Electrocraft E550

[Tinmuk]

I bought a used Master/Setco X-Y table this weekend and it had two Electrocraft E550-000 motor-generators on it. The p/n is 0550-00-000. I googled this and found some info, but not enough to know if I can use them.

I'll probably pull them and use steppers, but I thought I would ask around first and see if anyone knows how I would go about driving these.

Thes are labeled motor-generator, but my searches show these are servos. If so, can I use the the Maxon 108302 servo drivers I have? I have attached the Maxon manual.

[Tinmuk]

Well, I decided to play with them and see what happens. I know nothing about servos, so I just hooked up my 0-10vdc 20w supply to the red and black wires and slowly increased the voltage.

wow, at 2vdc they started turning. I ran them up to 8 volts and they whirred right along.

Then I hooked uo my voltmeter to the green and white wires and got a reading of about 50% of the input votage.

So now I know they work how do I control them? Will the Maxon work?

[Syp]

Hi Tinmuk!

So it sounds like you have a DC motor with a tach (the tach is probably the green and white wires..it gives you a voltage feedback proportional to the motor speed (like 7V/1Krpm or something).

I take it there are no other wires coming from the motor, other than the tach wires and the motor power wires? In other words, no encoder?

If you don't have an encoder, you can only really use the motor as a speed controlled device, not a precision positioning device. The drive you mentioned does seem to have a tach velocity mode, which is good.

If you do have an encoder on the motor, or can add one, then things are much happier if you want to use these motors to build some sort of cnc device.

Thanks!

Syp

[Tinmuk]

Hi syp,

Yes, there are only four wires. I knew thew the red and black would be power and ground. I guessed that the green and white would be some kind of out put. I didn't know if it would be an encoder pulse or what. so I just tried the VOM to see what came out. Would it help if I tried a scope on the out put signal? I really have no idea what should be coming out of those wires. I havn't really studied electronics since 1977.


Snce these are attached to an X-Y table I was hoping they would have some sort of position feedback.

You said a "tach velocity mode is good" why is that?

BTW, I have two DRC J42 incremental encoders. I havn't tested them yet (don't know how) but I can probably use them if I had to.

Thanks for your help!

[Al_The_Man]

As Syp indicated there is probabally a tach feedback on the motor, but most amps now use the torque or current mode of operation, when used in CNC applications, and this has made DC tach's redundant to some extent, that why most DC motors now sold come without. If you use them in this manner, just leave them disconnected.
In this mode the loop is closed via the encoder back to the control.
Al.

[Tinmuk]

Hi Al,

I'm afraid most of what you said went right over my head. I'm a mechanical designer and I've been using CNCs for over 25 years, but I've never got into the electronics of them before.

What I heard you say was,
"leave the green and white wires alone. Attach the DRC encoders to the motors and use them with the Maxon drivers"

Is that what you said?

[Syp]

What Al said in layman's terms, or at least as much in those terms as possible, is this:

"As Syp indicated, there is probably a generator on the end of your motor that produces an amount of voltage that is proportional (related) to the speed of the motor. This is called a tachometer, or tach for short. In the good old days, most motor amplifiers used this tach voltage (called feedback) to determine how fast the motor was moving. The amplifier would compare a input voltage (usually called a reference voltage) given to it by the user's controller (a computer with a digital to analog converter card) with this tach feedback voltage, and adjust the current (really it probably adjusted a PWM duty cycle, but don't worry about that) to the motor so that the tach voltage and control voltage agreed with each other. Modern amplifiers can operate in another mode, called Current Feedback (or sometimes called Torque mode) where the motor amplifier senses the current going to the motor and that is it's feedback. The controlling voltage (reference) from the user then tells the motor how much current to give the motor, and this is compared with the current sense feedback so they agree with each other. By controlling the amount of current going through the motor, you in essence controlling the torque of the motor, hence the name torque mode. In this mode, you do not need any other connection to the motor except for the two motor power leads. You still need some sort of feedback, mostly likely digital in the form of a quadrature encoder, to the user's computer control, so that it knows where the motor shaft is at any given time. With just the torque control of the motor you don't know how the load on the motor is affecting it's speed, so you need the encoder to "close the loop" back to your controller. Think of the encoder as your eyeballs, the controller is your brain, and your hand turning a manual handwheel is the amplifier in torque mode. Now close your eyes and try to turn the handle 1 revolution in 2 seconds. You get close maybe, but you are going to be off by a few degrees. You need your eyes (encoder) to go back to your brain (controller).

Ok, layman's terms are way too long.

Syp

[Tinmuk]

Ok Syp, that was long, but it made sense. Thanks for taking the time to decode that for me.

So heres the scenario. I have an X-Y table, 2 servo's (with tach), 2 Maxon 108302 servo drives, 2 DRC H15 (J42) 1000 count encoders. Will this particular combination of parts work together?

I've read the documetation on maxon and DRC stuff and it looks like it work, I would just like an opinion from someone with some knowedge of servos.

Thanks, I appreciate the help, Syp and Al.

[Al_The_Man]

Tinmuk, Looking over your drive manual they appear to be velocity mode drives, so unless you already have them or can get them cheap, personally these would not be my first choice.
For this mode you do require a Tach or, in the case of the Maxon, an encoder can be used, nevertheless, with this mode there is what is known as two control loops, the inner loop being the motor-tach-amplifier loop, and the outer loop which is the motor-encoder-controller loop. The inner loop has to be tuned first before the outer loop.
Although this method was used for many years for CNC control, it is no longer the prefered way, the Torque/current loop method is considered better for its tighter control around the zero point etc.
Also, if you are considering one of the control packages that use the paralell port for control, you will need amplifiers that take step and direction signals, rather than ±10v analogue control.
Al.