Rows and columns of force-sensitive elements are printed on Mylar and mated to create a sensor capable of measuring the magnitude, location, distribution, and timing of exerted pressure in ranges of 02 psi to 025,000 psi (014 kPa to 0175 MPa).
Charles F. Malacaria, Tekscan, Inc.
The heart of Tekscan's pressure measurement system is an extremely thin, matrix-based sensor consisting of force-sensitive resistive elements created by a printing process. Multiple layers of silver and a proprietary, pressure-sensitive ink containing semiconductive particles suspended in a polymer-based binder are printed onto two thin, flexible, Mylar sheets. The silver layers act as electrodes. There is a row pattern on one substrate and a column pattern on the other (see Photo 1 and Figure 1). The spacing between the rows and columns varies according to the intended application, but the spatial resolution can be as small as 0.02 in. (~0.5 mm).
The construction of the sensor is completed by adhering one sheet on top of the other. This yields a sandwich that is 0.004 in. (0.1 mm) thick. The resulting grid pattern has a sensing cell, or sensel, at each intersection. The sensels can be spaced as close together as 0.020 in. (0.50 mm)
The resistance of each sensel is inversely proportional to applied surface pressure. As force is applied to the sandwich, 8-bit electronics scan and measure the change in resistance from each sensel to determine the magnitude, location, distribution, and timing of the pressure exerted. Each sensel is
The sensing system is controlled by a PC running Windows-based software. The sensor is read sequentially by applying a known voltage to one of the rows and measuring current-to-ground on one of the columns. The microprocessor selects the row and column to be read by identifying the proper address for each sensel. The software and electronics work together to linearize the sensor's output.
The standard I-Scan sensor configuration's sampling rate is 250,000 sensels per second, and Tekscan has custom-manufactured systems that sample 50,000,000 sensels per second. This capability provides static or dynamic information on the temporal development of load profiles, peak load attainment, and the relaxation characteristics of materials.
The I-Scan system has often been used by automakers to measure clamping forces and sealing pressures and analyze engine gaskets (see Screen 1 and Photo 3). The 2D and 3D images depict the dynamic pressure changes of a center fire ring seal during assembly, simulated combustion, and relaxation. Data were recorded while test conditions were changed at certain stages. Once testing was completed, a pressure
In another application, I-Scan was used to electronically balance and measure the real-time force and pressure distribution at the contact area of pinch rollers during machine setup. Measurements were taken from two 1 ft wide sensors, placed at either end of the same 5 foot long roller (see Screen 2, below). The 2D images show the pressure distribution within the roller nip and permit accurate and fast adjustment of the contact pressure and area. Such a tool helps reduce machine setup time and minimizes product scrap by enabling dynamic adjustment of roller nip pressures for end-to-end alignment.
Other successful applications include evaluating the effects of high-speed impact; graphing the distribution of body pressure for use in comfort analysis of car and furniture seats; studying the pressure distribution between caster wheels and flooring materials; and optimizing the setup of chemical-mechanical polishing machines used in the manufacture of silicon wafers.Single-Element Model
The ELF (Economical Load and Force) measurement system combines a single-element Tekscan sensor called FlexiForce B101 with closely matched electronics and Windows-based software. Using the ELF system, the
The FlexiForce sensor (see Photo 4) is a flexible printed circuit 0.005 in. (0.13 mm) thick, 0.55 in. (14 mm) wide, and 8.25 in. (210 mm) long. Construction is similar to that of the I-Scan sensor, but the active sensing area is a single 0.375 in. (9.5 mm) dia. circle at the end of the sensor. A silver trace extends from the sensing area to the connectors at the other end of the sensor, forming the conductive leads. FlexiForce A101 sensors are terminated with a 3-pin Berg Clincher connector that allows them to be incorporated into a circuit. The two outer pins of the connector are active and the center pin is inactive. FlexiForce B101 sensors are available without the Berg connector and are used exclusively by the ELF system. The sensors can be mounted on plastic or metal substrates for increased stiffness or added protection from abrasion, and can be custom designed for specific applications, including experimentation and real-world measurements.
FlexiForce sensors are designed for OEM engineers and designers as well as the education, consumer, and testing markets. The sensors are in current use as tactile force probes; variable force controls for computer game joysticks; presence sensors; and measurement devices for the high-speed forces created by explosions and crash tests.
Charles F. Malacaria is Director of Sales & Marketing, Tekscan, Inc., 307 W. First St., South Boston, MA 02127-1342; 800-248-3669, 617-464-4500, fax 617-464-4266.