Pressure mapping system qualifies truck tire contact pressures on road surfaces.

Until recently, precise measurement of contact pressures (particularly those in lower ranges or speeds) in real time was difficult to accomplish. A new electronic sensor system using capacitance can now accurately measure and capture low tactile pressure information relevant to tire width pressure and road damage. The heart of the system is a thin, durable, flexible, high resolution and highly reusable electronic sensor element which is adaptable to the application being measured. The element contains sensing points or pixels capable of being as close as .04" (1 mm) apart and can capture data readings at up to 60,000 times per seconds. Pressure ranges can be selected for optimum viewing, with choice of printable screen displays in color or black and white, as well as 2D, 3D or histogramic formats for measurement. Therefore, the sensor is enormously helpful in monitoring precise force disbursement between truck tires and pavement or road surfaces. A method now exists to verify proper tire contact pressure and overload criteria for steering axle tires and four tired axles. Knowledge of tire contact pressure can also help plan for or prevent road wear damage, as well as assist updating of overload restriction criteria.

Precise measurement of low pressures between any two contacting surfaces as the event occurred was, until recently, difficult to accomplish. Traditional load cells or strain gauges are either unable to reach the desired pressure range, too bulky to accommodate the area being tested or too time intensive to be properly and efficiently used. Another difficulty was the tendency of capacitance sensor systems to provide false readings due to "noise" in the testing environment. With the development of real time capacitance tactile pressure sensor systems (CTPSS), evaluation of low contact pressures with almost instantaneous analysis of information useful in assessing potential road wear conditions, as well as approving entrance of trucks on state road nets, is now possible. The specific CTPSS which is discussed in this article is known as Tactilus (ref. 1). Using the particular CTPSS mentioned here, an engineer or technician can not only capture very low contact pressure data, but also confirm the predictions generated by the finite element analysis (FEA) component of the system. This feature of the CTPSS may assist with determining possible road damage problems caused by tire overload. Engineers and technicians no longer need sustain the iterative process of trial and error to obtain product optimization or the inability, even where low contact pressure information may be recorded, to efficiently and effectively obtain both pressure distribution and pressure magnitude measurements. Also, the problem of environmental "noise" has been overcome, allowing for confidence in obtaining precise information.

Relevance to tire contact pressure and road wear

The state of Washington has a criterion concerning maximum allowable pressure of tire width for steer axle tires and other axles for trucks. This criterion, shared by other states, is a force equivalent to 600 lbs. per inch of tire width for steer axles and 500 lbs. per inch of tire width for the other axles. The tire measurement simply is the overall size at the hub, such as 11". The 11" size, common for duals, actually has 8" of tread width. Thus, the tire width dimension used is not the one that directly describes tire contact pressure. While the criterion is not particularly accurate, the tire contact pressure parameter is the one directly relevant to road damage.

Measuring the load using the dimensions provided by the manufacturer can be highly empirical. It also does not give credit to the tire variation in contact area. A direct measurement system such as CTPSS would eliminate the variation in measurement (figure 1). Trucking companies would like to reduce front or steer tire sizes. A claim of more economical operation if front tires could be capped and reused on the rear axles has been made, leading to proposals for legislation to reduce front tire size. However, such a reduction in front tire size would markedly increase tire contact pressure.

[FIGURE 1 OMITTED]

Concerns over proposed legislation from the trucking industry, an immediate supervisor at WSDOT, as well as a colleague, prompted a search on the Internet for relevant systems allowing for such road wear damage testing in a dynamic fashion. The Tactilus CTPSS system seemed to be the only one which could do the job. With the specific project, both pressure distribution and magnitude needed to be assessed and were important. Pressure magnitude of overloaded tires can cause local damage and pressure distribution described the possible extent of such damage or wear. In using a load per unit width approach to the load restrictions, the information gained was only one-dimensional. A system using two or more dimensions would be more accurate and enable more accurate control. Ease of use and convenience made the CTPSS very attractive. The system clearly has relevance for research and development in the tire industry, in the tire production process and in testing for potential road wear.

Specifications of CTPSS

The CTPSS technology known as Tactilus offers, for the first time, the ability to capture low to medium pressure ranges (0.1 psi to 2,000 psi or .007 kg/[cm.sup.2] to 141 kg/[cm.sup.2]) efficiently, quickly and accurately. The system consists of a pressure mapping pad, a data collection controller, interface hub, power supply, interface cables and software. The 11 mils or .28 mm thin sensing element "skin" or pad is the heart of the system. The internal composition of this "skin" is very flexible, enabling placement in invasive intolerant environments and adaptation over curved surfaces. The "skin" itself is very durable, being utilizable for several hundred times or more before replacement is necessary. The "skin" is available in a minimum pad size of 1" x 1" (2.5 cm x 2.5 cm), customizable up to the maximum pad size of 32" x 80" (81 cm x 200 cm); the maximum temperature range for the "skin" is 165[degrees]F (73.88[degrees]C). With a query rate of upwards of 60,000 sensors points/second, information recording rate for interpretation is approximately [+ or -] 10%.

To successfully enable the software and hardware comprising the system, a 266 Mhz Pentium or faster computer or laptop with a serial port, Win 95 (or higher) OS and 64 MB RAM is required. The ease of installation and set-up makes it possible for the engineer or technician to use the system with minimal training. The entire system carries a warranty of one year parts and labor for the interface unit, with six months parts and labor for the pressure sensing pad or "skin." A valuable aspect of the system is said to be the ability to connect two sensing element pads to the master system, allowing the mapping of two applications simultaneously.

Another key aspect of this particular CTPSS is customization of the sensor element to the needs of the particular application or test under consideration.

Use and interpretation of Tactilus CTPSS

Unlike many other sensor systems which capture specific pressures within specific ranges at given moments in time (almost like taking a Polaroid), CTPSS records and interprets contact force in real time. As noted previously, the system is built around the durable, flexible and highly accurate pressure sensing pad. (This pad is so durable and rugged it can be crumpled like a piece of paper, smoothed out and still provide accurate compression mapping.) Pressure sensing is available in various sizes and detachable from the electronics, making this particular CTPSS attractive for environments where the testing area is either inhibitive of normal procedures to record/interpret the test information (temperature, humidity) or where space is at a premium. Within each specific pressure level, calculation of contact force occurrences is performed automatically, thereby enabling recording of average pressure information over a specific time interval from each sensing pixel. This information thus mapped by the sensing grid pattern can then be played back to review the entire scope of the data. Of special interest is the ability to display up to four images simultaneously for comparison. Finally, the mapped image in 2D, 3D or histogramic formats for dynamic measurement, along with numerical data and comments, can be printed in either color or black and white (figure 2).

[FIGURE 2 OMITTED]

There is literally no waiting /'or this particular CTPSS system to analyze and interpret the compressive load data captured by the sensing element. Other sensor systems usually require a set amount of time to capture the compressive load information and then another specific period of time to analyze/interpret the data. The speed of data collection is the real breakthrough in the efficient gathering of force information in low pressure environments. The system's ability to ignore environmental "noise" does contribute markedly to the successful capture of compression readings.

If a need exists for the engineer to display pressure images in variable viewpoints, the CTPSS allows rotation of the data in 90[degrees] increments for the most intuitive orientation. Similarly, pressure ranges can be selected to provide optimum viewing, regardless of weight. Contact force measurement data are able to be displayed via a mapping image with numerical data, color bar and comments. Images themselves are importable as bitmaps to other software documentation; the data itself may be exported in formats compatible with most spreadsheet software such as Excel, Lotus and dBase.

Conclusion

The CTPSS system briefly outlined here has enormous potential for revolutionizing how low to medium contact pressure information is both captured and interpreted. Employing this technique allows for the scientific and systematic collection and analysis of data in very low pressure ranges that was, until very recently, unattainable. Incorporating this system into an overall program of calibration of tire contact pressure criterion will improve one's ability to not only qualify truck tires at state weigh stations, but also alert road and bridge engineers to the compressive forces which, over time, will damage road and bridge surfaces. The use of this CTPSS system with its information capture and analysis abilities lends a very high degree of statistical control and qualification to this aspect of a vigorous QC program. The time saved in understanding the contact pressures at work in determining road wear will enable better understanding among trucking companies, as well as transportation engineers, of the factors involved in not only road and bridge construction, but also in forecasting when major maintenance or overhaul of such structures will be mandated. The CTPSS system herein described can markedly assist in prediction of road surface life, as well as guiding development of more resilient, economical methods of road construction.

Reference

(1.) Tactilus is a registered trademark of Sensor Products, Inc., E. Hanover, NJ.