Digit Evo 2 - A Step too Far

Digit Evo 2

Digit Evo 1, just wasn't fast enough or sophisticated enough for my liking, so I stored him away and began work on Digit Evo 2.

Evo 2 Knee

With Evo 2 I intended to give it proper legs with jointed 'knees', which would allow him to raise his feet off the ground, rather than leaning over to raise a foot clear.

However, the control of the servos relied on sequences of positions stored in tables in the micro-controller. The values were selected in real time for the servos, with no feedback. Feedback being necessary so that the robot knows how it is positioned within its environment and how it is walking. Many of us may recall walking up the stairs in the dark, and taking an extra step at the top of the stairs, where in actuality there is no extra step, and subsequently stumbling.

Our eyes normally tell us where the steps are, if being able to see the step is removed, we rely on memory and other senses. We also get feedback from the feel of our feet on the steps, Evo 2 has only the memory of where to place his feet. So if things start going wrong he doesn't realise and just carries on trying to walk, and has no ability to correct or accommodate things not being quite as he expects. All with a lot more servos, so considerably more complicated to get walking.

I did consider gyros and sensors etc, but at that time I didn't have the technology to proceed with that, and I was programming the micro controller in machine code, so it would have been a major leap in programming to accommodate the added complexity. 

Evo 2 Foot

I soldiered on trying to sequence the movements. Controlling the hip there is one servo for both legs striding, and one servo to twist one of the legs, for direction, then in each leg there is the  the knee servo and 2 ankle servos mounted on the foot.

Looking at the construction now, it does look a bit spindly, and the joints, although machined do not look like they would be stiff enough to precisely maintain the axial position, and would be a source of twisting etc.

Evo 2  Hips

The connecting rods are carbon fibre tubes 6mm in diameter and quite stiff. Another observation is that there is no shock absorption, so those poor servos had to put up with a lot of stress, and the servos range of motions look like they could have had a better arrangement.

Digit Evo 2 took a considerable length of time to build and wasn't as rigid as it could be. Instructing all the servos at the same time in a sequence, wasn't going to accurately position the legs and feet, as there is too much movement in the joints and twist in assemblies. So in the end all Evo 2 managed, was a quick stumbling fall every time.

I had a lot of fun working on him, but in the end we decided Evo 2 wasn't going to be finished in time for the 2006 UK Robotic Games, so we reverted back to Evo 1.

As a first attempt it wasn't too bad, but definitely needed development. However there wasn't another UK Robotic Games so poor Evo 2 has been stuck on the shelf along with all my other distractions.