Hi All,

The last time I posted, I was looking for help in troubleshooting a probe calibration issue with my old Tektronix 2213 oscilloscope. I very quickly got advice that worked, and saved me a lot of time and money!

This time, I'm posting because I'm working on a new project. It's a Vacuum Gravity Demonstrator. It is involving a 20" long X~6" diameter, thick walled, glass tube. I plan to pull a vacuum inside the glass tube, and drop a 1/2" diameter steel ball and a downy feather, and show (hopefully) that they fall at the same rate.

I'm thinking that I can pick up the ball and feather with small magnets (I will glue a very small piece of something that the magnet will be attracted to the feather). I want to lower the magnets using .005" diameter fishing leader to pick up the 2 items. I think that I can use microcontroller driven stepper-motors to do this. A friend of mine has suggested using a Raspberry PI microcontroller with a stepper-motor (maybe with driver). He said that I should look at Adafruit or Arduino for this.

So far, I haven't gotten any help from Adafruit (I haven't tried with Arduino yet). I am looking for help from you on what would be the best and cheapest components for me to acquire.

Regards, Bill

 

I would recommend https://randomnerdtutorials.com/ as a very good site for learning about all things micro-controller.
They have quite a few stepper projects.

 

I would recommend https://randomnerdtutorials.com/ as a very good site for learning about all things micro-controller.
They have quite a few stepper projects.

Thanks! I'll check it out.

 

I’m wondering if you really need a processor for this. A small geared motor with a switch which can turn it on in one direction and then the other should suffice, unless you want to automate and continuously repeat the experiment?

 

I’m wondering if you really need a processor for this. A small geared motor with a switch which can turn it on in one direction and then the other should suffice, unless you want to automate and continuously repeat the experiment?

Hi,

My first thinking, is that I want to start at a position where the ball and feather are pulled up, but not released. Then, a push on a momentary contact actuation switch would raise up the holding magnets a bit more to cause the ball and feather to release.

After the ball and feather have dropped, another push on the actuation switch would cause the magnets to lower, pick up the ball and feather, and return to the initial upward position, without dropping the ball and feather.

I that's why I think I want to use a microcontrolled stepper-motor. I was hoping with the posting that someone would save me the effort of choosing the cheapest effective components, and even save me the effort of writing the control software. What I mean by saving me the effort of writing the control software, is that I would just have to modify it.

Thanks, Bill

 

How will you release them from the magnets at the top? Indeed, what will be your release mechanism? Small electromagnets may well be the answer to both problems. Mass of 0.5" steel ball is about 9gm, so only a tiny 1 - 2N pull electromagnet needed. Finding one that small might be tricky - I'd look at repurposing the innards of a 5v PCB relay that can be driven directly from the Arduino - or just a release button.

A small DC motor driving a drum at the top is all that's needed to retrieve them. the e-magnet can be lowered (acts a cord tensioner) on its thin connecting cables - 30awg copper wire is 0.01" dia, can carry a few 10's of milliamps and has a breaking strength of about 1kg - perfectly usable for this.

EDIT - my post overlapped yours...

 

How will you release them from the magnets at the top? Indeed, what will be your release mechanism? Small electromagnets may well be the answer to both problems. Mass of 0.5" steel ball is about 9gm, so only a tiny 1 - 2N pull electromagnet needed. Finding one that small might be tricky - I'd look at repurposing the innards of a 5v PCB relay that can be driven directly from the Arduino - or just a release button.

A small DC motor driving a drum at the top is all that's needed to retrieve them. the e-magnet can be lowered (acts a cord tensioner) on its thin connecting cables - 30awg copper wire is 0.01" dia, can carry a few 10's of milliamps and has a breaking strength of about 1kg - perfectly usable for this.

EDIT - my post overlapped yours...

Hi Irving,

I'm thinking that I will release the magnets at the top, because when the magnets are raised further, the ball and feather will have reached a mechanical stop, and raising the magnets further will release them.

If I use permanent magnets, I can use very light test fishing leader (.005' diameter). If I use electromagnets, the raising and lowering lines would have to be much thicker, and less flexible.

By the way, the ball does weigh 9gm! I am using a very small rare earth magnet that barely picks up the ball. I will have glue a very small piece of metal to the downy feather, so I can pick it up also.

Thanks, Bill

 

Hi Irving,

I'm thinking that I will release the magnets at the top, because when the magnets are raised further, the ball and feather will have reached a mechanical stop, and raising the magnets further will release them.

If I use permanent magnets, I can use very light test fishing leader (.005' diameter). If I use electromagnets, the raising and lowering lines would have to be much thicker, and less flexible.

By the way, the ball does weigh 9gm! I am using a very small rare earth magnet that barely picks up the ball. I will have glue a very small piece of metal to the downy feather, so I can pick it up also.

Thanks, Bill

I'm not convinced a mechanical stop will release both identically - a rigid ball v a flexible feather? A magnetic release will be simultaneous. Over a 20" length I don't think the 30awg will be significantly less flexible; you could go down to 36awg which is 0.005" diameter.

 

I'm not convinced a mechanical stop will release both identically - a rigid ball v a flexible feather? A magnetic release will be simultaneous. Over a 20" length I don't think the 30awg will be significantly less flexible; you could go down to 36awg which is 0.005" diameter.

Hi Irving,

I think that I will be able to adjust the length of the 2 lines so that they will release at the same time, or I could use 2 small stepper-motors, and program their travel as required.

Bill

 

I think a couple of servos with grabbers, or a hinged platform with a locking pin will be better as a release mechanism as adding even a small magnet to the feather will skew the results.

 

How about, instead, inverting the glass tube end for end? Then both can be released from a shelf that rapidly rotates to dump both off at the same time? Also, consider the effect of the steel ball impacting the other end of the tube!

 

How about, instead, inverting the glass tube end for end? Then both can be released from a shelf that rapidly rotates to dump both off at the same time? Also, consider the effect of the steel ball impacting the other end of the tube!

Hi MisterBill2,
Thanks for your response. There are multiple reasons why I don't want to flip it over. I do know that I will have to dampen the steel ball's impact. What I was hoping for was a shortcut to the microcontroller and stepper-motor issue.
Regards, Bill

 

A microcontroller will only be needed if you choose to make the whole show automatic. A simple button arrangement to control the lift/lower motor is all that is actually required. A second switch to release the magnets , or one single electromagnet, will work, since mostly the electro-magnet would need to be off, to avoid running down the batteries.