Little walking robots

After seeing this robot by Pololu I suddenly found myself inspired to try and make a small walking robot. This post doesn't result in a finished robot, but it (hopefully) will act as a overview of starting points for small walking robots.

Initial idea
I wanted to make a 6 leg robot (for stability) using the smallest motors/gearboxes I could find. I was thinking of a design similar to this one where the middle pair of legs pivoted around an axis between the back and the front of the robot. I was thinking of using a cam mounted on the end of a pager motor to rock the tilt the leg back and forward - this was my starting point.

(Yes, this is my first model in SketchUp)

First effort
Soldering some paper-clips together led to the creation of the following disaster (AA battery for scale):


This initial prototype suffered from two major problems; it was almost impossible to tune the legs accuratly and the page motor had nowhere near enough torque to lift the leg.

The cam
The problem with the cam was that it was too abrupt in its transition from low to high for the low-torque motor to rotate. I set about fixing this with a small file with the following result.

The lower half of the cam is the original size, the top-half is the smaller profile.

The modified cam mounted on the pager motor

The chassis
The chassis was redesigned to be simpler, stronger and more accurate - this time using a stripboard as the base. This is the result:

Under construction

Posing for the camera

Results

Well, the modification to the chassis certainly worked. The legs were significantly easier to tune. On the other hand the same problem still exists for the motor. The cam is too large and generates too much friction where it connects with the top of the leg.

The next step will be to increase the motor size and to add gearing - probably with one of these from Solarbotics. This should allow me to up-size the cam to allow greater leg movement. From here I can work on the forward and backwards movement of the legs.

Conclusion
Whilst hardly a stunning success, this mini project was a great chance to learn how to fabricate on a small scale and I will try to keep posting about it as it progresses.

My first vibrobot

This is a small vibrobot (based on the awesome bristlebot over at evil mad scientist) that I started work on a couple of months ago but never finished. My initial aim was to make a simple vibrobot that was self-sufficient (BEAM-style) and photo-tropic. This required some sort of directional-control for the bot, a solar engine and light sensors.

I settled on a solar engine from Solarbotics, two Colgate "micro sonic power" toothbrushes and two LDRs that I had laying around. After wiring up the solar engine I wired up the vibrating motor capsules (from the toothbrushes) , the LDRs and a pair of IN4004 diodes as shown below.


The directional-control of the vibrobot is controlled by the two LDRs connected across their respective motors. When the LDR is exposed to sunlight it conducts more current than when it is in the dark. This increased conduction causes the voltage across the motor to drop, slowing down the motor on that side and turning the robot towards the light. The diodes allow the two motor/LDR circuits to act independently.

Now, the important part, does it work? Yes, fantastically well, in fact! Its initial movement is very vigorous and erratic but as the solar engine discharges the bot clearly starts to steer towards the sun. It orientates itself so well at the end of its movement that I will be shortly tilting the solar panel towards the front to speed up the charging time.

Finally, the design of the circuit has resulted in a few interesting benefits that I hadn't realised before which I thought I would list:

• The initial vigorous and erratic behaviour gets the bot out of tight situations, whilst the phototropic behaviour is more evident at the end of the solar-engine's discharge. This gives the bot an "obstacle-avoiding" behaviour that I hadn't planned for.
• The brighter the light, the slower the bot moves and the less distance it covers (due to the LDRs stealing some of the solar-engine's discharge). This has the unexpected benefit of keeping the robot relatively stationary within an area of bright light, whilst making it more active when it is in the dark.
• Finally, as mentioned above, the bot aims its front at the sun almost faultlessly. This will allow me to tilt the solar panel toward the front of the bot (and therefore the sun), increasing its efficiency and reducing the charge time.

I will upload a video in my next post, I am waiting on both consistent sunshine and a full camera battery...