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Communication

Rapid and Cost-Effective Fabrication and Performance Evaluation of Force-Sensing Resistor Sensors

by
Jinwoo Jung
1,
Kihak Lee
2 and
Bonghwan Kim
2,*
1
Department of Robotics Engineering, Daegu Catholic University, Gyeongju 38430, Gyeongbuk, Republic of Korea
2
Department of Semiconductor Electronic Engineering, Daegu Catholic University, Gyeongju 38430, Gyeongbuk, Republic of Korea
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(17), 7774; https://doi.org/10.3390/app14177774
Submission received: 10 July 2024 / Revised: 16 August 2024 / Accepted: 26 August 2024 / Published: 3 September 2024
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Abstract

:
In this study, we developed a cost-effective and rapid method for fabricating force-sensing resistor (FSR) sensors as an alternative to commercial force sensors. Our aim was to achieve performance characteristics comparable to existing commercial products while significantly reducing costs and fabrication time. We analyzed the material composition of two widely used commercial force sensors: Interlink FSR-402 and Flexiforce A201-1. Based on this analysis, we selected 4B and 9B pencils, which contain high concentrations of graphite, and silicone sealant to replicate these material properties. The fabrication process involved creating piezoresistive sheets by shading A4 copy paper with 4B and 9B pencils to form a uniform layer of graphite. Additionally, we prepared a mixture of 9B pencil lead powder and silicone sealant, ensuring a consistent application on the paper substrate. Measurement results indicated that the force sensor fabricated using a mixture of 9B pencil powder and silicone sealant exhibited electrical and mechanical characteristics closely resembling those of commercial sensors. Load tests revealed that the hand-made sensors provided a proportional voltage output in response to increasing and decreasing loads, similar to commercial FSR sensors. These results suggest that our fabrication method can produce reliable and accurate FSR sensors suitable for various applications, including wearable technology, robotics, and force-sensing interfaces. Overall, this study demonstrates the potential for creating cost-effective and high-performance FSR sensors using readily available materials and simple fabrication techniques.

1. Introduction

Force measurement is essential for numerous applications, including industrial automation, robotics, consumer electronics, and medical devices [1,2,3,4]. Accurate and reliable force sensors are vital for tasks such as load monitoring, robotic gripping, tactile feedback in touchscreens, and patient monitoring systems. Among the available force sensor technologies, force-sensing resistors (FSRs) have gained significant attention due to their thin profiles, flexibility, and cost-effectiveness [5,6,7]. FSR sensors operate on the principle that electrical resistance decreases as applied force increases, making them suitable for both static and dynamic force measurements [8,9,10,11]. Typically, an FSR sensor consists of a conductive polymer that alters its resistance in response to applied force, with a compact form factor often less than 0.5 mm in thickness, enabling integration into space-constrained devices such as wearable electronics and portable medical devices.
Commercially available FSR sensors, like those from Interlink Electronics (Camarillo, CA, USA) (e.g., FSR-402) and Tekscan (Boston, MA, USA) (e.g., FlexiForce A201), are widely used as benchmarks for evaluating alternative fabrication methods [8,9,11]. These sensors are employed in various applications, including robotics, musical instruments, and sports equipment, due to their robustness and ease of integration. However, their cost can be prohibitive for large-scale deployment or cost-sensitive projects.
The growing demand for cost-effective, flexible, and high-performance force sensors has led researchers to explore new materials and fabrication methods. Pencil graphite has emerged as a promising material due to its cost-effectiveness, accessibility, and excellent electrochemical properties, offering a potential solution to the limitations of conventional force sensors [12,13,14,15,16,17,18,19,20,21,22,23,24,25]. For instance, paper, with its durable and flexible mechanical properties, has been identified as an ideal substrate for FSR sensors [26,27,28,29,30,31]. Numerous studies have investigated the potential of pencil graphite in sensor applications. Srinivas et al. [12] focused on surface activation techniques to enhance sensor performance, particularly for dopamine detection. Kurra et al. [13] demonstrated a simple, cost-effective method for fabricating strain sensors, highlighting the feasibility of pencil graphite for sensor construction. Liao et al. [14] showcased the high sensitivity and flexibility of pencil graphite sensors in wearable applications, while Ren et al. [15] explored their use in structural health monitoring. Liu et al. [16] developed an integrated sensing platform with power and read-out, providing a low-cost solution for electrochemical sensing. Elangkovan et al. [17] and Zhang et al. [18] examined the use of graphite pencil traces in humidity sensors, emphasizing their cost-effectiveness, simplicity, and environmental benefits, with attention to how stroke count and pencil grade affect sensitivity and response time. Chen et al. [19] discussed pressure sensors that detect pressure through changes in contact resistance in graphite layers, demonstrating high sensitivity and fast response times, especially in wearable devices. Hussien et al. [20] utilized pencil graphite electrodes for electrochemical analysis, detecting substances like drugs and heavy metals. Zhang et al. [21] detailed pencil graphite-based gas sensors that alter electrical properties when interacting with specific gases, such as NO2, using a graphite pattern on silver interdigitated electrodes. Xu et al. [22] presented pencil graphite-based on-skin electronics for biosignal detection in wearable health monitoring devices, emphasizing their low cost and environmental friendliness. Annu et al., Ataide et al., and Kawde et al. [23,24,25] focused on the use of pencil graphite as an active material in electrochemical sensing, emphasizing its cost-effectiveness, simplicity, and environmental benefits.
Despite these advances, the discussed sensors exhibit several limitations. Humidity sensors based on 6B pencil graphite, while sensitive, may suffer from inaccuracies due to the manual fabrication process, which involves human hand-pressure rather than a controlled, automated system, leading to variability and affecting consistency. Pressure sensors, although flexible, may face mechanical durability issues under prolonged stress and have limited effectiveness in extreme pressure conditions. Electrochemical sensors are prone to surface fouling, impacting accuracy and requiring frequent maintenance, and may have lower sensitivity compared to more advanced materials such as modified carbon nanotubes. Gas sensors may encounter selectivity challenges in complex gas mixtures and are sensitive to environmental factors like temperature and humidity, necessitating additional calibration. On-skin electronics may experience reduced adhesion over time, particularly in moist environments, potentially affecting comfort and signal stability, and may encounter interference from other electronic devices, impacting biosignal detection accuracy.
Furthermore, paper substrates have been effectively utilized in conjunction with pencil graphite. Romanholo et al. [26] highlighted the mechanical strength and versatility of cellulose-based paper, making it suitable for forming various 2D or 3D shapes, and noted its compatibility with carbon and silicon sealant mixtures for FSR sensors. Yang et al. [27] used paper substrates to fabricate sensors for measuring the bending of fluidic elastomer actuators, leveraging the paper’s ability to support conductive materials. Zhao et al. [28] presented a high-performance, degradable all-paper-based pressure sensor. Chowdhury et al. [29] described a paper-based supercapacitive pressure sensor for wrist arterial pulse monitoring, emphasizing its high sensitivity for wearable healthcare. Gao et al. [30] introduced a fully paper-based piezoresistive pressure sensor for wearable electronics, focusing on its high performance under varied pressures. Li et al. [31] developed a capacitive flexible pressure sensor with silver nanowires and polydimethylsiloxane (PDMS), showcasing its potential in human-related measurements and artificial skin applications. Sakhuja et al. [32] discussed a multilayered, flexible, paper-based pressure sensor for human–machine interfacing, emphasizing its sensitivity and flexibility.
However, the primary limitations of these studies are potential long-term stability issues, mechanical durability concerns, and complex fabrication processes, which may affect scalability and production costs, impacting their practical application.
The objective of this study is to develop a cost-effective and rapid fabrication method for force sensors using readily available materials. To validate the feasibility of this approach, we compare the performance of these handmade sensors against commercial FSR sensors across various applications. The study addresses the following key aspects: (1) analysis of the material composition of commercial FSR sensors to identify suitable alternatives; (2) fabrication of FSR sensors utilizing 4B and 9B pencils along with silicone sealant; (3) evaluation of the electrical and mechanical properties of the fabricated sensors; and (4) comparison of the performance characteristics with those of commercial FSR sensors. The primary compositional difference between 4B and 9B pencils lies in the graphite-to-clay ratio, with 9B pencils containing more graphite, resulting in softer and darker lines, while 4B pencils contain more clay, making them harder and easier to control. This research aims to advance force sensor technology by exploiting the unique properties of pencil graphite and silicone sealant, contributing to the development of cost-effective, flexible, and high-performance force sensors that are easily fabricated and integrated into various applications, potentially transforming the field of force measurement technology.

2. Experimental Method

2.1. Fabrication of the FSR Sensor Sheet

The fabrication of the FSR sensor sheet focuses on materials that exhibit both conductivity and piezoresistive properties, allowing for resistance changes in response to applied force. The materials chosen for this study were readily available and inexpensive, making the process accessible and cost-effective. The fabrication process is detailed in Table 1 and illustrated in Figure 1.

Materials and Preparation

(1)
Force-Sensitive Conductive Sheet (Velostat/Linqstat);
Description: Velostat-1361 is a conductive polymer film made from polyolefin infused with carbon black, imparting electrical conductivity [33]. It is commonly used in packaging but has gained popularity among hobbyists for sensor applications due to its resistance-changing properties under bending or force.
Application: Velostat is utilized in wearable devices such as shoes that light up when stepped on and systems analyzing the load on the foot’s grounding surface.
Preparation: The 280 mm × 280 mm Velostat sheet was cut into 13.5 mm × 13.5 mm pieces to serve as the sensing part of the FSR sensor (Figure 1a).
(2)
Hand-made 4B Graphite Sheet;
Description: Cretacolor Monolith woodless graphite pencil, 4B, was used to create a conductive sheet on A4 copy paper.
Method: The paper was uniformly shaded horizontally and vertically, alternating directions for 10 strokes each. This sheet was then cut into 13.5 mm × 13.5 mm pieces (Figure 1b,c).
(3)
Hand-made 9B Graphite Sheet;
Description: Similar to the 4B sheet but using a Cretacolor Monolith woodless graphite pencil, 9B.
Method: The shading process was identical to the 4B sheet, with horizontal and vertical shading for 10 strokes each, then cutting into 13.5 mm × 13.5 mm pieces (Figure 1d,e).
(4)
Piezoelectricity Sheet;
Description: Manufactured by Tyco Electronics (TE), this sheet uses silver ink to exhibit piezoelectric properties under mechanical load.
Preparation: Specific details about the preparation are indicated in Figure 1f.
(5)
Mixture of 9B Pencil Lead Powder and Silicone Sealant;
Description: A mixture of 9B pencil lead powder and silicone sealant was used to create a piezoresistive sheet.
Method: The 9B pencil lead was shaved into a fine powder using a cutter knife. The powder was then mixed with silicone sealant in a 4:1 ratio to achieve a gel-like consistency. This mixture was evenly applied to copy paper using fingers to ensure uniform thickness and allowed to dry naturally for at least 12 h (Figure 1g,h).
As discovered [14,18], the thickness of pencil graphite can significantly influence sensor performance, particularly in terms of conductivity and resistance. Therefore, to ensure uniform thickness, we applied the same number of strokes when fabricating the 4B and 9B graphite sheets, using 10 strokes as similarly employed in [14].

2.2. Fabrication of the FSR Sensor

The assembly of the FSR sensor involves layering the prepared FSR sheets with conductive electrodes and securing them to form a functional sensor. The FSR sensor was fabricated using the same structural design of commercial products, as depicted in Figure 2. A double-sided tape with a thickness of 0.1 mm was applied as a spacer, positioning the electrode outside the conductive sheet and connecting the conductive side to the 0.25 mm wrapping, while the outer copper sheet electrode was grounded. This setup ensures proper grounding. The FSR sensor operates by allowing current to flow when force is applied. As shown in Figure 2, the top and bottom layers of the piezoresistive material are separated by a gap of approximately 0.1 mm, maintained by double-sided tape on either side. When force is applied to the upper electrode plate, the top layer bends and contacts the bottom layer, resulting in a change in resistance proportional to the contact area, thereby enabling current flow. This process is illustrated in Figure 3.

Step-by-Step Assembly

(1)
Electrode attachment;
Description: Copper foil (Cu foil) was attached to the back of each FSR sheet to serve as the electrode.
Procedure: The Cu foil was adhered to the FSR sheet using conductive adhesive (Figure 3a).
(2)
Wire connection;
Description: Wrapping wire (0.25 mm) was connected to the Cu foil and the FSR force surface to ensure electrical conductivity.
Procedure: The wire was securely attached to the Cu foil (Figure 3b).
(3)
Covering and securing;
Description: The Cu foil and wire assembly were covered and secured to the FSR sensor to ensure stability.
Procedure: The covering process involved adhering the Cu foil and wire assembly as shown in Figure 3c.
(4)
Opposite side assembly;
Description: The opposite side of the FSR sheet was assembled similarly, with the Cu foil and wire assembly.
Procedure: The same steps were followed to prepare the opposite side of the sensor (Figure 3d).
(5)
Double-sided tape application;
Description: Double-sided tape served as a spacer between the two FSR sheets.
Procedure: The double-sided tape was adhered to one FSR sheet, and the protective paper was peeled off to expose the adhesive (Figure 3e).
(6)
Final Assembly;
Description: The prepared opposite side was attached to the exposed adhesive side of the double-sided tape, completing the sensor assembly.
Procedure: The final step involved carefully aligning and attaching the two FSR sheets (Figure 3f).
By following these steps, a cost-effective and functional FSR sensor was successfully fabricated. The next sections will discuss the testing and performance evaluation of these hand-made sensors compared to commercial alternatives.

3. Results

3.1. Analysis of Piezoresistive Components in FSR Sensors

An analysis was conducted to compare the components and composition that determine the piezoresistive properties of FSR sensors under load, utilizing a field emission scanning electron microscopy (FE-SEM, Hitachi S-4800 (Tokyo, Japan)) and an energy dispersive X-ray fluorescence (EDXRF, Horiba EX-250 (Kyoto, Japan)). Figure 4 presents the SEM images of the Flexiforce A201 FSR Series (Tekscan, Inc., Boston, MA, USA) and the Interlink FSR-402 (Camarillo, CA, USA). Table 1 provides an analysis of the chemical composition. The composition ratio of each sensor was measured twice.
As shown in Table 2, the chemical composition of the FSR sensor is primarily based on the conductor carbon (C), with oxygen (O), silicon (Si), or tin (Sn) as additional components. Here, O is presumed to be a mixture of conductive and piezoresistive materials, while Si and Sn are considered to be the materials exhibiting piezoelectric properties under applied loads.
The Flexiforce A201 Series (Flexiforce A201-1, Flexiforce A201-25, Flexiforce A201-100, Flexiforce A201-125) features the same piezoresistive area while differing in allowable load. The components influencing these differences in allowable load indicate that the composition ratio of O has a minimal effect, whereas the composition ratios of C and Si significantly impact the range of allowable loads. For the A201-25, compared to the A201-1, the composition is adjusted by reducing the weight ratio of the conductive material C to decrease conductivity while increasing the weight ratio of the piezoresistive material Si to enhance the piezoelectric effect.
In terms of FSR sensor performance, it is inferred that appropriate adjustment of the composition ratios of conductive and piezoresistive materials can impact the allowable load and sensor linearity, assuming a uniform arrangement of constituent particles is achieved. Comparing the compositions of the Flexiforce A201 Series with the Interlink FSR-402 reveals that while C and O are consistent, the Interlink FSR-402 utilizes Sn as the piezoresistive material. Notably, the Interlink FSR-402 shows a nearly 1:1 composition ratio of conductive material C to piezoresistive material Sn, and the composition ratio of O is approximately 15% lower than that in the Flexiforce A201 Series. This difference in piezoresistive materials between the Flexiforce A201-1 and the Interlink FSR-402 results in distinctly different output characteristics under load.
Therefore, the output characteristics of the FSR sensor are determined by the composition ratio of the conductor C to the piezoresistive materials Si or Sn. Consequently, an FSR sensor was fabricated using C and Si, offering cost efficiency, easy accessibility, and rapid production.

3.2. Resistance of Fabricated FSR Sensors

The resistance value of the FSR sensing surface was measured before assembling the sensor and then compared with commercial products to determine the fixed resistance value for the voltage divider. As shown in Figure 5, the resistance was measured at five locations, each approximately 8 to 9 mm apart, on the FSR sensing surface using a digital multimeter (KeySight, Santa Rosa, CA, USA).
Table 3 presents the average resistance values for hand-made and commercially available FSR sensors. It is important to note that using a digital multimeter for these measurements may introduce slight inaccuracies. The resistance values for the hand-made FSR sensors, excluding the mixture of 9B pencil lead and silicone sealant, were in the lower kilo-ohm range. However, the mixture of 9B pencil lead and silicone sealant exhibited resistance values that fell between those of commercial products such as Interlink FSR-402 and Flexiforce A201-1. This phenomenon is attributed to the non-conductive silicone sealant infiltrating between the conductive 9B powder (carbon graphite) particles, thereby increasing the overall resistance.

3.3. Load Test of the FSR Sensor

To analyze the sensing characteristics of the manufactured FSR sensor according to load, a five-stage compression test was conducted using the setup shown in Figure 6. In order to perform continuous load tests, equipment was designed and manufactured to apply a fixed load of approximately 1 kg using a weighing holder and weights. First, the load tester was 3D modeled (Figure 6a). It was specifically designed to apply the load by moving 0.01 mm per pulse using a step motor. A load cell was used to log the applied load on the FSR sensor when a load is applied (Figure 6b). The load tester controls the travel distance using a TB6600 motor driver and an Arduino Uno board (Turin, Italy) (Figure 6c). Figure 6d shows the DAQ circuit using an Arduino Uno board to measure the pressing load of the load cell and the resistance change of the FSR sensor according to the load applied by the load tester.
This is a DAQ circuit using an Arduino Uno board to measure the applied load from the load tester, the pressing load of the load cell, and the resistance change of the FSR Sensor. The FSR sensor load test setup is driven by a stepping motor, with a punch end diameter of 10 mm to match the size of the manufactured sensor, which is 10 × 10 mm. The operation is programmed to pressurize in five stages and then decompress in five stages, with a holding time of 30 s per stage. The test was conducted with a movement step in the load application direction of 10 steps (approximately 0.1 mm per step).
Initially, a 0.2 mm gap was maintained between the FSR sensor and the force punch using a filler gauge. The 10-step moving distance is 0.1 mm, but the 0.2 mm interval is maintained to avoid damage to the sensor’s upper surface due to filler gauge measurement during test setup. Once the setup was completed, the five-stage compression was automatically operated by the program. Data acquisition was performed using an Arduino Mega board equipped with a 10-bit ADC, with a data acquisition rate of 10 Hz. The current graph displays data values that represent both numerical data and time. The data interval used for measurement was 1000 ms, meaning one data point was collected every second.
Figure 7 shows the output voltage of the Interlink FSR-402 sensor under a load cell load and a 100 Ω resistor. After calibration was performed at an initial load of 200 g, the load was gradually increased and decreased from 0 to 4800 gf. To represent the load and output voltage on the same scale, the output values were multiplied by 500 for the graph. The graph demonstrates that the same load increase and decrease pattern and corresponding output waveform can be observed for both loading cycles.
The resistance value of the Velostat FSR sensing surface was measured to be approximately 25 kΩ, so a fixed resistance of 100 Ω was used to measure voltage based on the voltage divider principle. To verify the repetition characteristics of the sensor during the continuous compression load test, the test was repeated three times. During testing, it was observed that the output voltage exhibited significant noise. The first test data was averaged and analyzed to check the change in output voltage according to the load, as shown in Figure 8. When Velostat was pressurized, the output voltage changed according to the load at each pressurizing step, though there was an unstable response when depressurizing (Figure 8a). The resistance value of the 4B pencil lead FSR sensing surface was measured to be approximately 37 kΩ, so a fixed resistance of 100 Ω was used for voltage measurement based on the voltage divider principle. In Figure 8b, there is a slight change in output voltage as pressurization begins and both sides of the sensing surface come into contact, but no change in output is confirmed for continuous pressurization. This indicates that the 4B FSR sensor lacks piezoelectric characteristics typical of general FSR sensors. For the TE piezoelectricity sheet, while it exhibited piezoelectric properties, it showed an increase in voltage rather than a decrease in resistance in response to pressing force due to piezoelectricity. This material was selected to observe the difference between piezoresistive and piezoelectric properties and is not suitable as an FSR sensor. The resistance value of the 9B FSR sensing surface was measured to be approximately 13 kΩ, so a fixed resistance of 100 Ω was used for voltage measurement based on the voltage divider principle (Figure 8c).
Figure 9 illustrates the voltage output characteristics of the FSR sensor made by mixing a specific amount of silicone sealant with 9B pencil lead powder. As shown in Table 3, the resistance value of this FSR sensing surface was measured to be approximately 0.8 MΩ, significantly higher than the other selected materials, so a fixed resistance of 1 kΩ was used for voltage measurement based on the voltage divider principle. Unlike the voltage output of the FSR sensor using only 9B powder in Figure 8c, it was confirmed that the output voltage was clearly responsive as the pressurized load increased, and the changes at the pressurization end were distinctly visible.
When applying a load using the load cell as in Figure 7, a graph similar to Figure 10 was obtained. The difference in the number of data points represents the number of data collected per second, so the difference in the x-axis can be disregarded. Figure 10a shows the output of the mixture of 9B pencil lead and silicone sealant for continuous pressurization and decompression load voltage (1st cycle). It can be clearly seen that the output voltage increases as pressurization starts and the load increases. During the 30 s pressurization maintenance period, the voltage value was not uniform, showing an initial slope. Figure 10b,c display the output voltage for continuous compression load of Interlink FSR-402 and Flexiforce A201-1, commercial products tested under the same conditions. The results exhibit a similar trend.

4. Discussion

This study aimed to investigate the potential of using easily accessible materials, such as pencil graphite and silicone sealant, for fabricating force-sensing resistor (FSR) sensors. The findings revealed that hand-made sensors, particularly those created using a mixture of 9B pencil graphite and silicone sealant, demonstrated performance characteristics comparable to commercial FSR sensors like the Interlink FSR-402 and Flexiforce A201 series.
One of the primary advantages identified in this research is the cost-effectiveness of using pencil graphite and silicone sealant. These materials are not only inexpensive but also widely available, which makes the fabrication process highly accessible. This stands in stark contrast to commercial FSR sensors, which can be prohibitively expensive for large-scale deployment or cost-sensitive applications.
The study highlighted the importance of material composition in determining sensor performance. The piezoresistive properties of pencil graphite were effectively utilized to create sensors that respond to applied force with changes in resistance. Furthermore, the inclusion of silicone sealant provided the necessary mechanical stability and flexibility, allowing the sensors to withstand repeated loading and unloading cycles without significant degradation.
However, some limitations were observed. The hand-made sensors exhibited higher noise levels during voltage output measurements compared to their commercial counterparts. This indicates that further refinement in the fabrication process, such as more precise mixing of the graphite powder and silicone sealant or improved assembly techniques, could enhance sensor performance. Moreover, graphite sensors can have issues such as limits of high resistivity, fair sensitivity, and moderate response time, as Hasnain et al. [34] noted when developing graphite-on-paper-based strain sensors. Thus, a highly conductive graphite pencil on a smooth paper substrate can be used to minimize these restrictions. In future research, the limitations should be carefully examined and considered to improve the performance of our FSR sensor.
Our experiments also showed that the conductivity of 9B pencil lead is higher than that of 4B pencil lead, resulting in better performance in terms of output voltage during continuous pressurization. The 9B FSR sensor displayed a clear increase in output voltage as force was applied, indicating its sensitivity and responsiveness. In contrast, the 4B FSR sensor exhibited minimal change in output voltage under force, suggesting that it does not possess the necessary characteristics to function effectively as an FSR sensor.
Velostat, a material commonly used in wearable devices, showed some promise due to its resistance-changing properties under flexing and force. However, the changes in output voltage were small and inconsistent, indicating that while Velostat can detect force, it may not be ideal for precise force measurement applications. The mixture of 9B pencil lead powder and silicone sealant emerged as the most promising material combination for creating an FSR sensor. This mixture demonstrated a voltage output that was proportional to the applied and decompressed force, closely mirroring the behavior of commercial FSR sensors. This suggests that the mixture of 9B pencil lead powder and silicone sealant can effectively measure force and respond to load changes in a manner similar to commercial sensors. In addition, Lee et al. [35] found that the roughness of pencil graphite can play an important role in enhancing pressure sensitivity and response time of the pressure sensor. It is expected that our FSR sensor may show similar characteristics related to the surface roughness. Thus, the effects of the rough surface of pencil graphite of our FSR sensor should be investigated in future research.

5. Conclusions

This research successfully demonstrated the feasibility of fabricating FSR sensors using cost-effective materials such as pencil graphite and silicone sealant. The hand-made sensors, particularly those utilizing a mixture of 9B pencil graphite and silicone sealant, showed performance characteristics that were comparable to commercial sensors. The results validate the potential of these alternative materials in producing efficient and reliable force-sensing devices. The cost-effectiveness and accessibility of the materials used in this study make the proposed fabrication method highly advantageous for various applications, particularly in fields where large-scale deployment is necessary. Future research should focus on refining the fabrication process to reduce noise levels and further enhance sensor performance.
In conclusion, the study provides a promising outlook for the development of low-cost FSR sensors, highlighting the potential for widespread adoption of these alternative materials in the force-sensing industry. The findings pave the way for innovative approaches to force measurement, which could significantly impact the design and implementation of sensors in diverse applications.

Author Contributions

Conceptualization, B.K. and K.L.; methodology, J.J.; software, K.L.; validation, K.L. and J.J.; formal analysis, K.L.; investigation, J.J.; writing—original draft preparation, J.J.; writing—review and editing, B.K.; supervision, B.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by research grants from Daegu Catholic University in 2022.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

All authors declare that they have no conflicts of interest.

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Applsci 14 07774 g001
Figure 1. FSR sensing sheet. (a) Velostat-1361, (b,c) 4B Pencil lead on a copy paper, (d,e) 9B pencil lead on a copy paper, (f) TE piezoelectricity sheet, (g,h) mixture of 9B pencil lead powder and silicone sealant on a copy paper.
Figure 1. FSR sensing sheet. (a) Velostat-1361, (b,c) 4B Pencil lead on a copy paper, (d,e) 9B pencil lead on a copy paper, (f) TE piezoelectricity sheet, (g,h) mixture of 9B pencil lead powder and silicone sealant on a copy paper.
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Figure 2. FSR sensor structure.
Figure 2. FSR sensor structure.
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Figure 3. Fabrication of the FSR sensor. (a) Cu foil electrode, (b) wire on electrode, (c) wrapping wire, (d,e) cover, (f) taping, and (g) final FSR sensor.
Figure 3. Fabrication of the FSR sensor. (a) Cu foil electrode, (b) wire on electrode, (c) wrapping wire, (d,e) cover, (f) taping, and (g) final FSR sensor.
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Figure 4. SEM images of (a) Flexiforce A201-1, (b)Flexiforce A201-25, and (c) Interlink FSR-402.
Figure 4. SEM images of (a) Flexiforce A201-1, (b)Flexiforce A201-25, and (c) Interlink FSR-402.
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Figure 5. Resistance measured by FSR sensing surface.
Figure 5. Resistance measured by FSR sensing surface.
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Figure 6. FSR sensor load test setup.
Figure 6. FSR sensor load test setup.
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Figure 7. Load cell load and Interlink FSR−402 output voltage. The load is the force applied to an area of 10 mm × 10 mm, which is equivalent to pressure. The measurement interval is 1000 ms (one data point per second).
Figure 7. Load cell load and Interlink FSR−402 output voltage. The load is the force applied to an area of 10 mm × 10 mm, which is equivalent to pressure. The measurement interval is 1000 ms (one data point per second).
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Figure 8. FSR sensor average output voltage for continuous pressurizing and depressurizing loads. (a) Velostat, (b) 4B, and (c) 9B. The load is the force applied to an area of 10 mm × 10 mm, which is equivalent to pressure. The measurement interval is 1000 ms (one data point per second).
Figure 8. FSR sensor average output voltage for continuous pressurizing and depressurizing loads. (a) Velostat, (b) 4B, and (c) 9B. The load is the force applied to an area of 10 mm × 10 mm, which is equivalent to pressure. The measurement interval is 1000 ms (one data point per second).
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Figure 9. 9B & Silicone Sealant Mixed for continuous pressurizing and decompressing loads output voltage. The load is the force applied to an area of 10 mm × 10 mm, which is equivalent to pressure. The measurement interval is 1000 ms (one data point per second).
Figure 9. 9B & Silicone Sealant Mixed for continuous pressurizing and decompressing loads output voltage. The load is the force applied to an area of 10 mm × 10 mm, which is equivalent to pressure. The measurement interval is 1000 ms (one data point per second).
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Figure 10. Continuous pressurization and decompression load voltage. (a) 9B + silicone sealant mixed, (b) Interlink FSR-402, and (c) Flexiforce A201-1. The load is the force applied to an area of 10 mm × 10 mm, which is equivalent to pressure. The measurement interval is 1000 ms (one data point per second).
Figure 10. Continuous pressurization and decompression load voltage. (a) 9B + silicone sealant mixed, (b) Interlink FSR-402, and (c) Flexiforce A201-1. The load is the force applied to an area of 10 mm × 10 mm, which is equivalent to pressure. The measurement interval is 1000 ms (one data point per second).
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Table 1. FSR sensor materials.
Table 1. FSR sensor materials.
FSR Sensor Material (Sheet)Product NameComments
Velostat-1361Adafruit-1361 (New York, NY, USA)Purchased
4B Pencil + Paper (copy paper)Cretacolor (Klagenfurt, Austria): Monolith 4BPurchased
9B Pencil + Paper (copy paper)Monolith 9BPurchased
TE Piezoelectricity SheetTE Conductivity (Schaffhausen, Switzerland)Purchased
9B + Silicone Sealant+Paper (copy paper)Silicone Sealant for bathroom: Ohgong (Gwangju, Republic of Korea)Purchased
Table 2. Chemical composition of the FSR sensors.
Table 2. Chemical composition of the FSR sensors.
Chemical
Composition (%)		Flexiforce A201-1Flexiforce A201-25Interlink FSR-402
1st2nd1st2nd1st2nd
C55.2359.3342.7146.2441.2139.39
O36.3233.5936.2737.9418.2519.54
Si8.457.0120.0215.81
Sn 40.5341.08
Table 3. Comparison of measured resistance between hand-made sensors and commercial FSR sensors (measuring distance: 8 to 9 mm).
Table 3. Comparison of measured resistance between hand-made sensors and commercial FSR sensors (measuring distance: 8 to 9 mm).
FSR Sensor NameAverage ResistanceComments
1Velsostat25.2 kΩHand-made sensor
24B37.12 kΩHand-made sensor
39B13.7 kΩHand-made sensor
4TE0.27 ΩHand-made sensor
59B + silicone sealant0.88 MΩHand-made sensor
6Interlink FSR-4020.2 MΩCommercial sensor
7Flexiforce A201-11.71 MΩCommercial sensor
8Flexiforce A201-2533.52 MΩCommercial sensor
9Flexiforce A201-10053.1 MΩCommercial sensor
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Jung, J.; Lee, K.; Kim, B. Rapid and Cost-Effective Fabrication and Performance Evaluation of Force-Sensing Resistor Sensors. Appl. Sci. 2024, 14, 7774. https://doi.org/10.3390/app14177774

AMA Style

Jung J, Lee K, Kim B. Rapid and Cost-Effective Fabrication and Performance Evaluation of Force-Sensing Resistor Sensors. Applied Sciences. 2024; 14(17):7774. https://doi.org/10.3390/app14177774

Chicago/Turabian Style

Jung, Jinwoo, Kihak Lee, and Bonghwan Kim. 2024. "Rapid and Cost-Effective Fabrication and Performance Evaluation of Force-Sensing Resistor Sensors" Applied Sciences 14, no. 17: 7774. https://doi.org/10.3390/app14177774

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