This project is BEAM robot made from a 555 timer, two photocells, two motors and pieces of a toy car mixed with pieces from a robo-vacuum cleaner. This robot is a so called PhotoVore. This means it follows light. It is pretty ugly, and I am happily calling it “Junkbot”, as it looks like it’s made of pieces of junk. Which was in fact exactly what I was trying to do. However it does work as intended.
What is BEAM?
BEAM is different way of making robots that sets itself apart by avoiding complex parts. Basically robots made with this idea, are made out of analog electronic components. This is in sharp contrast to the idea that Robots must be based on a CPU like architecture, where all the processing is digital. The result is very cheap, and very robust, though limited, and hard to augment robots. BEAM is a very interesting way of getting into robots, not lastly because it is very cheap. The godfather of BEAM is Mark Tilden, and the place-to-go on the internet is http://www.solarbotics.net/.
The Robot “Brain”
This is the circuit of the robot. As you see it is very simple. This is a key benefit of BEAM robotics. Simple circuits, and no programming. The circuit contains two resistors, 4 capacitors, 2 NE555 chips, and one diode. Off circuit there are also two photocells and two leds.
Allthough it is quite simple, I did manage to make a “spagetti board”. This is not recommended, as you get noise in the circuit, which leads to unexpected behaviour, and can be hard to figure out.
Understanding the robot
To understand this simple robot, there are some things first that you need to understand on how NOT to do it.
- To change the speed of the wheels, it is not enough to just reduce the voltage on the motors. You need to learn. about PWM.
- To move the robot, connecting the wheels directly to the motors is a BAD IDEA(tm). Gearboxes are MANDATORY.
- Gearboxes are best bought (not made).
This is basically the stuff I learned when “messing around” with the “prototype” of this robot.
Now what is PWM?
PWM is Pulse Width Modification. Or in other words, a very nifty way to change the speed of electric motors. In short, it means that you turn the motors on an off REALLY FAST. However the offtime and ontime can vary. More ontime means faster motor rotation. This difference between on and offtime is caled “duty-cycle”.
Yes, gearboxes. Tiny electric motors are NOT strong enough to move a robot, which contains at least the weight of these two motors, some electronics, a frame and relativly heavy battery. The motors spin fine with the wheels from the ground, but when the wheels are on the floor, it will not move at all.
I had the idea to “BUILD” a gearbox from the beginning. How hard could it be, add some axles, some gear wheels, and a plastic box, and done. It is probably not THAT hard, except for the fact that getting these parts is not that easy. And building a “plastic box” to house it in, is not that easy either. Eerything needs to fit very snug, so you need custom sizes, and cutting plastic may be easy, glueing plastic is definitly not. Getting a good bond on plastics glued on themselves may work, but you need a REALLY good bond, for it to hold together when forces are applied. I ended up with two different ratio gearboxes, and hacked one of them to accept it’s position on the robot. (It did not have a suitable axis protruding, I needed to add a an extra axle and gear). This reasonably went ok, using HOT GLUE (the wonder stuff for DIY robots). The only negative is that hot glue is a bit elastic, and gets deformed over time when force is constantly applied to it.
The NE555 Chip
This is the heart of the robot. The NE555 chip is not used alot in BEAM robotics, but I bought it in did not want to give up before I got my robot to work. The robot contains two of them. Why two? Well each wheel speed is seperatly being controlled by the light that falls on the photosensor. However the light on the photosensor needs to be translated into a PWM signal. Twophoto transistors, two motors, and thus two NE555 chips. In fact the robot has really two “brainhalves” which are identical. The NE555 is wired a an astable oscillator. The restistance of the photocells drive the frequency but also the DUTY cycle of the NE555s. A DUTY cycle is nothing more then the on versus the off time of an circuit, when switching the signal really fast on and off.