Torque-speed characteristics of rotary piezoelectric traveling wave motor
Simscape / Electrical / Electromechanical / Mechatronic Actuators
The Piezo Rotary Actuator block represents the torque-speed characteristics of a piezoelectric traveling wave motor. The block represents the torque-speed relationship of the motor at a level that is suitable for system-level modeling. To simulate the motor, the block uses the following models:
The motor is unpowered when the physical signal input v is zero. This corresponds to applying zero RMS volts to the motor. In this scenario, the block models the motor using the following elements:
An inertia whose value is the Rotor inertia parameter value.
A friction whose characteristics are determined by the parameter values in the Motor-Off Friction tab.
The block uses a Simscape™Rotational Friction block to model the friction component. For detailed information about the friction model, see the Rotational Friction block reference page.
When the motor is active, Piezo Rotary Actuator block represents the motor characteristics using the following equivalent circuit model.
In the preceding figure:
The AC voltage source represents the block's physical signal input of frequency f and magnitude v.
The resistor R provides the main electrical and mechanical damping term.
The inductor L represents the rotor vibration inertia.
The capacitor C represents the piezo crystal stiffness.
The capacitor Cp represents the phase capacitance. This is the electrical capacitance associated with each of the two motor phases.
The torque constant kt relates the RMS current i to the resulting mechanical torque.
The quadratic mechanical damping term, λωm2, shapes the torque-speed curve predominantly at speeds close to maximum RPM. ωm is the mechanical rotational speed.
The term represents the rotor inertia.
At model initialization, the block calculates the model parameters R, L, C, kt and λ to ensure that the steady-state torque-speed curve matches the values of the following user-specified parameter values:
Rated rotational speed
No-load maximum rotational speed
These parameter values are defined for the Rated RMS voltage and Motor natural frequency (or rated frequency) parameter values.
The quadratic mechanical damping term produces a quadratic torque-speed curve. Piezoelectric motors torque-speed curves can typically be approximated more accurately using a quadratic function than a linear one because the torque-speed gradient becomes steeper as the motor approaches the maximum speed.
If the rotor inertia J is not specified on the datasheet, you can select a value that provides a good match to the quoted response time. The response time is often defined as the time for the rotor to reach maximum speed when starting from rest, under no-load conditions.
The quality factor that you specify using the Resonance quality factor parameter relates to the equivalent circuit model parameters as follows:
This term is not usually provided on a datasheet. You can calculate its value by matching the sensitivity of torque to driving frequency.
To reverse the motor direction of operation, make the physical signal input v negative.
When the motor is powered, the model is valid only between zero and maximum speed, for the following reasons:
Datasheets do not provide information for operation outside of normal range.
Piezoelectric motors are not designed to operate in the powered braking and generating regions.
The block behaves as follows outside the valid operating region:
Below zero speed, the model maintains a constant torque that is the zero rpm torque value. The zero rpm torque value is the Maximum torque parameter value if the RMS input voltage equals the Rated RMS voltage parameter value, and the frequency input equals the Motor natural frequency parameter value.
Above maximum speed, the model produces the negative torque predicted by the equivalent circuit model, but limits the absolute value of the torque to the zero-speed maximum torque.
The torque-speed characteristics are most representative when operating the model close to the rated voltage and resonant frequency.
f— Motor driving frequency
Physical signal input port associated with the motor driving frequency, in Hz.
v— RMS supply voltage and direction of rotation
Physical signal input magnitude port specifying the RMS supply
voltage, and sign specifying the direction of rotation. If
v is positive, then a positive torque acts from
port C to port R.
i— RMS phase current
Physical signal output port associated with the RMS phase current.
wm— Rotor speed
Physical signal output port associated with the rotational speed of the rotor.
Mechanical translational conserving port associated with the motor case.
Mechanical translational conserving port associated with the motor rotor.
Motor natural frequency— Motor natural frequency
Frequency at which the piezoelectric crystal naturally resonates. For
most applications, set the input signal at port f
to this frequency. To slow down the motor, for example in a closed-loop
speed control, use a frequency slightly less than the motor natural
frequency. The default value is
Rated RMS voltage— Rated RMS voltage
Voltage at which the motor is designed to operate.
Rated torque— Rated torque
Torque the motor delivers at the rated RMS voltage.
Rated rotational speed— Rated rotational speed
Motor speed when the motor drives a load at the rated torque.
No-load maximum rotational speed— No-load maximum rotational speed
Motor rotational speed when driving no load and powered at the rated voltage and driving frequency.
Maximum torque— Maximum torque
Maximum torque that the motor delivers when actively driving a load
and powered at the rated voltage and frequency. The default value is
The Holding torque parameter value, the load torque the motor holds when stationary, may be greater than the Maximum torque parameter value.
Resonance quality factor— Resonance quality factor
Quality factor Q that specifies how torque varies as a function of driving frequency. Increasing the quality factor results in a much more rapid decrease in torque as driving frequency is moved away from the natural frequency.
Capacitance per phase— Capacitance per phase
Electrical capacitance associated with each of the two motor phases.
Rotor inertia— Rotor inertia
Rotor resistance to change in motor motion.
Initial rotor speed— Initial rotor speed
Rotor speed at the start of the simulation.
Holding torque— Holding torque
The sum of the Coulomb and the static frictions. It must be greater than or equal to the Coulomb friction torque parameter value.
Coulomb friction torque— Coulomb friction torque
The friction that opposes rotation with a constant torque at any velocity.
Viscous friction coefficient— Viscous friction coefficient
N*m/(rad*s)(default) | nonnegative scalar
Proportionality coefficient between the friction torque and the relative angular velocity.
Transition approximation coefficient— Transition approximation coefficient
The parameter sets the coefficient value that is used to approximate the transition between the static and the Coulomb frictions. For detailed information about the coefficient, cv, see the SimscapeRotational Friction block reference page.
Linear region velocity threshold— Linear region velocity threshold
The parameter sets the small vicinity near zero velocity, within which
friction torque is considered to be linearly proportional to the
relative velocity. MathWorks recommends that you use values in the range