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News:

Spectrum Software has released Micro-Cap 11, the eleventh generation of our SPICE circuit simulator.

For users of previous Micro-Cap versions, check out the new features available in the latest version. For those of you who are new to Micro-Cap, take our features tour to see what Micro-Cap has to offer.

 

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Stepper Motor Model

 

Here is a new model for Micro-Cap, the DC stepper motor. It is based on a model created by Dave Wilson, currently at TI. For more information, see http://e2e.ti.com/support/applications/motor_ drivers/f/38/t/283566.aspx.

This model handles detent torque and back EMF. It does not consider eddy current losses. To accurately simulate eddy current losses would require at least a rudimentary finite element analysis of the motor structure.

This model does not consider such nonlinearlities. It is a linear, parametric model that produces reasonably faithful results. Note that the model internalizes a constant viscous friction load and a physical angular stop.

Here is what the model looks like:

Stepper Motor Model

The model includes these parameters:

K Motor Constant = Ke (Volts/(radian/sec)) = Kt (Newton-Meters/Amp)

KD Damping Torque (Newton-Meters/(radian/sec))

STEP_ANGLE Angle width of one full step (Degrees)

RS Stator Coil Resistance (Ohms)

LS Stator Coil Inductance (Henries)

TD Detent Torque (Newton-Meters)

TLOAD Load torque (Newton-Meters)

JMOTOR Rotational Inertia of Motor Load (Kilograms-Meter^2)

LIMIT Hard stop position (radians)

HARDNESS Unitless factor which sets stop hardness

JLOAD Rotational Inertia of motor shaft (Kilograms-Meter^2)

TD Detent Torque (Newton-Meters)

How does it work? Well you can drive a stepper motor in a variety of ways. Here is one way using non-overlapping pulses.

One phase on drive

Here is what the waveforms look like.

One phase on transient analysis

Here we've plotted the voltage at VA and VB, which consist of two non-overlapping pulses. Next we've plotted the armature currents flowing from the generators. Since the current values are not brought out to pins, you must plot them by referring to the motor's name (X1) like this:

I(X1.L1)
I(X1.L2)

where X1 is the proper name for the motor macro. In your circuit it may be different.

Similarly, we've plotted the back emf of the A and B coils this way:

V(X1.BEMF_A)
V(X1.BEMF_B)

The last three plots show the acceleration (radians/sec/sec), the velocity (in radians/sec), and finally the angle (in radians). The angle increases until it hits the LIMIT parameter value, which in this case is 2 radians.

Note that the model has two input pins, one for each phase. It has four outputs: Torque, Velocity, Angle, and Accel (Acceleration). The outputs have been labelled with grid text to make it easy to identify the plots.

Here is another way to drive the motor, using overlapping phases.

Two phases on drive

Here is its transient analysis.

Two phases on drive

Here we've plotted the voltage at VA and VB, which consist of two overlapping pulses. Next we've plotted the armature currents flowing from the generators. As in the last circuit, the current values are not brought out to pins, so we must plot them by referring to the motor like this:

I(X1.L1)
I(X1.L2)

Similarly, we've plotted the back emf of the A and B coils this way:

V(X1.BEMF_A)
V(X1.BEMF_B)

The last four plots show the acceleration (radians/sec/sec), the velocity (in radians/sec), the angle (in radians) and the Torque (in Newton-meters). The angle increases until it hits the LIMIT parameter value, which in this case is 3 radians.

One of the most efficient ways to drive a stepper motor is called micro-stepping. The next figure shows a micro-stepped motor.

Microstepped drive

Here is its transient analysis. Note the stepped sine drive waveforms generated by the sine wave NFV sources, the ZOH macros, and the VCS switches.

Microstepped drive transient analysis

 
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