Strip and bar WIM Sensors
Strips and bars WIM sensors are installed in grooves made in the upper pavement layer and fixed with epoxy resins. Both the manufacturing and installation cost are reduced compared to the scales. The typical dimensions of a bar are: 3 to 4 cm (width and height) and the length vary between 0.75 to 1 m for a half lane (wheel weighing) or 3 to 4 m for a whole lane (axle weighing). They are mounted in grooves of 6 to 8 cm in depth and width. A strip is flat (below 2 cm in height) and 3 to 5 cm in width. Both strips and bars contain sensitive elements, capacitive, piezoelectric, strain gauges or fiber optics sensors, which measure pressure or strains variations while a wheel or an axle is passing on them. Strips and bars are mainly used for HS-WIM.
The most common technologies available on the market are:
- Capacitive strips consisting of 2 or more conducting plates separated by layers of di-electric material creating an electrical capacitor. A force applied to the bar will compress the di-electric material, decrease the distance between the plates and increase its capacitance. Capacitive sensors bars were mainly used before the year 2000;
- Piezo-polymer strips consist of a flat coaxial cable with a thick brass outer sheet, with a piezo-electric film spiral-wrapped around a silver plated copper wire. A force applied to the strip will result in a signal between the core and sheath of the sensor. These sensors are relatively thin and flexible and are less expensive technology, but are also less accurate and more sensitive to the temperature variations;
- Piezo-ceramic bars consist of a coaxial cable with the sheath made of a copper tube, which contains a polarised piezo-electric ceramic powder with a copper wire running through the centre. A force applied to the cable will result in a signal between the core and sheath of the sensor. The sensors offer an intermediate solution, both in accuracy and in cost;
- Piezo-quartz bars consist of a row of quartz discs mounted in an aluminium profile. When a load is submitted to the sensor, a charge is generated that is proportional to the load. Piezo-quartz strip sensors have a low sensitivity to temperature variations. The sensors have good accuracy at an above average cost. Subsurface piezo-quartz bars are installed below the road surface but are less accurate although they have a longer life span.
- Strain-gauge bars sensors consist of several strain gauges that measure the deformation (strains) of the bars as a result of the load on the sensor. The resistance of a strain gauge will change when it is deformed in a certain direction. Strain gauges have temperature compensation performed as part of the sensor. These sensors have good accuracy and average cost;
- Fibre optic cables or strips measure the force acting on the cable by measuring the changes in the characteristics of the light beam. This may be based on different principles: phase shift or changes in the polarisation of light, the spectral characteristics of the optical path under a load or changes in the optical path intensity (amplitude). Until recently fibre optic sensors suffered problems with durability and temperature compensation.