User Manual

Feedback Sensor Range Setting
Advanced Digital Motor Controller User Manual 147
A gear box is necessary to greatly increase the torque of the assembly. It is also neces-
sary to slow down the motion so that the controller has the time to perform the position
control algorithm. If the gearing ratio is too high, however, the positioning mode will be
very sluggish.
A good ratio should be such that the output shaft rotates at 1 to 10 rotations per second
(60 to 600 RPM) when the motor is at full speed.
The mechanical coupling between the motor and the sensor must be as tight as possible.
If the gear box is loose, the positioning will not be accurate and will be unstable, potential-
ly causing the motor to oscillate.
Some sensors, such as potentiometers, have a limited rotation range of typically 270
degrees (3/4 of a turn), which will in turn limit the mechanical motion of the motor/poten-
tiometer assembly. Consider using a multi-turn potentiometer as long as it is mounted
in a manner that will allow it to turn throughout much of its range, when the mechanical
assembly travels from the minimum to maximum position. When using encoders, best
results are achieved when the encoder is mounted directly on the motor shaft.
Feedback Sensor Range Setting
Regardless of the type of sensor used, feedback sensor range is scaled to a -1000 to
+1000 value so that it can be compared with the -1000 to +1000 command range.
On analog and pulse sensors, the scaling is done using the min/max/center configuration
parameters.
When encoders are used for feedback, the encoder count is also converted into a -1000 to
+1000 range. In the encoder case, the scaling uses the Encoder Low Limit and Encoder
High Limit parameters. See “Serial (RS232/USB) Operation” on page 161 for details on
these configuration parameters. Beware that encoder counters produce incremental val-
ues. The encoder counters must be reset using the homing procedure before they can be
used as position feedback sensors.
Important Notice
Potentiometers are mechanical devices subject to wear. Use better quality potenti-
ometers and make sure that they are protected from the elements. Consider using
a solid state hall position sensor in the most critical applications. Optical encoders
may also be used, but require a homing procedure to be used in order to determine
the zero position.
Important Warning
If there is a polarity mismatch, the motor will turn in the wrong direction and the
position will never be reached. The motor will turn until the Closed Loop Error de-
tection is triggered. The motor will then stop until the error disappears, the control-
ler is set to Open Loop, or the controller is reset.