Slip sensitive robot gripper system

United States Patent m

Daly

[li] Patent Number: 4,605,354 [45] Date of Patent: Aug. 12,1986

[54] SLIP SENSITIVE ROBOT GRIPPER SYSTEM

[75] Inventor: Timothy P. Daly, Fishkill, N.Y.

[73] Assignee: International Business Machines Corporation, Armonk, N.Y.

[21] Appl. No.: 635,988

[22] Filed: Jul. 30,1984

[51] Int. Q.* B66C1/00

[52] U.S. CI 414/730; 901/33;

294/907

[58] Field of Search 901/31, 32, 33, 34,

901/35, 36, 39; 414/730; 294/907, 402, 86 R,

106

[56] References Cited

U.S. PATENT DOCUMENTS

3,904,234 9/1975 Hill et al 901/32 X

4,306,148 12/1981 Ringwall et al 250/229

FOREIGN PATENT DOCUMENTS

0112729 4/1984 European Pat. Off. .
2542659 9/1984 France .
209409 9/1984 German Democratic Rep. .

52-33253 3/1977 Japan 901/33

52- 33254 3/1977 Japan 901/33

53- 13767 2/1978 Japan 901/34

433023 11/1974 U.S.S.R 901/34

OTHER PUBLICATIONS

T. G. Kennedy, "Force-to-Deviation Conversion Table for Robotic Gripping", IBM Technical Disclosure Bulletin, vol. 25, No. 6, Nov. 1982.

Primary Examiner—Leslie J. Paperner

Assistant Examiner—Dennis Williamson
Attorney, Agent, or Firm—Carl C. Kling

[57] ABSTRACT

A resilient gripper pad stores deformation energy just before the gripped object starts to slip, then rapidly accelerates as it suddenly releases the energy in the springback to rest position. An accelerometer in the resilient pad (corrected for ambient acceleration by differentiation with a nearby second accelerometer) provides information by which the control computer of a closed loop robot system mandates the manipulation of objects by controlled slipping. Controlled transitions of gripping (releasing) action evoked by the control computer provide related sequences of grip (slide) events, with one or more slip (pad chatter) events intervening at the threshold. Prior to the slide, there is an instant when the gripped object starts to slip on a slip pad on one digit of the gripper, but remains frictionally gripped by a resilient pad on the opposing digit. During this instant, the resilient pad deforms downward because of gravity on the gripped object. Next, when ■gripping force relaxes just a little more, the object slides free and the resilient pad springs back to its rest position. Two accelerometers, one mounted in the resilient pad to experience springback acceleration, and the other mounted in a complementary slip pad to experience ambient acceleration, are differentially compared. A computer manipulates signals related to motion experience of the gripper (grip or release) and detection (or absence) of springback acceleration, determines gripping, slipping or sliding status, and mandates the appropriate gripper action.

6 Claims, 2 Drawing Figures

SLIP SENSITIVE ROBOT GRIPPER SYSTEM

BACKGROUND OF THE INVENTION

1. Field of the Invention 5 This invention relates to slip sensing robot gripper

systems, and relates more particularly to a robot gripper system, operating on the springback phenomenon, the high speed return to rest configuration (springback acceleration) of a resilient pad which is suddenly re- 10 leased after being deformed linearly when a lightly gripped object suddenly releases energy stored by linear deformation of the gripper pad, which can determine slipping of an object lightly to facilitate controlled gripping, controlled slipping and controlled sliding of 15 objects.

2. Description of the Prior Art

There are a great number of robot gripper systems which have been deployed or at least described in publications, some with relatively sophisticated feedback and 20 servo systems to facilitate grasping of objects without damage. Remote pickup devices normally depend upon a human operator for intelligent control via various sensing and servo devices. Operating alone, however, the robot normally does not have sufficient sensing or 25 intelligence to make the necessary determinations for controlled slipping of objects.

Robots may be equipped with a variety of sensing mechanisms, including optical viewing, weight measuring devices, strain gages and length measuring devices, 30 among others. A particular problem with these prior art devices, however, is that they are not sensitive to the complicated dynamics of initial slippage of a gripped object during the processes of pickup and release. The human hand-brain combination handles initial slippage 35 easily and elegantly. A person may control the slippage of a pencil in hand, letting the pencil slip in a controlled slide so as to come to rest in the writing position.

Controlled slippage in robot grippers has not normally been deployed; there is no known body of slip 40 sensing art in robotics upon which to build. Certain vehicle safety programs have included skid sensing devices, again without common deployment; skid sensing devices tend to be optimized for rotating wheels and cannot be used directly in robotic grippers. 45

The following are representative of the prior art:

U.S. Pat. No. 3,744,850, Bartholome, AUTOMATIC TRACTION CONTROL SYSTEM, July 10, 1973, shows a wheel slippage detector comprising an angular accelerometer and a linear accelerometer which press 50 against one another to measure the ratio of wheel angular acceleration to vehicular linear acceleration. Closure of the loop is obtained by interrupting the vehicle ignition circuit to slow the vehicle upon detection of a skid, and to control flow of brake fluid, so as to control 55 slippage for maximum braking efficiency.

U.S. Pat. No. 4,306,148, Ringwall et al., TACTILE SENSOR, Dec. 15, 1981, shows a tactile feel system for a robot, in which a number of pneumatic passages are each equipped with a resilient light reflective tab and an 60 optical fiber, and deformations of the tabs, indicating the presence and shape of the object, may be sensed.

U.S. Pat. No. 3,904,234, Hill et al., MANIPULATOR WITH ELECTROMECHANICAL TRANSDUCER MEANS, Sept. 9, 1975, shows an array of 65 electromechanical transducers for force and slip sensing. Slip sensors include "... rotatable disks... extending outwardly from the opposing jawfaces." Hill does

2

not show any slip sensor operating on the springback phenomenon.

Japanese Patent No. 52-33253, Hoshino, shows a hydraulic technique for increasing pressure and moving the gripper upwards until no slip is sensed. Hoshino does not show any slip sensor operating on the springback phenomenon.

Japanese Patent No. 52-33254, Hoshino, shows a similar hydraulic technique for increasing pressure until no slip is sensed. Hoshino does not show any slip sensor operating on the springback phenomenon.

Japanese Patent No. 53-13767, Aoki, shows a pneumatic gripper which applies force controlled in response to position with respect to an array of photodetectors and light emitting elements. Aoki does not show any slip sensor operating on the springback phenomenon.

USSR author's certificate No. 433,023, Liberman et al., ROBOT MANIPULATOR, Urals Polytechnic Institute—Kirov, Nov. 18,1974, shows a robot manipulator with an electromechanical position transducer for slip sensing. A mechanical belt in one gripper face rotates the stem of a potentiometer for slip sensing. This USSR author's certificate does not show any slip sensor operating on the springback phenomenon.

SPRINGBACK PHENOMENON

The springback phenomenon upon which this slip sensor is based is the high speed return to rest configuration (springback acceleration) of a resilient pad which is suddenly released after being deformed linearly.

This slip sensor operates in response to the dynamic phenomenon of initial slippage of a relatively incompressible object under a linear force (e.g., gravity) as a gripper with a resilient face pad relaxes its hold.

As the gripper starts to relax its grip, just prior to initial slippage of the object, the resilient face pad deforms laterally and significantly; the object is solidly held.

Just before initial slippage, the object moves infinitesimally in the direction of linear force, leaving a slight linear deformation of the resilient face pad as the gripper relaxes and releases the resilient face pad from lateral deformation. This loads the resilient pad with a finite stored linear energy as a function of the linear deformation; the object serves as a cocked trigger for the stored linear energy in the form of deformation of the resilient face pad.

At the instant of initial slippage, the object breaks free of the resilient face pad, releasing the stored linear energy. The resilient face pad, loaded with stored energy is released by the slipping object, and springs back to its rest configuration. This springback is unrestrained and therefore quick.

The prior art does not suggest the use of the springback acceleration of a resilient pad to detect initial slippage of a gripped object.

SUMMARY OF THE INVENTION

The invention is a slip determining robot gripper system which operates by differentially detecting the springback acceleration of a first accelerometer, designated springback accelerometer, embedded in a resilient pad versus the ambient acceleration of a second accelerometer, a non-springback accelerometer, for comparison embedded in a non-resilient portion adjacent the resilient pad.