Sentinel Control For Cutoff Apparatus

Abstract

A sentinel control in cutoff apparatus having rotatable cutting means for cutting sheets of uniform length from a moving web of material fed to the apparatus to prevent the rotational velocity (revolutions or cuts per second) of the cutting means for exceeding a predetermined maximum or threshold limit rotational velocity and thereby prevent possible damage and destruction to the cutoff apparatus caused by the heavy inertia forces imposed upon the apparatus when operated at velocities in excess of maximum desired limit.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to a control system for preventing the operation of cutting means in cutoff apparatus in excess of a desired speed level measured in revolutions per second of the cutting means.

The employment of cutoff apparatus of the type herein disclosed is well known in the art. Such apparatus can be generally classified as processing apparatus and is used to cut selected sheet lengths from a continuous web of material as the moving web of material is fed into the cutoff apparatus.

Examples of such apparatus and control systems for preselecting the desired sheet lengths to be uniformly severed by such apparatus are disclosed in Drenning et al. 3,176,557 (83-76); Sterns et al. 3,181,403 (83-76); Paterson 3,195,385, (83-76); Neely 3,215,015 (83-363); Sterns et al. 3,267,781 (83-37); Rubinstein 3,355,973 (83-76); and Nido, patent application Ser. No. 848,469, filed Aug. 8, 1969.

In setting cutoff apparatus control systems to cut uniform sheet lengths from a moving web of material, such as, corrugated paperboard, these control systems are susceptible to operating the rotatable cutting means in the cutoff apparatus in excess of desirable rotational velocity limit as measured by revolutions (or cuts) made by the cutting means in a given time, such as a second.

If the cutting means, usually in the form of a pair of large rotatably synchronous cutting knives wherein the knife blades meet for shearing engagement for each revolution thereof, are operated in excess of the acceptable designed rotational velocity limit, the cutoff apparatus may be damaged by the tremendous inertial forces built into such apparatus, particularly when cutting certain prescribed sheet lengths, such as, for example, below sheet lengths of 60 inches. It is believed that one of the major costs today in repair of such cutoff apparatus is due to continuously operating such apparatus in excess of the designed limits of operation.

The inertial forces of the knives in cutoff apparatus of this type are very extensive. Power transmission means, which includes a variable speed transmission and a knife cycling mechanism driven by a variable speed motor, are designed to withstand the inertial forces developed by the rotating knives. As is well known in the art, the cycling mechanism is designed to bring equality in the rotational velocity of the knives with the lineal velocity of the moving web of material at the time of cutoff so that perfect shearing engagement (rather than ripping or tearing) )is made by the pair of knives upon cutoff. Approximately 20 times more torque is developed by the cycling mechanism when accelerating and decelerating the rotational velocity of the knives to bring about the above mentioned equality.

Thus, the inertial forces in the cutoff apparatus of this nature are of such a magnitude that when the apparatus is operated for extended periods of time over the acceptable speed limit, the apparatus literally is shaken apart eventually opening the door to possible self-destruction and final breakdown.

It is the common practice in the corrugated paperboard industry as well as other such processing type industries to post on the cutoff control system panel a table of safe operating speeds with corresponding sheet lengths so that the operator will be informed as to acceptable rotational velocity limits of the cutoff apparatus for a given sheet length. However, due to inadvertence or pressure placed upon the processing plant to meet order commitments on time, the cutoff apparatus is too often operated at speeds in the excess of the acceptable maximum or established threshold limit eventually bring about a breakdown of the apparatus and shut down of the plant's operation until repairs can be made by the manufacturer of the cutoff apparatus.

The present disclosure is directed to sentinel controls to automatically prevent the operation of such cutoff apparatus in excess of the acceptable maximum or threshold limit of operation.

SUMMARY OF THE INVENTION

The chief objective of this invention is the provision of a sentinel control to impose a maximum or threshold limit on the number of revolutions (cuts) per second made by a pair of rotating knives in a cutoff apparatus designed for cutting sheets of uniform length from a moving web of material fed to the apparatus. In this manner, the control will automatically prevent personnel from operating the cutoff apparatus in excess of the design speed tolerances of the apparatus. For example, when the sheet length selection control of the cutoff apparatus is adjusted for cutting sheet lengths of 30 inches, the designed tolerances of the apparatus may be such to limit its speed of processing the moving web of material at a maximum of 400 feet per minute or a little over 61/2 feet per second. On the other hand, a 40 inch sheet may be safely cut at 500 feet per minute. Anything in excess of 50 inches may be safely cut at a maximum speed, such as, for example, 650 feet per minute.

It has been generally found that as a safety factor, the rotational velocity of the cutting knives should not exceed 3 cuts (revolutions) per second, particularly when the moving web of material is moving at a rate of 100 to 500 feet per minute.

In controlling the operation of cutoff apparatus of the type referred to herein, a main drive motor, which is operative at different selected speeds, provides for both (a) the lineal velocity of the moving web of material to be processed, and (b) through adjustable power transmission means, the rotational velocity of the cutoff knives.

It should be noted from all of the foregoing that the revolutions or cuts made in any given time period by the cutoff knives is a function of the desired uniform sheet length to be cut and the velocity of the moving web of material fed into the cutoff apparatus. By the same token, the desired sheet length to be cut from the continuous web of material to be processed is a function of the number of revolutions or cuts to be made by the cutoff apparatus in a given time period and the velocity of the moving web of material fed into the cutoff apparatus. Thus, the sentinel control embodiments herein disclosed are capable of either (a) automatically detecting or measuring cuts made in any given time period by the cutoff knives, or (b) automatically detecting the desired sheet length to be cut from the continuous web of material to be processed, and thereby control, through the operational speed of the main drive motor, the velocity of the moving web of material fed into the cutoff apparatus. This latter function, again, has a direct bearing on what maximum velocity will be permitted by the rotating cutoff knives. Thus, the end result is the same -- limiting the maximum number of revolutions per second permitted by rotatable cutting means for a given sheet length.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages appear hereinafter in the following description and claims.

The accompanying drawings show, for the purpose of exemplification without limiting the invention or the claims thereto, certain practical embodiments illustrating the principles of this invention wherein:

FIG. 1 shows a sentinel control utilizing a speed sensing device for detecting the number of cuts (revolutions) made by the cutoff apparatus knives in a given time period and thereby controlling the circuit control means of the main drive motor to prevent the rotatable cutting knives from exceeding a maximum rotational velocity.

FIG. 2a shows a modified form of the sentinel control of FIG. 1 utilizing a speed sensing device of the digitizer type to detect the number of cuts made by the cutoff apparatus knives in a given time period and thereby controlling the circuit control means of the main drive motor to prevent the rotatable cutting knives from exceeding a maximum rotational velocity.

FIG. 2b graphically demonstrates the normal operation of the sentinel control of FIG. 2a when the velocity of the cutoff apparatus knives has not exceeded the acceptable maximum or threshold limit.

FIG. 2c graphically demonstrates the operation of the sentinel control of FIG. 2a when the velocity of the cutoff apparatus knives has exceeded the maximum or threshold limit whereby the circuit control means of the main drive motor is responsive to the sentinel control to decrease the rotational velocity of the cutoff knives.

FIG. 3 shows a sentinel control utilizing a sensing device in the form of a cam for detecting selected sheet lengths which is operative on the sentinel circuit of the circuit control means of the main drive motor to prevent the rotatable cutting knives from exceeding the maximum or threshold rotational velocity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Cutoff apparatus of the type herein referred to generally employs two sets of pairs of cutoff knives, one set being referred to as the upper cutoff knives and the outer set being referred to as the lower cutoff knives. Whereas the embodiments as shown in FIGS. 1 and 2a show the sentinel control detection in connection with both the upper and lower pairs of knives, it should be understood that, for the sake of simplicity, the description of the control in reality need only be directed to a sentinel control for one pair of knives. In this regard, the embodiment shown in FIG. 3 shows the sentinel control for only one set of pair of cutoff knives. The foregoing, therefore, also holds true in connection with the claimed invention.

Reference is now made to the drawings and more particularly to FIG. 1 wherein there is shown a sentinel control for detecting or measuring the rotational velocity of the cutoff apparatus knives. The upper knife speed sensing device indicated at 1 is rotatably driven by means of the belt and pulley arrangement shown at 2, which, in turn, is driven by the upper shaft 3 driving the upper rotatable cutoff knives (not shown). The lower knife speed sensing device 4 is also driven through a pulley and belt arrangement 2 by means of the lower shaft 5 which is driving the lower cutoff knives (not shown).

The speed sensing devices 1 and 4 are of the centrifugal switch type and have a contact element 6 therein which closes upon the sensing device being driven above a predetermined speed which is set to be the maximum or threshold limit of the upper and lower cutoff knives. These contact elements 6 are connected in parallel and connected to one side of supply source, or L.sub.1 with their other sides connected to the switching device 7. The switching device as shown in FIG. 1 is a relay K having a normally open contact K.sub.1 and a normally closed contact K.sub.2. The other side of relay K is then connected to the other side of supply source, or L.sub.2.

The remainder of the circuit shown in FIG. 1 is a portion of the circuit control means used in connection with the operation of the main drive motor shown at 8. The main drive motor 8 is a DC motor capable of being operated at various desirable speeds to drive the double facer rolls (not shown) that transfer, at a desired lineal velocity, the moving web of material into the cutoff apparatus. Also, the main drive motor 8, as shown schematically in FIG. 3, drives the power transmission means 12 comprising a variable speed transmission, usually of the Reeves type, and a cycling mechanism, the latter of which drives the pair of cutoff knives indicated at 10. Thus, the variable speed transmission and cycling mechanism represented power transmission means 12 capable of being adjusted to drive the pair of cutoff knives 10 at a desired rotational velocity in such a manner that this rotational velocity will be equal to the lineal velocity of the moving web of material fed into the cutoff knives (indicated at 11 in FIG. 3) at the exact time of cutoff.

As previously mentioned, the circuit control means for variably operating the main drive motor 8 includes the circuit as shown in FIG. 1 which comprises a Variac or auto transformer 13 connected to a three phase voltage supply P.sub.1, P.sub.2, and P.sub.3, the output of which, through an AC to DC rectifier 14 can vary the armature voltage of the main drive motor 8 and thus vary the speed of operation of the motor. The auto transformer 13 which controls the motor 8 is in turn, physically controlled by the auto transformer adjusting motor 15 which is a reversible AC motor. Motor 15 is provided with supply lines L.sub.1 and L.sub.2, from an AC supply source, L.sub.2 being the common supply line. Line L.sub.1 is connected to adjusting motor 15 through line 16 to operate motor 15 in one direction and is also connected to adjusting motor 15 by means of line 17 to operate adjusting motor 15 in a opposite or reverse direction. In this manner, the auto transformer 13 is either driven in one direction or the other in order to increase or decrease, respective, the voltage on the armature of motor 8.

Line 16 to adjusting motor 15 is provided with an increase push button 18 and a normally closed contact K.sub.2. Line 17, on the other hand, is provided with a decrease push button 20. Decrease push button 20 is provided with normal closed back contact 20a in line 16 in order to "override" the operation of the increase push button 18. Thus, it can be seen that upon operation of decrease push button 20, contact 20b of push button 20 will complete line 17 to adjusting motor 15 to operate the adjusting motor in the desired direction and at the same time release contact 20a to open line 16 and prevent the simultaneous operation of adjusting motor 15 in the opposite direction by attempting to also operate increase push button 18.

Normally open contact K.sub.1 of relay K of the switching device 7 is connected by means of lines 21 and 22 across normally open contact 20b of decrease push button 20.

From the foregoing, it can be readily seen that if, for example, upper shaft 3 driving the pair of upper cutoff knives is driven in excess of a predetermined maximum number of revolutions (cuts) per second, contact element 6 of the upper knife sensing device 1 will be caused to close thereby energizing relay K and closing its normally open contact K.sub.1 and opening normally closed contact K.sub.2. The resultant effect is that without actually manually operating decrease push button 20, contact K.sub.1 brings about the operation of auto transformer adjusting motor 15 to cause auto transformer 13 to decrease the voltage supply to the armature of drive motor 8. Simultaneously, contact K.sub.2 prevents the increase push button 18 from driving the motor. With the decrease in speed of motor 8, the rotational speed of the upper pair of cutting knives will be automatically decreased until the rotational velocity of the knives through their drive shaft 3 is reduced to below the maximum or threshold limit of revolutions per second causing operation of the speed sensing device 1. At this point contact element 6 of sensing device 1 will open deenergizing relay K causing the opening of its contact K.sub.1 and, thus, stopping the operation of adjusting motor 15.

From the foregoing description it can readily be seen that if the speed of the main drive motor 8 is attempted to be steadily increased through operation of the increased push button 18, the speed sensing devices 1 and 4 will limit the permissible level of operation of motor 8 to the predetermined maximum by speed or threshold limit operation of the sentinel control which "overrides" decrease push button 20 by means of the normally open contact K.sub.1 of relay K being closed.

Reference is now made to the sentinel control of FIG. 2a wherein the sensing means used in the sensing circuit to detect when the rotational velocity of the pair of cutting knives exceeds the maximum or threshold limit desired, are the proximity switches 23 and 24. These proximity switches may be, for example, of the transducer type. Each of the drive shafts 25 and 26 of the upper cutoff knives and the lower cutoff knives, respective are provided with a projection 27 which enters a magnetic field of the proximity switch upon every revolution of the shafts 25 and 26. Thus, upon every revolution of either shaft 25 or 26, the contact element 28 of the proximity switches 23 and 24 are closed producing a pulse which is detected by the sensing circuit.

As shown in FIG. 2a, the proximity switches 23 and 24 are connected from one side through line 30 to a DC supply source. The other side of the proximity switches 23 and 24 are respectively connected by means of lines 31 and 32 to the one shot flip-flop -1 and one shot flip-flop -2, respectively. Also, lines 31 and 32 are also respectively connected to one of the inputs of the AND-gates 33 and 34.

The pulses t.sub.P generated by the proximity switches 23 and 24 are used to respectively start the operation of the one shot flip-flop -1 and one shot flip-flop -2 which are in reality solid-state timers. As is well known, to those skilled in this art, the pulses t.sub.P are formed through a shaper circuit to produce a distinctly discrete "square shaped" pulse shown in FIG. 2b.

Each of these one shot flip-flops are a bistable device possessing two states, one of which is timed producing an output pulse along their respective lines 35 and 36. Means are provided in each of the flip-flops -1 and -2 for adjusting the time duration of the output pulse, which is designated as t.sub.D. The output pulse t.sub.D is then directed to the other input of respective AND-gates 33 and 34, along lines 35 and 36, respectively. In this connection one shot flip-flop -1 and flip-flop -2 are triggered to produce this timed output pulse t.sub.D on the negative going edge of the pulses t.sub.P produced by the proximity switches 23 and 24.

Without proceeding further into the discussion with the operation of the sensing circuit of FIG. 2a, it can readily be seen that if either of the AND-gates 33 and 34 are caused to operate to produce an output on line 37 this output will operate OR-gate 38 to produce a signal on the input of one shot flip-flop -3. Thus, regardless of whether the upper pair of cutter knives or the lower pair of cutter knives are operating at a rotational velocity in excess of that of the maximum or threshold limit, that is, one pair of rotating cutoff knives is operating at a faster rotational velocity as compared to the other pair of cutoff knives, the sensing circuit will operate to decrease the speed of main drive motor 8 if the rotational velocity of the faster operating pair of cutoff knives exceeds the maximum or threshold limit.

The one shot flip-flop -3 is also a bistable device possessing two states one of which is timed and the timed output of one shot flip-flop -3 is set to have a timed duration to produce an output pulse t.sub.S on line 40. The timed interval of pulse t.sub.S is equal to the time interval of the shafts 25 or 26 necessary to complete 1 revolution. Thus, it has been found that the threshold limit of rotational velocity of the pair of cutoff knives should not usually exceed 3 revolutions (cuts) per second particularly in connection with sheet lengths less than 60 inches. Thus, the time duration of the one shot flip-flop -3 output pulse t.sub.S should be set at 0.33 second, that is, one-third of the time interval necessary for the cutoff knives to negotiate three revolutions or one-third of 1 second (0.33 second).

As can be seen upon examination of FIG. 2a the circuit control means of this figure is identical to that previously described and shown in connection with FIG. 1 wherein the switching device 7 is utilized to "override" the decrease push button contact 20b by means of energizing the relay K to close its normally open contact K.sub.1 and open normally closed contact K.sub.2 when the rotational velocity of the cutoff knives 10 has exceeded the acceptable maximum threshold limit.

Reference will now be made more specifically to the operation of the sensing circuit of FIG. 2a by using the time diagrams of FIGS. 2b and 2c to illustrate the operation of the one shot flip-flops -1 and -2 in conjunction with their proximity switches 23 and 24 and respective AND-gates 33 and 34 and also in conjunction with the operation of flip-flop -3.

As shown in FIG. 2b for any given proximity switch, say, for example, proximity switch 23 of cutoff drive shaft 25, proximity switch 23 produces a pulse t.sub.P for each shaft revolution. The time duration or dwell between the successive pulses generated by the proximity switches is designated as t.sub.S or t.sub.S +, representing the condition where the maximum or threshold limit desired is 3 cuts per second to be made by the rotating pairs of cutoff knives, and the time duration between the proximity switch pulses t.sub.P should be equal to or greater than t.sub.S under normal operation. In the case of a threshold limit of 3 cuts per second, t.sub.S would normally be equal to 0.33 second.

As can be seen upon viewing FIG. 2b, the one shot flip-flop -1 or -2 is caused to change state and produce an output pulse T.sub.D along either line 35 or 36 which pulse is less than the time duration of t.sub.S. The time duration of one shot flip-flops -1 and -2 output pulse t.sub.D is calculated as follows: if pulse t.sub.P is the dwell of the proximity switches 23 and 24; pulse t.sub.D is the time duration or dwell of either one shot flip-flops -1 and -2; and t.sub.S is the dwell between the proximity switch pulses t.sub.P, then

T.sub.D = T.sub.S -t.sub.P.

In the case where t.sub.S the dwell between the proximity pulses t.sub.P, is equal to 0.33 second (3 cuts per second) at the cutoff knife), then,

t.sub.D =0.33-t.sub.P.

As mentioned previously, the pulse T.sub.D at the output of the one shot flip-flops -1 and -2 is triggered by the negative going edge of the proximity switch pulse t.sub.P, as illustrated in FIG. 2b. As explained above and also illustrated in this figure, the time duration of pulse t.sub.D is limited to be extinguished before the next succeeding proximity switch pulse t.sub.P. Therefore, assuming, as shown in FIG. 2b, that the proximity switch pulses t.sub.P are separated by sufficient time duration or dwell equal to at least t.sub.S, there will be no time coincidence of (a) the next succeeding pulse t.sub.P along either line 31 or 32 to respective AND-gates 33 and 34 and (b) the timed pulse t.sub.D from the outputs of the one shot flip-flops -1 and -2 along their respective lines 35 and 36 to their respective AND-gates 33 and 34. Therefore, AND-gates 33 and 34, whichever the case may be, are maintained at their low state as indicated at the bottom of FIG. 2b.

FIG. 2c, on the other hand, represents what happens in the operation of the sensing circuit shown in FIG. 2a when the proximity switch pulses with respect to either set of pairs of cutoff knives are caused to operate at a rotational velocity in excess of times interval of t.sub.S. For example, as can be seen in FIG. 2c the time interval or dwell indicated by the numeral 41 between the first two proximity switch pulses t.sub.P is substantially less than that of the timed pulses t.sub.S of the one shot flip-flop -3. This condition readily indicates that the rotational velocity of the cutoff knives is exceeding the desired maximum or threshold limit.

As the case in FIG. 2b, the timed output pulse t.sub.D of either one shot flip-flop -1 or flip-flop -2 is triggered by the negative going edge of the first proximity switch pulse t.sub.P. However, since the time duration of t.sub.D is met with the coincidence of the next promixity switch pulse t.sub.P, the respective AND-gate 33 or 34 is made to operate in view of the fact that there is a coincidence of inputs on the AND gate inputs and the AND gate operates from a "low" to a "high" as indicated in FIG. 2c at 42. This signal from either of the respective AND-gate 33 or 34 is passed by OR-gate 38 to the input of one shot flip-flop -3. The negative going edge of the involved AND gate pulse 42 triggers into operation the timed or "high" state of the one shot flip-flop -3. Thus, a pulse is sent along line 40 from the output of one shot flip-flop -3 equal to t.sub.S or, in the illustration herein mentioned 0.33 second, which operates switching device 7 to "override" the decrease push button 20b causing adjusting motor 15 to operate the auto transformer 13 to reduce the voltage impressed upon the armature of main drive motor 8. All this in turn reduces the rotational velocity of the pair of cutoff knives as well as the lineal velocity of the web of material fed to the cutoff apparatus.

As illustrated in FIG. 2c the correction by the sensing circuit of the rotational velocity of the cutoff knives is linear, each successive proximity switch pulse t.sub.P slowly being increased in time duration from its preceding proximity switch pulse t.sub.P by an incremental amount until the last proximity switch pulse t.sub.P has occurred over a sufficiently given time duration or dwell from the previous proximity switch pulse t.sub.P as to be equal to more than the dwell of the pulse t.sub.D or equal to at least t.sub.S.

Emphasis is again made in connection with FIG. 2c the operation of the one shot flip-flop -1 and -2 and -3 which are always triggered on the negative going edge of the proximity switch pulse t.sub.P, in the case of one shot flip-flop -1 or -2 and the negative going edge of AND gate pulse 42, in the case of one shot flip-flops are triggered into operation to produce their respective timed pulses t.sub.P and t.sub.S regardless of whether or not the previously triggered pulses t.sub.P and t.sub.S have been permitted to complete their full timed period. The retriggering of the one shot flip-flops -1 and -2 is insufficient, being in nano-seconds, but is indicated by the short vertical lines 43. The retriggering of one shot flip-flop -3 is indicated by the short vertical lines 44. Thus, when a series of corrective pulses t.sub.S are being made by the sensing circuit, as shown in FIG. 2c, the timed pulses t.sub.D of either of the one shot flip-flops -1 or -2 are for all practical purposes, of constant duration until the circuit control of the main drive motor 8 has been adjusted sufficiently to reduce the rotational velocity of the cutoff knives to the threshold limit. The same is true in connection with the operation of one shot flip-flop -3 wherein the timed pulses t.sub.S actually generated have an additive time duration indicated by the length designated as 45.

Although the switching device 7 shown in FIG. 2a is of the same nature and kind as shown in FIG. 1, it should be readily understood that the switching device 7 may be in the form of a solid-state rectifier, such as, an SCR having its gate controlled by the output pulse t.sub.S of one shot flip-flop -3. The anode and the cathode of the SCR would be connected by lines 21 and 22 across the decrease push button contact 20b. Thus, upon operation of the one shot flip-flop -3, the pulse t.sub.S would be sent along line 40 to the gate of the SCR causing the SCR to conduct and thus "override" the decrease push button switch 20 to operate adjusting motor 15 in adjusting for reduction in the rotational velocity of the cutoff knives.

Reference is now made to FIG. 3 wherein there is shown a third embodiment of a sentinel control comprising this invention for imposing a maximum or threshold limit on a number of revolutions per second that can be negotiated by the cutoff knives. The sentinel control shown in FIG. 3 differs essentially from those of FIGS. 1 and 2a in that the sentinel control of FIG. 3 is based upon the sheet length selection being made through a sheet length selection control which automatically makes adjustments to the power transmission means 12 so as to limit the operation of the cutoff apparatus knives to a maximum of threshold limit of specified revolutions (cuts) per second. The sentinel controls of FIGS. 1 and 2a are designed in such a manner to effect the circuit control means operating the main drive motor 8 by means of changing the armature voltage impressed upon the motor. However, in the case of FIG. 3, the circuit control means utilized controls the shunt field voltage of the generator 56. The motor field voltage is therefore maintained constant so that the main drive motor 8 cannot be driven above a maximum speed as governed by the resistance in the generator shunt field.

The motor-generator control system as shown in FIG. 3 is commonly known as the Ward-Leonard motor control system for DC motors. The armature of motor 8 is connected with its series field across the generator 56. The motor 8 may be started by means of the push button 57. The motor 8 is controlled by having its motor shunt field constantly excited with its armature circuit energized from the separately excited generators 56. The exciter armature with its series field and its excited shunt field is a regulating exciter for the generator 56. As the voltage of the generator 56 is increased, the speed of the motor 8 increases. If the voltage on generator 56 is decreased to zero, then the motor 8 stops.

The maximum speed level of motor 8 for a given selected sheet length is accomplished by placing in the generator shunt field of the generator 56 operating the motor 8, a plurality of additional resistances R1, R2 and R3 are shown in FIG. 3. These resistances are variable and are connected in series by means of lines 46 and 47 with resistance R3 connected to line 48, in turn, connected to one side L.sub.2 of a DC power source. Because these resistances are variable, it is possible to adapt the control for use with mechanisms whose maximum speed is not limited to three cuts per second throughout the range. Resistance R1 is connected by means of line 50 to the maximum speed trimming resistance 51 which is also connected in series to one side of the rheostat control 52. The other side of the rheostat control 52 is connected in series with the minimum speed trimming resistor 54 which, in turn, is connected to line 55 which is the other side L.sub.1 of the DC power source.

The point of interest with regard to the sentinel control of FIG. 3 is the generator shunt field in controlling the excitation of the generator 56. As shown, the generator shunt field is connected between line 55 and pointer 53 of the rheostat 52. Thus, as the pointer 53 of the rheostat 52 is operated, the voltage on the generator may be increased or decreased in order to control the speed of the motor 8. The minimum trimming resistance 54 and the maximum trimming resistance 51 provide the minimum and maximum speed levels of operation of the motor 8 through the generator 56 when the sentinel control of FIG. 3 is not in operation. The sentinel control comprises the additional variable resistances R1, R2 and R3 across which are respectively connected in parallel normally closed contacts C1, C2 and C3 of the limit switches LS1, LS2 and LS3, respectively. Thus, the amount of additional maximum trimming resistance placed in generator shunt field circuit is controlled by means of placing in this circuit any one of the resistances R1, R2, R3 or any combination thereof.

Reference is now made in connection with FIG. 3 to the sheet length selection control panel 57 having means for changing sheet length selection as indicated at 58. The operator changes the dial setting at 58 to correspond to the desired sheet length to be cut by the cutoff knives 10. Also, panel 59 includes the increase push button 18 and decrease push button 20 as previously explained in connection with the sentinel control embodiments of FIG. 1 and FIG. 2a. The sheet control length selection control 59 can be of the type found in Nido patent application Ser. NO. 848,469, filed Aug. 8, 1969, wherein change of the sheet length size causes operation of the transmission adjusting motor 60 which in turn properly makes adjustments to the variable speed transmission and cycling mechanism, generally indicated at 12, to bring about proper sequential operation of the cutoff knives 10 relative to material flow to cut uniform sheet lengths from the moving web of material fed to the cutoff apparatus. The cam disc 61 is also provided in connection with the power transmission means 12 to be also rotated by the adjusting motor 60 in making adjustments for sheet lengths through the power transmission means 12. The cam disc 61 is calibrated to approximate the sheet length selected at the sheet length selection control 59. A cam 62 is provided on the perimeter of the cam disc 61. The limit switches LS1, LS2 and LS3 are positioned radially and adjacent to the cam disc 61 so that they are selectively operated when cam 62 makes contact with the limit switches which occurs when shorter sheet lengths are involved thereby preventing the rotational velocity of the cutoff knives to exceed the maximum or threshold limit by increasing the maximum trimming resistance in the generator shunt field.

For example, if a sheet length of 60 inches is set at the sheet length selection control panel 59, the cam disc 61 would be rotated in such a manner that its cam 62 would take the dotted line position as shown in 63. In this position all three limit switch contacts C1, C2 and C3 would be closed as indicated in FIG. 3. Thus, the motor 8 will run at any preset maximum speed as governed by operation of the rheostat 52 and the maximum trimming resistance 51. If, however, a sheet length of 50 inches is called for at the sheet length control, cam 62 will operate limit switch LS1 opening contact C1 causing resistance R1 to be placed in the shunt field circuit of generator 56 and thus limiting the maximum speed of motor 8, for example, to 500 feet per minute. If the sheet length is progressively shortened, limit switches LS2 and LS3 will be consecutively operated opening their contacts C2 and C3 and, thus, placing their corresponding resistances R2 and R3 in the series with resistance R1. This, in turn, progressively limits the maximum speed obtainable by motor 8 due to the fact that additional resistance has been inserted in the generator shunt field of generator 56.

From the foregoing it is obvious that additional limiting resistances, such as R1 through R3, together with associated limit switches and caming means can be utilized so that smaller speed increments in regard to the maximum speed control on motor 8 can be acquired. Also, a solid-state rectifier could be used instead of the limit switch means as provided for in FIG. 3.

From all of the foregoing it should be readily seen that the maximum or threshold limit measured as the maximum acceptable cuts per second designed in the operation of the cutoff knife 10 is maintained in the FIG. 3 sentinel control by means of controlling field voltage, that is, the maximum resistance in the generator shunt field. However, on the other hand, the sentinel controls of FIG. 1 and FIG. 2a adjust for the maximum or threshold limit by means of controlling the armature voltage impressed on the main drive motor 8.

Claims

We claim:

1. A sentinel control to impose a threshold limit on the number of revolutions negotiated by rotatable cutting means in a cutoff apparatus for cutting sheets of uniform length from a moving web of material fed to the apparatus in a given time comprising circuit control means connected to operate a variable speed motor to drive the moving web of material to the cutoff apparatus and to drive, through adjustable power transmission means, said cutting means, a sensing circuit having sensing means associated with said cutting means to operatively detect when said cutting means exceeds said threshold limit, and switching means connected in said sensing circuit and responsive to said sensing means when made operative to override said circuit control means to control the speed of said motor and prevent said motor from driving said rotatable cutting means in excess of said threshold limit.

2. The sentinel control of claim 1 characterized in that said threshold limit negotiated by said rotatable cutting means is 3 revolutions per second.

3. The sentinel control of claim 1 characterized by centrifugal switch means comprising said sensing means and connected to be driven from said cutting means and responsive when said rotatable cutting means is driven in excess of said threshold limit to operate said switching means to override said circuit control means.

4. The sentinel control of claim 3 characterized in that said switching means comprises a relay in said sensing circuit and having a normally open contact, said circuit control means includes a reversible adjusting motor to control the speed of said variable drive motor and connected by two supply lines and a common line to a power source, a normally open switch in each of said supply lines to operate said reversible adjusting motor in a forward or reverse direction, said relay contact connected across one of said normally open switches.

5. The sentinel control of claim 1 characterized by means for producing a pulse signal indicative of each coincidental cutoff by said cutting means comprising said sensing means, said sensing circuit responsive to said pulse signals when the latter within said given time are in excess of said threshold limit to operate said switching means to control the speed of said variable speed motor to prevent said motor from driving said rotatable cutting means in excess of said threshold limit.

6. The sentinel control of claim 5 characterized in that said pulse signal producing means are proximity switches associated with the operation of said cutting means to produce a pulse upon each complete revolution negotiated by said cutting means.

7. The sentinel control of claim 5 characterized in that said switching means comprises a relay in said sensing circuit and having a normally open contact, said circuit control means includes a reversible adjusting motor to control the speed of said variable drive motor and connected by two supply lines and a common line to a power source, a normally open switch in each of said supply lines to operate said reversible adjusting motor in a forward or reverse direction, said relay contact connected across one of said normally open switches.

8. The sentinel control of claim 5 characterized in that said switching means comprises a silicon controlled rectifier with its gate connected to said sensing circuit, said circuit control means includes a reversible adjusting motor to control the speed of said variable drive motor and connected by two supply lines and a common line to a power source, a normally open switch in each of said supply lines to operated said reversible adjusting motor in a forward or reverse direction, the anode and cathode of said controlled rectifier connected across one of said normally open switches.

9. The sentinel control of claim 1 characterized by cam means comprising said sensing means and connected to said adjustable power transmission means for change in sheet length, said circuit control means includes a motor control system wherein there is provided an exciter generator for controlling the speed of said variable speed motor through the generator shunt field to control the excitation of said generator, a plurality of variable resistances connected in series with said generator shunt field, said switching means consisting of a plurality of limit switches positioned adjacent said cam means and each having a normally closed contact connected in parallel with one of said variable resistances to normally shunt the latter, said cam means operative on selected of said limit switches to change the shunting effect of said generator shunt field to control the speed of said motor and prevent said motor from driving said rotatable cutting means in excess of said threshold limit.