simple electric car

Need more details... you plan to ride this in the lounge, on the sidewalk, or on the freeway?

Bye. Jasen

Check out the Power Wheels cars for kids. The one that I have seen uses a 6V sealed lead acid battery under the seat, and 2 DC motors, one at each rear wheel, connected by a solid axle. The instructions state an upper load limit of something like 80 pounds. I assume that's due to motor overheating and the like. So maybe if you added 2 more motors (and probably another battery) for the front wheels you could expect 160 pounds or so.

Just a thought..

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Interesting idea.

You don't need 4 motors. The more motors you add, the more complex the car becomes because you'll have to syncronize the speed of the motors. If you're going to have a 4 wheel vehicle, then your best bet is to use a single motor and a simple drive train to drive both the back wheels at the same speed.

The battery is going to be a problem. It's the challenge of all electric vehicles. You'll have to find a balance between weight, power, and cost.

Not a good plan. First off you can't easily find a pot that can take the power required. Secondly, any resistive element for speed control will burn a lot of battery power up in heat, which you'll then have to dissapate.

The virtually universal technique for speed control in these situations is Pulse Width Modulation (PWM). With PWM you use a switch to either apply full power to the motor, or compeltely turn it off. The ratio of on/off time of these pulses will determine the speed of the motor.

Also if you plan to go backwards you'll need some direction control.

At the end of the day you'll need a somewhat sohpisticated motor controller to get it all to work. Take a read on H-bridges, which is a circuit that gives you forward/backward control and usually works will with PWM to govern speed.

For efficiency you'll want to check out Permanent Magnet DC motors. There are high efficiency motors that are often used in battlebots that can serve the need. Here's a sample E-Bay motor that's in the right ballpark:

Now this isn't going to be cheap. By the time you take care of Lead Acid batteries, the motors, the gears and the frame, you'll be a few hundred bucks in.

Good luck and be safe.

BAJ

--Have a look at battlebots people have built. Generally they have 2 motors but you might want to go with just one. The other thing you'll need is a big-ass motor controller, but there are some good ones that won't cost an arm and a leg.

You'll likely have the best results using two motors driven using PWM (as has been mentioned before). The added advantage of using PWM on two wheels is that it can be used for steering as well. By varying the speed between the two drive wheels, the vehicle will favor one side or the other. This would lead to some issues with the remaining two wheels, though, if none of the wheels turn.

Which ever way you choose, you'll have many design details to take into account.

Here are a few videos showing simple line-tracking, ball-collecting robots that use the type of two-motor, PWM controlled speed and steering I was talking about:

Even with these small bots, though, the weight vs. battery issue is a serious consideration. Batteries are heavy, and must be recharged. Of course, a passenger is heavy too...

If you're dead-set > I was thinking of building a simple electric car.

Equal torque is probably more useful than equal speed. (this is the basis of the differential used in conventional transmissions)

when cornering the front wheels (if they are for steering like in a normal car) travel further than the rear wheels.

Equal torque can be approximated by using the same PWM signal to drive all the motors. (which makes for a simple design that will work reasonably well)

That said, detecting a wheel that's turning much too fast or much too slow and correcting that is the basis of traction control and ABS.

Bye. Jasen