How to build your first mobile robot platform

This post is for my fellow junkies who are about to take a plunge into robotics. In strict sense, my definition of a robot is something that looks and thinks like the robot as shown in the famous movie named iRobot and the robot named Sonny 🙂

That is a pretty far fetched idea for today’s scenario, but we will reach there someday…who knows, may be you are going to take us there!

So in the hope, that you will some day build us a robot that we all dream of, I present you the details on how to build your first robot. It’s more of a mobile platform that is more closer to an RC car than anything else, but at-least a start.

We will try to meet the following objectives in this exercise:

1. How to build the chassis.
2. How to add wheels
3. How to provision for power supply
4. Placement of the motor driver circuit board and the microcontroller board for drive signals

I have uploaded a short video of the robot I built on YouTube. You can view it at Cellphone Controlled Robot

Also, the chassis I built looks like the one shown below:

Cellphone Controlled Robot Chassis

1.How to build the chassis

You have two options in this case – either buy a chassis from an online seller or build one yourself. The problem with buying online is that you miss the fun and your creativity is limited.

I took the second option…build my own. After a few failures, I finally managed to build one. Here is what you need to do:

• Buy 3mm plastic sheet. This is available online or in my case I got them from the local glass shops. If you live in India, then your local glass shop (the ones who make/sell mirrors and other clear glass items) will have them. Please note, buy the 3mm thickness ones. Not less, because they break easily and are not strong and not more (e.g. 5mm) because its difficult to work with them and you need bigger nuts and bolts. So, 3mm plastic sheet is just the right size. You get clear one or colored ones. I tried both clear and colored ones. Loved the colored one and chose the bottle green color.
• The most simple shape you can cut by hand using a small hacksaw is square. I would love to have a Dremel tool some day, but for now, even the hacksaw works great. First decide on the size. I chose 15 cm x 15 cm square. Almost double the wheel diameter.
• Cut two such 15cm x15cm squares. You need two because you need to house the power supply, drive motors and driver circuit in the bottom one, while the top one will hold control electronics. Here is how it looks like:
  
• Finally, you need to cut the corners of the square as shown in the picture above. The reason is that if the corners are at 90 degrees, the robot tends to get stuck in the corners of my house. Creating a 45 degree diagonal cut reduces that to a great extent. Here is how the sheet looks like after you saw off the corners at 45 degrees angle:
  
• You will be required to make many holes on these sheets. But we will cover them as we go along so that we understand why we are drilling.

2. How to add wheels

Adding the wheels requires careful planning. This requires you to get the following before you start:

• The motor clamp – These are the ones that you will bolt on to the chassis, and then you will attach the motors on these clamps
• The motors (two of them in my case) – For a start, try to get small, less powerful motors. I used plastic gear, BO motors (60 RPM). I had made the mistake earlier to buy powerful motors, only to realize that it needed more power and bigger batteries.
• The wheels – For starters, try to get small ones. I used 69cm diameter, plastic wheels

Here, we will have 2 wheels only, just to keep the cost low. This is going to be a “rear-wheel-drive”, meaning, the wheels will be on the rear end. I’ll tell you later on what to do for the front. Be careful in your measurements, the wheels must have proper horizontal and vertical alignment as shown in the picture below, else your robot will not move in a straight line (or as you command it to move)

After the measurements were taken, I mounted the motor clamps as shown below (Ignore the holes, those holes resulted in incorrect horizontal alignment, so I had to drill another set):

Once the motor clamp is done, you basically have to follow the instructions you received from the place you brought the motors. Its generally a good idea to buy the motors and the wheels from the same place. They all fit together nicely that way.

Once I fitted the motors and attached the wheels the chassis now looks like the image shown below:

Great, so far so good. You might have noticed, that the front part is now elevated because the weight has shifted to the back. This is not a problem!

We are going to put batteries in the front, that will take care of shifting the weight to the front. also, we now need a wheel in the front. The easiest solution is to put something called a “caster wheel” . If you don’t know what a caster wheel is, read the wikipedia entry.

Caster wheels are available online on all robotic stores. Just ensure that the wheel is center aligned. Drill a few holes in the front (center aligned) and with a few nuts and bolts, you have attached the caster wheel.

3. How to provision for the power supply

For the power supply, I’m using 4 AA batteries (alkaline) which fit nicely into a battery holder that was there in my junk box. I’ll use these batteries to drive my motors (To calculate how much mAH rating you need for your motors please see my post on battery selection ). I used another battery set as a power supply for my microcontroller. So, two sets of batteries will sit in the front. The picture below indicates the placement plan I had in mind (front view):

You can see the caster wheel in front (center aligned) The picture below is  top view view of the placement:

So you can see how the scheme nicely fits all pieces together and takes care of balancing too.

Now to hold the twin battery sets, you need to build a clamp. No issues! Because we selected a relatively thin plastic sheet, its easy. Cut two strips, around 5cm wide by 15 cm long.

Take appropriate measurements of the battery holder and bend the plastic sheet using a simple candle. Trust me, its very easy to bend. Just draw a line where you want the bend to be and put the plastic sheet over the flame. Quickly the plastic sheet will become soft and you can bend. Keep a small bucket next to you that is filled with water. The moment you bend it, put the plastic in water. It cools down instantaneously and the bend is permanent. With a little practice, you will become perfect. Just ensure, you do not overheat the plastic. I ended up with two bends as shown below:

Drill a few holes and use nuts and bolts to hold these bends. Please note, use a 3mm nut for this plastic sheet. Any smaller is too small and any bigger breaks the plastic sheet.

The assembly after joining the battery holders with the bends looks like the one shown in the picture below (You can also see the placement of the caster wheel too):

4. How to provision for motor driver and microcontroller board.

I created a L293D based motor driver circuit. The circuit board was small enough to fit in the space between the two motors. Exactly between the battery holders and the twin L-shaped motors. The microcontroller board that provides the signals to drive the motors needs to be placed on top of this contraption!

Remember, I asked you to cut two squares. Its time now to use the second one. Just drill 4 holes on the four corners. Use plastic or metal studs and place the second board on top of this board. The platform will look like the one shown below:

You can continue to add more boards on top. Just do not make this unstable. For additional information, refer to these two blogs:

1. How to build motor driver circuit board
2. How to build a basic microcontroller board
3. I brought the motors, clamps, wheels and caster wheel from Nex Robotics

You are now ready with your robot platform. You can begin your experiments on this basic beginner’s platform. Start with simple drive , make a line followers, create cellphone controlled one, obstacle avoiding….possibilities are endless, but this board remains more or less the same…enjoy and do send in your feedback on how else can I improve this post.