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Motor Control: More Than Just Switching MOSFETs

June 18, 2009
Motor-control design, with ilustrations from the Prius and Tesla Roadster

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.

There are 49 app notes on the Texas Instruments “high-voltage” motor-control site (focus.ti.com/docs/solution/folders/print/195.html). Analog Devices (www.analog.com/en/industrial-solutions/motorcontrol/applications/index.html) offers app notes, tech articles, and videos. Microchip has its motor-control center (www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en026178) online, and it regularly conducts classes at its regional training centers.

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.

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Displacement is now 1.8 liters, but the gearing was changed to lower RPM at high speeds, for a combined-cycle fuel economy of 72.4 mpg. Carbon-dioxide emissions are 89 g/km, beating the Euro 5 standard. Obviously, when operating as a purely electric vehicle, (speeds up to 30 mph), it produces zero exhaust emissions.

Toyota calls it a “synergy” drive because it can power the Prius exclusively from the gas engine, exclusively from the electric motor, or by powering the drive train simultaneously from both sources. Other hybrids use either a “parallel” arrangement, in which the electric motor is constantly in use as an auxiliary source of power for the gas engine, or a “series” topology, which uses the electric motor exclusively to power the drivetrain. Toyota says that its approach delivers the benefits of both.

The new version ups the displacement of the “variable-valve timing with intelligence” (VVT-i) gas engine to 1.8 from 1.5 liters. The engine generates 97 brake horsepower, or bhp (98 DIN hp) at 5200 RPM and 142 newton-meters (Nm) of torque at 4000 RPM, and it delivers higher torque at lower RPM—a reduction of 300 RPM at 75 mph. Toyota also says it’s quieter and delivers a 10% improvement in long-distance cruising fuel economy.

A new, more compact transaxle is 40 lb lighter and benefits from a 10% to 20% reduction in driveshaft energy losses compared to the current system. A new, cooled exhaust gas recirculation system boosts fuel efficiency and reduces emissions. Also, the new version of MG2 is bigger, rated at 60 kW/80 bhp. That’s 20% higher power, yet with a 33% size reduction.

The new Prius engine heat-management system improves fuel economy in cold weather as well as cabin comfort, employing a heatrecovery system and an electric water pump. Using an electric system in place of the waterpump drive belt reduces mechanical losses. As a result, the coolant flow rate can be controlled with greater precision for better fuel efficiency.

In the engine control unit, the new inverter is 37% smaller and 36% lighter, and it runs at a higher switching rate. Output voltage to the motors is now 650 V ac, up 150 V from generation 2. Similarly, the battery, still nickel-metal hydride (NiMH), has increased capacity up 2 kW to 27 kW. The battery also has been positioned to minimize its impact on cabin space.

The cooling system’s efficiency is improved with a significant increase in fan capacity. The controller has evolved, too. Drivers can now select EV, ECO, or POWER on the transmission control (Fig. 3). That’s electric-only (formerly available only as an aftermarket option in the U.S., though standard overseas), plus an economy/performance tradeoff option.

TESLA COURSE CORRECTION Mission does matter. The Tesla Motors Roadster is an entirely different beast from the Prius. To get some insights on Tesla motor control, I pulled some info from the extensive blogs maintained by firmware engineer Greg Solberg and chief technical officer J.B. Straubel.

Solberg notes that Tesla’s motor-controller algorithms handle both driving and regenerative braking. Throttle pedal position generates a torque command that’s applied to the motor, and the command can be either positive or negative, causing either acceleration or braking.

Interestingly, he says that since negative torque applied to the rear wheels can make the vehicle unstable (weight transfers from the rear to the front wheels), the Tesla controller uses the traction control system to limit regenerative braking if the rear wheels start to slip. In other ways, the control algorithms are like those in the Prius. For example, if the battery is so close to full charge that any additional charge from regenerative braking would cause a problem, the controller limits regenerative torque.

Solberg also discusses how market research affected the development of the algorithms. Most people “like the car to regen when you take your foot off of the throttle pedal, but \\[some\\] prefer the car to coast,” he says. “Ultimately, the Tesla Roadster is a sports car and the regen profile will be fine tuned for sports car driving.”

Straubel discusses the whole powertrain, which was redesigned to replace the original two-speed gearboxes with a shiftless, constantly engaged set of gears with an 8.2742:1 ratio (Fig. 4). That involved upgrading the power electronics module with more efficient IGBTs for switching and specifying a new motor, like the original, but with some changes so it can generate 850 A. All told, the new power train delivers 30% more torque (400 Nm at 14,000 RPM), but at the cost of a small hit in version 1’s foursecond 0- to 60 mph acceleration. The quarter mile still comes in at under 13 seconds.

For more, see “White Goods See Significant Motor-Control Innovations”.

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