IAN LANG ELECTRONICS
So far in these pages there's been a lot of theory about kinematics and electrical concerns but little in the way of practical concerns, so let's redress the balance now and introduce to the proceedings my experimental platform, whose name is Goliath because he looks a bit like those scary tracked mines that the Germans came up during the Second World War. There he is pictured below:
He's made from a Dagu Rover 5 chassis with a bit of foamboard screwed on top for a carrying plate, and the battery holder (6 x AA) is stuck down on the plate using a long strip from a roll of sticky fix. Shorter strips stick down the two motor connectors which are the white blobs you can see at the back there, and two jump wires have been inserted to make a lead-out for the motors which is surprisingly mechanically sound. Jump wires are necessary because stuck bang in the middle is a breadboard onto which I can insert and remove components with reckless abandon without having to solder/desolder every time I want to change something. There's tons of space for a microcontroller and some other bits too, and if I run out of space I can always build upwards by screwing cages on to the foamboard. If you're going to do anything with ROVs or robots, building yourself an experimentation platform like this one will pay dividends. If you are going to get a tracked chassis, I recommend the Rover 5 and furthermore if you are in the UK I recommend you get it from Robotbits, and if you click on the picture below it takes you to their webpage for it.
It's designed for student, amateur and hobbyist use but the quality is good enough for a marketable device. Robotbits sell it for £30 plus postage and they are very quick at despatching it- two days from the middle of darkest Wales to the wilderness of Yorkshire is not bad at all given the fact that the carriers don't like it where I am because they invariably get lost.
It isn't a flimsy thing either. It weighs not far short of 1kg and takes up an area of about 8 1/2 inches by 9 and in the default position (i.e how it is supplied) it stands about 3 1/2 inches tall. For our European friends one inch is 2.54 cm and so it's 21.6 by 22.9 by 8.89 cm which is pretty voluminous- you'll notice it trundling towards you sure enough. In theory you can alter the position of the wheel arms to get a higher ground clearance. This is useful if you dispense with the caterpillar tracks and use some wheels instead but
if you go too far back and try to use the caterpillars you find you lose tension and get a lot of slip. I've got mine set for a 1 3/4 inch clearance off the ground which should be enough to deal with most obstacles. The way you do it (and it isn't clear at all, which is one of the few irritations with this chassis) is to unscrew the metal locking plates on each of the four wheel arms; there's two screws per plate. Remove the locking plates by GENTLY prising upwards. I'd do them one at a time ifI were you, because when I did all four at one go and picked it up two dropped out which would have been annoying had I set them. There's a sort keyway on the chassis and a set of keys on the wheel-arms that looks like a cut-off gear. Place the arms to the keyways and count how many keys are not engaged into the chassis- you'll need to do the same on the other side otherwise your chassis will be lopsided. In this way you can actually have one end higher than the other. When you're done, put the locking plates back in position and screw them down again- they won't move anywhere after that. Speaking of moving:
These are free-runs, i.e. there's no control at all on Goliath. The motors are just being supplied with power and he's put on the floor and let go. He'll just keep on going like this until the batteries run out and there's nothing we can do to alter his course or stop him except pick him up and pull the wires out. That's no good for a robot but for the purposes of this exercise it's ideal because I'm now going to put forward two circuits and see if you can spot the difference. Here we go:
fig 1: Goliath going in a straight line
fig 2: Goliath turning
Spotted it? In fig 1, the terminals of the motors are inverse; motor A draws at the yellow wire where motor B does on the green, and motor B draws on the yellow wire where motor A does on the green. At this point, if you are thinking that the motors are spinning in opposite directions to each other, you are correct. If you are thinking that fig 1 should be representing a straight line, go to the top of the class. If you are ahead of this next bit, take five house points.
Because: the motors are indeed spinning in opposite directions to each other and if you viewed them side by side they'd look like this:
And those spirals on there indicate the directions the motors would go. If you attached them to your robot like this you'd end up with one very confused robot trying to do a cha-cha-cha in the middle of the floor and not succeeding in a very distressful manner. So let's spin one of the motors round 90 degrees:
Actually, this would make your bot a better dancer, unless it had to move to the right for some reason. Either way it's not going to win Strictly Come Dancing even if it paired up with a drop-dead-gorgeous Russian acrobatic danceuse. So let's spin the motor round another 90 degrees:
Aha! So although the two motors are moving in different directions, the fact that they are 180 degrees apart means that in relation to the viewpoint of the output shafts, they're actually spinning in the same direction.