High resolution linear encoder & 7i77

Out of curiosity why don't you use an interpolator from iC-Haus? They have a BISS interface that will connect to the ISS ports on the Mesa cards. And you can have as many as you need. And you won't have any issues with speed being a problem. They come with up to 16 bit interpolation, and isn't expensive (about $90 for an evaluation board ready to use).

I'm trying to figure out if anyone else have tested this, but so far I haven't found anyone. PCW mentioned in a post he was waiting for his eval. board. But I haven't seen any results from that. Did you do that test, and did it work PCW? I'd be very interested in knowing. And also know more about how to implement it.

Anders

akb1212 wrote: I'm trying to figure out if anyone else have tested this, but so far I haven't found anyone.

I know someone who is using the ICHaus board but I think he is using it in quadrature mode.

Yes, I have seen that. But for this topic, high resolution, it's irelevant. The whole point was to make use of the capability to increase the resoution to whatever you needed (and the hardware would allow taken noise and such in to account, and resolution of the interpolator).

By using the BISS protocol instead of normal quadrature the demand for high frequency communication would be lowered. In fact Renishaw states it's possible to have 1 nm resoluiton at 100m/s movement here: www.renishaw.com/en/resolute-absolute-op...onal-protocol--11041 .

The trick would be to use encoders with analogue 1Vpp output instead of quadrature TTL level. Then use the iC-Haus converter to increase resolution with up to 16 bit.

The resulting quadrature signal would be way higher than the Mesa encoder inputs would be capable of reading. But then you would be able to use the BISS interface, which the Mesa cards can read dirrectly on the SSI (Mesa ports for daughter cards) ports. The 7i84 have 8 such ports, so you can have as many encoders as you need.

So, it would be possible to use your existing encoder (providing it has 1Vpp or other interpolateable output). Then interface that to an iC-Haus card. Interface that dirrectly to Mesa cards and you wouldn't have to worry about frequency limitations on the encoder inputs. And as an added benefit you could (in theory at least) increase your resolution from 12 Bits on the 0.2 mm encoders to 16 Bits.

The question remains, is there anyone who have an idea on how this would work in real life? I for one would like to know as I have a Lathe head with a 1Vpp encoder (with 512 pulses/rev) I'd like to utilize like this.

Sorry for stealing this thread like this, but I would hope this is an idea worh looking in to, and also help you get the high resolution you want.

Anders

akb1212 wrote: By using the BISS protocol instead of normal quadrature the demand for high frequency communication would be lowered. In fact Renishaw states it's possible to have 1 nm resoluiton at 100m/s movement here: www.renishaw.com/en/resolute-absolute-op...onal-protocol--11041 .

Renishaw sent me a Resolute encoder to check the LinuxCNC interface with.
Very unfortunately it got lost in the post so I didn't get to try the experiment.

The question remains, is there anyone who have an idea on how this would work in real life? I for one would like to know as I have a Lathe head with a 1Vpp encoder (with 512 pulses/rev) I'd like to utilize like this.

I haven't tried it on actual hardware as I don't have any. But it ought to work.

akb1212 wrote: Out of curiosity why don't you use an interpolator from iC-Haus? They have a BISS interface that will connect to the ISS ports on the Mesa cards. And you can have as many as you need. And you won't have any issues with speed being a problem. They come with up to 16 bit interpolation, and isn't expensive (about $90 for an evaluation board ready to use).

I'm trying to figure out if anyone else have tested this, but so far I haven't found anyone. PCW mentioned in a post he was waiting for his eval. board. But I haven't seen any results from that. Did you do that test, and did it work PCW? I'd be very interested in knowing. And also know more about how to implement it.

Anders

Maybe i will use iC-Haus interpolator if I can't use mine, but in any case, it will be in quadrature output. I have no benefit to run BISS interface, my speed is limited by my ballscrew.
Beside this; 12 bits is already a lot, and you will need a very low amount of noise with 16 bits of interpolation.

I have read your topic, maybe you can run dual interpolation ?
A high one for low speed/positioning mode; and an lower/none (rail to rail comparator) one for high speed (inertia will help you for stability).
Btw it's strange to talk in mm for a rotary axis, arc-sec or radian is the unit for angle.
If my calculation are good you are asking for a sub arc second (0.36 arc sec) accuracy with a low resolution encoder ? If you can't correct positioning accuracy (like ballscrew mapping but for an axis) it will be impossible.
Take a quick look at this: www.heidenhain.co.uk/fileadmin/pdb/media...Integral_Bearing.pdf
They got nothing accurate to sub arc-sec, and they are high resolution. You can make one but you will require interpolation and correction in software; with of course the right metrology tools to measure/correct.

Not meaning to discourage you but you are asking to much for a little encoder.

I finally found a YouTube clip demonstrating something along the lines of what I want:

As you can see in the video this is a direct driven servo motor, not very unlike my setup where I have a direct driven induction drive. With high resolution encoders and a good drive I'm hoping to be able to get something similar, although I realize the resolution and accuracy here is more than I can hope to achieve.

But to give a quick summary, they use a high pole count (8 or 16) permanent servo motor. This makes it possible to get very high torque at the cost of having lower max speed. In this case 500 RPM. With a normal 2 or 4 pole motor we can expect to loose some low speed accuracy, but gain in the top speed. My spindle will have 5000 RPM at 254 Hz.

They also state they use sin encoders. We can assume they are interpolated, just like we talk about here. The question is how many periods/rev they use, and how many bits they use for interpolation.

The accuracy they get are 0.0001 inch, or 2.54 um at a radius of 27 inches, or 685.8 mm. This gives 1,696,460 counts/rev, or 0.76 arc sec resolution.

Now to be able to get motion with this repeatedly is impressive! That would mean the system have an internal resolution possibly 10 times that! after all it's not possible to get repeatability of 1 arc second if your encoder have 1 arc second resolution. To be able to achieve that repeatedly and reliably like they have in this clip they will have to have 10 fold the resolution internally.

Now since this axis has a limited speed of 500 RPM they can (and should) increase the resolution on the encoders used.

The Heidenhain encoders you linked to would most all of them fit right in this range. Many of them comes in resolution in the 10,000 to 150,000 pulses/rev, all in sin/cos. Which mean if you use 10 bit of interpolation you can get 10,000,000 to 150,000,000 counts/rev.

Now this is high enough to do what's shown in this video and then some. So I don't understand what you are saying when you say the mentioned encoders aren't accurate enough.

Now to get true micrometer accuracy at the radius mentioned here would obviously demand highly accurate mapping and temperature compensation and what not. Not something any of us using LinuxCNC would ever dream of.

The main goal for me with wanting this high resolution is to be able to calculate speed in an accurate fashion at low speeds. If you look at even low priced servo/drive system from china they use 20 to 22 bits/rev resolution encoder system these days. And that is because it's easily and cheaply available now.

500 p/rev sin/cos encoders and an iC-House chip is all it takes to get ~1,000,000 counts/rev resolution (not repeatable accuracy that is, resolution only, with spread and inaccuracy between steps). The BISS protocol is an open standard which will handle this easily and reliably. So there is no problem for any of the servo system manufacturers to take advantage of this. They also have magnetic solution with similar capabilities that are even cheaper (as you don't need an analogue encoder).

The question is when LinuxCNC users will start to do the same. I'm hoping we can get out of the limitations quadrature encoders impose. In the same way most of us now have gone beyond the limited frequency possible when using a parallel port for direct step/dir signals as used to be the standard way back when it used to be called EMC2.

Anders