Three Dimensional Mapping of Inductive Loop Detector Sensitivity Using Field Measurement

1. Three Dimensional Mapping of Inductive Loop Detector Sensitivity Using Field Measurement C. M. Day, T. M. Brennan, M. L. Harding, H. Premachandra, A. Jacobs,D. M. Bullock, J.V. Krogmeier, and J. R. Sturdevant Paper No. 09-0018 Joint Transportation Research Program of the Indiana Department of Transportation and Purdue University Inductive Loop Detector Operating Theory Motivation for Study Test Equipment Test Probe and Positioning Gantry Electrical Model Physical Model Inductive Loop Detector Test Bed There is a wealth of literature on the electromagnetic modeling of inductive loop detectors, but no set of 3D sensitivity maps that can be used for design guidance. Electrical model loop sensitivity definition: Loop Wiring Diagram Mutual Inductance between vehicle and loop Test probe: 3x10 ft anodized aluminum / galvanized steel sheet Self-inductance of the loop Self-inductance of the vehicle US Patent 3,984,764 (1976), Figs. 1 and 2 Physical equation for loop sensitivity: Inverse square relationship The 1976 patent for quadrupole loops shows electric field strength, but this does not necessarily equate with sensitivity to vehicle presence at the specified position above the loop. Rather than point field strength, the response of the loop circuit to a vehicle is the key factor to understand how to design a loop to detect vehicles. Loop sensitivity measurement: Test Procedure Example Field Data with Mitsubishi Galant traveling over 6x6 octagonal loopat selected lateral offsets 1. Identify appropriate cross sections, verify symmetry. 3. Record lateral cross section profile Measured points Implied points Highest sensitivity occurs when driving over center of loop (when coupled vehicle-loop area is maximized) Research Approach • We developed a set of test equipment and procedures to create 3D sensitivity maps of alternative loop detectors. • A testing apparatus was constructed to approximate a vehicle undercarriage situated at a fixed height from the pavement • Data consisted of loop sensitivity measurements (DL/L response to the metal probe) at various positions offset from the loop center • 3D maps of loop sensitivity were created for different heights in 6 inch intervals • In this study, we assumed loops would be symmetric with regard to sensitivity across the geometric axes of symmetry. This was verified during data collection. Direction of vehicle travel 2. Move Probe along grid at 6-inch intervals 4. Construct 3D map from cross sections Centerline Edge line Probe located at starting position 2 ft outside edge line Vehicle traveling across various cross sections of loop

2. Three Dimensional Mapping ofInductive Loop Detector Sensitivity Using Field Measurement Sensitivity Levels Quadrupole Galvanized Steel Sheet at 24 inches from pavement Long Rectangular Measured Loop Response to Actual Vehicles Galvanized Steel Sheet at 24 inches from pavement 6-foot Octagonal Galvanized Steel Sheet at 24 inches from pavement Conclusions • Vehicles with high ground clearance present challenges for accurate detection because loop sensitivity decreases by the inverse square of the distance between the vehicle undercarriage and the loop face. Although heavy vehicles are more massive, smaller vehicles typically create a larger response because they are closer to the ground. • There was little difference in the loop response to the aluminum and steel probes. • Although sensitivity decreases with vertical distance from the pavement, the overall shape of the response remains the same. • 6-foot octagonal and 6-foot round loops had very similar response. • Quadrupole loops were less sensitive overall than rectangular loops of similar size. This was true for both the test probe, and several different types of large vehicles that were driven over the loops. This finding runs contrary to the claim that quadrupole loops are more sensitive for vehicle detection. • The quadrupole loop had lower sensitivity along its central axis, which was where sensitivity was expected to be greatest. • The field sensistivity maps (edges) show no evidence that quadrupole loops are less susceptible than rectangular loops to false calls from vehicles in adjacent lanes. 6-foot Round Galvanized Steel Sheet at 24 inches from pavement