US3418895A - Control system for article stackers - Google Patents

Description

Dec. 3 l, 1968 3,418,895

C. G. PALMER CONTROL SYSTEM FOR ARTICLE STACKERS Filed June 20, 1967 FIGJ Sheei'I FIG.5

Dec. 31, 1968 c. G. PALMER CONTROL SYSTEM FOR ARTICLE STACKERS Sheet Filed June 20, 1967 INVENTOR Afro/@Ney Cham/ as' 6I HaMe-.e

United States Patent O 3,418,895 CONTROL SYSTEM FOR ARTICLE STACKERS Charles G. Palmer, 937 W. Collins, Orange, Calif. 92667 Filed June 20, 1967, Ser. No. 647,547 14 Claims. (Cl. 93-93) ABSTRACT OF THE DISCLOSURE This application describes a control system for article stacking machines. It is described in detail in its application to a newspaper stacker. The control is a servo system which incorporates an article or newspaper counter, a conveyor for holders on which the stacks are to be formed, a drive motor for the conveyor, a transducer for indicating conveyor motion, and apparatus for retarding and then advancing holders relative to the conveyor as stacking is transferred from one holder to another. In addition the system comprises electrical apparatus, for comparing the control and feedback signals, for developing the error signal, and for effecting control of the conveyor motor. The control scheme regulates holder movement from discrete signals to synchronize the average rate of holder displacement. It employs add-subtract pulse counting with adjustable ratio control on an energy balance principle.

Specification This invention relates to control systems for stacking machines and it relates particularly to systems for controlling displacement of holders which are carried by an endless belt and on which articles are to tbe stacked.

An object of the invention is to provide improved control systems of that kind which provide accurate and dependable control at minimum complexity and cost.

While the invention is not limited to application in newspaper stackers, it is well suited to control of such devices and the embodiment selected for illustration is shown in connection with a newspaper stacker.

In this connection, an object of the invention is t provide a novel controller capable of superior control of newspaper stackers and having the reliability, accuracy, and speed of response required for that application.

Another object is to provide an article stacker control capable of accommodating a wide range of article thicknesses, number of articles to be stacked, and article delivery speeds.

Another object is to provide an article stacker `control system which permits non-uniform motion of the article holder to facilitate cessation of stacking on one holder and commencement of stacking on a following holder without interruption in the stream of articles arriving at the stacker.

It is a feature of the invention that the speed of the article holders need not be synchronized with the arrival of articles at the stacker or at the holder. The invention will accommodate variations in spacing between arriving articles, in drive motor speed, in slippage between the conveyor drive motor and conveyor, in supply line voltage amplitude, and it will even accommodate errors in speed settings by the operator, all within a wide r-ange of the variations encountered in practice. Another object of the invention is to provide a control that will result in proper stacking despite such variations and adjustment errors.

These and other objects and advantages of the invention will be apparent in the detailed description that follows; it ybeing understood that while only one embodiment is ICC shown, various modications in that embodiment and other embodiments are possible within the scope of the invention.

In the drawings:

FIGURE 1 is a schematic drawing of a newspaper stacker;

FIGURE 2 is a schematic of a lost motion mechanism and related electrical control elements employed in the invention;

FIGURE 3 is a block diagram of an article stacker control system embodying the invention and suitable for controlling the newspaper stacker of FIGURE 1; and

FIGURE 4 is a schematic diagram of the control system of FIGURE 2.

Referring rst to FIGURE l, this schematic drawing shows a stream of newspapers 10 being delivered by a paper conveyor 11 to a stacker 12 where they are deposited on one of several holders which are mounted for movement on an endless type conveyor 13. The holder conveyor 13 moves clockwise in FIGURE 1 and carries the holders down slowly relative to the rate of newspaper delivery whereby papers are stacked up on that holder which is under the end of the paper conveyor 11. A control system is provided which adjusts and controls the motion of the holders such that each succeeding holder intercepts the next paper in the stream when a predetermined number of papers have been stacked on the preceding holder. In FIGURE 1, a stack 14 of papers is being arranged on a holder 15. Comparison of the height of stack 14 with the height of the preceding stack 16, which has Ibeen discharged yfrom the preceding holder 17 to a lateral conveyor 18, shows that stack 14 is nearly at its completed height. The holder 19, next behind holder 15, is shown in position ready to intercept the stream 10 as soon as stack 14 includes the requisite number of papers.

In the application of the invention selected for illustration and description, the rate of delivery of newspapers 10 on the conveyor 11 is usually controlled by the speed of a printing press so that the rate of delivery, article spacing and starting and stopping of delivery are variables that the stacker control must accommodate. In addition, the thickness of individual papers may vary from issue to issue whereby the height and weight of the paper stacks will vary. It will -be apparent to those skilled in the art that the article delivery conveyor and the article holders can `be modied to deliver and to stack or pile material of a Wide variety of sizes and shapes even including fungible goods. The invention is applicable to such uses. The application to newspaper stacking was selected because it illustrates some of the variables which can be accommodated and because of demonstrated success of the invention in that application.

Returning to FIGURE 1, the stacker 12 includes a frame 39 which carried lateral conveyor 18 and the end portion 20 of a delivery conveyor 11. The drive mechanism for neither of these conveyors is shown, having been omitted for the sake of clarity. The drive mechanism for the holder conveyor 13 is shown. It includes a motor 21, connected to a gear reduction and clutch assembly 22 by a drive belt 23. Another drive belt 24 connects the clutch 22 to a driver gear 25 which drives the chain belt 26 over the rollers 27 and 28.

The outer end portion 20 of the delivery conveyor 11 is mounted on the stacker frame 39 to insure that the height and orientation of the stream of papers is matched to the position and placement of the stacker holders. The very end roller 29 and its belt are spring mounted above the stream of papers and direct it downwardly at an angle toward the holders. The holders comprise a plurality of 3 spaced tines which are disposed on opposite sides of roller and belt 29. Thus in FIGURE 1, the tine of holder 19 is not connected (except through the frame 39) to roller 29. The position it occupies in FIGURE l is a special one from which it can be lowered rapidly into position to intercept papers from the stream.

FIGURE 1 depicts an instant of time in a dynamic condition. This is evidenced by the midair position occupied by the papers which have just cleared the delivery conveyor '11. The holder conveyor 13 is in motion, carrying the holders counterclockwise. Holder 19 has just move to a position in which its tines 30 are slightly above the level to which papers are limited by roller and belt 29 as they emerge from the delivery conveyor. Also the holder 19 has just come into engagement with an electromagnetically operated stop latch 32. This is evidenced by the fact that the spacing between holders 15 and 19 is the same as the spacing between any two of the holders as can be seen in the case of holders 33 and 34. However, as previously noted, stack 14 is not yet complete. Means are provided for interrupting the motion of holder 19 while permitting the holder conveyor and holder 15 to continue downwardly. Also means are provided, when stack 14 has the requisite number of papers, for moving holder 19 downward rapidly into its regular position relative to the holder conveyor and other holders. At that position it will intercept the next paper in the stream and begin another stack.

A preferred form of this means is shown schematically in FIGURE 2. The holder 15 has a delayed motion connection to the holder conveyor chain 26. The two are interconnected by a spring 35 which is sufficiently strong to hold the holder in a normal position on the conveyor chain. That position is represented by abutment of chain stop 36 against holder stop 37 in FIGURE 2. Means are provided for stopping downward movement of the holder without stopping the conveyor chain. Advantageously, as shown, that means comprises the electromagnetic stop latch 32. As shown in the schematic in FIGURE 2, this mechanism is spring biased to a position to stop the holder. It is released by energization of its coil.

In operation, when the stop 38 on the holder engages stop 32, the holder stops but the chain continues. Spring 3S is extended storing kinetic energy. Means are provided for energizing the latch 32 when the required number of papers have been deposited on the holder ahead. When the latch is released the holder is propelled forward along its conveyor to a level at which its tines intercept the next paper to leave the delivery conveyor 11.

Means are also provided for interrupting the forward progress of the holder conveyor, as well as the holder, after a predetermined degree of relative displacement between them and until the required number of articles have been delivered to the preceding holder. This means may comprise a switch 190 which, when opened, results in stopping the holder conveyor. Means, represented by the combination of holder actuator 41 and a chain trip 42 and overcenter mechanism 43, actuate the switch upon predetermined advancement of the chain relative to the holder.

Advantageously, the means for stopping the conveyor after it has advanced a predetermined distance relative to the holder, comprises a stop '189 on the conveyor which engages holder stop 37. Until the requisite number of articles has been stacked on the holder ahead, holder 19 is precluded from advancing by latch 32. The conveyor continues to advance until its stop 189 engages the holder at stop 37. Thereafter clutch 22 slips permitting the motor to continue while the conveyor is stopped.

For a reason to be described below a switch 4t) is included with the mechanism of FIGURE 2. It is normally open and it is closed during the period when the movement of the holder is stopped by latch 32.

Summarizing, the elements of the system shown are:

a plurality of holder means shaped to receive and hold a stack of the articles to be stacked or loaded; means for disposing the holder means, in turn, at the point at which articles are delivered; means for holding the holding means poised for a time in position to receive articles upon completion of a preceding stack or load; means for advancing the holding means rapidly into position to receive articles upon completion of the preceding load or stack; and means for controlling and coordinating action of the other means. In the preferred form the holders are moved at a uniform speed except that motion of the holder next to receive articles is retarded just before completion of the stack on the holder ahead and is eccelerated ahead to overcome the effect of retardation when the holder ahead has its assigned number of articles. In the event that the stack on the holder ahead is not completed, then progress of both holders is interrupted.

A counter mechanism is shown in FIGURE 1 just above the stream of papers in the end portion 20 of the conveyor. In one of the Variety of available counters, a spoked wheel sensor is rotated by successive papers in the stream striking the spokes. Counter 44 is that kind. It opens one electrical circuit each time a paper strikes a spoke. It closes a second circuit when a preselected number of papers have been counted. Another counter 45 is mounted on the stacker frame. It closes an electrical circuit upon a predetermined increment of displacement of the holder conveyor 13. These counters 44 and 45 are preferred forms, respectively, of means for indicating the number of articles delivered to the stacker and means for indicating position and motion of the holder positioning means.

A block diagram of the means for coordinating the various means of the stacker system, is shown in FIGURE 3, together with other elements of the stacker system. As previously described, the article counter 44 provides a signal indicating delivery of a paper to the stacker. This signal is applied to a multivibrator 46 which responds by application of a pulse of given duration to a constant current switch 47. The switch permits a pulse of current to flow in a current adding circuit represented by series variable resistors 48 and 49.

The drive motor 21 drives the holder conveyor 13 which drives counter 45 which supplies signals indicative of conveyor movement to a constant energy pulse multivibrator 50. The latter drives a constant current switch 51 which permits current flow in the current adding circuit represented by series variable resistors 48 and 49. One constant current switch furnishes negative current, the other furnishes positive current. The two currents need not and usually do not ilow in synchronism. They are added to form a composite signal in a current integrator 52. The composite signal is conditioned, and amplied in an amplifier 53, so that upon further conditioning in a trigger signal controller 54, a signal is applied to a power controller 55 Iwhich controls the amount of power furnished to drive motor 21 from a source of alternating electrical power.

Suppose that the system is in operation with the holder conveyor 13 running and papers being delivered on the delivery conveyor 11. Signals from article counter 44 trigger multivibrator 46 which triggers switch 47 with the result that a current flows in resistor 48 the average magnitude of which depends upon the number of articles counted, the value of resistor 48 and upon circuit constants. Similarly counter 45 triggers multivibrator 50 which triggers switch 50 with the result that current flows in resistor 49. The average magnitude of this current is determined by circuit constants, the speed of the holder conveyor 13, and the value of resistor 49. The use of constant energy pulse multivibrators and constant current switches insures that) the effect of circuit constants is indeed constant so that only signal frequency and resistor value are variables. Looking ahead in the system, the speed at which the article holder conveyor must move the holders is determined by the number of articles to be stacked on each holder and the rate at which they are delivered. Conveyor speed requirements dictate how fast motor 21 must operate. This required speed determines what power must be delivered to the motor 21 and the power requirement determines, in any given power source arrangement, the character of the signal to be supplied by the signal controller 54 to the power controller 55. Current integrator and amplifier 53 and controller 54 combine to provide the requisite signal as a function of the algebraic difference of the two currents in resistors 48 and 49. A preferred arrangement is shown in FIG- URE 4 and will be described below. The fact that the article counter 44 applies a summation signal to the latch 32 to permit progress of holders when previous holders are filled, is explained above.

The fact that limit switch is opened when the counter 44 does not cause the latch 32 to release is also explained above. The switch 40 is connected between resistor 49 and a voltage divider 57. It completes the circuit when closed and permits current flow in resistor 49. In the servo system, if paper delivery rate is increased, pulse rate in units 46 and 47 increases to increase current in resistor 48. Conversely, if holder conveyor speed is excessive, pulse rate in units and 51 is excessive and current 49 is excessive. An increase in resistor 48 current tends t0 increase the motor 21 and conveyor 13 speed but an increase in resistor 49 current tends to decrease the motor 21 and conveyor 13 speed.

A preferred form of electrical circuitry for the system is shown in FIGURE 4. Alternating electrical power from an external source is applied at terminals X-X, at the right in FIGURE 4, to the primary Winding of a transformer 61 having a center tapped secondary Winding 62. A first pair of back-to-back rectifiers 63 and 64 are connected such that the line 65 connected to the junction between them is positive with respect to common line 66 which is connected to the center tap of the secondary winding of transformer 61. A second pair of back-to-back rectifiers 67 and 68 are connected across the secondary winding 62 such that line 69, which is connected to the junction between them, is negative with respect to the center tap line 66. The full wave rectified voltage appearing between lines 65 and 66 is filtered by a filter capacitor 170 connected from line 66 to line 65. The full wave rectified voltage appearing between lines 69 and 66 is not filtered. However, a line 70 is connected to line 69 through a halfwave rectifier 71 and a filter capacitor 72 is connected between lines 70 and center tap line 66 so that a full wave rectified voltage appears between lines 70 and 66.

Supply lines X-X are also connected to A.C. power bus lines 73 and 74. These bus lines supply power to the direct current series motor 21. A first bridge rectifier cornprising half-way rectifier elements 75, 76, 77 and 78 is connected across bus lines 73 and 74 and its D.C. out- =put terminals are connected across the field winding 79 of the motor 21. A second bridge rectifier 80 comprising half-wave rectifiers 181, 82, '83 and 84 has its input terminals connected across bus lines 73 and 74 and its D.C. output terminal connected across the armature of motor 21. Rectifier elements 83 and 84, in adjacent legs of the bridge rectifier 80, are controlled rectifier devices. Advantageously as shown, they comprise silicon controlled rectifiers. Firing of `the silicon controlled rectifiers or SCRs is controlled by a Ipulse transformer 86 having a primary winding 87, a first secondary winding 88 connected between the firing electrode and cathode 0f rectifier 83, and a second secondary winding 89 connected between the firing electrode and cathode of rectifier 84. To `minimize the deleterious effect of transient voltages and currents in the several circuits, a rectifier 90 and a resistor 91 are connected in parallel with one another and the armature 85 from motor 21. For the same purpose the parallel vcombination of a capacitor 92 and a pair 93 of back-to-back rectiers is connected across power bus lines 73 and 74. Resistor 91 also serves as a dynamic braking resistor for motor 21.

The speed at which motor 21 operates is determined by the power delivered to its armature. The power is varied by varying the portion of the A.C. power supply cycle during which the SCRs are conductive. These units are rendered conductive by pulses applied to the primary winding 87 of pulse transformer 186. A pulse is applied to the primary winding in every half cycle of the supply cycle. The circuitry still to be described in FIGURE 4 determines at what time during a half cycle of alternating supply voltage the initiating pulse will be supplied to transformer winding 87.

That remaining circuitry includes type PNP transistors 100, 101, 103, 104, and 108 and type NPN transistors 102, 106, 107, 109 and 110. It also includes a unijunction transistor 111. Transistors 100 and 101 comprise a multivibrator which includes resistors 112 and 113 connected between the base of transistor 100 and negative line 70. The base of transistor 101 is connected to negative line 70 through a resistor 114 and, in a parallel circuit, through the `series combination of a capacitor 115 and a resistor 116. The emitters of these two transistors are tied together and both are connected to common line 66 through a resistor 117. The collector of transistor 100 is connected to the junction between capacitor 105 and resistor 116. The emitter of transistor 101 is connected to the junction of resistors 112 and 113. The base of transistor 100 is connected to common line 66 through a rectifier 118 and a resistor 119. A pair of capacitors 120 and 121 are connected in series across resistor 119. Bias for transistor 100 is established by current flow through a resistor 122 connected between positive line v65, as indicated by a plus terminal, to the base of transistor 100. Capacitor 121 is charged by current flow through a resistor 123 connected between that positive terminal and the junction between capacitors and 121. The normally open contacts of the paper counter 44 are connected across capacitor 121. When the contacts are closed the capacitor is discharged to provide a signal to initiate a pulse in the one-shot multivibrator 46.

Except that its initiating contact 45 is normally closed, its blocking rectifier 171 has reverse polarity, its transistors are of opposite type, and its applied voltages are of opposite sign, the multivibrator 50 is just like multivibrator 46. Advantageously its capacitors 124, 125 and 126 have the same value and circuit placement as do the corresponding capacitors 121, 120 and 115, respectively of multivibrator 46. Similarly resistors 127, 128, 129, 130, 131, 132, 134 and 135 have values and circuit placement corresponding respectively to resistors 119, 123, 122, 112, 113, 117, 116 and 114 0f multivibrator 46. Except for being reversed in polarity, rectifier 171 has a value and circuit connection corresponding to rectifier 118 of multivibrator 46. One-shot multivibrator 50 is pulsed by opening normally closed contact 45 of the conveyor speed counter.

The constant current switch 47 comprises the single transistor 102. The output of the multivibrator 46 is coupled to transistor 102 by a resistor 136 connected between the base of 102 and the junction between resistor 116 and capacitor 115 of the multivibrator. The emitter of a transistor is connected to negative line 70 through a rheostat 137 in series with a fixed resistor 138. The collector is connected to common terminal 139 at the input of the current integrator 52.

The other constant current switch 51 comprises a single transistor 108 whose base is connected by a resistor 140 to the output terminal of multivibrator 50 at the juncture of capacitor 126 and resistor 134. The collector of transistor 108 is connected to common terminal 139. The emitter is connected to positive line `65 through the series combination of a fixed resistor 141 and a tapped resistor 142.

Switch 40 is connected from common point 139 to the variable tap of a potentiometer 143. The potentiometer is connected in series with a resistor 144 between common line 66 and positive line 65 to form a voltage divider by which a voltage adjustable by moving the tap of potentiometer 143 may be applied to common point 139 by closing switch 40.

The current integrator comprises two emitter followers connected in series in a Darlington circuit, The base f transistor 109 is connected to common point 139 to a resistor 145. The emitter of transistor 109 is connected to the base of transistor 110 through a resistor 146. The collector of both transistors is connected to positive line 65. The output of the circuit appears at the emitter of transistor 110 and is applied through a rectifier 147 to the base of transistor 103 of the amplifier 53.

The base of transistor 103 is also connected by resistor 149 to the negative line 70. Diodes or rectiers 71 and 147 are connected to permit the iiow of. conventional current from positive line 65 through transistor 110 from collector to emitter, through rectifier 147, through resistor 149 to negative line 70, and through rectifier 71 to negative line 69. Rectifiers 63, 64, 67 and 68 comprise a bridge rectier whose unidirectional output is applied across the lines 65 and 69 without iiltering. Accordingly, the input signal voltage to transistor 103 is a full wave unrectied D.C. voltage whose magnitude varies with the impedance exhibited across the collector to emitter of transistor 110. This impedance varies with the average value of current input to the current integrator 52.

Amplier 53 and the trigger signal control 54 are integrated in a circuit which includes, in addition to transistor 103, transistors 104, 111, and 105. The collector of transistor 103 is connected through a resistor 150 to negative line 69 and through a resistor 151 to the base of transistor 104. The emitter of transistor 103 is connected by a resistor 152 to common line 66. The transistor 104 has its base connected to common line 66 by resistor 153 and has its emitter connected to common line 66 by a resistor 154. The collector of transistor 104 is connected to the control electrode of the unijunction transistor 111 whose base is connected to negative line 69 and through a resistor 155 to common line 66. A capacitor 156 connects negative line 69 to the juncture between the control electrode and the unijunction transistor and the collector of transistor 104. The coupling capacitor 157 is connected from the juncture between the base of the unijunction transistor and resistor 155 to the juncture between the base of transistor 105 and a resistor 158 which connects to common line 66. The collector of transistor 105 is connected through resistor 159 to the filtered negative supply line 70. The emitter of transistor 105 is connected to common line 66. The output of transistor 105 is coupled through a coupling capacitor 180 to one side of the primary winding 87 of the -pulse transformer 86. T he other side of the primary winding is connected to common line 66.

Input signal magnitude through the current integrator 52 is defined as the magnitude of the voltage appearing at common point 139. When switch 40 is closed the voltage at 139 is determined by the position of the variable tap of potentiometer 143 and can be envisioned as the voltage appearing across capacitor 161 which is connected from common line 66 to common point 139. When switch 40 is closed, the voltage across capacitor 160 is controlling and it fixes the output impedance of transistor 110. When switch 40 is open the relative currents through transistors 102 and 108 determine the voltage at point 139 and the impedance at point 110'.

Transistors 103, 104 and 111 are connected to a full wave uniiltered bias supply by virtue of their connection between common line 66 and negative line 69.

In one embodiment of the invention these Components were of the following value and type.

8 Resistors:

136 ohrns 12K 117, 132, 152 do 150 123, 12S, 144, 149 d0 47K 112, d0 4.7K 113, 131, 116, 134, d0 3.3K 122, 129 d0 24K 143, 137, 13s, 153, 155 do 10K 145, 14s` do 47o 142 ..-d0 68K 141 d0 750 Capacitors:

120, 125, 157 mmfd 2200 121, 124, 115, 126 mfd-- 0.1 72, rnfd 100 160 mfd l() 156, mfd 0.01

Transistors:

100, 101, 103, 104, 10S, 108 (type) 2N3638 102, 106, 107, 109, 110 (type) 2N3393 Rectifiers:

7s, 76, 77, 7s, s1, 82, 90 (type) MR1033B 93 (Thyrect'or) 20SP5B5 83, 84 MCR2305-5 The signal applied to transistor 103 has the shape of a full wave rectified but unfiltered sine wave whose amplitude varies with the voltage at common point 139. The signal is amplified in transistors 103 and 104 but the unijunction transistor 111 is connected in series with the transistor 104. The unijunction has high impedance until a predetermined signal voltage level is reached at which time its impedance is rapidly reduced permitting a rapid increase in current flow. A pulse corresponding to the rate of that current change is applied by the capacitor 157 to the base of transistor 105 wherein it is amplified and applied to the pulse transformer. The greater the amplitude of the signal applied at 103 the earlier in each one half of the supply cycle will the pulse be applied to the input winding of transformer 86 and the earlier in the supply cycle will the SCRs be rendered conductive. Rendering them conductive earlier increases the power applied to the drive motor 21.

Both secondary windings 88 and 89 are pulsed in each half of the supply cycle but only that SCR in each half cycle which is subjected to forward voltage will be fired. Thus there is delivered to the motor a quantity of electrical power which differs from the quantity selected by the setting of resistors 48 and 49 lby an amount which is proportional to the difference between the selected ratio of count to displacement pulses represented by those settings and the actual ratio of those pulses.

The switch 40 serves another purpose. It is closed periodically to establish, and re-establish, a reference level for electrical operation of the syste-rn. Since the control is an analog system in part it is advantageous periodically to begin afresh the comparison of count and displacement pulses. By this means any system error is prevented from becoming cumulative. When switch 40 is closed the standard reference or reset voltage at the tap of potentiometer 143 is applied to point 139. Switch 40 is opened each time the holder conveyor carries a basket to the position ahead of the conveyor 20 discharge point so the system operational level is reset in both an electrical and a mechanical sense before commencement of a new stacking event. Actuator 191 on the conveyor (in FIG- URE 2) holds switch 40 closed while holder progress is stopped by latch 32. This is the time when spring 35 is stretched. Thus the system provides a predetermined, standard input power to the motive power system during lost motion or delayed motion event.

I claim:

1. For an article stacker of the type in which articles delivered to the stacker are stacked on holders carried by an endless conveyor, a controller for controlling conveyor and holder displacement such that each holder is positioned to receive articles when a requisite number of articles 'have been stacked on the preceding holder, comprising;

(a) a motor operable at a speed determined by the quantity of electrical power applied to it and connected to drive the conveyor,

(b) displacement signal means for providing displacement signal pulses indicative of a predetermined degree of lineal displacement of said conveyor,

(c) `count signal means for providing count signal pulses indicative of the number of articles delivered to said stacker, and

(d) power application means for applying to said motor a quantity of electrical power which `difiers from a selected quantity of power by an amount proportional to the difference from a selected ratio in the ratio of count pulses to displacement pulses.

2. The invention defined in claim 1 in which means are provided for stopping displacement of said holders in a position that precludes receipt of papers and for displacing said holder to a position for receiving articles when said preceding holder has received the requisite number of articles.

3. The invention defined in claim `1 in which said power application means comprises voltage responsive means for varying the average current applied to said motor, means for applying to said Voltage responsive means a voltage whose magnitude is the composite of a uniform voltage value corresponding to a selected speed in operation of said motor and a voltage variable in magnitude and polarity corresponding to hte difference between a selected ratio of count and displacement pulse frequencies and their actual ratio.

4. The invention defined in claim 3 in which said voltage corresponding to a selected speed in operation of said motor comprises a voltage at which said motor drives said conveyor through a displacement distance in a given time exceeding the height of the stack of articles delivered to the stacker in that time.

5. The invention defined in claim 1 in which said motor comprises a D.C. shunt motor and in which said power application means comprises a rectifier circuit adapted for connection at its input to an alternating power source and having its output connected across the armature of said motor, said rectifier circuit comprising a rectifier of the type rendered conductive for a selected time segment of the power source cycle.

6. The invention defined in 4claim 5 in which said rectifier is of a type which is rendered conductive by application thereto of a control voltage and which includes means responsive to said difference from a selected ratio for applying a control voltage to said rectifier at a time during said power source cycle which increases with the magnitude of said difference.

7. The invention defined in claim 1 in which said power application means comprises means for altering the value of said selected ratio.

8. The invention defined in claim 1 in which said means for providing count signal pulses further comprises means for providing a summation signal indicative of the provision of a predetermined number of counts, and which further comprises means for stopping displacement of said holders in a position that precludes receipt of papers and responsive to said summation signal for displacing successive holders to a position for receiving articles when said preceding holder has received the number of articles represented by said summation signal.

9. The invention defined in claim 8 in which said power application means comprises: means including a rectifier for furnishing to said motor power from an alternating source during a portion of each source power cycle and means for adjusting the duration of said portion including means responsive to said count signal pulses for providing a first electrical current, means responsive to said displacement signal pulses for providing a second electrical curent, and means responsive to algebraic difference in 5 said currents for altering the time during said cycle at which said rectifier is rendered conductive.

10. The invention defined in claim 9 including means operative when displacement of said holders has been stopped for a selected time to alter the current to said means responsive to algebraic difference sufficiently to prevent said rectifier from being rendered ocnductive.

11. The invention defined in claim 8 in which said power application means comprises, a rectifier connected to supply power to said motor Iwhen connected to an alter- D nating power source and when rendered conductive, and

means for rendering said rectifier conductive during portions of the successive cycles of said power source including current responsive means responsive to average current magnitude for rendering said rectifier conductive at 90 a time during successi-ve supply cycles variable fwith said average current magnitude, means responsive to said count signal pulses for providing a first current whose average value varies with the number of said count signal pulses, means responsive to said displacement signal pulses D for providing a second current whose average value varies with the number of said displacement signal pulses, and means for applying to said curent responsive means a current variable -with the difference in said first and second currents.

12. The invention defined in claim 11 including means for supplanting said second current with a current of fixed magnitude, in the absence of a summation signal within a selected time following stoppage of displacement of a holder, sufficient so that the difference current applied d to said current responsive means is sufficient to preclude conduction in said rectifiers.

13. For an article stacker of the type in which articles delivered to the stacker are stacked on holders carried by an endless conveyor, a controller for controlling conveyor and holder displacement such that each holder is positioned to receive articles when a requisite number of articles have been stacked on the preceding holder, comprising:

(a) motive means for driving said conveyor;

(ib) displacement signal means for providing displacement signal pulses indicative of a predetermined degree of lineal displacement of said conveyor;

(c) count signal means for providing count signal pulses indicative of the number of articles delivered to said stacker; and

(d) means for comparing the actual ratio of count pulses to displacement pulses to a selected ratio and for adjusting the speed at which said conveyor is driven by said motive means.

14. The invention defined in claim 13 in which means are provided for stopping displacement of said holders in a position that precludes receipt of articles and for displacing said holder to a position for receiving articles Awhen said preceding holder has received the requisite number of articles. 60

References Cited UNITED STATES PATENTS GERALD M. FO'RLENZA, Primary Examiner.

R. J. SPAR, Assistant Examiner.

U.S. Cl. X.R.

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