Not sure I have a servo...

[Death Adder]

Ok... this is a weird one... I'm pretty sure that what I have are stepper motors but they 'say' servo on them. They use 4 wires. The four wires are seperated into pairs. One pair goes toward the bottom of the unit, one pair to the middle. Inside the unit are four permanent magnets fashioned into quarter circles. All four wires lead to spring loaded rods. The top pair has low resistance pretty much thoughout the spin (around 30 ohms or so if I remember right). The bottom pair has much higher resistance (like 500 - thousands of ohms).

They are EG & G Torque Systems units. They say something like Permanent Magnet Servo or something like that. Servos could not possibly be configured like the above could they?!?!

Wouldn't you know, they didn't bother to put any technical data onto the units either. Some companies are just great...

[Karl_T]

Sounds like a brushed dc servo to me.

A servo is really just a DC motor. Put the two high resistance wires on the + terminal of a car battery charger. And the two low resistance wires to the ground. Switch the charger on for a second. A servo will spin.

karl

[Death Adder]

Hmmm so likely it is a servo motor without the usual attached encoder. The machine is setup so that belts from the servo run, one to an encoder, and one to the gears to run the table. So chances are it's a servo and the encoder function is taken up by the linked encoder.... Interesting. That's what I needed to know. I'll test it tomorrow to be sure. Thanks!

[Death Adder]

Originally Posted by Karl_T Sounds like a brushed dc servo to me. A servo is really just a DC motor. Put the two high resistance wires on the + terminal of a car battery charger. And the two low resistance wires to the ground. Switch the charger on for a second. A servo will spin. karl

Well, if I send 12 volts through the low resistance terminals it spins. If I try through the high resistance it does not. I didn't try what you said and connect both highs to plus and both lows to minus. Should I really do that?

Also, shouldn't a motor need only 2 wires? I'm really new at this...

[Karl_T]

I'd best defer to somebody with more knowledge. All my servos have had four wires. Two each of one color and two each of another color. Generally red and black. I just put both reds on one connection and both blacks on another.

That said you're unlikely to hurt anything doing trials with a 12 volt battery charger.

Karl

[Al_The_Man]

The two low resistance wires are the armature and each one whould be connected across the battery, the other two should not be connected to anything as they will be the tach, You were trying to turn it with the Tach!.
If you spin the motor like I said, the high resistance ones should output a DC voltage, the polarity will depend on direction.
Al.

[Death Adder]

Originally Posted by Al_The_Man The two low resistance wires are the armature and each one whould be connected across the battery, the other two should not be connected to anything as they will be the tach, You were trying to turn it with the Tach!. If you spin the motor like I said, the high resistance ones should output a DC voltage, the polarity will depend on direction. Al.

Indeed you are correct. Thanks! The bottom two do create voltage as I spin it. Sounds like I really need to hit up google for some explainations of how servos work and what is what.

[Al_The_Man]

BTW if you use modern controllers with new drives now use the drive in the torque (current) mode, this means you do not use or need the Tach, if you go this route, just take the brushes out of the tach and leave disconnected.
Al.

[Death Adder]

Originally Posted by Al_The_Man BTW if you use modern controllers with new drives now use the drive in the torque (current) mode, this means you do not use or need the Tach, if you go this route, just take the brushes out of the tach and leave disconnected. Al.

Hmm.. Yes, I'm wondering what the old system is doing. It has a tach on each servo AND an encoder linked to each one. The info I can find seems to indicate that tachs and encoders pretty much do the same thing but this has both. It sounds like if I go w/ a Gecko 320 or 340 drive that I will not then need the tach's hooked up?

[Al_The_Man]

Older drives had a velocity loop which was closed by the tach, the encoder loop was closed to the controller, it was refered to as an inner (motor/tach) and outer (Encoder/control) loop.
The inner loop was usually tuned first, independantly, and then the outer loop was tuned.
The drive was operated in the voltage or velocity mode.
The torque mode system is usually a bit easier to tune.
One weak point in the tach mode was that the control tended to be 'soft' around the zero point.
Al.

[Death Adder]

Originally Posted by Al_The_Man Older drives had a velocity loop which was closed by the tach, the encoder loop was closed to the controller, it was refered to as an inner (motor/tach) and outer (Encoder/control) loop. The inner loop was usually tuned first, independantly, and then the outer loop was tuned. The drive was operated in the voltage or velocity mode. The torque mode system is usually a bit easier to tune. One weak point in the tach mode was that the control tended to be 'soft' around the zero point. Al.

I see... well, I will probably replace the drives then and use just the encoders for feedback. It's less wiring anyway. Thanks!