quick servo motor question

[im_noob]

Hi,

quick servo motor question. From what I know, servo motor have "work range" of max 300 degrees (180 would be the usual ones). The way servo motor is controled (widht of the pulse determine position of the motor) this is also confirmed. I heard (but I do not believe the source too much) that there is servo motor that will "rotate like any other dc motor", so in circles, but is controled like servo motor (to achieve rotation, after puls to drive it 360 degrees one send puls for position at 0degrees and then continue to drive forward)...

I know that in theory this can be achieved (using dc motor, feedback and some custom made controller) but, the questions are:

-does this type of motor exist (as a standard)?

-if this motor exist, is this also "servo motor" or just some hybrid controled similar to servo motor?

thanks in advance

 

[blueroomelectronics]

You can modify a servo for continuous rotation.
https://www.acroname.com/robotics/info/ideas/continuous/continuous.html

 

[Nigel Goodwin]

If we're talking radio control servos, they generally have a designed movement of about 90 degrees or so, although by extending the range of the input pulses you can stretch it further (but not 300 degrees).

As suggested, you can disconnect the mechanical connection to the feedback pot, but it's then no longer a servo - just a motor and H-bridge controlled by a pulse.

 

[im_noob]

Thanks, so in general, I can modify the servo or get already modified one, but that is no longer called servo.

 

[blueroomelectronics]

You still need to drive it like a servo (a 1.25ms to 1.50ms wide pulse stream about 20ms apart)


If you're just trying to run a motor look into H-Bridges instead. Much easier to control.

 

[jpanhalt]

Hi,

I heard (but I do not believe the source too much) that there is servo motor that will "rotate like any other dc motor", so in circles, but is controled like servo motor (to achieve rotation, after puls to drive it 360 degrees one send puls for position at 0degrees and then continue to drive forward)...

Sounds possible to me with something like an optical encoder instead of a pot. You would also need to keep track of the total revolutions. Why not just use a stepper motor?

John

Edit: I assume you are not talking about a simple hobby servo as used in model airplanes and cars.

 

[jpanhalt]

Double posted somehow. Sorry. John

 

[im_noob]

Thanks for the answers guy's.

to get into more details (I am only starting so I do not have a lot of experience), we were talking about "big" servo motors. I have never seen a servo motor in my life (that I knew it is a servo motor), but we were talking about "big" motors, big like - lot of torque and high speed (10Kg to 100Kg motors, 10-20Nm). Note that this was theoretical debate, I do not need this kind of power and if I do I would probably know much more then a noob. I was thinking about stepper motor, as I have experience with stepper motors (actually seen them, controlled them using 16F84), small ones, but the principle is the same, then the other party in the talk mentioned servo's. As far as I read on the net, servo is limited movement (90-270 degrees) feedback controlled motor (with potentiomenter or some other positioning encoder) that can have different torque, speed, accuracy, but it does not spin continuously.

Thing is, I defined the project with
- stepper or dc motor
- encoder (mechanical) that will get feedback on "position" or "movement increment"
- controller driving motor (stepper or dc) using the input from external source (for control) and encoder (to achieve precision)

and the answer I got is "you can find servo to do that for you"

That is why I posted the question.

In real life, I know how to drive DC and stepper. (not too good as I still need lot of practice, still have not figured out the difference between brushless dc, this dc, that dc, 2pole bi polar, 4pole unipolar, 4pole bipolar ... I know the theory, at least some of it, but still need to test all that in real life)

 

[jpanhalt]

Check out servo on Wikipedia.

The basic difference, since this is hypothetical, is that a servo has feedback and a stepper doesn't require it. Both can do positioning and both can do continuous rotation. Using a pot in a servo for continuous rotation raises a design problem, but using an optical encoder is a simple solution to that issue. BTW, continuous-rotation, servo motors in industrial applications have been around for a long time.

John

 

[Nigel Goodwin]

BTW, continuous-rotation, servo motors in industrial applications have been around for a long time.

And at industrial prices!

 

[im_noob]

continuous-rotation, servo motors in industrial applications have been around for a long time.

Thanks, you answered my question. The continuous rotation and term "servo" were just not going together in same sentence for me.

 

[Nigel Goodwin]

Perhaps you might mention what you're trying to do?, then we could offer suggestions.

 

[im_noob]

Perhaps you might mention what you're trying to do?, then we could offer suggestions.

I'm not trying to do anything and I apologize for not making that clear in first post (I was reading this forum for months but I registered just few days ago and this was mine first post).

I was just talking to a friend and he told me that "continuous rotation servos exist" and I was sure that they do not (that in that case the name is different - not "servo") so I wanted to ask if I was wright or he was. From the answer I got here, I was wrong and he was wright ... they exist, they are also called servos, I need much to learn.