I’ve opened up the digital encoder used in the Omni-E faceting machine. It’s a US Digital E5 Optical Kit encoder. Click on the link to have a look at the specifications or you can download the E5 specifications in pdf form here. The part number is E5-900, single-ended, 1/4″ shaft.
DO NOT REMOVE YOUR ENCODER DISK FROM THE FACETING HEAD SHAFT!
The disk needs to be a certain distance from the detector in order to operate correctly. If you do not have the spacing tool or a way to set this distance correctly after removing the encoder disk, your encoder may not function correctly. (If you happen to mess it up despite my warning, to re-assemble correctly the ink-side of the encoder disk should be 0.025″ from the detector – the side closest to you)
Faceting Head and Digital Encoder Disassembly Photos
Please do not try this yourself unless you know EXACTLY what you are doing and how to put it back together with the correct encoder disk spacing. And, in case you didn’t realize it, doing this will void your warranty.
Encoder Resolution and Techie-Talk
The most interesting thing about the Omni’s digital encoder is that it reads 900 counts per revolution. That means 900 little ticks per complete 360 rotation around the axis.
The Omni-E digital encoder disk is directly coupled to the faceting head shaft so there is a 1-to-1 correspondence between the movement of the faceting head angle and the counts that are recorded in the encoder. While the encoder can have an index pulse to keep track of where in the rotation the head is, the Omni does not utilize this. Thus the head and display needs to be zeroed every time the unit is powered on.
In order to get 1/10 degree readout from the encoder, the Omni electronics are reading the output signals from the encoder using x4 counting. Techie talk: this means reading both the rising and falling edges of the two square wave channels (the channels are offset so you can determine which direction the shaft is turning) in order to get 3,600 pulses per revolution.
This means the maximum resolution that the Omni-E read-out can be accurate to is 1/10 of a degree. With the E5’s stated worst-case quadrature variance of ±33%, this means that the 1/10th digit of the Omni-E can be up to 33% or 0.033 degrees out of accuracy. Another way of stating this is that the Omni-E’s digital encoder angle readout is accurate to 0.1 degrees ± 0.033 degrees or 1/3 of the smallest division.
The likelihood of this large of an error is low however; it is more likely to happen on high-RPM applications rather than the near-stationary use in the Omni.
It does give me pause however. The main selling feature of the Omni is the Digital Encoder vs. the majority of the competition’s analog potentiometers. At least one competitor’s potentiometer is able to repeatably resolve to 0.02 degrees.
To put this into perspective – a competitor’s potentiometer-based faceting head can accurately and repeatably resolve the facet angle that is smaller than the Omni’s potential error range!
As I’ve written before, I don’t care whether the ruler I’m using is plastic, wood or steel. I just want it to be accurate, precise, and repeatable.
If an analog potentiometer meets these requirements, it doesn’t matter that it’s not a digital encoder. Using the “it’s not truly digital” argument just becomes marketing hype and noise.