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issue: May 2005 APPLIANCE Magazine

Sensors and MCUs
Designing with DSPs


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APPLIANCE Best Practices Digital Signal Processors (DSPs) can play an important role in appliances. Manoel Soares, senior engineer at Whirlpool Corporation’s R&E Center, Advanced Electronics Applications, explains how to work with these powerful devices.

“More and more we are looking at home appliances from a ‘Signal and Systems’ perspective,” Mr. Soares says. “The availability of measuring instruments with faster data acquisition and more sophisticated signal processing features has allowed our engineers and technicians to take a closer look at washing machines, refrigeration, and other appliances, which, quite often, can be classified as multi-variable, non-linear time-varying dynamic systems.

“For example, a washing machine during spinning is subject to vibration caused primarily by unevenly distributed clothes inside the washer drum/basket. This vibration translates into oscillatory signals whose characteristics can be measured to determine the amount of off-balance present, and therefore, provide a remedial action. This could include redistributing the clothes or limiting the final spin speed to guarantee noise and vibration levels that are acceptable to the consumer.

“Measuring the signal characteristics is no trivial task, though. Sometimes the off-balance information will occur lumped into a single spot in the frequency spectrum of the signal. But sometimes it will occur spread across different frequencies. These may or may not be known a priority. Thus, some means of finding them is necessary.

“When there is a need for decomposing sampled analog signals into their respective frequency spectrums, the FFT (Fast Fourier Transform) is the method of choice, provided that computer memory and bandwidth are plentiful and available. Although microcontrollers in that category may exist, their programmers soon realize that implementing a simple FFT can be quite wasteful, therefore compromising the rest of the embedded software design. A DSP is a device that has its architecture designed around the requirements of the FFT and many other digital signal processing techniques.

“However, not all DSPs will meet the requirements for our candidate appliance applications. At a minimum, a 16-bit CPU running at 40 MIPS (million of instructions per second) is required. Beside it, the peripheral set must include a versatile module for PWM (pulse width modulation) generation, another one for the capture of digital signals, fast ADC (analog-to-digital converter) with multiple channels, and a SCI (serial communications interface). The product family design maturity and future development path is considered along with the relative acceptance by the industry.

“It is also important to consider good technical support from the supplier, especially because DSPs are relatively new to the appliance industry. For instance, many vendors will offer extensive software libraries containing benchmarked FFT and Digital Filter algorithms, ready to be linked to your software project. Oftentimes these libraries are already implemented in the DSP’s assembly language, thus guaranteeing optimal usage of CPU resources.

“A new trend among best practices when selecting DSPs considers the development environment. Real-time debugging and visualization capabilities are considered high-priority on our list. So we always remember to choose tools with plenty of extra memory for data logging, D/A converters, and communication interfaces—not to mention robustness to noise and availability of reference hardware designs for motor applications. Automatic code generation tools for DSPs and other embedded targets is another growing trend, yet the less expensive devices may suffer from bandwidth and memory starvation due to an extra amount of instructions created as a consequence of not manually writing them.

“Nevertheless, there is a huge amount of experimentation taking place in our labs, and we have to be able to prototype faster without having everyone become proficient in DSP assembly programming. At Whirlpool, all of this information and more is collected, and a comprehensive method of prioritizing and rating each candidate is used to make the final component selection.

“In addition to FFTs, signal conditioning, and other combined functions, DSPs are typically well suited for the so-called ‘sensorless’ applications, where expensive mechanical sensors are replaced with an algorithm in software that serves the same purpose. In our advanced motor control development, we no longer use a tachometer to measure speed of three-phase induction motors. Instead, lower cost and more highly reliable shunt resistors are being used to measure the motor currents. By using proprietary algorithms, we have successfully estimated the same motor speed without any loss of washing performance.

“The sensorless approach has also been accomplished with small brushless permanent magnet motors that run compressors in high-efficiency refrigerators, where the rotor position is determined by software. DSP-based systems also lend themselves very well to more advanced diagnostic systems. As we expect the cost of DSPs to keep going down and even as a result of more powerful 32-bit architectures becoming available, we will continue to look deeper and deeper at the signals to reveal new opportunities to provide more features and value to our customers. We have a pipeline filled with researched innovations and are now looking forward to moving them into the product development teams.”

 

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