Question for the encoder-savvy folks on the forum:

[DICKEYBIRD]

Later this year I’m planning to finally install the AMC BE25A20AC servo-drive on my ORAC CNC lathe’s Baldor ¾ hp DC spindle motor. I was originally going to use the tach generator that’s already mounted on the back of the motor but have changed my mind & am going to use an encoder since the drive is happy with either. Here’s what the AMC manual says (Incremental):



And this (Quadrature):



I know absolutely nothing about encoders other than the prices seem to vary wildly. I found “incremental” encoders on ebay in my price range but don’t know if this is the type I need. What do ya’ll electronic gurus think? http://www.ebay.com/itm/1-pc-OMRON-1...UAAOSwdzVXrBmk

[M_C]

Ideally you'd want an incremental encoder with differential / line driver output, as it greatly reduces the risk of noise problems, although on a spindle a bit noise causing a few extra/lost pulses probably won't cause any problems, unless you're planning on using it for positioning purposes. Unfortunately cheap differential output encoders are pretty rare.

The one you've linked to should work, although as it's open collector output, you'll need to add pull-up resistors.

[DICKEYBIRD]

Thanks M_C! No positioning capabilty at this point is needed, just as steady a low speed as possible with the motor I already have on the lathe. Someday I may run across a 1000w brushless AC servo for peanuts but I ain't holding my breath.

Thanks for the heads up on the pull-up resistors; I'll be asking for guidance when the time comes for sure. If you think the one I linked to will work, I'll look for the best deal on that type & pull the trigger. Looks like the best deals are sent via slow boat from China but that's OK; there's no hurry.

[Sparky_NY]

I am just finishing up a bridgeport retrofit using the same exact AMC drives you listed. I initially wired it up using encoder feedback to the drive but then switched over to tachometer feedback because of instabilities when the motor was stationary. Using the tachs worked much better in this particular case. That said, I have used encoder feedback on other retrofits using the BE25a20 drive (same but no internal ac supply) and it worked well.
If you already have a tach, I would just use that. The purpose is nothing more than velocity feedback to the drive. The feedback is not that critical. For a spindle drive, you will be running the drive in velocity mode. The AC version of the drives are pretty nice, I have several of them on the shelf.

Fleabay is probably the best source for a cheap brushless motor. They come along often. For that drive a servo motor of higher voltage is best, because the drive is limited to 12.5 amps output low voltage servos will need more current than the drive can supply with servos the size you will want.

George

PS, if you want to try encoder feedback, here is a popular one I have used many times, including on the current bridgeport. $23
http://www.digikey.com/product-searc...66131?k=AMT103

[macona]

Those AMT encoders have never given me anything but trouble on servo applications. Go to US Digital and get an optical encoder from them. The HP/Agilent compatible ones work great. Probably something around 500 line should do it. Differential is usually only about $10 more. A lot of single ended encoders
Are actually differential internally and they just connect wires to the A/B output lines of the driver cheap.

[Sparky_NY]

Those AMT encoders have never given me anything but trouble on servo applications. Go to US Digital and get an optical encoder from them. The HP/Agilent compatible ones work great. Probably something around 500 line should do it. Differential is usually only about $10 more. A lot of single ended encoders
Are actually differential internally and they just connect wires to the A/B output lines of the driver cheap.

I have used them on 3 mills and a lathe, all with excellent results, no complaints or issues. A friend has also used them on a lathe and a mill with similar results. I recall you had some problems with the AMT encoders but that is certainly not the norm, they are widely used.

The drive the OP is using accepts single ended encoder inputs only and they must be over 3.5 volts. A encoder with a line driver will not work with the drive unless you rig up a line receiver also. (fought that battle and had to ditch the line drivers) Line drivers usually meet the RS422 spec and are used with a line receiver.

[DICKEYBIRD]

I initially wired it up using encoder feedback to the drive but then switched over to tachometer feedback because of instabilities when the motor was stationary.

The reason I'm doing this George is for low spindle speed stability. Jitter at rest won't be an issue I think. The AMC data mentions possible noise/stability issues with tachs at low speeds & my tach is the lower range type (7v/1000 if memory serves) so I'm thinking an encoder is the way to go.

I'm real glad you've used these drives successfully, it gives me hope that even I can get mine working well!

[Sparky_NY]

The tach you have will work just fine, save your money for other toys. That is, unless you happen to also need a spindle encoder, the encoder could do double duty in that case if you mount it at the spindle.

I have used those drives with both brush and brushless motors, even did a Fanuc redcap with one for a friend earlier this year. You will be pleased with the results.


Isn't that lathe you have running under mach 3? If so, the motor control is probably 0-10V along with fwd and rev signals. Have you thought about how you will handle reversing the motor? Normally, + and - 10V is used for such purposes with that drive. Opposite polarity on the control voltage reverses the motor. IF, you are just going to use a speed control pot then its not a issue, if the breakout board supplies 0-10V then you have to come up with a plan.

[DICKEYBIRD]

Yup Mach3, hence the need for a steady speed on the low end when the lathe is configured in the belt reduced mode for threading. (No Linux gloating please.)

So you don't think that an encoder would be quite a bit more accurate & noise free than the tach-gen? It would be a little easier to stick with what I already have on it; couldn't hurt to try it that way first I suppose.

The C-11 BOB is definitely has a 0-10v spindle speed output. The reversing plan is to cobble up a DPDT polarity reversal relay to trick the drive into running the motor backwards upon M4 command. Shouldn't be too hard I hope. (?)

[MaxHeadRoom]

I have used the A-M-C fairly extensively on retrofits, many of the BLDC drives also have a Hall input tach feature on the mode-select dip sw, the one you have is one of the few that does not have that option, it is OK down to 300rpm for a 6 pole motor.
I also was not impressed with the AMT capacitive encoders, I generally stick with the optical style, the AMC BLDC type was a disaster, I contacted their support line and was told the guy that designed it bailed on them or some such story.
And were of no help.
Max.

[Sparky_NY]

Yup Mach3, hence the need for a steady speed on the low end when the lathe is configured in the belt reduced mode for threading. (No Linux gloating please.)

So you don't think that an encoder would be quite a bit more accurate & noise free than the tach-gen? It would be a little easier to stick with what I already have on it; couldn't hurt to try it that way first I suppose.

The C-11 BOB is definitely has a 0-10v spindle speed output. The reversing plan is to cobble up a DPDT polarity reversal relay to trick the drive into running the motor backwards upon M4 command. Shouldn't be too hard I hope. (?)

Your plan presents a problem. Reversing the motor leads by switch will indeed reverse the motor BUT you will have a runaway condition unless you also reverse the tach input. (same hold true if encoder feedback is used) The polarity of the feedback signal has to match the motor polarity or you get a runaway (motor comes up to full speed, in the case of a BE25...AC drive, 170 volts to the motor !) IF you switch the tack polarity at the same time, you are good to go. I don't think there is a easy/cheap way to get +/- 10V control signal out of a mach setup. Either a multi-pole switch or a relay with multiple contacts could be used. The only BIG warning is that this would have to be done with the drive not enabled, if Mach energizes the relay and with the drive enabled, expect a boom.

As for accuracy, yes encoder feedback is more accurate but in the real world, it only comes into play at extremely low speeds, say 10rpm and under.

Don't be too surprised if using the AMC drive does not result in good threading, the problem isn't spindle speed variation, its the approach Mach takes. Spindle speeds will vary a bit, its a fact of life, others thread perfectly despite that fact.

[Sparky_NY]

I have used the A-M-C fairly extensively on retrofits, many of the BLDC drives also have a Hall input tach feature on the mode-select dip sw, the one you have is one of the few that does not have that option, it is OK down to 300rpm for a 6 pole motor.
I also was not impressed with the AMT capacitive encoders, I generally stick with the optical style, the AMC BLDC type was a disaster, I contacted their support line and was told the guy that designed it bailed on them or some such story.
And were of no help.
Max.

I am guessing that by "BLDC type" you are referring to the encoders with commutation output capability along with the normal encoder outputs. As I am sure you know, those must be programmed with software they supply, the potential for errors in programming is huge without a deep understanding of BLDC motor theory.

[MaxHeadRoom]

I am guessing that by "BLDC type" you are referring to the encoders with commutation output capability along with the normal encoder outputs. As I am sure you know, those must be programmed with software they supply, the potential for errors in programming is huge without a deep understanding of BLDC motor theory.

Most of the BLDC drives I use in servo applications, I have converted some Fanuc also, but if requiring BLDC commutation the encoder has to be changed as they have a proprietary method of commutation and are actually 3ph sinusoidal motor originally.
As far as the AMT BLDC encoders, I purchased the required programming unit in order to program the correct commutation sequence when mounted and the pole count etc, I could not get consistent performance from them, the motor would run for a while, then suddenly reverse direction for no apparent reason.
That is when I attempted the support contact, with no success.
Max.

[DICKEYBIRD]

Your plan presents a problem. Reversing the motor leads by switch will indeed reverse the motor BUT you will have a runaway condition unless you also reverse the tach input.

Wasn't planning to reverse the motor lead polarity, just the input to the drive from the BOB. Won't that work or am I over-simplifying things?

Mine actually threads fairly decently now with v.062 after I got the spindle speed down via jackshaft belt reduction but the larger sizes in steel (40mm & above ) still give me trouble as the spindle load goes up when the cut gets to its deepest & the speed starts the wobblies.

My Mach threading problems have mostly been self induced or spindle speed related. Of course, I have always cut fairly short threads (1-2 dia's) and will always buy leadscrews if I need them.

[Sparky_NY]

That should work ok.