Precise measurement of interface pressure and force between two surfaces is always a challenge to engineers. However, several specific technologies exist for sensors dealing with interface force and pressure. Parameters such as form factor, precision and environment influence the selection and capabilities of such sensors.
A variety of applications requires measurement of pressure. These range from product development to medical research. Typically, pressure is the measurement of applied force over an area. With two objects held in contact, both exert force on the other. Therefore, the average interface pressure is the total force divided over the interface area. However, this interface pressure may not be distributed uniformly, creating the necessity of measuring localized interface pressure.
For measuring the force or interface pressure, chiefly three technologies are considered suitable – load cells, pressure indicating films and tactile pressure mapping systems. Although each sensing technology has some overlapping information, they all provide unique values when solving problems. Additionally, as the shape of the target becomes increasingly uneven, the ability of the sensor to match the overlap with the surfaces applying the force also becomes critical.
A load cell is the most common force or pressure sensor with which most engineers are familiar. The load cell has many varieties, the most useful being strain gages, piezo-electric elements and variable capacitance. Load cells may be utilized in multiple form factors depending on the force applied and the mechanics of the application. For example, measuring the deformation of a beam for qualifying the force of the load applied relies on load cells. Such compression, S- or Z-beam and shear beam load cells are all dependent on strain gages. Most reliable load cells utilize a full bridge of strain gages bonded on to the load-bearing structures.Force applied to the load cell deforms the structure and places a mechanical stress on the strain gages. This changes the resistance of the strain gages affecting their output signal. With calibration, the output voltage can correlate to the force applied on the load.
Pressure Indicating Film
These are useful when measuring interface pressure between two surfaces. A layer of polyester hides a color developing material layered next to tiny microcapsules containing staining ink. These microcapsules are designed to break under different pressures. With pressure applied to the film, the microcapsules rupture. This distributes the ink at the places where the pressure is applied. With more force being applied to a location, more microcapsules rupture increasing the intensity of color on the film. This gives an image of the force applied across the sensing area. Films are available for different sensitivities of pressure.
Tactile Pressure Sensor
Tactile pressure sensors are made of piezo-electric material. Two pieces of flexible polyester with printed silver conductors on each piece sandwich a unique piezo-resistive ink. The result is an extremely thin sensor, about a tenth of a millimeter thick. A signal is transmitted via the silver electrodes through the piezo-resistive ink. As pressure increases on the sensing area, the resistance of the ink changes and the data collected maps the pressure applied.