DSP technology widens prospects for stepper drives

This article shows how the latest DSP (digital signal processing) technology has been exploited in a new generation of direct digitally controlled stepper drives.

The use of DSP technology for digital stepper-motor control offers a number of advantages: * Ultra-smooth microstepping * Powerful control processing * Silent standstill operation * Very low noise at slow speeds * Adaptation to motor nonlinearities * Simple compact circuitry * High performance at low cost.

These benefits are realised in the latest development of SmartDrive's pioneering Taranis stepper-drive technology, which combines a powerful direct PWM (pulse-width modulated) driven 7.5 A 85 V MOSFET microstepping output stage with a fast BASIC machine and motion controller.

All this is integrated into a user-friendly compact package that sets new performance standards and takes steppers into a new application era.

The heart of this unique design is a super-fast digital signal processor.

The PWM generating structure of the DSP, intended for 3-phase AC/DC motor drives, has been adapted by SmartDrive to also generate the 4th-phase switching and control needed for the classic 2/4-phase stepper motor.

This adaptation allows the MOSFET power devices to be directly controlled by the DSP PWM outputs with none of the intervening analogue level comparators, pulse generating bistables or steering logic used in traditional designs.

The prime advantages of this innovative technique are circuit simplification and smooth motion, giving 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.

The Taranis 75-P module utilises this ground-breaking design, which is allied with a user-friendly and powerful floating-point maths processor, BASIC programmable motion and machine controller with opto-isolated I/O.

The result is a full featured integrated driver and controller package for simple single-axis standalone applications or multi-axis systems exploiting the dualled RS232-RS485/4W serial port.

Low motor noise 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 convertor 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 generally heard as varying pseudo-random 'shushing' pink noise or as 'singing' at an audible frequency.

These effects are completely eliminated by SmartDrive's direct digital drive design.

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 including magnetostriction in the motor magnetic paths, movement of the coils and torque variations due to rotor and stator tooth shapes.

However, the Taranis design has control of the AC waveshape both by predictive modification related to speed and by current feedback.

This allows adaptation of the parameters to better suit individual motor construc tion 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: here the Taranis DSP design also 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 submicrosteps, so that motion from one microstep to the next is graduated.

This results in ultra-smooth motion at slow speeds - equivalent to over 3 million 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 that some types of motor are much better than others.

By providing user adjustment 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.

The level of 'cogging' felt in overcoming the detent torque by hand rotation is a good first approximation to the suitability of a particular motor for smooth motion.

Generally, the more modern high-performance motors are better as attention has been paid to this feature, but some older designs of motors are also good by default.

When applying precise digital control, many new aspects of stepper drive and motor performance characteristics previously masked by analogue circuits became apparent, and these were subsequently solved by the development of advanced mathematical modelling techniques.

This 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.

Clearly, this innovative application of DSP technology widens the prospects for stepper control in today's economically sensitive and demanding motion control market.

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