10.2478/v10048-011-0013-2
MEASUREMENT SCIENCE REVIEW, Volume 11, No. 2, 2011
Improved Performance of 50 kN Dead Weight Force Machine
using Automation as a Tool
Harish Kumar1, Anil Kumar, Poonam Yadav
1
National Physical Laboratory (Council of Scientific & Industrial Research), New Delhi, 110012, India,
E-mail: kumarh@mail.nplindia.org
Continuously growing technologies and increasing quality requirements have exerted thrust to the metrological institutes to
maintain a high level of calibration and measurement capabilities. Force, being very vital in various engineering and non engineering applications, is measured by force transducers. Deviations in the values observed and mentioned in the calibration
certificate for force transducers may primarily be due to the creep, time loading effect and temperature effect if not properly
compensated. Beside these factors, machine interaction, parasitic components etc. pertaining to the quality of the force standard
machine used for calibration also contribute to the deviations. An attempt has been made by National Physical Laboratory, India
(NPLI) to automate the 50 kN dead weight force machine to minimize the influence of factors other than the factors related to the
machine itself. The calibration of force transducers is carried out as per the standard calibration procedure based on standard
ISO 376-2004 using the automated 50 kN dead weight force machine (cmc ± 0.003% (k=2)) under similar conditions both in
manual mode and automatic mode. The metrological characterization shows improved metrological results for the force
transducers when the 50 kN dead weight force machine is used in automatic mode as compared to the manual mode.
Keywords: Dead weight force machine, automation, force
The pneumatic system has been used for loading and
unloading the weights to minimize the oscillations so that
force is stabilized in the least possible time once applied or
removed. The force in range of 0.5 kN to 50 kN may be
applied by the force and up to 50 kN forces may be applied
in either mode (tension and compression) in any sequence
desired, depending upon the force to be applied [4]. The
maximum force applied is 50 kN and hysteresis calculations
can be done after applying forces in ascending order in the
series and then descending in the same order [5]. The 50 kN
dead weight force machine is able to perform calibrations
according to standards IS 4169-1988, ISO 376-2004 and
ASTM E-74. The expanded uncertainty associated with the
force applied is ± 0.003% (k=2) (Fig.1).
1. INTRODUCTION
F
ORCE MEASUREMENT plays a very important role in
various applications, necessitating its accurate and
precision measurement. The applications are for
example calibration of material testing machines, electronic
weighing apparatus, force measurement in various
machining processes (like turning / drilling / milling),
rolling mills, thrust measurement of jet engines and rockets,
installation of big or heavy machinery, weighing of aircrafts
etc. This has led to the thrust over metrological institutes to
maintain a high level of calibration facilities. The deviations
in the values of force transducers may be due to creep, time
loading effects, temperature effects, machine interaction
effect, parasitic components etc. To minimize these factors,
several efforts have been made, e.g., the automation of strain
gauge load cell force calibration, development of
computerized calibration system, development of
automation system for operating the force standard
machines etc. [1-3].
Though the National Physical Laboratory, India has the
facilities for the calibration of force transducers from 1 kN
to 3 MN, about 50% of the force transducers calibrated falls
into the range of 5 kN – 50 kN, which has made a precision
force standard machine a necessity in this particular range.
To cater the needs of precision and accurate measurement of
force, 50 kN dead weight force machine was developed by
the More House Corporation, USA as per the instructions of
the National Physical Laboratory, India. The main
components of the dead weight force machine are the
loading hanger, sets of dead weights and a rigid main frame
supporting these components. The dead weight force
machine employs 18 stainless steel dead weights of nominal
force values ranging from 0.5 kN to 5 kN which take into
account the local values of gravity and buoyancy correction
for the applied axial force by the dead weight force machine.
Fig.1. 50 kN Dead Weight Force Machine
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MEASUREMENT SCIENCE REVIEW, Volume 11, No. 2, 2011
The 50 kN dead weight force machine has been used for
calibration as per the standard calibration procedure based
on standard ISO 376-20046. The automated force machine
has been used to calibrate two precision force transducers of
20 kN and 50 kN capacity in manual mode and automatic
mode. The comparison of metrological characterization in
the form of relative repeatability error and the relative
reproducibility error has been presented and the effect of
automation over the performance of the 50 kN dead weight
force machine has been discussed here.
In manual mode, the operator has to apply forces after the
preset time interval, while in automatic mode the computer
itself controls the system and force is applied after the preset
time interval and reading is stored.
2. AUTOMATION OF 50 KN DEAD WEIGHT FORCE MACHINE
Calibration procedure is generally monotonous and labour
intensive. Any mistake during the calibration may lead to
improper metrological characterization including wrong
uncertainty evaluation and classification of the force
transducer under calibration. Human interference also plays
a vital role in the calibration process if the measurements are
carried out manually. Hence, efforts have been made for a
computerized calibration system [2-3]. Taking care of these
views, NPLI has automated a 50kN dead weight force
machine by customized hardware and software so that
calibration can be carried out with the least possible human
interference using the standard calibration procedure. The
actuation of pneumatically operated solenoid valves is done
through embedded server to a relay unit. The embedded
server, based on micro controllers, real time operating
system and semi-conductor based memory, has longer
reliability. The interaction of the embedded server with the
computer is through RS 232 port following the standard
protocol. While interfacing, Data Bits, Baud Rate, Echo,
Stop Bits, Com Port, Flow Control, Indicator Port and
Digital Indicator need to be specified. The control software
is a graphic user interface based user-friendly software with
help facility of the user manual. Various parameters like
waiting time, angle and sequence of weights need to be
defined before the calibration. A high resolution digital
indicator (Model DK 38, HBM Germany make) having
good stability and a resolution of 5 ppm with a data
sampling rate of 50 Hz has been used for indicating the
reading of the force transducer.
3.
Fig.2. Menu to Control / Monitor the
Observations during Calibration
4. CALIBRATION USING MANUAL & AUTOMATIC MODE
Two precision force transducers of 20 kN and 50 kN
capacity (compression type) were selected for calibration
using 50 kN dead weight force machine. The force
transducers have the uncertainty of measurement ± 0.004%
and ± 0.005% (at k = 2), respectively. Both force
transducers were calibrated as per the standard calibration
procedure according to ISO 376-2004 for manual mode as
well as automatic mode under controlled atmosphere, with
temperature 23 ± 1oC and relative humidity 50 ± 10% [5].
The calibration was conducted for 10% - 100% of the
capacity of the force transducers with 10% incremental
steps. The time interval between the readings was
maintained at 30 seconds.
The calibration procedure is as follows:
The digital indicator was switched on for 30 minutes to
warm up and stabilized for no load output before the start of
the calibration. The no load output was noted (before taring)
and the calibration signal was noted.
Before the application of the calibration forces, the force
transducer was preloaded thrice to its maximum capacity
and kept at full load for 90 seconds.
The calibration of the force transducer has been done in
tension mode as well as compression mode.
The calibration was carried out by applying two series of
calibration forces in ascending order from 10% to 100% in
steps of 10% at initial position, considered 0o.
Two series of calibration forces have been applied at
rotation positions 120o and 240o.
The force transducer was subjected to the full load once
for about 90 seconds each time before starting the
calibration to the new position.
Between the loadings, readings corresponding to no load
after waiting at least 30 seconds for the return to zero were
noted.
The observed data has been tabulated and used to evaluate
the relative error due to repeatability and reproducibility for
the force transducers [7-9]. The relative errors, due to
OPERATION OF AUTOMATED CALIBRATION SYSTEM
The operation of automated calibration system for force
transducers consists of the following stages (Fig.2):
• Select mode – manual or automatic
• Fill necessary information about force transducer,
capacity, calibrated for etc.
• Select force to be applied, sequence of force steps, waiting
time at no load / full load, waiting time in between force
steps
• Select the operation – preloading / get reading
• Calibrate the force transducer as per the standard
procedure
• Summarizing the calibration observations
• Uncertainty evaluation
• Preparation of calibration reports and classification of
force transducer
• Importing results in the form of MS Excel file
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MEASUREMENT SCIENCE REVIEW, Volume 11, No. 2, 2011
repeatability and reproducibility, have been plotted for the
force transducers in manual mode and automatic mode and
are summarized in form of plots (Fig.3-6). The relative
errors for the 50 kN force transducers have been plotted in
range of 20 kN – 50 kN only as up to as 20 kN, the force
transducer of 20 kN capacity has been considered. Relative
error due to repeatability has been computed by taking into
account the readings of force transducers at 0o, while the
relative error due to reproducibility has been computed by
taking the readings of force transducers at different
positions, e.g., 0o, 120o and 240o, into account.
50 kN Force Transducer
Reproducibility (%)
Manual Mode
0.0060
Automatic Mode
0.0050
0.0040
0.0030
0.0020
0.0010
0.0000
0
10
20
30
40
50
Applied Force (kN)
20 kN Force Transducer
Repeatability (%)
0.0125
Fig.6. Reproducibility Error for Manual & Automatic Mode
(50 kN Force Transducer)
Manual Mode
Automatic Mode
0.0100
5. RESULTS & DISCUSSION
0.0075
The 50 kN dead weight force machine has been automated
and is used for metrological characterization of two
precision force transducers of 20 kN and 50 kN capacity,
respectively. The results show a distinct variation between
the observations for manual mode and automatic mode
calibrations of the force transducers. The results indicate
about 0.0005% (5ppm) and 0.001% (10 ppm) variations in
relative repeatability error and relative reproducibility error
of force transducers, respectively, which may primarily be
attributed due to the automation of the 50 kN dead weight
force machine. Though the differences in the deviations in
manual and automatic mode may seem small, they are very
significant as the uncertainty of the force transducers is ±
0.004% and ± 0.005% (at k = 2), respectively, and they have
been found to have stable results over the years. The
deviations obtained may be due to minimizing the factors
due to human interference and factors other than the
standard force machine itself. The factors may further
include the errors due to operator while reporting the
observations and sometimes may be due to the time interval
in force application sequences. The uncertainties caused due
to operator are usually dominant and must be evaluated for
the given operator, but their evaluation is very difficult.
0.0050
0.0025
0.0000
0
2
4
6
8
10
12
14
16
18
20
Applied Force (kN)
Fig.3. Repeatability Error for Manual & Automatic Mode
(20 kN Force Transducer)
20 kN Force Transducer
Manual Mode
Reproducibility (%)
0.0200
Automatic Mode
0.0175
0.0150
0.0125
0.0100
0.0075
0.0050
0.0025
0.0000
0
2
4
6
8
10
12
14
16
18
20
Applied Force (kN)
Fig.4. Reproducibility Error for Manual & Automatic Mode
(20 kN Force Transducer)
6. CONCLUSIONS
The two precision force transducers of 20 kN and 50 kN
capacity considered for the study have been metrologically
studied and their findings are summarized in the form of
relative repeatability error and relative reproducibility error
for manual and automatic modes of the automated 50 kN
dead weight force machine. The repeatability and
reproducibility of the force transducers have been calculated
and plotted for both modes. The results were obtained for
both modes, the automatic mode indicates better
metrological results for both force transducers. Hence, the
present study discusses that the automation of the 50 kN
dead weight force machine and the automated calibration
system were able to calibrate the force transducers. The
metrological characterization of force transducers using 50
kN dead weight force machine has shown that the
automation was able to improve the performance of the
force machine by enabling measurements with complex
50 kN Force Transducer
Manual Mode
Automatic Mode
Repeatability (%)
0,0040
0,0030
0,0020
0,0010
0,0000
0
10
20
30
40
50
Applied Force (kN)
Fig.5. Repeatability Error for Manual & Automatic Mode
(50 kN Force Transducer)
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MEASUREMENT SCIENCE REVIEW, Volume 11, No. 2, 2011
sequences, precise control of loading time intervals, more
consistent indicator readings and limited / no operator
dependency.
[4]
ACKNOWLEDGEMENTS
[5]
The authors express their sincere thanks to Prof. R. C.
Budhani, Director, National Physical Laboratory, New
Delhi, India and Dr. A. K. Bandyopadhyay, Head, Apex
Level Standards and Industrial Metrology Group, National
Physical Laboratory, New Delhi, India.
[6]
[7]
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Received February 1, 2011.
Accepted May 30, 2011.
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