TechArchive

Original article date: February 2000

Does it make any difference if a stepper driver uses digital signal processing (DSP)? GEOFF SORE from Smartdrive explains why he believes that it gives many advantages

Using digital signal processing (DSP) technology for digital stepper-motor control offers a number of advantages to the machine builder and the end-user, but the key advantages are circuit simplification and smooth motion. This gives a significant reduction in acoustic noise from the motor when running at lower speeds, and silent operation at standstill and very slow speeds. This opens up many new applications where steppers have previously been excluded due to motor noise.

Low-noise motor operation is achieved because the PWM switching pulses applied to the power devices are generated directly by the processor, and can be precisely controlled in width and frequency so that they are inaudible at standstill. At slow speeds, the progression of changing pulse width needed to achieve the required pseudo-sine and cosine motor phase current waveforms can be made without deviation from a designed profile.

In a conventional design, a voltage comparator is used to adjust the PWM width on a pulse-by-pulse basis by comparing the measured current with a reference voltage from a digital to analogue converter in order to maintain the desired winding current. This use of analogue signals leads to pulse-width fluctuations resulting from random and switching transient induced signal noise. These effects are particularly difficult to minimise when one phase is at maximum current and the other is at minimum. The effect of these analogue induced fluctuations is to generate the classic acoustic noise at standstill or low speeds. These effects are completely eliminated by Smartdrive’s new Taranis 75-P direct digital drive design, which incorporates the latest in DSP.

There remains the acoustic noise produced by the alternating currents applied to the motor when it is rotating at speed. Unfortunately, this can never be completely removed as it is produced by a complex combination of effects. However, the Taranis design has control of the AC waveform both by predictive modification related to speed and by current feedback. This allows adaptation of the parameters to better suit individual motor construction types and thereby bring some reduction in acoustic noise at higher speeds.

Alongside low acoustic noise there is also the requirement in laboratory and technical applications for very smooth rotation at slower speeds and better stopping positional linearity. Again the Taranis DSP design brings substantial improvements. The program numerical position resolution is 51,200 microsteps per revolution (for a normal 200 step 1.8deg motor). However, when moving from one microstep to the next there is a further internal division of up to 64 sub-microsteps. This results in ultra-smooth motion at slow speeds – equivalent to over 3,000,000 microsteps per revolution!

At slow speeds, a stepper motor driven with a fixed-frequency pure sine and cosine microstep waveform current pair exhibits a small (50 per revolution) cyclic deviation in both incremental motion and torque stiffness from the uniformity predicted by a Lissajous diagram representation of the combined current vector. This deviation is primarily a function of the imbalance of sine and cosine components combining to generate the torque vector amplitude and angle resulting from nonlinearities as the stator and rotor teeth pairs pass in and out of alignment. The design of the rotor and stator teeth therefore has a major influence on this effect, so some types of motor are much better than others.

By providing a facility for the user to modify the wave shapes away from pure sine and cosine to compensate for motor design, the drive can be optimised for best linearity with a particular motor type.

In applying precise digital control, many new characteristics of stepper drive and motor performance previously masked by analogue circuits have become apparent. The pioneering investigation and development work over the past three years has moved forward the boundaries of stepper system performance, introducing new and exciting opportunities for stepper applications.

• Smartdrive

February 2000