Seeking a simple binary linear actuator

A solenoid with a 3" throw might be a bit too slam-bam violent. The actuators will be used to move components on an optical bench. There is a biological prep with microelectrodes nearby. Impacts and vibrations could cause problems.

And 3" is a lot for a solenoid. Don't recall ever seeing one with such a long pull.

Digressing, I recall wondering once (when experimeting with solenoids), whether there it would be possible to create some waveform that would pull in a solonoid plunger 'gently' but reliably?

I read in sci.electronics.design that Detector195 wrote (in ) about 'Seeking a simple binary linear actuator', on Thu, 12 Feb 2004:

They don't *have* to move abruptly; you can feed them with a current ramp. But in most applications, you want high efficiency and can tolerate the 'bang-bang' operation that gives it.

I read in sci.electronics.design that Terry Pinnell wrote (in ) about 'Seeking a simple binary linear actuator', on Fri, 13 Feb 2004:

I just bought an electric nail-gun which has about 50 mm throw. It failed after about 50 nails; the thyristor has probably died. So now I have a replacement; I hope this one survives. I didn't have time to try to repair the original, and the thyristor is *riveted* to the PCB, without any need for tight contact with a heat sink!

Yes: if there is a return spring. A current ramp pulls the armature in at a controllable rate. Obtaining the right current waveform is, however, not straightforward. A starting pulse may be necessary to overcome 'stiction', and then the inductance increases as the armature moves into the coil, but it can be done. It's possible, clearly, to use a position sensor to provide feedback, and then you are into PID controller territory and you can make the thing do tricks.

The problem with solenoids is that there is a trade-off between force and travel. As mentioned in the other twig of this particular branch of the thread, the OP wants a three inch (76 mm)travel which is a lot for a solenoid - I looked into this for the pen drive in a 100mm pen-recorder some thirty years ago, and the force you could generate with a simple solenoid was pretty pathetic.

I rather fancied the linear stepping motor approach used in the Hewlett Packard X-Y recorder of the same period, but for our application a bit of stainless steel wire and a capstan driven by an Escap DC motor was a lot cheaper.

------- Bill Sloman, Nijmegen

Ah I see, the travel is a bit too much for a solenoid, hence the need to examine all these other options.

Well I wont throw my ideas in, I'm sure there are enough already in this thread!

Hugh

I can't find the site now, but the other day I did a google search and found a simple servo controller made from a 555 chip that controlled a servo in two positions. The servo position was controlled via the value of a resistor. A low resistor held the servo in one position, and a higher value moved it to another position. I can't remember if the design flip/floped between two resistors, or if a second resistor was put in parallel with the first to get the low resistance value. With a setup like this a $10 servo could be used to push/pull the device between two positions in a fairly gentle way. The below links have similar setups using a 555 chip. I haven't tried using the 555 chips, so don't know just how well they work controlling servos.

Circuits like those can be found all over, & they're not limited to just 2 positions. My personal favorite, & quite easy to build, used to be #4 at the 2nd source above, but I see it's not there anymore for some reason (found it with a google search for "servo drive, 4" at:

JM

I would guess that's the circuit I posted about early in this very thread!

Thats it! Guess I'm a victim of old timers disease. ;-) Got the link saved now. I think that would be the best fit if the sliding gizmo is fairly light.

were considered. Solenoids might be too violent,

I'm pretty lazy and cheap. Go to an automotive junkyard, pick up a old automotive heater and A/C case. They are covered with cheap vacuum acutators to control mode and temperature doors. If you find a unit with an digital controlled system (in production since the 80's...) that unit will have a solenoid pack attached to the case. Open the pack and use the individual solenoid valves to drive individual vacuum actuators. The solenoids use... you guessed it... 12v dc.

For a vacuum source pick up a cheap venturi device and hook it up to shop air compressor. For moving things, these HVAC vacuum acuators are pretty nice... they don't move too fast, are pretty flexible and if you push against a stop, no big deal. You don't get current draw issues, etc... they will move a considerable force. I've used them numerous times in projects (of course, it helps to work at an automotive company, where you can walk up to the shop floor and get these parts out of the parts bin...)

LB Detroit.