[Technology Report]
Motor Control: More Than Just Switching MOSFETs
designing for certain motor-control systems can whet the engineer’s appetite more than others—and present the greatest challenges.
Enter “motion control” or “motor control” into your favorite search engine, and you’ll be rewarded with links to an ad-hoc encyclopedia of solid design information. Freescale’s site (www.freescale.com/webapp/sps/site/overview.jsp?nodeId=02M0zpbnQXGM0zpqCKS2&tid=tMCdr) is broad, deep, and far more than a product selection guide—which it also is. Information ranges from brief descriptions of the different types of electric motors to comprehensive application notes.
That’s just a partial list of resources from controller-IC companies. More is available from the power-semiconductor companies who make the driver chips’ MOSFETs and insulated gate bipolar transistors (IGBTs) that interface to the motors. However, all of this information still fails to provide a picture of the current challenges facing real-world motor-control designers, who must deal with more subtle application issues than feedback-loop response and switching losses.
PRIUS MAKEOVER—AGAIN Controllers for the motor generators in hybrids and the motors in all-electric vehicles provide good examples of complexity. Toyota’s Hybrid Synergy Drive (HSD), which has been in use since the 2004 model year when the company updated the Prius’ original Total Hybrid System (THS), is also used in the Highlander and Camry hybrids, as well as the Lexus RX 400h, GS 450h, and LS 600h/LS 600h, in addition to the Nissan Altima hybrid (Fig. 1). For the Prius’ 2010 model year, the engine has entered its third iteration, offering a choice of driving modes—drivers can opt for more zip or higher economy.
As in previous generations, it’s a driveby- wire system. The gas and brake pedals and the shifter connect to the engine control computer. The brake pedal also operates front disk and rear drum brakes that back up the regenerative braking part of the drive.
The HSD replaces the conventional drive train with a pair of motor-generators (MG1 and MG2), a “power splitter” differential gearbox, and a battery pack. Regenerative braking works by putting MG1 and MG2 into generator mode, which captures kinetic energy to recharge the batteries while arresting the vehicle. MG1 is also the starter motor for the gas engine, supplying electrical power to drive MG2. As its speed and load vary, it controls the transaxle’s continuously variable transmission (CVT) as well.
MG2 and the engine work together to drive the wheels. In fact, the elegantly clever aspect about the HSD design is the way the transmission allows the mechanical power from the engine to be split three ways: extra torque at the wheels (under constant rotation speed), extra rotation speed at the wheels (under constant torque), and power for the MGs in generator mode. This is where the motor control comes into play. The controller drives mechanical actuators to direct the power flow. It’s like a mechanical CVT that uses an electric motor instead of a cluster of gears.
Not that the drive is totally gearless. A transaxle mixes the torque from the engine and MG1/MG2 at the final stage. MG2 couples torque into or out of the drive shafts. MG1 and the engine share a differential that relates their RPM to the RPM of the wheels, and MG1 absorbs the difference between wheel and engine RPM. That’s the mechanical “power-split” part. Sensors also feed back data about RPMs and torque to the control computer.
What’s the point? The design enables M2 to provide power boosts that allow for more acceleration than the fairly anemic gas engine could otherwise put out—and that engine is there to provide nice, high fuel mileage numbers. Regenerative braking is really the source of the fuel efficiency.
Then there’s the advantage of being able to turn off the gas engine at traffic lights and use the electric motors for good, high-torque acceleration as soon as the light turns green. That’s also why there are two motor/generators. MG2 gets you going, and MG1 cranks the gas engine so it’s running when it’s time for it to take over.
Other cool aspects include low- and highgear emulation and “engine braking.” Lowgear emulation, naturally enough, deals with acceleration at low speeds under gas-engine power. The driver wants reasonably snappier acceleration than the limited torque available from the gas engine. But while the torque isn’t available, RPMs are, so the gas engine is allowed to rev higher.
The extra rotational speed, powering MG1 in generator mode, then powers MG2, which “helps” the gas engine. The total torque boosts acceleration. It sounds like perpetual motion, but it’s just using electronics to duplicate the effect of a conventional transmission’s lower gear ratio.
High-gear emulation is the complementary process. Tooling along the flats at high speed, the engine has more torque available at the high end of its RPM range. The controller uses that to run MG2 as a generator and feeds that to MG1, which keeps the wheels going at the desired rate while providing enough energy to overcome aerodynamic drag. When you need to pass, or come to a hill, the controller adds battery power. In fact, the controller is always dynamically shifting power between the gas engine, the two motor-generators, and the battery.
Compression braking isn’t an emulation mode. Coasting downhill, the driver can select “B” on the gear shifter, drawing power from MG2 and shunting it to MG1. This raises the engine RPM with the throttle closed, burning off kinetic energy (Fig. 2). The controller also does this automatically when the battery is at full charge.
EVOLUTIONARY STEPS The HSD debuted in 2004 and was named International Engine of the Year. It also has won the Green Engine of the Year title in subsequent years. For the third product generation (starting with the 2010 model year), it has been reengineered. More than 90% of its components have been updated to make it lighter and more compact and give it more power, at the same time further cutting fuel consumption and improving cold-weather operation.
I just purchased a 2010 Pruis over the weekend and the three way power options are vary nice to select the power needed for a given driving scenerio. The EV mode is very limited in range and I have yet to use it for more than a few hundred feet before the system is switched to gas engine mode. The ECO mode is the most usefull, this switches between EV and gas mode and adjusts for the power needed for acceleration or hills. The power mode switches to gas/electric boost and has more power all the time.
Anonymous -June 30, 2009
"It’s like a mechanical CVT that uses an electric motor instead of a cluster of gears."
This shows that the author does not fully understand the hybrid synergy drive. The "power split device" is a planetary gear set, which could be described as a "cluster of gears". It is the proper application or subtraction of torque by MG1, MG2, and the ICE to the sun, ring and planet carrier GEARS that creates the CVT in the Prius.
Anonymous -June 29, 2009
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