There are two well-established approaches to measuring forces: quartz piezoelectric sensor technology, and the strain gauge method. For dynamic force measurements (when the forces to be measured move or change very quickly), quartz piezoelectric technology shows unrivaled advantages. Weigh In Motion applications are a good example of this: vehicles passing over the sensors at speeds of 100 km/h or more generate very fast force impacts, and the sensor can be exposed to loads of several tonnes within a matter of milliseconds. This is just one of many reasons which prove that quartz piezoelectric technology offers far greater benefits than the strain gauge method. What are the precise reasons for preferring quartz piezoelectric technology over strain gauges in a WIM system?
Quartz piezoelectric technology: the solution of choice for Weigh In Motion (WIM) systems
Road installation depth
Quartz piezoelectric sensors for Weigh In Motion applications are less complex in design than strain gauges, so they are smaller and more compact. This reduces the effort needed to install them under the highway's surface: they can be installed in the road pavement at depths of as little as 55 mm (2.17 in), so the work can usually be completed in one or two days with minimal disruption to traffic. The dimensions of strain gauges, on the other hand, have to be increased in order to lower tensions – and, as the U.S. Department of Transportation notes in its "Weigh-In-Motion Pocket Guide": "The strain gauge strip sensor installation currently recommends 3-inch-deep channels (= 76 mm) cut in pavement for installation. This installation requirement limits the applicability of strain gauge strip sensors for thin pavement types." Kistler also offers a new service – Structural Road Analysis (SRA) – to help customers identify and qualify the most suitable locations for installing their WIM systems: Learn more here.
Durability and robustness
Quartz piezoelectric sensors have a proven standard life span of over five years. In the USA, where Kistler WIM technology is operated in 43 states, the first quartz PE sensors were installed in 2003 – and the system is still operating with the same sensors today! Lifetimes of ten or more years are not unusual, so there is ample experience to show that quartz piezoelectric sensors offer our customers excellent value for money. They also feature overload protection. The structure of one single sensor can withstand axle loads of 30 tonnes or even more for short periods. By contrast, the yield point/elastic limit of the metals (such as constantan) used in strain gauges cannot be extended to infinity, and this is one of several reasons why they are affected by fatigue and their lifetimes are limited.
Wide measuring range
Probably the best-known advantage of quartz piezoelectric sensor technology is its wide measuring range. In other words, the consistent sensitivity of quartz piezoelectric technology means that trucks, cars and even bikes passing the WIM site will have their weight measured and recorded with equal accuracy. Many piezoelectric materials feature a high modulus of elasticity; quartz PE sensing elements show almost zero deflection, and the sensors benefit from extremely high natural frequency and excellent linearity over a wide amplitude range. Quartz piezoelectric technology is also insensitive to electromagnetic fields and radiation. Strain gauges can only achieve comparable measuring ranges and performance if a much more complicated design is implemented – so load cells of this type are heavy and difficult to handle. And although strain gauges perform well at the maximum end of the weight scale, their efficiency decreases sharply for lighter vehicles. See: Lineas quartz sensor data sheet.
Fully controlled production process
As a leading provider of quartz piezoelectric measurement technology, we are in full control of the entire manufacturing process at our Swiss headquarters. Disk-shaped crystals, the same size as a contact lens, are installed over the whole length of the sensor – and Kistler even operates its own facility for growing crystals according to a proprietary formula. Cutting-edge production equipment operated by highly qualified specialists ensures that potential fault sources are excluded at the earliest possible stage; in-depth final inspections include a tightness check and various function tests; uniform sensitivity throughout the sensor’s entire length is verified on the calibration system. Over 50,000 Lineas sensors for Weigh In Motion (WIM) have already been manufactured in-house and shipped from our plant in Winterthur, Switzerland. Systems based on Lineas sensors have regularly set global benchmarks for accuracy, reliability, lifetime, and ease of use. Learn more about our sensor production.
Global support and service
Kistler offers a portfolio of added-value services to enhance the usage and lifetime of WIM systems based on quartz piezoelectric technology. Our aim: a carefree user experience throughout the lifetime of every WIM installation. We provide assistance to determine the best location for the system installation, as well as training and certification so customers' own staff can execute high-quality installations. Our services also include calibration and optimization to ensure maximum accuracy, and warranty extensions of up to five years to guarantee that the system functions reliably for minimal cost of ownership. Quartz piezoelectric installations in over 50 countries on all continents prove that we can provide local support for system integrators authorized by governments or road operators. With a presence in over 60 countries, Kistler is excellently positioned to provide on-site installation support and additional services for Weigh In Motion across the globe. Discover our WIM services.