Servo Motor questions

PFloyd36069

hi guys,

I have a few questions about servo motors. Is the continuous rotation servo that parallax sells still a servo or is it just a variable speed/direction motor? What is the difference between a hobby servo (like the ones sold here) and the larger type used in CNC machines?

Thanks,

Bryan

wabbit

I'm a newbie too, but heres what I have learned.

Normal servos work by receiving a signal pulse and translating that into a position to turn.·Normal servos cant rotate 360 degrees due to a mechanical stop inside and some other electronic details I am not 100% clear on.

By modifying the servo to remove the mechanical stop and modify the electrics a little it causes the servo to translate the incoming pulse into speed rather than position. SO a pulse that would normally cause the servo to turn 10 degrees instead will cause the servo to rotate slowly 360 degrees.· As far as I know once a servo is modified for continous rotation it can not be used to go to a specific position and stop.

To have continous rotation and specific position you will need some kind of stepper motor.



Like I said I'm no expert, but I thought I would try to help out a fellow newbie. If you search the net for Modifying a servo for continous rotation you will find many tutorials on the subject.



-wabbit

Ugha

Wabbit is pretty much right except there ARE 360 degree standard servos... they just can't rotate beyond the 360 degree
mark. Meaning they can't spin like tires... they can just turn all the way around, stop, and then you can reverse it to spin
all the way around the other direction.... good for an Exorcist-inspired robot... You can even waterproof servos so you
can include the projectile vomiting [noparse]:)[/noparse]

Franklin

Or something like this. www.servocity.com/html/hs-785hb_3_5_rotations.html

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- Stephen

whicker

PFloyd36069:

to answer your question, I need to start with the fact that yes the term "servo" doesn't mean the same thing in different applications. The exact kind of motor or method of control is not the same, either.

Hobby servos use pulse length (PPM) to determine what angle to position the output shaft. Velocity is generally fixed (however many degrees/sec the rating is). Industrial type servo systems may use an analog voltage (or a digital numeric position from a serial bus) to tell it where to position, or to tell it what velocity to travel at.


The point of using a servo system is to be able to accurately position the output shaft, and to keep it in that position regardless of distrubance (push or pull on the output shaft). A hobby example of this is of course the steering of an RC car. This is done by a so-called "closed loop" control system. A servo is more than just the motor, it's the complete feedback loop. In the case of a hobby servo, the feedback device is a normal potentiometer (cheap but effective). In the case of an industrial servo, there is a "real" position encoder, (Incremental, Quadrature, Resolver, EnDat, SinCos, etc.)

Now the confusing part comes in where hobbyists like the premade package that hobby servos come in. I mean, you've got gears and a motor in a nice compact, easily mountable box. They "break" the closed loop control in one way or another, causing it to just become a normal electric drive motor with some gearing reduction.


To answer your question, the servo that parallax sells is just going to either sit still, run forward at full speed, or run at reverse full speed. Completely open loop and not like an industrial servo at all. Without any feedback loop, it's going to slow down with a heavier load put on it.

To get speed and positioning control, you need to add the feedback loop with an external encoder and probably PWM the motor voltage.