Standard MCU simplifies efficient motor control

Standard MCU simplifies efficient motor control

MCU-driven field-oriented control yields responsive speed control and optimized efficiency

BY VINCENT ONDE
STMicroelectronics
Rousset, France
http://www.st.com

When discussing energy usage, a commonly cited figure is that more than half of electrical energy is used to drive electric motors. Industry is heavily contributing to this, mostly with integral horsepower ac induction motors, but it is very likely that household appliances will also contribute significantly in the coming decades, with their increasing adoption in emerging markets. These two market segments have very different requirements with regards to power rating, lifetime, and connectivity of the motors.

Still, in terms of energy savings, a domestic air-conditioning unit and a conveyor for the paper industry do have some similarities. They are both often powered-on 24 hours a day, and every percentage of efficiency gained on the motor and drive will represent a significant number of kilowatt-hours saved.

Motor and drive topology

Drive efficiency can be improved by several means, which are often combined. If we discard process level improvements (such as replacing fixed-speed mechanically controlled processes with variable-speed control), the motor topology comes first.

Brushless motors are by far the preferred choice, either three-phase ac induction in their high-efficiency form (with around 80% efficiency in best cases), or even better permanent magnet synchronous motors (PMSM), which can reach more than 90% efficiency (see Fig. 1 ). The PMSMs are less widely used, but they are gaining market acceptance thanks to their better power-to-size ratio and high torque at startup.

Fig. 1. An efficient PMSM control design needs high-performance processing.

Pricewise they are becoming competitive, partly because they are less sensitive to the booming copper price, as they require a smaller quantity of copper compared to an ac induction motor. Their rotor flux comes from a magnet and does not need to be induced by stator windings.

The downside is that these motors do require a complex drive circuitry — basically consisting of an inverter power stage made up of six switches and a processor able to control them in real-time. Considerable efforts have recently been made to decrease dynamic and conduction losses of switching elements (power MOSFETs or IGBTs), and modern power converters are typically more than 95% efficient in their nominal operating conditions.

Find the best control strategy

The last, but not least, step is to adapt a given drive plus motor to its load, and choose the most adequate control strategy. This is where the processor is crucial, and hardware design techniques are complemented by algorithms and software.

The simplest way to achieve variable-speed operation for a three-phase ac motor is to implement a so-called voltage/frequency control (or scalar control), where the principle is to maintain a constant ratio between the frequency and the voltage applied to the motor. This produces a constant stator flux, and nominal motor torque is available on the rotor shaft.

This is a very popular control method for low-cost drives that are suitable for applications having well-known load characteristics and are not very demanding in terms of control bandwidth (such as low-horsepower pumps and fans). Unfortunately, not all applications can afford such simple control and its limitations.

Since it is based on steady-state motor equations, scalar control does not guarantee the optimum motor behavior (torque, efficiency) during transient operations. To overcome these limitations, other control strategies have been developed, taking into account the dynamic characteristics of the motors.

The field–oriented (also called vector) control is the most widely used. It allows driving any ac machine (either induction or permanent magnet based) in the same way as a separately excited dc machine, with two decoupled control variables. The magnetizing current, Id , corresponds to the dc main flux and Iq controls torque just like the armature current does in the dc motor.

Field-oriented control allows precise and responsive speed control when the load changes, and guarantees optimized efficiency, even during transient operation, by maintaining the stator and rotor fluxes in perfect quadrature. Energy optimal control strategies can then be added on top of vector control, by reducing magnetizing flux in case of low load conditions — typically below 60% load torque.

Vector control algorithms must be continuously recomputed, at a rate of between 1 and 20 kHz, depending on the final application bandwidth, which requires intensive numerical computations, such as trigonometric functions or PID control. This is made even more complex when sensorless operation is requested, both to cut cost and boost reliability. This is why DSPs, MPUs, and FGPAs were used in the past as controllers.

High-performance processors

High-efficiency motor drives must have very good real-time responsiveness (low interrupt latency), pure processing capabilities (such as single-cycle multiply or HW divide), as well as excellent performance for control (for conditional branch and nonsequential execution flow).

Cost-effective industry-standard MCUs are now gaining the performance and peripherals required for high-efficiency motor control. An example is STMicroelectronics’ STM32 32-bit flash microcontroller. It features a 72-MHz ARM Cortex-M3 that can perform a sensorless field-oriented control loop for a permanent magnet motor in less than 22 µs, corresponding to 22% CPU load at a 10-kHz sampling rate. With more than 75% headroom for the rest of the application, this shows that standard MCUs now have the performance to be viable alternatives to proprietary DSP or digital signal controller architectures.

As for peripherals, the processors A/D converter is a key element of the system — the most sophisticated algorithm will not be able to compensate for poor analog measurements. To the overall drive performance often depends on A/D quality. The STM32, with two 1-Msample/s 12-bit A/Ds each with a typical accuracy of 3 LSBs over the full temperature and voltage range, is able to achieve simultaneous conversions with sampling times below 125 ns.

Furthermore, when reading the current in the motor phases, one must deal with the noise created by the transient voltage on the power switches (typically hundreds of V/µs in off-line applications). The solution is to synchronize the A/D with the timer controlling the power stage. Given that the commutation point can be anticipated (defined using the three PWM timer’s compare registers), the A/D conversion can be triggered slightly before or after this event using an extra compare channel of the PWM generator.

As a side effect for efficiency, this synchronization allows a 100% modulation index (full dc bus voltage usage) above a 10-kHz switching frequency. For a given power rating, having a higher voltage available for the motor means a lower current and therefore lower losses in both the converter and the motor.

To complement the A/D converter on the control side, the STM32 includes an advanced control timer with state-of-the-art features for three-phase motor-control PWM generation. It has a 16-bit counter with auto-reload capabilities and up to a 72-MHz clock, an up/down counting mode for optimum switching losses versus acoustical noise tradeoff, six outputs with programmable polarity, and provisions for dead-time insertion and emergency shutdown input for glueless interface with the power stage.

As mentioned, a fourth timer channel is available on the PWM generator, commonly devoted to the A/D trigger and used to apply a pre- or post-delay to precisely position the sampling points versus the PWM edges. Here again, this timer is able to support energy optimum sine wave generation schemes, such as space vector modulation (maximizing motor voltage with a given dc rail) or clamped modulation (minimizing the number of PWM switching components in high-power converters).

Development kits are key

Most processors have a motor control evaluation kit available with all the necessary hardware for a quick evaluation, from the JTAG probe to the permanent magnet motor and a versatile power stage. The STM32 design kit includes sensorless field-oriented control source code for no cost, an intuitive human interface on the kit’s LCD display, and easy-to-customize header files for tuning the motor control libraries. They support both ac and PM motors, with various kinds of feedback sensors and current measurement techniques, and allow developers to focus on their application and saving every watt-hour possible. ■

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