Sunday, 13 July 2014

Wrist Designs

Over the past couple of weeks I have been mostly testing Armdroid electronics and going completely round in circles...

I've somehow destroyed an Arduino, questioned my sanity, sacrificed several chickens, and finally, have the interface electronics from TNMOC's Armdroid now running.  But, I'm facing what appears to be a timing problem as soon as the 74LS366 is inserted into the empty IC5 socket that enables the feedback sensors.

I was wondering why TNMOC's interface was missing this chip, because it doesn't make a great deal of sense when the arm has already been fitted with sensors.  When you look back at the photographs it's quite obvious the previous owner bypassed the interface circuit and controlled the steppers and sensors  from some other source.  This is not the first time I've witnessed this when making a post-mortem analysis of a dead looking Armdroid.

Enabling IC5 and testing the same combination with my Armdroid interface circuit, everything is working as expected, so assuming I do have my facts correct, I will write up another day more information about the wiring up of these sensors.

I suspect the timing problem relates to the 74LS123 (IC4) Monostable Multivibrator and delay determined by the accompanying capacitor and resistor network.   Testing this capacitor with my ESR meter in-circuit proved inconclusive, so I'll probably swap this out as soon as I have replacements available and try again.

Anyway, having spent many hours looking at the circuit board, decided I would take a break and start cleaning up the mechanics as everything is really dusty and dirty....   This morning I spotted a very subtle difference between my Armdroid 1 hardware and TNMOC's regarding the finger supports.

On the surface, the design of Armdroid 1 hardware didn't actually change a great deal after going into production, so this is worth a quick mention here...

Spot the Difference

As you can see in the following photograph - my Armdroid consists of an aluminium support flange for the three fingers:

Finger Support Flange - my Armdroid
Ignoring the fact the DELRIN gearing have different colours, this design consists of two different types of bevel gears - flanged gears (the ones with string wrapped around them) and one non-flanged gear, called the hand gear, which this finger assembly is attached with machine screws.

This must have changed at some point to the following arraignment which appears much simpler.  In this case, we have three identical bevel gears with much larger flanges, and the aluminum support is no longer necessary because the fingers are now bolted directly to the gearing:

Simplified finger supports on TNMOC's Armdroid
Another view of the flanged bevel gearing

Looks obvious this must have been a cost cutting exercise, but did Colne Robotics ever update their documentation and blueprints to reflect this ?     Answers on a postcard please...