It is extremely difficult to collect and analyse data on the fatigue history of high-speed railroad components such as rail, bridge members, and wheels using traditional data acquisition systems. Strain gauge systems typically require specialized signal processing devices, large power sources, and extensive wiring, which are generally not durable.
A Uni-Axial Strain Transducer (UAST) is a Micro-Electro Mechanical System (MEMS) with characteristics such as high resolution and high sampling rate, absolute encoding, no calibration requirements, no drift over time, and less measurement noise than analog-based strain sensors The UAST measures substrate strain by accurately measuring the displacement between two attachment pads. The distance between the pads is the gauge length for the device. When the substrate strains, displacement of one pad, relative to the pad on the other side of the package, causes the emitter, which is connected to the one pad through the moving flexture, to translate over the surface of the UAST IC chip. Strain is calculated by taking this measured transitional displacement and dividing it by the gauge length.
The UAST exploits the capacitive coupling between an array of electrostatic field emitters and an array of 64 field detectors on a UAST IC chip. The slightly different array element spacings form a vernier scale, and the system uses digital signal processing of the detector outputs to calculate the displacement of the emitter array relative to the detector array on a UAST IC chip. Displacements as low as 2.5 nm (10-9 m) can be resolved. Because UASTs have low DC power requirements, they can be used in remote locations.
The research goal was to develop a prototype Hybrid Uni-Axial Strain Transducer (Hybrid UAST) that includes non-volatile RAM to store strain cycling history (e.g., tracking how many times the UAST crosses each of specified strain thresholds across its dynamic range), and to temporarily store the preprocessed data. The objective of this research was to determine the potential of the Hybrid UAST as a new tool to continuously monitor, analyze, and store the strain history of components such as rail. The prototype Hybrid UAST consists of three parts: a UAST sensor, a networking controller box, and a communication cable. A load cycle counting algorithm is integrated into a microcontroller, which is programmable using configuration switches.
Laboratory tests using an aluminum beam equipped with UASTs and conventional foil strain gauges demonstrated the accuracy and repeatability of the UASTs. A series of cyclic loading tests was performed to simulate a moving trainload applied on a rail using an MTS fatigue loading machine in the laboratory. Overall, these laboratory test results indicated that the UAST is accurate and repeatable in a wide range of strain values from 0 to 2,000me. A prototype Hybrid UAST package suitable for field application was fabricated, and field data were collected. The Hybrid UAST data compared favorably with strain gage data. Results of both laboratory and field testing of the prototype system were encouraging with respect to its repeatability, accuracy, and viability.
An ideal technique for mounting the UAST on a rail was also developed by creating separate detachable mounting pads for the UAST. The proposed Hybrid UAST could be suitable for monitoring track and bridge structures at remote locations, and for estimating their remaining service life in the interest of maintenance planning. Additional potential application areas include a train presence detection device, wireless instrumented wheel sets, a portable weigh-in-motion device, and a device to predict buckling of the rail.
The final report for this IDEA project can be found at:
https://www.trb.org/studies/idea/finalreports/highspeedrail/HSR-15Final_Report.pdf.
