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.
Omni-E Faceting Machine Head - Digital Encoder Cover Removed
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.
Omni-E Faceting Machine Head - Digital Encoder End View
Omni-E Faceting Machine Head - Digital Encoder Disassembly and Removal
Omni-E Faceting Machine - Faceting Head with Digital Encoder Removed
Omni-E Faceting Machine - Faceting Head Bearing and Shaft
Omni-E Faceting Machine - Faceting Head Bearing Exposed
Omni-E Faceting Machine - Faceting Head Shaft and Bearing Cover
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.
First I wanted to say that you took alot of time and I really made alot of use with your posts. Having said that I thought I would try to help with some of your issues with the digital encoder from US Digital. First I dont have an Omni Machine, I have my own custom faceting machine and it is actually the prototype for another faceting manufacturer and I did the designs on there digital system so I will leave it at that. I am also working on another digital design that will change the way many do faceting in the future. My current faceting machine currently operates at better than .001 acurracy or under 5 arc seconds and is fully digital and is much more accurate than any potentiometer design and your faceting machine is also more accurate than a potentiometer. In your post you ask why Omni did not use the indexing feature of the code wheel ? That is an easy answer. Simply it would be next to impossible to install the index with any accuracy. If you placed the index at 90 degrees or 45 degrees your head would have to be at that same exact position and the index would have to be placed at the exact point of reading for it to be accurate in its reading of your index preset degree. So in a nutshell, none of us could do it with any accuracy and your index angle would always be off and so would any other angles because it would never be installed right. Omni could have chose an absolute encoder vs the incremental but that would have been 4 times as much cost. Next is the post about resolution. Yes the E5 encoder is set to 900 CPR x 4 quad for 3600 ppr. Omni could have chosen a higer code wheel reading up to 1250 cpr for the E5 but they chose the 900 because of the scale factor. Because the quadrature readout is 3600 your scale factor is .1 and therefore you get 360 degrees at .1 resolution. HoweverOmni could have chosen a US Digital readout that allowed you to change the scale factor to a higher number say 10.0 and you could have simply adjusted the decimal point to give you a better readout down to .01. Remember the E5 encoder is designed to operate at a much higher rpm and most are using these encoders at 0 rpm for faceting. From an inside source at US Digital these encoders have virtualy no error at this low of a speed. Because the read head reads so many pulses per second and the head tends to stay in a very close range the accuracy of the readout is that much better because there is no rotation and a consitent readout of the same code line on the wheel. In reality if the readout was capable of the correct scale factor you could do much higher resolution. Because there is no speed associated with faceting angle rotation, the accuracy factors the factory gives are not correct. Remember any statement they make is a guarantee of their product and they need to leave fudge factor. The E5 encoder is much more accurate that its given accuracy. Now onto the competitor machines and potentiometers or hybrids. They are nowhere near as accurate as the US Digital encoders. The competitors potentiometer whether is be Ultra tec or graves or facette. They read down to .01 because of the readout device not the potentiometer. Currently the limits of even the best potentiometer are .3% of linearity. They can accuratly work to that level but operate on a wave output pattern and that can create huge problems for accurate readout. Also they suffer from significant degradation from rotating parts that contact each other and that translates into errors. Currently the best potentiometer faceting machine out right now is not operating at .3% of linearity and it is not operating accuratly at .01 of a degree. No matter what they claim or there readout says it isnt accurate. I have personaly checked the ultra tec and the graves machines when brand new and both are lucky to read .1 degree accuratly and across the full range of 90 degrees they both have serious errors. Ultra tec is reading off of 2 gears and we all know that spells disaster. In reality your Omni is reading more accuratly and much more correctly even at the .1 degree. If you want the .01 degree output you need an adjustable scale readout to get that reading. You can also change the code wheel of you encoder to a 1250 cpr disk and that would get you down to a scale factor or .072 of a degree. If that isnt accurate enough you can always go to a Reinshaw Encoder ring and head and that will set you back $2,000 dollars just for te encoder, but that is the only way to be any more accurate than what you currently have. I currently use a Reinshaw Ring on one of my machines and a US Digital E6 encoder at 2500 cpr with Quad 4 reading 10000 ppr. Both are incredibly accurate but at a certain point the accuracy really makes no difference because none of us can see it.
Thanks for your comments.
My point when I wrote this article was to clarify exactly what was being done to get the 0.1 degree resolution. At the time there was a lot of dis-information being spread around about the Omni-e’s accuracy and resolution so this was aimed at clarifying exactly what it was able to do.
In my opinion a read-out that that is 0.001 degrees is extreme over-kill as most laps aren’t going to be anywhere near that flat, nor will be the runout on any spindle/platen assembly. And as you say, no one will be able to tell anyhow.
In fact 0.1 degrees is probably good enough for most everything. With all the other accumulated error in the total system (faceting head, quill, dop, etc.) there is still a need for tweaking to make meetpoints, which means cut-adjust-look. In my experience knowing it’s 42.13 degrees doesn’t matter, once I’ve set the angle to make the meet. I know I need to get to 42.1 degrees plus a bit of look-and-see tweaking on the first facet and then it should be set.
The only thing that reliable 0.01 degree resolution will do is reduce or even eliminate the need for a dial indicator or a strain-guage EMS-type indicator. I’ve seen the Graves Mark V head in use and the nicest part of the 0.01 resolution is that the final digits perform the same function as a dial-indicator to show when the cutting has more or less stopped and a given depth of cut has been reached.
-Allan