Pressure sensing is the capacity for some system to sense the force exerted on a surface per unit area and express that force in the strength of an electric signal. For example, pressure sensing is what allows a robot to tell when it collides with something, or when something pushes against it. Pressure sensors can be used to measure force, and in some cases, to determine the contour of an applied force.
A capacitive pressure sensor employs two metal plates separated by a layer of nonconductive foam. This forms a capacitive transducer. The transducer is connected in series or parallel with an inductor. The resulting inductance-capacitance (LC) combination determines the frequency of an oscillator. If an object strikes or presses against the transducer, the plate spacing decreases, causing the capacitance to increase. This lowers the oscillator frequency. When the pressure is removed, the foam springs back, the plates return to their original spacing, and the oscillator returns to its original frequency. A capacitive pressure sensor can be fooled by metallic objects. If a good electrical conductor comes very close to the plates, the capacitance might change even if physical contact is not made. If this occurs, the sensor will interpret this proximity as pressure.
An elastomer pressure sensor solves the proximity problem inherent in the capacitive device. The elastomer is a foam pad with resistance that varies depending on how much it is compressed. An array of electrodes is connected to the top of the pad; an identical array is connected to the bottom. Each electrode in the top matrix receives a negative voltage, and its mate in the bottom matrix receives a positive voltage. When pressure appears at some point on the pad, the material compresses at and near that point, reducing the resistance between certain electrode pairs. This causes a current increase in a particular region in the pad. The location of the pressure can be determined according to which electrode pairs experience the increase in current. The extent of the pressure can be determined by how much the current increases. If the electrode matrix is fine enough, the contour of the pressure-producing object can be determined by a microprocessor that evaluates the electrode current profile.
The output of a pressure sensor is analog, but it can be converted to digital data using an analog-to-digital converter (ADC). This signal can be used by a robot controller. Pressure on a transducer in the front of a robot might cause the machine to back up; pressure on the right side might make the machine turn left. The presence of pressure might be used to actuate an alarm, or to switch a device on or off. Calibrated pressure sensors can be used to measure applied force, mass, weight, or acceleration.