help with servo.

[Drools]

I'm not sure if this question should be asked here or in the servo & driver forum.
I have a working CNC router using steppers but I want to learn about servos. I have decided to build a small 12x12 CNC just to do PCBs because I do not want to change from my Porter Cable spindle to my Wolfgang every time I want to do a PCB, the PC spindle is the one that is usually mounted.
I have 2 of these servos:

Globe Motors, 537A250, 30.3VDC The encoder is an HP HEDS-5505-G06 The motor has a no-load speed of 3220 rpm @ 30 VDC & no-load current draw of 0.25 amps. At 20 oz-in the motor runs at 2840 rpm and draws 1.95 amps

I understand they are pretty weak but for learning purposes they should do. I need to know what encoder I should buy that works with Mach3. I'm not sure if the encoder and driver have to be matched or not.
I also want to build my own driver but I'm not sure which open source design I should use. If possible I want the driver I build and the encoder I buy to be compatible with an upgraded servo should I decide to upgrade.
Can I use ACME threaded rod with servers? I want to upgrade my bigger router's screws to 5 start and use the single start screws on the smaller one.
Thanks for the help.

[H.O]

Hi Drools,
Normally the encoder has nothing to do with Mach3. Mach3 doesn't know the difference between steppers and servos - it only sends step and direction signals to the drive and IT drives the motors and reads the encoder. (It closes the position loop)

Incremental encoders are by far the most common ones and is definitely what you should use if building your own drive. (The one you have is the "correct" type). A quick Google search on your encoder part number indicates that it's a 360cpr encoder meaning you will get 1440 "steps" per revolution of the motor. If the motor is capable of 2800rpm that equals 67kHz which is in the upper regions of what Mach3 can produce.

You can definitely use ACME screws with servo motors but the (un)effeciency of the ACME screw compared to a ballscrew means you'll need larger motors to get the same performance.

I don't know if I can recommend any particular open source servo drive, personally I'm running HP-UHU drives in my big machine and although the design is "open" the control chip isn't as you have to buy it from Uli. (I've also rolled my own servo control chip for compatabillity with the UHU but that isn't open source either). There's a DS-PIC servo project here on the zone but I haven't been followin it so I don't know much about it. Which ones are you considdering?

/Henrik.

[ger21]

From what I've read, Ideally, you want to run your servo's at about 80% of their max rpm. In order to do this, you should use a belt reduction to your screws so you get the rapid speed you want with the servos at about 2200rpm. This will also increase your torque by the reduction ratio. 4:1 reduction will give you 4 times more torque at the screw.

[Drools]

Thanks for the help guys!
I will look at some sort of belt/pulley for these, does anyone have a site which might sell pulleys /belts?
I would still like to buy 2 matching encoders, I understand they are anywhere from 20.00 to 100.00 or more. Is there a recommended encoder in around the lower end of the spectrum?
I have seen the DS-PIC web site and it is within the scope of my abilities. I might go that route, unless something else comes up that it better.

[H.O]

Hi,
One source for belts and pulleys are SDP-SI and another one is Technobots in the UK but I'm sure you'll be able to find one a bit closer to you.

The cheapest encoder that I know of is the AMT102/103 from CUI, they are available from Digikey at $30 and they have setable resolution which is kind of cool. Sometimes they develop a slight slop between the hub and shaftadapter which causes oscillations/vibrations but it is easily cured with a dab of hot melt glue etc - been there, done that.

USDigital are popular as well, personally I've had nothing but trouble with the ones I bought though (E7P). I replaced them with Renco R35i and everything's fine. Hengstler, Scancon, Omron and GPI are other brands that springs to mind but they are all a bit more "industrial" and the price reflects that. You might get lucky on EBAY.

[Drools]

Thanks for the help H.O
I have a question in the CUI encoders, do they require a code wheel that is sold seperately?

[H.O]

Hi,
You get everything you need in the kit, encoder, mounting tools/spacers, connector and shaftadapters it will fit a whole bunch of different shaft sizes and mounting instructions. I beleive the AMT102 even comes with two different hosings/flanges providing several different mounting hole patterns.

All you need to do is extend the wires so they reach all the way to your drive or buy spare connectors and make up your own cables. There's also a differential line driver available for a couple of extra bucks.

[Iron-Man]

Originally Posted by Drools I'm not sure if this question should be asked here or in the servo & driver forum. I have a working CNC router using steppers but I want to learn about servos. I have decided to build a small 12x12 CNC just to do PCBs because I do not want to change from my Porter Cable spindle to my Wolfgang every time I want to do a PCB, the PC spindle is the one that is usually mounted. I have 2 of these servos: I understand they are pretty weak but for learning purposes they should do. I need to know what encoder I should buy that works with Mach3. I'm not sure if the encoder and driver have to be matched or not. I also want to build my own driver but I'm not sure which open source design I should use. If possible I want the driver I build and the encoder I buy to be compatible with an upgraded servo should I decide to upgrade. Can I use ACME threaded rod with servers? I want to upgrade my bigger router's screws to 5 start and use the single start screws on the smaller one. Thanks for the help.

I am building a system from scratch.
See my work here: http://www.cnczone.com/forums/bencht...running-3.html
Also check out this link: dspic - servo


Hope this helps,

Iron-Man

[Drools]

Hi Iron-man yep I have the dspic-servo site bookmarked I'm pretty sure this is the same site H.O mentioned earlier.
I have seen your work and it looks great but is it open-source? Are there eagle files available?

[Drools]

This may be a silly question but do servos work on step/direction signals similar to steppers? I would guess there must be a direction but the steps would tranlate into the encoder?

[Iron-Man]

Originally Posted by Drools This may be a silly question but do servos work on step/direction signals similar to steppers? I would guess there must be a direction but the steps would tranlate into the encoder?

The servo amp takes step and dir signals. The servo motors actual position is compared to that obtained from MACH. A PID routine functions to maintain them as close as possible. My current work is based on the dsPIC link. I have made numerous changes. Mr Gaister's work is open source. You will also need Microchip software to program the dsPIC. This is not open source.


Hope this helps,

Iron-Man