Consumer Reports for EMC2

[greeder88]

This post is (I Hope) for people to weigh-in on what components would make the best CNC MACHINING CENTER (VMC 20-40 amp motors) you can build and still fit in the garage. There are countless combination's of parts, controller cards, drives, encoders 200-2000ppr,servo motors, etc that you can use. The last factor I want to impose here is COST! Biggest Bang for the Buck! If something works just as good and costs less it wins. So let me guess the winners just to get things going:

I personally am ruling out AC Servo for bang for buck reasons and other items too since we are all on a budget but nobody wants junk when they are done.

Controller card- 1st- Mesa 5i20
2nd- Pico universal PWM
Drives- 1st- Servo Dynamics type that came with machine
2nd- Pico PWM amplifier
3rd- Larkin Viper 200 with Pico "Gecko Interface" card.

Encoders- 1st-1000ppr USDigital
2nd- What came on machine 500ppr.
Servo Motors- 1st- What cam on machine if good.
2nd- Used SEM, Baldor,etc you can find cheap.

This is all the high ticket parts I can come up with that will eat up your budget and I don't know that much about electronics to evaluate but I do know cnc milling and what happens if you choose poorly... .

[greeder88]

Oops! I forgot the VFD (Variable Frequency Drive) for the spindle. No self respecting system can be without one of these.
1st- Any Flux Vector model $$$ with spindle orient for tool changer.
2nd- Any Sensorless Vector model. Rigid Tapping but no spindle orient available.

[jmelson]

Originally Posted by greeder88 Controller card- 1st- Mesa 5i20 2nd- Pico universal PWM

Hate to keep blowing my own horn, but the Pico PWM controller has more things included
in it, that you have to pay extra for on the Mesa boards. So, it considerably narrows the
cost in most applications.

Encoders- 1st-1000ppr USDigital 2nd- What came on machine 500ppr.

I have recently tried the Cui AMT102 encoder that is available for about $29 from Digi-Key, and they seem to work well. It looks like they should be able to fit on a number of motors. They go to 2048 cycles/rev, you surely won't find another new encoder that cheap with that resolution.

Servo Motors- 1st- What cam on machine if good. 2nd- Used SEM, Baldor,etc you can find cheap.

There was a time servo motors went relatively cheap on eBay, now they are flooded with
brokers selling used motors for thousands of $. There is still occasionally a bargain, but they are harder to find. Keling has some very nice motors that go QUITE cheaply. I have tried their size 23 motor for $56, they also have a size 34 motor a little over $100. These need encoders added, the Cui fits nicely. IMTT USA has a quite awesome 80 mm motor for $135 I think, but it is a lot harder to put an encoder on it. If your install can use an encoder on the end of the leadscrew, this is a good source.

Jon

Jon

[greeder88]

Anyone have experience with different servo drives? Drives run from very cheap to really expensive. What do you get for your money?

[roger_e]

greeder,

why would you go with a PWM controller over a PPMC? It was my understanding that you would have a little more choice as far as components goes with PPMC than PWM....and it seems the accumulated cost of components is what gets yo uin the end..

Do I have this wrong?

[jmelson]

Originally Posted by roger_e greeder, why would you go with a PWM controller over a PPMC? It was my understanding that you would have a little more choice as far as components goes with PPMC than PWM....and it seems the accumulated cost of components is what gets yo uin the end.. Do I have this wrong?

The PPMC is an analog interface to velocity servo amps, and the basic set is $780.
You still have to come up with a set of velocity servo amps.

The Pico Systems PWM controller is $250, and Pico Systems brush servo amps are $125
per axis. The brushless servo amps are $150 per axis. So, you can get the PWM controller and
3 brush motor amps for less than just the PPMC interface boards alone.

If you want the performance or other features of a velocity servo amp, then the PPMC is a good choice. If you just want a cheap servo system, the PWM controller and amps are cheaper.
The PWM system needs fairly high resolution encoders to run smoothly, as it lacks the velocity feedback inherent in a velocity servo system. If you will be using low resolution encoders, the performance may not be acceptable. That's where a velocity servo shines, making a system with low-res encoders still run smoothly.

Jon

[greeder88]

What do you mean by "performance or other features"? Compare the two systems assuming you buy 1000ppr encoders ($75.00/axis) for both systems.

[jmelson]

Originally Posted by greeder88 What do you mean by "performance or other features"? Compare the two systems assuming you buy 1000ppr encoders ($75.00/axis) for both systems.

A true velocity servo amp is what is called a "continuous-time system", so that it doesn't have
to wait for the encoder to cross over to a new count to sense movement. Using only an encoder for velocity sensing, it is impossible to detect movement until the encoder hops from one count to the next. So, the servo loop is essentially running open loop until the count occurs. With a DC tachometer sensing the velocity and feeding it back to the velocity error amplifier, it is possible to constantly monitor velocity even between encoder counts. I did some tests while working on my own velocity servo amps about a decade ago, and was able to have totally smooth motion from 100 IPM down to 0.01 IPM, at which point the stick-slip friction finally overcame the finite gain of the system, and movement started to get slightly lumpy. This is a 10,000 : 1 velocity ratio! The encoder was providing about 3 counts per second at this rate. Without the tachometer feedback, it would be impossible for a servo to maintain smooth velocity with only 3 position updates a second. I couldn't measure it, but the DC tachs were calculated to be providing 7 micro-volts of output while moving at 0.01 IPM.

You may wonder why I'm worrying over performance at 0.01 IPM, but when boring circles or cutting angles close to one axis, the other axis may be required to move extremely slowly. You don't want to have steps there, or you might as well use a stepper motor.

This was with 1000 cycle/rev encoders directly mounted on the end of the ballscrews.
This is why some control vendors now supply insane encoder resolutions on their systems, to
provide velocity feedback.

Jon