Documentation

Generic Rotary Actuator

Model generic rotary actuator driven from DC voltage source or PWM driver

Library

Rotational Actuators

Description

The Generic Rotary Actuator block implements a model of a generic rotary actuator designed to be driven from a DC voltage source or PWM driver. You define torque-speed characteristics in terms of tabulated values for powering the motor at the rated voltage. This functionality allows you to model a motor without referencing an equivalent circuit.

The motor or actuator architecture determines the way in which electrical losses depend on torque. For example, a DC motor has losses that are proportional to the square of the current. As torque is proportional to current, losses are also proportional to mechanical torque. Most motors have an electrical loss term that is proportional to the square of mechanical torque. The Generic Rotary Actuator block calculates this loss term using the Motor efficiency (percent) and Speed at which efficiency is measured parameters that you provide.

Some motors also have a loss term that is independent of torque. An example is a shunt motor where the field winding draws a constant current regardless of load. The Torque-independent electrical losses parameter accounts for this effect.

The motor efficiency is the mechanical power divided by the sum of the mechanical power and both electrical loss terms. The block assumes that the speed at which the motor efficiency is defined is in the motoring quadrant and, therefore, positive.

You can operate the block in the reverse direction by changing the sign of the voltage that you apply. The H-Bridge block, for example, reverses motor direction if the voltage at the REV port is greater than the Reverse threshold voltage parameter. However, if you are using the block in reverse, specify the torque-speed data for forward operation:

  • Positive torques and positive speeds in the motoring quadrant.

  • Positive torque and negative speeds in the generating counterclockwise quadrant.

  • Negative torque and positive speed in the generating clockwise quadrant.

Thermal Port

The block has an optional thermal port, hidden by default. To expose the thermal port, right-click the block in your model, and then from the context menu select Simscape > Block choices > Show thermal port. This action displays the thermal port H on the block icon, and adds the Temperature Dependence and Thermal port tabs to the block dialog box.

Use the thermal port to simulate the effects of copper resistance losses that convert electrical power to heat. For more information on using thermal ports and on the Temperature Dependence and Thermal port tab parameters, see Simulating Thermal Effects in Rotational and Translational Actuators.

Basic Assumptions and Limitations

This block has the following limitations:

  • The torque-speed curve data corresponds only to the rated voltage, so the block produces accurate results only when driven by plus or minus the rated voltage.

  • In this block requires, you must provide torque-speed data for the full range over which you use the actuator. To use the actuator in the generating and braking regions, provide additional data outside of the normal motoring region.

  • Model behavior is sensitive to torque-speed data. For example, no-load speed is correctly defined and finite only when the data crosses the speed axis.

  • To drive the block from the H-Bridge block:

    • Do not place any other blocks between the H-Bridge and the Generic Rotary Actuator blocks.

    • In the H-Bridge block dialog box, set the Freewheeling mode to Via one semiconductor switch and one freewheeling diode. Selecting Via two freewheeling diodes does not set the bridge output voltage to zero when the PWM input signal is low.

    • In the H-Bridge, Generic Rotary Actuator, and Controlled PWM Voltage block dialog boxes, ensure that the Simulation mode is the same for all three blocks.

Dialog Box and Parameters

Electrical Torque Tab

Speed values

Specify a vector of speeds, including their units, for your torque-speed data. The default value is [ -1.5e+03 -1000 -500 0 500 1000 1.5e+03 2e+03 2.5e+03 3e+03 ] rpm.

Torque values

Specify a vector of torques, including their units, for your torque-speed data. The default value is [ 0.04 0.035 0.03 0.025 0.02 0.015 0.01 0.005 0 -0.005 ] Nm.

Rated voltage

Indicate the voltage for which the device you are modeling is rated. The default value is 12 V.

Motor efficiency (percent)

The efficiency that the block uses to calculate torque-dependent electrical losses. The default value is 80.

Speed at which efficiency is measured

The speed that the block uses to calculate torque-dependent electrical losses. The default value is 2e+03 rpm.

Torque-independent electrical losses

Fixed electrical loss associated with the actuator when the torque is zero. The default value is 0.1 W.

Simulation mode

If you set the Simulation mode parameter to PWM, apply a PWM waveform switching between zero and rated volts to the block electrical terminals. The current drawn from the electrical supply is equal to the amount required to deliver the mechanical power and to compensate for electrical losses. If the applied voltage exceeds the rated voltage, the resultant torque scales proportionately. However, applying anything other than the rated voltage can provide unrepresentative results. PWM is the default setting.

If you set the Simulation mode parameter to Averaged, the torque generated in response to an applied voltage Vav is

VavVrated×T(ω)

where T(ω) is the torque value at speed ω. The current drawn from the supply is such that the product of the current and Vav is equal to the average power that is consumed.

Mechanical Tab

Rotor inertia

Rotor resistance to change in motor motion. The default value is 1e-04 kg*m2. The value can be zero.

Rotor damping

Rotor damping. The default value is 1e-08 N*m/(rad/s). The value can be zero.

Initial rotor speed

Speed of the rotor at the start of the simulation. The default value is 0 rpm.

Ports

This block has the following ports:

+

Positive electrical conserving port

-

Negative electrical conserving port

C

Mechanical rotational conserving port

R

Mechanical rotational conserving port

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