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Boost current control for better stepper motor motion

Posted: 16 Feb 2016 ?? ?Print Version ?Bookmark and Share

Keywords:Bipolar stepper motors? windings? microstepping? MOSFET? H-bridge?

Bipolar stepper motors are employed in a number of applications, from driving paper through a printer to moving an XY stage in industrial equipment. Typically, the motors are driven and controlled by inexpensive and dedicated stepper motor driver ICs. Unfortunately, most of these ICs use a simple current control method that causes imperfections in the motor current waveforms and results in less-than-optimal motion quality. Implementing internal, bi-directional current sensing inside a stepper motor driver IC results in improved motion quality with lower system cost than legacy solutions.

Bipolar stepper motor basics
A bipolar stepper motor contains two windings. The motor is moved by driving varying currents sequentially through the two windings. To make the motor move smoothly, the two windings can be driven with sinusoidal currents that are 90 out of phase C sine and cosine.

Usually, steppers are not driven with analogue linear amplifiers; they are driven using a PWM current-regulating driver with discrete current values that break the sine wave into straight segments. This is called microstepping. The sine wave may be broken up into any number of segments, and the waveform approaches a true sine wave as the number of segments increases. In practice, the number of segments varies from 4 to 2048 or more, with most IC stepper drivers implementing between 4 and 64 segments. Since one sine wave generates four steps (mechanical states in a stepper motor), a 64-segment sequence is called a ? step operation (figure 1).

Figure 1: Microstepping Current Waveforms.

Why current-control accuracy is important
The position of a bipolar stepper motor's rotor is dependent on the magnitude of the currents flowing thorough the two windings. Normally, if a stepper motor is being used, there is a requirement for accurate mechanical positioning or accurate speed control of some mechanical system. So it is only logical that the accuracy of the motion is determined in part by the accuracy of the winding currents being used to drive the motor.There are two problems that inaccurate current control causes in the mechanical system:
???At slow speeds or when a stepper motor is used in a positioning application, the motor steps a different amount at each microstep. This causes an error in positioning.
???At higher speeds, the non-linearities cause short-term speed variations within a single rotation of the motor. This adds undesired components to the torque that increase noise and vibration of the motor.

PWM and decay modes
Most stepper motor driver ICs rely on the inductive nature of the stepper motor windings to implement PWM current regulation. Using an H-bridge arrangement of power MOSFETs for each winding, the supply voltage is applied to the winding at the beginning of a PWM cycle, causing the current to build through the inductance of the winding. Once the current reaches the desired level, the H-bridge changes state to reverse the current buildup. After a fixed period of time, a new PWM cycle begins, and the H-bridge drives current through the winding again.

This process is repeated, so the winding current goes up and down with the peak current programmed by a state machine and a DAC that sets the desired current for each segment. When the state machine advances to the next segment, the regulated peak current changes accordingly.

After the desired peak current is reached, the H-bridge can drive the winding current down in one of two ways:
???If the winding is short-circuited (by turning on both low-side or both high-side MOSFETs), the current will decay slowly.
???If the H-bridge is reversed or if the current is allowed to re-circulate through the MOSFET body diodes, the current will decay quickly.

These two options are called slow decay and fast decay (figure 2).

Figure 2: H-Bridge States.

Since the motor winding is an inductor, the rate at which the current changes is proportional to the applied voltage and its inductance. To move a stepper motor quickly, it is desirable to be able to drive the current changes in a very short period of time. Unfortunately, there is another factor that works against the changes in current. When the motor is in motion, a voltage is induced in opposition to the current C the back EMF. This back EMF effectively reduces the voltage available to increase the current in the windings, so the faster the motor turns, the longer it takes to force a change in the current through a winding.

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