Idea for damping solenoid kick and slow down the punger movement - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: July 15, 2022, 05:14:14 am
content: Hi,I am making a simple eject system that pushed upward a small glass vial when the user needs to change its content. Said glass vial is contained in a vertical cylinder with a stopper at the bottom to avoid falling out, and open at the top so it can be ejected/extracted. The vial has clearance or 1-2mm to the cylinder around it.Due to the low cost and limited space available I would prefer to use a solenoid instead of a linear actuator (or version thereof such as those dc motors with a thread as shaft).But solenoids have quite a kick and to avoid the glass vial being broken and/or being "kicked out" of its container because too much force, I am thinking of adding some simple damping mechanism.The simplet thing I thought of is a o-ring coupled to the fixed frame surrounding the moving plunger and the plunger sliding inside it in such a way that it creates friction. As well as a rubber disk on top of the plunger to protect the glass when the plunger hits it.Has anybody done anything similar and/or have any better ideas?Thank you

author: Berni
author_rank: Super Contributor
timestamp: July 15, 2022, 05:26:20 am
content: The problem is that solenoids very a very non constant force when turned on.At the start of the stroke they tend to be the weakest and then get stronger as more of the iron comes inside, then in the last tiny bits of travel they become very strong as the two pieces of iron inside snap together.Friction will likely be difficult to get right. I would probably instead go for something like a spring damper. You can place a regular coil spring between the solenoid and the vial to take up and store the force of the solenoid, releasing it gradually as it pushes up. Then also place a piston between the spring and vial. The piston rides in the same cavity as the spring. As air leaks around the edges on the piston it slowly rises up under spring force and pushes the vial up.You can probably go by with using just a spring with no piston, but the vial will likely end up bouncing on the spring for some time after movement since there is nothing to damp out the energy.

author: ricko_uk
author_rank: Super Contributor
timestamp: July 15, 2022, 05:48:50 am
content: Thank you Berni,not sure I understand the setup/configuration using the piston. Could you please explain it a bit more.But using "just the spring", do you mean placing it inline with th plunger so that one end of the spring sits on the plunger's tip and the other end of the spring touches the vial's bottom? If so, because of the non-linear behaviour you explained, perhaps a non-linear spring could work even better. Would that be correct?Thank you

author: Berni
author_rank: Super Contributor
timestamp: July 15, 2022, 06:56:14 am
content: The idea stems from these door dampers that stop a door gently without a loud slam:https://www.dande.co.uk/index.php?route=product/product&product_id=185This is just a piston in a closed cylinder with a spring trapped inside. When a high speed door comes at it and hits the piston it gets pushed in, compressing air inside that pushes back against the piston to stop it. Once the air pressure has done its job is leaks out around the piston (that doesn't actually seal properly on purpose). After the door moves away the spring then pushes the piston back out all the way to ready it for another cycle (sucking the air back in slowly in the process)We can use the same mechanism to achieve smooth motion by placing a solenoid on the end of the spring for moving the piston into 2 positions.The benefit here is that friction is not used (hard to make consistent). The energy dissipated is trough the air leaking around the piston, so as long as you have a precise gap around the piston a precise amount of air will leak and you get a precise amount of resistance.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 15, 2022, 07:03:51 am
content: Slowing down / dampening the solonoid is possible, but needs quite some extra effort and it adds to the size.A spring can soften the force peak, but would still add oscillations. It would be more like viscous friction to slow down the movement.To limit the work done, it may be a good idea to have the iron core of the solonoid as a kind of counterbalance to the vial. A rather soft spring may be also work, but also need space.Anyway a solonoid only works once per cycle and thus is rather large for a given power level. Slower action also reduces the power for a given solonoid.Small DC motors can be relatively cheap. So it is not clear that the solinoid solution with damping is cheaper or smaller. The weight and needed way of travel are important parameters.If low speed and power consumption is not an issue, there may be a possible thermal actuator as an option.

author: themadhippy
author_rank: Super Contributor
timestamp: July 15, 2022, 07:41:22 am
content: 2 bits of string and a spring,solenoid fires pulling the string to open the vial,another bit of string with a spring attached pulls the vial shut when solenoid is turned off

author: CaptDon
author_rank: Super Contributor
timestamp: July 15, 2022, 01:13:59 pm
content: I have seen this done with an air cylinder as a damper. The cylinder was a small one made by Bimba. The air inlet/outlet had a threaded plug with a small hole. The plug was on the end where the piston rested so when the solenoid activated it created a 'vacuum with a leak' behind the piston which tended to not induce the oscillations that would occur if the piston was building pressure. Of course there was a return spring involved and a long cycle time until the piston came back to the resting position. I saw the same physical configuration used in a tape transport on the pinch roller actuator that brought the roller against the capstan shaft without a 'slam'. That cylinder was made of glass. It would also be worth noting that in both cases the solenoid plunger was arranged with limited travel so it never was fully against the backstop of the solenoid while energized.

author: beanflying
author_rank: Super Contributor
timestamp: July 15, 2022, 01:19:41 pm
content: Have a watch of TOT for some ideas. Rather than use the Solenoid to eject it use it to reset the mechanism and soft lift with a spring/damper.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 15, 2022, 01:26:43 pm
content: Search: "micro mini linear actuator"There are some about US$30 12V DC with speed related to load.Search: "voice coil motor /actuator"These are not the audio speaker parts, they are industrial actuators with force proportional to current.Probably more costly being industrial items with prices not listed.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: July 15, 2022, 01:51:17 pm
content: For soft eject and cheap, I was thinking on the line of a model makers pneumatic ram/arm - even Lego make them. Question, how does the user reset the mechanism once filled? Otherwise, what about a soft spring loaded carrier that is held in place by a solenoid operated trigger?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 15, 2022, 04:32:39 pm
content: People have used these for years. No need to reinvent the wheel.https://www.airpot.com/product-category/applications/damp-motion/You might also do it using a true damping grease. All you need is a close fit piston and the right grease. Again, been done for years.https://www.nyelubricants.com/damping

author: ajb
author_rank: Super Contributor
timestamp: July 15, 2022, 05:41:03 pm
content: What about a small motor with a cam? You can adjust the cam profile to determine the force curve through the motion (providing more mechanical advantage at the start of the eject cycle, for example), and if the forces are low enough, you can probably get away with a low power motor that can simply stall against end stops. Add a current sense resistor and you could detect the current increase when it hits the end stop and then PWM the motor current down to a lower 'hold' value--or just turn the motor off, especially if the cam profile is such that it can't be backdriven by someone pushing the vial down while the cam is up. Feedback, if you need it, could be a single switch or optointerruptor that is actuated at both ends of the range of motion with a little bit of logic to identify different failure conditions.

author: beanflying
author_rank: Super Contributor
timestamp: July 16, 2022, 02:07:21 am
content: And because it was late on a Friday night forgot the obvious one a small R/C Servoneeds a 1-2mS pulse train for travel adjustment and you can control the transition of that rate change to control speed. If you just want to bang it left and right a 556 Timer with two trim pots to set the limits. A lot slower transition even then compared to a solenoid.Cost from $3 but for not a lot more you can get metal gears and a ball bearing output shafthttps://hobbyking.com/en_us/corona-cs238mg-metal-gear-servo-4-6kg-0-14sec-22g.html?queryID=0844e27ce3d2a5cb9b99e93c0917d04a&objectID=35613&indexName=hbk_live_products_analyticsTiny end of the market but it might be ok depending on your loadeBay auction: #144481730740The linear motion might make your linkages easier.

author: Psi
author_rank: Super Contributor
timestamp: July 16, 2022, 02:16:38 am
content: Some sort of rubber + PWM on the solenoid is probably how I would solve it.If you want to avoid needing a MCU for the PWM you could use an adjustable voltage regulator with a capacitor added to its Vset resistors so the output voltage ramps from 0 up to Vout over time at turn on.

author: David Hess
author_rank: Super Contributor
timestamp: July 16, 2022, 03:12:52 am
content: This is an electronics forum. Add a shorted turn to the solenoid. Make it a nice thick one, and then add some more. This will limit the rate of change of the magnetic field.

author: ricko_uk
author_rank: Super Contributor
timestamp: July 23, 2022, 02:03:50 am
content: Thank you all for the many diffferent end interesting solutions!I have ordered most of them and will compare the performance, reliability, size with any additional required coupling parts etc.@David, that seems an interesting solution but not sure I understand exactly. Do you mean just winding a second solenoid around the first one?Thank you

author: David Hess
author_rank: Super Contributor
timestamp: July 23, 2022, 02:12:25 am
content: Quote from: ricko_uk on July 23, 2022, 02:03:50 amIt means exactly that; wind a thick single turn short around the existing solenoid. This "shorts out" the dynamic magnetic field limiting the rate of change, which will limit the rate of change in force on the armature.When the magnetic field is increasing or decreasing, it induces a voltage in the shorted turn, which induces the opposite magnetic field cancelling the magnetic field from the powered solenoid winding. Since the shorted turn has some resistance, the cancellation is not perfect and the field slowly increases or decreases. If the shorted turn was a superconductor, then it could block the field completely.I do not really know from experience that it will work well enough for your application, but I have sometimes seen solenoids with a thick copper shorted turn presumably to do exactly what you want, and it is easy enough to test.Transformers sometimes use a shorted turn going through part of the core to act as sort of a "magnetic resistor" to balance flux, like putting a resistor in series with the emitter or source of a transistor to balance current. Shaded pole motors do the same thing to induce a phase shift in the magnetic field driving the rotor.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 23, 2022, 06:50:51 am
content: I like stepper motors, and have a couple of the mini variety with screw shafts. The motor is 6mm or 8mm in diameter (size of the tip of your little finger, or smaller), and the shaft is typically 8mm long, although you can find ones with up to 40mm long. You can find these on fleaBay ("mini stepper motor", "stepper motor linear") for around $1 apiece, and drive them using a H-bridge or even an ULN2003A at 5V. I understand they're extremely common in cameras with automatic focus or physical zoom. The ones used in CD and DVD drives have much longer strokes and a sparser thread.The annoying part is that the thread is usually not well specified, so it may be difficult to make a new "block" to ride on the thread; so better look for one that comes with a nut on the thread.The ones with a planetary gearbox (giving more torque) do cost in the $3-$5 range apiece, but do often have (sintered?) metal parts and a support bar to keep the nut from rotating. Depending on the weight of your vial, I'd look into one of these. I suspect having the vial lift be occasionally turned into a vial launcher is undesirable...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2022, 07:07:02 am
content: FYI, I did some playing around recently, albeit with a much smaller armature (small solenoid valve) operating in ~5ms. By modulating applied voltage (as PWM into a transistor, with catch diode to circulate current between pulses), I was not able to affect the bulk of the swing -- the force just goes from "barely moving" to "train crash imminent" in too little time to control. In addition, operation was too unreliable to do by open-loop control, anyway: due to friction and wobbly motion and whatnot I assume, the timing varies +/- several ms from cycle to cycle.An active circuit may be feasible; the armature motion is sensible as the coil current dropping momentarily. A linear circuit (or at least a moderately fast PWM (>20kHz say) -- with a wide output voltage range, including negative*) can servo on this, significantly reducing coil current once the drop has been detected, and then perhaps rebounding slightly over time as it settles down into whatever holding current should be.*Normally, coil voltage is only allowed to reverse by Vf or so (clamp diode). If you use a zener in series with the diode, this can be raised, albeit at a significant hit to efficiency -- unimportant for infrequent, momentary operation, but significant if you're using, for example, current reduction in the holding state. Maybe in that case, the zener can be bypassed once the pull-in cycle has completed.So, a mechanical solution is definitely the better option.Regarding a shorting ring, it might help, but keep in mind the magnetic field isn't following the armature like in an induction motor, it redistributes through the material fairly quickly; the point of course is to get some of that induction behavior, but it may need more than one ring. As a fairly extreme example, consider an armature completely plated in copper (pretty heavy copper at that, like 20+ oz): it doesn't matter from which direction the field is coming, it has to go through the copper "armor", which takes time (field strength and direction literally soak in, diffusing through the layer). This roughly limits velocity proportional to applied current, I think.And the armature being something like solid steel, does help a bit -- the skin depth is quite shallow, so it may take some 10s of ms for field to reach the center of the armature -- but there's still plenty of force available just by magnetizing a fairly shallow surface, to get it going dangerously fast. Obviously, this is the baseline condition (more or less), so, whatever it's doing, it's not enough and something more is needed.And keep in mind, a solid layer of copper cladding would increase the distance to the armature core, increasing reluctance, reducing the maximum holding force. The copper acts like air gap at DC*. I suppose ideally, there would be rings on the armature, evenly spaced along its length (ribbed for her pleasure?!...), so that field can only flow down it, to any given position, at the combined time constant of all those shunts.*Technically, a little worse (diamagnetic), but not by nearly enough to care.All to say, I don't think a single ring is enough, since whatever core is ahead and behind that ring can still be freely magnetized -- but restricting it section by section, with a stack of rings, evenly spaced, ought to do the trick.There's also magnetic field change in the stator, as the incoming armature changes the intensity and angle of the field in it; so, similarly, rings there will help. That may be harder to arrange, though.In combination with some mechanical abatement (rubber bumpers, dashpot?*), there should be a good combination in here.*Most solenoids, as I recall, have a modest clearance fit for the stator around the armature (air gap), and have a hole in the end to relieve gas pressure. Perhaps tightening these tolerances, closing the restriction (so it acts somewhat as a gas spring -- pushes back harder as the armature comes in tighter -- perfectly opposing the rapid increase in force as it closes, perhaps?), but not completely, so that most of the air squeezes out just as it begins to rebound. If it can be sealed, it might also be filled with (thickened?) oil, to get more shear between armature and stator wall, and better control the nozzle restriction. (Dynamics will also be different with relatively incompressible fluids; analogous to, instead of an optimally dimensioned RLC snubber, just throw on a big fuckin' electrolytic and let its ESR dominate the discharge rate.)Edit: That said, it is true that reluctance of the total magnetic path is falling, thus its field strength increasing, so a shorting ring can be applied anywhere -- but be careful of where this is. It has to make a shorted turn around the core. Usually, the stator is a serif-U-shaped steel frame, with the serifs pinched in around the armature, where it enters (and air gap transfers field to it). The path is closed when the armature seats fully inside the 'U', making a sideways figure-8 magnetic path, with the coil wound through the holes of that figure. Just like any E-core based transformer.But then, the coil is already in precisely that location -- any shorts added around the core, would be exactly equivalent to (i.e., in parallel with) the coil acting as a short, which, you usually apply a fixed voltage to, so, it is.The coil usually has a fairly modest electrical (L/R) time constant, so there is that; but any shorts you put around the core will be using less metal, how can they hope to have a longer time constant? Well, material can be thicker and denser at least, but it'll take a lot -- on the order of the skin depth at that frequency -- so, for actuation in the 10s of ms range, we're talking... really thick copper? Well, width counts for something as well, recall the demo of a magnet dropped down a not-terribly-thick copper pipe; not really sure. Well, probably the "full metal jacket" armature I imagined earlier, wouldn't work out too badly, but how much you can go down from there (in terms of ring width and spacing, as opposed to the "all width and no space" of a solid wrap), not sure.In any case, it's definitely not going to do anything, wrapping theoutsideof a solenoid with copper (outside the legs of the 'U'); just in case that idea came to mind. There should be fairly low external field from a well made solenoid. This is all about internal fields, you'll need to work pretty invasively, wrapping parts with solid copper or whatever, to do anything.Also a note on this post, I'm making no attempt to write a coherent structure; I'm just reasoning it out as I go, stream of consciousness. Expect earlier overly-simple statements to be refined by later insights. Or forgotten, as the case may be..Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 23, 2022, 07:24:36 am
content: There are lots of variants of quite small stepper motor based actuators on Ali / Ebay / China Express.Some examples:

author: mikerj
author_rank: Super Contributor
timestamp: July 23, 2022, 09:46:17 am
content: One of the little dampers used on self closing kitchen cupboard doors and drawers might be worth investigating. Very cheap and easily available.

author: Seekonk
author_rank: Super Contributor
timestamp: July 23, 2022, 01:39:38 pm
content: Cheap and expensive consumer electronics have done this for years with two sliding plates and dampening grease. no need to overthink this.

author: ajb
author_rank: Super Contributor
timestamp: July 27, 2022, 04:26:00 pm
content: Quote from: T3sl4co1l on July 23, 2022, 07:07:02 amDrifting a bit off topic, but to expand on this idea a little bit: if you're trying to servo the solenoid current, you want to dump the energy stored in the inductor as fast as possible (fast decay), so that your loop can reduce the drive current more quickly in response to the solenoid's motion. To do that, you want to allow the coil reverse voltage to be as large as possible--solenoid current is initially constant, so higher voltage -> higher power dissipation-> shorter current fall times. Oh the other hand, if you're modulating the steady-state current via PWM eg to reduce holding power, you want to keep that energy in the solenoid as long as possible (slow decay) because any energy dissipated during the PWM off-time will need to be replenished from the supply during the next on-time, which reduces efficiency. To do that, you want to minimize the coil reverse voltage to minimize power dissipation. Getting only fast decay or only slow decay is easy--it's a transistor and a diode either way, plus or minus a zener. Getting both is a little trickier.I've experimented with this in a recent solenoid driver design, albeit in my case the goal was toincreasethe mechanical speed of the system when the solenoid is released, while allowing efficient peak-and-hold drive--a rubber bumper on the end stop is all the soft landing we need. You can use a full bridge as a mixed-decay solenoid driver, and in fact some full bridge ICs have this functionality designed in, where they can switch between conventional transistor bridge and diode bridge operation. But unless you need to drive the solenoid in both directions you don't need a full bridge, with the associated drive complexity, and can use a simpler quasi-full bridge like so:When Q1B is on, Q1A can be PWMed, with current circulating through D11 for slow decay, and when the solenoid needs to be released both Q1A and Q1B are turned off, allowing the coil voltage to fully reverse to -Vmain via D8 and D11 for fast decay. All jellybean parts, and simple to control.Ultimately in my case the inertia of the mechanical system totally swamps any benefit faster decay has on the release speed--and I'm using a rotary solenoid, which is actually a very short stroke axial solenoid with an added ball bearing and sloped plate to convert linear motion to rotary, so ESPECIALLY unsuited to trying to servo the position due to the short solenoid stroke and added friction--but I ended up keeping the basic bridge topology for other reasons related to how the circuit is being controlled.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 27, 2022, 06:33:33 pm
content: Yes indeed, a two-switch converter configuration will do the job. Assuming ~equal up/down slew rates are acceptable. Believe that's been used for injector drivers -- fast operating, and often, so the power savings doesn't hurt either.If you want to know coil current between pulses (continuously), try a shunt resistor on low side diode, then a diffamp between shunts. (Assuming complementary drive so both sides are constantly switching together.)Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: July 30, 2022, 06:24:17 pm
content: Thank you all again!replies below.@Tim, @David and @ajb:David suggestion is a nice idea and Tim's and Ajb' detailed points and suggestions also quite relevant and interesting. Definitely worth trying. For the final product though not an option because it would make it difficult and expensive to do on large quantities.@NominalAnimal and @Doctoramus,I didn't know they were going that small. Some on order to see how they can fit in the mechanics and how I can best couple with custom a thread/nut. That might end up being too expensive solution (for the project cost constraints) having to make a custom nut coupler but maybe I can integrate some other mechanicals functionality required so might make it okAt the moment the simplest and preferred one because of cost and compactness might be the dampening grease. But when the remaining parts will come in I will also pay a bit more with those.Thank you all again, great suggestions, very much appreciated as always!!