Tension Springs and Measurements

Do it with rubber bands to start with, cheap, simple easy to adjust.
Alternatively, drive the motor with a long enough pulse to rotate it back round to the start position, this can be indexed with a microswitch or sensor arrangement, or just rely on momentum and a soft pad to catch it.

The problem with returning the cog is that, whtever plucks the string (plectrum?) it is likely to be hitting it twice, once on the pulse and once on the return.
I can encapsulate the best gameplan into one word...
Simplicity.

Is the motor really just a motor or is it a motor/gearbox? A motor gearbox may be better as it will be more controllable and repeatable and have more torque.

Good luck and keep us posted, it sounds like a fun project.
Del

I think some pictures may be of use at this juncture, probably the worst photo ever but should be just enough to give you an idea. I would like to stress that this is a very messy prototype BUT it works, the cog strikes the string cleanly, and bounces back. Occasionally it seems to strike the string twice, but i think this would be fixed with the correct spring.

So you're going for more of a hammer dulcimer type sound, rather than plucking?

I think Del's right about the 'lastic bands - experiment 'til you get one that works, then either measure the characteristics, or take/send it to a spring supplier/manufacturer and ask for "6 steel extension springs like this, please".

Then again, if you can get hold of a (broken!) big wind-upwrist watch or a pocket watch (broken beyond repair!), the mainspring could provide good source material for torsion spring experiments.

Another possibility is asking a spring manufacturer for a few samples.

Then again, RS Components do spring kits in various ranges.

Where are you located? Please register on CR4.

Hi guest! Interesting idea there.

Which direction is the motor rotating in? I would think to simulate a plucked or strummed guitar string you would have counter-clockwise rotation. Since the harmonics in a guitar string are typically formed when the string is released from a deflected state and allowed to return to its rest state. In this rotation the string would be released when the step on the cog is turned past the string.

If you are rotating clockwise, as the peg and spring seem to suggest, you would get more of a piano string type impact from your cog. and i would think depending on the spring reversing the cog it would be difficult to correctly pull the cog away to allow good vibration.

From reading your description a little more closely it does appear to be the latter action. I have very little experience with electronics but I would think the pulse of current as well as the motor could result in differing torque durations and strengths, have you considered a set up where the cog would be allowed to rotate without a spring attached, building inertia, and to have a "return" spring which would be strong enough to return the cog to position after it strikes the string?

I agree with Del that some experimentation with either rubber bands, or springs you already have on hand, followed by measurement of the force exterted by the "correct" number of rubber bands, could assist you in finding a starting range for purchasing a set of springs. As the website listed springs in N/mm units i would find a set of bands that emulates good performance and then estimate that sets "spring rate" by adding a known weight and measuring deflection. 1 Kg here on earth that stretched your "correct" set of bands 10 mm would result in a 0.98 N/mm spring rate, if you use Lb weights the calculation isn't as clean but it is the same concept.

Spring rate = Force/Deflection = (Mass*Acceleration)/Deflection

The example is easy because 1Kg here on earth exerts 9.8 Newtons due to gravity's 9.8 m/s^2 acceleration.

Hope this helps. -T

Part of the comment by JohnDG (post #3) made me think... solenoid.

Will return to neutral position every time. Will only strike string once per cycle. Can be held on string, if desired. Re-cycle time would (I believe) be faster than the spinning cog.

It would not give you the pluck type sound, tho. Strumming? Maybe.

Not sure if these handy little gadgets fit into your plan or not. Hope it is useful.

Best of luck with your project... looks like fun!

@Doorman. Solanoids where something that was considered early on, but they are quite a lot more expensive than dc motors. The other thing is that we have already constructed the instrument, it has 18 strings and 18 motors and we thought that it would work. Unfortunatley the plucking mechanism was far from reliable and i have been playing for the last two days with a bag of little springs to make it better. Basically all that needs to happen is after the motor moves it needs to snap back. All i need is a way of figuring out how much force will be applied to the spring from the motor, surely there is a pure mathematical way of figuring this out?

surely there is a pure mathematical way of figuring this out?

Aaaaarrgghhh, Yes I'm sure there is if you measure all the variables, but in the real world, you will find that experimentation is sometimes often quicker and more reliable in problems of this scale..
If you really want to 'work it out' you will need a very small spring balance to measure the restoring force required*, you can also use it to measure the force generated by the motors.

Remember maths is just a way of describing reality, not the other way round.

* The resoring force will need to be sufficient to overcome friction but less than the force exerted by the motor.
Rubber bands and a spring balance...trust me I'm a cat engineer.
Del

BTW, I'll bet you a tin of my finest Tuna that if you do it all by calculation it won't work

BTW2, Rubberbands are less likely to oscillate/ring/effect the pickups. Just what have you got against rubber bands for experimentation? Ok it may look a bit 'Blue Peter' but it works...ever tried bunjee jumping?

The torque from the motor can be used to determine the force exerted on the point where the spring is attached.

Force= Torque/Moment arm

Force is typically in Newton-Meters (N-m) but your original post, it is yours correct?, specs the motor torque as 10.9 g/cm. I am unfamiliar with this way of speccing a motor, but i would think it means with a 1 cm moment arm a 10.9 gram-force could be exerted.

If this is the case you need only adjust this force at 1 cm to whatever moment arm your spring is actually attached at. The problem arises that this is a tangential force, acting tangential to the circular motion of the cog, where the spring is fixed at one end and not always tangential to the cog, thereby complicating calculation of the actual force on the spring.

A way around this would be to find a torsion spring as JohnDG suggested, or just buckle down and work through the equations at specific points in the cog revolution to determine the deflection of the spring and the linear spring force, and then converting this vector into the solely tangential force exerted on the cog. An excel spreadsheet could easily be set up so that you could more easily iterate through different spring rates.

-T

Thanks al lot guys! @Del I am completely sure that calculations will be no replacement for experimentation but i thought as many angles as poss would be sensible.

I have a big bag of springs (unfortunately almost all of them are too tough for the job, actually as are most elastic bands). I did find one spring that almost works perfectly and i measured its strength (N/mm) with 100 grams of 1p and 2p coins to give me a bench mark.

I'll get it figured out today and let you know how it turns out!

Meewo.

Actually what i need is a spring that will exert as little force as possible so that the motor can move with as little inhibition as possible. However the spring must have a little pull to take the motor back to its starting position after the movement, which i suppose can be a very minimal exertion.

I was going to go for a spring with a resistance of 0.05 N/mm, as the spring i have experimented with had a resistance 0.1 and i was a bit too tough.

Ok, I think I've got the hang of what you're doing, and offer a suggested arrangement which I think would be easier to control.

Is the upper illustration roughly what you've got?

My suggested arrangement in the lower sketch gives a much more linear relationship between cam/hammer angle and spring extension, and a good mechanical advantage (you haven't got a huge amount of torque/force to play with). The stop is so that the spring can be under slight tension in the rest position, which should also make the torque vs angle relationship more linear. Also, the 60° rotation from rest to striking the string would be better defined.

How are you powering the motor? I assume it's a fixed duration pulse. Having the angle well defined should make the timing easier, and I think could also help eliminate your double-strike problem.

Good luck,
John.

This shows the (normalized) %age extension of the spring for the two arrangements:

Have you thought about using a simple pad type brake/clutch?

It will end the problem of double string strikes that Del brought up and allow the motor to move in either direction.

The felt disc will press against the pick wheel with the constant tension of the compression spring which is adjustable. The flat stock is used to prevent spinning of the brake disc. Of course, A piece of threaded rod with the end milled flat will work the same and allow the use of a lock nut as the spring stop.

Just something else to contemplate. Good luck with your project!

Have you thought about a flat sheet spring working on the cam which has a flat in its stop position?

When the cam turns the spring is deformed and generates a drive-back torque, when it comes on the flat it stops the motor in same position every time.

It is easy to manufacture, positions the cam and can be easily replaced.

You can use even plastics if torque has to be small and this can also offer the anti-friction at sliding place. In fact what you need is a drive back torque and a positioning of the cam at back stroke but you are not OBLIGED to use a helicoïdal spring which presents many inconvenient aspects.

If it is not clear then ask and I shall make a sketch.

It is definitely not clear! :P

When the hammer turns (to dotted figure) the flat spring is bend and applies a return torque F x L on the shaft.

When the hammer is again in its "0" position it is maintained by the spring itself.

The hammer comes on its right side to the contact with the string.

It is only a principle sketch so that you understand how it works.

Thanks all for your splendid helps! i have now sourced some perfect springs and pretty much finalised the cog mechanism design. I will upload a photo very soon,

I have, however a new question!!! (yay)

What are those little plastic cogs made from? You know the little hard white things that you would put on a little dc motor (like the one in the photo in my second post).

I think it is nylon, But i don't know what type, if it is nylon. At any rate i need a material suitable for the cogs, as the acrylic ones that i have already just slip of the drive shaft after a little while.

Nylon-6,6 was my first idea - but I thought it was probably a bit soft, so I looked around and found this:

http://www.motionco.co.uk/?gclid=CIS38K6C_50CFYwA4woduDbkpQ

They list gears made of Delrin Acetal (whatever that is!).

Edit: Any of these 'soft' plastics materials will eventually slip on a cylindrical steel shaft - you'd be better filing a flat on the shaft & using a gear with a grub-screw to hold it, or maybe using a hard resin-based wheel permanently bonded to the shaft* (but that will also need some kind of key to prevent rotation).

*Car engine ignition distributors sometimes use this approach.

Delrin is the most common trade name for Acetal. From DuPont I believe. It is Poly OxyMethylene or POM but sometimes known as Polyacetal. It has better chemical resistance than nylon and is approved for food use, but typically Nylon is a more robust material mechanically. Acetal is easier to use in injection molding which is why it is used for plastic gears in many applications.

-T

Yup i think it shall be acetal mmmm, now all i have to do is find someone with a big f*** off laser to cut them for me :P