Force Sensing Resistor FSR402
| Autor: | Sherina Uwera |
| Betreuer: | Prof. Dr.-Ing. Schneider |
| Sprache: | 🇩🇪 🇬🇧 |
Introduction
The FSR402 is a polymer thick-film force-sensitive resistor consisting of conductive electrodes and a force-sensitive resistive layer. When a mechanical load is applied to the sensing area, the electrical contact characteristics within the sensor change, causing its resistance to decrease. Its resistance must be converted into a measurable electrical signal using an appropriate interface circuit.
The objective of this investigation is to characterise the FSR402 under different known loads and evaluate its suitability for force measurement. The sensor output will be acquired using an Arduino microcontroller, analysed and visualised in MATLAB, and subsequently processed using Simulink.
The investigation focuses on the sensor's response to applied force, calibration, repeatability, hysteresis and measurement accuracy.
Technical Overview
Operating Principle
T The FSR402 exhibits a nonlinear relationship between applied force and electrical resistance. As the applied force increases, the sensor resistance decreases. The corresponding electrical conductance generally increases with force over part of the operating range.
The sensor's response is influenced by the magnitude of the applied force, the contact area and the mechanical loading conditions.
Electrical Measurement Principle
For a configuration in which the FSR is connected to the supply voltage and the fixed resistor is connected to ground, the output voltage is:
Vout = Vcc × Rfixed / (RFSR + Rfixed)
Where:
- Vout = measured output voltage (V)
- Vcc = supply voltage (V)
- Rfixed = fixed resistance (Ω)
- RFSR = force-dependent sensor resistance (Ω)
As the applied force increases, the FSR resistance generally decreases, producing an increase in the output voltage.
The Arduino analog-to-digital converter (ADC) converts this voltage into a digital value for further processing.
Technical Specifications
These specifications describe sensor behaviour under particular test conditions and do not represent a universal force-measurement accuracy.
| Specifications | ||||||||||||||||||
|
Components Requirements
| Component | Quantity | Purpose |
|---|---|---|
| FSR402 | 1 | Force-sensitive measuring elemen |
| LM393 interface module | 1 | Sensor interface and threshold detection |
| Arduino Uno R4 WiFi | 1 | Analog data acquisition |
| 3D-printed sensor holder | 1 | Mechanical positioning |
| Reference masses | Several | Application of known loads |
| Jumper wires | 3 or more | Electrical connections |
| Arduino IDE | 1 | Initial data acquisition |
| MATLAB / Simulink | 1 | Visualisation, calibration and modelling |
Pin Configuration
The FSR402 is a two-terminal resistive sensor connected to an external interface module.
The interface module provides four connection pins:
| Module Pin | Function | Puroposed Arduino connection |
|---|---|---|
| A0 | Analog output | A0 |
| D0 | Digital threshold otput | D2(optional) |
| GND | Ground | GND |
| VCC | Suppply Voltage | 5 V, subject to module verification |
For the initial experiment, only the analog output, supply voltage and ground connections are required.
Measurement Method
The FSR402 will be characterised by applying a series of known masses to its active sensing area and recording the corresponding analog output.
The reference force is calculated using:
F = m × g
where:
- F = applied force (N)
- m = reference mass (kg)
- g = gravitational acceleration (9.81 m/s²)
The experimental procedure consists of:
1.Recording the unloaded sensor output.
2.Applying a known mass to the sensing area.
3.Recording the corresponding Arduino analog readings.
4.Repeating the measurement for several reference masses.
5.Repeating measurements to investigate repeatability.
6.Removing the masses in reverse order to investigate hysteresis.
7.Saving the recorded data for MATLAB analysis.
The load application method and contact area will be kept as consistent as possible.
The resulting measurements will be used to establish a calibration curve and evaluate the sensor's response and measurement uncertainty.
Measurement Circuit
Software
The FSR402 sensor measurements are acquired using the Arduino IDE and analysed using MATLAB. The Arduino reads the analog output of the sensor interface and converts it into digital values. MATLAB is used to visualise the measured sensor response and investigate its characteristics under different applied loads.
Arduino IDE
Data Acquisition
The Arduino IDE was used to program the Arduino Uno to read the analog output of the FSR402 sensor through the LM393 interface module.
The analog signal was connected to pin A0. The Arduino's 10-bit analog-to-digital converter (ADC) converts the input voltage into a digital value between 0 and 1023.
The readings were displayed using the Serial Monitor.
Arduino Program
The following program was used to acquire and display the sensor measurements.
ADC-to-Voltage Conversion
The Arduino converts the analog input voltage into a digital value between 0 and 1023.
To estimate the corresponding voltage, the ADC reading is divided by 1023 and multiplied by the reference voltage.
For example, with an ADC reading of 644 and an assumed reference voltage of 5 V:
Voltage = (644 ÷ 1023) × 5 = 3.15 V
This allows the measured sensor output to be expressed in volts rather than only as a digital number.
Experimental Measurements
Different known masses were applied to the FSR402 sensor, and the corresponding ADC and voltage values were recorded.
The measurements were then used for further analysis in MATLAB.
Initial Observations
The recorded ADC values generally decreased as the applied mass increased.
At low loads, the ADC reached its maximum value of 1023. At higher loads, the readings decreased, indicating a change in the analog output voltage.
The force–voltage relationship was subsequently analysed using MATLAB.
MATLAB
Analysis of the Force–Voltage Characteristic
The experimentally recorded measurements were imported into MATLAB to investigate the relationship between applied force and output voltage.
The resulting graph shows a nonlinear decrease in output voltage with increasing applied force.
At low applied forces, the ADC reaches its maximum reading, resulting in a region where different loads cannot be distinguished. At higher forces, the output voltage decreases more gradually, indicating reduced voltage sensitivity.
These findings suggest that a nonlinear calibration model may be required to estimate applied force from the measured voltage.
Further experiments are necessary to evaluate repeatability, hysteresis and measurement uncertainty.
Experimental Observations and Evaluation
The FSR402 sensor was investigated using different reference masses and manual pressure. The analog output was acquired using an Arduino Uno and visualised in MATLAB.
The initial force–voltage characteristic showed a nonlinear decrease in output voltage with increasing applied force.
Further experiments were performed to investigate the stability of the sensor response over time. The live MATLAB measurements showed significant voltage fluctuations, sudden signal changes and extended periods at approximately 5 V.
During manual pressure tests, the sensor responded visibly to changes in applied force. However, when reference weights were applied, the output did not consistently settle at a stable value.
Repeated loading measurements also produced different voltage readings, suggesting limitations in measurement repeatability under the tested conditions.
The observed variations may be influenced by the sensor's mechanical characteristics, load positioning, contact conditions, electrical interference or the LM393-based interface circuit.
The experiments therefore demonstrate that the investigated measurement setup can detect changes in applied pressure but has not yet demonstrated the stability and repeatability required for reliable quantitative force measurement.
Measurement Limitations
The following limitations were identified during the investigation:
- Nonlinear response: The relationship between applied force and output voltage was nonlinear, requiring calibration for quantitative force estimation.
- Signal fluctuations: The measured voltage varied during testing, preventing consistently stable readings under the tested loading conditions.
- Limited repeatability: Repeated loading experiments produced different output values.
- ADC saturation: The output reached approximately 5 V at low applied loads, limiting the ability to distinguish small forces.
- Mechanical sensitivity: The sensor response may depend on the position, distribution and duration of the applied load.
These limitations indicate that further optimisation of the mechanical loading arrangement, signal conditioning and calibration procedure would be necessary before using the system for quantitative force measurements.
Simulink
Datasheets
https://www.interlinkelectronics.com/downloads/datasheets/fsr-400-series-datasheet.pdf
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