Sensor Products has recently filed two patents for, and introduced, the ASET® sensor for measuring interfacial stresses in real time. The sensor does not measure force, but rather the amount of surface area under compressive load. The principle of the ASET, which is a hybrid of a position and a displacement sensor, lies in treating the entire sensing element as a potentiometer. In the typical linear format design of the ASET sensor, not only can surface area of contact be determined, but position along the line can be sensed as well.
The one caveat of the sensor's capacity to detect surface area is that the tactile surface of the object coming in contact with the sensor must be a solid continuous surface. The ASET sensor is available in thicknesses similar to the force sensing resistor (FSR) and piezo sensors described below. Unlike the resistive ink sensors (RIS) and piezo sensors, the ASET does not require complicated or time-consuming calibration. The ASET sensor is a rugged, durable device that can withstand repeated applications of contact and impact under a wide range of environmental conditions.
As noted above, ASET, conceptually similar to a linear potentiometer, reveals both interpolated force and position of an object (or object dimensions), and can sense the position and interpolated force, by virtue of surface area of contact on a deformable mass, of a single- or multiple-contact actuator. The ASET sensor system does not rely upon a reading of electrical resistance to determine the shunting level (percentage of circuit surface area that is shorted), but rather a ratio of the starting resistance to the final resistance.
In addition, a typical RIS sensor has a limited life, as the elastomeric compound that is integral to it begins to fatigue and compress with use. Thus these sensors are not well suited as embedded devices in a product or system.
ASET bears comparison with other sensor technologies on the market. Magnetic RIS sensors convert mechanical stresses into electrical charges. RIS sensors consist of two sheets, one with a semiconducting polymer, the other with a printed conductive ink (dielectric). Typically these two compounds are coated onto Mylar substrates, resulting in a total package thickness of 4 to 6 mils. A traditional RIS sensor measures force disbursement by measuring actual pressure or force load applied to the sensor element. This method is fraught with difficulties: extreme environmental sensitivity, low repeatability, sensor deformation over time, acute and frequent calibration requirements, and low accuracy. Furthermore, the resistance-pressure relationship for RIS sensors is hyperbolic and changes with time.
Capacitive sensors are relatively similar to RIS sensors but much more accurate and stable. At the same time they are much thicker. These sensors cannot be made on flexible substrates, and therefore are difficult to use to monitor interfacial stresses.
Piezo-film sensor elements are used to reveal tactile force distribution as well, but given their abrupt and unpredictable decay time, are primarily relegated to very brief event recorders. Neither piezo-film or the RIS are instrument-quality devices suited for precision measurement.
Potential applications for the ASET are checking roller pressure in real time, and in others where it would be embedded into smart structures or objects. It could serve any application requiring a ball or ball joint, where it is essential to know the relative position and force being applied on the ball. Another use would be in any application involving linear motion of an object that requires knowledge of that object's position or amount of force or surface area of the object.
Another application area would be where a solid or semisolid object is moving in three-dimensional space and there exists a need to monitor where and when a component of the object is striking or mating with another solid surface. Yet another would be where an object changes dimension as a result of compression or impact and there exists a need to measure this dimensional increase. Finally, a robotics application exists where object grip and surface area of contact and other important considerations need monitoring.
Some product-specific aspects of the ASET sensor include the following: it has both dynamic and static load capability; its linearity and hysteresis combined are ±0.2 percent of full scale; it is self-calibrating, relying on ratios of starting-state resistance versus finished resistance; it is environmentally stable, with an operating range of -40 to +225 °F (storage: -55 to 225 °F); its accuracy is ±1.6 percent of full scale; its sensitivity is 0.01 in.; its repeatability cycle to cycle is ±0.2 of full scale; its repeatability part to part is the same; and its creep is 0.01 in. per day.
These sensors can be made as thin as 4 mils. Virtually any shape, size, and resiliency can be manufactured. The sensor elements are made in a carefully controlled environment to ensure uniform resistor and dielectric deposition onto the Mylar substrate. Temperature, humidity, and mixing ratios of the various compounds have a considerable effect upon the final accuracy of the sensor element, and consequently need to be monitored. After the resistor and dielectric depositions are applied and dried, the sensor is then laser-planed to remove surface aberrations. After each lot of sensors is manufactured they are calibrated and individually identified.