View Full Version : Servos and DC Motors

03-15-2007, 08:52 PM
Point simply, what would be a few reasons for using a servo vs. a plain dc motor and vice versa.

I am willing to read and research on my own and I will so links are great.

But I wanted to hear in plain terms from people who know the in&outs.


T Chap
03-15-2007, 09:00 PM
Do you have a specific goal in mind? Sometimes it's easier to work backwards from what you want to do. There are different types of motors referred to as servos, you'd need to narrow that down. In some cases, a servo is just a DC motor with an encoder. In other cases, a servo is a DC motor that does not have encoders for feedback(closed loop). There are continuous motion servos, fixed travel servos, etc. Too broad a question I think.

A servo without feedback is only marginally accurate compared to a closed loop system with encoders. A DC motor with no feedback has no precision, no repeatablility.

A DC motor can turn on with a wire and a battery. A servo needs electronics.

Post Edited (originator) : 3/15/2007 2:05:17 PM GMT

03-15-2007, 09:32 PM
Okay, then let me give a couple things and see if it is a little more clear.

1. The motor in a RC Helo is called a servo. Why?
(1. -this is where my main question stems from what is the advantage of a servo over a motor w/ wire and battery)
2. The motor in a RC car that turns the wheels is call a DC motor. Why?
3. The motor in a robotic arm that is used a shoulder or elbow is called a servo. Why?

I am guessing that I just don't have the basic concept of servos yet.

I know some people will not want to take the time to explain, that is okay just provide a link that would.
I would enjoy (as stated earlier) to hear in plain terms the reasons.

T Chap
03-15-2007, 09:42 PM
Think of it this way, for your helo RC motor, it "tracks" very closely to your remote control joystick "position". Those motors do not do continuous motion, they turn in some cases only 0-180 degrees based on a pulse length from a processor. If you go to the main Parallax site, there is a seach window. Type in servo or motor, find something that looks like it is servo related and read it. The Parallax servos like this one:

www.parallax.com/detail.asp?product_id=900-00008 (http://www.parallax.com/detail.asp?product_id=900-00008)

they will generally have some Stamp code included on the page, or in the manual. If you know about Stamps, then you can then see how the servo is being operated. Note that Parallax sells both continuous and limited movement servos, but the operation is very similar with the code. That is a good place to start.

You should reread my first response. The servo usually "tracks" either in some precise or semi-repeatable manner, what some controller tells it too. A DC motor either turns or it doesn't. Granted, there are speed controllers for DC motors, and Hbridges that can reverse the direction. But, a DC motor can be a servo! If it has some feedback, and processor control. Google servo motors too after you look at the Parallax code and servos.

Post Edited (originator) : 3/15/2007 3:09:21 PM GMT

03-15-2007, 10:18 PM
One last time and I will leave you alone.

Servo is best used when a controlled motion or timing is needed.
DC motor is best used to set a rpm speed (or continuous motion) and let the motor run.

Here is what I was think, I wanted to try to build up my RC helo w/ two props. Now, I wasn't planning on controlling it through a joystick but by sending a command to something like a Motor Mind B. So that the BS2 Stamp could send the command to 'spin-up' to a speed that would be just slow enough not to lift the object and then increase rpms to a speed that would lift the object and keep it a set elevation.

Your thoughts.

God Bless,

T Chap
03-15-2007, 10:26 PM
That sounds close. I suppose you mean two large "rotors". The Stamp can control several motors at a time separately. With some sensors it could be autonomous, with an Xbee it could be controlled manually too.

03-15-2007, 10:34 PM
"Rotors" - yes you are correct, I kept typing before I was thinking about what I was doing.

Thank you for your help, I have found that any question I have posted here has been answer and done so very quickly.

God Bless,