Finding Home Position

The encoders used to control the servo motors on the ZY system are incremental encoders. When the system is first powered on, the position they happen to be in becomes the home position (encoder coordinate 0). This is a purely arbitrary position. Before the encoders can be used to point the laser at any absolute coordinate, the servo systems must first undergo a procedure that locks in their true home positions. This is accomplished with the `FHM' command for each axis. FHM will rotate the specified axis until a special index pulse is noted. The servo controller's position register is then altered so that it reads 0 at the location of that index pulse. Since the location of the index pulse is physically fixed, the encoder's coordinates can now be considered absolute. This procedure needs to be performed only once after a system startup. As long as the ZY program is running, the servo controllers will remember their respective home position. The ``homed'' condition of the servo system can be determined by reading the system status word, so that a client program need not perform this procedure unnecessarily (see STS, STW).

NOTE: Some commands will fail if this procedure was not performed or if it failed.