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Author Topic: Lets talk about motors..  (Read 10771 times)

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Offline ART

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Lets talk about motors..
« on: October 11, 2006, 09:52:03 AM »

   Well, motors and drivers are something that are frequently asked about. Allot of people have tried to go too small on a system,
and surprisingly, allot of them try to go too big. Yes, you can go too large. At least in steppers. You can make errors here, a stepper
can be switched to a servo at any time with a new driver and a bit of rewiring, or a smaller stepper can be swapped for a larger one if your
power supply will handle it. Lets talk steppers first. Theres allot to be said for Servos, I like them myself, but many getting into CNC will
be using steppers.

   Steppers come rated at a voltage and current. Its typical to see 4.5Volts at 1Amp and such written on the motors faceplate. The rule is:
NEVER go over the rated current, if it calls for 1 amp, don't go over that 1 amp, it will overheat the motor. But voltage is a different matter. You can go
up to 25 times the voltage on a motor. Typically, most steppers like about 48Volts or so. Ive used from 24Volts to 65 volts quite successfully. The
Higher the voltage, the more power you ll have and the higher acceleration you'll be able to get. (Remember..acceleration is good.. :) )

   I used AC steppers rewired to DC for my first table. Big Honking 72RPM steppers. They used to be easy to get and a simple rewiring of the
coils in the back made them work fine. In fact, their still in use. But you'll want to use normal steppers, 4 wire, 8 wire..doesn't matter really. I
wont get into all the arguments on Bi-Polar ..etc.. Usually most will use what they have. Nema 32 are pretty good for a router table, if the gantry
is heavy you'll want about a 4 amp motor running at 48 volts at least. Don't be too tempted to get Nema42's with 1200oz inch power, as steppers
get larger, the detent torque (that clicking you feel as you turn a stepper) increases, and it can be a devil to tune them or get great acceleration.
I recommend (just me..) that you don't go higher than 800oz in for a stepper. If you need more, think about servo's.

  Servos' are great. Harder to hook up, but they work very well, and once you've used them, your unlikely to go back. Out table will be using Technique
SST-1500 servos for the X and Y. Still not sure for the Z yet. But we'll probably use a servo there as well. (Though a stepper is tempting on the Z as its easier..).

  For either stepper or Servo, I usually use a Gecko stepper (201) or Servo( G320) driver. Ive had very good experiences with Gecko drivers, great support,
and they work very well. But I happen to have some SST's here, so we'll use them. Ebay is great for getting motors , you can often get a servo and matching driver
at a fraction of the cost of a new one. They can all be the same or different, doesn't matter as long as they do the job. If you ask around, people will tell you
how much power you may need, Ive never had Anything that a 200watt servo wouldn't drive. But with servo's , you can go as large as you can handle.

   Steppers, (or servos) are pretty easy to hook up. If using Servos, make sure the driver is a step/direction interface. Otherwise, you'll need a step/dir converter to make
the step/dir signal from Mach3 an analogue output for the driver. Your power supply should have enough oomph for the motor. If you have 3 steppers at 4 amps, you
don't need a 12 amp supply. (many just add them up, but it doesn't work that way..), you'll need about 60% to be safe, so 8Amps at speced voltage would drive 3
4 amp steppers. For servos you'll need lots of power. They can draw 20-30 amps when they need to kick in current, so check with the driver maker as to specs of that.
The servos Ill be using come with a power supply, so we wont have to worry about that one.  We'll go into tuning when the motors are all attached, and Ill show you
 what trouble we hit. Servos can be a real pain to tune in depending on the system, some are easy, some are hard. Persevere, in the end it'll work great usually.

    The only other thing you need to get movement is a breakout board. Check the web links on the artofcnc website. Many makers there and their all pretty good. A
breakout boards job is to give you a wiring platform AND to isolate signals, and condition them from your computer. IF your computer is 3.5 volts only on its printer port
(use a meter and measure a few pins from pin 25 , if you see 5 volts on any, you have a 5 volt motherboard, if you see 3.5 volts on one, then you have a 3.5 volt port.)
then you need a breakout board that will accept 3.5 volts and condition it to 5. Many are available. You CAN just cut off a printer cable and wire it direct, but I don't recommend it,
you'll be confused enough at times, the breakout board really helps calm the confusion and is easier to wire up, it also allows for future additions, so trust me, don't cheap out here,
their only about 125.00 or so typically, and well worth it..

 More on motors and breakout boards soon, specially as we begin to hook them up. Ive asked Bob to take pictures of the old table as examples so Ill post them soon, and as we go, we can draw
comparisons between old and new to show various differences between a Servo and a Stepper operated table. As well as wiring and performance.. In the end, we'll have 2 tables that can cut our
jobs and you'll have seen our mistakes. (allot of them to show you. :) ), hopefully we'll all learn something..

  Bobs great at the mechanical work, I'm more the theory guy and the software guy these days. But I wont code anything special for this table, Ill show you how it works out of the box, and
we'll solve any troubles we get along the way showing you everything we hit, in hopes you'll be better prepared for what you may hit..

to be continued...

Re: Lets talk about motors..
« Reply #1 on: October 16, 2006, 12:51:10 AM »

It would be worth mentioning the modification and use of Permanent Magnet DC motors as Servos and how they differ from the regular ones, and what to keep in mind while using them...........

Love U for what U do
Re: Lets talk about motors..
« Reply #2 on: March 06, 2007, 09:46:38 AM »
YEs, would very much like to hear more on BLDC (brushless DC motors) & their app. as servos...