Seeking a simple binary linear actuator

Use a spring to bias against the inner stop. Pull the carriage away from the stop with a piece of string attached to a servo (look at servos intended for sail control for model yachts). Make sure that the string is slack at the IN position. Control the servo with one of the servo drive units attached to a serial port (or parallel if you want).

Dave

I'd go for the simple servo approach too, as Dave recommends. That's essentially the approach I used for my electronic shed lock. Here's a circuit you can use for the servo.

SPST 'Up/Down' switch would of course be replaced by your PC signal.

Another method, one you're already considering I think, would be to use microswitches at either extreme position and a cheap, suitably geared DC motor, again with a spring tensioning the IN position. In that case you could adapt the circuit I've used for various projects, such as an automatic bedroom window controller.

uses a DPDT switch (C1,N1,T1 etc refers to 'Common, Normal Transferred), and a couple of normally closed SPST microswitches. You would again replace my manual on/off switch with a PC control signal, either directly or via a relay.

You need to remember that microswitches don't last forever.

In the U.K., my central heating system relied on a microswitch in its electrically controlled valves.

The cheap-skates who designed the valve assembly used microswitches rated for only 500 operations, which lasted for about a year. I eventually got sick of replacing them with the original parts, and found something that was good for 100,000 operations.

Optical forks don't wear out in the same way, but the LED emitters lose brightness over time, and have been known to fill up with dust.

I tend to stick a magnet on the moving bit and use a stationary digtal Hall effect sensor to detect when it gets within range.

------ Bill Sloman, Nijmegen

The designers must have looked long and hard to find a microswitch that only lasted 500 operations! :)

Yeah, maybe in 10 years of continuous operation! These are used ina all sorts of consumer and industrical equipment.

Magnets may loose strength over time. ;-)

--- sam | Sci.Electronics.Repair FAQ Home Page:

Repair | Main Table of Contents:

+Lasers | Sam's Laser FAQ: | Mirror Site Info: Important: The email address in this message header may no longer work. To contact me, please use the feedback form on the S.E.R FAQ Web sites.

Thanks for the suggestions. The spring and string combo is appealing for simplicity and ease of construction. I would need to make sure that the spring does not cause the slider to creep back if the servo motor is depowered.

I had already considered a sail winch servo. But (since we don't need proportional control) I'm not sure this offers any real advantage over a bang-bang controller with limit switches and a (non-servo) motor. Of course, the servo solves the spring creep issue - as long as it is powered on.

...

Yow. The switch in a cheap flashlight probably lasts longer than that.

For this project I am not too worried about longevity - the slides will be moved only a few times per week.

With a limit switch I am more worried about on/off oscillation at the flipover point. But a good microswitch should have a crisp overcenter transition. And I also would need to make sure that the limit switch at the IN position caused the slider to stop at the same spot (within our

0.020" tolerance). I could do this by using a spring and string (as suggested by Dave), or by spring-loading the nut on a lead screw. Then I could move the slider to a hard stop, overdriving the transport mechanism a bit before turning off the motor.

You could use a magnet instead of the spring. Position it so that the force pulls the moving part in at the approprate distance. When it is farther away the force will be negligible.

Mitch Berkson

WalMart did have a $10 Black&Decker 6v cordless screwdriver that might be useful. You could make a setup using two SPDT relays and limit switches to operate it (simple schematic below). If you need something strong the screwdriver could spin a threaded rod with nuts on it attached to the sliding device. For less strength, a plastic jar lid could be put on the end and be used as a pully to pull a string attached to the sliding device.

Having gone to some trouble to replace the first failed microswitch with an identical part, I was a bit pissed off when I realised how easy it was to find an interchangeable part with a better specification.

When Hewlett-Packard published their first application notes on LED life-times, back around 1975, I seem to remember being told that a LED run at its rated current could lose half its brightness in a year. Dust build-up depends on the environment.

Know any application notes that talk about mechanisms and rates of loss? I've yet to hear about any problems, but I've always used neodinium-iron and samarium-cobalt magnets which are difficult to demagnetise, short of heating them above the Curie point.

------ Bill Sloman, Nijmegen

Car door actuators - about as cheap and nasty as it gets ;-)

It *could be* that you need an RC network across the switch points to prevent arcing from destroying the switch - there are some standard "black boxes" for that purpose, in case it is mains voltage, safety, etc.

Or simply a freewheeling diode to dump the back EMF

Steve

In my application the switch was switching UK mains voltages, 230V AC, about 375V peak to peak, which is above the Paschen minimum for air, so no sort of catching diode would have been to be able to prevent an arc between the contacts.

In the original posters application, the voltages should be low enough that a catching diode could help, but most microswitch contacts are pretty robust.

------ Bill Sloman, Nijmegen

I found something even cheaper and nastier: a motorized linear potentiometer.

For $7, I could not resist. We have a couple on order.

We really don't need the potentiometer part of this thing - only the motor and slide mechanism. It looks like the motor does not have gear reduction. So I will need to see if it has enough torque to pull a couple of ounces of (linear) friction force, and without slamming our optical components too hard. Maybe a PWM drive signal will help with that.

Dear sir, We can quickly find the actuator you need at the best price on the market.

This is how we work at PhotonicSourcing (our services are FREE for a limited time).

Try us, you will be convince:

1) We will search for you across the 15,000 suppliers in PhotonicSourcing database, send the request for quotation to some potential suppliers and place it in your account on our website. 2) You will receive an email when someone is answering your request (quotation or asking you some questions). You will need to access your account on PhotonicSourcing. Feel free to answer their questions. All info you need to contact them will be available to you. 3) During this process, your organization and contact name will be available to the potential suppliers. Your email, tel and fax will NOT be visible unless you made it available.

Let me know if you want us to move forward.

Waiting for your reply,

Sincerely yours,

Marc Duquette

snipped-for-privacy@photonicsourcing.com

We source any photonics, opto-electronics, imaging or laser products.

How about a cordless screwdriver turning a threaded rod with nuts on it, which push/pull some object.

Someone else suggested that. I took a quick look at cheap cordless screwdrivers at Sears and Home Depot. But the ones that I saw were too big, too expensive, and/or the chucks were designed to accept hex-ended bits, etc. We could probably cobble up one of these things and make it work. But for the amount of work involved, it would probably be easier just to buy a naked motor with gear-reduction.

Maybe it's one of those companies where Purchasing gets to overrule Engineering

--and they found such a bargain.

I may have missed soemthing, but why isn't a simple solenoid suitable for this application?

There are only two positions (from what I can gather).

The solenoid of course has an "unpowered" position so when power is off it returns (via spring pressure) to its unactivated state. But I see no reason why this wouldnt do.

As for the 0.02" (approx 0.5mm) tolerance required for repeatability, this too should present no problem, the solenoid will always be in one of two extremes and these will always be identical no matter how many times its operated.

Unglamorous perhaps, but suitable, I think.

Hugh