Tactile Sensors: What They Are and How They Work

Overview

Tactile sensors, also called touch sensors, are widely used in everyday applications. Many common systems—mobile phone touchscreens, biometric security systems, pressure measurement, force measurement, and robots—rely on this type of sensor.

What is a tactile sensor?

A tactile sensor provides information about an object contacting the sensor surface. That information can include the object's shape, size, and sometimes material type. The sensor responds to touch, pressure, or any force applied to its surface. These characteristics are typically detected as changes in electrical parameters such as capacitance or resistance. After sensing, the signals are processed by associated electronics to generate a virtual image of the contact pattern.

Structure of tactile sensors

Tactile sensors are built from basic electronic components such as capacitors, piezoresistive elements, or optical light sources and photodetectors. The choice of components depends on the sensor type. These basic elements are arranged in an array, typically as multiple rows and columns. Associated circuitry interfaces with the array to process the data captured by the capacitors, piezoresistors, or optical detectors.

what is a tactile sensor?

How tactile sensors work

Tactile sensors operate on basic physical principles: changes in capacitance, changes in resistance, or changes in light intensity caused by an object contacting the sensor surface. These changes are measured and then used to form a virtual image that represents the contact. The virtual image can include information such as pressure distribution at all contact points, the contact object's shape, and its size. A common example is a fingerprint sensor, which scans a finger as the contacting object and generates a virtual image using the sensor array and supporting electronics.

To detect a contacting object, the sensor uses an array of basic sensing elements arranged in rows and columns. When the object touches the array, the electrical parameters at the corresponding elements change. The associated electronics detect which elements changed and then create a virtual image of the contact for further processing. The main purpose of the sensor is to convert the physical contact into this virtual image or data representation.

Types of tactile sensors

Depending on the sensing elements used, tactile sensors are commonly categorized into three main types:

Capacitive tactile sensors
Piezoresistive tactile sensors
Optical tactile sensors

Capacitive tactile sensors

Capacitive tactile sensors are among the most common sensors on the market. They detect touch, pressure, or force by measuring changes in capacitance. External contact, pressure, or force alters the distance between capacitor plates, causing a change in capacitance.

C = ε0 × εr × (A ÷ d)

where

C = capacitance of the plate
ε0 = permittivity of free space
εr = relative permittivity
A = area of the plate
d = distance between the two plates

In typical tactile sensors, area, ε0, and εr are constant, so capacitance changes only when pressure or force changes the distance d. Capacitor arrays are used to build capacitive tactile sensors, with each capacitor identified by its row and column. When capacitance changes are detected, the processing circuitry determines the capacitor location and generates a virtual image of the touch, pressure, or force pattern. Compared with some other types, capacitive sensors can have slower response times.

Piezoresistive tactile sensors

Piezoresistive tactile sensors are similar in structure to capacitive sensors and are widely used. They typically offer higher sensitivity than capacitive sensors. Piezoresistive elements change resistance when external pressure or force is applied. These elements are arranged in rows and columns to form a grid. The circuitry senses resistance changes and identifies the exact element locations. A virtual image of the contact pattern is then created from that data. Another advantage of piezoresistive sensors is their low hysteresis.

piezoresistive tactile sensor

Materials used in these sensors include conductive rubber, carbon fibers, and other engineered conductive materials.

Optical tactile sensors

Optical tactile sensors operate by detecting changes in light intensity. A light source continuously illuminates an area monitored by photodetectors. When touch, pressure, or force deforms the surface between the light sources and detectors, the received light intensity changes. Multiple source-detector pairs are used across the sensing area. The detectors' changing light intensities are converted by dedicated circuitry into a virtual image. The figure shows how deflection changes light intensity when external force is applied to the sensor surface.

optical tactile sensor

Optical tactile sensors typically offer resolution and sensitivity slightly lower than piezoresistive sensors. However, their relatively lower cost compared with other sensor types can be advantageous. These sensors are commonly used in robotic systems and advanced research applications.