PlantSecondOrderMechanical¶
A second order mechanical plant model with actuator and sensor models.
The PlantSecondOrderMechanical 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 structure of the mechanical plant and its typical location in a control loop is shown above. The inputs to the mechanical plant can be either force, torque or current. The outputs of the mechanical model are position, velocity, acceleration and sensor outputs, which include an index pulse and limit switches.
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:
Inverse actuator model
Second order mechanical model
Encoder index pulse model
Limit switch
Inverse sensor model
The implementation of the processing block¶
Inverse actuator model¶
The processing block can handle various input signal units, due to the inverse actuator model.
The ActuatorScaleFactor is used to convert the data on the Actuator input to force, via \(Force = \frac{Actuator}{ActuatorScaleFactor}\).
The implementation of the inverse actuator model¶
The quantization function allows for simulating an actuator’s resolution. The quantization block is parametrized by the ActuatorResolution. Note that (default) value 0.0 bypasses quantization.
Second order mechanical model¶
The mechanical model is based on a discrete-time implementation of the following second order transfer function:
Where \(m\) is Mass, \(d\) is Damping and \(k\) is Stiffness.
Each parameter can be described using the corresponding signal.
Two optional mechanical end stops are included in this model.
The locations of the end stops can be defined using the PositiveEndStop and NegativeEndStop signals.
The end stops are ignored if NegativeEndStop >= PositiveEndStop.
End stops are ignored by default since their locations are initialized at zero.
If the position of the plant reaches one of the defined end stop positions, the EndStopSaturationExceeded event is raised.
The acceleration and velocity of the plant are immediately set to zero such that the position remains constant.
Note
The model for the end stop does not include dynamical characteristics of the end stop, such as damping and stiffness. It is intended to test homing functionality and therefore implemented as a rigid body.
The implementation of the second order mechanical model¶
Encoder index pulse model¶
The encoder index pulse model can be used to obtain index pulses in a simulation, which correspond to the pulses in the actual system.
The model requires three configuration signals:
Signal |
Description |
|---|---|
IndexPulsePosition |
The exact location of the index pulse within one encoder ‘revolution’. |
IndexPulseDistance |
This is the distance between two index pulses, given in the same unit as the position input. It defines a single turn in case of a rotary application, i.e., \(2 \pi\) [rad]. |
IndexPulseWidth |
The width of the pulse in distance, given in the same unit as the position input. |
The signals are described by the figures below (left for linear applications, right for rotary).
The index pulse definition for linear applications¶ |
The index pulse definition for rotary applications¶ |
The implementation is given below, where the behavior of the comparator is depicted above.
The implementation of the encoder index pulse model¶
Note
The main functionality of the simulated index pulse is to simulate homing procedures, which require an index pulse.
Attention
An index pulse is skipped if the IndexPulseWidth is smaller than the distance covered in one sample period.
Limit switch¶
The limit switch model consists of a PositiveLimitSwitch and a NegativeLimitSwitch.
The positive limit switch can be configured using PositiveLimitSwitchPosition and PositiveLimitSwitchPulseWidth.
The same terminology is used for the negative limit switch.
Note
Although the names suggested otherwise, the defined positions of the limit switches can both be positive or negative.
The PositiveLimitSwitch and NegativeLimitSwitch signals are high when the corresponding switch is active.
The index pulse definition for linear applications¶
Hint
The main functionality of the simulated limit switches to simulate homing procedures, which require one or more limit switches.
Inverse sensor model¶
The inverse sensor model can be used to convert the unit of the output of a plant, such that it can be matched to the encoder configuration in the actual system.
The SensorScaleFactor is used to convert the data on the Position signal to the data on the Value signal, via \(Value = \frac{Position}{SensorScaleFactor}\).
The implementation of the inverse sensor model¶
The quantization function allows for simulating a sensor’s resolution. The quantization block is parametrized by the SensorResolution. Note that (default) value 0.0 bypasses quantization.
Integration¶
The integration of this processing block in the platform is depicted in The integration of plant second order mechanical in the software platform. 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 plant second order mechanical in the software platform¶
See also
- Closed-loop system
Using PlantSecondOrderMechanical to create a simulated closed-loop system guide via API.
- Closed-loop system
Using PlantSecondOrderMechanical to create a simulated closed-loop system guide via Tooling.
Interface¶
Inputs¶
Name |
Description |
|---|---|
Actuator |
Actuator input (i.e. force, torque or current). |
SamplePeriod |
Input to configure the period at which the plant is sampled. |
Read only signals¶
Name |
Description |
|---|---|
Acceleration |
Acceleration of the mass. |
Force |
Force generated by the actuator. |
IndexPulse |
Index pulse value. |
NegativeLimitSwitch |
Negative limit switch value. |
PositiveLimitSwitch |
Positive limit switch value. |
Position |
Position of the mass. |
UnsaturatedPosition |
Position of the mass before end-stop saturation. |
Value |
Raw sensor value. |
Velocity |
Velocity of the mass. |
Read-write signals¶
Name |
Description |
|---|---|
ActuatorResolution |
Actuator resolution. |
ActuatorScaleFactor |
Actuator scale factor. |
Damping [N*s/m] |
Damping. |
IndexPulseDistance [m] |
Index pulse distance. |
IndexPulsePosition [m] |
Position at which index pulse is generated. |
IndexPulseWidth [m] |
Pulse width of index pulse. |
Mass [kg] |
Mass. |
NegativeEndStop [m] |
Position of negative end-stop. |
NegativeLimitSwitchPosition [m] |
Position of negative limit switch. |
NegativeLimitSwitchPulseWidth [m] |
Pulse width of negative limit switch. |
PositiveEndStop [m] |
Position of positive end-stop. |
PositiveLimitSwitchPosition [m] |
Position of positive limit switch. |
PositiveLimitSwitchPulseWidth [m] |
Pulse width of positive limit switch. |
SensorResolution |
Sensor resolution. |
SensorScaleFactor |
Sensor scale factor. |
Stiffness [N/m] |
Stiffness. |