Many absolute encoders are equipped with a counter which records the number of revolutions. These devices have one weakness: if the actual position changes in case of power failure, the encoder will 'lose its place' unless it is supplied with battery power. New technology provides a solution that does not require batteries. If that reminds you of bicycle dynamos, you are not entirely wrong.
Optoelectronic absolute single-turn encoders currently available measure shaft position by means of complex optical sensors. Multiturn encoders can record the absolute position value for several turns of the shaft. These multiturn devices employ a combination of optoelectronics and a mechanical reduction gear to measure the angular position of the gearwheels by means of the same optoelectronic principle that is used for single turn devices. Optoelectronic encoders typically provide a 16 bit (0,0055°) or better resolution and precision. Magnetic encoders are a more cost-efficient alternative for measuring rotary motion. The most common magnetic devices have a 14 bit (0,022°) resolution and a precision range of 8-10 bit (1,4-0,35°) per revolution. These encoders do not require gear mechanics. A battery buffer, however, is essential.
Motion generates voltage
To avoid complex gear units or battery buffers, the rotary motion of the shaft can be used for the power supply of the counting electronics, in a similar operation to that of a bicycle dynamo. This results in independence from external power supply units, eliminates the need for homing an axis after power failure. At a near-zero speed, however, this method of energy generation usually fails. Posital has developed the new Magnetocode (MCD) magnetic encoder which is based on a Wiegand sensor. This proven technology generates short, powerful voltage pulses which are sufficient to power the counting electronics. Normally, the counting electronics of the MCD are in a dead state. When the shaft moves, they are 'brought to life' for a short time by a voltage pulse and then analyse the rotating direction. The number of revolutions stored in a non-volatile memory in the encoder is then incremented or decremented accordingly.
Mere fractions of a second
With each revolution, the Wiegand sensor supplies the revolution counters in MCD encoders with energy for only a fraction of a second. This process takes 100 μs at most. The system is not influenced by errors during the dead state. The encoders do not require gear units or batteries, which minimises production costs. Because only one permanent magnet is required to operate the Wiegand and Hall sensors, all elements can be fitted into a very small space and consequently material costs are minimised. Thanks to their simple design, Wiegand sensor encoders are robust and reliable. They are suitable for applications where the use of standard encoders would be too costly and they can be used when replacing incremental encoders or as analog potentiometers.
Unlike most multiturn encoders featuring gearwheels, MCD units can record a practically unlimited number of revolutions.
Wiegand sensors
By means of a special treatment, a wire made from a suitable ferromagnetic alloy acquires a magnetically hard outer zone (shell) and a magnetically soft centre (core). A strong magnetic field along the wire makes it possible to magnetise the shell and core with the same polarity. The soft core’s magnetic field will switch polarity if a relatively weak magnetic field of the opposite polarity is applied to the wire, while the polarity of the shell remains unchanged. This change of polarity generates a voltage pulse in a coil wound around the Wiegand wire. Height and amplitude of the impulse are practically uninfluenced by the speed of polarity change in the external magnetic field.
Eißler, Werner, et al.: Praktischer Einsatz von berührungslos arbeitenden Sensoren.
Renningen: Expert Verlag, 1996.
For more information contact Wim Annandale, ATI Systems, +27 (0)11 824 1315, technical@atisystems.co.za, www.atisystems.co.za
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