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Looking for a miniature actuator

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jjellin

jjellin

Mechanical
Jun 4, 2001
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I'm looking for a miniature linear actuator that has a max cross-sectional size of .250" square and is no longer than about 3". I need it to exert between 1 to 2 pound-force and have a stroke of at least .188". Additionally it needs a fast response time (actuates and releases in less than 0.5 sec). Cycle time will be once every 5 seconds.

Actuation can be either electro-mechanical (solenoid) or pneumatic (cylinder).

I've been looking into Shape Memory Alloys, but they don't seem to be the right fit. Anybody know of any tiny solenoids? Any suggestions?

Thanks, Jason
 
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We have done some special fluid power cylinder designs with small bores and strokes. Making a special unit the size listed will be relatively expensive. I doubt you will find a solenoid to fit in your envelope.
How many units do you need?
Does it have to be double acting, single acting, or spring return?
Does the rod have to guide anything?
 
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Well it now seems I do not have a compressed air line, I only have a vacuum line (25 in-Hg). So, are there any vacuum operated cylinders?

To answer the questions above: the cylinder would need to be spring return or single acting (spring on mechanism) and the mechanism will guide itself. Quantaties are currently unknown, so for now its strictly a prototype (maybe two).

Festo actually has a very small rectangular actuator and a cylindrical actuator that might have fit, but they operate using compressed air.

I know of the venturi vacuum generators that convert compressed air into vacuum, can these be operated in reverse or is ther a device out there that can create compressed air from a vacuum?

The only solenoids I've found do not provide near enough force.
 
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Vacuum is the easy way to go, but your dimensions indicate that you are on the ragged edge of the thrust spec, with a /single/ piston, even ignoring friction.

A tiny electric motor with a reduction gearbox and a worm drive as Ed suggests is my second choice. Sounds like it would meet the spec, but would cost a lot.

SMA tends to be slow and short stroke, high thrust. Some sort of clever arrangement like a temperature compensated pendulum might ncrease the stroke, but the sped is problematical for an electrically heated thermal system - bearing in mind it needs to cool down within 5 seconds.

I think your spec needs more work, how much thrust do you really need? You quote some dimensions to 3 significant figures, but the thrust has a 100% unilateral tolerance!





Cheers

Greg Locock
 
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Several years ago I had a similar problem. Used the solenoid out of an electric contactor. Worked fine once I sorted out the cushioning. There are plenty of solenoid vendors around.

cheers

Steve
 
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Thanks for all the help so far. I've checked out many of the suggestions. Festo has good air cylinders, but according to their tech, will not work with vacuum. I'm waiting for return calls from SMC and Clippard. I cannot find a solenoid that meets both the size and force requirements. The peizo actuators look interesting, but the largest stroke I've seem is 300 micro meters (.01"). Finally, the gearbox, or leadscrews I think would be too big for the size constraints.

Let me give a little more detail on this project:

My company makes tools (mostly manual/hand-held tools) for handling semiconductor wafers. We make a couple different automated machines, but what we're known for is the hand-held tools.

A company that makes SCARA robots needs a special end-effector for their machine and they are talking to us about making this end-effector for them. Basically they want to take one of our hand tools and automate it with some sort of actuator. They have very little room for an actuator and only a vacuum line or electrical. The electrical is 12 volts and comes from a 26 AWG ribbon cable. I don't have specifics on the power supply or cable, but let's say 2 amps are available and the cable can handle that ampacity.

With regard to the ambiguity of the spec, I am trying to determine whether or not it is feasible for our company to design this tool. Therefore, I'm trying to find out if there are a few possible actuators that would fit within the loose specs I've given. So here's a little refinement of the spec: I know the max height is .25", the max width is 1" (I have a wider cross-section than originally thought), and the max length is 3" (the smaller the better). A stroke range of .15" to .38" (the more the better). The force should range from 1.5 to 2 pound-force (again the more the better).

So here's a spec summary:

Required force: 1.5-2 pound-force
Max Cross section: .25" square (or diameter of .25")
Max length: 3"
Stroke range: .15"-.38"
Response time (approximation only): Actuation/release time is less the .5 sec, (will remain actuated for .25 to .5 sec), idle time 2 sec.
Vacuum: 25 in-hg
Electrical: 12V/2A

Agian, thanks for the help so far!!!

 
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How many cycles must it last?

What is the flow rate/pressure capability of the vacuum line?

I disagree with your conclusion on the electric motor idea, I don't think you have checked out what is available in toys.

So far as pneumatics go I doubt that a conventional ram will have the right friction characteristic. I'd look at a multi piston design using very light sealing. Or a syringe.

Cheers

Greg Locock
 
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Seems to me the robot people could plumb an air line or separate electrical down the arm external like welding robots.
If your end-effector is hand operated, you should have more room than described. Can the actuator be connected similar to a hand and of a similar size?
I don’t think I would run 2 amps of 12V through #26 wire.
 
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jjellin :

Assuming 0.25" X 1" X 3" actuator size I believe it can be done with SMA.
- Actuation time of 0.5 sec can be achieved.
- Release time for less than 0.5 sec can be achieved too using forced cooling or even without it.
- The stroke range of .15" can be achieved too, 0.38" probably.
- The force requirements too can be met.

However, you didn't specified how many actuations thE actuator has to survive what is the price target?
 
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Cycle life is a good question. I would like to have a cycle life in the millions, but I may be able to lower that, if I can make the mechanism cheap enough to sell it as a consumable/replaceble item. Ideally though we would have at least a 1 million cycle life.

I do not have info yet on flow rate of the vacuum line. I have asked if they would plumb the existing line to house air, but they said they cannot (or will not).

I will have to check out toy motors, do you have a recommendation on a source.

Although our tools can be hand-operated, the customer wants two of them mounted at a 10 mm vertical distance. With other constraint it only leaves the size profile I mentioned for the lower tool. The upper tool has unlimited room above for mounting an actuator, but the lower tool has near zero room below and only about 6mm (.25") above. Our tool will be modified by removing the handle portion. It will essentially be a custom version of our hand tool, but customization of our standard tools is a large part of our business.

I got the 2 amps from a spec on 26 awg ribbion cable that rates ampacity to 3 amps. This is probably peak ampacity, not constant ampacity. However, the 2 amps, will be a peak, and cycled, not constant. Either way, I'm just trying to find some options for actuators, understanding that I may not get a perfect fit. I can then tell the customer; I have option a but I need x, or I have option b but I need y, or sorry we can't do it. In fact, it may not be feasible, as the customer has already had designs from internally and other companies that won't fit into their size profile.

My understanding of SMA's is to get a long cycle life (in the millions) the SMA can only be elongated by 4% of its length. For .15" I would need a 3.75" long SMA wire. I could probably squeeze that in, but I have other tradeoffs to consider such as cooling time. I've contacted a couple SMA actuator companies, but I was told by one that their SMA was not right for my application. I've just received in some Flexinol NiTi ( muscle wire that I'm going to be experimenting with. If I get good results I'll let you know.

Again thanks for all the suggestions,

Jason
 
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jjellin :

My previous response was based on Dynalloy Flexinol wires. If you look at their technical data you can see the cooling rates at still air. 5 to 6 wires in parallel of the 150 gram wire having two or three folds over guided wheels will give you the desired length for the desired stroke. This is where the 1" width will be used (to allow for the folds). However the diameter of the guided wheel should be at least 50 times larger than the wire diameter to avoid overstressing the wire.

It is tricky to work with those wires and especially with more than one in parallel. I do not know if you have used those in the past but you have to use them in a very specific way to get the forces and strokes repeatedly. I suggst that you study the matter throughly and know exactly what you are doing to avoid frustration. Or seek the help of someone who used have experience with those wires. Or the Dynalloy company if they supply such a service.

 
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The melting/expanding wax "power pill" in automobile cooling system thermostats (Robertshaw design?) seems to meet your requirements, including MTBF, except the 500ms operate time. You could supply the thermal energy through a nickel chrome ribbon wire with your 12V 2A (24 Watts), but higher wattage would obviously give better operate times.

You can stack piezo actuators to get more stroke than .01".
 
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