Tactile pressure sensors are used to detect the pressure distribution between a sensor and a target. They are often used as robot grippers or flat tactile arrays. Flexible sensors can be molded to curved surfaces such as the human body. Solid-state sensors can be etched into silicon chips. Most tactile pressure sensors use resistive-based technologies where the sensor acts as a variable resistor in an electrical circuit. A small deflection of the diaphragm causes implanted resistors to exhibit a change in ohmic value. The sensor converts this change into a voltage that is interpreted as a continuous and linear pressure reading. When tactile pressure sensors are unloaded, their resistance is very high. When force is applied, their resistance decreases. Pressure sensitive film is used to create a direct, visual image of the pressure distribution. Active pressure sensor arrays consist of multiple sensing elements packaged in a single sensor.
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Specifications for tactile pressure sensors include width, length, and thickness; pressure range and allowable over-range; and sensing area and spatial resolution. Additional considerations include saturation force, linearity error, drift, repeatability, hysteresis, response time, force range, and operating temperature. The saturation force is the point at which the output no longer varies with the applied force. Repeatability measures a sensor’s ability to respond in the same way to a repeatedly applied force. Linearity refers to the sensor’s response to the applied load over the range of the sensor. Hysteresis is the difference in the sensor’s output response during loading and unloading, measured at the same force. Drift is the change in output when a constant force is applied over a period of time to tactile pressure sensors.
Tactile pressure sensors provide different types of electrical outputs. Analog current levels such as 4 – 20 mA are suitable for sending signals over long distances. Feedback is used to provide the appropriate current regardless of line noise and impedance. Analog voltages are simple, usually linear functions of the measurement. Modulated analog output signals are encoded, but still analog in nature. Examples include sine wave, pulse wave, amplitude modulation (AM), and frequency modulation (FM) signals. Several types of digital outputs are available. RS232, RS422, and RS485 are common serial, digital protocols. Popular parallel protocols include the general-purpose interface bus (GPIB), a standard which is also known as IEEE 488. Tactile pressure sensors with outputs that change the state of a switch or alarm are also available.