Seeking a simple binary linear actuator

My workgroup is interfacing some equipment on an optical bench. We have three small elements (lenses and filters) that need to be moved, on
command from a computer, in or out of the optical path. The elements are on ball-bearing linear slides, movement is horizontal, and it only takes a few ounces of force to move them. All the off-the-shelf actuators that I have found so far are very pricy, and overkill for what we need. Our requirements are minimal:
a) IN position can be set by a hard stop, and must be repeatable to 0.02". OUT position is not critical, as long as it is about 3" away from IN.
b) No need for proportional control or stopping at intermediate positions between IN and OUT.
c) Transit time is not important: 10 seconds or less.
d) Ideally, each actuator would be controlled by one or two TTL control lines from our interface.
After some googling and brainstorming, I can think of a couple of designs. A motor and leadscrew is the obvious starting point. A small DC motor could be driven in both directions by an H-bridge, plus a couple of transistors or logic gates to handle the polarity switching, limit switches to stop the motor at each end, maybe a flip-flop to keep track of the current state.
Or I could use a stepper instead of a DC motor. HSI has small steppers with integral leadscrews:
<http://www.hsimotors.com/linear-actuators/26000-Ncap.htm
I found a nice stepper driver chip (Allegro 3967):
<http://www.allegromicro.com/sf/3967/>
It needs only two TTL control lines: one selects direction, the other counts pulses. Perhaps with a stepper, I would not need the limit switches.
Other solutions were considered. Solenoids might be too violent, pneumatic gizmos too complicated. An R/C hobby servo uses proportional control, but it could be rewired as a bang-bang. I even thought about scavenging the tray actuator from a compact disk drive.
Any other ideas? Thanks in advance.
--
Julian Vrieslander

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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
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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.
--
Julian Vrieslander

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Julian Vrieslander wrote:

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
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<snip>

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.
http://www.terrypin.dial.pipex.com/Images/ServoUpDown.gif
The 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.
http://www.terrypin.dial.pipex.com/Images/AutoMotorExtremes.gif
That 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.
--
Terry Pinnell
Hobbyist, West Sussex, UK
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wrote:

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
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snipped-for-privacy@ieee.org (Bill Sloman) writes:

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: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html
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(Bill Sloman) writes:

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
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Maybe it's one of those companies where Purchasing gets to overrule Engineering --and they found such a bargain.
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snipped-for-privacy@ieee.org (Bill Sloman) wrote:

...
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.
--
Julian Vrieslander

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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.
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snipped-for-privacy@removethis.ted.ericsson.dk (Frithiof Andreas Jensen) wrote:

Or simply a freewheeling diode to dump the back EMF
Steve
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snipped-for-privacy@dea.spamcon.org (Steve) wrote in message

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
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On Tue, 27 Jan 2004 07:31:43 GMT, Julian Vrieslander

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.
http://www.geocities.com/zoomkat/switch.htm
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Car door actuators - about as cheap and nasty as it gets ;-)
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wrote:

I found something even cheaper and nastier: a motorized linear potentiometer.
<http://www.allelectronics.com/cgi-bin/category.cgi?category=search&itemMSP-10K&type=store>
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.
--
Julian Vrieslander

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On Wed, 11 Feb 2004 05:22:41 GMT, Julian Vrieslander

How about a cordless screwdriver turning a threaded rod with nuts on it, which push/pull some object.
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shb*NO*SPAM*@comporium.net (Si Ballenger) wrote:

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.
--
Julian Vrieslander

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snipped-for-privacy@photonicsourcing.com (Marc) wrote in message

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
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