EE Times Under The Hood - October 8, 2007 - (Page 8) under the hood: FEATURE www.eetimes.com • www.techonline.com ance that it would not be fun to ride and would be able to compete only with small Euro-scooters rather than the targeted internal-combustion machines. The engineers found their solution by astutely blending bike structure and materials; battery technology; a correctly sized patented motor and transmission configuration; and control electronics and software. They came up with a design for a machine that delivers enough speed for highway driving (speed algorithm limited to 100 kph, or 62 mph) and with range to satisfy the majority of commuting customers (upwards of 72 miles, depending on factors such as prevailing wind, etc). Enablers To get the project moving, the designers relied on a lightweight (20-pound) aluminum frame, a nickel metal hydride (NiMH) battery pack produced by Gold Peak in Hong Kong, and a hub-mounted motor/transmission, all of which form an integrated package ideal for the task. For instance, in a typical bike/motorcycle, the engine is below the rider, near the center of gravity (CG), and a chain drive delivers power to the rear wheel. But because the 102 cells in the 140-volt battery take up such a large volume and weight (to provide adequate range), the three-phase brushless, reversephase dc motor was designed for mounting on the wheel hub and drives the wheel directly via a 6:1 planetary gear—with the weightier batteries closer to the CG, cradled within the frame. The Vectrix-designed electric motor is supplied by SBA Parker Hannifin (Milan, Italy). David Dugas, Vectrix electrical engineer, notes that the motor has to work across a broad range of speed. The motor controller, with a Texas Instruments TMS320F241 16-bit DSP on board, handles this task, which entails operating from 0 to 1 kHz and 0 to 375 amps peak by controlling gain in the current loop. Because an electric motor produces its maximum torque at zero speed, torque is limited at low vehicle speed so the bike is less likely to get away (accelerate) from the rider and put excessive strain on the motor. Motor rpm is limited to 5,000, about 1,000 rpm below its capability, which results in the 62-mph top speed of the VX1. (For more on the performance capabilities of the bike, see the table in the online version of this feature at www.techonline.com/underthehood.) Quarterbacking many of these and other electronic functions is the interface control module (ICM). This component handles throttle inputs and outputs to the motor controller; regenerative braking to conserve energy; instrument cluster displays; turn-signal blinkers and high and low beams; horn; and battery temperature and charge sensors. It forms a critical network “I/O box,” as Dugas terms it. A deterministic CANbus network ties together the VX1 electric scooter’s electronic controls and functionality. Exposing the ICM printed-circuit board reveals it to be fairly conventional. Microchip Technology PIC MCUs are featured for control and CAN func>>10 8 Electronic Engineering Times, TechOnline | October 8, 2007 http://www.eetimes.com http://www.techonline.com http://www.techonline.com/underthehood http://www.techonline.com/underthehood
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