Let's Make Robots!

How would you register the sligtest touch?

UPDATE BELOW!

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I have an idea for a really fantastic little project (IMO :) I'll show it later, but I have run into a little challenge that I did not see when I woke up in the middle of the night with the overal concept crystal clear in my head.

I need a "probe" - a small thin stick - like a long fat needle - to move forward towards an object, and with none, or only a slight touch, register when it touch the object.

Now, in my dream that was easy, because I'd just make some form of push switch; Either out in the end of the probe, or at the other end of it -- A mechanical sollution, simple, no problem.

However, in reality it turns out that this object to be touched quite often is rude enough not to be at a right angle to the probe, and so it is just sliding off, which is devastating for the project.

Imagine a ping pong ball, with a knitting needle moving towards it like a rocket - only very slowly :D I need to know when that needle touches anywhere - even the outside edges - of that ball.

I do not mind what the needle is made of (could be a laser, or a tentacle from a space monkey, what ever, just that long thin shape) - and I just need to know when it is touching something.

A good idea would be to make an apparatus shaped like the letter U, with a laser on one end, pointing towards a sensor in the other end. That would give the effect: Put down the ping pong ball between the two, and you would interrupt the laser, and know someting was there.. Only that would not do, because I need to know the distance - how far did the needle go before it touched the freakin ball.

 

I made a mechanical solution with a combined touch-and side-move switch, reistering when the needle is either stopped, or pulled slightly to one side or the other. I just do not hink it is very elegant.

 

I cannot hook up the ping pong ball with electric current, though that would be a perffect solution: The instant the needle touch the object, a current could flow. But electrifying the object (ping pong ball in the example) will not do.

 

Damn, I am stuck on this one. I keep moving my finger slowly towards stuff, and sense just the moment I touch it. Why can I not do that with a sensor? (No; HOW do I do that with a sensor?)

 

I think I will send a present to the person who can give me a good idea here :)

Thanks!

 

UPDATE :

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First I want to thank you all for some excelent thinking! I was merely hoping to get a single respond, and figured that perhaps it could inspire me to think different.. but I received some very, very good and concrete ideas, really fast!

You guys are very talented and nice persons, thank you :)

Then, since you take me so serious, so you actually come up with good ideas, I figured I'd better share my idea:

I had this idea: Did you guys see the Tintin cartoon, where the professor makes a machine to copy things? He has 2 plates, on one he places an object, and the machine copies it to the other. I would like to make something like that :)

If I made a very simple system that would enable a probe to poke at all possible angles on a rotating object - just by using 3-4 servos in total, not regarding angles much, just making sure the the object was poked in all sorts of angles and directions, one servo step inside another servo loop at a time.. Could take all night! Poke, change angle, poke, poke, poke..

When the probe reaches the object, it instantly retracts, servo moves, and the next point, just somewhere next to it is touched.

 

Where would that take us?

 

Well.. Based on the fact that 2 servos whith the same signal will move the same way, I would just make 2 of these machines. Only the other would have a drill instead of a probe. And instead of the object, a block of an easy to drill-material should be.

Drill till the surface on the sensor was reached, drill, baby, drill. From that direction, and from that :D

 

Extras:

The 2 machines could have long wires between them, one in another house. Almost teleportation! :D And since it's only simple servo signals, it would be easy to transmit over air as well. Or hey - over the Internet?

And as the machines would be made by simple servos, you could build one as well, and I could teleport my shape to your machine :D

And what about detecting colors on the tip as well? And on the shaping-end, an ink-jet head. That would be sick!

 

When done with the copying things, instead of a surface detecting probe, I could place perhaps a light detection. And then I could physically shape the rays from a flashlight shining on a diamond-shaped crystal..

Or how about sensing magnetism, and shape it in ral form? 

Heck, place a noisy speaker there, and detect sound, see how it would go out from the speaker :D

 

But first I wanted to copy objects, and I need a good way to detect them. I could use laser, see when the ray was broke, but that would not give me "introvert" shapes - only the outsides of a cup, not the insides..

:D

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A concern that comes to mind with this approach is I don't think a copier based on using a common servo signal will be precise due to servo tolerances (unless you are using some nicely matched servos?)- I guess it comes down to what your tolerance requirements are.  To achieve more accurate duplication with servos I think it is probably necessary to use an absolute encoder (or something similar), and teach each servo's home position so you have an offset to go by, and no linear variance.

I would think a pro solution to this would be to use lasers to create a 3d model such as .stl (stereo lithography) and then print the object out of abs plastic or something similar using a 3d printer.  The 3d scanner could be a challenge as well as expensive, so I expect this to be unuseful but I thought it was worth mentioning.  The premade, expensive solution is to use a 3d scanner and then 3d printer.  Of course there is cheap do-it-yourself 3d tools out there as well.

An other idea i just think of:

A stift that slides inside another, with a spring. Then somehow put a led and an ldr inside it so that when it slides, the stift blocks the light and changes the resistance of the ldr. Then you will get an analog value. By the slighest change it will react I think.

However the piezo idea is also a good one i think.

Linear variable differential transformers (LVDT's)    used in weight scales , load cells , works like a metal detector with a pair of receiver coils and a movable core ,,,,,(Practical Robotics p158 ,by Bill Davies published by WERD Technology 2002 ) 

The probe could be the movable core with some kind of rubber spring system    :)

I know I have seen what you are talking about but it was mechanical. I am sure some of the older members probably know better than I do. I remember seeing it on a show. It was used to replicate something... Maybe coin designs carved on a large surface and then transferred to smaller dies. my memory fails.

One spring mounted inside another on a stick-The springs are selected so the inner spring is stiffer than the outer, but both are the highly flexible thin wire compression type springs-Works every time. Solder or wrap a wire arond each-Its an infinite trip hard contact switch that is very sensitive.

I used to do that when I built lots of wheeled bases robots.

Give it a try-Its cheap and effective.

Hey - I really like that!!!

Yes! So simple.

Wow, yes, it would be easy to build, and very hard to touch anything from any side without causing a read!

It takes guts to admit that i did not think of that - I always had the problem with the hard center core that would be a kille ron one direction or the other.

2 springs!! One twisting one way, the other the other, perhaps. or one pre-stretched a bit of the same kind as the other.. Very nice, androidman.

Heck, I am going to try that!!! :D

I'll be back!!

where does one get these springs? do you have a picture or anything?

Shop for springs at the hardware store-or salvage them.....or wind them yourself out of solid thin wire?

Ballpoint pens! No problem! They are everywhere!

Pen springs might be sorta stiff....but may be good for internal one.