1 Weigh-in-Motion (WIM) Sensor Based on EM Resonant Measurements C. R. Liu 1 , L. Guo 2 , J. Li 3 , X. Chen 4 1 Dept. of Electrical and Computer Engineering, University of Houston, Houston, Texas, United States 2 Hardware Division, BakerHughes, Houston, Texas, United States 3 Resistivity Modeling, Halliborton Energy Services, Houston, Texas, United States 4 Electronics Technology, Texas Southern University, Houston, Texas, United States ABSTRACT — A novel microwave WIM sensor is presented in this paper. The microwave WIM sensor is an active sensor based on the perturbation theory of microwave resonant cavities. The microwave signal generated by a DDS-based synthesizer circuit is coupled into the sensor and the returned signal is measured to determine the load applied to the sensor. The microwave WIM sensor was designed, simulated and tested in the lab. The TE 11L cavity mode of the sensor was found to be the best choice for WIM applications. The sensor shows an excellent linearity, uniformity and measurement accuracy from the results of the lab test. Index Terms — weight measurement, microwave sensors, cavity resonators, perturbation methods. I. INTRODUCTION A WIM system is a dynamic scale which measures the weight of a passing vehicle. WIM sensor can overcome the limitations of static weighing scales. The high-speed WIM system can even conduct the weight measurement of a vehicle at speeds above 70 MPH which makes it possible to weigh vehicles without slowing down the traffic flow. Current WIM sensors include bending plate, piezo-electric ceramics, and load-cell and fiber optic sensors [1]. These sensors usually require special sensor holders and careful installations in order to obtain accurate measurement. The cost of these sensors is rather high due to the manufacturing and installation complexity and accuracy. One of the main requirements for WIM sensors is the length. A WIM sensor should at least cover half of a wheel path (longer than a meter). Microwave cavity is a natural candidate for WIM measurement. Microwave cavity is made of metal and can be easily installed inside pavements. The pressure on the wall of the cavity will slightly change the resonant frequency. By detecting the resonant frequency change, the impact applied to the cavity can be calculated. By choosing correct frequency range and resonant mode, a microwave WIM sensor can be made low cost, rugged and accurate. In this paper, we report our efforts in developing a microwave WIM sensor system and test results of the sensor in lab conditions. II. THEORY OF MICROWAVE WIM SENSOR The microwave WIM sensor was designed based on the microwave cavity theory. The sensor body was designed as a cylindrical microwave resonant cavity. There are many applications using the cavity in measurements, such as the measurement of electron densities in a shock tube and complex permittivity of dielectric materials [2]. When the pressure is applied to the outside wall of a resonant cavity, according to slight disturbance in cavity shape, the resonant frequency is changed correspondingly. The change of resonant frequency can be calculated using the following perturbation equation [3]: ( ) ( ) ∫ ∫ ⋅ ⋅ + ⋅ ⋅ ⋅ - ⋅ = + ∆ - ∆ = - ∆ 0 2 0 2 0 2 0 2 0 0 0 V V e m e m dV E H dV E H W W W W ε μ ε μ ω ω ω (1) 561 1-4244-0878-4/07/$20.00 ©2007 IEEE