Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Reexamination Certificate
1999-11-19
2001-04-24
Donels, Jeffrey (Department: 2837)
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
C318S567000, C901S004000, C700S261000
Reexamination Certificate
active
06222338
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and apparatus for programming an automatic machine, particularly but not exclusively a robot, and more particularly, for teaching a robot the operative positions and orientations of the robot manipulator arm, by a direct teaching method
BACKGROUND OF THE INVENTION
Computer operation of robots must be programmed by teaching them a cycle of operations and the positions and orientations in which the manipulator arm, which supports the tools for carrying out the desired operations and places them in the appropriate positions, carries out each operation. In view of the importance of teaching the robot in a precise manner and without excessive effort on the part of the operator, and further, of avoiding possible damages or danger resulting from unpredictable reactions of the robot, various methods have been developed in the art. One method involves guiding the robot indirectly by means of a pendant, which may comprise, e.g., three position control buttons and three orientation control buttons. Commands are given by means of the pendant and are fed into a computer to command movement of the arm by coordinate extrapolation and transformation. As soon as an operative position or orientation is reached, the position and orientation of the end of the robot arm in a coordinate system, which may be Cartesian or cylindrical, are memorized and become part of a program. The computer calculates a path between the points in the program. Instructions regarding the operations to be carried out at each point so defined and memorized, are separately given. Inventions of this kind are described, for instance, in U.S. Pat. Nos. 3,920,972 and 4,757,459.
Another method of teaching is the so-called “direct” teaching, in which the operator directly leads the end of the manipulator arm to the desired locations and the coordinates of said locations are memorized Direct teaching is described, for instance, in U.S. Pat. Nos. 5,051,675, 5,103,149 and. 5,363,475. U.S. Pat. No. 5,051,675 discloses a system in which the power lines for the electric motors driving the moving arm are cut off in the teaching mode. U.S. Pat. No. 5,103,149 discloses breaking means provided in the servo driver for breaking a current generated by the electric motor when the motor is rotated in response to the teaching operation, so as not to flow into the servo driver, to reduce the force which the instructor must use to move the robot arm during teaching. U.S. Pat. No. 5,363,474 discloses a system for controlling industrial robots which comprises sensor means for detecting the amount of external force applied from the forward end of the manipulator arm, means for controlling a driving force of said arm in accordance with the amount of external force detected by said sensor means, and emergency cut-off means for cutting off the supply of said driving force in an emergency.
All of the above systems, however, are not fully satisfactory. Either they do not reduce the force to be exerted by the instructor or reduce it only to an insufficient degree, and/or they render the servo system totally or partially inactive during teaching, which is undesirable and may even be dangerous, and/or they require complicated and expensive mechanisms. Further, generally they do not permit to position the robot arm with complete accuracy, so that the memorized coordinates are often inaccurate. This is due to the facts that: a) if the motors are left inactive, the operator must equilibrate by his own effort the arm and payload weights—if he discontinues this effort, the arms or the whole robot may collapse and thus cause a danger; b) the friction of the gears, and particularly the transition from static to dynamic friction, makes it more difficult to move the arms precisely.
This invention has the purpose of eliminating the defects of the prior art robot teaching methods and apparatus, and to provide a direct teaching method and apparatus that is simple, permits to reach the desired positions and orientations with absolute accuracy, does not require the introduction of complicated and/or expensive structural components in the robot, reduces the operator's effort and eliminates any possibility of danger to him.
SUMMARY OF THE INVENTION
It should be understood that the invention applies to any automatic machine or automatic mechanical system that can be “taught”, viz. programmed to carry out a desired series of work stages by defining a succession of geometric stages, generally defined by positions and orientations, and a series of operations to be carried out in said geometric stages. However, for the sake of illustration, reference will be made to robots, which are typical automatic machines to which the invention is applicable. Robots are constituted by a plurality of elements, hereinafter generally “arms”, that are connected to one another, generally by pivots, to form a kinematic chain. They have, among others, the following three features: 1—They comprise actuators—“actuator” meaning herein an electrical motor which can cause relative motion of two arms, generally about the pivot joining them—which actuators produce torques that can be calculated from the variables of the robot control loop; 2—Their mechanical system is reversible, viz. if a load is applied to an arm, it can be sensed on the corresponding actuator; 3—The mechanical model and mechanical equations of the robot system are known. This invention can be applied not only to robots, but to any automatic machine that has the same three features, with modifications (if any) that persons skilled in the art will have no difficulty in understanding. Examples of such machines are: a) cranes for heavy material handling, wherein a servo system will keep the payload still, until the Direct Teach system of the invention is activated by the user, and allows to balance the payload without effort; b) laser cutting machines, wherein a metal sheet is placed on a table driven by a servo system, and the positions and points of the cutting trajectory can be taught using the Direct Teach system of the invention.
Furthermore, this invention, though mainly directed to the teaching process, has applications in other phases of operation of robots or other machines, as will be explained hereinafter.
The teaching method according to the invention, as applied to a system that comprises a number of relatively movable elements (hereinafter “arms”) and of actuators for holding said elements in a given position against payloads and moving them to operative positions, whenever desired, essentially comprises the steps of:
a—placing the system in a configuration, that will be called “the reference or starting configuration”, equilibrating its arms, and memorizing for each arm what will be called “the reference torque”, viz. the equilibrating torque that must be produced by the corresponding motor to maintain the arm in its reference position, viz. in its position in the reference configuration of the system;
b—when it is wished to carry out the teaching, placing the system in the Teach mode;
c—manually applying an external force to an arm of the system in the direction desired for the purpose of the teaching;
d—for each arm, determining what will be called “the actual torque”, viz. the torque that would have to be applied by the motor, according to the mechanical equations of the system, to maintain the arm in its reference position in spite of the application of said external force;
e—calculating what will be called “the displacement torque”, viz. the difference between said reference and said actual torque;
f—causing the motor to displace the arm in the direction of said displacement torque and by an amount proportional to it; and
g—repeating the above operations c- to f- by the successive application of an external force, until the position that it is desired to teach has been reached.
Thereafter, the system is placed in the Record mode and the taught position is memorized.
In particular, the system is a robot and, more particularly, the arm that to which the ex
Altera Law Group LLC
Donels Jeffrey
Duda Rina I.
Eshed Robotec
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