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If you're buying Ultimate Real Robots magazine, you'll know that I'm showing you how to build a small but feisty version of Cassius. Called Cassius Junior, it's made to a simple design formula that uses mostly re-cycled materials and requires very little technical know-how to make. The idea is for people to build machines to the same formula and then pitch them into battle against one another to the death!!
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Well, we've reached the exciting final
stage fitting the electronics to control the robot, and for that I've enlisted the help of electronics engineer, Team Chaos's Ian Swann. He's come up with a prototype for two boards: a micro-controller and a speed controller - finished boards now available from www.technobots.co.uk
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MICRO CONTROLLER AND SPEED CONTROLLER BOARDS THE BEST CHOICE
What we were after was a more effective alternative to the rather crude 'bang-bang' approach, where you steer by simply switching the motors on and off one side 'bang' and then the other 'bang' (hence the name)! Although this is the cheapest way to control your robot, it's hard work for the driver because there's no proportional control over the speed of the motors they're either full on or off, nowhere in between.
(I'll cover the 'bang-bang' approach from issues 32 onwards of the magazine for those that can't afford the boards.)
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HOW THE BOARDS WORK
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PROPORTIONAL CONTROL
Here the idea is that instead of steering your robot
using both joysticks, you use just one.
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A) Pushing the stick forwards makes the robot go straight. The further you push it, the faster it goes. That's proportional control.
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B) If you push the stick a little to the right, the robot will turn to the right; push it a little to the left; it'll go to the left.
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C) With the joystick at 45º to the right, the robot spins in a clockwise direction about the right wheel. (Only the left motor runs in a forward direction.)
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D) Push the stick hard to the right and the robot spins clockwise about its own centre. (The left motor runs forwards, the right one runs in reverse.) Again, the further you push, the faster your robot spins.
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It's a totally intuitive and logical system, and one that's natural to drive with. But achieving this result requires sophisticated electronics.
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TRIED AND TESTED
Ian made up prototypes of the two boards. We tried them out in Cassius Junior and, after changing a few components and fine-tuning the micro-controller, soon had the little fella driving round better than I'd ever expected! Since then the boards have been improved, refined and manufactured to professional standards, and when connected together correctly, they're very robust.
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LOGICAL CHAIN OF COMMAND
The chain of command is as follows. Your transmitter 'talks' to your on-board receiver. The receiver passes on signals to the micro-controller board. The main PIC chip on the micro-controller decodes the signals and, according to the chip's logic circuits, converts the signals into a set of commands that the speed controller can interpret and act on.
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The PIC chip is the 'brains' of the whole system, and the secret to its processing power is in the lines of source code written on a computer and used to program it.
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The speed controller controls the amount and direction of current
flowing to each motor. Changing the speed and direction of each motor is what steers your machine. The boards are installed by simply connecting them together with plug-in connectors according to the instructions provided.
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INTERFERENCE FREE, PROTECTED ELECTRONICS
Why two boards? The radio receiver must be isolated from any source of electrical noise that might interfere with it. Two such sources are the windscreen wiper motors that power your robot. Since these are connected to your speed controller, the micro-controller must be separate from it. Another reason is because any high currents flowing back into the board (such as when the motors suddenly stop) would almost certainly destroy it. So the micro-controller and receiver have to be protected.
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By making the speed controller handle all the high power side, the micro-controller is left to all the low power, highly-intelligent stuff. But of course, if the micro-controller is going to 'talk' to the speed controller, it has to be linked in some way.
This is done using an electronic device called an opto-isolator (actually sited on the input to the speed controller board). The micro-controller uses very rapid pulses of light to 'wink' at the speed controller, and the signals are then converted back to electrical currents to drive the speed controller. Even a tremendous current surging back towards the micro- controller would not bridge the gap.
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A KIT TO SUIT YOU
There are two packs available. For the least technically able, both boards come ready-made and tested. They are installed simply by plugging together the connectors according to the instructions provided. For the most competent, there are packs containing two bare PCBs and a pre-programmed PIC chip. Both packs are available EXCLUSIVELY from the website www.technobots.co.uk
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WARNING
Please note that making these circuit boards requires a very high level of technical skill and should not be attempted by anyone doesn't feel up to it. In spite of their design, ready-made boards may be damaged if they are not connected correctly or if the robot has not been assembled correctly. You must follow the manufacturer's instructions.
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Disclaimer
Eaglemoss International Ltd are in no way associated with the manufacture or manufacturers of these products. Eaglemoss International Ltd disclaims all responsibility for any loss, injury, claim, liability or damage of any kind resulting from the purchase or use of the product, or for damaged or faulty boards and parts. All enquiries regarding this product should be made via the www.technobots.co.uk website.
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