Scale WIM Sensors
WIM scales are instrumented plates mounted in a rigid frame fixed in the pavement structure, which measure the wheel/tire vertical forces of the running vehicles. The scale length in the traffic direction is more than a wheel/tire imprint (≈ 0.30 m), but less than the minimum axle spacing (0.80 m). The scale width is either below 1 m (wheel scale) or above 3 m (axle scale). Therefore it measures either the wheel or axle impact force at once.
The main technologies of WIM scales are:
- Load cell scales: a load cell is a transducer unit that converts a load or force acting on it into an electronic signal. The measurement principle may be based on different technologies: capacitive, hydraulic, piezo-electric, pneumatic, strain gauge or vibrating wire. In a WIM scale, one or more load cells are mounted between a rigid steel plate (on top) and the support frame (at the bottom). The load cell(s) measure the vertical forces transferred from the wheel/axle to the plate and then to the frame. They are used both for low speed (LS-)WIM and high speed (HS-)WIM conditions, mainly for enforcement and industrial purposes;
- Bending plates: the plate is simply supported by the frame at its edges, and instrumented with stain gauges, which measure the bend of the plate while a wheel or axle is crossing it. The bending strains are proportional to the vertical force if applied at the same location. Combining several strain gauges, the wheel transverse location may be estimated and the measured vertical force becomes almost independent on it. They are mainly used for HS-WIM and as axle weighing scales.
- Capacitive mats consisting of two or more conducting plates separated by layers of di-electric material creating an electrical capacitor. A force applied to the mat will compress the di-electric material, decreasing the distance between the plates and increase its capacitance. It is used both in fixed and portable installations and is mainly used for enforcement purposes.
- Hydraulic plates consisting of two steel plates with regularly-spaced elastically deformable tubular spring elements placed between the plates. The elements are filled with a hydraulic liquid and connected to a gauge measuring the changes in volume resulting from a deformation of the elements. A force applied to the plate will cause a deformation in the tubular spring elements that is linear to the weight on the plate. They are used for static and low speed WIM conditions, mainly for weight enforcement and industrial purposes.