
To re-establish the default mode where the negative gain tracks the positive
gain, issue the negative gain command with a minus sign (-) in the command
field for the affected axis (e.g., SGPN,- restores axis 2 to the default mode).

Gain Sets
An added dimension to the control techniques discussed in the previous
section is to group the gains into “gains sets” that can be invoked to affect
motion under certain conditions. Gain sets may be useful for applications in
which you would like to invoke different gains a different portions of a move
profile, or at rest, or based on an external process, etc.
The SGSET command allows you to save the currently active gains, control
signal offsets (SOFFS and SOFFSN), and maximum position error (SMPER)
setting, to a specified gain set (see list below).
SGPN ........Proportional Gain Negative
SGVN ........Velocity Gain Negative

➃ Servo Tuning 41
 SGIN ....... Integral Gain Negative
SGVFN...... Velocity Feedforward Gain Negative
SGAFN...... Acceleration Feedforward Gain Negative
SGILIM .... Integral Windup Limit
SOFFS...... Servo Control Signal Offset
SOFFSN .... Servo Control Signal Offset Negative
SMPER...... Maximum Allowable Position Error

The gain set saved with the SGSET command can be enabled/recalled later
with the SGENB command or the SSWG command. Using the SGENB command,
the gains can be enabled during motion at any specified point in the profile,
or when not in motion (see programming example below). If using the SSWG
command, the gain set is referred to as the “setpoint window gain set” and is
invoked after the commanded profile is complete (see Setpoint Window
Gains below for details).

NOTE
The tuning gains saved to a given gain are specific to the current feedback source (selected
with the last SFB command) at the time the gains were saved with the SGSET command.
Later, when you enable the saved gain set, make sure that the gain set you enable
is appropriate to the feedback source you are using at the time.

To display the gain values currently in effect, use the TGAIN command. To
display the contents of a particular gain set, use the TSGSET command.

Setpoint Window Gains

You may activate a specified set of gains to be used when with a defined
region either side of the position setpoint (“setpoint window”).
The setpoint window is defined by the SSWD command, which establishes the
distance on both sides of the position setpoint (“setpoint window”) in which
the gain set specified with the SSWG command is used. Specifically, the gain
set is automatically invoked by the controller after the commanded move
profile is complete and the position error is within the setpoint window. The
SSWG gain set accommodates different proportional, integral, and velocity
gains and offsets for each direction.
The setpoint window includes a hysteresis loop equal to 25% of the value
used in the SSWD command.
If you want to disable the servo control loop when the position error is
within the setpoint window, set all of the gain values to zero—you can do this
by executing all the individual gain commands or by executing the SFBØ
command. If you want to disable the offsets (SOFFS & SOFFSN) when the
position error is within the setpoint window, set them to zero volts.
To disable the To assign a gain set as the setpoint window gain set with the SSWG command,
setpoint window gain
feature, use the you must first define/save the gain set with the SGSET command (see
SSWGØ command. programming example below).

42 6270 Motion Controller User Guide

 DAC
Output
(+)
Normal Gainset

DACLIM

SSWD SSWD

Setpoint Window
Hysteresis (25%)
Setpoint Window
SOFFS Gainset (SSWG)

Position SOFFSN Position

Error (-) Error (+)

Setpoint Window

DACMIN

DAC
Normal Gainset Output
(-)

The diagram above makes two assumptions. First, for simplicity, only a proportional gain is being used. Second,
related to the SSWG gain set, the proportional gains (SGP and SGPN) are lower than those used in the normal gain
set and the offsets (SOFFS and SOFFSN) are set to zero.
The arrows on the above diagram illustrate the hysteresis loop. The SSWG gains are used until the position error
increases to a value of 25% greater than the number specified in the SSWD command. At this point, the normal
gain set is automatically substituted until the position error falls below the value in the SSWD command when the
SSWG gains are returned.
Finally, the maximum positive DAC voltage is determined by the value in the DACLIM command and the
maximum negative value is determined by the DACMIN command.

Target Zone — An alternative to SSWD

You can use the target zone settling mode to override the setpoint window distance (SSWD)
and introduce distance and velocity end-of-move criteria to define when the controller
switches to the setpoint window gains. When using the target zone settling mode (enabled
with the STRGTE command), after completion of the commanded move profile, the actual
position and actual velocity must be within the “target zone” (i.e., within the position band
defined by STRGTD and within the velocity band defined by STRGTV) before motion can be
determined complete. After that point, the controller will switch to the SSWG gains. For more
information on the target zone mode, refer to the Target Zone section later in this chapter.

Programming Exam p le D escr ip t io n

Example > SGP35 Set proportional gain to 35
> SGI3 Set integral gain to 3
> SGSET3 Save current gains as gain set #3 (to use when within the setpoint window)
> SSWG3 Assign gain set #3 as the setpoint window gain set
> SGP5Ø While moving, use higher proportional gain
> SGIØ While moving, use no integral gain,
> SGV4 While moving, use introduce velocity gain
> SSWD1ØØ After the commanded move profile is complete, the controller will use gain
set #3 if within 100 counts (125 counts including hysteresis) of the
setpoint position

➃ Servo Tuning 43
Open Loop Operation
When the control algorithm is used, the whole servo system is a closed-loop
system (see diagram below). It is called closed loop because the control
algorithm accounts for both the command (position, velocity, tension, etc.)
and the feedback data (from the LDT, encoder, or ANI input); therefore, it
forms a closed loop of information flow.
Closed Lo op System
Offset
Control Drive Command = Servo Hydraulic
Command Digital Signal Control Signal + Offset Valve Cylinder,
Control or Motor, Load
Algorithm Drive etc.

Feedback Data Feedback Device

(LDT, Encoder, or AN Input)

When all gains are set to zero, the digital control algorithm is essentially
disabled and the system becomes an open loop system (see diagram below).
Open loop operation could be used during system setup or troubleshooting so
that you can independently test the drive/motor or valve operation (the
Tuning Setup Procedure below demonstrates open-loop operation).
Offset
Open Loop System SOFFS Servo Hydraulic
Offset Drive Command = Offset Valve Cylinder,
or Motor, Load
Drive etc.

Feedback Device
(LDT, Encoder, or AN Input)

There are methods of entering the open loop mode:

• Disable each individual positive and negative gain value. This allows the
controller to monitor the position error and signal a fault if the maximum limit is
exceeded.
• Issue the SFBØ command. This automatically invokes the following conditions:
- WARNING — The hardware and software end-of-travel limits are
disabled. Make sure that it is safe to operate without end-of-travel limits
before using the open-loop function.
- Gain values (SGILIM, SGAF, SGAFN, SGI, SGIN, SGP, etc.) set to zero (open-
loop operation).
- SMPER value set to zero (position error is allowed to increase without
causing a fault).
- Subsequent attempts to change gain values or SMPER will cause an error
message (“NOT ALLOWED IF SFBØ”).
- SOFFS and SOFFSN set to zero, but allows subsequent servo offset changes
to affect motion.
- SSWG set to zero (disables the Setpoint Window Gains feature).
- Disables output-on-position (OUTPA - OUTPD) functions.
- Any subsequent changes to PSET, PSETCLR, SCLD, SCLA, SCLV, SOFFS, and
SOFFN are lost when another feedback source is selected.
Recommendation — Use the Disable Drive On Kill mode, enabled with the
KDRIVE command, so that the 6270 will shut down the valve/drive if a kill
command (e.g., !K) is executed or if a kill input is activated. CAUTION:
Shutting down a valve/cylinder system returns the valve to the null
position; shutting down a rotary drive system allows the load to freewheel if
there is no brake installed.

44 6270 Motion Controller User Guide

Tuning Setup Procedure
Use the following procedure to set up your servo system before completing the
tuning procedures. You can perform this procedure for both axes
simultaneously.

Before you set up for tuning:

Do not begin this procedure unless you are sure you have successfully completed these
system connection, test, and configuration procedures provided in Chapter 3:
• Connect the valve or the drive (especially the drive's shutdown output).
• Connect and test the feedback devices.
• Connect and test the end-of-travel limits.
• Test the 6270's analog output.
• Attach the load and the feedback devices as required for your application.
• Configure the number of axes in use, drive fault level (if using a rotary drive), and
feedback device resolution.
• Select the appropriate feedback source per axis with the SFB command (tuning
parameters for each axis are specific to the currently selected feedback source).

WARNING
The tuning process requires operation of your system's electrical and mechanical
components. Therefore, you should test your system for safety under all potential conditions.
Failure to do so can result in damage to equipment and/or serious injury to personnel.
E M E R G E N C Y S H U T D O W N : You should be prepared to shut down the valve or drive
during the tuning process (for instance, if the system becomes unstable or experiences a
runaway). You can use the E N B L input (disconnect it from ground) to disable the 6270's
analog output signal. An alternative is to issue the @DRIVEØ command to the 6270 over the
communication interface, but this requires connecting a shutdown output to the drive. If the
drive does not have a shutdown input, use a manual emergency stop switch to disable the
valve's/drive's power supply.

Step 1 Drive Users Only: If you are using a rotary drive, make sure the power to
the drive is off.
Step 2 Apply power to the 6270 only and issue the DRIVE11 command. Measure the
6270's analog output between the CM D + and CM D - terminals on the D R IVE
connector with both an oscilloscope to check for noise and a digital volt-meter
(DVM) to monitor the analog output. Both readings should be very close to
zero. If an offset exists, ignore it for now; it will be taken care of later in step 8.
Step 3 If your system has mechanical stops, manually move the load to a position
mid-way between them.
Step 4 Enter these commands to zero all the gains and run the system in open loop:
Open-Loop Operation ☞ C o m m an d D escr ip t io n
> SGPØ,Ø Set the proportional feedback gain to zero
> SGVØ,Ø Set the velocity feedback gain to zero
> SGIØ,Ø Set the integral feedback gain to zero
> SGVFØ,Ø Set the velocity feedforward gain to zero
> SGAFØ,Ø Set the acceleration feedforward gain to zero
Step 5 Drive Users Only: Apply power to the drive. The motor shaft should be
stationary or perhaps turning very slowly (velocity drives only). A small
voltage to a torque drive, with little or no load attached, will cause it to
accelerate to its maximum velocity. Since the torque demand at such a low
voltage is very small, you can prevent the shaft from moving by holding it.

➃ Servo Tuning 45
 Step 6 Observe the 6270's analog output noise level on the oscilloscope. Typically, the
ideal noise level should be below 3.0mV, but inevitably you must determine the
☞ acceptable noise level for your application.
The control method (voltage
or current output) is selected If the noise level is acceptable, proceed to Step 7. If the noise level is too high:
with internal jumpers. The
default is voltage output. If
you required current output, a . Turn all the power off and tie the earth grounds of all the electrical
refer to the instructions in components of your system to a single common earth ground point.
Chapter 6 to change the
appropriate jumpers. b. Shield the drive or valves properly and shield all the wiring that
interconnect the components.
c. After you have completed a and b above, turn on the controller only and
start over from Step 2. If the noise level is still unacceptable, consult the
noise suppression techniques described in Appendix A.
Step 7 The purpose of this step is to ensure that a positive voltage on the 6270's
analog control signal output (from the CM D + and CM D - terminals) results in
the feedback device counting in the positive direction.
a . Using the SMPER command, set the maximum allowable position error to 1
inch by entering the SMPER1,1 command. This assumes the default scaling
factor (1 distance value = 432 counts, or 1 inch) is still in effect.
b. Enter the TFB command to check the current position of the feedback
devices. Record this number for later use.
c. CAUTION
The offset introduced in this step may cause an acceleration to a high speed, if there is
little or no load.

Enter the SOFFSØ.2 command to introduce an offset DAC output value of

0.2V to make the servo mechanism move slowly in the positive (clockwise
or extension) direction. (Motion will stop when the maximum allowable
position error is exceeded.) If the load has a large stiction component, you
may need to use a larger offset (SOFFS command) to overcome stiction and
affect motion.
d. Use the TFB command again to observe the feedback device's position.
The value should have increased from the value observed in Step 7.b.
If the position reading decreases when using a positive SOFFS setting, turn
As an alternative to
swapping CMD+ and off the 6270 (and the drive, if using one) and swap the CM D + and C M D -
CMD-, you could use the connections either at the 6270 or at the valve/drive, whichever is more
appropriate feedback accessible (this will not work for servo valves/drives that do not accept
polarity reversal
command (LDTPOL, differential input). Then turn on the 6270 again, enter the DRIVE11
ENCPOL, or ANIPOL). command, and repeat Steps 4 through 7.d. before proceeding to Step 8.
e. Enter the SOFFSØ command to stop the motor, and enter the DRIVE11
command to re-enable the drives.
Step 8 Having set the servo output offset to zero with the SOFFSØ command (see Step
7.e.), read the 6270's analog output with the DVM to determine if there is any
offset caused by the electrical interconnections between the 6270 and the valve
or drive.
☞ If the DVM reads anything other than zero, enter the DVM's reading (but with
If you are using current control, the opposite polarity) as the offset adjustment with the SOFFS command. For
convert the offset from
milliamps to volts (output range example, if the DVM reading is 0.015V, then enter SOFFS-Ø.Ø15. If, after doing
in mA ÷ 10V = mA/V) and enter this, the reading is still not zero, then fine-tune it by trying SOFFS entries of
the result in the SOFFS
command. slightly different values until the DVM reading is between ±3.0mV.

46 6270 Motion Controller User Guide

 Step 9 Drive Users Only: If you are using a velocity drive, motion may still be
occurring due to the drive's balance/offset setting. If so, adjust the drive's
balance/offset until motion stops. Consult the drive's user documentation
for instructions.
Step 10 Proceed to the Drive Tuning Procedure section to tune the velocity drive (if you
are using a torque drive or a valve, skip to the Controller Tuning Procedure).

Drive Tuning Procedure (Velocity Motor Drives Only)

The goals of the Drive Tuning Procedure are to:
1. Tune the drive to output the desired velocity at a given voltage from the 6270.
2. Tune the drive (iteratively) to achieve the desired response.

NOTE
Be sure to complete the Tuning Setup Procedure before proceeding with the following drive
tuning procedure. Unlike the Tuning Setup Procedure, you must tune one axis at a time.

Step 1 Tune the drive to output the desired velocity at a given voltage from the 6270:
a . If your system has mechanical stops, manually move the load to a
position mid-way between them.
b. Enter the SOFFS command to set the 6270's output voltage to its maximum
level, 10.0 volts (SOFFS1Ø for axis 1, or SOFFS,1Ø for axis 2).
c. Adjust the drive gain factor (sometimes called the tach gain) such that when
the 6270's command output is 10V, the velocity just reaches its maximum
value (check the velocity with the TVELA command). Refer to your drive's
user documentation if necessary.

EXAMPLE
Suppose your drive can run at a max. velocity of 7000 rpm (or 116.67 rps). If the drive
gain factor is 20 rps/V, then the drive will reach the maximum velocity (116.67 rps) when
the 6270's command output is only 5.833V. This means the full range of ±10V is not fully
usable. To use the full range of ±10V, the gain factor has to be adjusted to 11.667 rps/V.
Drive manufactures usually provide a potentiometer for adjusting this gain factor. Some
manufacturers provide a few preset values selectable with jumpers or DIP switches.

Step 2 Tune the drive (iteratively) to achieve the desired response:

a . Enter the following commands to create and execute a step velocity
command:
C o m m an d D escr ip t io n
> DEF STEP Begin definition of the program called STEP
- @SGPØ Set the SGP gain to zero
- @SGIØ Set the SGI gain to zero
- @SGVØ Set the SGV gain to zero
- @SGAFØ Set the SGAF gain to zero
- @SGVFØ Set the SGVF gain to zero
- @SMPERØ Disable checking the maximum allowable position error
- @SOFFSØ.5 Set the command output to 0.5 volts
- T1 Wait for 1 second
- @SOFFSØ Set the command output to zero volts (stopping the motor)
- @SMPER1 Re-enable checking the maximum allowable position error
- END End definition of the program called STEP
> STEP Execute the program called STEP (the motor will move for 1
second and then stop)

➃ Servo Tuning 47
 b. Observe the plot of the commanded velocity versus the actual velocity on
the oscilloscope.
Using the tuning methods specified in the drive's user documentation,
tune the drive to achieve a first-order response (no overshoot) as
illustrated below—repeat Steps 2.a. and 3.b. as necessary.
Command Velocity

VELOCITY
Actual Velocity

TIME

Step 3 Proceed to the Controller Tuning Procedure section to tune the 6270.

Controller Tuning Procedure

The Controller Tuning Procedure leads you through the following steps:
1. Setup up for tuning.
2. Select the 6270's servo Sampling Frequency Ratios (SSFR).
3. Set the Maximum Position Error (SMPER).
4. Optimize the Proportional (SGP) and Velocity (SGV) gains.
5. Use the Integral Feedback Gain (SGI) to reduce steady state error.
6. Use the Velocity Feedforward Gain (SGVF) to reduce position error at constant
velocity.
7. Use the Acceleration Feedforward Gain (SGAF) to reduce position error during
acceleration and deceleration.

Before you tune the 6270:

Be sure to complete the Tuning Setup Procedure (and the Drive Tuning Procedure, if you are
using a velocity drive) before proceeding with the following tuning procedure. Unlike the
Tuning Setup Procedure, you must tune one axis at a time.
If your application requires switching between feedback sources on the same axis, then for
each feedback source on each axis you must select the feedback source with the SFB
command and repeat steps 3-7.

Step 1 Set up for tuning:

Use a computer (with a terminal emulator) or a dumb terminal to enter the
commands noted in the steps below. To monitor system performance, you
may use visual inspection, or use an analog type position transducer
(potentiometer, LVDT, RVDT, etc.) to pick up the load's or motor's position
displacement and monitor the transducer output on a digital storage
oscilloscope.
Step 2 Select the sampling frequency ratios (SSFR) and max. position error (SMPER)
The 6270's control signal is computed by the digital signal processor (DSP).
The velocity of the commanded position, the velocity of the feedback device's
position, and the integral of the position error are used for various control
actions. These measurements are derived by the DSP from the position
values sampled periodically at a fixed rate; this sampling rate is called the
servo sampling frequency (samples/second).

NOTE
The SSFR setting affects the dither frequency ratio (SDTFR setting) and the LDT position update
rate (LDTUPD setting). If the sampling rate is too fast for the LDT, position errors or bad LDT
reads will occur. The occurrence of read errors can be monitored with the TAS and [ AS ]
command bit #27. Refer to the SSFR, SDTFR, and LDTUPD command descriptions in the 6000
Series Software Reference Guide for more details.

48 6270 Motion Controller User Guide

 Higher sampling frequencies improve the accuracy of the velocity and
integral values derived. A higher sampling frequency can also improve the
tracking of a rapidly changing or oscillating position. Therefore, the servo
sampling frequency is a key parameter that influences the servo system's
stability and closed loop bandwidth.
In addition to computing the 6270's control signal, the DSP also computes the
commanded position trajectory. When the servo sampling frequency is
increased, the motion trajectory update rate has to be decreased, and vice
versa. The ratio between the servo sampling frequency and the trajectory
update rate, called the sampling frequency ratio, depends on the requirements
of your application and/or the dynamic characteristics of the system. The
Servo Sampling Frequency Ratio (SSFR) command offers four selectable ratio
settings. These four ratios and the actual sampling frequencies and sampling
periods (reciprocal of sampling frequency) are shown below.

NOTE
Changing the active axes with the INDAX command will change the SSFR ratio.

# of Axes SSFR Servo Sampling Update Motion Trajectory Update System Update
Active Command Frequency Period Frequency Period Frequency Period
(INDAX) Setting (samples/sec.) (µsec) (samples/sec.) (µsec) (samples/sec.) (µsec)
INDAX1 SSFR1 3030 330 3030 330 757 1320
INDAX1 SSFR2 4000 250 2000 500 500 2000
INDAX1 SSFR4 4651 215 1163 860 581 1720
INDAX1 SSFR8 4878 205 610 1640 610 1640
INDAX2 SSFR1 1667 600 1667 600 417 2400
INDAX2 SSFR2 2272 440 1136 880 568 1760
Default → INDAX2 SSFR4 2500 400 625 1600 625 1600
INDAX2 SSFR8 2667 375 333 3000 333 3000

The general rule to determining the proper SSFR value is to first select the
slowest servo sampling frequency that is able to give a satisfactory response.
This can be done by experiment or based on the closed-loop bandwidth
requirement for your application. (Keep in mind that increasing the SSFR
value allows for higher bandwidths, but produces a rougher motion profile;
conversely, decreasing the SSFR value provides a smoother profile, but makes
the servo system less stable and slower to respond.)
As an example, if your application requires a closed-loop bandwidth of
300 Hz, you can determine the minimum servo sampling frequency by using
the rule of thumb—setting the servo sampling frequency at least 8 times
higher than the bandwidth frequency—the required minimum servo
sampling frequency would be 2400 Hz. If two axes are running (INDAX2), then
you should try using the SSFR4 setting.
The table below provides guidelines for various application requirements.
Application Requirement SSFR1 SSFR2 SSFR4 SSFR8
XY Linear Interpolation ✔ ✔
Fast point-to-point motion ✔ ✔
Regulation (speed, torque, etc.) ✔ ✔
High natural frequency system ✔

Setting the Sampling Frequency Ratio

Select a sampling ratio (with the SSFR command) appropriate to your system now, before you
proceed to tune each gain.
If you change the sampling frequency ratios (SSFR) after the tuning is complete and the new
servo sampling frequency is lower than the previous one, the response may change (if your
system bandwidth is quite high) and you may have to re-tune the system.

➃ Servo Tuning 49
 Step 3 Set the Maximum Position Error (SMPER):
The SMPER command allows you to set the maximum position error allowed
before an error condition occurs. The position error, monitored once per
system update period, is the difference between the commanded position and
the actual position as read by the feedback device selected with the last SFB
command. Larger values allow greater oscillations/motion when unstable;
therefore, smaller SMPER values are safer.
When the position error exceeds the value entered by the SMPER command, an
error condition is latched (see TAS or AS bit #23) and the 6000 controller
issues a shutdown to the faulted axis and sets its analog output command to
zero volts. To enable the system again, the appropriate DRIVE1 command
must be issued, which also sets the commanded position equal to the actual
feedback device position (incremental devices will be zeroed).
If the SMPER value is set to zero, the position error condition is not
monitored, allowing the position error to accumulate without causing a
fault.
Step 4 Optimize the Proportional (S G P ) and Velocity (S G V ) gains (see illustration for
tuning process ):
a . Enter the following commands to create a step input profile (use a comma
in the first data field when tuning axis 2—e.g., D,1ØØ):
C o m m an d D escr ip t io n
> A999 Set acceleration to 999 units/sec2
> AD999 Set deceleration to 999 units/sec2
> V3Ø Set velocity to 30 units/sec
> D1ØØ Set distance to 100 units

b. Start with an SGP command value of 0.5 (SGPØ.5 or SGP,Ø.5).

c. Enter the GO1 or GO,1 command depending on which axis is being tuned at
the time.
d. Observe the plot of the commanded position versus the actual position on
the oscilloscope. If the response is already very oscillatory, lower the
gain (SGP); if it is sluggish (overdamped), increase the SGP gain.
Repeat Steps 4.c. and 4.d. until the response is slightly under-damped.
e. Start with an SGV command value of 0.1 (SGVØ.1 or SGV,Ø.1).
f . As you did in Step 4.c., enter GO1 or GO,1.
g. Observe the plot on the oscilloscope. If the response is sluggish
(overdamped), reduce the SGV gain. Repeat Steps 4.f. and 4.g. until the
response is slightly under-damped.
☞ h . The flow diagram below shows you how to get the values of the
Refer to the Tuning proportional and velocity feedback gains for the fastest, well-damped
Scenario section response in a step-by-step fashion. The tuning principle here is based on
later in this chapter
for a case example. these four characteristics:
• Increasing the proportional gain (SGP) can speed up the response time and
increase the damping.
• Increasing the velocity feedback gain (SGV) can increase the damping more so
than the proportional gain can, but also may slow down the response time.
• When the SGP gain is too high, it can cause instability.
• When the SGV gain is too high, it can cause the motor (or valve, hydraulic
cylinder, etc.) to chatter.

50 6270 Motion Controller User Guide

 START

Increase SGP
UNTIL

OR

OR

Decrease SGV
UNTIL Increase SGV
UNTIL

OR

Decrease SGV
UNTIL

OR
STOP

Decrease SGP
UNTIL

OR

Increase SGV
UNTIL OR

Decrease SGV
UNTIL

Step 5 Use the Integral Feedback Gain (S G I ) to reduce steady state error:

☞ a . Determine the steady state position error (the difference between the
Steady state position error is commanded position and the actual position). You can determine this
described earlier in the error value by the TPER command when the load is not moving.
Performance Measurements
section.

NOTE
If the steady state position error is zero or so small that it is acceptable for your
application, you do not need to use the integral gain. For hydraulic
applications, it is usually best to use a small SGI value, or use SGIØ while moving and
use SGIn when stopped. The use of the Target Zone Settling Mode (STRGTE) is
recommended.

➃ Servo Tuning 51
 b. If you have to enter the integral feedback gain to reduce the steady error,
start out with a small value (e.g., SGIØ.1). After the gain is entered,
observe two things from the response:
• Whether or not the magnitude of steady state error reduces
• Whether or not the steady state error reduces to zero at a faster rate
c. Keep increasing the gain to further improve these two measurements until
the overshoot starts to increase and the response becomes oscillatory.
d. There are three things you can do at this point (If these three things do not
work, that means the integral gain is too high and you have to lower it.):
1st Lower the integral gain (SGI) value to reduce the overshoot.
2 nd Check whether the 6270's analog output saturates the ±10V limit; you can
do this by observing the signal from a digital oscilloscope. If it saturates,
then lower the integral output limit by using the SGILIM command. This
should help reduce the overshoot and shorten the settling time.
Sometimes, even if the analog output is not saturated, you can still reduce
the overshoot by lowering SGILIM to a value less than the maximum
☞ output value. However, lowering it too much can impair the effectiveness
If you are using current control, of the integral feedback.
convert the offset from
milliamps to volts and enter the 3 rd You can still increase the velocity feedback gain (SGV value) further,
result in the SGILIM
command. provided that it is not already at the highest possible setting (causing
the motor or valve to chatter).
Step 6 Use the Velocity Feedforward Gain (SGVF) to reduce position error at
constant speed:
a . Execute a continuous (MC1 command) move, setting the acceleration,
deceleration and velocity values appropriate to your application. Set the
SGVF value to be the product of SGP ∗ SGV (if SGV = zero, set SGVF equal to SGP).

b. Check the position error at constant velocity by issuing the TPER

command.

c. Increase SGVF to reduce the position error (repeat steps a and b as

necessary).
Step 7 Use the Acceleration Feedforward Gain (SGAF) to reduce position error
during acceleration:
a . Execute a continuous (MC1 command) move, setting the acceleration,
deceleration and velocity values appropriate to your application. Set SGAF
to 0.01 (SGAFØ.Ø1).

b. Check the position error during acceleration by issuing the TPER

command.

c. Increase SGAF to reduce the position error (repeat steps a and b as

necessary).

52 6270 Motion Controller User Guide

Tuning Scenario
This example shows how to obtain the highest possible proportional feedback
(SGP) and velocity feedback (SGV) gains experimentally by using the flow
diagram illustrated earlier in Step 4 of the Tuning Procedure.

NOTE
The steps shown below (steps 1 - 11) represent the major steps of the process; the actual
progression between these steps usually requires several iterations.

The motion command used for this example is a step command with a step
size of 100. The plots shown are as they might appear on a scope (X axis =
time, Y axis = position).
Step 1 For a starting trial, we set the Commanded Position
proportional feedback gain (SGP) to
2. As you can see by the plot, the
response is slow.
In the next step, we should increase
SGP until the response is slightly SGP = 2
underdamped.
Actual Position

Step 2 With SGP equal to 15, the response

becomes slightly underdamped (see
plot).
Therefore, we should introduce the
velocity feedback gain (SGV) to damp SGP = 15
out the oscillation.

Step 3 With SGV equal to 2, the response

turns out fairly well damped (see
plot).
At this point, the SGP should be
raised again until oscillation or SGP = 15
excessive overshoot appears. SGV = 2

Step 4 As we iteratively increase SGP to

105, overshoot and chattering
becomes significant (see plot). This
means either the SGV gain is too low
and/or the SGP is too high.
SGP = 105
Next, we should try raising the SGV SGV = 2
gain to see if it could damp out the
overshoot and chattering.
Step 5 After the SGV gain is raised to 2.6,
the overshoot was reduced but
chattering is still quite pronounced.
This means either one or both of the
gains is too high.
SGP = 105
The next step should be to lower the SGV = 2.6
SGV gain first.

➃ Servo Tuning 53
 Step 6 Lowering the SGV gain to 2.3 does not
help reduce the chattering by much.
Therefore, we should lower the SGP
gain until chattering stops.
SGP = 105
SGV = 2.3

Step 7 Chattering stops after reducing the

SGP gain to 85. However, the
overshoot is still a little too high.
The next step should be to try raising
the SGV to damp out the overshoot. SGP = 85
SGV = 2.3

Step 8 After raising the SGV gain to 2.4,

overshoot is reduced a little, but
chattering reappears. This means
the gains are still too high.
Next, we should lower the SGV gain SGP = 85
until chattering stops. SGV = 2.4

Step 9 After lowering the SGV gain to 2.2

(even less than in Step 7—2.3),
chattering stops.
Next we should lower the SGP gain.
SGP = 85
SGV = 2.2

Step 10 Overshoot is reduced very little after

lowering the SGP gain to 70. (The SGV
gain might have been lowered too
much in Step 9.)
Next, we should try raising the SGV SGP = 70
gain again until the overshoot is SGV = 2.2
gone.

Step 11 When we raised the SGV gain to 2.52,

the step response became fast and
very stable.

SGP = 70
SGV = 2.52

54 6270 Motion Controller User Guide

Target Zone (Move Completion Criteria)
Under default operation (Target Zone
Mode not enabled), the 6270's move
completion criteria is simply derived Commanded

Position
from the move trajectory. The 6270 Move is actually
Completed
considers the current preset move to be Actual
Time
complete when the commanded
trajectory has reached the desired target When the Target Zone Mode
is not enabled, the move is
position; after that, subsequent considered to be complete
and subsequent moves can
commands/moves can be executed for now be executed.

Velocity
that same axis. Consequently, the next
move or external operation can begin
Actual
before the actual position has settled to Commanded
the commanded position (see diagram).
Time

To prevent premature command execution before the actual position settles

into the commanded position, use the Target Zone Mode. In this mode,
enabled with the STRGTE command, the move cannot be considered complete
until the actual position and actual velocity are within the target zone (that
is, within the distance zone defined by STRGTD and less than or equal to the
velocity defined by STRGTV). If the load does not settle into the target zone
before the timeout period set with the STRGTT command, the 6270 detects a
timeout error (see illustration below).
Refer to the Error If the timeout error occurs, you can prevent subsequent command/move
Handling section in
the 6000 Series execution only if you enable the ERROR command to continually check for
Software Reference this error condition, and when it occurs to branch to a programmed response
Guide for error
program examples you can define in the ERRORP program.
As an example (assuming scaling is enabled and default scaling values are
used), setting the distance zone to ±0.01 inches (STRGTD.Ø1), the velocity zone
to ≤0.5 ips (STRGTVØ.5), and the timeout period to 1/2 second (STRGTT5ØØ), a
move with a distance of 8 inches (D8) must end up between position 7.99 and
8.01 and settle down to ≤0.5 ips within 500 ms (1/2 second) after the
commanded profile is complete.
Damping is critical. To ensure that a move settles within the distance zone, it must be damped to
the point that it will not move out of the zone in an oscillatory manner. This
helps ensure the actual velocity falls within the target velocity zone set with
the STRGTV command (see illustration below).

Fa ile d Move Comp le tio n S ucce ssful Move Comp le tio n

STRGTD STRGTD
(Distance Zone) (Distance Zone)

Commanded Commanded
Position

Position

Move Move
Completed Completed

Actual Actual

Time Time

Actual Timeout Occurs, Actual

STRGTT Commanded STRGTT
Velocity

Error Bit Set

Velocity

(Timeout Period) (Timeout Period)

Commanded
STRGTV STRGTV
(Velocity Zone) (Velocity Zone)
Time Time

TSTLT
(Actual Settling Time)

➃ Servo Tuning 55
 Checking the Using the TSTLT command, you can display the actual time it took the last
Actual Settling move to settle into the target zone (that is, within the distance zone defined
Time by STRGTD and less than or equal to the velocity defined by STRGTV). The
reported value represents milliseconds. This command is usable
whether or not the Target Zone Settling Mode is enabled with
the STRGTE command.

56 6270 Motion Controller User Guide