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Voltage mode driving

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Voltage mode driving

This section describes the basic principles of Voltage mode driving and its implementation in

STMicroelectronics devices with a focus on the compensation of:



The back electromotive force (Section 1.2 on page 7)



The motor supply voltage variation (Section 1.3 on page 11)



The thermal drift of the phase resistance (Section 1.4 on page 12).

1.1

Basic principles

The classic current mode driving method limits the phase current to a reference value using

a comparator and a current sensor (usually an external resistor). This control is the most

intuitive but brings with it some drawbacks: the current ripple can be significant and

obtaining an acceptable control of the current can be challenging. In trying to solve these

problems, current control algorithms were made more and more complex, including

techniques such as fast decay and mixed decay. With the introduction of microstepping in

stepper motor driving a new current control algorithm limit became evident: the analog

circuitry and the control loop should be able to manage lower currents with higher

resolution.

Voltage mode totally changes the control approach implementing an open-loop control:

a sinusoidal voltage is applied to the motor phases and the electro-mechanical system

response with a sinusoidal current.

Note:

Due to its principle of operation, Voltage mode driving is not suited to full step driving. The

best performance is always obtained using microstepping operation.

This result can be obtained through the analysis of the stepper motor electrical model.

Equation 1, extracted from the model in Figure 1, shows how the current of a generic motor

phase is related to:





Back electromotive force (BEMF)



The back electromotive force is typically a sinusoidal voltage with frequency and amplitude

the hypothetical current sine wave that should be applied to the motor phase in order to

and structure (rotor material, coil turns, etc.).