ExMod The experimental module development platform

From ExMod

ExMod is a platform that was designed to be experimental but easy to use and build on. The design allowed for a large space to add sensors and boards. As well as a 3 omniwheel design to give it good maneuverability but also allow for both simple and complex movement to be tested. The name can from EXperimental MODule.

 

 

 

1. Frame Construction

Frame Construction

 

This platform needed a lot of flexibility and room to add components such as sensors and breadboards. It needed to be stable and easy to use. I also wanted the ability to test complex motion controls. The omniwheel design allows for either simple forward motion with the third wheel used to steer or true omnidirectional motion.

 

From ExMod

The frame is constructed from PVC. I had some laying around but any material that is light and durable would work. Acrylic or wood are good options. There are 2 main pieces to the frame. They are identical pieces to allow them to be layered on top of each other. This creates a top surface to mount electronics to as well as an inside gap to mount the motors.

 

 

I chose a triangular design to allow for the three wheel design. Using a jig saw I cut the frame out and then spend several days trying to figure out a way to mount the wheels. I did find an interesting thing to note. A piece of U channel from Lowe's just fits over the end of a standard servo. So after drilling holes in the channel and sandwiching it between my frame pieces I was able to wedge my servos into the channel. After tightening the bolts down the servos are held secure enough to not come out but loose enough to allow for minor adjustment if needed.

 

From ExMod
From ExMod
From ExMod

A later addition to the frame that came in handy was a battery holder cutout. It secured my batteries and didn't look so bad. I simply cut the top layer of the frame and slid the holder into the slot. To add electronics to the frame you simple drill holes where needed and add spacers if you want. The frame is non-conductive so spacers are not necessary.

From ExMod
From ExMod

2. Wheel Mounting

Wheel and Sensor Mounting

 

The frame is designed to be experimental. Nothing is permanent and can be moved where needed. The wheels are about the only important part that cannot be moved. As I mentioned earlier I mounted the servos in Aluminum U channel. However servos don't get you very far without wheels. I found a tutorial on mounting the omniwheels to the servos here: http://www.acroname.com/robotics/info/ideas/omnimod/omni_mod.html

 

However I didn't like the design so I modified it slightly. I didn't use the finishing washer. I just used a standard locking washer. Also I tried the nylon bolts but found them lacking in stability and strength so I upgraded them to steel bolts. These work great and have not failed me yet.

 

From ExMod

This small metal connector can be found at your local hardware store. Its called a T-nut. They work great. I bought this one at Lowes.

From ExMod

A big issue was finding a round servo horn. I finally had to break down and buy a bag of miscellaneous servo horns from two different hobby stores in two different states. Hobbytown USA sells them but I couldn't find them for much cheaper individually online. This way I have a few extra horn types for other projects.

 

The small metal peice is called a T-nut. They can be found at hardware stores. I purchased these ones at Lowes. They work great.

From ExMod
From ExMod
From ExMod
From ExMod

The servo horn screw didn't fit through the T-nut so I had to put it under the T-nut and bolt that down before I screwed it into the servo. A bit of a pain but the added benefit that you can't loose the screws. That was the hard part the rest is done by threading the other T-nut onto the bolt and screwing the bolt into the servo horn's nut. Don't forget the lock washer to keep things tight. Remember the idea is not to tighten it down as tight as possible but to secure the omniwheel to the servo. Watch you don't get over excited and damage your wheels.

 

3. Adding Electronics

Now on to sensors. Currently I have only mounted a single EZ1 Sonar sensor to the platform. I used some erector set parts and a bit of left over pvc to create the bracket. This provides a stable platform and it can be moved where ever needed by adding some holes to the top frame. Additional sensors can be added where needed. The frame is meant to be flexible and reusable.

From ExMod

 

The other major electronics part that needs to be mounted is the Axon and any OSCAR modules needed to complete the design.

This is a simple process of drilling some holes and inserting spacers if desired. I connected power and in moments I was programming

the Axon. As future modules are design for the OSCAR system additional mounting points can be added as needed.

 

From ExMod
From ExMod

Now you have a simple and easy to use base to design and build any type of robot you want. This frame will assist me in creating new modules for use by the community.

 

As a side note if you liked this tutorial please vote for it in the contest this year.