Pointing the Laser Beam

Once the servo system has been properly initialized and homed, it can be used to aim the laser to a target. The ZY does this by loading the LM628 with the desired Azimuth and Elevation encoder counts and issuing the LM628 STT command. The ZY obtains the Azimuth and Elevation encoder counts for the desired target in one of two ways:

1. They are calculated from the $XYZ$ coordinates of the retroreflector target (see section 2.4).
2. They are provided directly by the ZY's client

The LM628 will then compute a trajectory to the destination and start the motors moving. The LM628 will indicate that the move is complete by issuing a Trajectory Complete Interrupt (TCI), which sets a flag that the ZY looks for. Unfortunately the TCI tells only of a completed move, which occurs when the servo crosses the destination, not when it is settled on the destination. It may take several more milliseconds for the servo to stabilize in its new location, particularly if the LM628's PID filter parameters are not just right. For a measurement to be successful, the servo system must be on target, within a reasonable tolerance. Thus, the ZY's measurement commands (CMEAS, SCN) call on the servo system's LM628::ServoWait() routine to determine when it is safe to start a measurement. The ServoWait() routine works as shown in figure 2.6. The ZY's client can set a tolerance for the destination count (see WTOL). For instance, if the tolerance is 2 encoder counts, the servos will be considered on target if their actual reading is the destination $\pm 2$ counts. If the ServoWait() routine can make $n$ straight readings that are within this tolerance, where $n$ can be set by the ZY's client (see WCNT), then ServoWait() considers the servo to be ``on-target,'' and a measurement can be made. The ZY's client can also set other parameters that affect the operation of ServoWait(): A time out value (WTMO), and a strict flag (WMD). The time out value causes the ServoWait() routine to abandon the servo-wait procedure if the $n$ straight in-tolerance measurements cannot be made within the time-out period. The `strict' flag affects the return value of ServoWait() if it times-out. If `strict' is set, then ServoWait() returns failure if it times-out. This should be the usual operation. If `strict' is not set, then ServoWait() return 1 (success) even if it times out. This mode of operation is useful for testing purposes, but probably should not be used for normal operation. Failure to settle within a reasonable time period is most probably due to poor tuning parameters for the LM628's PID filter. Some or all of the steps described above are performed automatically by the ``cube-based'' pointing and measuring commands CIL, CMEAS, CWT, and SCN. Even when using these commands, though, it is important to understand how the system works, especially how the LM628::ServoWait() routine and the PID filter parameters work, to tune the system for maximum performance. The above steps can also be explicitly performed by the low level servo commands ABP, STT and WAI. These are provided for special purposes or for testing the servo system.

Figure 2.6: Flowchart for the ZY Servo System settling routine.