Multiturn absolute encoders

April 12, 2007
Multiturn absolute encoders resemble any absolute encoder where light shines through an etched disc that has a combination of ON and OFF signals.

Each gear in a multiturn encoder is an etched position disk that turns in relation to the encoder shaft. Optical sensors mounted on the circuit board read the etched patterns on each disk to generate a specific coded value. The value represents the number of complete turns of the shaft along with its current angular position.

resemble any absolute encoder where light shines through an etched disc that has a combination of ON and OFF signals. The disk indicates shaft position by encoding a unique binary word that matches the amount of rotation. However, multiturn encoders not only determine shaft position, but also how many times the shaft rotates 360°.

Most multiturn encoders use a system of gears to count the number of complete turns. A primary gear meshes with the encoder shaft which, in turn, moves a secondary gear, and so on. Each gear is an etched disc whose rotation is tracked by the encoder electronics. The encoder combines the output of all discs to count the number of turns the shaft makes.

Typically, multiturn encoders are larger than standard single-turn encoders because they must hold a gear train. In the past, large gear circumferences typically didn't permit encoders to have a through-hole design. Early gears had to be metal to endure high-speed operation. However, metal gears proved susceptible to EMI. Attempts to use plastic gears failed because of the interaction between the metal shaft and plastic. Any backlash or slip between the shaft and gears degraded the plastic leading to gear failure.

However, new plastic formulas now let plastic gears work with metal shafts without breaking down. Specially designed bearings for the primary gear lets it hit speeds of up to 6,000 rpm. As the gears are plastic, magnetism or EMI does not affect them. The much smaller primary gear permits shrinking all of the other gears to create a true through-shaft encoder that mounts inside the machine.



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