Let's Make Robots!

motors vs. servos

Im learning as I go here, and recently learned in another thread that motors and servos are very different in that servos can be driven directly from the board while motors require an H bridge or similar to protect the board.

When designing a beginner robot, what are some other differences between motors and servos that may not be obvious to a newbie?

 

thanks

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Talk about a supplier not giving you useful information on a product web page...

sry..here is a better one

 

http://www.pololu.com/catalog/product/77

 

 

these motors run veeeeeery fast, I thing near 12000 RPM. I have some of then

Of course, to drive something "heavy" like more than 300 grs, you need gearboxes to increase torque.

 

This gearbox: http://www.pololu.com/catalog/product/114 use same size motors you posted... can drive 2 motors in 2 diferent channels(ideal for a tank robot or a diferencial drive robot), and can be monted with 4 diferent gear ratios: 12.7:1(fast, less torque), 38:1, 115:1, or 344:1(slow, but HUGE torque by the size).

You may experiment to learn... Just hear us talking is not enouth, you may "feel" what we are talk about.

Off the top of my head,

  • There are many types of h-bridge, but for simplicity when I talk about them here it will be limited to a dual-h-bridge integrated circuit like the L293.
  • Motors don't necessarily have gearboxes included.
  • All servos have motors in them, and include also control circuitry and gears of various constitution, from plastic on the cheapie end to metal in more expensive ones. There are both digital and analog varieties,,and it's important to know which one you have when programming. 
  • Most servos have a limit on their range of motion, but not all. Some are "continuous rotation" (which are often used as a "drive wheel" for a robot) or "linear" which move a lever on a screw back and forth in a straight line.
  • A stepper motor is kind of like a servo in that it can be more precisely controlled in terms of speed and holding position.
  • Servos are rated in terms of torque (mass/distance.). They are controlled by pulsing the electrical signal over the control line, while feeding a continuous current through the power circuit.
  • Motors do not necessarily need a h-bridge to run, but an h-bridge is an effective way of controlling them via a similar (but also quite different) pulsed electrical signal to the one above mentioned. This kind of signal requires at least one extra "pin" from your micro controller over a servo's requirements, and in most cases will require two more pins than a servo if you want to control vector. If you only want to control one vectoral factor (ie, either speed or direction of rotation) you can get away with just one pin to control an H-bridge. Conventional wisdom holds that you shouldn't try to power your motors from the "rail" of your micro controller, but that you can power your h-bridge from it. This is can be confusing-an h-bridge generally requires two power sources to be effective. One lets the logic of the IC work, the other feeds the motors. It's like having a light switch to turn on a light switch.
  • Motors vary in feed voltage required to run them and are rated as such. Most servos are safe to run from 4.5-6v and will have torque capabilities listed for each of those voltages.

I hope this is the kind of information you were looking for.

Maxhirez-  man that is a very complete and helppful answer.

Specifically for drive wheels, what are some pros/cons for servos vs. motors?

Thanks again for sharing your knowledge so completley.

Motors are usually cheaper (including a gearbox) for the size of unit needed to drive a robot of a given mass, but servos will be easier to use and program if you're using a microcontroller.  Past a certain mass, you really can't find continuous rotation servos at the right size and torque or expect to modify them yourself.  If you're making a SPURTbot of some type or non-walker BEAMbot, the servo advantage dissappears. 

when you use a motor without a gearbox (say a 130 motor for example) what are you giving up and/or gaining in terms of functionality?

 

You give up some RPMs to get some more torque with a gearbox.

I don't know what you mean by a 130 motor.  It could be a model number or RPMs or any number of things, but generally if you tried to use an ungeared motor as a drive motor you would lose all functionality.  Most hobby-class motors are going to run way to fast to be useful but not generate enough torque if you try to just slap a wheel on them.  Results would vary from comic to catastrophic. Continuous rotation servos on the other hand (since they have the gearing) are often designed to have just exactly that done to them.  It is as easy to get ahold of a motor with a gearbox as it is without though, and the prices aren't all that different.