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issue: June 2006 APPLIANCE Magazine

Motor Technology
Blended Control


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New digital signal controllers (DSCs) combine the benefits of microcontrollers and digital signal processors to provide cost-effective, efficient motor control.

The two new 16-bit DSCs—the dsPIC30F5015 and dsPIC30F5016—are said to have the “heart” of a 16-bit Flash microcontroller (MCU), with robust peripherals and fast interrupt handling capability, and the “mind” of a digital signal processor (DSP) that manages high-computation activities. Part of the dsPIC® family developed by Microchip Technology Inc. (Chandler, Arizona, U.S.), the controllers boast several features, including performance speeds of 30 million instructions per second (MIPS), advanced pulse width modulation (PWM), 1 MSPS 10-bit A/D converter, and full-speed operation using an internal oscillator.
Steve Marsh, product manager of the Digital Signal Controller Division, says that the new controllers’ top benefit, however, depends on the application. Washing machine engineers, for example, may benefit most from the devices’ low cost. “Performance allows ‘sensorless’ techniques to reduce the cost of Hall-effect sensors, and in some applications, a lower-cost motor can be employed using the compensating performance of the DSC,” he explains.
An air-conditioner engineer, on the other hand, might find the devices’ efficiency to be the key attribute. According to Marsh, variable-speed compressors for air-conditioning and refrigeration require complex control, and the power of a DSC is the optimal solution. “For example, Field Oriented Control (FOC) methods can help the efficiency of most applications,” he notes. “This is due to the FOC software algorithm always keeping the rotating field 90 degrees ahead of the actual rotor position. This results in the best torque and, hence, best efficiency from the motor. It takes the speed and fast math capability of the dsPIC DSC to do this, especially at higher motor RPMs.”
Marsh notes that blending MCU and DSP capabilities was no easy task, as the technologies differ in many ways. For example, microcontrollers are interrupt-intensive while DSPs are flow-centric. “Constant interruption of a DSP will degrade performance significantly,” he says.
To address this challenge, Microchip architects developed a list of strategies to reduce interrupts by installing buffers on peripherals or direct memory access. “If you do get an interrupt, you at least want it to be fast,” Marsh explains. “Deep stacks, which impact the interrupt latency of competitive solutions, were not implemented on these DSCs. The result is a fixed five-cycle latency to enter an interrupt, and at 30 MIPS, those five cycles are achieved very quickly in real time.”
Another design feat was the high-performance PWM peripheral. “It comprehends that there are different motor types and that even within each category there is a lot of variation between individual motors,” Marsh says. “Overlay that with the myriad of possible control strategies and you have an architectural challenge that is not for the faint of heart. A lot of time, customer input and inherent talent was applied to this module to make it as near to universal as possible.”
The end result is a 16-bit controller that features eight-output PWM, with complementary or independent modes, four duty-cycle generators and programmable dead time. The controls also have
66 KB of Flash program memory and are available in 64-pin and 80-pin packages.

Suppliers mentioned in this article:
Microchip Technology Inc.
 

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