Editor's Note: For further details on the Vectrix VX1 electric scooter, including images and information on the frame, instrumentation panel and a system block diagram, click here.
When design engineers were developing the Vectrix VX1 battery-powered electric "maxi-scooter" to compete with the likes of Honda, Yamaha and Moto Guzzi 400-cc gasoline-powered motorbikes, they faced a dilemma. They could build a machine that would outperform internal-combustion machines in acceleration and speed, but it would have so little range as to be useless to everyday drivers (except for drag racing). Or they could build a more practical machine with long range but with such weak performance 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, reverse-phase 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.
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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.
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 functions, along with LED drivers and FETs.
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Some of the unique design elements of the VX1 stand out in the software. Take the throttle control on the handlebar, for instance. Vectrix engineers could have used a conventional mechanical control link. However, they decided to take advantage of the scooter's electric drive motor to function as a generator and charge the battery pack by regenerative braking, and thus conserve energy when slowing the vehicle.
An Austriamicrosystems absolute rotary magnetic encoder provides a PWM signal of throttle-handle position that the ICM sends to the motor controller via the CANbus. To accelerate, the rider twists the handlebar grip downward, as with an internal- combustion engine. The difference comes when an upward twist is applied while traveling forward, which puts the powertrain into the "regen" braking mode.
The versatility of the electronic throttle-by-wire control architecture allows the electric motor to drive the VX1 slowly in reverse if the upward twist is applied when at a dead stop, which riders find useful when backing out of tight parking spaces for two-wheeled vehicles that are at nearly right angles to curbs.
The throttle encoder PWM signal is check-summed for integrity. For safety purposes, the bike turns off in the event of a complete failure, rather than having the system rely on the throttle position input by the rider.
In other examples of hardware/software flexibility, the ICM outputs charging data to the speedometer to display "fueling" current (as opposed to mph or kph) when the bike is plugged in for a battery recharge. The "fuel" gauge function measures and displays amp-hours remaining in the battery. Because the system is not measuring battery charge in µAmp-hours (as with handheld devices), the gauging is relatively easy and accurate.
Safety first
The ICM also quarterbacks safety functions and displays in conjunction with the motor controller. If the charge state is low, or a hot battery condition is detected, power is "gracefully degraded" to prolong range or allow the battery module(s) to reach an acceptable equilibrium temperature. (For temperature monitoring, there is a sensor on each of the 12 modules in which the cells are housed.)
ICM integration makes possible another safety function--monitoring turn-signal activation. If a blinker is on for a long time, the horn sounds to alert the driver. The blinker is not automatically turned off because the switch could end up in the "on," or thrown, position when the rider goes to use it again, leaving the bike without the signal function in that direction.
In discussing the major engineering hurdles faced in development of the VX1, Dugas is quick to note that, in addition to balancing performance and range, ensuring cool running even under Death Valley conditions was a primary goal. The solution was to employ current-control algorithms to handle battery heating and clever physical measures such as heat sinks, fins and fans (without major current use). The fans, interestingly enough, were implemented without the use of computational fluid dynamics but by knowledgeable trial and error by the design team.
These heat-handling measures were born in the company's Pink Room--an insulated enclosure where the VX1 demonstrated continuous operation at temperatures of 105°F with simulated cooling flows over the battery. Dugas noted that it was truly a worst-case scenario since not all airflow around the bike was replicated.
At the other temperature extreme--winter cold--operations and charging were modified. While range was down a bit, it came back up after a warmup period, Dugas said. And charging takes longer because of a battery preheating routine. (Normal charging, on 120 or 220 V, takes two hours to 80 percent charge and 3.5 hours to full charge.)
In another adroit maneuver in handling adverse conditions, the system will check switch states (such as the turn and horn switches) with high-current bursts into them to see if they are closed or open. The reason is that in seacoast salt air (i.e. such as the Massachusetts south coast), deposits can produce false switch-closed indications if low current is used.
Development surprises and the road ahead
When asked if any pleasant surprises fell into place during development of the VX1, Dugas said that the range went up after production bodywork with tighter fit--as well as production weight components--were available for the bike.
Looking to future developments of the electric bike concept, Vectrix is continuing development on a hybrid version with Parker Hannifin, whose fuel cell would be used to trickle-charge the battery.
Perhaps there will be a three-wheeler concept in the more near term, but with twin, steerable and articulated front wheels rather than a traditional tricycle having a pair of wheels in the rear. The idea would be that in some jurisdictions only a standard driver's license and not a special motorcycle ticket would be required. This arrangement would have greater stability, but would still be able to lean in turns with the closely spaced front wheels.
While the initial cost of the VX1--at around $11,000--is higher than that of competing gas-engine bikes (which run about $7,000), the overall operating cost of the VX1 should be less since it doesn't require gasoline or maintenance (except for brake pads), Vectrix said. So if you like emission-free, nearly silent running, take a hop on a VX1.
Rick DeMeis (rdemeis@cmp.com) is editor of AutomotiveDesignLine.com
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