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Author Topic: Lifting and dropping a weight twice per second (DIY tapping machine)  (Read 1930 times)
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Austin, TX
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I'm looking into using Arduino as part of a low cost tapping machine for impact noise tests.  The machine has a pretty simple job, it lifts little hammers up and drops them on the floor underneath at regular intervals.  While the machine taps the floor, someone in the unit below measures the resulting sound, which determines the impact isolation class (IIC) of the floor/ceiling structure.

All of the examples I've seen of tapping machines operate completely mechanically, with a series of cams that lift the hammers and drop them at the required speed.  This requires a lot of precision machined parts, which causes these devices to cost thousands of dollars.  This is typical in acoustics, where machines are all very expensive with not much effort is put into cost savings.

Here are the basic rules to the tapping machine.
  • 5 hammers in a line that extends 400 mm between the two outer hammers
  • Each hammer is 0.5 kg and drops 40 mm to create the floor impact.  The drop should be "free fall."
  • The machine must drop a hammer every 100 ms, so each individual hammer must complete 2 cycles per second

My thinking is that, by controlling the motion of the hammers electronically, the precise timing of the tapping machine's operation can be handled by the Arduino, with no need for any custom machined parts aside from the hammers themselves.  Having a micro-controller in charge of the device would also allow me to easily include some convenience features, like wireless control.

My first thought on a method to pick up each hammer and drop it is to use a solenoid.  The solenoid plunger could be connected to the hammer and included in the 0.5 kg.  Energizing the solenoid would pick the weight up, de-energizing it would drop the weight freely.  The trick is finding a solenoid that can lift 0.5kg the required 40 mm reliably without an impact that could cause the machine to move.  Any thoughts on this idea?

Any other ideas for ways to handle a weight in this way?
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Manchester (England England)
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I would say eccentric cams are the simplest and cheapest way to do this. A solenoid would have to be very big to move such a weight through such a distance. Also the release time of a solenoid is probably not going to be fast enough to allow you true free fall at the rate you are looking for.
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Eccentric cams will not work in this application. You need the hammers to drop, not just come back down.

Unfortunately, a mechanical setup is probably the best for this application. You have a shaft with several cams that are a spiral - the cam starts out close to the shaft and rises to a step, the cam clears the pin on the side of the hammer and the hammer drops.

What is important is the weight of the hammer, and the distance it falls. Speed of rotation of the shaft only controls the rate of the taps, not the intensity. Does your college have a machine shop? Non of this has to be very complex and could be done in a couple of hours and would last forever.
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Manchester (England England)
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Eccentric cams will not work in this application.
The depends on how eccentric they are, sure clinically depressed cams won't cut it but true nut cases will.
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Austin, TX
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I have no access to a machine shop (except through normal commerce).  I was hoping to be able to build something like this with (mostly) off the shelf parts.

I would say eccentric cams are the simplest and cheapest way to do this. A solenoid would have to be very big to move such a weight through such a distance.

This one seems like it could do it.  Maximum stroke of 62.5 mm and over 20N of force at 40mm, if needed.  They're big, but not so big they couldn't be used in this application.  Other tapping machines are very heavy.

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Also the release time of a solenoid is probably not going to be fast enough to allow you true free fall at the rate you are looking for.

This is the part I guess I need help with.  Can the plunger not move freely through a de-energized solenoid?
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This is the part I guess I need help with.  Can the plunger not move freely through a de-energized solenoid?

I found the answer to my question.  Of course the solenoid is an energized inductor, which takes time to de-energize when you flip the switch.

I think I also found the solution to doing this electro-mechanically.  The solenoid can be controlled by a solenoid driver.  The driver activates the solenoid with full voltage/current but after a certain amount of time (determined by attached capacitor) it switches to PWM mode (duty cycle of which determined by attached resistor).  The PWM mode can be set to put just enough current through the solenoid to keep the weight up, so that when the signal comes to release, the release time is kept to a minimum.

To account for the slightly sluggish start, an appropriate amount of height can be added, which will probably only be a few millimeters.  What matters is that the hammer hits the ground with speed as if it had been in free fall for 40 mm.  If its first few mm of travel were under an acceleration slightly less than g, then a few extra mm of distance can compensate for that.
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Well if you can calibrate that then it would give you a good solution.  smiley
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How big a solenoid and what kind of currents will you need to work with? That might be where the real problems lie.
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How big a solenoid and what kind of currents will you need to work with? That might be where the real problems lie.
Indeed.  The solenoid I linked to would work with 80 Watts in (voltage anywhere from 35 to 340), which isn't too awful, but getting that from an off the shelf solenoid drive IC is probably asking for trouble.

Perhaps I can use the arduino to drive a MOSFET directly and control PWM duty cycle in  the code.  Any reasons this wouldn't work?
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Dubuque, Iowa, USA
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Put a magnetic head on the solenoid to lift the hammer. Have the hammer restricted to only lift 40mm. Your solenoid has a stroke of 62mm, but after 40mm of lift the magnet will detach and the hammer will freefall.
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This requires a lot of precision machined parts, which causes these devices to cost thousands of dollars.

Way back in the past drop hammer equipment were actually made from wood and powered by water wheels and such.

http://en.wikipedia.org/wiki/Trip_hammer
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Put a magnetic head on the solenoid to lift the hammer. Have the hammer restricted to only lift 40mm. Your solenoid has a stroke of 62mm, but after 40mm of lift the magnet will detach and the hammer will freefall.

That's not the worst idea I ever heard.

The problem that comes to mind is dropping the solenoid plunger back down to pick up the head again would cause a secondary impact that might screw up the test results.
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That's not the worst idea I ever heard.
Geez, tough crowd! smiley-grin
The problem that comes to mind is dropping the solenoid plunger back down to pick up the head again would cause a secondary impact that might screw up the test results.
You can set the solenoid to drop so that it doesn't drop onto the hammer but rather stop just a little short so the magnetic attraction lifts the hammer. Or simply put a piece of felt on top of the hammer to deaden the click.

The way I see this built is that you'd make a 2kg cylinder from cold roll steel and have it sliding up and down in a piece of vertically-oriented PVC. Sleeve that pipe with a larger diameter PVC -- I'm pretty sure you can find plumbing parts that allow an easy, adjustable length junction between different sizes of PVC. That would allow you to finely adjust the height. Would make for a very small footprint, although you'd probably need to give it some kind of weighty base for stability which would more than likely include the battery.
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How big a solenoid and what kind of currents will you need to work with? That might be where the real problems lie.
Indeed.  The solenoid I linked to would work with 80 Watts in (voltage anywhere from 35 to 340), which isn't too awful, but getting that from an off the shelf solenoid drive IC is probably asking for trouble.

Perhaps I can use the arduino to drive a MOSFET directly and control PWM duty cycle in  the code.  Any reasons this wouldn't work?

That sounds entirely feasible to me. Of course, if your code leaves the solenoid at full power for too long, the solenoid will burn out. So you need reliable code and/or a protection mechanism.

You may find that with the usual diode connected across the solenoid, the solenoid doesn't switch off quickly enough. If so, you can speed up the current decay by using a second mosfet to switch the diode out of the circuit when you turn off the solenoid, together with a resistor to limit the back emf. Preferably, you should measure the inductance of the solenoid so that the speed at which the current decays can be calculated. If the current doesn't decay fast enough, the motion of the still-magnetised plunger leaving the solenoid will generate more current and there will be a braking effect.

I suggest you choose the minimum winding resistance from those on offer (i.e. 34v at 2.3A), then you can more easily find a mosfet with enough voltage headroom to allow a reasonable back emf.
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You can set the solenoid to drop so that it doesn't drop onto the hammer but rather stop just a little short so the magnetic attraction lifts the hammer. Or simply put a piece of felt on top of the hammer to deaden the click.
Hey, good idea.  That might be the way to go if I can't get a solution involving the plunger attached to the hammer to work.  Thanks!

You may find that with the usual diode connected across the solenoid, the solenoid doesn't switch off quickly enough. If so, you can speed up the current decay by using a second mosfet to switch the diode out of the circuit when you turn off the solenoid, together with a resistor to limit the back emf. Preferably, you should measure the inductance of the solenoid so that the speed at which the current decays can be calculated. If the current doesn't decay fast enough, the motion of the still-magnetised plunger leaving the solenoid will generate more current and there will be a braking effect.

This is good advice that would have taken me a long time to figure out on my own.  Thanks!

This device does not need to be battery powered.  I can plug it in, so that should help with the high current requirements.
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