Force-Sensitive Resistor
- Also called: FSR, force-sensing resistor, force sensing resistor, pressure-sensitive resistor
- Category: Sensing › Contact & force
- Tags: force-sensing, tactile-sensing, sensors, electronics, grippers
A thin, flexible sensor whose electrical resistance drops as force is applied to its surface, giving a simple, low-cost way to detect touch and estimate contact force.
Definition
Force-sensitive resistors (FSRs) are thin polymer sensors whose electrical resistance decreases when they are pressed. A typical FSR is a sandwich of layers: a flexible substrate printed with interdigitated conductive traces, a spacer, and a layer of conductive or semiconductive polymer ink. With no load the resistance is very high. As force increases, more of the ink touches the traces and the contact area grows, so the resistance falls.
FSRs are read with simple electronics. The most common circuit is a voltage divider with a fixed resistor, connected to a microcontroller's analog-to-digital converter; an op-amp current-to-voltage stage gives a more linear output. The response is strongly nonlinear, with resistance roughly inversely proportional to force over much of the range, so calibration against known loads is needed to turn readings into force values.
Compared with load cells and force/torque sensors, FSRs are inexpensive, very thin and easy to place on curved or small surfaces such as fingertips and foot soles. In exchange, their accuracy is limited. They show hysteresis, drift under constant load (creep), sensitivity to how the force is distributed over the active area, and variation from one unit to the next. They measure only the force normal to the surface and give no information about shear.
For these reasons FSRs are best suited to detecting contact, sensing whether a grip or footstep has occurred, and giving relative force readings, rather than precise force measurement. Arrays of FSRs and related piezoresistive films are also used as simple pressure-mapping tactile skins.
Why it matters
FSRs give robots a sense of touch at very low cost and with almost no space or wiring burden, which makes contact sensing practical on hobby robots, prototype grippers and wearable devices. They are often the first tactile sensor a team adds when it needs to know whether, and roughly how hard, something is being touched.
Key applications
- Fingertip contact detection on robot grippers and hands
- Foot-contact sensing for legged robots and gait analysis
- Data gloves and wearable devices that record grip force
- Simple pressure-mapping tactile arrays
- Touch input on hobby and educational robots
In practice
A team adding contact sensing to a gripper typically mounts FSRs on the fingertips under a thin rubber or silicone pad that spreads the load evenly across the active area. Each sensor is wired in a voltage divider to an analog input, with the fixed resistor chosen to give good resolution in the expected force range. The team calibrates each sensor with known weights, fits a curve, and uses thresholds on the reading to detect contact or to stop closing the gripper once a target grip level is reached. On legged robots and in gait studies, FSRs in the feet or shoe insoles give footstep timing. Because of drift and hysteresis, engineers avoid relying on FSRs for absolute force values and re-zero them regularly.
Common misconceptions
- FSRs are precision force sensors — they suit contact detection and relative force; load cells or force/torque sensors are needed for accurate measurement
- An FSR's output is linear in force — resistance falls nonlinearly with force, so readings must be calibrated or linearized
- FSRs sense force in any direction — they respond to force normal to the surface and do not measure shear
Who should know this
- Hardware engineers designing grippers and robot hands
- Makers and students building low-cost robots
- Wearable and data-glove developers
- Legged robot and gait analysis researchers
Related technologies
- Load Cell
- Tactile Sensor
- Force/Torque Sensor
- Voltage Divider
- Piezoresistive Sensor
- Capacitive Touch Sensor
Related terms
Related projects
- Gazebo: Industry-standard open-source robotics simulator with multi-physics and ROS integration
- ORCA Hand: Open-source tendon-driven robotic hands with tactile sensing from ETH Zurich
- Carbon Aeronautics Quadcopter: From-scratch quadcopter build that teaches aeronautics, electronics and programming
- Robotic Origami Challenge: Fold Plane Demonstrations: Bimanual paper-folding dataset: 682 LeRobot episodes with vision and touch
Videos
- How to use Force Sensitive Resistors (FSR) with Arduino - Simple Force Sensing Arduino Tutorial: Science Fun