DSPs in Motors: Are you kidding? Why bother?
by Paul G Schreier, Consulting Editor
Over the past year or two, there's been a distinct trend towards the development of DSPs targeted at niche markets. Most of the ones I recall are for high-volume consumer markets such as settop boxes, audio players and, most recently, digital still cameras. In those cases, I can clearly appreciate the value a DSP offers in being able to accelerate algorithms vital to the application. But then I see some other dedicated DSPs and wonder just what is going on. For instance, motor control. Motor control? C'mon! Isn't putting a DSP into a motor controller just a little bit of overkill? Sort of like putting a Pentium III in a toaster? To find out what's going on, I did a little refreshing from my college days and also tapped into some people knowledgeable in the latest DSP trends.
Aren't motors pretty much the same as they've been for years and years? Isn't there only so much you can do to a rotor or a stator or an armature winding? I'm also thinking back to my time in the heavy machinery lab during my undergrad studies more than 25 years ago. We would take jumper cables with conductors the thickness of your thumb and switch them around to account for different speeds and loads -- but do you need to replace those simple actions with a DSP?
Again I'm thinking back to some basic principles. With a traditional 3-phase AC induction motor (squirrel cage), the primary way to control its speed is by changing the frequency of the voltages that excite the windings. That fact, however, makes it difficult to vary the speed unless you have an expensive controller that knows how to modify those voltages. Also, if you're running these motors from single-phase line voltages such as in the home, you must also add an inverter circuit that generates and feeds the three phases properly. Further, these motors don't work well at low speeds, and their positioning control isn't adequate for many high-precision applications. On the plus side, AC induction motors are one of the most reliable electromechanical things mankind has ever designed, they can be quite inexpensive to produce, and I've seen statistics that say that more than 90% of all motors are of this type. You'll find them in many home appliances as well as in industrial settings.
The big advantage of the other major class of motor, the DC motor, is that it offers easily variable speed and torque capabilities. Control can be quite simple -- on a brushed DC motor, the higher the armature voltage, the faster the rotation, while output torque is proportional to current. Note that most industrial DC motors operate reliably over a speed range of about 20:1, partially because they have added heat-dissipation features that allow operating speeds lower than those on an AC motor. However, DC motors typically can't provide the high torque needed in some applications.
A big drawback, though, can arise when you add physical contact to power the windings in the rotor. Any motor must create a field in the rotor that reacts with magnetic fields in the stator. Here DC motors generally follow one of two approaches: those with electromagnetic windings require brushes to commutate the voltages to windings on the rotor; the alternative consists of rotors that instead use permanent magnets and that perform commutation electronically rather than through physical contact. Because of its simplicity, cost effectiveness and reliability, the brushless version has become the motor of choice for applications requiring fractional horsepower DC motors.
I'm not trying to present a detailed tutorial on motors here. What I'm still trying to do is think through where and why you might need the horsepower of a DSP. We've been building and controlling motors for well over a century and they all seem to be spinning nicely, thank you. So what's the problem? To get a better perspective, I spent some time talking with Dr Aengus Murray of Analog Devices Inc. He's the Product Line Manager for Motor Controls in the Embedded Control Systems Group.
He explains that DSPs are enabling a major transition for applications that require the best of both worlds -- variable speed and torque with high performance -- all while slashing energy costs, using AC induction drives. The previous discussion might have you asking how that's possible. Thanks to accurate mathematical models of motor operation and the knowledge of how to set voltages and currents on the stator, it's possible to run AC induction motors at variable speeds.
Years ago researchers came up with sets of equations that describe the induction motor. But only with the arrival of DSPs has it become possible to solve these equations in real time to implement a scheme known as sensorless control. Instead of sensor-based feedback on motor position, the latest systems use these mathematical models to predict what the voltage and current on the drive voltages should be. The controller then compares them to actual measured values; the error helps the algorithm calculate a correction signal. Note, though, that these motor models can also change with differing conditions of load and other operating variables. As you can see, the computations involved in performing this modeling in real time can become quite involved and complex.
The latest DSP-based motor-control ICs are up to the job. The chip reads the current or voltage and, using the control equations, calculates the required winding voltage magnitude and frequency to operate the motor at the desired speed. To vary the magnitude of the motor voltages, a PWM modulation circuit controls the on/off duty cycle of the power-inverter switches. Now you've got a variable-speed AC drive.
Do these controller chips add lots of cost to an application? Surprisingly little. Dr Murray talks about one of ADI's devices that integrates a DSP core, an A/D to read feedback signals as well as a block that generates the PWM control outputs -- and it sells for roughly $15 in large volumes for a high-end chip with 12-bit resolution needed for machine-tool apps where accuracy and wide dynamic range in positioning is mandatory. But for your hair dryer or refrigerator, controller chips with a 10-bit A/D can sell for $3 in quantity.
Hold on a minute -- a refrigerator? Nothing terribly sophisticated here, right? The motor runs and keeps the icebox chilled. Isn't that one of those cases I started off this column with where the phrase "technology overkill" comes to mind? Well, How does the prospect of a 30% energy savings sound? That's possible, says Dr Murray, by switching from a fixed-speed compressor motor to a variable speed DSP-controlled motor.
Let's look at some major contributing factors to these energy savings. First, note that a standard compressor runs at a set speed, perhaps 3600 rpm, and turns on and off to meet cooling requirements. It's much better to run the compressor at a lower speed but without constant starts and stops. Second, these motors can work with permanent magnets, so there's no need to send current through windings to generate a field, thereby cutting energy requirements. And let's not forget about the power-factor correction these DSP algorithms can implement. These items all combine to achieve that 30% energy savings. And with the brownouts we're seeing this summer and can expect for the next few years, that number is starting to sound quite nice.
The advantages are becoming clear to me now. You'd think that appliance designers would be jumping on this new technology. Apparently they're starting to, and according to Murray this is the year the trend is really taking off. By the end of the year he expects that ADI will have shipped more than 1,000,000 DSP-based motor controller ICs. He estimates the total number of compressors being manufactured worldwide at 80 million, but of those roughly 2 million will have variable-speed motor controllers. There seems to be a big market to be tapped, and I'm starting to realize why manufacturers of DSPs are so excited by what might initially seem a mundane niche.
Meanwhile, you might want to learn more about this technology. ADI has set up a web page (http://www.analog.com/motorcontrol) with plenty of informative white papers and datasheets. Recently the firm added an online motor-control seminar that you can "attend" whenever you have a spare moment and that you can run through at variable speeds.
chipcenter.com
Jim |
