ARMazing! (Part Four: Arm + Hand = oh noes )

in ARMazing! (Part Three: the nervous system), we understood the nervous system of Mentor Arm. The motors and Potentiometer are connected to the front panel.

Next we wanted to put the 3D printed hand and mentor arm together!

LOOKS good... but will it work?

We created a mount for the servos with Andrew Lizner's help. After sketching with pencil and paper the dimensions, we created a 3D sketch on Google sketch up (Figure 1)

Figure 1. 3D modelling in Google Sketchup

We used the CNC (computer numerical control) router machine with Andrew's help to cut out the parts. The parts are made out of aluminium.

Figure 2. CNC machine

Then we bent the parts in the work shop and used A LOT of 3mm screws and nuts to put the servos together.

Figure 3. Putting the pieces together

And finally it can go on the Mentor Arm like so...

Figure 4.Overview

When we were putting everything together we were very excited. We thought it will work as follows (numbers match up with the image below):

1. Servos will pull the string (green line) causing tension. 
2. Mechanical force will transmit mechanical force from the servos to the 3D hand via a bowden cable where the inner string will move relative to the outer hollow cable. Ray gave us the inner string (nylon wrapped steel string) and we found the outer hollow cable (plastic tube) from the Chemical Engineering Department. 
3. The string will come out from a small hole at the end of the arm and into the 3D hand.
4. The 3D hand will contract in tension (servo pulling).

Figure 5. How it works

However there were several problems:

• Since the plastic tube (#2 in figure 5) curves on the path shown in the picture, when the string is in tension it will rub against the softer outer cable causing friction (#A in figure 6)
• More friction (#B in figure 6) when the string exits the small hole.
• More friction (#C in figure 6) when the string enters the 3D hand.
  

  

  Figure 6. Friction Problems

The friction was HUGE! We could not move the finger from #1 (figure 5). Most of the friction was due to #A (figure 6) where the string is rubbing against the plastic tube. Therefore:

K2 = K1 -  μ2.

μ2 = Friction from the #A, #B and #C (figure 6)
K2 = mechanical force the hand receives
K1 = mechanical force the servo produces

Therefore K2 is too small to make the fingers move!

Figure 7. Energy loss diagram

Should we continue with this MADNESS!?!?!?