Serial PWM motor controller

[dtengineering]

After looking at the pages you guys showed me I've decided to attempt to build my own serial based motor controller from this article: http://www.rentron.com/SerialServo.htm

Good on ya' mate! That'll keep you busy for a while.

One question about that article, the diagram shows a PIC: http://www.rentron.com/Files/Stamp/pc-servo2.gif

I've located this PIC and priced it (around $5, not bad) but won't I need to program the PIC itself before it does anything?

Yes, you will need to program the PIC. Fortunately there are a number of simple PIC programming circuits you can build. One that I have had luck with (note the caveats on the main page) is the NoPPP (No Parts Pic Programmer) There are also other designs available. Googling "DIY PIC Programmer" or something similar should get you good results. Some of the newer PICs (the 16f627a, for instance) offer Low Voltage Programming, but that still requires that you program them with a "high voltage" (12v) programmer at least once to set them into LVP mode. Some of the newer PICs also have internal timing circuits, removing the need for a ceramic or crystal resonator unless you require highly precise timing. The catch in using a newer PIC, however, is that if the code is written for a 16f84, then you don't want to have to re-write it. You've got enough to do on this project, already!

Another small question, the diagram shows one pin from the microchip connecting to each PWM output, well it's my understanding that PWN cables have three wires (black, white and red). If the microchip only outputs to one wire then I'm guessing that it's the white one and that black and red are battery and ground?

Yes.

Also, what is a 4MHz Ceramic Resonator? I've looked one up, a little $1 part that I've seen before but what does it do?

This controls the timing for the base level of operations on the PIC. If you put a 10MHz resonator on the PIC, you would essentially be "overclocking" it. As the resonator "vibrates" the voltage changes, providing a stable timing circuit to the PIC. The PIC then uses this to control all communications and data movement within the chip. You can actually use the frequency of the resonator to determing how long it will take the PIC to execute any specific machine language command quite accurately.

- I need a way to recharge the battery without a car, I'd like to build a battery charger that could charge it using a household 120V AC plug. How simple would that be? I assume I can't just plug one end of the plug to the + and - ends of the battery.

Well, no... but it is almost that simple. All you need is a DC voltage source of about +13.6 volts or so. A variable power supply, a "wall wart" transformer of the kind that takes up all the space on your power bars when you plug in the scanner, speakers, etc., or the battery charger used by your FRC team will all work well. You can also find trickle chargers, and fairly cheap car battery chargers at a hardware/automotive store like Canadian Tire (not that you have a Canadian Tire in Palm Bay, but you definitely have something similar. UAP-NAPA dealers might be a good start point.) Just be cautious that if you are using a "dumb" charger (ie a constant voltage source, or anything that does not specifically advertise itself as either a trickle charger - where the current is so low that it doesn't much matter - or a smart charger) then you have to be careful to not overcharge the battery. The scientific way to determine a car battery's charge is by measuring the density of the electrolyte (acid). Your school's auto shop probably has a small turkey-baster like device for doing this.

- I'd like to be able to make something, perhaps a small LCD screen that could tell me the amount of charge left in the battery. So that I never drain it all the way and so that when it's charging we know when it's done. I believe the drop in voltage is how you can tell the amount of charge left? Like when the Vex controllers show voltage on the little LCD.

This would be nice, and once you've finished all the rest of this project I suspect you will have no problem building one of these. But do the meat and potatos first, and save this for dessert.

Good luck,

Jason

[smartkid]

Extremely useful information, thanks dtengineering.

To Daniel_LaFleur:

Car batteries usually have 40 Amp hours.
20% of that is 8 Amp hours.

Yes but what I said is that the battery cannot be drained lower then 20% charge. So if I'm right then that gives me 80% usable charge with is 32 Amp hours. Of course I'm the newbie here, maybe I'm wrong.

While I'm here, whats the difference between a stepper motor and a regular DC motor? I remember finding a small stepper motor in a hard drive and doing a bit of research on it but besides that their foreign to me.

Also, where would be a good place to look for cheap DC motors? I need something in between Vex motor size and FRC motor size. We'll call Vex small and FRC extra large, I want a medium.

Thanks,
-Smartkid

[smartkid]

I went ahead and whipped up a small 3D model showing what I have in mind for the bot design. This model is very incomplete but it does show the computer components to scale. The red part is the frame and wheels I hope to build for the project later on. The whole thing is about 24in x 24in x 16in. I believe that I've allowed plenty of room for custom made electronic components and a car battery.

The design allows the motors to be in their own little area in the omni-directional wheel pattern. I'm going to have to figure out a way to make my own omni wheels but at least I know what I'm doing there.

Any comments?

[dtengineering]

Yes but what I said is that the battery cannot be drained lower then 20% charge. So if I'm right then that gives me 80% usable charge with is 32 Amp hours.

The figure of 20% is somewhat arbitrary, and refers to the amount of charge that you can use from a car battery without eventually causing damage to the plates. So you CAN extract 100% of the charge and drain a car battery to zero... but the deeper you drain it the more rapidly damage accumulates. Looking at what you have in mind, a car battery will be more than sufficient. I suggest that the power source is not going to be the "bottleneck" that holds you back.

While I'm here, whats the difference between a stepper motor and a regular DC motor? I remember finding a small stepper motor in a hard drive and doing a bit of research on it but besides that their foreign to me.

Did your "bit of research" include checking wikipedia? Simply googling "stepper motors" brought that up as one of the first links. Remember to read to the bottom of the wiki entry for the links if you don't get it just from the article. One of the reasons you are getting good answers to your questions is that you have done some research and are asking intelligent questions. Please don't stop now. You might have luck for some of this at howstuffworks as well, or by googling "stepper vs. servo".

Also, where would be a good place to look for cheap DC motors? I need something in between Vex motor size and FRC motor size. We'll call Vex small and FRC extra large, I want a medium.

Did you look for motors at the robot shops such as www.robotshop.ca as mentioned earlier in this thread? You may also wish to try out banebots. You will probably be wanting a gear motor of some kind unless you are planning to build your own gearbox... which is probably more expensive than just buying the gearmotor. I can't promise that you will find any gear motors that fit your definitition of "cheap", however.

Perhaps you need to contemplate what is important about your project. Is it the size of the project, or the skills you are learning? Scaling back the size of your robot to use vex sized components (there's your omni wheels for you...) could be done without losing any of the educational or technical value and may put the project within your financial reach.

You may also wish to consider checking books such as "Robot Builder's Bonanza" from your library and reading them before carrying on further.

Jason

[Kevin Sevcik]

If you're looking for motors and any of this other stuff, find out if you have an electronics surplus or hobby store in your area. They're somewhat more rare than they used to be, but they're often a good resource for parts and motors if they exist. There's one in houston that has a decent selection of all sorts of surplus motors for pretty ridiculously cheap prices. Amazingly there's apparently one in your town. Check out AstroToo

[smartkid]

Thanks again. I did do a Wikipedia search on stepper motors, at the time I really didn't understand much of that article but now I should be much better off.

As far as the Vex parts go, well I don't personally own any of those either. I do have some of my schools vex parts but those are just to work on our FTC bot for this year. I would consider using the vex parts but I'd still want to use the motherboard instead of the vex microcontroller. Which could still be done however it would make a rather large vex bot.

But in this case small might be good. The question is will the vex motors be able to handle moving the computer components and a battery to power the whole thing... I don't think so.

I think it's time for me to stop asking questions and start finding answers on my own. You guys have been so helpful, thanks!

-Smartkid