GB2049227A - Control of the speed of a cut-off knife - Google Patents

Abstract

The speed of a rotary cut-off knife (12, 14) which cuts sheets of preselected length from a constant speed moving web (16), begins to decrease, at the start of a cutting cycle, to a preselected speed. Pulses representing the speed of the moving web and the speed of the rotary knife are separately counted. When the count of the web speed pulses equals a preselected fraction of the length of sheet to be cut from the web, the rotary knife speed begins to increase to the speed of the knife at the start of the cutting cycle. When the latter speed is reached by the rotary knife, the speed of the rotary knife is controlled in response to the web speed and a position error signal (PE), derived from a preset count which is based on the difference between the desired length of sheet to be cut from the web and the circumference of the rotary knife and is modified in dependence on the knife speed pulses.

Description

SPECIFICATION Control of the speed of a cut-off knife The present invention is directed to a direct drive cut-off control.

In particular, the invention is directed to a method and apparatus for controlling the speed of a rotary knife which cuts sheets of preselected length from a moving web.

Heretofore, the speed of a rotary cut-off knife was controlled in response to speed and position error signals. At the start of a cutting cycle, the rotary knife speed began to decrease to a preselected reduced value. While the rotary knife remained at the reduced speed, the web would continue to move at its initial speed and the position error signal would gradually decrease to zero. The position error signal was derived from a count of the knife speed pulses, the web speed pulses, and a preset count representative of the difference between the desired length of sheet to be cut from the web and the circumference of the knife. When the position error signal was reduced to zero, the rotary knife speed would begin to increase from the reduced value to the knife speed at the start of the cutting cycle. This is the "line" speed or the speed of the moving web.As the knife speed increased toward the line speed, the position error signal passed through nominal zero error and then reversed sign. The position error increased, making it necessary to drive the knife beyond the line speed and then back toward the line speed to null out the position error.

At the end of the cutting cycle, the knife speed would be near the line speed so that the knife could cut the moving web. However, because the knife had to be driven beyond line speed to null out the position error, the knife would reach line speed only near or at the end of the cutting cycle and would not have enough time to stabilize at line speed prior to cutting the web. Accordingly, the position error signal might not in fact be completely nulled out at the end of the cycle.

Any residual position error, of course, would result in an inaccurate cut of the web and a sheet which was offsize.

The present invention overcomes the faiiings of prior art cut-off controls, such as that previously described, by ignoring the position error signal during a portion of the cutting cycle and by increasing the speed of the rotary knife towards line speed when a count of the web speed pulses equals a preselected fraction of the desired length of sheet to be cut from the web. Preferably, the point at which the knife speed is increased towards the line speed occurs at or approximately at the mid-point of the cutting cycle. At this point, the position error signal is not necessarily zero. This affords the cut-off control sufficient time to stabilize with the knife at line speed well before the end of the cutting cycle.

The position error signal is used to control the knife speed only after the knife has reached line speed. As a result, the position error signal is completely nulled out with the knife stabilized at line speed well before the end of the cutting cycle. This ensures consistent increased accuracy of the size of the sheets cut from the moving web.

Summary of the Invention At approximately the start of a cutting cycle, the speed of the rotary knife begins to decrease to a preselected reduced value. The knife speed is maintained at the reduced value until a count of web speed pulses equals a preselected fraction of the desired length of sheet to be cut from the web. At that point, the knife speed begins to increase to the speed of the knife at the beginning of the cutting cycle. When the latter speed is reached by the knife, the knife speed is controlled in response to the web speed and a position error signal which is derived from a preset count representative of the difference between he desired length of sheet to be cut from the web and the circumference of the rotary knife, the web speed pulses, and the knife speed pulses.

Brief Description of the Drawings For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

Figure 1 is a block diagram of a conventional direct drive cut-off control.

Figures 2A and 2B are charts of the speed and position curves for the conventional cutoff control shown in Fig. 1.

Figure 3 is a block diagram of the direct drive cut-off control of the present invention.

Figures 4A and 4B are charts of the speed and position curves for the direct drive cut-off control shown in Fig. 3.

Figure 5 is a chart of the microprocessor pulse outputs and the flip-flop states during a cutting cycle.

Detailed Description of the Invention Referring to the drawings in detail, wherein like numerals indicate like elements, there is shown in Fig. 1 a conventional direct drive cut-off control 10 for controlling the speed of a pair of rotary cut-off knives 12 and 14 in a corrugator. The knives 12 and 14 repetitively sever a moving web 16 into sheets of preseiected or desired length. The knives 12 and 14 are driven by a d.c. motor 20 through a gear reducer 22. A pulse tachometer 24 is mechanically coupled to the drive for the knives. The pulse tachometer 24 generates a stream of pulses at a repetition frequency which varies with the speed of the knives 12 and 14.

A measuring wheel 26 is disposed upstream of the knives 12 and 14. The measuring wheel 26 is maintained in rolling contact with the top surface of the moving web 16. A pulse tachometer 28 is coupled to the measuring wheel 26. The pulse tachometer 28 generates a stream of pulses at a repetition frequency which varies with the speed of the moving web 16. The speed of the moving web is presumed constant.

The repetition frequencies of the pulse train outputs of tachometers 24 and 28 are sensed by D/A converters 30 and 32, respectively.

The numbers of pulses generated by tachometers 24 and 28 per inch of travel of the knife 12 or 14 and per inch of travel of the web 16, respectively, are assumed to be the same. Appropriate scaling circuitry (not shown) may be provided for this purpose. The D/A converters 30 and 32 generate analog signals representative of the repetition frequencies of the outputs of tachometers 24 and 28, respectively. Accordingly, the outputs of D/A converters 30 and 32 indicate the actual speeds of the knives 12, 14 and the moving web 16. The speeds of knife 12 and knife 14 are the same.

The outputs of D/A converters 30 and 32 are combined by an analog difference circuit 34. The analog difference circuit 34 senses the difference between the analog outputs of D/A converters 30 and 32 (the difference between web speed and knife speed) and generates an analog signal representative of that difference at the input of a speed regulator circuit 36.

The speed regulator circuit 36 drives an SCR power unit 38 which controls the current signal to the motor 20.

The output of a D/A converter 40 in a position controller circuit 42 is added to the difference between the outputs of the D/A converters 30 and 32. The position controller 42 typically includes a latch 44 and up/down counter 46. The latch and up/down counter are multiple-bit input and output devices. The multiple bit inputs to the latch 44 are provided by a set of thumb wheel switches TW located on a main operator's panel 48. The thumb wheels switches TW are set by the operator to the desired length of sheet to be cut by the knives 12, 14 from the moving web 16. At the start of a cutting cycle, the thumb wheel switch outputs are transmitted by the latch 44 to preset the up/down counter 46 to a preset count.Although not shown in Fig. 1, the position controller circuit 42 includes digital scaling circuitry which con verbs the thumb wheel outputs to a number which represents the number of tachometer 28 pulses equivalent to the desired sheet length. In addition, the circuit 42 includes circuitry which offsets the thumb wheels output by the circumference of the knife 12. As a result, the up/down counter 46 is preset to a preset count representative of the difference between the desired sheet length and the circumference of the knife 12.

The preset count represents the position or sheet length error which would be obtained if the knives 12, 14 were driven at the speed of the moving web 16 throughout the cutting cycle. The pulse outputs of tachometers 28 and 24 increment or decrement the preset count in -up/down counter 46 depending on whether the desired sheet length is greater than or less than the circumference of the knife 12. The output of the up/down counter 46, therefore, represents the position or sheet length -error throughout the cutting cycle.

For purposes of explanation, it is assumed that the speed of the moving web 16 is constant. Accordingly, the rate at which the tachometer 28 generates the web speed pulses is constant. The knife speed pulses generated by tachometer 24, however, are generated at a varying rate depending on the actual controlled speed of the knives 12, 14.

If the desired sheet length is greater than the circumference of the knife 12, the average speed of the knife during the cutting cycle must be less than the speed of the moving web 16. Also, the number of knife speed pulses generated by tachometer 24 must be less than the number of web speed pulses generated by tachometer 28 by the amount of the preset count to completely null out the preset count, i.e., the preset position error. On the other hand, if the desired sheet length is less than the circumference of the knife 12, the number of knife speed pulses must exceed the number of web speed pulses by the amount of the preset count to null out the preset position error. The up/down counter 46 maintains a running count of the knife speed pulses and the web speed pulses for this purpose.

At the end of a cutting cycle, the knives 12 and 14 sever the moving web 16 to produce a sheet of the desired length as set by thumb wheels TW. To make the cut, the speed of the knives 12 and 14 is brought into synchronism with the speed of the moving web 16 at the end of the cutting cycle. During the cutting cycle, the speed of the knives 12, 14 is determined by the speed regulator circuit 36 in response to the feedback signal provided by the analog difference circuit 34. A typical speed v. time curve A for the knives 12, 14 is shown in Fig. 2A for the case of a desired sheet length of 80 inches and a knive circumference of 40 inches. The speed of the moving web is indicated by the horizontal line B throughout the cutting cycle. Initially, at time O (the start of the cutting cycle), the speed of the knives begins to decrease from the web speed W to a preselected reduced speed R which is assumed to be zero.

The preset count or position error which is preset into the up/down counter 46 at the start of the cutting cycle is indicated by the horizontal line C in Fig. 2B. The preset count is presumed to have a value P. The curve D represents the preset count P incremented by the knife speed pulses. The curve E represents the count of the web speed pulses. The difference between the values of curves D and E throughout the cutting cycle represents the position error signal generated by the D/A converter 40 in the position controller circuit 42 and is indicated by the letter F.

As the speed of the knives 1 2, 14 is reduced to the preselected reduced speed R in Fig. 2A, the position error F is relatively large and gradually decreases. The speed of the knives 12, 14 is maintained at the reduced value R until the curves D and E intersect.

This occurs at a time TO past the mid-point T/2 of the cutting cycle. At this point, the position error F is zero, and the speed of the knives 12, 14 is gradually increased by the speed regulator circuit 36 to bring the speed of the knives up to the web speed W to cut the web at time T (the end of the cutting cycle).

As the speed of the knives is increased from the preselected reduced speed R toward the web speed W, the position error F passes through nominal zero at time TO and reverses sign. At time T1, the speed of the knives reaches the web speed W but the position error F is appreciable. Although the speed of the knives may reach the web speed W at time T1, the knives do not stabilize at the web speed W until the position error F is completely nulled out. To null out the position error F between times T1 and T, the speed of the knives must be increased temporarily beyond the web speed W and then returned to the web speed at the time T (the end of the cutting cycle). The position error signal F may not be nulled out by the end of the cutting cycle so that the knives may not stabilize at the web speed, with nominal zero position error, prior to the time of the cut (time T).As a result, the length of sheet cut by the knives will vary from the desired sheet length set by the thumb wheels TW.

To minimize the inaccuracy in the cut, the control circuit parameters (gain, proportional band, stability, lead network time constants, and so forth) may be "tuned" to enable the speed regulator circuit 36 (and the knives 12, 14) to approach a stable condition as close to the end of the cutting cycle as possible. To date, however, all attempts to tune" the control 10 to achieve stable operating conditions at the end of the cutting cycle for as many combinations of web speed and desired sheet length as possible have proven unsuccessful and have not resulted in consistently accurate sheet lengths.

The direct drive cut-off control of the present invention is shown in Fig. 3 and is designated by the numeral 52. Like elements in the cut-off controls of Figs. 1 and 3 have been designated by the same numbers 12-48. The direct drive cut-off control 52 ensures stabilization of the knives 1 2, 14 at the web speed W well prior to the end of the cutting cycle T. As a result, the moving web 16 is severed at the end of the cutting cycle under stable conditions to produce a consistently accurate sheet length. As will be described more fully hereinafter, this is accomplished by increasing the speed of the knives 1 2, 14 from the preselected reduced speed R towards the web speed W at or approximately at the mid-point of the cutting cycle.

The sequence of operating states of the cutoff control 52 is determined by a programmed microprocessor 54. In describing the operation of the cut-off control 52 shown in Fig. 3, reference is made to the sequence of microprocessor output signals shown in Fig. 5.

At the start of the cutting cycle (time O) or a short delay interval thereafter, the microprocessor generates a ST pulse. The ST pulse sets a set-reset flip-flop 56 which in turn sets a set-reset flip-flop 58. When flip-flop 56 is set, the flip-flop gates an analog switch 60 off. The gate 60, therefore, prevents the analog output of D/A converter 32 from reaching the analog difference circuit 34. This removes the web speed reference signal from the difference circuit. At the same time, the flip4lop 56 gates an analog switch 62 on.

The analog switch 62 transmits the analog output of a D/A converter 64 to the difference circuit 34. The output of the D/A converter 64 is the analog equivalent of the digital signal CSR generated by the microprocessor 54. The CSR signal represents the preselected reduced speed R to which the speed of the knives 1 2, 14 must be brought.

It can be shown that the reduced speed R is equal to twice the average speed of the knives 1 2, 1 4 during the cutting cycle minus the web speed W. The average speed of the knives 12, 14 is given by the equation: average speed = (circumference of knife 1 2 12 desired sheet length) x W Referring to Figs. 4A and 4B, there are shown the characteristic speed and position curves respectively for the cut-off control 52 for the case of a desired sheet length of 80 inches and a knife circumference of 40 inches. The reduced speed R is therefore zero in Fig. 4A.

Since analog switch 62 is gated on when the ST pulse is generated, the speed of the knives 1 2, 1 4 is controlled in response to the analog output of the D/A converter 64, that is, in response to the CSR signal. Accordingly, the speed of the knives 1 2, 14 is gradually reduced to the reduced speed R (zero in this special case). During this time, the flip-flop 58 is set, keeping analog switch 66 off.

Once the speed of the knives 12, 14 reaches the reduced speed R, the speed is maintained at the reduced speed until the mid-point T/2 of the cutting cycle is reached.

At the mid-point of the cutting cycle, the count of web speed pulses (curve E) reaches the value of one-half the desired sheet length or TW/2. The microprocessor 54 counts the web speed pulses generated by tachometer 28 when the ST pulse is generated at the start of the cutting cycle. When the count reaches the value TW/2, the microprocessor generates a zero crossing signal ZC. The zero crossing signal ZC is a pulse which resets the flip-flop 56 while flip-flop 58 remains set.

When reset, flip4lop 56 gates analog switch 62 off and, simultaneously, gates analog switch 60 on. Analog switch 60 transmits the output of D/A converter 32 to the analog difference circuit 34. Thus, the web speed pulses become the speed reference signal for the speed regulator circuit 36, and the speed of the knives 12, 14 is gradually increased towards the web speed W.

When the speed of the knives 12, 14 reaches the web speed W at time T1, the microprocessor 54 senses this condition and generates a sync speed signal SS. The sync speed signal SS is a pulse occurring at time T1 as indicated in Fig. 5. The SS signal is passed by an AND gate 68 to the reset input of flip-flop 58. Flip-flop 58, therefore, becomes reset and gates the analog switch 66 on. Analog switch 66 transmits the position error signal PE to the analog difference circuit 34. Accordingly, the speed of the knives is now controlled in response to the web speed pulses and the position error signal PE.

The microprocessor 54 computes the preset count or position error P based on the difference between the setting of the thumb wheels switches TW and the circumference of the knife 12. The microprocessor increments the preset count P by the knife speed pulses (for desired sheet length > knife circumference) and subtracts the web speed pulses to obtain the position error signal PE. A D/A converter 70 converts the PE signal to an analog signal for transmission through the analog switch 66 to the analog difference circuit 34. The analog output of the D/A converter 70 is indicated by the letter F in Fig. 4B. The position error gradually decreases as the mid-point T/2 of the cutting cycle is approached. However, the position error is not nulled out at the midpoint of the cutting cycle.

When the speed of the knives 12, 14 is restored to the web speed W at time T1, the position error is zero or very nearly zero. As previously indicated, at time Ti, the position error signal is transmitted by the analog switch 66 to analog difference circuit 34 to completely null out any residual position error while the knives 12, 14 are stabilizing at the web speed W. Accordingly, the knives 12, 14 stabilize at the web speed W with zero position error well before the end of the cutting cycle T.

Although in the preferred embodiment described herein, the mid-point T/2 of the cutting cycle is chosen as the point TO at which the speed of the knives 12, 14 begins to increase towards the web speed W, it should be understood that restoration of the speed of the knives towards the web speed may commence at other points in time TO of the cutting cycle which do not coincide precisely with the mid-point T/2, within the spirit and scope of the invention. Thus, the microprocessor 54 could be programmed to generate the zero crossing ZC signal when the count of web speed pulses (curve E) reaches a value other than one-half the desired sheet length set by thumb wheels TW.The particular fraction of the desired sheet length reached by the count of the knife speed pulses for purposes of initiating restoration of the speed of the knives towards the web speed W at time TO would be chosen to ensure that the knives could stabilize at the web speed with zero position error well before the end of the cutting cycle. For example,- choosing a value of 5 TW/8 instead of TW/2 would result in a shift of the point TO to the right of the midpoint T/2 of the cutting cycle while ensuring stabilization of the knives at the web speed at time T1 well before the end of the cutting cycle.

In the description of the preferred embodiment herein, it is presumed that the slopes of the falling and rising portions of the curve A are equal in magnitude but of opposite sign.

If the slopes are not of equal magnitude, this will result in a residual position error at time T1. However, the residual position error is easily nulled out by the control 52 sufficiently in advance of the end of the cutting cycle so that the knives 12, 14 are stabilized at the web speed W at the end of the cycle.

In addition, although the invention has been described in terms of a pair of cut-off knives 12, 14, it should be appreciated that the speed of a single rotary cut-off knife associated with a fixed or stationary anvil could be controlled in the same manner.

Finally, it should be appreciated that although the line TW/2 and the line P have been shown at different ordinates in Fig. 4B to illustrate the general relation between the lines, for the special case of a desired sheet length of 80 inches and a knive circumference of 40 inches, the lines will coincide since TW/2 will be (80 inches/40 inches) = 40 inches and the preset count P will be (80 inches - 40 inches) = 40 inches.

Claims (10)

1. Method of controlling the speed of at least one cut-off knife during a cutting cycle to cut at least one sheet of preselected length from a moving web including reducing the speed of the knife from a first speed to a preselected speed, characterized by: detecting a preselected point of the cutting cycle; restoring the speed of the knife from the preselected reduced speed to the first speed after the preselected point of the cutting cycle is detected; generating a position error signal based on the speed of the knife, the speed of the web, the preselected length of sheet to be cut from the web and the circumference of the knife; and controlling the speed of the knife in response to the position error signal after the speed of the knife reaches the first speed.

2. The method according to Claim 1 further characterized in that the preselected reduced speed of the knife is equal to twice the average speed of the knife during a cutting cycle minus the speed of the web.

3. The method according to Claim 1 or 2 further characterized in that said step of generating the position error signal includes generating a preset count representative of the difference between the desired length of sheet to be cut from the web and the circumference of the knife, generating plural web pulses indicative of the speed of the web, counting the web speed pulses, generating plural knife speed pulses indicative of the speed of the knife, counting the knife speed pulses, and varying the preset count by the count of the knife speed pulses and the count of the web speed pulses.

4. Apparatus for controlling the speed of at least one rotary cut-off knife during a cutting cycle to cut at least one sheet of preselected length from a moving web according to the method of Claim 1, comprising means for generating plural web speed pulses representative of the speed of the moving web, means for generating knife speed pulses representative of the speed of the knife, means for counting the web speed pulses, means for counting the knife speed pulses, and means for reducing the speed of the knife from a first speed to a preselected reduced speed, characterized by:: means for detecting when the count of the web speed pulses reaches a value which is a preselected fraction of the desired length of sheet to be cut from the web; means for restoring the speed of the knife to the first speed from the preselected reduced speed after detecting that the count of the web speed pulses reaches said value; means for generating a position error signal based on the preselected length of sheet to be cut from the web, the circumference of the knife and the web speed pulses and knife speed pulses; and means for controlling the speed of the knife in response to the position error signal after the speed of the knife reaches the first speed.

5. The apparatus according to Claim 4 further characterized in that said value is approximately one-half the preselected length of sheet to be cut from the web.

6. The apparatus according to Claim 4 further characterized in that said value corresponds to the mid-point of the cutting cycle.

7. The apparatus according to Claim 4 or 6 further characterized in that the preselected reduced speed is twice the average speed of the knife during a cutting cycle minus the web speed.

8. The apparatus according to any of claims 4 to 7 wherein said means for generating the position error signal includes means for generating a preset count representative of the differences between the desired length of sheet to be cut from the moving web and the circumference of the knife, and means for varying the preselected count in response to the knife speed pulses and the web speed pulses.

9. A method of controlling the speed of a cut-off knife substantially as hereinbefore described with reference to Figs. 3 to 5 of the accompanying drawings.

10. Apparatus for controlling the speed of a cut-off knife substantially as hereinbefore described with reference to Figs. 3 to 5 of the accompanying drawings.