How to build a Pneumatic Force Applicator Machine on a Budget !
_Raquel Barras
One of the greatest struggles when doing my PhD on Triboelectric Nanogenerators (TENGs) was to find a proper way to apply consistent amounts of force to test my devices. The problem was that by that time I did not have the right tool available at my institute. Specific tools for mechanical tests like this can be really expensive.
If like me, you are also struggling to find a proper and accurate way to test any type of force actuated devices but you are on a budget, look no further! I prepared this post with clear and simple instructions on how I built a simple pneumatic force applicator machine that anyone can follow, even if you have no prototyping experience.
Using this tool I was finally able to test and properly characterize the devices I developed in my PhD. It saved my (PhD) life and now I want to share it so everyone can do it too. You only need to have access to a pressurized gas line which is a standard in most labs. If you do not have access to this, follow this link for an alternative system that does not require gas pressure.
Let's get started !
1. What are TENGs
2. What is a force applicator machine
3. How it works
4. What materials do I needed
5. How to assemble it
6. How to program the arduino
1. What are TENGs ?
TENGs are devices capable of converting mechanical to electrical energy by using the triboelectric effect. Because they are a recent approach to a rather old scientific finding (the existence of static electricity), they are not commercialized yet, lacking sufficient output power for being able to serve any real electronic application. TENGs are a hot topic in scientific research and new papers are published every month with new findings on this matter.
Because TENGs convert mechanical to electrical energy, to test its conversion performance it is necessary to apply a mechanical force, many times on the form of vertical tapping.
2.What is a force applicator machine?
Chances are, if you work on an electrical, material or sensor characterization laboratory yourself, or even other field of research, perhaps you've also experienced the need to apply a controllable force to perform some force dependent characterization test.
The force applicator machine feature in this post is a system designed to apply a fixed but controllable tapping force to a sample, at an also controllable frequency.
The following video is the PFAM beeing operated to test linear shaped devices:
3. How it works?
The force applicator machine is composed of a pneumatic cylinder actuated by an electrovalve, connected to a relay and an Arduino.
An electrovalve is an electronic actuated valve mechanism that controls the flow of a pressurized gas. The opening and closing of the valves direct the gas towards one of the cylinder compartments, which results in the extension or contraction of the rod stroke. The electrical signals to command the cylinder are processed by the programing of the arduino and sent to a relay that amplifies the signal and actuates the electrovalve directly.
The resulting force can be controlled by varying the pressure of the gas line. It can be calculated by using the simple formula (Eq. 1) or measured directly using a force sensor.
F = P x A (Eq. 1)
(Eq. 1) Formula to calculate the Force from the pressure of the gas line
Based on Eq. 1 we can see that the cylinder dimensions will also impact the force applied. This should be kept in mind when choosing the right cylinder for the project. Below you can find a table relating the cylinder diameter and pressure and its effect on the range of forces that can be used.
|
Pressure (bar) |
D = 1.6 cm F (N) |
D = 2 cm F (N) |
D = 2.5 cm F (N) |
D = 3cm F (N) |
D = 4 cm F (N) |
|
0.5 |
10.1 |
15.7 |
24.5 |
35.3 |
62.8 |
|
1 |
20.1 |
31.4 |
49.1 |
70.7 |
125.7 |
|
1.5 |
30.2 |
47.1 |
73.6 |
106.0 |
188.5 |
|
2 |
40.2 |
62.8 |
98.2 |
141.4 |
251.3 |
|
2.5 |
50.3 |
78.5 |
122.7 |
176.7 |
314.2 |
|
3 |
60.3 |
94.2 |
147.3 |
212.1 |
377.0 |
|
3.5 |
70.4 |
110.0 |
171.8 |
247.4 |
439.8 |
|
4 |
80.4 |
125.7 |
196.3 |
282.7 |
502.7 |
|
4.5 |
90.5 |
141.4 |
220.9 |
318.1 |
565.5 |
|
5 |
100.5 |
157.1 |
245.4 |
353.4 |
628.3 |
|
5.5 |
110.6 |
172.8 |
270.0 |
388.8 |
691.2 |
|
6 |
120.6 |
188.5 |
294.5 |
424.1 |
754.0 |
In the image below you can find the theoretical force range when using cylinders with bore size of 6 and 16 mm.
Be aware that the initial impact force may be superior to the pressure applied after impact.
4. What materials do I needed
For assembling the force applicator machine you will need the following materials listed above, with some suggestions from amazon listing products:
- Pneumatic cylinder actuator
- Pneumatic electrovalve
- Grove 30V Relay
- Arduino microcontroller
V-Slot - assembly parts for the structure:
- 2x C-Beam V-slot 400 mm
- 1x C-Beam V-slot 285 mm
- 2 x 20x40 V-Slot 285 mm
- 5 x 20x40 V-Slot 205 mm
- 2 x 20x40 V-Slot 200 mm
- Bolts and Nuts as needed
3D Prints:
5. How to assemble it
First you need to assemble a robust structure where you can attach the cylinder, the arduino and electrovalve, as well as a platform where you can introduce your sample to be tested.
There are several possible ways to assemble the cylinder and the structure, and you can use the mounting that is more suitable for your application.
Bellow you can find how I assembled mine, for a vertical tapping mode on the sample, that would sit on the red base.
For a better visualization and help with the assembly, check this 3D model of the PFAM: PFAM
6. How to program the arduino
You can use the following code to obtain operation frequencies from 0.5 to 3 Hz.
/* Driver controller for Pneumatic Force Actuator Machine - PFAM */
/* Raquel Barras, 2022 */
int dataPin = 10; // set data output pin name
void setup() {
pinMode(dataPin, OUTPUT);
}
void loop() {
//Choose frequecy: 1750 = 0.5Hz || 750 = 1Hz || 250 = 2Hz || 83 = 3 Hz
int a = 1750; // 0.5 Hz
int b = 750; // 1 Hz
int c = 250; // 2 Hz
int d = 83; // 3 Hz
digitalWrite(dataPin, HIGH); // turn on
delay(250); // in millisec
digitalWrite(dataPin, LOW); // turn off
delay(c); // < < < < < < < < < < < < Change the frequency(a, b, c or d) accordingly
}