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issue: October 2006 APPLIANCE Magazine - Part 2: Motors & Air-Moving Devices

Motors and Air-Moving Devices
It Doesn’t Take a DSP

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Figure 1.

A few years ago an ARM-powered refrigerator or washing machine would have seemed unrealistic—this processor architecture was mostly used in cell phones and PDAs, where its computational performance and lower power consumption were highly appreciated. But things have changed, and 32-bit ARM-based devices may soon pervade household appliances, motivated by two main factors.
First, appliance requirements continue to evolve. New display options such as dot matrix LCDs demand more memory, particularly when several languages must be supported. A higher-performance MCU with more memory is needed for the options expected in today’s, and tomorrow’s, appliances.
The second factor is the emergence of ARM processors as a standard for embedded applications, such as those now seen in appliances. The controllers are designed to have a simple and effective architecture and are being built with large peripheral sets and proposed with wide memory and pin-count portfolios, while prices can be competitive. A range of tools, a growing developers’ community, and the ARM7 direct migration path to ARM9 products add to its versatility for medium/high-end home appliance platforms.
But how well does the ARM core fulfill motor control requirements such as vector control algorithms? Isn’t a proprietary, dedicated DSP typically used for those functions?
Widely used in high-performance drives, vector or field-oriented control algorithms are designed to provide precise and responsive speed control and guarantee optimized efficiency during transient operations. They also have the advantage of using the same framework to control either an asynchronous or synchronous motor, making it easier for development teams that must deal with various motor types and applications. Additionally, most sensorless algorithms are based on field-oriented methodology, which is also of interest when trying to increase cost-effectiveness of the drive.
Vector control relies on the fact that by changing reference coordinates from fixed stator coils to the moving rotor frame, the equations describing the motor are drastically simplified. “Clark” and “Park” transformations convert variables with fixed 3-axis, 120-degree shifted coordinates into 2-axis orthogonal rotating coordinates. These last variables are DC, or slowly varying values, which can be regulated by means of simple PID controllers and are then transformed back to the fixed stator windings frame using reverse transforms, as shown in Figure 1.

Figure 2.

The inner control loop, in charge of current components regulation, is typically executed every 100 µs, while the speed control loop is in the milliseconds range. The control flow requires intensive math computations (trigonometric functions, multiple PID regulators, speed calculation). The minimum resolution required for the main control variables is 16-bit, with a need for 32-bit intermediate results, such as integral terms. Finally, some free CPU load must be kept for the remaining applicative tasks, such as communication and user interface.
This explains why fast and powerful processors are mandatory for vector drives, and the usual market offerings are 16/32-bit MCUs, or DSPs. These last are usually associated with advanced motor control, but a standard 32-bit ARM7-based MCU could perform just as well.
An embedded ARM7TDMI core running from on-board flash at 60 MHz can execute a tachogenerator-based field-oriented control loop within 30 µs, which gives a 30-percent CPU load with 10 kHz sampling rate. The ARM architecture is mostly responsible for those results. The barrel shifter, for example, provides an optimized variable resolution all along the processing flow and can change format within a single clock cycle. Some multiplication is also sped up; r0= (r1<<4)-r1 can be executed in one instruction while r1=15*r0 requires several.
In general, low-cost DSPs have 16-bit fixed-point cores that are stalled by multiple 16-bit loads, whereas the ARM7’s 32-bit data path avoids multiple loads when dealing with the integral terms of the PID regulators or when extended precision is needed. Moreover, hardware looping and double-addressing modes, common features in DSP architectures, are irrelevant for motor control. The ARM7 core, equipped with peripherals dedicated to motor control, can well serve the appliance motor control market.
Illustrated in Figure 2, such an ARM7TDMI core-based device providing motor control peripherals minimizes the overall system cost, while fulfilling the necessary high-performance requirements. A device such as the STR750 from STMicroelectronics, supplies the following benefits:
• a 60 MHz clocked PWM generator, with dead time insertion, asymmetric PWM generation capabilities
• a 4µs 10-bit ADC, with channels scan and sophisticated PWM triggered sampling
• safety features such as emergency stop input or protected critical registers
• peripherals to implement connectivity, power factor correction, etc.

This information provided by Vincent Onde, applications engineer for STMicroelectronics.

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