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28 • Chapter 2 Getting Started DMC-1700/1800
the command , OE 1. If the motor runs away due to positive feedback or another
systematic problem the controller will disable the amplifier when the position error
exceeds the value set by the command, ER.
Step D. Disable motor with the command MO (Motor off).
Step E. Connect the Motor and issue SH
Once the parameters have been set, connect the analog motor command signal (ACMD) to the
amplifier input.
To test the polarity of the feedback, command a move with the instruction:
PR 1000 <CR> Position relative 1000 counts
BGX <CR> Begin motion on X axis
When the polarity of the feedback is wrong, the motor will attempt to run away. The
controller should disable the motor when the position error exceeds 2000 counts. If the
motor runs away, the polarity of the loop must be inverted.
Inverting the Loop Polarity
When the polarity of the feedback is incorrect, the user must invert the loop polarity and this may be accomplished
by several methods. If you are driving a brush-type DC motor, the simplest way is to invert the two motor wires
(typically red and black). For example, switch the M1 and M2 connections going from your amplifier to the motor.
When driving a brushless motor, the polarity reversal may be done with the encoder. If you are using a single-ended
encoder, interchange the signal CHA and CHB. If, on the other hand, you are using a differential encoder,
interchange only CHA+ and CHA-. The loop polarity and encoder polarity can also be affected through software
with the MT, and CE commands. For more details on the MT command or the CE command, see the Command
Reference section.
Sometimes the feedback polarity is correct (the motor does not attempt to run away) but the direction of motion is
reversed with respect to the commanded motion. If this is the case, reverse the motor leads AND the encoder
signals.
If the motor moves in the required direction but stops short of the target, it is most likely due to insufficient torque
output from the motor command signal ACMD. This can be alleviated by reducing system friction on the motors.
The instruction:
TTX (CR) Tell torque on X
reports the level of the output signal. It will show a non-zero value that is below the friction level.
Once you have established that you have closed the loop with the correct polarity, you can move on to
the compensation phase (servo system tuning) to adjust the PID filter parameters, KP, KD and KI. It is
necessary to accurately tune your servo system to ensure fidelity of position and minimize motion
oscillation as described in the next section.