Electrocraft servo - How do they work?

[Chris64]

I'm really just curious how these servos work? It's an Electro-craft servo / Brushless Servo motor tach.

I'm familiar with how motors work (ie. ac/dc, brushed and brushless). Does anyone know how these type of motor/servos work?

I can assume they are brushless, from the name...and based on they're age I can assume they're sensored brushless. And Probably DC.

Am I right so far?

How does the Tach sensor work? How does it know it's speed or position? Or does it just work off the brushless sensor and esentially so the same thing as a stepper motor with a sensor? What wires usually come out of the servos themselves? I've tinkered with RC servos but they use PCM to set the angle and don't rotate continuously so I'm assuming these servos work on an entirely different principle.

My CNC controller has a questionable future and I'd like to investigate my options. I've tried searching for tutorials on how to work with these motors and haven't found a thing (and I don't really want to take them apart for research...it's not broken yet [knock on wood]). I've tinkered with electronics for many years and could probably put something together my self for basic controls since a PC could probably handle the sequencing. Anyone?

[Al_The_Man]

Not quite sure what you are describing, a photo would help and any connections,
I have the Electro-Craft catalogues going back some time, including their motor theory manual, and they do not really describe what you are mentioning.
Very early on, there was a tendency to design some DC brushless motors that included either a DC or AC tach as well as hall commutation, but I have never seen an ElectroCraft with this design.
Al.

[Chris64]

I'm sorry, I was just speculating what I thought was inside a servo. I was making assumptions (and you know how that goes) about how they work just off the name...DC brushless tach.

Can you elaborate on what it is that makes these servos work? What makes it go? What makes it hold position? How you know whether it's turning or not?

From what I'm reading (about a similar servo) it seems like it may have two optical sensors that tell it what position it's in and you sequentially give it power to the coils to make it go...like a sensored brushless motor (very common Radio Control Plane/Heli part). It also explains how they can hold position. While this makes perfect sense - it may not be how they work at all...hence the question.

[Chris64]

Actually I found this post and it is full of all the information I'm asking.

http://cnczone.com/forums/printthrea...1&page=1&pp=15

The main thing I'm wondering now is what type of signal does the tach accept to find it's destination?

Edit: From what I understand...you feed it an intended destination and then it must return to you a value of whether your are CW, CCW or at the destination. What type of signal does it want for a destination? An analog signal? If so, Just DC? Or are there multiple legs for a digital signal?

[Al_The_Man]

As far as DC Brushless, I think you can disregard the tach, A DC brushless motor is identical in appearance to a three phase star connected servo/motor.
Because it does not have a comm. like a Brushed motor, it requires electronic commutation, often refered to as Hall effect sensors, three of them, one for each winding.
You can control them this way, open loop, just the same as a brushed motor, but the common way is to 'servo' them or use a quadrature encoder back to a servo control of some kind, that will issued the correct motion signal, based on the command and the encoder feedback.
Of course, in there somewhere you require a Dc brushless drive or amplifier.
There is a downloadable handbook of AC servo sytems from the Motiononline.com web site.
Al.
.

[Chris64]

OK, now that I'm home it appears my memory didn't serve me that well.

The motor is an Electrocraft Permanant Magnet Servo Motor (model 072311...discontinued). It appears to be a straight DC motor with brushes...I was way off.

I started following the wire to see where it goes (and to count the wires). It routes out of the motor and goes right back into the mount fixture, and then goes to the computer box. Well, right where it goes in and out there's a small service panel. I popped it open and there were just three big wires (10 gauge maybe) coming from the motor (+/- and ground maybe?) and in this box was some type of little sensor that obviously detected rotations.

Anyway, I don't know how well this setup would accomodate a controller upgrade...but at least it seems pretty straight forward if I wanted to make a controller myself.

I'm just thinking that if I set up a simple logic circuit to send any of the 3 axis 1 frame in either direction it would require at least 2 bits (00=lock,01=CW,10=CCW,11=unused). On a parallel port you can have 12 bits really easy so that would allow to control 4 axis without breaking a sweat and leave 4 bits for other accessories (coolant etc.). This seems pretty feasable.

[linatrol god]

The tacho has nothing to do with positioning, it is to regulate the speed of the motor. It is a generator that produces a dc voltage in proprtion to the speed the motor is turning. This voltage is fed back into the amplifier and used to keep the motor speed constant. Positioning is done by an encoder, the motor you are describing does not seem to have an encoder on it, if it did it would have another cable with upwards of 9 wires in it.