ROBOT CONTROL APPARATUS

Abstract: A system consists of an input device (a), a CPU (b), a sensor (c) which detects whether a robot hand is holding a work or not, a servo control circuit (d), a servo motor (e) which drives the robot arm, a pulse coder (f) which detects the rotation angle of the servo motor (e) and a tacogenerator (g) which detects the number of revolution of the servomotor. The drive torque for the servo motor is calculated using a motion equation, of which the inertia term depends on the position of the robot arm. Since the variation of the robot arm position is relatively slow, the operation period on the inertia term is set longer than that of the torque to reduce computation.

Description: This invention relates to a robot control apparatus for servo-controlling various drive means by calculating, on-line, the torque command of a robot arm on the basis of the motion equation of a manipulator. Horizontal articulated-type robots of the kind shown in Fig. 4 have recently come into use on factory assembly lines and the like. A robot of this type comprises a post 1, a first arm 2, a second arm 3, a hand 4, a joint shaft 5, a cable 6, a base 11 and covers 22, 32. When the arms and the hand are driven by servomotors, not shown, to grip a workpiece and perform other motions, a torque command for the robot arms is calculated on-line by a microcomputer for servo-control based on a manipulator motion equation, whereby each of the drive means is servo-controlled. The drive torque of a servomotor used for robot arm drive is determined as a function of the weight of the workpiece gripped by the hand and inertia decided by the length of the arm. However, the torque necessary for motion when the workpiece is being gripped differs from that when the workpiece is not being gripped. Accordingly, the operator provides a control unit with inputs indicative of whether the robot arm is gripping a workpiece, the weight of the workpiece, etc., and the control unit calculates inertia by computing complicated simultaneous equations, thereby commanding a drive torque of the servomotor. Consequently, the operation for commanding the drive torque of the servomotor is troublesome and the processing performed by the control unit is complicated. In order to compute other parameters such as control current during execution of this processing, a considerable high-speed processing capability is required. The present invention has been devised to solve the foregoing problems and its object is to provide an inexpensive robot control apparatus capable of shortening the processing time required for torque calculation and of executing highly precise control without interfering with other processing for current control and the like. The robot control apparatus of the present invention is constituted as follows: Specifically the invention is characterized by having a servomotor for driving a manipulator through the intermediary of a robot arm, arithmetic means for computing drive torque of the servomotor in accordance with a motion equation, memory means for executing a computation, with regard to an inertia term included in the motion equation, at a predetermined period greater than a drive torque computation period in dependence upon the position of the robot arm, and for sequentially updating and storing the results of this computation, and control means for performing feed-forward compensation of the servomotor based on the motion equation. Fig. 1 is a block diagram of the present invention, Fig. 2 is a schematic diagram, Fig. 3 is a flowchart illustrating an example of operation, and Fig. 4 is an external view of a horizontal articulated-type robot. An embodiment of the invention will now be described in detail with reference to the drawings. Fig.1 is a schematic block diagram of the invention, in which a represents an input unit, b a CPU (central processing unit), and c a sensor provided on a robot hand for sensing that the robot hand has gripped a workpiece. For example, a weight sensor provided between the robot hand and wrist can be used as the sensor. Further, d denotes a servocontrol circuit, e a servomotor for drive along each axis of the robot, f a pulse coder for detecting the rotational angle of the servomotor, and g a tachogenerator for detecting the rotational speed of the servomotor. A method of commanding torque for the servomotor e by means of the robot control apparatus of the invention will now be described. The horizontal articulated-type robot shown in Fig. 4 can be expressed by a simplified diagram, as shown in Fig. 2. In the diagram, we have the following: ℓ₁: length of first arm ℓ₂: length of second arm ℓ₂': length from rotational axis ϑ₂ to hand (where we let ℓ₂ = ℓ₂' for simplicity) m₁i, m₂j: i-th and j-th weights obtained by dividing the weights of the arms of the respective axes by k, n, respectively ℓ₁i, ℓ₂j: lengths from the rotational centers of the axes to weight dividing points Considering the weight of a workpiece as being the weight m₂n of an n-th material point of the second arm, drive torque T₁ of a first axis ϑ₁ is expressed by the following motion equation: The coefficients A, B, C and D of each of the non-linear terms represent the inertia corresponding to workpiece weight and arm position. When a horizontal articulated-type robot of this kind is used on a factory assembly line or the like, the operator inputs the weights m₂n of various workpieces to the CPU b in advance by means of the input a. On the basis of the weight of each workpiece, the CPU b determines the coefficients A, B, C and D of the non-linear terms, which are dependent solely upon the position of the manipulator, in the abovementioned torque computation equation, and stores these coefficients as inertia terms corresponding to workpiece weight and arm position. Next, when the robot is operated to grip a workpiece, the sensor c sends a signal, which is indicative of the fact that the workpiece has been gripped, to the CPU b. In accordance with this signal sent by the sensor c and a signal indicative of manipulator position obtained from the pulse coder f, the CPU b reads out the corresponding inertia term and sends a control signal, which applies drive torque compensation to the servomotor e by feed-forward, to the servo-control circuit d. In the robot operation for gripping a workpiece, the robot control apparatus checks the control program of the robot and on the basis of this is capable of discriminating the type of workpiece about to be gripped or the type of workpiece being gripped. Fig. 3 is a flowchart illustrating an example of a processing operation performed by the above-described robot control apparatus. In order to apply a torque command to the first axis, T₁ must be computed from the torque computation equation. However, in an ordinary robot motion equation, the rate of change in arm position may be assumed to be smaller than the rate of change in velocity or acceleration. Therefore, the position of the second axis, namely the rotational angle ϑ₂, is obtained (step a). On the basis of this angle, the corresponding sine and cosine values are calculated from a function table or the like (step b). The inertia terms corresponding to the weight of the workpiece are then calculated (step c). Next, the velocity and acceleration ϑ ˙₁, ₁ along the first axis and the velocity and acceleration, ϑ ˙₂, ₂ along the second axis are obtained (step d). The torque T₁ is calculated from the inertia values calculated and stored at step c (step e). The torque command is applied to the first axis (step f). This calculation cycle (steps d - f) is repeated n times (step g), during which identical interia values are taken as a reference. The program subsequently returns to step a. Letting B represent the calculation time at step (e), the flow of Fig. 3 will be repeated at a period A (>nB). It should be noted that it is unnecessary to fix the inertia term computation period A to n times the torque calculation period B, as is done in the abovedescribed embodiment. For example, it will suffice to set the inertia term computation period to a large value with respect to the drive torque computation period in accordance with the motion of the robot hand. The present invention is not limited to the foregoing embodiment and can be modified in various ways. without departing from the scope of the claims thereof. Thus, in accordance with the invention as described above, a rate of change in position is smaller than a rate of change in velocity or acceleration when torque is calculated using a robot motion equation. The inertia term calculation period A therefore can be set to be longer than the torque calculation period B, and the time required for torque calculation can be shortened. As a result, a highly precise torque command can be formed without interfering with other processing such as for current control. The invention is convenient in that processing for torque control and current control can be performed simultaneously. Though the present invention relates to a horizontal articulated-type robot, the invention can also be applied to industrial robots of polar coordinate-type as well as other forms of industrial robots.

Claims: A robot control apparatus for computing, on-line, a torque command of a robot arm based on a motion equation of a manipulator, and servo-controlling each drive means, comprising: a servomotor (e) for driving the manipulator through the intermediary of the robot arm (2, 3); arithmetic means (b) for computing drive torque of the servomotor in accordance with the motion equation; memory means for executing a computation, with regard to an inertia term included in said motion equation, at a predetermined period greater than a drive torque computation period in dependence upon the position of the robot arm, and for sequentially updating and storing the results of this computation; and control means (d) for performing feed-forward compensation of the servomotor (e) based on said motion equation. A robot control apparatus according to claim 1, characterized in that the computation period of the inertia term performed by said arithmetic means (b) is controlled at a fixed period longer than the drive torque computation period. A robot control apparatus according to claim 1, characterized in that the computation period of the inertia term performed by said arithmetic means (b) is controlled at a variable period dependent upon the operating velocity of the robot arm.

Inventors: IWAMOTO, TAKASHI, KURAKAKE, MITSUO, SAKAMOTO, KEIJI
Applicants: FANUC LTD.
Application year: 1987
Main classification: B25J9
IPC codes: G05D3, B25J9, G05B19, B25J13