Flexible measurement of motor speed

16 April 2013

As the exclusive UK distributor for Contelec, Variohm EuroSensor presents an application note on motor speed measurement

Motor speed is usually measured by means of an internal sensor bearing or an optical encoder with fixed pulse counts, whereby a separate sensor or code disc (code wheel) is required for each pulse count. This has a negative effect on flexibility and storage.

When sensor bearings are used, the electronic unit is frequently not rugged enough and so is susceptible to external causes of malfunction. In addition, the time involved in servicing internal sensors and consequent loss of availability in the event of a sensor malfunction brings considerable expense.
The use of optical encoders and encoder modules requires precision installation, which in turn demands precision bearings and extremely accurate alignment of the code disc to the optical module.

However, the temperature fluctuations which are common in motor operation – high temperatures while running, cooling while stationary – cause material expansion in encoder components which is far in excess of the installed tolerances for optical encoders, bringing the risk of failure. Their use is further restricted by their vulnerability to dust, with the concomitant risk of failure.
Just as critical is the fact that customers are restricted to the pulse counts of the relevant manufacturers. Then, if production of the sensors ceases and they are no longer supported, users are faced with the high cost and not inconsiderable time involved in finding an alternative.

To tackle this problem, Contelec has launched an innovative sensor concept. This new ground-breaking solution makes use of a slim external sensor to measure the speed of the main shaft – without any physical contact between the magnetic actuator and sensor unit. All that is needed to attach the magnetic actuator is to tap a hole in the main shaft.

The sensor unit itself is attached to the reverse of the motor or to the fan housing and aligned.

The shape of the sensor has been designed to have a negligible influence on the air flow.

To simplify installation, Contelec supplies pre-configured sensors. A broad range of pulse counts may be configured by keeping always the same magnetic actuator.
The rotary sensors available from Contelec for flexible motor speed measurement have impressive characteristics:
• Resolution: up to 12 bit
• Possible interfaces: incremental or SSI (gray code)
• Possible output stages: TTL, RS422, HTL, OC
• Pulse counts for incremental output (pulses per channel and rotation): 1 to 128, 256, 512, 1024
• Incremental high speed operation (>10’000 rpm) at max. resolution

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