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14/07/2008
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Tom Shelley reports on linear and rotary position encoders that are immune to shock and magnetic interference
Linear and rotary absolute position encoders, based on capacitance, are rugged enough to survive being thrown at a wall, have no bearings and still work even when substantially misaligned.
Developed originally for military projects in Israel, they owe their durability to being made out of PCBs – with no wires attached to moving parts – and can remember their ‘taught in zero’ positions, in the event of power failure.
They occupy very little space and can be made lightweight, making them attractive for aerospace design. And because they are unaffected by magnetic fields, they are also proving of interest to designers of medical scanners and other critical system products.
Yishay Netzer, founder and chief technology officer of manufacturers Netzer Precision Motion Sensors, who originally came up with the idea of the sensors in 1994, recently demonstrated both linear and rotary types in the offices of agents Micromech in Braintree, Essex. It was here that director Trevor De Wilde duly hurled a rotary encoder at a wall to prove its mettle.
The ‘Linear Electric Encoders’ come in two types: one with a single track and moving head with wires attached, and the other with two tracks, one of which transmits while the other receives, with a small PCB head that travels between the tracks with no wires. Position readout then comes, wires attached, to the receiving track.
The rotary electric encoders also come in a number of configurations and varieties. In one case, there is a transmitter disk and receiver disk, with an injection-moulded rotor in between, which is equipped with a series of lobes. The conductive pattern on the rotor can be grounded, literally or virtually. Alternatively, the pattern on one disk can be read by its interactions with the pattern on a second disk.
An important aspect of the concept is that, whereas the sensing system looks at only one small sector of the disk in an optical encoder, the whole area of one disk interacts with the other in the Netzer designs. Hence, if the two disks become displaced relative to each other, displacement of one half can be compensated by the displacement of the other in the equal and opposite sense. It is this that allows for misalignment and does away with the need for shaft bearings.
Most applications are in defence and aerospace, but a linear encoder is also being used in a CT scanner developed by Philips Medical Systems of Akron, Ohio. Here, says CEO of Netzer, Omri Messer, “it replaced three costly linear encoders with one unit”. This was possible because Netzer could come up with a linear encoder, with three read heads on one scale.
Meanwhile, rotary encoders with arc-second resolution have been supplied for the X, Y and Z axis controls for Orbit Technology Group satellite systems. This, explains, Messer, was because there was not enough space for traditional optical encoders “whereas ours can be made only 4mm thick”.
Rotary encoders have also been supplied to several US gimbal manufacturers for accurate aiming of weapon systems.
“It is a 247mm diameter encoder – the largest we make – and has an accuracy of two thousandths of a degree,” he adds. “We are also involved with several space telescope projects in Europe and the USA. The encoders are used in critical systems, because they show oblivion to magnetic fields and can be used over a temperature range of –55ºC to +150ºC, which also allows their operation immersed in hot oil.”
Pointers
* Linear and rotary encoders, based on capacitance technology, are unusually robust
* Magnetic fields have no effect
* Rotary versions work well, even if misaligned, and require no bearings
* Versions are available with no electrical connections to moving read heads or disks
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Author
Tom Shelley
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