DC Server motors for CNC - Source? why so pricey...

countryguy

countryguy

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DC Servo motors for CNC - Source? why so pricey...

Ok, Newb w/ a new 80's servo system on a Bandit III CNC controller. Learning about the Servo world. I come from a Mach 3/ hobby stepper setup. Now I am trying to see if I'll want to upgrade to a ajax centroid setup. I like what I see there. I am finding out that a DC Servo, 40inlb+ w/ encoder diff/quad from the CNC retro site is $1,300 a piece from that Vendor. I simply cannot afford that. The controller and driver, no motors, w/ Centroid is about $2K for the kit.

Q1- Any vendors for a DC servo 40+ inch pounds welcome.

I'm told that my mill has DC 90v motors w/ the diff/quad encoders ( hope I have that right). I did not get a pic of the badge on those... Check out the size of the motor off the X & Y mounts on the table.

As usual- Just trying to learn in advance of the units arrival next week. a Bandit III manual, Comms and cable set will be about $400 That is almost 1/5th the ajax retro kit. See why I'm struggling? :)

OEM kit:
I can supply my own DC server motors and even put on their encoders (for $225 apiece) if I cannot get the diff/quad and DC servo aio (all in one) .

Hope to reuse the Motors already on there. If so, No clue on the encoder counts if that even matters. The OEM is 2500 or 2000 on their add-on encoders and I could put the onto these motors as well.

I think I'm realizing that while the Bandit III would be a fun 'old school' project - I simply do not have the time and $$ to toss-out on this.

Q2: And I see far more AC servo motors than DC? Not sure what's up there.. For CNC is the rule usually DC based Servo setups for safety & digital readiness reasons?

WWYD (what would you do) Input welcome as always gang. If you know of some OTS (off the shelf) vendors for Servo motor and driver setups . Send a shout pls.



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Lets start with the assumption that the existing DC servos and drives are OK. In that case there is no reason to replace them. The drives don't care what controller is connected to them, as long as it will output a +/- 10 volt command signal. I didn't read through all of the Ajax hype, but they may have a controller that would run your drives with a command signal. Something like the All-In-One DC without the built in drives.

Quad/diff encoders are a good thing. The output is A, -A, B, -B, pretty much noise(electrical) immune. Most controllers will accept that encoder configuration.

It looks like the Ajax All-In-One DC would run your existing motors. It also looks like the All-In-One will accept your existing encoders. As long as you can scale them in the software, there should be no problem with the PPR.

Another option would be to replace the existing motors/drives with steppers and drives and run the mill on Mach3. High quality NEMA 34, 80 lb/in stepper motors are around $180, or you can get them on fleabay for a lot less. You normally want to double the lb/in requirement when using steppers to keep them from decoupling under heavy load. Doing this using Mach3 would preclude the use of the encoders. There are other controller options available that would accept the encoders. As has been stated before, Mach3 does not play well with closed loop systems.

If one of my DC servos die, I'm going to replace them with steppers, but will be operating them in velocity mode with a closed loop, thus they become a servo. The controller won't know it's running a stepper motor. I am already using a stepper in this configuration for my Z-axis.

AC servo motors (BLDC) have pretty much replaced DC servos. These are actually 3 phase motors with a permanent magnet rotor (armature), and today you find them in all kinds of applications, from very tiny, to large industrial uses. They are the latest and greatest technology. Functionally they are pretty much interchangeable. In general be prepared to sell your first born to buy them. When you add an encoder to a BLDC motor, it then can become a servo.

Servo means that the system is a closed loop (feedback). Theoretically any motor could be a servo motor as long as you can feed back the position information and have a way to control the position. Your heating system in your house is also a servo system. The feed back device is the temperature sensor in the thermostat.

I hope this helps.
 
Jim. Thanks. I am intrigued by the stepper setup as that is so much less. Do you have a controller inside a sidecar then with your software over it? Or do you plan to follow some type of known stepper w velocity mode closed loop system I can read about?

Thanks for the explanations! Jeff and Mitch.

Another option would be to replace the existing motors/drives with steppers and drives and run the mill on Mach3. High quality NEMA 34, 80 lb/in stepper motors are around $180, or you can get them on fleabay for a lot less. You normally want to double the lb/in requirement when using steppers to keep them from decoupling under heavy load. Doing this using Mach3 would preclude the use of the encoders. There are other controller options available that would accept the encoders. As has been stated before, Mach3 does not play well with closed loop systems.

If one of my DC servos die, I'm going to replace them with steppers, but will be operating them in velocity mode with a closed loop, thus they become a servo. The controller won't know it's running a stepper motor. I am already using a stepper in this configuration for my Z-axis.

AC servo motors (BLDC) have pretty much replaced DC servos. These are actually 3 phase motors with a permanent magnet rotor (armature), and today you find them in all kinds of applications, from very tiny, to large industrial uses. They are the latest and greatest technology. Functionally they are pretty much interchangeable. In general be prepared to sell your first born to buy them. When you add an encoder to a BLDC motor, it then can become a servo.

Servo means that the system is a closed loop (feedback). Theoretically any motor could be a servo motor as long as you can feed back the position information and have a way to control the position. Your heating system in your house is also a servo system. The feed back device is the temperature sensor in the thermostat.

I hope this helps.[/QUOTE]
 
When my bandit died in 1995, I had no choice but to go to AHHA control with steppers. This was before Mach or EMC were around

Big mistake, they would stall and ruin my parts. I HATED them. In 2002 I put the servos back on and went with a rather expensive control, Camsoft. Couldn't be happier and i now have done five machines this way plus several more for other shops.

You've got good motors and encoders. By far your best bang/buck is LinuxCNC (same as EMC) If you're afraid of this job, I'd suggest Mach with special step servo drivers as a second choice. I'd beware of the other company you mention.

Karl
 
countryguy said:
Jim. Thanks. I am intrigued by the stepper setup as that is so much less. Do you have a controller inside a sidecar then with your software over it? Or do you plan to follow some type of known stepper w velocity mode closed loop system I can read about?

Thanks for the explanations! Jeff and Mitch.
Click to expand...

I'm not sure what you mean by ''sidecar''.

I'm sure that someone else is doing it the same way I am, but I have never seen anything written up about it. On the other hand, I haven't looked for any information either. Maybe I should write something. I just designed my system from the ground up based on my years of motion control experience, and reading the specs on the available hardware.

The required major components are, 1) a servo capable motion controller that outputs an analog command signal and has encoder inputs, a stepper drive that will accept an analog command input, and 2) some type of encoder system (could be a rotary encoder, or glass or magnetic scales on the table), and 3) the software to run the system.

What I am using is Automation Direct stepper drives, http://www.automationdirect.com/adc...ystems/Stepper_Drives_-z-_Power/STP-DRV-80100 driven directly from a Galil DMC-1846 motion controller. There are a few other Galil motion controllers models that would work also, this one just happened to fit my needs at the time because I had the breakout boards and cables on the shelf.

I have seen these type of drives on fleabay also, but haven't had enough guts to buy one. These have a built in voltage controlled oscillator that drives the the motor at a speed directed by the controller analog (normally +/- 5 Volt) command signal, this is called velocity mode. This is how most CNC systems work. In one case I'm running a 4 axis router with these, and the other case is my mill Z-axis.
 
Some info on closed loop stepper systems:
https://www.damencnc.com/en/components/motors-and-drivers/closed-loop-stepper-systems/530
And their PDF is attached. some neat reading in the PDF and what this Co. is doing.
View attachment Hybrid closed loop System.pdf

And here is a full 3 & 4 axis kit for $1.6K and $1.9K from Automation Technology.
http://www.automationtechnologiesin...op-stepper-motors-3-axis-cnc-kit-110vac220vac

Of course, I'm unsure yet of what control software is out there to use as yet. some note that Mach 3 and closed loop used w/i Mach3
http://www.machsupport.com/forum/index.php?topic=15204.10

Here is a cool video of a leadshine closed loop stepper setup and noted positional accuracy? Do you believe? ;-) If its on the net it's true right. :)
https://www.youtube.com/watch?v=cPEuE8P1g0M


Not even sure the large electrical cab is a sidecar but that's what I called the doghouse sized bandit controller and supply box. ;-)
 
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I would just call the ''sidecar'' an electrical enclosure, or less correctly but more used in the industry, ''control panel'' once it has some parts stuffed into it. But I'll remember sidecar.:))

That system from from Automation Technology looks pretty good and less cost than what I have been buying. I'll look at that system the next time I need to build a mid-range CNC machine. That system looks like it would work just fine with Mach3, it takes a step and direction input. No expensive controllers required, pretty much the best of both worlds. The internal encoder feedback insures that the motor reaches it's commanded position. In addition, you could also run it with the higher cost controllers if the software needed that.

I don't agree with their negative hype about servo motors, a properly sized and tuned AC or DC servo system will not act that way. When not properly tuned, a closed loop stepper system will react exactly the same way.

What I did works exactly like what they have done, with the exception that I have the encoders (mag scales) directly on the load (table and quill). That way, there is no worries about backlash, it is automatically compensated for.
 
Have you considered DMM Tech? Proper closed loop servo motors with rotary encoders - but can also be controlled with pulse / direction inputs as well as analog signals. I've bought some of these myself but can't say much about them yet as I haven't had a chance to fit them yet - I'm in a short term rental home for now, away from my machines.

They appear to offer full torque up to max speed (3000rpm?), unlike steppers which suffer rapid torque fall off with speed.

They now offer a breakout board but I reckoned that a Mesa 7i76 offered more flexibility at not much more money.

Happy New Year!
 
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I recently upgraded the old brush motors on my Bridgeport to 750w dmm ac servos with 14bit encoders. I highly recommend the new 70v dmm dyn2 drives and motor combo.
They are super easy to setup with no tuning required. Just set a few params in the drive and your ready to go.
 
If you're not in a hurry watching ebay will net you some nuggets. I spent several months sourcing motors. I eventually got 2 pairs of motors for my 4 Axis setup for $200. the Drivers I found on Ebay were AMC branded and I picked those up for $50-$70 each. downside is I had to create my own wiring harness for all of this but that was childs play after emailing the manufacturers for a pin out diagram.
 
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