# Looking for a certain transducer

Usually I give a more specific title to my posts, but the problem is I don't even know the name of what I'm looking for. So I'm going to have
to describe it...
Summary: I am looking for a small device which can "quickly" move a weight (about 1/4 kg) back and forth along a short distance (about 3 to 5 cm).
Imagine that you have a steel rod hanging from a ceiling. It's not a very big rod, only about 18cm (7") long and at most twice as thick as that part that contains the ink in ball point pens. At the other end of this rod (pendulum) is a weight (about 1/4 kilo) that can slide up down the rod, only I want to anchor this weight to behave as follows:
When the rod moves through the vertical position (ie. vertical from the ceiling), the weight should move closer to the ceiling by about 3 to 5 cm. When the rod has reached a high point in its path (and is therefore no longer in motion), the weight should retract to its starting position (see diagram, below).
The question is where can I find (and how is it even called) a device that will be able to move the weight as I described? The exact times when the weight movements happen may vary (ie. I am not concerned about the measurement portion of this problem. We can assume that there is a signal to the transducer that says 'extend' or 'retract'. But (1) the rising should be "quick" even though it may be acting against up to 5g (due strictly to the swinging) (2) the weight has to be able to maintain that 'risen' position until it gets the signal to retract (3) the retraction is never against more than 1g (if you imagine that the pendulum can rise above the level of the ceiling).
Apologies if I have not adequately described what I am looking for. In that case, please ask for clarification and I will provide it.
Thanks for any tips, Csaba Gabor from Vienna
Ceiling | | | | | weight (rises as it swings through) | | -
Ceiling \ \ \ \ \ \ \ weight (retracts to end at high point) -
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Hi! If you are permitted a disc at the high point and a smaller retaining disc at the low point then you could do it with an electromagnet in the high point disc and make your 1/4 kilo weight a permanent magnet with a hole through the middle ?
Best of Luck - Mike
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Hi Mike,
Since I'm the one making this thing, I'm permitted to have those discs, and I'm open to your idea. If I understand you right, when I want to make the weight extend (move towards the ceiling), then I should pass a current through a wrapped wire to the high point disc which will create a temporary magnet, attracting the half pound weight (permanent magnet) over the 3cm distance. When I want it to go back to its original position (at the end of the rod), I could just reverse the current.
Perhaps more importantly, what is/are the term(s) I should search under to find this type of item for purchase (and I'm happy to receive the name/site of specific recommended stores - don't worry, I'll do my homework before purchase). At this point, I just want to experiment around to come up with proof of concept model.
Thanks, Csaba
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In article <1114698527.182408.254020

I'd start by Googling "voice coil actuator". If you find you need longer strokes or higher forces than you can get with this type of device, search on "linear motor" or "linear servo motor".
I'm assuming you need smooth, controlled motion between the two end positions. If not, the problem is much simpler.
Can we also assume the actuator must swing with the pendulum, or might it be possible to transfer forces through the pendulum's hinge?
Ned Simmons
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snipped-for-privacy@z6.com wrote:

Hi, I think that you could do what you are describing with a solenoid. Pinball machines used them extensively for for actuating the flippers and bumpers and such. A solenoid has a steel or iron plunger inside a short tube with magnet wire wrapped around the outside of the tube. If the plunger is not all the way into the tube when power is applied to the magnet, it will be pulled until it is. It has been many years since I have designed a mechanism with a solenoid in it, so I have forgotten the amount of force availible from them, but as I remember, it can be pretty substantial.
To make your mechanism work, I would get a steel rod that is the correct diameter to go into the solenoid and tap the end of it to take a machine screw. Use a nylon or aluminum (non-magnetic) bolt threaded into the end of your steel rod to support the rod at it's longest extension. I am guessing that the steel rod will need to be about 1/3 of the way into the solenoid tube at rest. _____ / \ ------------- ------- ^ | | | | | Solenoid | | Nylon or aluminum Bolt length | | | --- | | | ------------- | | | | | | | | Steel Rod | | | | | | -------- | | | | Weight | | --------
Next, make a pivoted mounting bracket for the solenoid so that it can pivot back and forth with the swinging weight below it.
Bob
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This sounds a lot like the Foucault pendulum driving mechanism described in "The Amateur Scientist: How to Make a Pendulum That Will Demonstrate the Rotation of the Earth" by C.L. Stong from Scientific American pp 115-124, June, 1958.
It would probably be worthwhile for you to take a look at that and see how they did it. Although it may not be a requirement for your project, an interesting aspect of what they did was the consideration that the mechanism which kept the pendulum swinging not have any directional preference.
Mitch
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Solenoid, 24VDC, 0.77" dia., 3.7lb pull, 3" plunger, \$3.30 each.
http://www.73.com/a/0201.shtml
John Nagle
snipped-for-privacy@z6.com wrote: