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issue: December 2009 APPLIANCE Magazine

APPLIANCE Engineer - Motor Controls
Motor Control Doubled


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 An MCU boasts improved operational efficiency and enough computational power to drive two motors.

A high level of computational capability allows the new 32-bit MCU to control two motors with a single IC.

Toshiba America Electronic Components Inc. (TAEC; Irvine, CA, U.S.; www.toshiba.com/taec) recently launched a 32-bit MCU designed for high-efficiency AC and BLDC motor applications such as washing machines, refrigerators, and air-conditioners. Said to be the first ARM 80-MHz Cortex-M3 processor–based MCU to include a hardware Vector Engine (VE), the TMPM370 device allows appliance engineers to use field-oriented control (FOC) techniques effectively to optimize motor efficiency. Operational efficiency is improved as a result, enabling lower-cost motors to be used. Moreover, simultaneous vector (field-oriented) control of two permanent magnet synchronous motors can be achieved with a single MCU, thanks to the additional computational power.

“Field-oriented control requires conversion between Id/Iq state space coordinates and physical voltages and currents. This conversion is computationally intensive, adding to the list of the many tasks the processor needs to handle,” says Steve Williams, product manager at TAEC. Toshiba created the Vector Engine by identifying certain tasks specific to FOC motor control that could be implemented in a hardware coprocessor. The VE takes over computation and real-time control functions that can bog down the main CPU, such as the vector space transformations, PI control loop tasks, and space vector modulation. In the end, William says, “it reduces the load on the CPU by 50%.”

Because the TMPM370 offloads the coordinate conversion, sequencing, and PI control loop tasks to the hardware VE, the CPU can focus on analyzing the dynamic state of the motor and planning ahead to compensate for load changes or to implement a variable speed profile. For example, “Is the motor changing speed in accordance with plan? What compensation is needed? Are there any early warnings of a load shift (in a washer) or pressure-wave oscillations (in a compressor)? The software that does this analysis can be focused on it without having to baby-sit the motors as well. As a result, these tasks are simplified,” says Williams.

The simplification of those tasks also yields benefits for software engineers. “The TMPM370 makes the routine tasks of field oriented control truly routine. The integration of analog input conditioning, along with flexible ADC triggering and the Vector unit, lets the engineer concentrate on the part of the product that is unique to the application,” Williams tells APPLIANCE. “The engineer saves design time during development and the execution time in the final product to ensure that he or she has enough time to spend on the portions of motor control that set the product apart, such as velocity and position estimation and speed control.”

The new 32-bit MCU runs on a single 5-V power supply and has 5-V, fully compliant outputs. It is the first of the firm’s 5-V family of Cortex-M3 processor-based MCUs for industrial and home appliance control. “With its 5-V VCC, TMPM370 has inherently better noise immunity than similar ICs with 3-V VCC,” says Williams. The MCU also has other system-level features to help mitigate against noise. “The PI controller integrates a number of input samples over time, which will tend to filter out some noise. The timing of the ADC triggering can be varied to work around synchronous noise. And, of course, the CPU can be used to run noise filters like a Kalman filter,” he adds.

Williams says integrating the new MCU into an existing product should be quite simple at the hardware level. “TMPM370 has programmable gain on the analog inputs so it can be interfaced to a range of sensors. Typically, all that is needed is to adjust the values of a few biasing resistors.” The software task, on the other hand, will require more effort. “But reference code and our TMPM370 starter kit will help get the motors spinning quickly so you can get right to work on porting the control algorithms and motor models. ARM development tools are quite advanced, and the TMPM370 includes real-time debug and trace support.”

 

 

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