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 Motivation for StudyTest Equipment Inductive Loop Detector Operating Theory Paper No Joint Transportation Research Program of the Indiana Department of Transportation and Purdue University 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. Inductive Loop Detector Test Bed Test Probe and Positioning Gantry Loop Wiring Diagram Example Field Data with Mitsubishi Galant traveling over 6x6 octagonal loop at selected lateral offsets Test Procedure 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 (  L/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. Test probe: 3x10 ft anodized aluminum / galvanized steel sheet 1. Identify appropriate cross sections, verify symmetry. 2. Move Probe along grid at 6-inch intervals 3. Record lateral cross section profile 4. Construct 3D map from cross sections US Patent 3,984,764 (1976), Figs. 1 and 2 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. Probe located at starting position Highest sensitivity occurs when driving over center of loop (when coupled vehicle-loop area is maximized) Direction of vehicle travel Electrical ModelPhysical Model Physical equation for loop sensitivity: Electrical model loop sensitivity definition: Loop sensitivity measurement: Mutual Inductance between vehicle and loop Self-inductance of the loop Self-inductance of the vehicle Inverse square relationship Measured points Implied points Vehicle traveling across various cross sections of loop Centerline Edge line 2 ft outside edge line

Three Dimensional Mapping of Inductive Loop Detector Sensitivity Using Field Measurement Quadrupole Long Rectangular 6-foot Octagonal 6-foot Round Vehicle Type Clearance (inches)  L/L Value 6 ft Round 6 ft × 6 ft Octagonal 20 ft Quadrupole 20 ft Rectangular Passenger Car (Mitsubishi Gallant) Pickup Truck (Chevrolet Silverado) Front End Loader > Axle Dump Truck > Bicycle (Loop Edge) On Pavement Bicycle (Loop Center) On Pavement ∆ L/L (percent) Reno AE Sensitivity Level EDI Sensitivity Level Sensitivity Levels Measured Loop Response to Actual Vehicles 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. Galvanized Steel Sheet at 24 inches from pavement