Galil Motion Control DMC-18 2 Series User Manual page 26

Step C. Set Torque Limit as a Safety Precaution
Note: Once the correct polarity of the feedback loop has been determined, the torque limit
Step D. Connect the Motor
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, respectively. 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 desired commanded motion. If this is the case, reverse the
motor leads AND the encoder signals.
DMC-18x2
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If the motor runs away and creates a position error of 2000 counts, the motor amplifier
will be disabled. Note: This function requires the AEN signal to be connected from the
controller to the amplifier.
To limit the maximum voltage signal to your amplifier, the DMC-18x2 controller has a
torque limit command, TL. This command sets the maximum voltage output of the
controller and can be used to avoid excessive torque or speed when initially setting up a
servo system.
When operating an amplifier in torque mode, the voltage output of the controller will be
directly related to the torque output of the motor. The user is responsible for determining
this relationship using the documentation of the motor and amplifier. The torque limit
can be set to a value that will limit the output torque of the motor.
When operating an amplifier in velocity or voltage mode, the voltage output of the
controller will be directly related to the velocity of the motor. The user is responsible for
determining this relationship using the documentation of the motor and amplifier. The
torque limit can be set to a value that will limit the speed of the motor.
For example, the following command will limit the output of the controller to 1 volt on
the X axis:
TL 1 <CR>
should, in general, be increased to the default value of 9.99. The servo will not operate
properly if the torque limit is below the normal operating range. See description of TL in
the command reference.
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.
Chapter 2 Getting Started • 15