Recommendations for ad-hoc fast movement test fixture - Page 1
Forum section: electronics / general

author: Poe
author_rank: Frequent Contributor
timestamp: January 28, 2025, 08:21:06 pm
content: I need to slap together a test fixture that can linearly pull a 15gram metal rod 7mm, then push it back.It needs to complete the entire cycle in 500uS!There's no significant friction or forces other than inertia to worry about.We only need to use it to test 800 assemblies, so it doesn't need to be pretty but we only have a week.Cost isn't really an issue since I'm not paying, but could only find long-travel and slow solutions.I was thinking about repurposing a nail/brad gun, but have no clue what acceleration they're capable of, or if they can even pull.Any other ideas? Preferably something that I can just buy without much testing or messing around.Unfortunately, we can't 'fake' the movement by trying to mess with electrical sensors (there are none) or something else that would be easier. Customer wants to see this rod moved that fast and their assembly confirm.

author: TimFox
author_rank: Super Contributor
timestamp: January 28, 2025, 08:31:59 pm
content: I haven’t done any engineering calculations, but a voice-coil actuator should be able to work that fast.You need to develop an equivalent electro-mechanical circuit including the voice coil’s electronic parameters along with the mechanical inertia and compliance.By voice coil, I mean a solenoid moving in a uniform magnetic field, as in a normal loudspeaker.Try Thorlabs. They make actuators with up to 25 mm travel.

author: ajb
author_rank: Super Contributor
timestamp: January 28, 2025, 09:42:37 pm
content: At that travel/time you'd need an average linear movement speed of at least 28m/s, or 100km/h, depending on acceleration rate and dwell times at each position. That's pretty fast!Do you need to do a single motion cycle at a time, or repeated motion?Thorlabs is a good suggestion -- they are friendly and helpful in my experience, so wouldn't hurt to ask them if they have anything that will do this off the shelf.A pneumatic solution like a brad gun will be limited by valve actuation time and air flow, and the time it takes to accelerate the driving piston. Since you need very little force, stiction will also be a problem. There are small air cylinders with special low-friction/stiction seals that would be a better bet -- or it might be possible to remove the seals from a standard cylinder and accept the air loss. For valves, your probably want something direct-acting -- most air cylinder control valves are piloted spool valves. With the smallest possible cylinders, short air path, and maybe some other tricks like overdriving the valve coils/tuning pulse times very carefully, it might be workable.

author: Poe
author_rank: Frequent Contributor
timestamp: January 28, 2025, 11:19:51 pm
content: Quote from: ajb on January 28, 2025, 09:42:37 pmGood question. It needs to only do one cycle at a time and hopefully hold the end/starting position against a mechanical stop.

author: Smokey
author_rank: Super Contributor
timestamp: January 29, 2025, 01:47:16 am
content: Um.... Is physics different in your part of the universe?Don't you need like 100kW to do that move?

author: Someone
author_rank: Super Contributor
timestamp: January 29, 2025, 02:19:05 am
content: Quote from: ajb on January 28, 2025, 09:42:37 pmCalculate the acceleration needed to move that part the specified distance. Force required is insane!

author: Someone
author_rank: Super Contributor
timestamp: January 29, 2025, 02:20:41 am
content: Quote from: Smokey on January 29, 2025, 01:47:16 amThere is a reason speakers aggressively reduce the weight of the moving mass! 2kHz at 7mm displacement is pretty wild.

author: coppercone2
author_rank: Super Contributor
timestamp: January 29, 2025, 11:35:48 am
content: I think your manager confused your project for a DIY towel rack. Its understandable because it is 'linear' in it and has a rail.I think they had a amazon.com stepper in mind.

author: Poe
author_rank: Frequent Contributor
timestamp: January 29, 2025, 05:34:31 pm
content: Quote from: Smokey on January 29, 2025, 01:47:16 amI don't think so. 100kW appears correct.

author: Poe
author_rank: Frequent Contributor
timestamp: January 29, 2025, 05:36:01 pm
content: Quote from: Someone on January 29, 2025, 02:19:05 amTo move a 15g mass 7mm in ~200uS?Online calculators say..Acceleration ~200,000 m/s^2 or ~23,000 g'sFor a 15g mass that would a require a force of ~350kg (~750lbs)Over 7mm in 250uS is ~100kWAbout ~20J of energy. About the same energy stored in a 1mF capacitor at 200V no?

author: Poe
author_rank: Frequent Contributor
timestamp: January 29, 2025, 05:37:26 pm
content: Quote from: coppercone2 on January 29, 2025, 11:35:48 amPossibly. Thank you for your input.

author: Poe
author_rank: Frequent Contributor
timestamp: January 30, 2025, 01:55:12 am
content: Bought a Harbor Freight brad nailer. How can they sell it for $14? Anyway, it's fast, but too slow and next to no force on the pull.Borrowed a friend's pneumatic chisel and it might actually be perfect except it's not fast enough the first dozen cycles and there's no easy way to make it only do one cycle.Coworker recommended a spring & hammer. My guess is something like a 30lb spring, 30g weight, over about a foot will do it, but that's only one half cycle. Maybe a stronger torsion spring with a crude cam/tappet, but I have no machining so I'm headed back to Harbor Freight to see if there's something close I can modify.

author: Marco
author_rank: Super Contributor
timestamp: January 30, 2025, 09:46:45 am
content: Quote from: TimFox on January 28, 2025, 08:31:59 pmBut which have a light and sturdy enough coil assembly? This is an electromagnetic captive bolt gun.

author: Marco
author_rank: Super Contributor
timestamp: January 30, 2025, 10:05:39 am
content: Quote from: Poe on January 30, 2025, 01:55:12 amIf it works with a solenoid, you can take them apart and bodge stuff together with a bit of welding. When one solenoid pulls in, the other pulls out. Then just drive them harder.

author: Nominal Animal
author_rank: Super Contributor
timestamp: January 30, 2025, 10:12:38 am
content: At constant acceleration followed by constant deceleration, with 100µs dwell time, you have t = 0.0001s = 100µs from standstill to travel x = 0.0035m = 3.5mm. The acceleration needed is a, x = a·t²/2, i.e. a = 2x/t² = 0.007m/0.00000001s² = 700000 m/s² ≃ 71356·9.81 m/s².The accelerating force is constant, and only changes direction. For 15g, thus assuming no extra weight, the force is 0.015 kg · 700000 m/s² = 10500 kgm/s² = 10500 N = 10.5 kN. It's quite a lot of force, similar to that of a 100 kg/200 pound weight in standard gravity.At the fastest point, midway through the motion, the velocity is at = 700000 m/s² · 0.0001s = 70 m/s ≃ 250 km/h (or about 157 MPH).(With zero dwell time, a = 448000 m/s² ≃ 45668 G, the force is a third less at 6720 N, and the maximum velocity 56 m/s ≃ 200 km/h or 125 MPH.)Consider reciprocating movement with drive pin at radius 3.5mm = 0.0035m. It would, of course, be continuously repeated movement, and for 500µs cycle time it would need to cycle 2000 times per second. That's 120000 RPM, pretty much outside the realm of possibility for a piston mechanism.Consider a large flywheel with a straight groove on the outer perimeter, doing a suitable 7mm jog, smoothed-out _/\_ curve, and for the rod, a carrier with a drive pin. This converts the rotary motion to linear motion, parallel to the axis of rotation. If we assume the jog is some 0.030m = 30mm long (along the circumference), then the surface speed of the flywheel needs to be 0.030m / 0.0005 s = 60 m/s. At a diameter of about 47.7cm = 0.477m the circumference is about 1.5m, so the flywheel needs to turn forty times per second, or at 60·40 = 2400 RPM. The motion will also repeat forty times each second. The axial forces on the flywheel will be quite large, too, so you'll need good bearings, similar to lathes. Hmm.. lathes... Yes, doing the setup on an industrial lathe might actually work.If non-repeated measurements are needed, then a cross-shaped setup, with the drive being a long beam you accelerate to suitable final speed, with a groove for a drive pin or some kind of drive dogs to move the rod carriage in the perpendicular direction. Why perpendicular? Because you need movement in opposite directions, and there just isn't time for any kind of reciprocating movement. To ensure good transfer of energy, the slopes need to be gentle, and thus the speed of the drive beam should be greater than the speed of the rod. The drive beam could be actuated by some kind of a shooting mechanism, perhaps an explosive, or two high-speed rollers coming into contact.Note: I didn't check any of the math above; there could be typos in there.

author: tszaboo
author_rank: Super Contributor
timestamp: January 30, 2025, 10:18:29 am
content: So, you convert the linear motion into a circular motion, give it a gearbox or maybe a belt ratio. And then run it only for a short time, because obviously it is going to shake itself apart. And when they ask you why didn't you only do one motion, you just shrug.I wouldn't even try it with a motor, just a hand wheel. Make them sweat for the impossible requirements.

author: Smokey
author_rank: Super Contributor
timestamp: January 30, 2025, 10:33:11 am
content: what is the length and diameter of this "metal rod"?

author: Analog Kid
author_rank: Super Contributor
timestamp: January 30, 2025, 11:33:07 pm
content: From what the OP wrote:QuoteI'm assuming their test is a one-time affair, in and out, not continuous motion. (It would be nice if they could confirm or deny this.)

author: Poe
author_rank: Frequent Contributor
timestamp: January 31, 2025, 01:00:17 am
content: Unfortunately THORLABS didn't have anything and the speaker coil would require way too much power.Quote from: Smokey on January 30, 2025, 10:33:11 amI haven't seen it yet, but assuming it's quite short and strong since this is supposed to be how it operates in the field.Quote from: tszaboo on January 30, 2025, 10:18:29 amI think we need to minimize the mass as much as possible.Quote from: Analog Kid on January 30, 2025, 11:33:07 pmSorry. Yes, just one cycle at a time. We'll put the assembly in, press go, one cycle, then spend a few minutes loading another assembly.Quote from: Nominal Animal on January 30, 2025, 10:12:38 amI think this is where we are going. Since it only needs one cycle, we're discussing just placing a flywheel on an a large stepper motor we have. 300 degrees of rotation get the flywheel up to speed, then the last 60 degrees smash it open/closed. Then we have 300 degrees to slow it back down..........Ya I don't like it either, but it's what we have so far.Thank you everyone for the input.

author: Kasper
author_rank: Frequent Contributor
timestamp: January 31, 2025, 04:09:41 am
content: Quote from: Marco on January 30, 2025, 09:46:45 am2 pipes with different diameter could help. 6" speaker into a 6" pipe. 6" pipe connected to smaller pipe. DUT (device under test) sits in smaller pipe, as a piston. Bolts through pipe on either end of device create stops to limit the travel.I only looked at hydraulics briefly, about 10 years ago so I may be way off with this. In a perfect world, a 6" speaker moving 1mm could move the DUT 9mm in a 2" pipe.Piston leak, piston friction, and air compression will of course weaken this.Use pipes that are as short as possible to limit the compression. You should be able to drive the speaker above rated power since it's pulse not constant. Could also add multiple speakers.

author: coppercone2
author_rank: Super Contributor
timestamp: January 31, 2025, 06:43:17 am
content: I thought the requirement is 15 grams.Time is running out for your deadlinehere is some inspirationhttps://www.tiktok.com/@its_certifiedjay/video/7209720937481014533

author: Poe
author_rank: Frequent Contributor
timestamp: January 31, 2025, 03:47:10 pm
content: Quote from: coppercone2 on January 31, 2025, 06:43:17 amThe thing that has to accelerate quickly is only 15grams.There's always unpaid overtime on the weekend....

author: Marco
author_rank: Super Contributor
timestamp: January 31, 2025, 04:04:38 pm
content: Google found something, looks expensive.https://www.moog.com/products/actuators-servoactuators/actuation-technologies/high-force-voice-coil-actuators.htmlAs I said, it's an electromagnetic captive bolt gun so that's how I imagined it would look.

author: Kasper
author_rank: Frequent Contributor
timestamp: January 31, 2025, 05:41:13 pm
content: Quote from: Poe on January 31, 2025, 03:47:10 pmWhat are the dimensions of it?