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Microcontroller Fun: The Hexapod Robot Hexapod? It’s a weird name for a weird looking animal. It “walks” on six legs. How on earth can something walk on six legs? Build it and find out! By Ross Tester 84  Silicon Chip The kit unpacked: the large yellow sheet contains the mechanical components to be assembled onto the Hexapod body. The three servos and their actuator arms are at the top – the circular actuators already fitted are discarded. The manual on the left is for the BASIC Stamp controller, the manual on the right is for the robot itself. T he Hexapod Walker is a fasci nating little kit which will pro vide a lot of enjoyment – not only in building it but seeing what it does. And it will give you a good insight into basic robotics (and you will see shortly that basic plays a significant part!) plus computer control. When completed, the Hexapod Walker also looks like a large insect. In operation it looks somewhat like a large insect, whirring along as it somewhat clumsily moves along one step at a time. But it’s no accident that the Hexapod looks like an insect: that was obviously the designer’s idea. In fact, options are given in the instructions to make it look even more insect-like. What you get The Hexapod Walker kit is supplied as a number of bags of “bits” and the walker components themselves stamp-ed from a large-ish sheet (about 300 x 200mm) of bright yellow plastic, about 3mm thick. The Hexapod “body” is of the same material but about 7mm thick. Incidentally, if you want to change the colour (bug black, maybe?) it can be spray-painted with acrylic lacquer such as auto touch-up spray paint. The bags of bits contain almost all the components you need to put the robot together. For example, there are three servos (the type used in most radio controlled cars/planes/boats, etc), along with various control arms to suit. You’ll only need one type of arm so the rest can go into your junk box – just in case. There’s another bag containing a small PC board (about 37 x 57mm) and all the components you’ll need to build the BASIC Stamp robot controller. Did we forget to mention that’s how the robot is driven? Sorry! Yet another bag contains “hardware” – Nylon screws and nuts, a couple of battery holders, some rubber feet, tinned copper wire and so on, with all the above housed in a large bag which also contains an assembly manual and microcontroller manual, along with a program floppy disk. What you don’t get There are a couple of things you’re going to have to buy, scrounge or otherwise procure before you can build the robot. We’ll warn you about them now because when we started to put the kit together it was after the local shops had closed and we had to wait! The most important thing is some double-sided foam adhesive. This is sold by hardware stores for sticking photos, mirrors and anything else to walls. It’s also available from large supermarkets. We used a packet containing 32 mounts pre-cut into 10 x 20mm rectangles. It’s a handy way to buy them (ours were “Permastik” brand and cost about three dollars for the packet). You’ll also need some super glue – not just ordinary super glue, but the gel variety. Super Glue Gel gives a slightly longer working time and is less likely to stick your fingers together. But like ordinary super glue, it does go off fairly quickly and it can stick your fingers together if you let it. We used Selleys Fix’n’Go Supa Glue Gel – a 3g tube also cost about three dollars. A sheet of sandpaper is also required to smooth the edges of the robot components where they break away from the carrier sheet. JULY 1999  85 The kit requires two power pins (the gap between the pins sources – a 9V battery for the is very small). processor and 4 x 1.5V “AA” Don’t place the ICs in their cells (6V) for the servos. None sockets yet. of these are supplied in the The pin spacing for the kit. Alkaline batteries would three sets of header pins is of course be preferable. also very close, so be extra As far as tools are concareful when soldering these cerned, you will need a sol(especially the middle row dering iron (with a nice fine of the three-way set!). The tip) and solder (electronic short-er pins go through the type, of course!), a PhilipsPC board to be soldered. head screwdriver and a pair Place and solder in the RJ11 of pliers with cutting blade (to connector (it can only go one cut and bend the wire). way) followed by the two One option which is almost The assembled First Step BASIC Stamp controller. It 3-pin semiconductors (these essential (unless you want to is programmed from a PC via the large socket on the could, with difficulty, be inright which looks like a telephone connector. make your own) is the proserted incorrectly!). gramming cable: a short cable Finally, insert the 18-pin fitted with a standard computer parallisted in the manual are actually sup- and 8-pin ICs into their respective lel port plug one end and an RJ-11 plug plied. The way the bags are sealed sockets. This is one area where a lot (what looks like a modular tele-phone of problems occur; you have to ensure it would be very surprising to find plug) the other. Be warned: it’s NOT to anything missing (but stranger things all the IC pins are inserted into their be plugged into the telephone socket; have happened. . .) mating sockets. It’s easy for pins to this is the plug which enables you to Start by placing and soldering in the be bent in this process, so take care. program the Hexapod from your PC. You have now finished the controlresistors except R4 & R5 (10kΩ). These Oh yes, you need a PC of some sort, are not required in this version of the ler board. After thoroughly checking too (just about any IBM-compatible kit. They won’t do any harm but they your soldering and component placewill do). In fact, it’s a perfect appliment, put it to one side while we get will drain the battery slightly. cation for that old XT gathering dust Next, fit and solder the three capac- on with the fun bit: the robot! in the back shed! itors (one is an electrolytic, so watch Building the robot its orientation) and then the ceramic Starting construction Unfortunately, the instruction manresonator (not polarised). You can start with either the robot or Both ICs are provided with sockets; ual for the robot is a little disjointed the microcontroller board – but seeing the 8-pin socket for the EEPROM and and some important information is we’re an electronics magazine, we’ll the 18-pin for the PIC processor. Care- unclear. So we will try to cover that start with the controller. fully solder in the sockets, making here. First, verify that all the components sure you do not bridge between the The first step, according to the man- Fig.1: the circuit diagram of the Hexapod’s microcontroller which itself is programmed from a PC via the modular programming port, CN1. While the output header has labels for four servos (0-3), only three are used in the kit. Other pins can be used for microcontroller ­­ I/O – RB6 and RB7 can have bumper microswitches fitted if you wish. 86  Silicon Chip The first steps in assembling the Hexapod are the mounting of the servos on the underside of the base panel (Fig.2, left) and the gluing in place of the vertical leg supports and spacer bracket (Fig.3, right). ual, is to centre the servos by applying a 1.5ms pulse every 10-15ms. How? You need to do it via the controller board you have just finished and by running a program on your PC which downloads the appropriate program to the controller board. Great if you’re into BASIC programming; hardly the stuff beginners or even the average kit constructor will get their head around quickly. If you wish to do this correctly, you should jump over to the “programming” section at the end of this article and follow the instructions there. But if you’re like us, in a hurry to get the robot going, you can cheat a lot and centre the servos by eye. Sure, it won’t be exactly correct but our kit worked doing it this way, so what the heck? Screw one of the servo actuators onto the servo arm with the screw provided. Turn the servo actuator all the way clockwise and place a reference mark on the servo arm at the end point. Turn the actuator all the way anti-clockwise and place another ref- erence mark on the servo arm at that end point. Half way between those two marks will be close enough to the midpoint for our purposes. Repeat for the other two servos. Now it’s time to carefully break all of the components from the carrier sheet. We didn’t have any difficulty doing this – just take your time and don’t force any pieces. If necessary, help them a bit by cutting with a knife. The largest piece is, not surprisingly, the robot “body”. There will also be four identical back-and-forward legs, two identical up and down legs, eight leg support brackets, two identical vertical leg supports and a leg support spacer. Smooth any carrier sheet remnants from all the components with sandpaper before continuing. Put all of the legs and support pieces to one side for a moment. Now we have to fit the servos to the robot body with double-sided adhesive foam. First, with a pencil and straight edge, mark a centre line right down the length of the body (there are reference marks each end to help you do this). Then mark one line across the body (exactly at right angles to the first line) 85.5mm from the front and another 25.5mm from the back. Next you will need one of the servos, one of the slim actuator arms (not the circular ones) and the actuator arm retainer and screw. Place the servo on its side with the shaft pointing towards you and the wires emerging from the right end. Keeping the servo shaft in mid position, place the retainer onto the shaft and screw the arms on so the actuator points 90° straight upwards. Tighten the screw holding the actuator arm in place, making sure you don’t move the servo off mid position. Once all servos are in position you won’t have access to this screw, so make sure it is right first! In fact, it’s a good idea to do a “dry run”, placing all the servos without adhesive to make sure you understand how they all fit together. When ready to permanently mount Next comes the mounting of the support brackets (Fig. 4, left) and the fitting of the legs (Fig.5, right). Note the way the holes in the support brackets all face towards the middle legs. JULY 1999  87 Once all the legs are fitted, you need to bend the pushrods from the wire supplied so that the servos can drive the legs. The two diagrams here show how those pushrods are fitted. When you get to this stage, your robot is mechanically complete – all you need to do now is add the electronics and batteries. the first one, attach a couple of adhesive foam tabs to the underside of the servo and fix it exactly to the centre of the robot body so that its back lies along the line you ruled closest to the front of the body. When mounted, its actuator points down (away from the robot body). The other two servos are prepared and mounted in a similar way, except that when mounted, the servo actuators point straight up with the servos back-to-back along the centre line. Their back edges follow the line you ruled towards the back of the robot body. This means the adhesive foam pads actually stick to opposite sides of the servos. Now you have to make the vertical leg supports, using the two support pieces with their central spacer. Before gluing, place the two vertical legs in position with their two Nylon nuts & bolts. It’s vital that you don’t get any glue on the legs themselves, otherwise they won’t be able to move. When this assembly is dry, glue it to the TOP side of the robot body (it fits into notches on the body). Again, keep glue away from the legs. Next come the horizontal leg support “hinges” which are glued directly to their respective legs. You will note that there are two holes on the hinges – these holes must be aligned in the same direction for each hinge. The hinges on the front legs have their holes to the rear; the hinges on the back legs have their holes to the front. It’s also important that the hinges 88  Silicon Chip are assembled exactly in line with each other – placing the Nylon bolt through each will line them up for you. Finger-tighten all of the Nylon nuts and bolts and glue the hinges to the legs, making sure you don’t get glue on the faces of the hinges or on the base. Now all the legs and their fittings are assembled, it’s time to make the pushrods which connect the legs and their respective servos together. This is done with the tinned copper wire. Be careful here: there is just enough supplied to do the job. Before using the wire, it’s a good idea to straighten it by nipping one end in a vice and pulling the other end hard with a pair of pliers. The legs can be mounted at 90° to the body, which is most efficient, or they can be mounted at, say, 10 degrees offset – which looks more like a bug! It’s up to you which way you go. Cut two 200mm lengths and mark them (with a Texta or similar) at 30, 80, 130 and 180mm. Bend the wire at 90° at 30mm and push the longer end through the two holes in the centre leg brackets. Push the other end right through one of the pair of holes in a front leg hinge set and bend it back to make it captive in the hinge set. Bend the wire at the 80mm and 130mm marks about 15° in a horizontal direction, with the mark at 180mm at 90° in the vertical direction. The free end then passes through a pair of holes in the rear hinge set and is bent over underneath to make captive. Snip the ends off the wire to make sure they don’t foul anything as the legs are moved backward and forward. The other piece of wire, for the opposite side, is prepared the same way. Exact angles are less important than making the two pieces of wire symmetrical. The rear legs connect to their respective servos with short (83mm) lengths of the same wire. These go through the other holes in the rear hinges and connect to one of the holes in the servo actuators. We used the second hole from the top which seemed to work pretty well. The centre legs connect to the centre hole in the remaining servo with the remaining length of wire. It must be bent in an elongated “S” shape as per the diagram. Naturally, you will have to thread the wire through some of the holes before bending – the angles are too acute to allow it to pass through otherwise. Now see if you can move the front legs by gently pushing on the rear legs and vice versa. Don’t push too hard because you’re also turning the servo. Wiring the beast It really is starting to look like a beast, isn’t it? ­ The controller PC board and 4 x AA cell holder are glued to the top side of the body with the same adhesive foam we used to glue the servos in place (lucky there were 32 foam tabs in the pack!). The 9V battery holder is glued to the underside of the body in front of the vertical leg servo. Now we have to run the wiring from the servos to the controller – and here’s where you can come unstuck. We believe the instructions are not clear enough in telling you which way around the 3-pin servo plugs go on the header pins. The circuit diagram in the “First Step” manual has the wire colours shown but doesn't tell you which way around they go on the socket – and they could be placed either way around. The wiring diagram in the kit manual is not 100% clear, either. It would be too easy for anyone not familiar with electronics to get it back to front. And then there is the dire warning about not getting it back to front . . . In all cases, the black wires in the servo connectors go to the header pins closest to the edge of the PC board. This makes the red wires go to the middle pins while the yellow wires, the ones which receive direction information from the controller, connect to the pins closest to the controller IC. It is also possible to get the wrong servo on the wrong set of header pins. The left servo goes to the pins labelled Servo-0, the right to the pins labelled Servo-1 and the middle to the pins labelled Servo-2. When mounted, both their actuators point up, alongside the body. The servos are taken care of, now for the power wiring. In the kit we built, two power switches were included which make it very easy to turn power on and off. The alternative is to whip a battery out but that is sometimes not quite so easy with the thing going walkabout! Wiring the battery connectors to the switches is the easy part. Connecting the switches and negative supplies to the PC board – well, that wasn’t quite so simple. We could only find two header connectors supplied in the kit – one red, one black. And there are four connections to make: +9V, 0V, +6V and 0V. What to do? We cheated. We cut the header connector leads to a suitable length, giving us four header connectors. These we soldered to suitable lengths of insulated hookup wire and connected those to the switches and battery holders. Of course we also insulated the soldered joins. We’ve been assured by the suppliers that more connectors will be supplied in future kits so this problem should not occur. The switches themselves are the standard mini toggle switches, complete with nuts and washers. However, we found that they were such The battery holders and the microcontroller PC board are attached using doublesided foam tape. Be careful to keep wiring away from the pushrods or legs. a snug fit into the two holes right at the back of the robot that no nuts or washers were required. That bit is up to you. Finally, we used a couple of cable ties to tidy up all the wiring. Because the servo leads can’t easily be shortened, there is a fair amount of excess wire around. And the last thing you want is a wire dragging along the pushrods as they move back and forward, back and forward . . . The penultimate step is to check your wiring and all clearances, making sure that the legs move in unison with each other. If all is OK, insert the 9V battery and the four AA batteries and switch on. Hopefully, nothing at all will happen! Programming it That’s because you haven’t programmed the Stamp controller yet, so the beast hasn’t got a brain to tell it what to do. The first thing to do is to load the supplied software into a directory on your hard disk drive (it will work from floppy if you must!). Make a directory called stamp1 and copy all the files from both the “stamp1” and “Hexapod” directories on the supplied floppy disk to that directory. Connect the supplied cable to your computer’s parallel (printer) port and the RJ-11 socket on your robot. You OK, How Does Hexapod Walk? The principle behind walking with the 3-servo robot is simple. One of the servos is used to provide vertical lift to legs 1,3,5 or 2,4,6. The other servos provide the horizontal shift for the left legs, 1 & 3, or the right legs, 4 & 6. By cycling through the sequences to the right the robot can walk forward, reverse or turn left or right. To walk forward, follow the sequence 1,2,3,4 then repeat. To walk backwards, follow the sequence 4,3,2,1 then repeat. The same rule applies to turning sequences. You can experiment with the amount of throw for the servos and the type of feet with different floors. JULY 1999  89 These are the screens you should see on your PC: above, we have loaded stamp1.exe and then pressed “ALT-L” to list the available PBASIC programs. Selecting “WALK.BAS” loads the program to make Hexapod walk. This is displayed in the screen top right. Pressing “ALT-R” will download this to your Hexapod, as shown bottom right (assuming, of course, that the cable is connected AND the 9V supply is turned on). Turning on the 6V supply should start Hexapod walking. It’s wise to disconnect the cable first, though! Turning off the power switch or even removing the batteries will not alter the program: it will stay in the robot’s memory for at least 40 years or until it is replaced, which ever comes first . . . will need to turn the 9V supply on but the 6V supply doesn’t need to be on yet (in fact, it’s more convenient not to have it on unless you want to be chasing the little beast all over the place!). Run the stamp1.exe program. This brings up the screen shown above. Load the appropriate BASIC program. Alt-L will list the available files for you; Alt-H will give you a list of valid commands. After loading the program, press Alt-R to download it to the controller on your Hexapod. It will begin running automatically – as soon as you turn the 6V supply on, the legs should start to move. If it doesn’t, you will probably already have received a “hardware not found” error on your screen. Check that the 9V battery is OK, that it is turned on and that there is power getting to the board. You can also check that the on-board regulator is working by measuring the voltage between pin 6 and pin 8 on header H1 – you should get very close to 5V. If this doesn’t work, make sure that the cable is properly plugged into both your parallel socket and the RJ11 socket on the robot. If all else fails, go over your soldering once again and check the placement and polarity of the components on the board. Assuming that everything is now working properly, disconnect the programming cable. Next time you turn Hexapod on, he/she/it will go 90  Silicon Chip lumbering away again, exactly as before. That’s because the program stays in memory until erased (or another program is loaded, which is effectively the same thing). In fact, the manufacturers of the BASIC Stamp say that if you come back in 40 years time and turn the Hexapod on, it will still have the program in memory. We think you might need some fresh batteries, though! What to do next Once you are completely satisfied with Hexapod’s operation, we suggest once again tightening up the Nylon Where To Get It: Our Hexapod Robot Walker Kit came from the Australian distributors of Lynxmotion products, RobotOz, 7 Felgate Place, Warwick, WA 6024. Phone 08 9243 4842; fax 08 9246 1563, email kits<at> robotoz.com.au Recommended retail price of the kit, including the BASIC Stamp microcontroller, is $320. The optional infrared proximity detector sells for $65 and the programming cable $10. For more information, visit the RobotOz website www.robotOz. com.au The kit is manufactured in the USA. Assembly drawings in this article courtesy Lynxmotion, Inc. nuts & bolts (finger tight), then melting them slightly with your soldering iron. We found in operation the nuts continually working loose – and every now and then poor old Hexapod would “throw a leg”. An alternative to melting (and therefore damaging) the nuts and bolts would be a tiny dab of glue. There are quite a few programs to try out on the disk which make the robot do various things. Or if you have web access you can try downloading others from the manufacturer’s website, www.lynxmotion.com You can also add other hardware to your robot: an optional infrared sensor is available which stops the robot hitting objects. A cheaper option is to fit a couple of microswitches to the front of the robot as bumper switches, connected to I/O pins 6 and 7 of the header socket. If the robot hits anything, the switches tell the legs to stop walking. In this case, those two resistors (R4, R5) we said to leave out at the beginning, need to be fitted! Remember, too, that the controller on the robot is a full-blown PBASIC Stamp microcontroller, not dissimilar to that we used in the article “Getting Going With BASIC Stamp” in the January 1999 issue of SILICON CHIP. You can write PBASIC programs or download loads of them from websites to do a whole lot of things apart from SC move your robot’s legs!