US3728597A

US3728597A - Servo motor controlled transfer system for automatic press line

Info

Publication number: US3728597A
Application number: US00160799A
Authority: US
Inventors: J Cummens; F Heiberger
Original assignee: Danly Machine Corp
Current assignee: CONNELL INDUSTRIES LP ONE MASS TECH CENTER BOSTON MA 02128; Wabash Alloys Inc; Avondale Industries Inc
Priority date: 1971-07-08
Filing date: 1971-07-08
Publication date: 1973-04-17
Anticipated expiration: 1990-04-17

Abstract

A transfer system for an automatic press line which includes a gripper for transferring workpieces between presses. The gripper is supported on the ends of a pair of transfer arms projecting from a corresponding pair of pantographs. The pantographs are manipulated by a pair of ball screw assemblies operated by a corresponding pair of servo motors. The servo motors are electrically connected to the press drive for synchronizing movement of the transfer assembly with the press line. A pair of cams connected to the press drive are associated with a tachometer and a resolver for generating electrical input signals for the control system associated with the servo motors.

Description

United States Patent Cummens et a1.

1 SERVO MOTOR CONTROLLED TRANSFER SYSTEM FOR AUTOMATIC PRESS LINE Inventors: Joseph R. Cummens; Francis E.

Heiberger, both of Elmhurst, Ill.

[73] Assignee: Danly Machine Corporation, Chicago, Ill.

Filed: July 8, 1971 Appl. No.: 160,799

[56] References Cited UNITED STATES PATENTS 8/1965 Danly ..318/85 X 11/1962 E1161] et a] 318/578 Primary Examiner-T. E. Lynch Attorney-Wolfe, Hubbard, Leydig, Voit & Osann, Ltd.

[ 5 7] ABSTRACT A transfer system for an automatic press line which includes a gripper for transferring workpieces between presses. The gripper is supported on the ends of a pair of transfer arms projecting from a corresponding pair of pantog'raphs. The pantographsare manipulated by a pair of ball screw assemblies operated by a corresponding pair of servo motors. The servo motors are electrically connected to the press drive for synchronizing movement of the transfer assembly with the press line. A pair of cams connected to the press drive are associated with a tachometer and a resolver for generating electrical input signals for the control system associated with the servo motors.

5 Claims, 11 Drawing Figures W J/PV/ l/fZZ/fl/f 1? //Z na /z; u/rzm/mrw f/JfiJ/f/ 1 PATENTEBAPR 1 71975 SHEET H (]F 6 rem afz/wffwzz wad SERVO MOTOR CONTROLLED TRANSFER SYSTEM FOR AUTOMATIC PRESS LINE DESCRIPTION OF THE INVENTION The present invention relates generally to transfer mechanisms for power presses and, more particularly, to an improved transfer mechanism for an automatic press line, and an improved control system for synchronizing the operation of the transfer mechanism with operation of the press line.

In US. Pat. No. 3,199,439 to James C. Danly, a system is disclosed for synchronizing the operation of a line of independently driven presses, each equipped with a variable speed drive, at any desired operating speed up to a limit imposed by the mechanical capabilities of the presses. The synchronization is achieved by feeding back to each press drive a phase error signal obtained by continuously comparing the press phase position with an externally generated reference phase position. With that arrangement, if any given press tends to get out of step, a phase error signal is supplied to the press drive to increase or decrease the press speed as necessary to maintain the press in step. Thus, any corrective action required is provided when only minor changes in press speed suffice and, therefore, the necessary corrections are readily made despite the high inertias of the presses.

Although the control system described in the aforementioned patent provides excellent synchronization, it is relatively costly in that it requires a complex power take-off from the main press drive.

Accordingly, it is one of the primary objects of the present invention to provide an improved transfer mechanism and control system, for use with a synchronized automatic press line, which simplifies the power take-off from the main press drive system.

It is another object of the invention to provide such an improved transfer mechanism, and associated control system, which has minimum power requirement.

It is a further object of the invention to provide such an improved transfer mechanism and control system which can be efficiently and economically manufactured, installed, and operated.

Yet another object of the invention is to provide such a transfer mechanism and control system which is relatively light in weight, and well balanced.

A still further object of the invention is to provide such an improved transfer mechanism and control system which has good working rigidity and in which the travel of the actuating component is relatively short in comparison with the travel'of the working components. In this connection, a related object of the invention is to provide such a mechanism which can provide any desired working stroke.

Still another object of the invention is to provide such an improved transfer mechanism which can be easily and quickly adjusted or modified to transfer different types of work pieces, and to provide accurate positioning of the work pieces. 1

A further object of the invention is to provide such an improved transfer mechanism which has a compact overall assembly, thereby permitting relatively close spacing of adjacent presses, with attendant reductions in floor space requirements.

Other objects and advantages of the invention will become'apparent from the following detailed description taken in conjunction with the attached drawings, in which:

FIG. 1 is a side elevation, partially in section and partially diagrammatic, illustrating a portion of a typical press line embodying the present invention;

FIG. 2 is an enlarged vertical cross section of one of the gripper assemblies utilized in the press line of FIG.

FIG. 3 is an enlarged side elevation of one of the transfer mechanism assemblies utilized in the press line of FIG. 1;

FIG. 4 is an enlarged end shown in FIG. 3;

FIG. 5 is a section taken along line 5-5 in FIG. 4;

FIG. 6 is a section also taken along the line 5-5 in FIG. 4, but with the transfer mechanism in a different operative position;

FIG. 7 is a section also taken along the line 5-5 in FIG. 4, but with the transfer mechanism in another different operative position;

FIG. 8 is a side elevation of the same transfer mechanism shown in FIG. 3, but in a different operating position;

FIG. 9 is a block diagram of an electronic control system associated with one of the servo motors included in the transfer mechanism assembly shown in FIGS. 1 through 8;

FIG. 10 is a diagrammatic perspective of the driving mechanism for one of the presses shown in FIG. 1; and

FIG. 11 is an enlarged vertical cross-section taken along line 11-11 looking into the cam box in FIG. 10.

While the invention will be described in connection with one particular embodiment, it is to be understood that it is not intended to limit the invention to any particular embodiment. To the contrary, the intention is to cover all alternatives, modifications and equivalents falling within the spirit and scope of the invention.

Turning now to the drawings, and referring first to FIG. 1 there is shown a portion of an automated press line intended for performing successive operations on a workpiece W. For purposes of illustration, only three presses have been shown, indicated as press A, press B, and press C, together with their associated transfer and conveyor mechanisms, but it will be understood that the press line may in practice be extended to include any number of similar units.

As here illustrated, the presses are identical and of generally conventional construction. For example, the press A includes a massive base 10 which is anchored below the floor 11 and a frame 12 topped by a crown l3. Reciprocatingly mounted in the press frame 12 is a slide 14 with cooperating upper and lower dies 16 and 17, respectively. To drive the press A, a motor 18 is mounted on the press crown 13 and coupled to the press drive which, in turn, is coupled to the press slide 14. FoFcTa'iity, corresponding parts in the three presses have been identified in the drawings by identical reference numerals with the addition of the distinguishing suffix b for elements of press B, and suffix c for elements of press C.

The coupling between the press motor, press drive, and slide 14 is not shown in detail, since it may be entirely conventional. If the details of this drive system and coupling are of interest, reference may be had to elevation of the assembly the trade brochures and descriptive bulletins of the press manufacturers, aswell as to Danly US. Pat. Nos. 3,199,439 and 3,199,443. For the present purposes it suffices to simply note that the press drive motor and its associated speed controls provide an electrically responsive, variable speed drive for the presses A, B, C, and that the drive for the press A is independent of the similar variable speed drives provided for presses B and C and the other presses in the line.

As illustrated, for transporting the workpieces W down the press line, there are respective conveyors 21 leading up to and away from the input and output sides of each press. Further, each press is equipped with an input transfer mechanism 22 and an output transfer mechanism 23. In the illustrative transfer mechanism, the workpiece W is gripped between a pair of jaws 24 and 25 (FIG. 2) in a conventional gripper assembly 26. Since the gripper assembly 26 does not form a part of the present invention, it need not be described in detail herein, and it will suffice to simply note that when the pneumatic actuator 27 for the movable lower jaw is advanced, the jaw 25 is pivoted upwardly to .grip the workpiece against the underside of the upper jaw 24, as illustrated by the broken line drawing of jaw 25 in FIG. 2. When the movement of the pneumatic actuator 27 is reversed, the movable jaw 25 is returned to its open position by means of a biasing spring 28. The entire gripper assembly is mounted on a gripper mounting plate 29 carried on the lower end of a transfer arm 30 which is an integral part of the transfer mechanism to be described in more detail below.

In accordance with one important aspect of the present invention, the transfer mechanism includes a pantograph comprising four pivotally interconnected links, with the elongated transfer arm extending from one of the links of the pantograph and carrying the gripper assembly on the end thereof. Thus, in the illustrative arrangement, a pair of pantographs and 40a which are mirror images of each other are interconnected to work together in unison. Each pantograph 40 and 40a comprises four pivotally interconnected links 41, 42, 43 and 44, and 41a, 42a, 43a, and 44a, respectively. Both pantographs 40 and 40a are mounted on a common main pivot shaft 45 which extends through, and is rigidly connected to, a pair of spaced frame plates 46 and 47.

For the purpose of actuating the pantographs 40 and 400, the links 42 and 42a thereof are connected .to a pair of integrally connected pins 48 and 49 which can be displaced both vertically and horizontally, relative to the main pivot shaft 45, by means of a biaxial ball screw drive arrangement mounted between the spaced frame plates 46 and 47. When the pins 48 and 49 are moved either vertically or horizontally, the entire pantograph 40 is pivoted around the fixed pivot shaft 45, and the individual links of the pantograph are pivoted relative to each other about the four pivot points at the four corners of the pantograph where the links are connected to each other.

One of the advantages of the pantographic linkage between the biaxial drive system and the gripper assembly 26 is that the pantograph amplifies any given displacement of the pins 48 and 49 by the biaxial drive system. Consequently, the actuating portion of the transfer mechanism, i.e., the biaxial drive system connected to the pins 48 and 49 between the frame plates 46 and 47, is operated at a lower velocity and acceleration than the working portion of the mechanism, i.e., the gripper assembly 26. For example, if a working stroke of 72 inches is desired for the gripper assembly 26 mounted on the end of the transfer arm 30, the pantograph 40 may be designed to provide a 4-to-1 amplification, so that an actuating movement of only 18 inches is required at the pins 48 and 49 to provide the maximum working stroke.

A further advantage of the illustrative arrangement is that the two pantographs 40 and 40a are mounted on opposite sides of the spaced frame plates 46 and 47, thereby providing a relative wide spread between the bearings of the pantograph links, while still providing a relatively compact overall assembly. This spacing of the two pantographs, combined with the relatively wide spread of the interconnections between the various pantograph links, provides desirable stability and rigidity to the overall system, while still maintaining a relatively compact size.

For the purpose of further stabilizing the pantograph assembly 40, a first stabilizing rod 50 is mounted in parallel with the-transfer arms 30. The lower end of the stabilizing rod 50 is pivotally connected to the gripper mounting plate 29, while the upper end of the rod is pivotally connected to an L-shaped bracket 51 which is rigidly mounted on a pin 52 interconnecting links 43 and 44 of the pantograph. As can be seen most clearly from the sequential views in FIGS. 5 and 6, the bracket 51 is always maintained in a fixed vertical-horizontal orientation, regardless of the position of the pantograph assembly, so that the horizontal leg 53 of the bracket effectively maintains the gripper mounting plate 29 in a corresponding horizontal position. That is, the horizontal arm 53 of the bracket and the gripper mounting plate 29 always remain horizontal, with the transfer arm 30 and the stabilizing rod 50 pivoting therebetween as'the pantograph assembly is displaced,

e.g., from the position illustrated in FIG. 5 to the position illustrated in FIG. 6.

To further stabilize the pantograph assembly, and to maintain the bracket 51 in the desired orientation, the I vertical leg 54 of the bracket is connected to a second stabilizing rod 55, the upper end of which is pivotally connected to a fixed point on the frame plate 46, with the lower end of the rod 55 being pivotally connected to the upper end of the vertical leg 54 of the bracket 51.

It will be appreciated that a similar arrangement of stabilizing rods is associated with the second pantograph 4011, which is a complete mirror image of the first pantograph 40.

Turning next to the biaxial drive system that is mounted between the frame plates 46 and 47, for the purpose of driving the pins 48 and 49 both horizontally and vertically, the drive system basically comprises a pair of ball screw units operated by a pair of servo motors 62 and 72. The horizontal ball screw unit comprises a ball screw nut 60 threaded on a horizontal drive screw 61. The screw 61 is rotated in either direction by means of a reversible horizontal servo motor 62 which is rigidly mounted on the end of a housing having an upper section 63 and a lower section 64. The housing formed by the sections 63 and 64 is mounted between the frame plates 46 and 47 for vertical movement relative thereto, as will be described in more detail below. When the horizontal servo motor is actuated to turn the screw 61, the ball screw nut 60 is moved horizontally along the screw 61, with the direction of movement being determined by the direction of rotation of the drive screw 61 by the reversible servo motor 62. The pins 48 and 49 which couple the ball screw drive unit to the pantograph assembly are formed as integral parts of the ball screw nut 60, so that any movement of the nut 60 causes a corresponding movement of the pantographs 40 and 40a.

For the purpose of moving the pantograph in the vertical direction, a second ball screw unit includes a ball screw nut 70 fixed to the upper housing section 63 and threaded on a vertical drive screw 71. Rotation of the vertical drive screw 71 is controlled by means of a reversible vertical servo motor 72 rigidly mounted on the frame plates 46 and 47 via plate 72a. As the drive screw 71 is rotated by the motor 72, the ball screw nut 70 is moved vertically along the screw 71, thereby effecting vertical displacement of the entire horizontal ball screw unit including the housing sections 63 and 64 and the motor 62. Of course, the vertical movement of the horizontal ball screw unit also moves the pins 48 and 49 connected to the pantographs 40 and 40a, thereby effecting corresponding vertical movement of the pantographs.

To permit vertical movement of the horizontal ball screw unit, the two housing sections 63 and 64 are provided with a plurality of

vertical guide rods

65, 66 and 67, 68 which are slidably mounted in corresponding bushings 65a, 66a, 67a, 68a within a plurality of corresponding guide cylinders 69, 70 and 71, 72. The guide cylinders 69-72 are rigidly mounted in a housing assembly including a pair of plates 73 and 74 connected between the frame plates 46 and 47, substantially enclosing the horizontal ball screw unit.

As can be seen in FIG. 3, windows are formed in opposite sides of the housing formed by sections 63 and 64 to permit horizontal movement of the coupling pins 48 and 49, and a similar window, larger in the vertical dimension, is formed in the frame plates 46 and 47 to permit vertical movement of the coupling pins 48 and 49. Whenever the vertical servo motor 72 is operated, the entire horizontal ball screw unit, including the housing sections 63 and 64, is displaced vertically, with the guide rods 65-68 sliding up and down within the bushings 65a-68a in the guide cylinders 6972. The electrical system for controlling and driving the servo motors 62 and 72 is synchronized with the main press drive system, a portion of which is illustrated in FIG. 9. Knowledge of only the main elements of a power press will permit full understanding of the present invention, and for the details of a commercial or practical press, cross-reference may be made to the trade literature and descriptive brochures of press manufacturers. Referring to H6. 10, the press driving motor 80 is shown feeding astep down drive connection including a belt 81 and pulleys 82, 83. Pulley 83 is connected to a clutch 85 having an input shaft 86 and an output shaft 87. The clutch output shaft 87 is coupled to an output pinion 90 which drives a pair of intermediate gears 91, 92 which mesh with the main press drive gears 93, 94

respectively. In a conventional press, such gears are connected to the press pitman, and the movement and phasing of the slide are under the joint control of timing cams and operating pushbuttons which, through appropriate and interlocked circuitry, control the energization of the clutch 85.

One of the main press drive gears, in this case the gear 94, is meshed with a power take-ofi' pinion 95 secured to a horizontally extending power take-off shaft 96. For driving the first transfer mechanism, the power shaft is coupled to a servo cam box 97 to be described in more detail below. The shaft 96 also drives a bevel gear 960, which meshes with a bevel gear 98 having a horizontal shaft 99 which is coupled, via a pair of bevel gears 100, 101, to a rotary cam limit switch drive shaft 102. At is lower end the latter carries a worm gear 103 meshing with a worm wheel 104 which drives a timing cam assembly 105. The assembly 105 maycorrespond to timing cams used for interlock purposes in a regular power press, and is driven via

bevel gears

106, 107 which rotate a vertical shaft 108.

Returning now to the servo cam box 97, one of the cams in this box, illustrated as cam 110 in FIG. 9, drives a tachometer 111 (e.g., a model No. SU-780D-1 Servo-Tek tachometer) and a resolver 112, for the purpose of generating electrical input signals for controlling the servo motor 62. More particularly, the tachometer 111 responds to rotation of the cam 110 to generate a d-c. signal representing both the magnitude and direction of the velocity desired at the output of the servo motor 62. This d-c. signal from the tachometer 111 is supplied to an amplifier 113 which drives the servo motor 62, with the magnitude of the d-c. signal being proportional to the magnitude of the velocity of the servo motor 62, and the polarity of the d-c. signal determining the direction of the velocity. More specifically, the slope of the cam 110 controls the magnitude and polarity of the d-c. output from the tachometer 1 1 1 which causes the amplifier 113 to drive the servo motor 62.

In order to provide a position feedback loop to compensate for system errors, such as might be caused by drift of the amplifier 113 for example, the resolver 112 is connected to a second resolver 114 driven by the servo motor output screw 61. (It will be understood that both resolvers 112 and 114 are electromechanical transducers which develop output voltages proportional to the input voltages and sine of the angles of displacement of the input shaft.) As long as the mechanical output displacement sensed by the resolver 1 14 corresponds to the mechanical displacement of the cam 110, the two resolvers 112 and 114 are balanced and, consequently, there is no output signal from the resolver 114. However, whenever a differential exists between the two mechanical inputs to the resolvers 112 and 114, the resolver 114 generates an a-c. output signal which is supplied to a discriminator l 15. The discriminator 115 converts the a-c. signal to a d-c. signal, which is'supplied to the amplifier 113 to compensate for any system errors.

For the purpose of preventing the servo-motor 62 from overshooting the commanded displacement, which in turn would result in hunting of the system, the servo motor output screw 61 is mechanically coupled to a tachometer 116 which produces a feedback signal to the servo motor 62.

In FIG. I1, one illustrative arrangement for providing a mechanical connection between the cam 110 in the servo cam box 97 and the tachometer MI and the resolver N2 is shown in more detail. In this arrangement, the cam 11f) cooperates with a cam follower on the end of a rack 121 which is biased against the cam surface by means of a spring 122. As the rack I21 is moved linearly in response to rotation of the cam 110, the rack turns a pinion 123 which is connected at opposite ends to the mechanical input shafts of the tachometer 111 and the resolver 112, respectively. As described previously, rotation of the input shafts to the tachometer and resolver results in electrical output signals representing both the magnitude and direction of movement of the cam I I0.

It will be understood that the control system of FIG. 9 controls only the servo motor 62, and that a similar control system, driven by a second cam in the servo cam box 97, controls the vertical servo motor 72. It will also be understood that the entire control system, including both the pantograph and the servo motors associated therewith, have been described with reference to the control of only one transfer assembly, since the control systems for the other transfer assemblies are identical, and thus the description of the one exemplary control system is equally applicable to the control system as applied to any of the other assemblies.

Since the cam unit-97 controls the cyclic movement of the servo motors 62 and 72, and thus the transfer assembly, it will be appreciated that the transfer assembly may be programmed to follow different cyclic paths simply by substituting different cams in the unit 97. This is a relatively low cost substitution, in comparison with the overall system, and thus it does not impose any great economic burden on the user to have different cams for each different part that is to be operated on by the press line. Moreover, substituting one set of cams for another is a relatively quick and easy operation.

We claim as our invention:

1. A transfer system for an automatic press line comprising the combination of a transfer arm having gripper means mounted on one end thereof for gripping the workpieces to be transferred, said transfer arm being mounted on a pantograph comprising four pivotally interconnected links for both vertical and horizontal movement, a pair of servo motors operatively connnected to said pantograph for effecting both vertical and horizontal movement of said pantograph, said pantograph and said transfer arm being dimensioned to amplify any movement of said pantograph effected by said servo motors, and a servo control system for each of said servo motors including program means connected to the automatic press line for generating electrical control signals according to a predetermined program in synchronization with said press line, said control signals including both a velocity signal and a displacement signal, means for driving said servo motor in response to said control signals, velocity feedback means responsive to the velocity of the mechanical output of said servo motor for generating a velocity feedback signal, displacement feedback means responsive to the displacement of the mechanical output of said servo motor for generating a displacement feedback signal, and means for combining said feedback signals with said control signals to modify the effect of said sigznals on said driving means.

. A transfer system as set forth in claim 1 wherein said servo motors are located at said transfer arm and said program means is located at the drive means for the press line with said servo motors and said program means being electrically connected.

3. A transfer system as set forth in claim ll wherein said program means includes a pair of cams connected to said press drive means means, one of said cams varying the electrical signals controlling operation of one of said servo motors, and the other cam varying the electrical signals controlling operation of the other servo motor.

4. A transfer system as set forth in claim 3 which includes a pair of racks driven by said cams and a pair of pinions driven by said racks, and a pair of tachometers operatively connected to said pinions for generating electrical control signals in response to rotation of said pinions.

5. A transfer system as set forth in claim 1 which includes a pair of ball screw drive units both connected to said pantograph at a common connecting point, said ball screw drive units being driven by said servo motors for pivoting said pantograph in response to operation of said sen/o motors.

Claims

1. A transfer system for an automatic press line comprising the combination of a transfer arm having gripper means mounted on one end thereof for gripping the workpieces to be transferred, said transfer arm being mounted on a pantograph comprising four pivotally interconnected links for both vertical and horizontal movement, a pair of servo motors operatively connnected to said pantograph for effecting both vertical and horizontal movement of said pantograph, said pantograph and said transfer arm being dimensioned to amplify any movement of said pantograph effected by said servo motors, and a servo control system for each Of said servo motors including program means connected to the automatic press line for generating electrical control signals according to a predetermined program in synchronization with said press line, said control signals including both a velocity signal and a displacement signal, means for driving said servo motor in response to said control signals, velocity feedback means responsive to the velocity of the mechanical output of said servo motor for generating a velocity feedback signal, displacement feedback means responsive to the displacement of the mechanical output of said servo motor for generating a displacement feedback signal, and means for combining said feedback signals with said control signals to modify the effect of said signals on said driving means.

2. A transfer system as set forth in claim 1 wherein said servo motors are located at said transfer arm and said program means is located at the drive means for the press line with said servo motors and said program means being electrically connected.

3. A transfer system as set forth in claim 1 wherein said program means includes a pair of cams connected to said press drive means means, one of said cams varying the electrical signals controlling operation of one of said servo motors, and the other cam varying the electrical signals controlling operation of the other servo motor.

5. A transfer system as set forth in claim 1 which includes a pair of ball screw drive units both connected to said pantograph at a common connecting point, said ball screw drive units being driven by said servo motors for pivoting said pantograph in response to operation of said servo motors.