Saturday, 25 February 2012

Weight and balance

A hexapod scorprion with large pincers and tail walks less well that I had hoped. It seems very difficult to walk with any significant weight forward or backward of the legs. So although I like the look of this model and spent a load of time getting the chassis right, it's back to the drawing board... I'm going to investigate 8 legs (octopod) and less weight out beyond the legs.

Saturday, 18 February 2012

Buying S2 bolts in bulk

The Bioloid kits ship with a vast number (200?) of 'S1' bolts (machine screws). S1 are used to join brackets to servos, and some brackets to brackets. S2, S3 and S4 are really useful when two brackets need joining. But Robotis only include 20 of each! This isn't enough for 4 per leg on a hexapod. You can buy a complete set of additional bolts, but you still only get 20xS2, 20xS3 and so on.

Fortunately the bolts are standard sizes:

S1 = M2 x 6mm
S2 = M2 x 8mm
S3 = M2 x 10mm
S4 = M2 x 12mm

So a quick eBay search for "M2 X 8 PAN HEAD POZI" turns up packs of S2. The ones I bought from jf_tools have a slightly larger head than the Robotis ones, but in many places this is a benefit (they sink into the plastic less when done up tight)

Leg design

I've chosen a 3 DOF (degrees of freedom) leg because this is sufficient to place the tip of the leg (i.e. foot) anywhere in a bounded 3D space. I will be able to control how far forward, out from the body, and up in the air the leg is. 4 DOF would be cool but the servo count (and therefore the weight) would be high as would the complexity of the control algorithms. The three degrees of freedom are:

* Swing (leg backwards/forwards), driven by AX-18A servo

* Hip (lifting of the entire leg up and down), driven by AX-12A servo

* Knee (lifting of the lower half of the leg), driven by AX-12A servo

Choosing the feet

The Bioloid kits don't include good feet for multi-legged robots. The Robotis multi-legged designs tend just to use pointy plastic parts which have no grip on the floor. They also make a loud tapping sound on hard surfaces. I therefore decided to search for softer, more grippy alternative. Two evenings of web searching later I found these anti vibration mountings for industrial equipment. They are mainly a dense rubber, but with a threaded (M8) male insert. By chance the M8 insert passes easily (but snugly) through the large hole in the Bioloid F9 bracket. The pointy end to the foot means that it can make contact with the floor at various angles which will be important for smooth walking with 3 servos per leg.

Thursday, 16 February 2012

The inspiration

This project is heavily based around the Bioloid robotic system from Robotis in Seoul, South Korea. I got my Bioloid Premium kit from RoboSavvy in the UK (excellent service by the way). Amongst other models, the Premium Kit can build a robot scorpion - video here. It looks like this:

It's a fun model and for a 18 servo limit (what the kit is supplied with) it's not too bad. However it could be so much better. My main gripes are:
  • Only 2 degrees of freedom per leg. This means the feet skid badly when it walks
  • Poor walking gait - just an inefficient tripod gait run at a frantic rate
  • The pincers are fixed into place - no moving arms
  • No remote control except for responding to clapping
The Pointless Robot will expand on this in many ways.

The beginning...

First post of a project blog


If all goes well, this blog will track the design and building of a robot scorpion with:
  • Over 30 Robotis Dynamixel networked servos
  • Inverse kinematic walking (probably just 6 legs)
  • Arms, pincers and a stinging tail
  • Robust 2.4GHz remote control with over 10 channels (hacked)
  • Multiple microprocessors including an Arduino Mega
  • Attitude and guidance system
  • Autonomous and user-controlled behaviours
  • Large Li-Po power cells
  • ...more stuff I haven't thought of yet!