Optical Flex Sensor
About Flex Sensor
The "Flex Sensor" or "Bend Sensor" are flexion, curvature or torsion sensors. Actually, these are variable resistors; the greater the bending, the greater the resistance.
There are many "Flex Sensor" on the market today for commercial prices between 10 and 20 euros [1,2,3] depending on their size and design. They are based on conductive ink deposited on a polymer film and their design dates from the 90s [4]. They are light, thin, can works under the stress of several Newtons of force, are reproductible, and can go up to torsion angles of 90° [5].
The resistance of these sensors moves from ten kilo Ohms with no stress or deformation to a hundred kilo Ohms for an angle around 90°, i.e. nearly 1 kilo Ohm per degree of deformation.
There are various applications of these “Flex Sensors” [6] for human health, but also for industrial applications. The first is a goniometer (angle measurement). But there are other applications as motion detector or angle change, as intrusion detection system, as vibration measurement, etc [7].
Finally, there are numerius DIY tutorials [8] on the internet for making these sensors with aluminum or copper foils, plastic supports and conductive rubber (Velostat or Linqstat [9]). Their low precision and repeatability limit their applications to coarse actions.
Our purpose is to design a reliable flex sensor for light deformation with very low angles (a couple of degrees).
Basics of our optical flex sensor
The optical flex sensor uses path of light theory and principes. It uses light emitting diodes (LED) and photo resistors (LDR):
The LEDs are placed in the middle of the probe. The LED face the LDR. A plastic film (PET from a bottle) is placed between the two probe's supports. A dark mask limits the surface of photoresistors expose to the light. This plastic film is fixed on one side only, which allows sliding during bending. The device is supposed to be symmetrical and the two photoresistors give a final resistance of [R0].
During bending, the optical path of light from the LEDs is changed and it changes the exposure intensity of the photoresistors. The sliding the intermediate mask also modifies the exposure of the photoresistors. This sliding provokes an amplifying effect. The higher the bending, the higher the resistance [Rd] will be.
Since photoresistor has short response time (around 50 ms), the inertia of the probe is due to the plastic properties of the probe.
Supplies
Our home made Optical Flex Sensor needs the following supplies :
- Recycled LED strip[[i]]. 3 items will be needed;
- 2 (LDR) [[ii]] (light-dependent resistor) ;
- PET Strip (15 mm wide - 80 mm long). This strip is from a recycled water bottle
- a home made and 3D printed designed strip to house the LED. The design is linked to the LED positions;
- a home made and 3D printed designed strip to house the LDs itself also designed according to the LED.
- translucent epoxy glue
- 2 M3 screws and nuts
- Electrical wires (from an used Cat 5 LAN cable)
LED & LDR Probe Strips and Final Assembling
The LED Strip
LED strip is 0.7 mm thick in PLA. Our LED strip is made up of a set of 3 , 4x4 mm, LED blocks. They can be cut apart and the separation between blocks is 11 mm. Schemas below gives the dimensions (in mm) of this LED strip:
Before being fixed in its housing with epoxy glue, the LED are connected with thin wires needed for energy supply (wire are from a used Cat5 LAN cable) :
Please note that the PLA parts were printed in yellow for photographic reasons. Yellow on a black background increase the contrast and hillight details of the parts. In reality, it is wise to print the parts in black PLA in order to isolate the photoresistor from outside and ambient light.
The LDR Strip
The LDR strip is 0.7 mm thick in PLA. The strip of photoresistors is design for the symmetry that we have mentioned. Propagation of light to the surface of the photoresistor is done through a hole 2 mm diameter. Schema below give the dimensions of this LDR strip :
Final assembling
The LDR are fixed on its strip with the translucent epoxy glue. The two LDR are connected in "series" mode. All the assembly is blocked by the two M3 screws and nuts.
At the end, the final device is isolated from outside ambiant light by a black adhesive tape for electrical application :
Measurements, Further Improvements & Bibliographic References
Measurements
Probe efficiency is first evaluated by measuring the LDR resistances. When the LED are OFF the resistance LDR should not be measured with avoltmeter (therefore greater than 2 mega ohms). When the LED are ON, resistance of the two LDR in series should be close to 100 k Ohms.
The second stage of the evaluation is the simulation of the final application.
A force is apply in the center of the probe. this force causes a bending of a few millimeters. the variation of the resistance is then measured.
Undoubletly, the sensor has a high sensitivity for the first few millimeters of flexion because,for the first five millimeters, we observe an increase of 14 kilo Ohms for each millimeter of deformation.
Beyond the threshold of 5 millimeters, the sensitivity drops to 2 kilo Ohms per millimeter of bending.
This sensor has a high sensitivity for small deformations. Indeed, this few millimeters present a deformation of less than 10°.
In comparison with the commercial "Flex Sensor" which presented a sensitivity of the order of 1 kilo Ohm per degree of deformation, this optical "Flex Sensor" we have a sensitivity of the order of 10 kilo Ohms per degree of deformation, ten times higher.
It can be used to detect mechanical vibrations or thermal or sonic deformations.
References and Notes
Further Reading : « 100 Sensors In Action, Electronic For Chemists » - Gérard Bacquet- SciencExpert Edition - 2024
[1] https://www.gotronic.fr/art-capteur-de-flexion-74-mm-fs2l055-30852.htm
[4] Langford G. B. 1996. Flexible potentiometer. US 5 583 476.
[5] Mohamed Aktham Ahmed, Bilal Bahaa Aws Alaa Zaidan, Mahmood Maher Salih; Muhammad Modi bin Lakulu “ A Review on Systems-Based Sensory Gloves for Sign Language Recognition State of the Art between 2007 and 2017. “ Sensors2018, 18, 2208; doi:10.3390/s18072208
[6] Alapati Sreejan, Yeole Shivraj Narayan M. Tech. “ A Review on Applications of Flex Sensors “ International Journal of Emerging Technology and Advanced Engineering, Volume 7, Issue 7, July 2017
[7] Giovanni Saggio, Francesco Riillo, Laura Sbernini andLucia Rita QuitadamoDe, “Resistive flex sensors: a survey”, Smart Mmaerials and structures, 25 (1), January 2016
[8] https://www.youtube.com/watch?v=nXON9INI4Io
[9] https://ca.robotshop.com/fr/products/pressure-sensitive-conductive-sheet-velostat-linqstat