Re-design of the caster ball wheel re-design

The even newer ball caster wheel

Yes, this is another ball caster wheel and yes also, it does look exactly the same as it did last time. However, it is actually ever so slightly different from the previous version (v0.3)

The main differences are that this has fewer ridges to help reduce surface area in contact with the ping-pong ball, therefore increasing the fluency of it's rolling. Also, the ridges are now deeper and more prominent, this is in order to to make them look more purposeful; the bumps on the previous version looked like they were were accidental, thus making it look like the part was poorly designed.

To help reduce the chance of it snapping, I have reverted back to the slots being deeper and therefore the design has more structural strength throughout. On the other hand, this would cause the robot chassis to be uneven again, as it would be sloped backwards. Unfortunately, it still snaps, even though it has a thicker structure at the bottom supporting the ball; I think the cause of the weakness now is the slot which turns in the centre of the platform, as this may be causing unnecessary strain on the design.

It still holds the ping-pong ball nicely

The design files for this version of the ball caster wheel (v0.4) can be downloaded from Dropbox, along with the previous versions of the ball caster wheel. 

Caster ball wheel re-design

The newest ball caster wheel

I have yet again re-designed the ball caster wheel, this time I incorporated bumps into the design in an attempt to reduce the surface area in contact with the ping-pong ball and therefore aiming to make the ball run more smoothly. You might also notice that this design has two bolt connections, this is to help reduce movement of the acrylic parts. I have also made another small edit in the design, this is to make it slightly stronger so that it wont snap as easily, however, it was slightly stronger, but it was still not strong enough and broke fairly quickly.

A side-on view of the caster, you can see the bumps quite clearly

The design files for this ball caster wheel (v0.3 - v0.1 and v0.2 are implemented in the chassis designs) can be downloaded from Dropbox.

Re-designed chassis

This is the second version of the chassis, including a menacing 'grill' on the front

Since I posted the feedback of the modular circuits and the chassis design, I have been working on an updated chassis. Here are a few images of the chassis, currently without any electronics.

Here is a list of updated design features:

• The chassis is both thinner and shorter as I have no removed dead space on either side of battery and moved the caster ball wheel move closer to the battery so that it can be shorter
• It now has a sectioned battery compartment with a removable access panel, so that the battery does not move or fall out unless you remove the panel
• The motor mounts have been moved outwards and altered slightly, so that the wheel can be pushed fully on and will not rub with the chassis
• An integrated 'bumper' has been added, which is designed to slide back and make contact with a touch sensor
• The ball caster wheel has been updated, so that the ping-pong ball is slightly less loose
• Slots have been added in the centre of the chassis layers so that wires, such as the battery connections, can be wired through to the middle section
• The orientation of the LDR mounts have been altered, to point slightly more outward
• I made the ball caster wheel mount have a thinner section, so that the robot is more level

The battery compartment with the panel...

...and with it removed

There are already some known issues with the current design:

• The centre support of the bumper prevents it from sliding backwards
• I think that the bumper could look slightly less menacing, removing half the supports may fix this
• The ping-pong ball in the ball caster wheel now has some play, however, when the robot is resting on it, it causes enough pressure that it will not move as freely, one way to fix this would be to add bumps (ribs?) to the contact point of the ball caster wheel as to reduce the surface area that is in contact with the ping-pong ball
• The mount of the ball caster wheel is now thin enough to break easily
• Friction on the bumper prevents it from moving backwards and forwards easily, this could be helped by adding a layer of paper to act as a spacer

A close-up of the front 'bumper'

The two versions side-by-side

This chassis design (v0.2) can be downloaded from Dropbox.

What needs to be improved on the modular robot?

I have now had a working modular robot for a few days and I now know a few problems wioth the current design and had feedback from others. So this post is simply a list of potential improvements that could be implemented in later designs. Some parts of the robot require more work than others.

Edgebot potential improvements:

• Remove the need for a jumper for when the Photovore shield is not used, this could be replaced by two transistors and act as a switch when the Photovore shield is added
• Remove the two vias on the PCB

Photovore potential improvements:

• Find another source of LDRs, so that the sensitivity is altered so that it can be used in lighter conditions
• Convert the circuit to an Eagle schematic and produce a PCB

Motor controller potential improvements:

• Remove the voltage regulator, as it is not needed for the current boards and takes up a large amount of space on the PCB
• Add a second trim-pot, so that you can calibrate both wheels to ensure that it can go in a straight line
• Re-arrange various components
• Re-do all the tracks, in an attempt to get rid of vias
• Move the switch so that it is not obscured by the shields and add pin spacing so taht you can have an external switch
• Change the mounting hole layout, so you cannot attach it to the chassis in an incorrect orientation
• Add PCB to where it is currently not, at the sides of the board, as it is a paid-for area, which is currently not used

Improvements that would affect all the boards:

• Alter the stackable header format so that one of the 6 pin headers become an 8 pin header, this would cause less confusion when it comes to the orientation of the boards and it would also allow for more pins to transfer signals between boards
• Change the pad size to be larger, as this would make for easier soldering for people who are less capable

Chassis potential improvements:

• Make the chassis smaller, as there is currently a lot of wasted space, which is doing nothing apart from making the robot larger
• Make a battery compartment, so that the battery does not rattle around when it is moving, but make sure that it can come out when you want to change the batteries
• Change the motor mount design, so that the wheel can go on fully
• Add an integrated bumper at the front of the chassis, this would trigger the touch sensor when it drives into an object
• Alter the ball caster wheel design, so that the ping-pong ball is less loose
• Change the design so that wires are not wrapped around the chassis
• Rotate the LDR mounts to point slightly more outwards
• Alter the design so that the robot has a more level base

Photovore in action

Here is a short video to show to Photovore shield working both on it's own, and then with the added capabilities of the Edgebot:

Revised Photovore shield

The perfboard Photovore with diagnostic wires to figure the input/output wiring

As the Photovore shield failed, I had a look at the various components on it which could have failed, due to the little amount of parts on the modular Photovore PCB, it indicated that either the dual comparator was blown, or I had implemented the comparator chip into circuit incorrectly.

I started by simply swapping the comparator chip out and replacing it with another, completely new chip and the result? No difference in result. This caused me to presume that I had made an error when designing the PCB of the Photovore, so I made a mock-up of the circuit on a solderless breadboard which again, did not work, even when I attempted multiple circuits found online and within the simulation software.

Now without a Photovore shield, I needed another circuit that would perform similarly; I looked back through my notes on various original circuit design concepts and found a suitably simple circuit which I decided not to use previously as it required only 2 inverters and inverters are only packaged as a large hex NOT gate, therefore I would be using a large chip which could be replaced with a smaller DIP chip.

This new circuit is simple, it sets two LDRs up as a voltage divider, and the first inverter takes the split signal and turns it into a clean, digital signal, this is then fed to one of the motors, while that signal is inverted again for the other motor.

I quickly wired it up on a solderless breadboard to test the circuit, then continued to transfer it onto a perfboard shield. As you can see in the images, the perfboard shield is similar size, and no moderation to the headers was required to make them slot together correctly, due to the layout of the PCB.

The original Photovore PCB next to the freshly cut perfboard

The perfboard shield with the stackable headers in place

The finished DIY Photovore

The new Photovore shield on the chassis

As of the time of writing, it is fairly bright and it is hard to follow a light in bright light, as the difference between the two LDRs' resistance is minimal, so I will post a video when I have chance to.

It's alive!

The very first working modular robot

As I have previously posted, I have now finished the current versions of the Edgebot shield and motor controller board and they're working correctly together. I quickly designed a chassis for the robot that would act as a platform for the electronics to be attached to. The design includes and integrated caster roller that uses a common 40mm ping pong ball as the main pivot, this is inserted with a little force and a reassuring 'pop'. Once this current design can still be improved greatly, for example, the pin-pong is very loose and rattles a fair bit when not in use, etc.

The ping-pong roller ball, still to be finalised, but working.

The main section of the chassis is made up of 2 layers, the first of which is the base, this is what the gear motors are mounted to and the battery mounting rests upon, while the front section of this layer is slightly obstructed by the roller wheel fastenings, it does contain space to put other sensors or parts.

The top layer currently only accommodates the stackable PCBs, the motor controller is fastened to the acrylic using four M3 bolts and some nuts. Then other layers are stacked on top of that to provide other behaviours to the robot, such as light seeking and object avoiding, whilst the motor connections could be attached to the motor controller permanently, the other behaviour modules' inputs are interfaced by right-angle headers pointing outwards from the board. You can then plug in the sensors as they are required.

The files for the chassis design featured in this post  (v0.1) can be downloaded from Dropbox.