Make a versatile 6 wheel drive chassis for your bots
Now with cool video of a real life military prototype! This is an easy to make 6 wheel drive robot platform designed for rough terrain. The chassis is made from 3 Tamiya universal plates, 6 plastic "T" sections of pipe with suitable plugs and clothes peg springs. Add 6 geared motors with wheels and your ready to rock!
The first video shows the suspension systems articulation. This is designed to try and keep all 6 wheels on the ground for maximum traction. In this video there is no control circuitry. the motors are connected directly to a 9.6V battery. It goes even better on 12V.
This would be an ideal system for the mighty bot challenge as the 6 wheel drive provides good traction and the overall weight is very light, especially if using a LiPo battery pack.
The second video is a quick demonstration on how to bend Tamiya universal plates.
The third video is a better demonstration of the clothes peg suspension system at work.
The fourth video is an off road test. The weight of the 12xAA NiMh batteries did make it prone to getting bogged in sand. A LiPo battery pack will give this a much better power to weight ratio.
After tearing half of my motors out while preparing for the Mightybot challenge I ended up doing almost a complete rebuild including suspension improvements. This video shows the front/rear suspension modification. Remember when watching this video that the suspension has been mounted upside down so you can see it easily.
The fifth video shows a military version of my design, it is one hell of a cool bot but mine is cheaper to make :p
I started with 6 small geared motors. These have nice brass gears for long life and strength. You can buy these or similar geared motors at most robot electronic suppliers. Mine are small 12V motors 20mm in diameter. They are rated at 70RPM with a torque of 2.1Kg/cm. Their stall current at 12V is less than 400mA. They may not sound very powerfull but with 6 working together the platform is hard to stop. In my demo video they were only running on 9.6V.
The decision of what motor and wheels to use is up to you and depends on what you can get in your country. Remember, google is your friend!
Pololu sell a terrific set of far more powerfull motors with wheels although they are expensive.
Sparkfun sell a similar motor here.
Solarbotics have a good range of geared motors.
Jaycar / Electus is where I got my motors from.
I then looked for suitable wheels. My wheels with the tyre on, have an outside diameter of 70mm. You could go bigger but you will need to get motors with higher torque if you do. I originally looked for wheels in RC hobby shops and on the net. I was looking at about $10 AUD per wheel (hub and tyre).
In the end, since I use meccano a lot it was better value for money for me to get another meccano set with 6 wheels included. The motor shafts also happened to be the right size for meccano wheels. You can see in this photo that I've drilled a hole in the hub and inserted a grub screw after tapping a thread.
When I attach the wheel to the motor I'll add some thread lock to prevent the screw from working loose. I also put a small amount on the shaft to help ensure the wheel does not come off accidently.
I then looked for plastic pipe "T" sections to house them in. I found that although a slightly loose fit, the black polypipe used for garden automatic watering systems was very cheap compared to electrical conduit. I could not find plugs I liked for the pipe ends so I bought some replacement plugs used in outdoor furiture. These were also a slightly loose fit but some hotmelt glue around the outside when I installed them fixed that. In the picture below you can see that I filled the plugs with polymorph so that the mounting screws will have a solid plug to dig into. This is important for strength. I've also trimmed the ends of the "T" section so that the length was 65mm. With the plugs in, the total length is 75mm.Depending on the size of your wheels you may need to adjust this. A total length of about 5mm greater than your tyre diameter should be about right.
The microwave is ideal for melting polymorph into the plugs.
About half way through the melting process, as the pellets melt you may want to add more.
You can see here that I've drilled a hole in the top of the intersection for a rubber gromet. The gromet stops the insulation of the wires being damaged by the sharp sides of the hole. Insert your polymorph filled plugs at each end and then drill a small hole in the top for a self tapping screw. The screw at each end performs two functions. Firstly they hold the plug and it's polymorph filling in place. I found that once the polymorph cooled it could fall out of the plug. Secondly they provide an attatchment point for the suspension. For this reason make sure they're at the top of the assembly as shown.
I decided to mount my motors with a small self tapping screw since the gears were all to one side of the gearbox.
You can see in this photo that all the gears are away from the hole in the plastic "T" piece. Make sure your screw is short enough so that it does not hit any gears when it is inserted.
When you drill your holes, make sure not to drill into the gears or to drop swarf into the gears.
I took my gearbox housing off to drill it and then carefully cleaned out any burrs etc.
After re-assembling I then re-sealed with silicon.
Although this is more difficult than glueing your motors in it is now stronger and it can be undone for repairs. If you do decide to glue your motor in then use some sand paper or a dremel tool to roughen up the inside first for a better bond.
Seal your motor assemblies with automotive silicon to keep water and dust out.
When marking out your tamiya plates for bending, allow for the wheels in the middle section to be mounted slightly lower. This makes it easier for your robot to turn on the spot if most of the weight is on the middle wheels. Make these big enough for the motor assemblies to fit comfortably and allow enough room for the suspension.
Once you've bent your Tamiya universal plates (see the second video) and cut off the excess then you will be ready to make your suspension springs. The suspension springs are clothes peg springs that I've bent slightly with some pliers.
After bending them and crimping some lugs on the end they should look like this.
Notice that the lug at one end is at 90 degrees to the lug at the other end for the middle suspension. The lugs are loosely mounted to the chassis so they can pivot slightly toward the centre. To stop the nuts vibrating loose I've used thread lock. I also used threadlock on the ends of the springs before crimping to help ensure the ends don't come off. If you don't have thread lock then some supaglue gel will probably be best as it doesn't set too quick. The lug not mounted to the chassis will attach to the motor assembly via the screw used to hold the plug in.
If you have some of those rubber mounts that come with servos spare then put one in your crimplug. It will stop the thread of the screw catching on the lug for a smoother action as well as taking up some slack.
The front suspension (below) is slightly different as it doesn't attach to the chassis. This allows the wheels to twist freely. The spring is the same except that the lugs are not at 90 degrees to each other.
With the springs tensioned to support heavy loads such as batteries the wheels tend to hang low. This can cause problems when turning as the front and rear wheels drag too much on the ground so I've added a strap made from a cable tie offcut and some more crimp lugs.
These straps stop the front/rear wheels from hanging too low and catching on the ground when turning.
You can see how the springs connect to the motor assembly's. The springs in the middle section attach to the motor assembly the same way but by attaching the other end to the chassis they prevent the whole robot flopping over to one side. Now all the chassis sections can be joined together. You can see in the picture below the spacers made from the offcuts of the front and rear chassis sections. Each offcut was cut in half and cleaned up. The spacers alow room for the screw heads.
Notice the holes drilled in the chassis for running the motor cables.
This is a closeup of the chassis sections joined together. Notice that the middle section is deeper with the wheels mouted lower. As mentioned earlier, this is so that most of the weight rest on the middle wheels allowing for easier turning on the spot. The tension of the middle suspension is also greater as they will support most of the weight.
Notice that while the front and rear suspension is held level by the straps the middle suspension is free. This helps keep all wheels on the ground when crossing a ditch. Since most of the weight rest on these wheels they tend to level out when upright.
Good luck and enjoy :-)
Going into production:
I am pleased to say that my 6WD chassis is now going to be produced by DAGU. Along the way there is going to be a few improvements.
BIGGER WHEELS, these bad boys are 125mm with spikes!
Because the wheels are bigger we have more room underneath so there are now 5 segments, 2 of which are free to hold batteries or circuitry. Check out this early draft:
We can also make a 4WD version just by removing 2 sections.
I have revised the design and a sample is now being made. The new design has two compartments, each big enough for a 7.2V Ni-Mh battery pack like those used in radio controlled models. A cover plate has been added, mounted on spacers. This provides a better surface for mounting robotic components with room underneath to run wires and mount PCBs.