Hypertherm V9 Series Phoenix Rev.11 - Manual (Page 210)

210 Phoenix 9.76.0 Installation and Setup Manual 806410
9 – Motion Control
Following Error
Following error or servo error is the distance between the position that the CNC commands and the actual position of
motion. Some following error is normal because the resistance of the load prevents the response of the motor to reach
the ideal command of the system. This latent response is similar to driving a car away from a stop light. It takes time for
the engine to produce the force that is required to move the weight of the car to the desired speed. Following error for
each axis can be selected to be viewed in the Watch Window / Status Window of the CNC. This is a quick reference tool
for monitoring the performance of the actual machine motion as compared to the CNC command.
The important consideration for X/Y coordinate motion is that the response for the X and Y motion is similar. If the
response is dissimilar, poor results for commanded motion will result. An example of a dissimilar tuned response would be
that when a circular motion is commanded, an oval or elliptical motion will result as one axis out-performs the other.
Position and Servo Error
A position, 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, was executed too slowly or was executed incorrectly. The Servo Error Tolerance is a user-definable
value within the CNC and is usually set to a value twice the 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.
 It is important to remember that in the closed position loop, the output command adjusts to maintain speed and
positioning. Thus, a loss of encoder feedback causes the control to send its maximum motion command to the motor
so the motor “runs away” without control. Conversely, if the motor fails to turn while feedback is enabled, a position
error results and the command output stops when the fault occurs.
Encoder Counts and Maximum Machine Speed
Remembering that the position loop uses the pulses or 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. The
following formula illustrates this relationship:
counts/line x counts/rev x 1 rev/inch = encoder counts/inch
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
Determining Maximum Machine Speed
The following formula is used to establish the proper value for determining maximum machine speed.
max RPM of the motor x number of inches of travel per revolution = maximum machine speed in inches per minute.