DSPs stimulate advances in motor control

The real-time computing power of digital signal processors (DSPs) is being harnessed to increase the efficiency of electric motors and David Melder outlines the benefits they provide.

The number crunching ability of digital signal processors in the control of motors offers a number of powerful advantages over traditional microcontrollers.

While microcontrollers are either general purpose or optimised for control functions, the strength of DSPs is that they are designed for high performance, numerically intensive tasks, which enables them to perform in software many functions that have previously been carried out by expensive hardware.

This leads to lower system cost, together with increased performance and power efficiency.

The advanced algorithms required for real-time control of devices such as motors are very demanding of processing power, and it is only comparatively recently that the cost of DSPs has fallen to make these solutions economic.

Furthermore, as DSP technology continues to advance, the chips are becoming so powerful that they have increasing amounts of spare processing capacity, so that they can take on additional 'intelligent' functions and background tasks.

* DSP advantages - as an example, bulky temperature sensitive Hall-effect sensors have been traditionally used to determine the position and speed of the rotor in brushless DC motors.

However, by using high speed algorithms such as Kalman filters, DSPs can calculate the rotor's position in real time from the voltages and currents induced in the phase windings by the rotating magnetic field.

Another advantage is ultra-smooth operation for stepper motors.

A DSP can directly generate waveforms of the precise shape and frequency optimised for the particular motor design, without requiring any of the intervening analogue processing used in traditional designs.

This ensures operation is smooth and quiet due to reduced jitter, and opens up the potential for stepper motors to be used in a much wider range of applications.

This smart control enables the motor to operate more efficiently, which frequently means that a smaller motor can be used, producing savings in space and cost.

The cost of the associated power electronics is also reduced.

A number of other benefits are also possible, such as increased stability and reproducibility because of the elimination of analogue signals; lower component count, producing enhanced reliability; operation over much greater speed ranges; higher efficiency, resulting in lower motor temperatures at high torque; and sophisticated feedback and error reporting.

In addition, greater control over intellectual property is possible since the source code can be protected.

* Increased processing power - the diagnostic output available from DSP controlled stepper motors can also be used to optimise motor performance in real-time applications.

This can be achieved by providing continuous feedback to the drive power supply in order to optimise the electrical input to the motor windings.

In the event that a failsafe shutdown is triggered, the diagnostic reporting can provide details of the winding voltage and current at the point of failure.

The nature of the error mode can be reported by the diagnostic display, eg over voltage, over temperature, over current, open circuit, phase short circuit, DC bus voltage ripple or CANbus error.

To suit the needs of specific applications, such as textile, packaging or printing equipment, intelligent DSP drives can be programmed with standard profiling commands and controlled in a fieldbus net (e g, CANopen).

The transition from analogue to digital control is enabling the characteristics of stepper motors and drives to be studied in more detail so that their performance can be refined, leading to a much broader range of motion control applications, in particular those requiring smooth, quiet and reproducible precision.

* About the author - David Melder is with Astrosyn.

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