Micro switching guide:
(For the budget minded builder)
So… you’ve decided you want to build a robot, learnt how they work, read up on the rules, maybe even attended an event. As you sit down to think about your first design, you realise a major flaw... Most of the parts for your featherweight are easy to get, and cheap. Save the speed controller, a key component in your combat robot. A quick browse through the net and you find the cheapest you can get is about $315 Aus! But don’t despair, there’s hope.
Although an ESC (electronic speed controller) would be the best and safest choice for your bot, micro switching is a tried and true method of driving your bot for minimal cost. It’s not that bad either. In Australia, micro driven bots are still competitive, the 2004 NSW series was won by “Sorry”, a servo switching robot!
What is micro switching/servo switching?
Micro switching involves using a standard radio control servo to acuate two micro switches on each side of the servo using a servo horn. These two micro switches are connected in an H bridge format to the battery(s) and motor(s), and incorporates forward, reverse and brake. One servo switching setup controls one motor, so using the standard two drive motors in a tank steering format, two servos and two separate H bridges will be needed (however there are variations allowing you to drive multiple motors using the one servo, more on that later).
If it’s so good why doesn’t everyone use it?
This method of bot control, while fairly decent, has a fair amount of disadvantages, which is why they are placed lower in desirability to ESC’s:
• They lack proportional control, making driving somewhat hard (each motor can only be either off or full on)
• No “mixing” is included. This means you either buy a tank or plane style mixer, or drive your bot using tank steering. Most people find due to the subtle inaccuracies caused by mixing that it is unsuited to micro switching. Tank steering gets the best results, but is hard to master and needs a lot of driving practice.
• MICRO SWITCHING WILL NOT FAILSAFE!!!
Although your standard 12v dual drill driven wedge is semi ok if it goes out of control, if you intend to have a bot with a weapon it is absolutely necessary that you either buy an ESC (most have a failsafe built in), or fabricate some kind of failsafe using a bought failsafe module (such as setting it up to trigger a “kill switch” with another servo. I’ve seen an out of control bot weapon flailing… its not a fun experience and could be potentially deadly, not to mention how embarrassing it is (oh, and you’ll probably get disqualified too).
this control setup is perfect for beginners. If you can afford to build a bot with a deadly weapon, you could probably afford an electric speed controller too couldn’t you?
Ah that’s the intro done… now to build one eh?
(assuming for a 12v dual cordless drill drive featherweight or smaller):
• 4 x 10A minimum micro switches. 5A switches will weld contacts and be permanently on, causing hell to your batteries. If you can find 15A or 20A switches you’ll have no problems with welding at all at 12v with drills.
• 2 x standard size hobby servo’s (free if they came with your radio system)
• A sheet of either thin polycarbonate (3mm works really well) or plastic. Basically you need something for the whole setup to sit on. People have used many other materials too. Try to avoid Perspex; it will crack easily so is very hard to work with.
• 4 x small long bolts to mount the micro switches. They need to be thin enough to pass through the mounting holes in the switch. You will also need a packet of nuts to suit the bolt chosen to raise the micro so it aligns with the servo horn. The 3mm brass bolts from jaycar along with the matching nuts are appropriate for this.
• Mounting bolts for the corners to mount the switch setup, however if your lazy you can usually get away with just sticking down the base of the servo and packing them tightly into the robot (make sure there’s clearance for the servo horn to move freely).
• Decent gauge wire. 12g is heavily recommended. In both red and black colours to identify polarity.
Assuming you already had servos, the above setup can cost as low as $10 or if you buy all the gear from jaycar/dicksmith it could cost a fair bit more… but either way, it’s cheaper than an ESC!
Cutting the board to shape:
Due the subtle dimension difference between separate micro switches and servos, instead of a print out picture you can follow the steps and get guidance from the following images from my micro setup.
1. Hold the micro switches near the servo to get a good idea for the spacing. Keep in mind you want the micro to engage fully (“click” on) every time, but you don’t want to strain the servo trying to push in a fully pressed in switch.
2. After you get your suitable rough layout, you can mark out the board size, but leave it uncut till the rest of the fabrication is done; it gives you more space to clamp down the material and makes it generally easier to work with.
3. Measure the length and width of your servo base and draw it up on the board where you want it to be positioned. Then cut it out either by using a jigsaw, small hand saw or by drilling heaps of holes and joining them then filing down the sides (this method is very inaccurate and messy). A dremel and carbide cutting bit is another method for doing this. however it is messy and melting polycarb flies everywhere so wear protection.
4. Once the servo fits in snuggly, mark out the screw holes and drill them, then screw it into place.
5. Determine the holes for the switches using your previous experiments as a guide. It helps to wedge two tooth picks in the holes so you can get accurate holes with the switch at the accurate hight, if you’re using a soft material pressing hard will leave little pocks for a drilling guide, otherwise use a marker to carefully mark the hole positions.
6. Drill the holes, taking care the bit doesn’t slip, and that it leaves room for the bolt to fit through, but is small enough to hold it in place.
Assembling the board:
1. Assembly is fairly straight forward; just make sure the servo horn is in the correct place. Plug the servo into a receiver, set the adjustment tabs on the radio to centre and turn it on. If the horn goes way off centre, take it off the servo (reciever still on) and place it back on dead centre.
2. Now mount the micro switches. Using the extra nuts purchased, add enough so the switch sits on a little raised platform and is level with the servo horn. Too high or low and the servo may slip under the tab or worse, get jammed behind it. Tip: A dot of hot glue on the securing nut will help hold it tight during shock impacts, and is easy to remove if needed.
Wiring the board:
By far the most difficult part of the whole setup, and can easily become a mess of wild tangled wires prone to failures. However there are a few things you could try to stop it looking like a pile of…err… yeah!
• Whilst screw terminals will work ok, it’s a good idea to invest in connectors of some sort. Not only does this make for a more professional looking project, it makes hasty disconnections faster, changing batteries or parts easier and effectively stops accidental shorts from your battery packs.
• The wiring requires various joins. One good trick is to make one wire with a long soldered end, and to solder the other to the base of it, making a secure and strong join. Simply bundling them together and jamming them into a screw terminal is asking for failure. Another effective method is to use spade connectors to join wires.
• Keep wires to a suitable length. Too short and you’ll have to add extensions, too long and it’ll just add to the rats nest.
<WIRING DIAGRAM HERE>