By Andreas Schroter
Peter Fuchs
Heidenhain Corp.
Schaumburg, Ill.

Edited by Miles Budimir

ECI/EQI Series inductive single and multiturn rotary encoders from Heidenhain are equipped with EnDat interface. The single-turn version has a resolution of 131,072 steps/rev, whereas the multiturn can distinguish 4,096 individual revolutions.

The inductive rotary encoder sits between resolvers and optical rotary encoders. It is an absolute position device. By virtue of integrated electronics, it can generate an electronic ID label and handle diagnostic functions as well. Inductive rotary encoders are also good for applications where accuracy requirements are relatively low, generally in the range of 10 arc minutes or higher. The only real limitations are susceptibility to extreme temperatures and shock, which makes resolvers a better choice in harsh environments.

INSIDE THE INDUCTIVE ENCODER

There are three basic components to an inductive encoder; an excitation coil, a series of receiving coils, and movable shielding disks. A signal with an 800-kHz carrier frequency is applied to the excitation coil, generating a magnetic flux. The receiving coils sit within the excitation coil. A current is induced through the existing alternating field into the receiving coils.

The excitation and receiving coil are on the same PCB. The conductive shielding disk is located on a separate PCB that is attached to the shaft so that signals do not have to be transmitted to any moving parts. As the shaft rotates, the shielding disk passes over one of the two receiving coils. This induces a current in that coil, weakening its magnetic field. The induced voltage around the remaining surface of the receiving coil predominates, modulating a signal onto the 800-kHz carrier. As the shielding disk moves, the amplitude of the signal changes depending on the proximity of the shielding fields relative to the receiving coils.

This modulated carrier signal then feeds into a mixedsignal ASIC where it's demodulated to yield a sine wave. There are two tracks on the shielding disk; one track has a resolution of 32 signal periods and the other has one signal period. The absolute value is determined by evaluating the signals from both tracks in the ASIC. The absolute position value is then transmitted via the EnDat interface (see box). The transfer time for the position value in the multiturn version (29 bits), at a clock frequency of 2 MHz, is only 23 sec.

An excitation coil is energized with an ac signal, inducing a current in the receiving coils. Passing a shielding disk over the receiving coils alters the magnetic field, producing a modulated sinusoidal voltage at the output terminals.

The encoder also has the raw 32 signal periods of the first track available in a 1-V peak-to-peak, sinusoidal output. This can be interpolated within the subsequent electronics to determine either position or speed. This feature is for applications that require absolute data only at power up at which point they go back to incremental data for either speed control or position. Using 12-bit interpolation with the 32 1-V peak-to-peak signals yields a resolution of 131,072 counts/rev.

There are two forms of absolute feedback with rotary encoders: single turn and multiturn. Applications requiring angle measurement work within one rotation. These are considered single-turn applications because there is no need to determine the total number of revolutions.

Applications involving motor position require multiturn capability. The controller needs to know the position within a revolution as well as the number of revolutions. This particular encoder is 29 bit; 17 bits/turn with 12 bits or 4,096 distinguishable turns. This gives 536,870,912 positions within one complete cycle.