GPS Guided Robotic Car

[imagitronics]

A quick question about linear actuator force:

The Article "Turn a Kid's Ride-on Car into a GPS Guided Robotic Car" in the April 2008 issue of Servo Magazine left me salivating. I decided to give it a while myself.

Yesterday, I picked up a used Power Wheels car from the classifieds. The only issue that I foresee is the steering. I'm thinking about using a linear actuator to control the car. The car's steering is pretty straight-forward as you can see below:



The "center hole," used to steer the car, moves 1.25" maximum. The Firgelli Technologies L12 6V 30mm 298:1 PLC/RC Miniature Linear Actuator looks as though it may fit the bill. Operating on 6V (the battery voltage) with 30mm (1.18") of linear motion, this may be the ideal solution. However, the maximum peak power point is 67N (~15lbs) of force. I'm not much of a physicist, and I was wondering if anyone thinks this would be an acceptable amount of force for this project.

Any information that you could provide would be extremely helpful.
Thank you,
Tim

[MrAl]

Hi there,

15 pounds is quite a bit of force for a small car, but then again we
dont know how much the car weighs or speed or what effort arm you
intend to use which affects turning radius.
Worst case if it did not have enough force at a particular mounting
you might shift the end connected to the axle away from the rotation
point. What this does is increases the apparent force according
to the amount moved. The downside is that the turning radius
decreases. I dont know what turning radius you are after either though
but you will at least end up with something that works.

One test you might do is to connect a spring to the axle in the way
you expect to connect the device. Use a spring that has say 10
pounds of force when it starts to move or at some small stretched
length. The spring will be attached such that it forces the car to
"always" turn (say to the right). You can then run the car up to
speed and see if the car turns as expected. If the car starts to
straighten its path too much the spring is being stretched too far.
If not, 15 pounds will do it.
You could also experiment with different effort arms, that is,
the distance between the end of the spring (on the axle) and the
center of the rotation of the axle (where the axle turns when the
car is making a turn). Increasing this length increases the force
applied to the axle and decreases turning radius, while decreasing this
length decreases the force but increases turning radius.
You could also try to turn the axle (as the car would turn) by hand
to get a feel for how the effort arm affects the turning power and the
radius.