The multi-turn absolute encoder

Start-up of machines need some adjustments, to find the starting position of mechanism. For this, a so-called Homing process must be performed at every power-ON. Motors move the mechanical parts, sometimes a single axis at a time, to reach the homing sensor position and then move to the starting point. This process is time consuming, and in production environment time is money. How to shorten the start-up phase? A possibility is using multi-turn absolute encoders, which keep in memory the motor position at power-OFF, thanks to a backup battery. In this way, if the axes position are adjusted by hand during power-OFF the real motor positions are updated, and at next power-ON all the drive systems deliver the real position.

A multi-turn absolute encoder in combination with 4-pole brushless motor and ball screw allows to get a high-performance, high-accuracy drive system for linear axis

The actual position is available immediately at power-ON in numeric format via Serial Synchronous Interface, compatible with the BISS-C standard protocol. Reading the position value is not so complicated: the communication protocol is explained in the encoder data-sheet, downloadable from the FAULHABER website:

AEMT means Absolute Encoder, with Multi-Turn. 12 is the number of bits for single-turn position information (from 0 to 4095 counts) and 16 is the number of turns for the number of revolutions made by the rotor (from 0 to 65535 revolutions).

The motion controller must generate a Clock signal (max. 2MHz) and for each clock pulse the encoder responds with one bit of information on the Data line. Differential lines for Clock and Data signals allows for reliable communication also with long wires (as usual, the shorter the wires, the better the feedback signal quality).

A termination resistor of 120 Ohm on the receiver side reduces reflections and signal distortion due to high frequency signals on transmission lines. Between Data lines it must be installed at the controller side.

MT15..MT0 is the information data for the rotor multiple revolutions (MT15 is the most significant bit) and ST11...ST0 is the information data for single-turn rotor position. A Cyclic Redundancy Check field is then available to secure the communication between encoder and drive electronics against possible disturbance.

This application note explains how to install the multi-turn absolute encoder AEMT-12/16L and give tips and tricks for its best use. Thank you for your time!