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Encoders on extra-rugged duty

08 February 2010

The smart ‘fortwo’ EV from Zytek Electric Vehicles has set a new benchmark in the electric vehicle sector. The 55kW drivetrain delivers a top speed of 120km/h and at just 12kWh per 100km, the vehicle has a range of 110km making it ideal for short journeys in urban traffic.

The electric motor/gearbox replaces the engine and drivetrain in a normal fuel powered car or van. This electric ‘engine’ is located in the rear of the Smart ‘fortwo’ EV, installed into the three engine mounts exactly where the combustion engine would be in a conventional Smart vehicle. Motor, power and control electronics are all combined within a single assembly that requires only three connections: one for water, and two electrical (300V and 12V). The transmission, which is locked in second gear, is likewise located in the rear of the car.

The motor is a 16 pole brushless dc unit. With such a high pole count, efficiency ultimately depends upon the accuracy with which the rotor position is known. To maximize the power output, Zytek needed a high-resolution feedback device for motor commutation and accurate velocity control.

Following the initial success of rotary optical encoders supplied by Heidenhain company, Renco for this development, it has now received a substantial order for rotary encoders ahead of the pre-production manufacturing, scheduled to take place later this year.

Zytek selected Renco’s 35mm diameter R35i rotary optical encoder for this demanding application as it met the former’s requirements for compact size, integrated commutation outputs, and high-resolution in a low cost package. The encoder provides a resolution of 8,192 counts per revolution (13bit) and features a metal code disc for extra ruggedness.

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