HEIDENHAIN ND 1300 CNC Setup - Manual (Page 25)

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CNC Setup
CNC PID control loops
The QC-320 software includes independent PID (Proportional Integral Differential) loops to control accelera-
tion, velocity and position on each CNC axis. When a new position is requested by a CNC program, joystick
or digital positioner, the QC-320 program predicts a series of points along the axes to the requested position.
The program then compares the predicted positions to the actual positions measured by the encoder as the
axes are moved. The differences between predicted and actual positions (following errors) are processed by
PID control terms, amplified and drive the axis motors to minimize the difference. A simplified PID feedback
control loop is diagrammed here.
Axis motion occurs roughly as the trapezoidal velocity outlined and diagrammed below.
• A new position is predicted by the CNC module of the QC-320 program
• A following error is generated by the difference between the predicted and actual positions
Following error = Predicted position - Actual position provided by encoder
• The following error is processed by the PID control terms, amplified and used to drive the motor
(and encoder) in the direction of the predicted position
• The motor accelerates the axis to a specified maximum velocity at a nearly constant rate
• As the axis position approaches the final predicted position, the motor decelerates the axis at a
nearly constant rate
• Axis motion finally stops within a specified radius around the final position (continuation zone) and
the following error becomes insignificant
Simplified PID control loop diagram
Trapezoidal CNC axis motion
CNC PID Control Loops