Encoders? Karl?

Ignoramus21167 fired this volley in news:2oOdnSCH_ckGppTRnZ2dnUVZ snipped-for-privacy@giganews.com:

Thanks for doing the math, Iggy.

Bingo! You win the big teddy bear.

If a servo misses re-positioning 128 (or 256!) times in a full stroke of the machine, it's undersized, or being driven too fast.

I have a couple of machines that would upchuck if they detected TWO missed steps in 38".

LLoyd

"Pete C." fired this volley in news:4c093eef$0 $20588$ snipped-for-privacy@unlimited.usenetmonster.com:

If, in fact, that's the case, then it's a huge difference.

But still, that only gets you to roughly 11:1 better than the .02 error... and a thou in CNC work is NOT good enough!

The thing about a servo missing up to 256 counts in a full-excursion run is nuts. My OLD R2E4 doesn't do that sort of thing. Newer, more capable servo drivers and faster encoders couldn't possibly miss that bad.

LLoyd

"Pete C." fired this volley in news:4c094cf5$0$1092 $ snipped-for-privacy@unlimited.usenetmonster.com:

yep... and I have some cheap motors from old TI printers that have 720 LINE encoders. These are from the mid-80's, and they were not high-end expensive equipment like CNC mills or lathes.

LLoyd

The resolution (within reason) isn't the expensive part, the rugedizing to survive a machine tool environment is.

+/- 128 counts isn't the same as missing by 256 counts.

Some servo drives will have much tighter tolerance, and that is the limit under load, not normal. I used +/- 128 in the example since that is the max error of the popular Gecko servo drives, more than 128 off and it will generate an alarm to stop the control.

You're confusing cumulative error in a stepper system with the non-cumulative following error in a servo system.

A servo system needs fine encoder resolution so that the servo loop can function properly and hold position against loads. The max following error is the point that the servo will alarm and shutdown. If the system is designed correctly, 128 counts should translate into something like

1/10 of the nominal position resolution of the system, like 0.0001" if you expect to utilize 0.001" positioning.

Would you have any use for some of these?

Integrated power designs 65 Watt switching power supply: Model # SRW-65-4006 4" X 6" X 1.25" +5V @ 5A / +24V @ 1A / +15V @ 2A / -15V @

2A

The website is screwed up. Right click to save and add .pdf, then select all file types before saving.

I still have 25 NOS units on hand for $25 each.

Yep, the "big boys" are way up there in resolution and accuracy. Like I said, for a home machine, 0.001" accuracy is a reasonable target, especially considering the "iron" is typically 40 years old or more and worn. The goal for encoder resolution should be for one encoder count to translate into something like 0.00001", finer than the hardware would even move, but also fine enough to keep the servo happy.

The line / count confusion is there, so 2000 line = 8000 count, so more like .001" error to shutdown. The +/- 128 count is based on the alarm threshold in the popular Gecko servo drives, not the higher end stuff and also isn't the normal following error.

Great prcie but i need more amps @24V

Karl

No problem. I just thought I owuld ask. They are low noise, and were being installed in some sensitive audio equipment.

I may still have some good 8" floppy drive power supplies. They have more current at 24 volts. They usually have +5 & and adjustable +12 to

+15 output that will go down to +10.

What do you mean by "sinusoidal encoders"? Synchro transformers running at 60 Hz or 400 Hz single phase? What is the make and model of the encoders?

If it's a synchro, there will be five wires (perhaps plus a safety ground). Two wires will go to a single coil on the rotor. Three wires will go to a Y or Delta sator coil.

Synchros are still used because they are simple and extremely rugged.

Anyway, one can buy chips that will convert synchro to angle, and vice versa, and the wiring isn't that hard to figure out, so making a converter isn't that hard.

Joe Gwinn

The 1/10th rule of thumb is what I always used, I hardley ever see an error of more than 2 counts at any kind of low speeds. I don't recommend trying .0001 tolerance at rapid speeds. With 1 count error, my system slowly corrects, with 2 counts it corrects quite rapidly, you have to be on the ball to see it. I don't think I've ever seen 3 count error unless I use controls to see it.

If I got a 128 count error then I would tune or replace the control.

Sounds reasonable to me, I don't recommend feeding 250 ipm if you have .0001 tolerance unless you have a machine specifically designed to do this. I may get 10 count error in rapids but not on any speed I would cut at. I haven't even optimized the PID gains on my CNC lathe but it gets within 0.0001 within a second or so.

RogerN

Where do you get this "servo error tolerance +/- 128?" That sounds like you are assuming the Gecko 320 series of drives, which Iggy will not be using.

But, anyway, the US Digital encoders have caused all sorts of problems in a variety of applications, mostly due to a total lack of decoupling capacitors in the encoder. I have worked with the CUI AMT102 encoders, and they seem to work well. But, they only go up to 8 mm ID on the wheel adaptor. They have a DIP switch inside to set the resolution, and also have the index pulse. There is a cable adaptor that converts the output to differential. They are available from Digi-Key, and quite affordable, just over $30 with either the plain or differential cable.

Jon

The PPMC has jumpers to set each encoder for differential or single-ended, and provides up to 200 mA at 5 V to each encoder with a self-resetting thermal "fuse" (PolySwitch). It also has a termination resistor (120 Ohms) when in differential mode to absorb reflections on the lines.

Jon

These have incandescent LIGHT BULBS in them, so there is NO DOUBT, whatsoever, they are optical, and also OLD. LEDs came out in the late

1960s, and due to the safety aspects of encoder failures, this is one of the first applications they went into. So, these must be from the late 1960's or very early 1970s.

Heidenhain made analog output encoders, that either produced a current or voltage output, but was not converted to digital signals in the encoder. The eclipsing of the light path by the disc caused the signals from the encoder to vary in an approximately sinusoidal pattern. It was possible to interpolate these signals to increase encoder resolution, but some systems used the basic resolution without interpolation.

There are a LOT of these Heidenhain analog or sinusoidal encoders out there!

Synchro transformers are fairly rare in shaft angle measurement in the machine tool business. A Scott-Tee transformer is needed to convert to quadrature-related signals. Simply winding the stator with two coils at

90 degrees provides the quadrature relationship directly without the Scott-Tee, and so is much more commonly used in this industry. That is called a resolver. They put a rotary transformer in there, too, to get rid of the brushes, thus making a "brushless resolver". These typically have 6 wires, but can be made with 5.

Jon

I have been using HP (Agilent) HEDS 5645-H06 encoders for years with Geckodrives. Here is the datasheet:

These are 400 CPR quadrature encoders, so you get a resolution of 1600 per revolution, which with a 2:1 pulley translates to 3200 per rev, which on a Bridgeport table with 0.2" pitch screws (ballscrews or leadscrews) gives you about 0.0004" resolution. Given that backlash and rigidity slop, even with ballscrews and belt drive, is much more than that, this is plenty of resolution and an excellent match to the Geckodrive methods. The 1/10 rule-of-thumb is arbitrarily costly, less reliable, and of little value, in my engineering opinion. There should be an "impedance match" between the digital control resolution and the mechanical tolerances, for optimal economy and performance.

I have a surplus quantity of the new HP items for sale at $50/each.

These are made for standard 1/4" encoder shaft mounts, but if you are equipped for metalworking you can make your own adapters for other size shafts and mounts. I've even inserted 1/4" dowel pins into non-servo DC motor shafts to convert them to inexpensive high-power servos: This page shows how I did this with the HP HEDS item:

It looks like you have provided everything, and that with differential encoders, no additional noise suppression is needed. Neat. Thanks

i

Now _that_ would be a fun machine to do the control design on -- or a nightmare, depending on whether I came in at the "blank sheet of paper" stage or if I came in at the "we blundered through this without knowing what we were doing and it's all screwed up, how little can you do and have it all meet spec?"

Granted, the latter case is often more lucrative, but no one comes out of it just thrilled with life.

A second seems like a long time -- but then 0.0001 is a small error. You do realize that 0.0001 error in the measurement may be much bigger by the time you get out to the cutting tool?