Electric Vehicle C

[gh]

When driving servos, the speed/time curve translates directly into pulse width input to the servos. e.g. A 1.5ms pulse commands 0 speed, a 2ms pulse commands full speed, and a .5ms pulse commands full speed in reverse. (check the pulse widths are correct, I'm going strictly from memory). Acceleration is the rate of change (slope) of the pulse width.

Note: in unmodified servos, the input pulse width represents a position error. The servos drive to a position (rotation angle from center position) relative to the pulse width. Servos modified for continuous rotation use a fixed position signal equivalent to the center position. This makes the input pulse width a speed command rather than a displacement command.

Full reverse is usually 1.0 ms, but I suspect that was just a typo.

How a servo reacts to modification to continuous rotation really depends on the controller, but most of the time it's a close approximation to a simple P (for proportional—the velocity response is linear to the error) controller. Most of the time they will actually hit full speed forward/reverse even if the error isn't yet equivalent to 500µs of displacement, so servos will probably most responsive to control than you would expect.

BTW, the Basic Stamp 2 in the BOE bot uses the PULSOUT command for servo control, which takes a 16-bit number as the number of TWO µs, if you're wondering why the program would have PULSOUT 750 instead of the 1500µs we're using here. The Vex system abstracts it more, and you just set motor speeds in a signed 8-bit number (-128–127).

I posted my EV's progress to the Image Gallery: 26/EV_foil_2009.png

[captbilly]

I am a senior and this is the first time my high school has a Science Olympiad team. I have no knowledge of using electronic components like a board of education or micro controllers and servos. I am working a partner who knows a lot about this electronics and we are using a Boe-bot kit from parallax, and we need servos that turn faster. I know that we need the vehicle to go at most 10 meters in 45 seconds and we can currently get it to travel 8.427 meters in 45 seconds. How can we get the wheels to turn faster? We are using Basic Stamp 2 to code our vehicle and are using the Board of Education Rev C. if it helps. And we do not have a breaking system for our vehicle, is it a must because our vehicle is able to stop instantly. Any help would be great. Thanks.

You actually need to be able to go 10 meters in 15 seconds if you want to compete at Nationals. If you believe that Nationals is out of the question then 45 seconds will be fast enough. There are all sorts of ways to got your vehicle to go faster. The simplest method for a vehicle that is already built would be to increase the voltage to the motor/s. You can use a DC - DC converter to increase the voltage. There are ready made converters or you can build one fairly simply.

By the way that's some fancy measurin you've been doing (8.427 meters in 45 seconds). How did you get such an accurate measurement of time, did you have a built in timing circuit.

[captbilly]

So I am not actually going to nationals this year, but I want to do one event just to keep my problem solving skills fresh. I was wondering if anyone here had worked with stepper motors before. I understand how they work and they look ideal for this sort of application due to their low speed, high low speed torque, and the ability to precisely control speed. I was wondering though how accurate they are over long distences. Basically can I rely on them totally to get distance or should i put the 500 pulse count encoder that I have from last year on my car as well.
One last thing. I am considering, in the coming month or so, of designing up a Dc-Dc board for ev in the coming month based on some of the chips they offer from linear devices as you can get samples for $4 dollars on some of them. I am looking at a device that would output somewhere in the range of 10-15 volts at three amps or so. I know its kinda late but would anyone else be interested in a PCB after I test it to make sure it works (I can borrow almost any electronic test equipment I need from my dads work) I have never done PCB layouts before (but have played around with eagle) and I dont know as much about electronics as some of you so perhaps it could be sort of a community project? EV might go away after this year but a well documented project based on readily available parts could come in handy in latter years.
Oh one last thing about these things I know its kinda obvious but dont use too large of a capacitor with them. I make that mistake last year and smoked a dc-dc converter that cost me over 100 when I connected a 20,000 mF filter capacitor... (Sometimes I dont think clearly at 4am)

If you don't accelerate too fast of run the stepper too fast then they can be absolutely accurate. If you attempt to accelerate faster than what the motor can do then you will skip steps and your distance and time will be off (and unpredicatable). Likewise if you try to do too many steps per second then the motor may skip steps or do more than one step per pulse. The question boils down to whether or not your stepper is fast enough and powerful enough to get the job done without any missteps (pun intended).

[r00bin]

Just for rule clarification, if we bring a laptop to program a Lego RCX, do we have to impound the laptop with our vehicle?

Thanks

[gh]

Just for rule clarification, if we bring a laptop to program a Lego RCX, do we have to impound the laptop with our vehicle?

Nothing posted on this forum can really be counted as official, and that sort of thing is determined by your tournament directing people anyways.

[r00bin]

Just for rule clarification, if we bring a laptop to program a Lego RCX, do we have to impound the laptop with our vehicle?

Nothing posted on this forum can really be counted as official, and that sort of thing is determined by your tournament directing people anyways.

ok, thanks

[Mike4192]

We had regionals last saturday. My purely mechanical vehicle got a raw score of I think 197.23 and got first (from what I remember). Distance was 6.5 m, we were off by 0.5 cm and time was off by 0.6 seconds.

With the mechanical bonus added in, that gives me a score of around 198. Wonder what place that will translate to in states.

[Dark Sabre]

Just for rule clarification, if we bring a laptop to program a Lego RCX, do we have to impound the laptop with our vehicle?

Thanks

As gh said, the answers here are not official in the "clarification" sense [warning: hot word].

The rules are, however, pretty specific on this point...you need to impound: 1) Your vehicle 2) Your batteries
The rules also give examples of what you don't need to impound: Tools, data, and calculating devices.
It also says that "Contestants may bring tools for adjusting the vehicle." What you are doing with your laptop is adjusting your vehicle's settings.

I think it is pretty clear, but you can submit the question for clarification if you think it is necessary.

We had regionals last saturday. My purely mechanical vehicle got a raw score of I think 197.23 and got first (from what I remember). Distance was 6.5 m, we were off by 0.5 cm and time was off by 0.6 seconds.

With the mechanical bonus added in, that gives me a score of around 198. Wonder what place that will translate to in states.

Very nice. What kind of braking system were you using?

[sirup96]

Try having a car that do 8.5 and gets 8.49 and a p time of 33 and get 33! the only thing wrong was it was off to the right by two cm!

[sirup96]

Is any one having problems with two servo moters and runing them at the same time?