Motor control solutions

Motor control solutions

Motors are becoming more intelligent in response to demands for greater energy efficiency in order for companies to meet green targets and to improve overall production efficiency. The ability to respond to commands sent over a network in real time so that the motor is only running when necessary calls for efficient digital motor control.

 

Effective and efficient motor operation relies on an understanding of the needs of the application and increasingly close cooperation between the control electronics and the design of the motor itself. For example, many precision motion-control applications use DC motors, such as those made by Faulhaber.

 

With DC motors, the most common form of control is through pulsewidth modulation (PWM). This makes it possible to control the current to the DC motor and, with that, control its rotational speed. By increasing the width of pulses during which current flows from the power supply to the motor, the current smoothed by the output capacitance of the circuit increases. Many applications call for AC motors often because they cost less to manufacture and may be used in applications where the requirement for precision is different to that of a DC motor. The speed of AC motors can be controlled using simple techniques such as volts-per-hertz.

 

Traditionally, AC motors suffer high losses and issues with torque at low speeds. Field-oriented control and flux-vector control techniques have emerged as key techniques for increasing the efficiency of AC motors. These techniques use higher-performance MCUs or digital signal processors (DSPs) to implement the more complex algorithms they employ than those typically used to support PWM control.

 

A key benefit of field-oriented control is the improved control of the AC motor at low speed. Under traditional volts-per-hertz controller algorithms, the motor typically needs to run at higher voltages than ideal. This can lead to coil saturation and the motor can overheat. Under flux-vector control uses a mathematical model of the motor’s magnetic fields to better control the relationship between voltage, speed and torque. Closed-loop control algorithms are applied to maximise torque rather than applying force along the axis of the stator, a mode that does not affect rotation but is a side effect of less precision control methods.

 

High processing performance, available on DSP-MCU devices such as Texas Instruments’ C2000, the Infineon Technologies XMC or Analog Devices Blackfin, is required to deal with the challenges that flux-vector or field-oriented control preset. A key issue is that the speed of the rotor inside an AC induction motor does not match the speed of rotation of the magnetic flux driving it. The mechanical speed lags slightly. It is possible to use sensors to detect the physical rotor position at any point in time to calculate the difference but this adds cost.

 

Through the application of DSP power, feedback from the voltages and currents developed within the motor can be used to build a sensorless motor-control architecture. For example, the measured back-EMF can be used to estimated the rotor slip, although there is a computation cost with increasing accuracy. However, the more accurate the slip prediction, the less energy that will be wasted, making processing performance a key consideration for energy-efficient motors.

 

The need for efficient field-oriented control has inspired the design of products such as Texas Instruments’ InstaSPIN-FOC software. Written to run on the C2000 DSP, the software uses sensorless feedback takes advantage of the company’s FAST software sensor for rotor-flux estimation. It provides motor identification, automatic current control tuning and sensorless feedback in a field-oriented control torque controller.

 

As well as the overall control strategy, an important consideration in motor control is the safe delivery of power to the motor itself. Responsive gate drivers, such as the Analog Devices ADuM4135, are essential for delivering on-off commands to power transistors – whether they are MOSFETs or IGBTs – to ensure that PWM or field-oriented control switching algorithms can work at peak efficiency. Development boards such as the BTM7752G from Infineon or the Analog Devices EV-MCS-ISOINVEP-Z provide the ability to prototype high-power motor controller solutions easily.

 

The motor-control portfolios of Analog Devices, On Semiconductor and STMicroelectronics further allow the ability to choose the right isolation strategy for the motor control board to ensure that sensitive communications channels and MCU-level electronics are not affected by spikes and other problems on the high power side. Analog Devices, for example, identifies four key isolation strategies, each of which suits a particular combination of peak motor power and system performance. For example the combination of high-speed communications protocols and earth-referenced, isolated control and feedback ensure effective motor control in high-power situations.

 

Overall, the wide variety of controller and power solutions available ensure that there is a package of components and development support for practically all motion control applications.