FHM

Causes the specified axis to set its home position at the encoder's index pulse position.

Mnemonic:

Find HoMe

Syntax:

FHM $<$n$>$

n: Axis number (0:azimuth, 1:elevation).

Return values:

Success:

FHM 1, n

Failure:

FHM 0[, n], err_msg

n: Axis number.

err_msg: Should the command fail, this value will be an error message string, explaining the cause of failure. FHM has two basic failure modes. The first is if improper parameters are given it. In this case, FHM returns the failure code and an error message. The second is if the axis actually failed to find home for any reason. FHM then returns the failure code, the axis number, and then the error message. FHM may fail for any of the following reasons:

• The wrong number of parameters were supplied.
• A non-numeric value is provided for any of the parameters.
• The axis number in n is not `0' or `1'.
• The axis failed to find home. This could indicate problems with the LM628 tuning parameters, or hardware problems with the encoder, such as a miswired index pulse line or other defect.

Remarks:

The servo system must be homed prior to any use that requires absolute encoder coordinates. Once homed, the servo system need not be homed again unless the LM628 controller chip is reset or in the unlikely event that it should lose count. The `homed' status of each axis can be determined by reading the system status word. The find home operation works by moving the servo until the index pulse is detected or until it has moved one full revolution. Upon detecting the index pulse, the position counter reading is latched into the index position register and the servo is moved back to this position. When the servo has settled on this position, the position register is cleared to zero. Encoder counts can subsequently be treated as absolute encoder coordinates. FHM attempts to complete the find home operation within 30 seconds. It can fail to do so if the servos do not move, if the index pulse is not detected within one full revolution, or if the servos cannot settle on the index pulse position. If FHM fails, or if FHM is never executed, operations that require absolute coordinates will also fail, with an error message indicating that the servos are not `homed.'

Example:

     send: FHM 1
  receive: FHM 1, 1

See also

STS, STW, VHM