PositionControlSimple

A simple position control implementation.

The PositionControlSimple processing block can be found in the ‘Templates’ feature in the PMP installer.

The control loop structure where a feedback controller controls a mechanical plant

The most basic version of the typical feedback control structure is shown above, where a reference is tracked by a mechanical plant using sensor feedback. For position control, the reference signal is the demanded position and the output of the plant is position, measured using an encoder.

Description

The implementation of the processing block is shown in the figure below. The block consists of multiple components, which are further explained in the following sections:

• Tracking error calculation, loop control & homing

• Feedback controller

• Saturation and limits

• Signal propagation

The implementation of the processing block

Consider to use the advanced version of this processing block if one of the following additional extensions or features are required:

• More filters for the control algorithm.

• Separate tracking error limits and control output saturation limits during movement and standstill.

• Delay in the reference signal for feedforward timing optimization.

• Waiting for settling before starting a new move.

Tracking error calculation, loop control & homing

The tracking error is calculated using the following equations:

TrackingError = (DemandPosition - ActualPosition) * IsClosedLoop

ActualPosition = PositionSensor + HomeOffset

The position controller can be in open loop or closed loop state. A request for a certain loop state is done via the CloseLoopRequest input. The IsClosedLoop signal is used to inform PMP about the active loop state. The behavior for different loop states are as follows:

Loop state

Loop state	Move commands	Feedforward	Feedback
OpenLoop	Allowed	Enabled	Disabled
ClosedLoop	Allowed	Enabled	Enabled

The HomeOffset can be used to add a value to the relative position measurement to translate it to an absolute value. This absolute position is called the ActualPosition.

When closing the loop, the tracking error is set to zero to prevent an initial step in control output which could result in unexpected movement. This is achieved by setting the demanded position equal to the actual position during the first calculation cycle in closed loop. The DemandPositionOffset signal is used to inform the trajectory interpolator of the change in demanded position. Similarly, the demanded position is updated when applying a home offset such that the home offset does not cause a step in tracking error.

Feedback controller

The tracking error is forwarded to the control algorithm. The feedback controller has a PID controller and a generic filter.

The filter and PID controller are implemented in series. The filter can be excluded from the control loop by configuring it as pass-through.

Generic filter

The generic filter can be used to apply one of the following filters: 1st or 2nd order lowpass or highpass, 1st order lead/lag, 2nd order notch or 2nd order custom filters in either the continuous or discrete domain. By default, the generic filter type is set to PassThrough, which means that it’s disabled and the signal passes through the block.

PID

The PID controller combines a proportional, a derivative and an integral control term with a second order low pass filter. The second order low pass filter is applied to the results of the proportional and derivative terms, but not to the result of the integral term. The integrator output can be clipped to configurable limits. An IntegratorSaturated event is raised if the output exceeds the limits, i.e., IntegratorOutput < IntergratorLowerClip or IntegratorOutput > IntergratorUpperClip.

If the PID controller is not used, it can be configured as pass-through by using a proportional gain of one and setting the integrator and differentiator frequencies to zero. In this pass-through mode it is still required to configure the second order low pass filter.

Note

The PMP Generic Filter block is a GenericFilter. The PID block is a PIDLowPass. Both blocks are discretized using Tustin’s method.

Saturation and limits

Limits are available to evaluate if the tracking error and control output stay within defined bounds. Event are raised if these bounds are exceeded.

Tracking error limit

The purpose of the control loop is to keep the tracking error small. If the tracking error becomes very large, then it’s likely that some part of the loop is not correctly functioning. For instance due to a failing actuator, a noisy sensor or obstructed mechanics.

Implementation of the tracking error limit

The limit can be configured using the TrackingErrorLimit signal. The TrackingErrorLimitExceeded event is active if the tracking error exceeds the configured limit:

• |TrackingError| > TrackingErrorLimit

Saturation

The control output saturation limits make sure that the position controller is aware of the physical limitations of the connected actuator. If the control output saturates, then the integrator of the PID controller is locked to prevent ‘integral windup’.

Attention

The control output saturation is not intended to protect the actuator against thermal overload. These protections should be configured on the actuator interface.

Implementation of the saturation

Note

The values of the saturation limits depend on the unit of the control output.

The saturation limits can be configured using the ControlOutputUpperSaturation and ControlOutputLowerSaturation signals. The CoutSaturationExceeded event is active if the control output exceeds these limits:

• ControlOutput < ControlOutputLowerSaturation

• ControlOutput > ControlOutputUpperSaturation

The saturation is disabled if ControlOutputUpperSaturation =< ControlOutputLowerSaturation.

Hint

If possible, design the controller such that the control output saturation limit is never exceeded during normal operation. Then use this event to disable the axis. This way, unsafe high forces due to mechanical obstruction, instability or magnetic misalignment are detected and captured early.

Signal propagation

The processing block has inputs that can be used to combine the feedback control output with other signals that must be propagated to the actuator (see The implementation of the processing block). These inputs are:

Input	Description
FeedbackOffset	Input to apply an offset before the FeedbackControlOutput.
FeedforwardControl	This can be used to connect the feedforward controller to the position controller. It is added after the FeedbackControlOutput.

Integration

The integration of this processing block in the software platform is shown in the figure below. Most connections between the processing block and other software components are required for correct operation, but some are optional. The optional connections are depicted using dashed lines.

The integration of position control in the software platform

Hint

In case a three phase actuator is used, the control output must be connected to the quadrature axis, often denoted as ‘DemandIq’.

Note

The unit of all signals is not defined by the processing block. The unit of the control output, feedforward control and feedback offset depends on the unit of the Demand input of the actuator. The unit of the demand position, demand position offset, home offset and position sensor depends on the unit of the Actual signal of the sensor.

Interface

Inputs

Inputs

Name	Description
CloseLoopRequest	Input to request a ClosedLoop loop status.
DemandPosition	Demand position input used for tracking error calculation.
FeedbackOffset	Input to apply an offset before the FeedbackControlOutput.
FeedforwardControl	Input to connect the feedforward controller to the position controller.
HomeOffset	Home position offset value, used to add an offset to the relative position measurement to translate it to an absolute value
PositionSensor	Sensor value input used for tracking error calculation.

Read only signals

Read only signals

Name	Description
ActualPosition	Actual value of the position sensor after home offset.
CombinedControlOutput	Combined feedback and feedforward controller output.
ControlOutput	Output of the position controller.
DemandPositionOffset	Offset for the demand position of the trajectory interpolator.
FeedbackControlOutput	Feedback controller output.
GenericFilterOutput	Control output of the sequence of generic filters.
IsClosedLoop	Indicates if the feedback control loop is open or closed.
PidOutput	Output of the PID controller.
TrackingError	Tracking error value.

Read-write signals

Read-write signals

Name	Description
ControlOutputLowerSaturation	Controller output lower saturation value.
ControlOutputUpperSaturation	Controller output upper saturation value.
TrackingErrorLimit	Tracking error limit for feedback tracking limit determination.

Events

Events

Name	Description
TrackingErrorLimitExceeded	Active if a configurable feedback tracking error limit is exceeded.
IntegratorSaturated	Active if the integrator output reaches the saturation limit.
CoutSaturationExceeded	Active if a configurable control output saturation limit is reached.

Filters

Filters

Name	Type	Discretization type
GenericFilter	BiQuad	Tustin
PID_LowPass	PidLp	Tustin