Table of Contents

Photo 1. In a Tekscan 5051 matrix-based sensor, the sensitive region appears as the grid lines on one end of the photo. At the opposite (or dotted) end is the sensor's connector or tab end that is inserted into the Tekscan A/D converter, or "handle," that optimizes the sensor's performance.
A Thin, Flexible,
Pressure Sensor

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)
Figure 1. The silver conductive leads of a Tekscan sensor are oriented on two flexible polyester sheets in a row pattern and a column pattern. A proprietary presure-sensitive ink is applied over these conductors, also in rows and columns. When one sheet is placed on top of the other, these orthogonal lines form a grid with sensing elements at the intersections. The electrical resistance of these elements changes in proportion to the applied normal force.
or as far apart as desired, depending on the application. The sensor is available in a variety of shapes and sizes (see Photo 2, below), and can handle pressure ranges as low as 02 psi (014 kPa) or as high as 025,000 psi (0175 MPa). The sensor's extreme thinness and high spatial resolution make it minimally intrusive and a good choice for measuring dynamic pressure distribution between mating surfaces.

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
Photo 2. The I-Scan system consists of matrix-based sensors of various shapes, sizes, resolutions, and pressure ranges; an 8-bit A/D converter that connects to the sensor; a specially designed serial, parallel, or ISA (and soon to be released PCI) interface card; and Windows-based software. The software enables the user to view in real time, record, play back, and analyze the tactile pressure distribution measurements taken with the sensors.
actually a variable resistor whose value is high when no force is applied to it.

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 software displays force in real time on the computer screen in vibrantly colored 2D or 3D images. System controls are similar to those on a VCR. Tests can be recorded, stored, played back, and
magnify Screen 1. This screen shot from the I-Scan software shows the pressure pattern under the center fire ring of the head gasket. It also illustrates the sequence of pressure patterns that result from different clamp loads and engine pressurization settings. The pressure graph in the lower right of the screen summarizes the data taken during the test, but the dynamic sequence of the pressure images shows how a picture can really enhance the information derived from numerical test results and summaries.
graphically analyzed. The software allows multiple windows to be opened and force and pressure information to be viewed in user-defined focus areas. Data graphs and images can be printed for inclusion in reports. The system can also graph information in several ways, including force vs. time, pressure vs. time, peak pressure vs. time, and pressure profile vs. sensor length. The system can also export data in bitmap or ASCII format for postprocessing.

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
Photo 3. A sensor was custom designed to help analyze the cause of an automotive engine block and head sealing problem. The sensor was placed between the engine block and the head gasket as the engine was being assembled, thus instrumenting the deck face and three fire ring seals of the block with more than 18,000 force sensing elements. When connected to the I-Scan system, the sensor allowed the user to "see" the changing pressure pattern between the mating surfaces of the assembly as the engine was built, pressurized, and run.
graph was created, providing the automaker with clamp and seal pressure distribution data used to optimize the engine gasket design and assembly specifications.

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.

magnify Screen 2. Viewing the nip pressure images taken from both ends of a roller makes it easy to see the difference in roller nip pressures and area before adjustment. Notice how much wider the roller contact area is in the measurement from one end of the roller (top screen) than it is from the other end (bottom screen).

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
Photo 4. Extremely thin and flexible, the single-element FlexiForce sensors have numerous test, measurement, and OEM applications. The sensors are available as part of the ELF (Economical Load and Force) measurement system or independently as variable resistive force-sensing elements for use with the user's own circuitry.
FlexiForce B101 sensors can be calibrated and their sensitivity adjusted to match the application. Sensitivity adjustment optimizes the sensor's dynamic range for the particular application; calibration translates the system's digital output into engineering units.

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.

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