SimElectronics™

Servomotor - Model brushless motor with closed-loop torque control

Library

Actuators & Drivers

Description

The Servomotor block represents a brushless motor with closed-loop torque control. This block abstracts the torque-speed behavior of the combined motor and motor driver in order to support system-level simulation where simulation speed is important.

The block allows the range of torques and speeds defined by the torque-speed envelope that comes from the motor manufacturer. You specify this data in the block dialog box as a set of speed data points and the corresponding maximum torque values. The one in the following figure shows a typical torque-speed envelope for a servomotor.

The block limits any demand applied to its reference demand port Tr to values within the defined torque-speed envelope.

Note For numerical reasons, you must not specify an infinite slope at N_max.

The block models the electrical losses in the motor using an equivalent resistance R in series with the DC supply to the motor and driver. Compute the equivalent resistance for your motor in terms of the manufacturer-quoted efficiency level E at some rated torque T and speed ω as follows:

Equate the power used by the servomotor to the mechanical power plus the electrical losses. In terms of the DC supply voltage (V) and current (I), this means
Define the efficiency of the servomotor as the mechanical power into the motor divided by the total electrical power supplied:
Solve the preceding equations for R:

The block produces a positive torque acting from the mechanical C to R ports.

Basic Assumptions and Limitations

The model is based on the following assumptions:

The motor driver tracks a torque demand with a time constant Tc.
The motor torque tracking is not affected by motor speed fluctuations due to mechanical load.
Motor electrical losses are proportional to the square of the DC supply current.

Dialog Box and Parameters

Electrical Torque Tab

Vector of rotational speeds in RPM: Rotational speeds for permissible steady-state operation. The default value is [ 0 3.75e+03 7.5e+03 8e+03 ].
Vector of maximum torque values in Nm: Maximum torque values for permissible steady-state operation. These values correspond to the speeds in the Vector of rotational speeds in RPM parameter and define the torque-speed envelope for the motor. The default value is [ 0.09 0.08 0.07 0 ].
Torque Control time constant, Tc: Time constant with which the motor driver tracks a torque demand. The default value is 0.02 s.
Supply series resistance R to model electrical losses: The equivalent resistance used in series with the DC supply to model electrical losses in the motor. The default value is 3.5 Ω.

Mechanical Tab

Rotor inertia: Rotor inertia. The default value is 5e-06 kg*m². The value can be zero.
Rotor damping: Rotor damping. The default value is 1e-05 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

The block has the following ports:

+: Positive electrical DC supply.
-: Negative electrical DC supply.
Tr: Reference torque demand.
w: Mechanical speed output.
C: Mechanical rotational conserving port.
R: Mechanical rotational conserving port.