Motion Overview 408
circular motion is commanded, an oval or elliptical motion will result as one axis out
performs the other.
1c) What is a Position / Servo Error?
A Position Error or Servo Error occurs when the distance between the ideal motion position and the
actual motion position exceeds a pre-defined Servo Error tolerance value. A Position Error indicates
that the response from the motion command was not executed, executed too slowly or executed
incorrectly. The Servo Error Tolerance is user definable value within the control and is usually set to
be a value twice the noted following error during normal operation. This allows the system to operate
without nuisance faults caused by temporary resistance to the motion, such as dirt on motion rails or
temporary tension from motor cabling, etc.
Note: It is important to remember that in the Closed Position Loop, the output command will adjust to
try and maintain the desired speed and positioning. As a result, a loss of Encoder Feedback would
result in the control sending out it‟s maximum motion command to the motor causing it to “run away”
without control. Conversely, failure of the motor to turn while feedback is enabled, will result in a
position error and the command output will stop when the fault occurs.
2) Determining the Encoder Counts per Inch Parameter and Maximum Machine Speed
Remembering that the position loop uses the pulses/ counts from the encoder to
calculate distance, it is important to determine how many encoder pulses are equal
to a specific distance of machine motion. The control uses the Encoder Counts Per
Inch/mm value as the constant to calculate distance and speed.
The Encoder Counts per Inch/mm is a calculation based on the number of pulses
generated by the encoder for one revolution of the motor and how much distance is
traveled from that motor motion through the gearing being used.
So this relationship shown as a formula would be:
Counts / Line x Counts / Rev x 1 Rev / Inch = Encoder Counts/
Inch
encoder mode e.g 4x Pulses per Rev gear box ratio &
rack /pinion ratios
For example: The resolution of a 4X – 1000 line encoder counts both edges (lines)
of channel A and channel B to equal 4 counts per line multiplied by the 1000 lines
per revolution equaling 4000 counts per revolution. If the encoder revolutions per
inch of travel are 1:1, we would have 4000 encoder counts per inch of travel.
4 Counts/ Line x 1000 Lines/ Rev x 1 Rev/ Inch = 4000 Encoder Counts/ Inch
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