Building a Robocar: Turning Challenges and Solutions

[anonymoussome]

I am ona project of making a robocar and I want it to turn.
After my first attempt I realized that it should be a three wheel car with a ball bearing in the front as the third wheel.
But I want to make it a four wheel drive and the problem is:
I use simple motors and they are free to rotate in both directions.So Even if I stop motors on one sid's set of the wheels tehy continue to move in the forward direction.
Any suggestions?

 

[Danger]

To start with, you cannot have 4-wheel drive on a 3-wheel vehicle.
If you insist upon using a ball bearing as your 3rd wheel, I'd recommend using an inverse version of a computer mouse. (ie: Put drive wheels where the sensor wheels would be on a mouse.) That, combined with differential braking on the back wheels, should give you all of the manoeuvrability that you could hope for.

 

[rcgldr]

Assuming these are DC type motors, you need a speed controller that shorts them out (or a low resistance connection) to get a braking effect.

 

[anonymoussome]

Please elaborate

"To start with, you cannot have 4-wheel drive on a 3-wheel vehicle."

As four your comment, I am not trying to use a four wheel drive on a three wheel robocar.
I have used a three wheeled robocar previously and now I wish to use one with four wheels.


"Assuming these are DC type motors, you need a speed controller that shorts them out (or a low resistance connection) to get a braking effect."
I am using DC motors,but I don't understand how to do that braking effect. If you are saying to connect a shunt across the motor then also the motor would stop to rotate but the wheels of the other side would tend to pull it along, and as there is no restriction on the rotation produced mechanically, the wheels would continue to move even when motors on that side has been turned off or has been shunted.

There is another technique:
Make one motor move forward while other moves back, which wolud make it turn on the same place, but I don't wish to use it.

 

[Jwink3101]

With regular DC motors, shorting them is not the same as turning them off. DC motors act as generators and when they are shorted, it is pulling a larger load and acts like a brake.

I do not know how much you will see it with cheap or small motors but it is there. For example, with Lego motors if you cut the cable and short it, they stop immediately and are very hard to turn.

If you are more interested in why, let me know. Basically, it has to do with the motional emf and if current flows then back emf is created. It isn't complicated but i have to read about it again.