engineer

tim   .

                                                      


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.   hunkin

                                                         


cartoonist

 

MY-NUKE REMOTE MANIPULATOR ARM

 

       

Three years ago I made a claw to handle ‘radioactive fuel’ in a glove box. I returned to this last winter, but soon found the glove box was clumsy and unsatisfying to use. Then these photos reminded me that ‘remote manipulator arms’ rather than glove boxes are used for working with strongly radioactive materials. This was more ambitious, but the previous job I’d been doing had involved too much time on the computer and I was feeling like a challenge to keep me in the workshop for a while.



Remote manipulator arms are an ideal project
for a mad inventor   


My initial prototype arms were moderately encouraging, but it was Sam, my six year old grandson, who really persuaded me they were worth pursuing. Even in a very rough form, Sam was completely absorbed playing with it for 45 minutes. It only took me two weeks to get this far, but making an arm strong enough and making the whole machine practical took months, and it became a lot more complicated than I ever imagined. In this respect its like a real nuclear reactor – a simple idea that gets very complicated trying to make it foolproof. 

I had initially assumed I would have two arms like real nuclear manipulators, but I found this needed considerable skill. I remembered Sam had held the prototype with both hands, so I tried a single arm made to be held with both hands. This was much easier and more satisfying to use. 

I  gradually refined the linkages from the handle to the remote ‘arm’. The steel tie rods got replaced with nylon cord to make everything more springy and more resilient to being roughly handled. I was delighted to find it was completely intuitive to multiply a movement (so a small rotation or tilt of the arm’s handle makes the remote arm turn or tilt much further). 

Discovering this was exciting. It gave me a new appreciation of the human brain. Its amazing we are so competent doing something as indirect as steering a car by rotating the steering wheel – where the movement of our arms bears no simple relationship to the car’s movement. Its because we have such amazingly sophisticated eye, brain, muscle co-ordination. When we eat, we are completely unaware of what our hands are doing – we only focus on what the tips of the knife and fork are cutting or shovelling. The steering wheel or cutlery implements are effectively bionic extensions of our arms. And then we are often hardly conscious of what we are doing, our attention is directed on what we are listening to or talking about while we eat or drive. (though driving is obviously more demanding than eating because we can’t read or watch TV while driving, which we do while eating.)      

As I developed the arm, I became increasingly aware of subtleties like its friction and springiness. They made a big difference to how intuitive and satisfying the arm was to use. It made me more aware of the limitations of computer human interfaces. I use an expensive Wacom cintiq tablet for drawing sometimes, but even this has time delays and parallax problems between the stylus and the ‘drawn’ line on the screen so I still prefer to draw on paper and scan the image.

Using computers to control physical machines involves much more compromise. I’m sure remote surgery robots must have sorted this out, but on a limited budget it doesn’t work well. Radio controlled planes and helicopters are tricky to master mainly because the relationship between the joystick and the movement of the craft involve delays and are not proportional. They are certainly not instinctive like a car’s steering wheel.

  Even expensive robotic servo systems have small delays between sensing and reacting. Its hard for a walking robot to react fast enough just to keep its balance. I was amazed by the video of the Big Dog walking robot recovering itself when slipping on ice. I thought computer servo systems must have finally speeded up. But then I found its recovery was mainly a result of cleverly placed big springs. Springs react without any time delay. This encouraged me. Kids used to computer interactivity might be impressed by the sensitivity and immediacy of the direct mechanical linkage of my remote manipulator arm.  

I fretted a lot about whether I could ever make the arm strong enough to survive rough handling. Part of the answer was to make it as easy as possible to use – people are much rougher if they are frustrated! Kids can also be rough if they aren’t playing the machine, just messing about. So I decided the arm had to be clamped when the machine wasn’t in use. This had the added advantage of forcing people to watch the instruction video – without clamping the arm, people get so absorbed inserting the fuel pellets that they never glance at the screen.   

Deciding the task to perform with the arm was surprisingly easy. I had tried putting little cylinders into tubes in an early prototype and the idea stuck. I was unsure about it until I worked out a mechanism to get the cylinders back inside a box at the end of each go. Psychologically, this seemed to make the fuel seem more dangerous and the task more interesting.

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