H-bridge

The H bridge is best bet for bidirection turn of motor. Of cause you can use two transistors as a switcher, which link to different pole power. one is positive power and another si negtive power supply to motor. There are lots of driver for sevomotor at market. Most of them are worked in way of PWM. they can control sevomotor's rotation speed and direction easily by PWM, either by close loop or by open loop, if you neednt precise and stable control. you can also use a steppping motor for your robert.

Instead of using servo motors have you thought of using stepper motors. If you engineer the stepper motors correctly you don't need any feedback.

To control the speed you just adjust the frequency rather than trying to mess about with trying to control the current through a DC motor. Controlling the frequency with software is a hell of a lot easier than controlling the current with hardware.

Also with stepper motors you don't need a breaking mechanism to stop it from rolling down a slope. All you need to do is keep one phase energized, that will stop the motor from freewheeling. If you really want to get sophisticated you can put a rotary encoder on the stepper motors for feedback

You don´t need any servo motor driver if you have enough CPU time available on your PIC: Servos are driven by a PWM signal that can be generated by the PIC. You just needa transistor on the output so you don´t demand an excesscurrent from the PIC. (I did so without the transistor and the servo behaves erraticly because (in my case) the 555 timer could not hold the output).

If you want to try PWM, here is a simple circuit for motors. I just used it with a servo motor and it worked out great!

http://www.dprg.org/tutorials/2005-11a/index.html

What are you trying to do with the servos? As most people have suggested, a stepper might be better suited to your application.

Regards.
Alex.

Actually those servomotors are use to wheeled my robots. what the driver is suitable to controls my servo rotation and speed? does any h-bridge needed. I'm still unclear. Let's forget about the stepper motor coz i have decided to choose the servo.

If you are going to wheel your robots with servos you have to modify them a little. Servos have both a mechanical limitation and an electronic feedback that prohibit them to spin freely like reguolar motors. They just sweep an arc from one end to another and they stop there.

If you remove the mechanical limitation and the potentiometer you can use them as regular motors. Look this up on the web to get the idea.

About the driver, you don´t need a circuit for it. Read a little about PWM, servos are controlled by a pulse modulated signal. You can get it from your PIC by software and save on a driver.

Are you clear on how a servomotor and PWM work?

Regards.
Alex.

Slipknot, It sounds like most here are assuming that you are talking about the servos that are often used in Radio Controlled cars & airplanes. Like this one:

If that is the case it is true they only need a PWM signal to controll the angle that they are at, not the most ideal for wheel motors. These are better suited for something that only needs to rotate about 200 degrees. Here is a site I found quick using "RC servo PIC" in a google search

http://www.piclist.com/techref/microchip/servos.htm

Now if you're making a realy big robot and you are actually using "Servo Motors" like this one:

then the controll is entirely different! These are more like a 4 phase motor with encoder feedback. I really wish they made mini versions of these, because they would beat the tar out of Perm Magnet DC motors!

Anyways, back to your task at hand; if it is the first servo that you are trying to use for wheel motors you will have a fair bit of work to make them work for you unless you buy some already converted. Steppers might work better to get you started. I have a link to some robotics store that handles this stuff and all in PIC :( (I dealt with bank switching five too many times, now revolve around AVR) I will try to find that site and post a link.

I could be wrong but the photo of the larger motors look like 4 phase stepper motors to me.

As for getting small stepper motors would the one in your watch be small enough. They move in 6º steps and run on 1.2 Volts.

you're right, those do look alot like the big steppers. And I suppose one could argue about the usage of the term "Servo Motor" to mean any type of motor used in a "Servo" according to Webster, or WikiPedia, be it a stepper, DC per mag, AC 3ph or 1ph, BLDC, sycronous... It seams that in industry automation has tagged the term servo motor to a family of sychronous motors with integrated resolvers that make them ideal for the "precise" postition, velocity, accelleration, and jerk controll that is expected of automated systems. These motors are generaly large and very expencive they act much like a freq controlled 3ph motor, but can handle a much larger range of RPM. However, they require a driver to run.

I found this site that explains the basics of the type of industry automation servo motor I was refiring to in the photo and post:

http://zone.ni.com/devzone/cda/ph/p/id/233

Here is baldor's site for this type of servo motor.

http://www.baldor.com/products/servo_motors.asp

Sorry Slipknot, this is way off topic what what your post was looking for.

here are some links that should help explain RC Servos and PWM (pulse width modulation):

http://www.lynxmotion.com/images/html/servo01.htm

This link has a servo that has been modified to be capable of continuous rotation. I didn't read all that they did to it, but it might be a place to start:

http://www.acroname.com/robotics/parts/R174-CONT-RO-SERVO.html

Here is a link to a PIC / Robotics online store, they have a LOT of code available, just look around there.

http://www.parallax.com/index.asp

And since I can't help myself; a great place for help if you decide to go with AVR microcontrollers is avrfreaks.net

This is a little off your request of help for Servos, but these are some fair prices for gearhead DC motors that would work great for fast and/or powerful wheel drives.

http://www.lynxmotion.com/c-50-motors-wheels-props.aspx

The problem with DC motors is that unless you incorporate some feedback to control the RPM, it is almost impossible to get good control of RPM. Their output RPM (revolutions per minute) is not proportional to voltage, or current. Steppers work much better in open loop (meaning no feedback) because you "step" the motor through its four phases. Unless the load exceeds its torque capabilities, which will cause it to miss steps, you can assume that as you energise the next phase the motor shaft will move. For info on stepper motors search, here are a couple links to get you started.

Here is a page comparing several types of motors commonly used in robotics:

http://www.robotics.com/motors.html

Always a great place to look for good help is a site called Wikipedia, this entry should give you the nuts and bolts of how a stepper motor works. Let it be known that there are two types of stepper motors; unipolar and bipolar.

http://en.wikipedia.org/wiki/Stepper_motor

As always, Google is your friend.

thanks a lot. more infos are welcome.