Position Control Loop

A set velocity is transmitted to the drive controller via the fieldbus. The actual position of the axis is transmitted to TwinCAT as feedback via the fieldbus, creating a position control loop. The position control can be carried out using this circuit.

The position controller of the TwinCAT axis is only active if the drive controller is operated via the set velocity. If the drive controller is controlled via the set position, the position control loop in the drive controller is closed and must be parameterized there. The control parameters described here are then not effective.

Position control: Dead Band Position Deviation

Position Control Loop 1:
Position Control Loop 2:
Position Control Loop 3:

The dead band position deviation defines a range in which the controller is inactive. This parameter is a symmetrical window based on the assumption that the position lag error with respect to the position control (position control deviation) within this window is zero. Position control is therefore disabled within the window.

This functionality is required because certain axes and mechanical systems may cause a stationary oscillation around the target position. The parameter can be used to enforce "calm". The parameterizable deviation around the exact target position is accepted.

There are axes that are not capable of position control or have insufficient holding torque around standstill, so that a certain inaccuracy around the target position is accepted.

For other real effects, such as pronounced static friction (with tear-off torque) or pronounced slack, dead band position deviation can also be used to enforce "calm" for an accepted accuracy.

Position control: Proportional Factor Kv

Position Control Loop 4:
Position Control Loop 5:
Position Control Loop 6:

The Proportional Factor Kv is the proportional gain factor of the P part of the P controller. Output velocity = feed forward velocity + Kv * following error.

Position control: Proportional Factor Kv (standstill)
Position Control Loop 7:
Position Control Loop 8:
Position Control Loop 9:

The Proportional Factor Kv (standstill) Kvs is the proportional gain factor of the P part of the P controller when the axis is stationary. Output velocity when stationary = feedforward velocity + Kvs * following error.

Position control: Proportional Factor Kv (moving)

Position Control Loop 10:
Position Control Loop 11:
Position Control Loop 12:

The Proportional Factor Kv (moving) Kvf is the proportional gain factor of the P part of the P controller when the axis is moving. Output velocity when moving = feedforward velocity + Kvf * following error.

Position control: Velocity threshold V dyn [0.0 … 1.0]

Position Control Loop 13:
Position Control Loop 14:
Position Control Loop 15:

With a PP controller, two P constants

and a velocity threshold value vdyn define a function for a velocity-dependent control factor kv. If the value of the quotient "set velocity/reference velocity" is in the velocity threshold interval [- vdyn ... + vdyn], the gain factor of the P controller is linearly interpolated from the proportional factor Kv (moving) Kvf to the gain factor Kv (standstill) Kvs with zero velocity. The relationships are shown in the figure below.

Position Control Loop 16:

The example was parameterized with Kvs = 50, Kvf = 10 and v_dyn = 0.2.

Position control: Integral Action Time Tn

Position Control Loop 17:
Position Control Loop 18:
Position Control Loop 19:

Integral action time Tn of the I part of the PID controller. Integration time.

Position control: Derivative Action Time Tv

Position Control Loop 20:
Position Control Loop 21:
Position Control Loop 22:

Derivative action time Tv of the real D part (D-T1 component) of the PID controller.

Position control: Damping Time Td

Position Control Loop 23:
Position Control Loop 24:
Position Control Loop 25:

Damping time Td of the real D part (D-T1 component) of the PID controller.

Position control: Min./max. limitation I-Part [0.0 … 1.0]

Position Control Loop 26:
Position Control Loop 27:
Position Control Loop 28:

Limitation of the I part of the PID controller.

Position control: Min./max. limitation D-Part [0.0 … 1.0]

Position Control Loop 29:
Position Control Loop 30:
Position Control Loop 31:

Limitation of the D part of the PID controller.

Disable I-Part during active positioning

Position Control Loop 32:
Position Control Loop 33:
Position Control Loop 34:

The I part of the PID controller can be disabled during active positioning.

Feedforward Acceleration: Proportional Factor Ka

Position Control Loop 35:
Position Control Loop 36:
Position Control Loop 37:

The proportional factor Ka of the acceleration pre-control is the gain factor when a control with NC acceleration pre-control is used. Output velocity component = Ka * setpoint acceleration.

Further information about acceleration pre-control can be found in the section Acceleration Feedforward.

Feedforward Velocity: Pre-Control Weighting [0.0 … 1.0]

Position Control Loop 38:
Position Control Loop 39:
Position Control Loop 40:

This parameter is a relative weighting of the pre-control. Here, the default parameter value 1.0 corresponds to a weighting of the pre-control of 100 %. The weighting of the velocity pre-control can be reduced to avoid overshooting in position.