You’ve built a machine to frost the market’s largest apple strudel. Prefect swirls of gooey confection fall into place during the customer demo. Motion is smooth and accurate in part because of the linear incremental encoder for position feedback. The line driver interface is fast, simple, tested, and inexpensive. Nothing-extraordinary here. It’s not rocket science, it’s strudel frosting!

Then the simulated crisis: you cut power, everything stops…. The pattern of movement is interrupted.

Calibration is lost! The frosting procedure must begin again. Typically the machine must be homed. The first step to recovery is removal of the partially frosted scrap from the machine. Someone is going to eat some Danish.
Wait! From the interrupted state, the controller sends a serial ASCII command to ask the same linear encoder its absolute position. The RS485 interface is slow, simple, tested and inexpensive. The incremental pattern of motion picks up right where it left off—without homing. The machine resumes frosting mid-swirl.

There is a mix of wonder and disappointment in the room. The customer realizes two things: This machine will have very little down time, and nobody at this demo gets to eat scrap strudel…

The heroic encoder: an absolute/incremental hybrid
The same sensing elements are used to produce fast quadrature pulses (or 1vss) and a binary (or Gray scale) address. Serial communication is RS485 ASCII or SSI. Duplicating this solution with two independent encoders invites the challenge of synchronizing remote feedback sources with mechanical variations. This single device’s absolute response always defines the appropriate incremental calibration state.

It can do more than just frost your strudel! Imagine looking up commutation state of a linear motor. Automate movement of a heavy part from current position to target position without homing. Easily recover from an E-stop. Adjust the position of a tool in the middle without disturbing the tools on either side…

Darrell Davey–Applications Engineer
darrell@sikoproducts.com
734-426-3476
www.sikoproducts.com