Making an up and down mechanism

[android90]

Hello there,

I am in no way shape or form a scientist/engineer. (I am making a prop for a film) I am filming a xylophone and need a very smooth movement of the stick/drum stick hitting the xylophone plate. (The movement has to be quite small from 10cms between each point, up and down)

I came across this site from another post about a similar object -

http://www.physicsforums.com/showthread.php?t=254310

Now I know that I need to construct some sort of contraption to get this smooth move, but I have no idea where to start,


So I was wondering whether any kind souls out there might be able to help,

thanks

[mgb_phys]

Not clear what you mean by smooth?
Do you just need the stick to hit the xylophone at a repeatable time/speed/position - in which case a pivot at one end and allow the stick to fall and hit the bar would work (and is closest to a real player)
Do you need the stick to slowly and controllably move down and hit the bar - in which case you probably need some sort of pnuematic cylinder or electrical motor control (this isn't going to sound correct)

[Bob S]

You certainly do not want a sinusoidal motion (like a cam), because the stick will be in contact with the xylophone plate too long, and it will damp many of the harmonics (above say C above middle C). So do you want a short repeatable impulse motion? Upward or downward? Can it be electric, like a solenoid?

[android90]

Hi, well the sound is not an issue because I will be tinkering with that in my editing software. I just need the stick to make contact with the xylophone, in a smooth slow motion - up and down.

[android90]

Not clear what you mean by smooth?
Do you just need the stick to hit the xylophone at a repeatable time/speed/position - in which case a pivot at one end and allow the stick to fall and hit the bar would work (and is closest to a real player)
Do you need the stick to slowly and controllably move down and hit the bar - in which case you probably need some sort of pnuematic cylinder or electrical motor control (this isn't going to sound correct)


Possibly the first idea is the best one for me because the sound of the stick hitting the plate is not an issue

[mgb_phys]

Drill hole near middle of stick and pivot on a steel pin.
Fix an electric motor near rear end with an bar/cam attached to the spindle that pushes down the rear end of the stick and then lets it drop so that the front end hits the instrument. Adjust the speed of the motor to get the beats/second you want.

It won't give you perfectly accurate timing because of friction but it will be as accurate as any real player.

If you need specific timed beats rather than a regular sequence replace the motor with a solenoid that pushes down the rear end of the stick and then releases it to fall back.

[android90]

Whoa! Fantastic! Thanks for the tips, is there a place that you can buy bits and bobs for this kind of thing? Like a tailored website somewhere..?

[mgb_phys]

Model car/aircraft shops are a great place for robotics stuff.

[Danger]

Does the stick just have to hit the same plate repeatedly, or do you want it to simulate playing an actual tune with different plates? The latter would require a bit more complexity.

[minger]

In order to play a tune, I imagine something like a circular nut on a screw. As the screw rotates it translates down the screw. You then use the cam type idea such that you can place arbitrary cam lobes in a spiral pattern.

Little more complicated than a simple cam-follower mechanism though.