Hypertherm Phoenix 8.0 - Manual (Page 447)

Motion Overview
440
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 control 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.
Note: 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 of inches of travel per revolution = maximum machine speed
in inches per minute.