Serial PWM motor controller

[dtengineering]

Originally Posted by smartkid

No way


A question: Is there a major difference between USB and RS-232 serial?

Yes. At least if you have to do the programming. RS-232 is dead simple on a PIC, or other microcontroller, especially if you are using a high-level language like PIC Basic Pro. While some PICs are USB capable, at least from what I have read, I have been hesitant to jump in as the protocols do seem to be a bit more complex.

The question that comes to mind though is do I really need to buy 4 $100+ speed controllers? I've worked on our teams FRC bot, what we're hoping to do is about 1.5 times smaller then that and shouldn't need quite as powerful motors.

There are a number of sub-$100 options. You could check out www.robotshop.ca or a number of other on-line robotics suppliers for options. Or you could take a look at building your own. Here are some links that might help, the first one is truly worth checking out:

http://www.mcmanis.com/chuck/Robotics/
http://www.barello.net/Papers/H-Bridge.pdf
http://www.armory.com/~rstevew/Publi...d/h-bridge.htm

There is also an online collaboration to create an "open source motor controller", or try googling robotics clubs such as the Seattle Robotics Society. They have some excellent information and links on their web site.

Another small question that I'm sure someone here knows is, how long would you think a car battery would power the motherboard and the motors?

The short answer is "Long enough!" If you are thinking of doing deep discharges on the battery (beyond 20% of capacity), however, you may want to go with a deep cycle battery. If there is any danger of the the battery being tipped over, you probably want a gel cell. A typical car-sized deep cycle gel cell (such as used in Electrathon Racing) will have a capacity of 40 amp hours. IE you could theoretically draw 40 amps continuously for one hour. That should be more than enough to smoke your motors! On the other hand, if you are using a typical car battery with a 40 amp hour rating, you would only want to use 20% of that (8 amp hours... or 16 amps for half an hour) before recharging or risk quickly damaging the plates in the battery after just a few deep discharges. Perhaps there is an old FIRST battery sitting around that your team doesn't need for a while, or perhaps you want to buy a new one when the KOP arrives.

And finally one last question, when working with Vex parts I noticed that the servos can work directly with the controller inputs (PWM output value of 0 set it all the way in one direction, PWM output of 255 spun it around the other way), do real servos work just like this? And do servos also need speed controllers (I assume yes and yes).

Well, first of all the VEX servos are "real" servos. If you are referring to Radio-Control servos, the answer is a qualified, "Yes, as far as I know." When you refer to a PWM output of 0 or 255 you are referring to what fraction of the time the Pulse-Width-Modulated (PWM) signal is "high". At 255 it is essentially always high, at 0 it is essentially always low, and at 127 it turns on and off rapidly, being high 50% of the time and low 50% of the time. Unless there was some weirdness to the VEX servos, I would expect the VEX output would work with any standard RC servo. The RC servos, like the VEX servos, would not need a separate speed controller. In fact, the Mini-RC system, from which IFI developed the VEX systems, was built around standard radio-control servo technology.

Sounds like you've got a great project ahead of you and lots to learn,

Good luck,

Jason