The autonomous vehicle project is moving along faster than I can keep the blog updated. There are a lot of topics that I’ve been planning to write about, but just haven’t taken the time to get them out.
Our biggest concern so far has been the steering. As it turns out, the steering mechanism in a PowerWheels car is about as basic as it could possibly be. As you can see in the image below it’s essentially a single metal rail that moves psuedo-linearly from left to right when the child spins the wheel.
What concerns me is that I don’t have the mechanical engineering background to really plan ahead for this. At first, I thought I could simply replicate the steering system used by radio control cars. Unfortunately, it seems as though several people have attempted this route, and wound up with broken servos. The torque required is just too high for a hobby servo, and I’m at a major disadvantage because I don’t have the foggiest idea how to mathematically determine the required torque, under a given weight, and with variable road conditions beneath the car. HiTec does make a relatively high-torque servo, but I haven’t had any luck finding a servo horn large enough to be practical.
Next, I tried to create linear motion from a servo by modifying the servo for 360 degree rotation putting it together with a flange nut, threaded rod, and coupler. 
This setup didn’t really suit our purposes however. Modifying the servo means losing positioning information, which means I would have had to find another way to monitor the position of the wheels. The linear motion also proved to be painfully slow. Although I can imagine there are plenty of interesting uses for this configuration, I didn’t think it would be practical for our autonomous vehicle.
In the end, we decided that a linear actuator would probably be the cleanest possible solution. There were a few downsides though. The biggest downside was simply cost. Even small, relatively low power actuator costs around $90 US. Those prices don’t leave much room for error. The other major concern was speed and torque. The Firgelli Tech L12 actuator that we ordered was the highest torque available in that price range at 67 Newtons (~15lbs) of force. It’s not quite as powerful as the HiTec HS-805BB which can handle a reasonable 21lbs of force, but the linear actuator removes the much of mechanical complexity from the design. At the end of the day, I feel that the actuator will be more stable and less prone to breaking. If 15lbs of force turns out to be insufficient, then we’ll have to consider one of the jumbo servos.
Later this week I’ll have pictures of the actuator installed on the vehicle.
Until then,
Tim
