optical encoders + gecko's = too slow

[fatal-exception]

Hey all,

I'm kinda stumped here. I received 4 nema 34 servo motors, 4 g320 drives, and 4 US Digital 500CPR encoders from Keiling late last week. Friday, I tuned one of the motor loops and was using Mach3 as the step/direction generator.

The stats of the motors say 4200rpm, which I'm sure they would do, but using mach as the digital driver in 45khz mode, I was only able to spin the motor at 2000rpm, as measured on the scope. (I tried everything I know with mach3 to try to increase the speed, but this seemed to be the max it would do) My original thinking was that 45,000/500cpr*60 = 5400rpm, more than enough. Not so.

First off, does this seem right?

One of the 4 motor systems was destined to be the spindle driver, but with only 2000rpm, I would have to step up the speed with belts to get the desired 4000rpm I want at the tool. Because the small pulley needs a 1 3/16 bore, the minimum pulley diameter is too large for my design. As far as the xyz axis' go, no big deal, I will step them up appropriately to get 600ipm rapids.

So, what are my options? Will a coarser digital encoder (say 200cpr) give higher rpms at the expense of rotary accuracy? Any other ideas?

Thanks,
Paul

[ger21]

Doesn't a 500cpr encoder with a G320 need 2000 steps per revolution? What is your steps/unit set at in Mach3? Also, what voltage are you supplying. A servos top speed is related to the voltage you supply it. And lastly, the newer versions of Mach3 have a 100Khz mode, if you have a fast PC.

[fatal-exception]

Hmm, thanks for the info. I have a 72vdc supply (70vdc measured), 20amp. Another rating of the motor is ~1000rpm/15vdc. The steps/unit is where things didn't make any sense. At 500 steps/unit, I was able to get the highest speed, but not the first time I tried setting it to 500. I basically resorted to trial an error to allow mach3 to give me the highest ipm/rpm.

I don't have the exact number here, but there didn't seem to be a theoretical way of setitng the steps per unit and what mach3 would let me set the max ipm at.

Has anyone been able to get more than 2000rpm out of a g320 + mach3?

Paul

[Mariss Freimanis]

Mach3 max freq is 45kHz. 4,200 RPM is 70 revs per second. 45,000 / 70 is 642.8 steps per rev. Divide it by 4 to get a 160.7 line encoder. Pick a 150 line encoder as the best standard value.

A 500-line encoder gets you only 1,350 RPM at 45kHz.

Mariss

[fatal-exception]

Thanks Mariss, that helped a bunch. I ordered a 100cpr unit, since it was the closest available resolution.

One more quick question, can the index pin from the encoder be tied directly back into the mach 3 for speed feedback? The manufacturer says that it gives one pulse per revolution. When we had a look at it on the scope, it look like more of a sine wave rather than a square wave.

Thanks again,
Paul

[Mariss Freimanis]

No. The index pulse isn't used by the drive or Mach3. Leave it unconnected.

Mariss

[keebler303]

If you are using a G320 to drive your spindle, mach will already know how fast it is going because it will be controlled with step/dir signals from mach.

Mariss

Wouldn't a G902 be a better solution than dropping the encoder count. I would think this will give the best of both worlds as the Gecko will still have a high count encoder to minimize dither while the pulse train would be multiplied to get the desired speed.


Matt

[fatal-exception]

I think your right Matt. I wasn't aware they had one. The guys at work offered to make me a pulse multiplier, but who knows how long they would take...

Have you ever used the G902? Are they thumbs up?

Paul

[keebler303]

I haven't used it but everything is thumbs up from Mariss!

The G340 is the same drive as the 320, but with the step pulse multiplier onboard, so I think its proven technology.

You are still limited somewhat by the encoder you pick. The cheap ones from usdigital are only rated to 100kHz. You need 140 kHz to go 4200 rpm on 500 line encoders. Maybe they have ones that will go higher, never looked.


Matt

[fatal-exception]

Yep, the spec on the US Digital encoder is 100khz. Anyways, I'm hooking it all up tonite and will do some actual speed tests with a reflective photo sensor before ordering the spindle drive components.

I wish I would have found a decent engineering guide on selecting servo systems before I jumped the gun...

Thanks for all your help.

Paul