The single motor bot is straightforward little simple bot that is a great stepping stone for learning robotics. I choose the term "Simple Bot" very intentionally.
Simple Bots are a style of robotics I have come up with that incorporate the following guidelines:
They need to be able to be quickly and easily manufactured with readily available items.
They need to be easily understood.
They need to be mobile.
They need to be autonomous.
They need to be built in such a way that it can later incorporate more complex logic and/or be used as a fundamental element of a more complex organism.
This 'straight forward' Single Motor Bot is a classic example of a Simple Bot. It is like a robot in many ways, but lacking in intelligence, and responsiveness. Nevertheless, bots like these are great springboards for jumping into the wild world of robotics. If you can build this, you are a long stride towards building your first full-fledged robot.
Step 1: Materials
For this project, you will need:
(x1) Fly swatter
(x1) 1/4" x 5-1/4" turnbuckle with eye bolts
(x2) 2" caster assembly
(x1) 3" caster assembly
(x2) 3/4" x 4-40 flathead bolts
(x2) 3/4" x 4-40 washers (optional)
(x2) 3/4" x 4-40 nuts
(x1) Assorted zip ties (not pictured)
(x1) Modified continuous rotation servo and 4 x AA battery pack**
(x4) AA batteries (not pictured)
**The modification of the motor is covered in steps 13 through 17 of the Basic Electronics Skills for Robotics lesson.
Step 2: Free the Wheel
Remove the two 2" wheels from their respective caster assemblies.
Typically this just involves removing a bolt and pulling out the axle bolt.
In some cases, there is not a bolt to remove and the axle is rivited in place. I won't lie, this is annoying and you should avoid getting these. If you did happen to get something like this, you have a few options for freeing the wheels. The easiest is to cut away the rivits using a rotary tool such as a Dremel and a cutting wheel. Should you not have a rotary tool, you could also saw through the shaft with a hacksaw. This is foolproof, but may keep you busy for a while.
Step 3: Mount the Wheels
Remove the eyelets from the turnbuckle, and slide each respective eyelet shaft through the center of one of the wheels. Once you have done this, thread the eyelets back onto the turnbuckle. It is now an instant robot axle shaft that is stable, rigid, and perfectly aligned.
Step 4: Remove the Other Wheel
Remove the 3" wheel from the caster assembly by loosening the nut and bolt holding it in place.
In this case you likely don't need to worry about rivits, because wheels of this size rarely have them.
However, you will notice that this wheel has a bearing instead of a thick plastic hub. Sometimes this can make it more difficult to attach a motor because bearings are meant to spin in place, which is the opposite of what we want. We want something we can attach the motor to that won't spin at all.
If you find your wheel has a bearing, it is not the end of the world. For starters, the motor's attachment bracket might be wide enough that it is bigger than the bearing. In our case, we will discover it is. What this means is that we can simply drill through the plastic on the side of the bearing and zip tie our motor in place. Problem solved.
If this is not the case, also no big deal. Simply find a piece of plastic that will fit inside the wheel. Later on, you will then attach the motor to one side of this piece of plastic, and the wheel to the other. This intermediate piece will hold them together.
When using pre-made parts, you can't be daunted by the unexpected. You simply have to be willing to be flexible and adapt your plans. Most problems are easily to solved.
Step 5: Modify the Servo Horn
The gear-like thing attached to the servo's motor shaft is called a horn. Most servo horns come with tiny mounting holes which are more or less useless. In order to make them useful, it is generally important to widen them to be large enough to pass a fastener through.
For a standard servo, this typically means using a 1/8" drill bit. This hole is large enough for both small zip ties and 4-40 bolts. In our case, we will be fastening things to the servo horn with zip ties
In order to span the bearing in the center of the wheel, we want to select the servo horn's outer mounting holes. These holes align with the plastic rim around the outside of the bearing.
To widen the holes, simply use a power drill with a 1/8" bit and quickly drill through. The existing hole should serve as a very good pilot, and the softness of the material should offer no resistance.
Step 6: Mark and Drill
Center the servo horn upon the wheel's plastic hub and mark the center of each hole in the servo horn using a mini screwdriver. Typically a center punch and hammer is used for this task. However, the wheel's hub is made of a very soft plastic and we can get away with making a pilot hole using a mini screwdriver. Just a light tap on the screwdriver with a hammer should be more than enough to make a small indent.
Once all the marks are in place, drill straight through each marking upon the wheel with a 1/8" drill bit.
Step 7: Shape the Fly Swatter
The fly swatter is going to serve as the frame or "body" for the bot. Since it is still a fly swatter and yet, a bot, it not entirely a perfect fit quite yet. It needs to be shaped.
We will be mounting the turnbuckle wheel axle where the handle meets the paddle. Basically, we want to make cuts such that the wheels can spin freely when the axle is centered. To do this just get a pair of scissors and trim away enough of the fly swatter to allow for this.
Also, don't forget to keep in mind aesthetics! Like anything else in life - having style is classy. A simple thing to go for is simply to make sure the two sides you are cutting mirror one another. You can also try following the patterns on the swatter itself.
Step 8: Mark the Motor Mount
Place the motor at the end of the fly swatter's handle and trace its outline along one of its edge.
Step 9: Trim the Edge
Trim away the edge of the fly swatter in-between the two mark lines with a pair of cutting pliers. Continue trimming until you have neatly cut away enough material to snuggly seat the servo motor inside.
Depending on the type of plastic the fly swatter is made out of, it may crack if you try to cut it directly with cutting pliers. To prevent stress fractures, a trick is to drill a series of small 1/8" stress relief holes along the bottom edge. You can then snip the plastic towards these holes, which will absorb the stress and keep the plastic from cracking.
Step 10: Mark and Drill Motor Mounting Holes
Place the servo inside the opening you have just cut. Use a pen to mark the servo mounting holes that overlap with the plastic.
Drill through each of these marks with a 1/8" drill bit.
Step 11: Attach
Attach the motor to the frame using zip ties. Pull them as tight as possible, and then trim away the excess tails.
Step 12: Attach the Wheel
Insert two zip ties both down through the wheel, and the corresponding holes on the servo horn. Next, pass each respective tail back up through the servo horn and the wheel using one of the two free holes. Once passed through, fasten the wheel tight.
You may do this and realize your zip tie is not long enough. Fret not. This is an easy problem to solve. Just slide the head of a new zip tie on the tail of the last. You have just extended it. Now you can now close it pull it tight like you would any other zip tie.
Again, trim way any unnecessary excess plastic once the wheel is completely attached.
Step 13: Attach the Axle Shaft
By now you may be beginning to notice a trend. Most things are easily attached using zip ties. The reason I like using zip ties is that they are strong, cheap, relatively small, and easy to undo. You can zip tie two things together, decide you don't like it, and undo it rather painlessly. Zip ties are both a quick way to protoype and make structural joints. They are fantastic, and we will continue to use them extensively throughout these lessons.
Zip tie the turnbuckle wheel shaft to the underside of the fly swatter such that the wheels are able to spin freely.
Step 14: Mount the Battery Holder
The one part that does not get attached with zip ties is the battery holder. Bolts are used to mount the battery holder because battery holder tend to have mounting holes directly below the batteries. Using zip ties will push the batteries upward and cause them to pop out. This is no good. There is typically not a good or easy way to zip tie battery holders down without interfereing with the batteries. Bolts are typically the better option when dealing with these.
The use of washers is optional depending on the size of the holes in the swatter. Initially, I thought I might have needed them, but it turned out they were not necessary. If the holes in the swatter turn out to be bigger than the size of the 4-40 nut, you will need to use them.
Step 15: Clean Up the Wiring
It is good form to bundle of the loose wires and tidy them up using zip ties. This helps to prevent them from snagging and breaking.
Step 16: Batteries
When everything is all cleaned up, power it on by inserting the batteries.
I kept things simple. There is no on/off switch in this design. Simply remove a battery to turn it off.
Step 17: Troubleshooting
If you followed my instructions to a 'T' you may currently be noticing your bot is going backwards!
Normally, I would have gone back and corrected my instructions so that everything worked perfectly the first time, but after thinking about it for a moment, I decided there was a teachable moment here.
First off, maybe it's not actually going backwards. Maybe that is the way you wanted it to go to begin with. In which case, you are done. Congratulations.
Or maybe it is not the way you wanted it to go, but you like it better like this. In which case, celebrate happy accidents, and pat yourself on the back.
Or perhaps - like me - you wanted it to go the other way and this is annoying you.
If this is the case, let's try to figure out what is going wrong.
Remember that the direction of a motor is determined by the polarity of the power supply connected to it. So, if the motor is spinning the wrong way, that means that power and ground need to be reversed. This is simple to do with a little rewiring.
To begin, snip the wires, and clip away any of the old solder connections.
Strip the ends of all the wires, add some more shrink tube, and then solder them red to black, and black to red.
Make sure they are nicely insulated with shrink tube.
Lastly, clean up the wiring once more with a with zip tie.
The problem should now be solved.
To avoid this problem in the future, simply check polarity and orientation before you solder the wires and mount the motors.
Anyhow, to reiterate, I left the instructions incorrect so that you can understand:
1) When building robots, things go wrong.
2) That is okay.
3) Sometimes you can simply embrace the mistakes.
4) Even should you not, mistakes are often easy to fix.
5) And can be avoided with good planning.