One of my first attempts at building a robot was a project I had found in a book. Basically the idea was to scratch build an electric motor powered car, a simple electronic circuit with the key components being two photocells at the front of the car. I pretty much just built exactly what the book showed in the diagrams. I made a wooden triangle base, I rounded up two electric motors of matched type and size which I had from a radio controlled cars that had long since been busted up. (2) two and a half inch model air plane wheels, one small caster, one small electronics experimenters board, batteries and battery holders, wire, and a handful of small electrical components which would be needed to be assembled on the experimenters board.
I attached the wheels with small axles at the rear of the triangle piece of wood and mounted the motors on top of the wood so that the two gears would mesh up for the motors to supply power to the gear on the axles. I mounted the small caster at the front point of the triangle base. In the middle of the triangle I mounted the experimenter's board and the batteries. Assembly went well with no problems popping up I wasn't able to figure out how to work around.
When I was finished I turned the car on and nothing. The car just sat there. The theory behind the project was light was supposed to be picked up by the photocells and then based upon the amount of light being received by each photocell that controlled the amount of power to go to the opposite wheel at the back of the car. So if it was receiving more light from the photocell on the right side of the car, the left motor would get more power which would in turn move the car to the right. If more light was getting picked up from the left side photocell it would provide power to the right side of the car. It should in theory chase down the light source.
The book mentioned shutting off the lights and having the car chase a flash light which you would hold. So I shut the garage, turned the car on, shut the lights out and walked to the other side of the garage. I turned around and faced my project and turned on the flashlight. I heard the motors start up and a slight yelp from the tires burning rubber on the concrete floor. I saw in the light provided by the flash light this car screaming across the floor towards where I was standing. As I instinctively jumped out of the way of this triangle (pointed front) car the beam of the flash light hit the wall on the right as I jumped to the left. The car jerked left coming off the floor and smashing into the wall of the garage as if it had been a model airplane and flown into the wall.
The project was toast! It took me quite a while to figure out exactly what had happened. It had worked! It did move and chase the light! What I couldn't understand was why it had moved so fast and blasted itself into the wall. It hit the wall because that’s where the light was shining the photocells were picking up. So then I had to discover why it had traveled so terminally fast.
The book had not specified the type or size of motors to use for the project. Neither had it specified the size or type of wheels to use at the rear of the car. I had used radio controlled car motors, but I hadn't taken into account is most radio controlled cars have a gear set up which reducers the revolutions provided by the motors and provides a bit more power in the driving of the car. The gear set up I used was the small gear on the wheel axle because of the small space from the bottom of the car to the floor. I had used a big gear to make up the distance from the motor drive shaft to the axle gear because there was plenty of room on top of the car. I hadn't considered that this was actually gearing the speed up, increasing the revolutions. So say the motor turned at 2,500 rpm and my gear set up increased this by about 3X that would be 7,500 rpm.
The wheels used were model aircraft landing gear with a diameter of 2 1/2 inches. The circumference of the wheel was 7.85 inches. Which means the car would move 7.85 inches per revolution of the wheel. 7.85 X 7,500 translates to 58,875 inches per minute. Divide this number by 60 and the car would travel 981.25 inches per second. A fifty foot garage is 600 inches in length. This creation of mine was going to attempt mathematically to cross that distance plus another 31.7 feet in the first second. As I said it didn't get the best traction in the world and the squealing from the tires was evidence of some slippage and loss of traction.
I should have geared the thing down, and maybe used much smaller wheels in the back. That's why my creation attacked me. It did work very well for a fraction of a second! It left a gouge in the wall of the garage and I learned more about gears and the importance of looking at all aspects of a design than I did about the electronics and photocells. My crazy scientific learning and discovery didn't stop there and hasn't gotten much safer over the years. My family has presented me with a fire extinguisher and a full blown first aid kit for my garage! It is well known to not let me near super glue either because of the tantrums and cussing involved when I glue my fingers together. I also was given a box of latex gloves to use in case I sneak any super glue into the garage as the gluing the fingers together has happened on multiple occasions!
Last night's study was Verbs, Action Verbs, and Linking Verbs. I found out how to pronounce Gerund, and had to look up the definitions for transitive, intransitive and participle. I thought editing sucked!
Posted in: photocell motors gears rpm's robot
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