Electric Vehicle C

[gh]

I have another question about determining what resistor to use.

I let P=15 and V=9.6. I then used P=V^2/R to solve for R. My resistance was calculated to be 6 ohms. This is vastly different then the 3.65 Kiloohms specified in the data manual. The data manual also states that I should use the equation R=15000ohms*2/(V-5)-2.94 to determine the resistance based on my desired output voltage. I'm confused as to how this will output will have enough current and power to run the RCX when power is conserved.

describes the characteristics of a simple closed DC circuit. The resistor specified by the TI converter datasheet is a reference resistor used by a more complex circuit inside the converter to sense and regulate the output voltage. The idea is that they are two different things.

For now, just calculate what voltage you need to power up the RCX/NXT, then figure out using TI's equation what your reference resistor's value should be. does not apply here.

[captbilly]

Remember that your processor must never drop below a safe operating voltage, even for a microsecond. Motors are notorious ofr generating electrical noise so if you are using the same DC-DC converter to drive your processor and motor you will need to be very careful to have pleanty of power and or filtering. I am not sure where the 4 cell battery limit comes from but it will certainly cause some serious problems for many electric vehicles. I would think that the ideal solution would be for a DC-DC converter with a large filter cap to power the processor and a direct to the battery connection for the motor (or a separate DC-DC converter). For reasons that I am not familiar it is fairly difficult to find DC-DC converters that will supply substantial current from 5 volts to a higher voltage, but it is easy to find a converter that will easily supply the processor alone. Most of the companies that sell stand alone DC-DC converters (Vicor, etc.) do not sell anything that will output more than a few watts with an input of 5 volts and an output of over 5 volts. It would be no big deal to make your own DC - DC converter to makes several amps at 9 volts or more, but it's another project on top of designing and programming and calibrating your vehicle. Sort of makes you wonder why a bonus is given for people who don't use electronics.

[WrightStuffMonster]

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)

[gh]

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)

Heh, that's what I meant by:

Bypass capacitors never (well, rarely) hurt anyone.

Uncharged bypass capacitors act as (sort of) short circuits for a very brief amount of time, causing a spike of current known as inrush current. A big capacitor would take longer to charge up, and so the current would be very high for a longer amount of time.

BTW, 20000 mF would have been HUGE, physically. I think most people would realize it's overkill (WSM-style) as soon as they saw one. :P

[WrightStuffMonster]

Uncharged bypass capacitors act as (sort of) short circuits for a very brief amount of time, causing a spike of current known as inrush current. A big capacitor would take longer to charge up, and so the current would be very high for a longer amount of time.

BTW, 20000 mF would have been HUGE, physically. I think most people would realize it's overkill (WSM-style) as soon as they saw one.

They were only rated for 15v so they were not that large...at least compared to the 20,000mF 60v power supply caps I had sitting around. I only decided to use them because starting out the 4 motors I was using would draw 7amps combined for a few seconds when the car got started moving and the voltage output of the converter decreased to the point where my motor controller would shut off. After the dc-dc board caught fire I quickly realized my mistake and put a resistor in the circuit with a switch when i first turned my ev on. It was my second worst electrical disaster last year (the time i short circuited a 10,000 mAh LiPo gets those honors).

Over Engineering works great for events like robots but it sort of failed me in this event last year. This year I am going to try to limit the current draw of the electronics to reasonable levels and spend the money I will save on a machined car base so that my ev goes straight.

[lightning922]

I guess I could tell everyone what I'm going to be doing, since it's so close to the competitions now. I'm going for the mechanical braking system, and basically what I'm going to do, is have a big spinning gear. The gear will have a magnet on it, and I'm going to put a magnet switch which is hooked up to the batteries, the motor, and an on/off switch. When I press the on/off switch, the motor would spin the wheels, which in turn would spin the axle, spinning the gear. When the magnet on the gear comes around to the magnet switch, it would stop the vehicle. There you go, no electronic components used, but a clever braking system!

[gh]

So, just to clarify, you have a magnet on a geared down wheel. The spinning of the wheel makes the magnet approach a magnetic reed switch, which, when actuated, will break your EV's drive circuit (and maybe close a braking loop, a la [wiki]Image:EVcircuitTransparent.png[/wiki])?

If that's right, then it's a very clever braking system, and I think it definitely qualifies for the mechanical bonus. How consistent is it, and how do you set it?

[lightning922]

So, just to clarify, you have a magnet on a geared down wheel. The spinning of the wheel makes the magnet approach a magnetic reed switch, which, when actuated, will break your EV's drive circuit (and maybe close a braking loop, a la [wiki]Image:EVcircuitTransparent.png[/wiki])?

If that's right, then it's a very clever braking system, and I think it definitely qualifies for the mechanical bonus. How consistent is it, and how do you set it?

It's actually pretty consistent. I have a point marked on the gear, and a toothpick attached above the gear. the toothpick would be a "standing point", so all I have to do is match the point on the gear with the toothpick. Afterwards, I marked additional points so that I could determine how far away the magnet has to be in order to turn off the motor.

[astrospirit]

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.

[fleet130]

Use wheels that are 1.187 x the diameter (10m/8.427m)or larger than your current drive wheels. It might be a good idea to use 1.25 x the diameter to compensate for any decrease in speed due to the additional loading and to provide a margin for error.

If you're using servos, a braking system should be unnecessary. The servos should hold their position when their speed is commanded to zero (this can be harder to accomplish than you might think.) Once the vehicle is stopped, turning off the power to the servos should hold it in position.

For most accurate results you might want to consider trapezoidal speed curve as your braking system.



This is often used when driving stepper motors to prevent skipped steps that can happen when you try to accelerate the motor too fast. Skipped steps can produce strange, unpredictable results.

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.


Edit: OOPS! That should have been 1.0ms for full speed in reverse. Speed vs pulse width is usually fairly linear, but different manufacturer/model servos (and individual servos to a lesser extent) can vary greatly.