Sinusoidal current-mode control lets steppers perform like servos in many applications, including this three-axis milling machine.

For the last 40 years, stepper motors have been used in applications requiring tight position control at an affordable price. The advantageous economics stem from simplicity. Steppers deliver accurate positioning without feedback, and can be driven by squarewaves, which almost any digital controller can supply.

But don’t underestimate the complexity of these electromechanical devices; controlling them is often more difficult than controlling other motors. For starters, inherently low damping leads to frequent resonance and acoustic noise problems. Another problem is their limited speed range. On a typical 200-step/rev stepper, reversing the current in one of the stator coils results in only 1.8° of shaft movement, compared to 100 times that amount of movement when the current is reversed in the stator of a two-pole ac design. As the step frequency increases, a point is reached (at a fairly low rpm) where the current simply can’t commutate in and out of the coils fast enough - the inductance of the coils oppose rapid current changes — and motor torque decreases.

For better performance, steppers are often driven in current mode with sinewaves instead of squarewaves. This not only offers smoother performance over a wider speed range, but also increases positional resolution. Current mode control also reduces sensitivity to system fluctuations, such as power supply disturbances, coil crosstalk from high mutual inductance, or changes in coil resistance because of heating. But achieving top performance with a current mode controller requires careful attention to design details.

Better behaved stepper

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Consider a singlechip sinusoidal current- mode stepper controller in which motor current is regulated via digital control loops running in software. Reference sine and cosine signals are supplied to the inputs of the servo loops, which are generated from a sinusoidal look-up table in memory. Instead of maintaining a separate wavetable for each reference signal, data is retrieved from a single table by using two pointers offset by 90°. It’s not necessary to fetch every point in the table when reconstructing each cycle of the reference waveform. Because table access occurs at fixed intervals, more waveform points will be skipped at higher speeds. To change the phase relationship between the coil currents required for reverse operation, the wavetable pointer is decremented, instead of incremented, at each interval.

An on-board analog-to-digital converter acquires samples of the motor phase currents and software digitally compares them to the sine and cosine reference signals. The result of each comparison is an error signal, processed by a simple gain stage. As with most current-mode controllers driving inductive loads, the only pole in the open-loop transfer function is that formed from the inductance L and resistance R of the motor windings. A stability analysis shows that, as a result, a gain stage is all that is needed for stable operation; no phase-lead compensation is required. By selecting a sampling frequency much higher than the bandwidth of the servo loop, the sampleand- hold delays associated with the a/d converter and PWM modules are insignificant in terms of their impact on the loop phase margin.

One step at a time

The output of each gain stage is processed and supplied to the onboard PWM module, which generates the switching signals necessary for controlling both H-bridge power stages. The PWM module is capable of generating PWM signals in several formats, so it’s important to select a mode optimized for the application. Specifically, the PWM module should generate high-resolution output that delivers fast current slewing, economical motor current detection, and control of both H-bridges with the available six PWM signals.

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