I want to design a system that allows a 1kg metal brick to fall under gravity at variable speeds. The brick initial position is 20cm above a flat table. I want to have control on the duration (to reach the tabletop) anywhere from 1 second to 5 seconds approximately (it doesnt have to be exact).

So what is the simplest way to achieve this? I have read its possible to do it using an Arduino + stepper motors, but i am scratching my head thinking there must be a better way to do it without the need of electrical parts. Is there some sort of inexpensive vertical damping to achieve this?

Here's a simple diagram in case i wasn't clear enter image description here

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    $\begingroup$ A friction brake would work. $\endgroup$ – Solar Mike May 23 '20 at 12:58
  • $\begingroup$ Even driving a fan would work., $\endgroup$ – Solar Mike May 23 '20 at 13:37
  • $\begingroup$ Thanks for your suggestion! I did a bit of research but could not find a cheap DIY way of building a friction brake system. I'd like to use a screw to adjust the amount of friction. $\endgroup$ – Jon Sat May 24 '20 at 6:52
  • $\begingroup$ I haven't got any information regarding Driving a fan. Could there an example you could send, that'd be great! $\endgroup$ – Jon Sat May 24 '20 at 6:53
  • $\begingroup$ Check out early shock absorbers: two metal plates separated by a cork disc and the clamping pressure provided by a bolt and nut - you could even add a compression spring... $\endgroup$ – Solar Mike May 24 '20 at 6:54

You have many options from friction brake, magnetic brake, oil damper or pneumatic damper shown below.

enter image description here

Figure 1. The pneumatic solution.

Toggling the manual valve will send air into the bottom of the air cylinder sending it up at a speed determined by the adjustment of the speed controller at the top of the cylinder. Switching it off again will allow the load to fall at a rate determined by the setting of the lower speed control which regulates the rate of exhaust from the bottom of the cylinder.

  • $\begingroup$ Is there an industry consensus standard that provides definitions for pneumatic like these symbols in a way similar to how the ISA 5.1 document provides definitions for P&ID symbols? $\endgroup$ – baltakatei May 23 '20 at 16:16
  • $\begingroup$ Probably, but I'm an electrical engineer. $\endgroup$ – Transistor May 23 '20 at 16:28
  • $\begingroup$ Thanks for these suggestions! however it seems a bit pricey to achieve. I want the absolute cheapest way to solve this problem. $\endgroup$ – Jon Sat May 24 '20 at 6:57
  • $\begingroup$ If low cost is a requirement then that needs to be specified in your question. $\endgroup$ – Transistor May 24 '20 at 7:51
  • $\begingroup$ @Transistor you are right, sorry about that! what i meant by an easy way was in fact cheap as well $\endgroup$ – Jon Sat May 26 '20 at 5:35

Before anything else, just to be clear. I don't know your application, the solution I'm posting is cheap and easy to create but is not safe. During operation there's lots of ways this thing can fail.

First of all, we need to understand the physics in your situation. For an object with 1Kg of mass under effect of pure gravity (vertical acceleration of 9,81m/s²) and the course of 20cm (0,2m).
With our object free falling there's:

  • Travel time of t= (0,2[m]*2 / 9,81[m/s²]) = 0,04s
  • A force of Fp= 1[Kg] * 9,81[m/s²] = 9,81N

To reduce the falling time we need to control the speed and to achieve this a counter force is needed, and the way I'll show generate this with the friction effect.

  1. You can work with the simplest way, the rope pass inside two pieces of soft material (like rubber) and you can increase the pressure with a screw. (this one works exactly like when you use your own tumbs to create pressure and decelerate)

  2. You can work with a wheel apllying friction (the system @Solar Mike sugested if I'm not wrong).

  3. This one I'm not sure if will work, but based on what I understood until now (about electrics), you can use an electric dc motor (operating like a generator) with resistors to control the speed, the more resistance the greater the torque necessary to rotate the shaft.

Below I'll put images of system 2. The 3rd is the same way, but using a electric motor. And the first system I think is kind of self explanatory. Braking system Source: Myself

Obs.: These systems aren't failprof and even the best solution. But if you want some crude and cheap solution these options are the best way I know.


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