Ball Thrower Bot

Pretty recently we got Lux, our new puppy. She has been a ball of energy ever since. The only thing that keeps her from doing mischief is playing fetch. Sometimes, though, none of us have the time to throw the ball and go through the routine of no-take-only-throw again and again. Hence the idea: an automatic ball thrower that we can teach her how to use!

For this project you will need:

1. Tools

• Ruler
• Drill
• Drill bits (misc.)
• Hacksaw
• Glue-gun
• Wire strippers
• Soldering iron
• Pencil
• Utility knife
• Multitool (screw drivers, pliers, etc.)

2. Materials and Electronics

• Insulated wire
• Barrel adapter
• RS 775 Motors x2
• Toggle switch
• Electrical tape
• Cello tape
• Masking tape
• Glue
• Glue sticks
• 2 inch wood screws x2
• 10mm wood screws (4mm diameter) x4
• Machine screws x4
• Sun board
• Plywood (misc.)
• RS 775 motor brackets
• 70mmx40mm wheels
• Small box
• counterweight (can be anything)
• 5V power supply

Since we are using a pair of flywheels to toss the ball across the room, we need to have them spinning in opposite directions such that the ball is pushed out of the middle. Hence we wire the motors using opposite polarity so they spin in opposite directions. following the diagram shown above, wire your motors in a similar way. Adding a toggle switch on the live wire helps too, since I have found that the contraption is kind of noisy and sometimes needs to be connected to the power even when not in use. A barrel adapter is added so that the 5V power supply can be used.

After you have made all connections and tested that the circuit works, solder it together. Make sure to use electrical tape to insulate it after.

Now that the motors are wired properly and have been soldered, we need a way to hold them in the right position for the ball to pass through. We will use a plywood scaffolding to hold the motors in place.

The distance between the motors depends on two factors, the diameter of the ball and the diameter of your flywheels. Here, I am using 70mm wheels for the flywheels and a 65mm ball, so the distance between the two shafts of the motors should be 35+65+35=135mm. Make your calculations accordingly, and mark the plywood for the motors.

To mount the motors we will be using a RS 775 motor bracket. The width of this bracket is 42 mm, hence the center lies at 21mm. Adding 21mm on either side of the motor shafts we have 135+21+21=177mm. This is the final length of plywood I require. It is also a good practice to reduce the distance between the two motor shafts by a millimeter or two so that the ball may be gripped better.

Using a motor bracket as a template, mark the points where you would need to drill holes in the plywood. Use a 3.2mm drill bit to drill about 10mm into the plywood. Then use a screwdriver and 4mm screws to fix the motor brackets to the plywood. Then mount the motors (without using screws) and check if they are at the required distance.

Once this is done, we will make the rest of the scaffolding, namely the support. If you had measured out the distance between the two motor shafts, mark the midpoint using a pencil. Take a new piece of plywood, and drill a hole in the center lengthwise. The hole should be about 3/4th the length of the wood screw, in my case 1.5 inches deep. Now, drill a hole at the base of the plywood, as shown in the picture above (pic 3). Finally, use a wood screw to attach the two pieces of plywood. If you feel that the attachment is unstable, use a little bit of glue while putting the screw in.

Now, for the final frontal support. I ended up freestyling it, but you may use any method you please. I screwed in two wood screws in a small scrap of plywood I had left over, and covered the heads on hot glue to prevent damage to surfaces, then hot gluing the scrap to the assembly.

Since my build was top heavy, I ended up having to glue a small box to the back so that it wouldn't topple over.

To collect the ball, and to pass it between the flywheel arrangement, we require a hopper. I decided to make my own out of sun board. It was a simple box design, with a 70mm x 70mm hole offset to a side. This is so that once it is tilted to a side, the ball may roll into the slide instead of getting stuck at the top.

To ensure that the ball gets passed through the flywheels, we need a slide. I designed mine as shown in the picture above. Since the two pieces of sun board needed to be held at a 90 degree angle and hot glue alone would not suffice, I decided to use elbow supports to hold the sun board in place. Two pieces of sun board were glued together to make one support.

Once the slide was prepared, it needed to be fit onto the plywood support. A quick trial and error determined where the slide needed to be cut such that a snug fit may be made. After this, another problem arose, that is, when the ball would be put on the slide, it would sometimes bounce off the plywood, or get stuck in between the slide and the plywood. Hence a ramp needed to be designed, such that the ball rolls into the flywheels instead of bouncing or getting jammed. After the ramp was designed, it was hot glued to the slide.

After confirming that the modified slide fits on the plywood assembly, we can assemble the hopper. The box and the slide are hot glued together, without tilting. Since the area of contact between the slide and the box is very small, we need to reinforce the hopper. I did this using a few scraps of sun board I had left over.

Finally, we can assemble the build. The slide is attached to the plywood and the ramp is aligned to the motor mount so that the ball may pass through. Then we can hot glue the parts together.

The Ball Thrower Bot test videos can be viewed here.

Overall, I have a very satisfied puppy. She loved the build, though she still needs to learn how to use it properly. Since that is a lengthy process, I will not be including that here.

• Using better motors
• Using Arduino and sensors to detect the ball and dispense treats
• Using motor drivers to make the overall contraption less noisy
• Using batteries to make it more portable
• Using a better power source to make the motors run faster
• Making an enclosure to protect the build and the user