1. Field of the Invention
The present invention relates to a robot control apparatus, and more particularly, to a robot control apparatus having a backward operation function for operating a robot to retrace a history of program execution.
2. Description of Related Art
A robot control apparatus has a function called a backward operation function as one of general functions thereof. This function of control is in a contrast to a forward operation function for operating a robot by reading an operation program forward, and is used for operating a robot to retrace the forward operation.
For example, the backward operation function is used when it is necessary to return the robot to an intermediate position or an initial position on a taught path so as to confirm the taught path or modify taught positions in a teaching operation, or when it is necessary to retract the robot which has been interfered and stopped in emergency from the interfered position.
The conventional backward operation function is, however, associated with the functional limitations mentioned below.
(1) In the backward operation, only motion commands are executed, and no consideration is given to motions involving input/output of external signals. For example, when the backward operation function is activated with respect to a forward operation including a closing (or an opening) action of a hand or a spot gun by an output signal to outside, an opening (or a closing) action to return the hand or the spot gun to the original state is not performed.
It is therefore necessary for an operator to separately carry out an operation for returning (opening or closing) the hand or the spot gun in the middle of the backward operation. If the operator omits to perform the returning operation or the returning motion is incomplete. (e.g, the hand or the spot gun is half-open), an interference may possibly be caused.
(2) If a branch command is executed during a forward operation, a backward operation which does not exactly correspond to the execution history is performed. Namely, according to the conventional backward operation function, a set of commands written in the operation program for the forward operation are simply executed in reverse order. Accordingly, in the case where the backward operation function is activated after a branch command is executed during a forward operation, a command that was skipped because of the branch command and was actually not executed (command which is not in the forward operation history) is executed during the backward operation.
The robot can therefore move to a position unexpected to the operator during the backward operation. To avoid this, the operator has to suspend the backward operation at a position immediately following the branch command, manually move the robot to a position immediately preceding the branch command, and then to restart the backward operation, thus requiring complicated operation.
(3) It is not possible to teach a backward mode executable program which is prepared specially for backward operation to be executed only during backward operation, and thus to start/execute such a backward mode executable program at a predetermined stage in the process of backward operation. For example, it is difficult to cause the robot to automatically take a "roundabout course from a certain robot position to the next robot position (immediately preceding position) via another robot position" only during backward operation.
(4) Similarly, it is not possible to teach a command to be executed only during backward operation (hereinafter referred to as backward mode dedicated command), and thus to execute such a backward mode dedicated command at a predetermined stage in the process of backward operation. For example, it is difficult to cause the robot to output a predetermined signal to the outside before the movement thereof from a certain robot position to the next robot position (immediately preceding position) only during backward operation.
(5) In connection with the problem that motions involving the input/output of external signals are not taken account of during backward operation, the output states of commands (hereinafter referred to as I/O commands) that dominate over the states of external devices during backward operation can not be selectively specified (e.g., specification of hand open state) at liberty.
(6) Since motions involving computation of register values are also not taken into account during backward operation, backward operation entailing restoration of register values cannot be executed.