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Clear as Crystal
Quartz: Leading Technology in Weigh-In-Motion
David Cornu, Kistler Instrumente AG
Special print from Traffic Technology International,Aug/Sept 2007,UK International Press
Special print920-359e-06.13
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2013
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FranceKistler France91976 Les Ulis CedexTel. +33 1 69 18 81 [email protected]
GermanyKistler Instrumente GmbH73760 OstfildernTel. +49 711 34 07 0 [email protected]
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Kistler Instrumente AGExport SalesEulachstrasse 22, 8408 WinterthurSwitzerlandTel. +41 52 224 11 [email protected]
Headquarters
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www.kistler.com
TECHNOLOGYPROFILE
Clear as crystal
he sensing element used within the well-known Lineas weigh-in-motion sensor, manufactured by
Kistler, is made from quartz. Each Lineas sensor consists of 20 quartz discs mounted in a special aluminum profi le, which allows the very precise measurement of vertical forces, such as wheel loads (Figure 1). Quartz (SiO
2) is a piezoelectric material.
This means a charge is generated at the surface of the crystal when submitted to a mechanical load. This generated charge is exactly proportional to the load.
QUARTZ TECHNOLOGYAs one of the most common minerals found on earth (Figure 2), natural quartz has gradually been replaced by synthetic quartz in many sensing applications; it can be produced with perfect atomic structures and almost no defects (Figure 3 and 4). As a result of the linear response of quartz over a very wide measuring range, it is possible to accurately measure both the weight of a locomotive train (100 ton) and the weight of a coin (10g) using the same sensor.
Another outstanding feature of quartz is its stability. The properties (e.g. sensitivity) do not change during the whole lifetime, which means that the sensors do not need to be recalibrated over the years. And unlike many other piezoelectric materials, quartz is not pyroelectric, meaning that no electric signal is generated as a result of temperature changes. The result of this is that there are no errors due to changes in temperature. The high accuracy and stability of the Lineas sensor originates from the perfect atomic structure of the quartz crystal (Figure 4).
Such excellent properties are particularly benefi cial for weigh-in-motion applications, where the sensors can be used over many years, under a wide range of temperature variations (day/night, summer/winter, etc), in locations where it is not possible to regularly perform calibration and maintenance, and where reliable data is expected.
T
Those in the fi eld of WIM technology are only too aware that accuracy and stability are crucial factors. The case for quartz sensors – which offer an advantage in both of these areas – is therefore quite straightforward
by David Cornu, Kistler Instrumente AG, Switzerland
79TRAFFIC TECHNOLOGY INTERNATIONAL AUGUST/SEPTEMBER 2007
Figure 2: The picture shows quartz crystal in its raw state, as it is found in nature
Figure 3: Synthetic quartz crystals machined and cut into disc plates by Kistler
OTHER TECHNOLOGIESOther piezoelectric materials are used as sensing elements, such as piezoceramics and piezopolymers. Both of these materials are used in the so-called ‘piezo cables’ that are employed in WIM applications.
Unlike quartz, piezoceramics are not piezoelectric by nature. To show piezoelectric properties, the piezoceramics must be
Figure 1: The cross-section shows the design of the maintenance-free Lineas sensor. For installation, only a small intrusion in the road is performed (no frames or screws are required)
Load-bearing coat(can be ground)
Elastic material (foam)
Quartz (sensing element)
Special alloy profi le
Sand-epoxy grouting
75mm
55m
m
TECHNOLOGYPROFILE
Figure 4: Extraordinary properties due to the perfect atomic structure of quartz (SiO2)
to measure deformation (strain) of an object, such as bending. It typically consists of a foil with a metallic wire, which changes its resistance when deformed.
This technology is suitable for measuring static forces. For dynamic processes, though, it is less appropriate. The other main disadvantage is the limited measuring range. A strain gauge sensor designed for high loads cannot measure small forces. Furthermore, strain gauge technology is very sensitive to temperature changes. As it is also subject to ageing, the sensitivity changes over time and makes periodic calibration necessary.
MAKING SENSEThe sensing element is the heart of the sensor and should be selected carefully. It is diffi cult for errors generated by ‘bad’ sensing elements to be compensated for by electronics or software. Since 1959, Kistler has developed sensors to measure force, pressure and acceleration for all kinds of applications, mainly using quartz. Lineas is unique in the WIM fi eld, as it is the only sensor available with quartz. The quartz discs are integrated into a smartly designed construction and – with the appropriate electronics and software – provides the best results in terms of accuracy, stability and longevity.
The Lineas WIM sensor (Figure 5) is accepted as a technology leader and is being successfully used all over the world in applications such as traffi c data collection, overload detection, bridge protection, tolling, traffi c management duties and research. ■
To fi nd out more, please contact Kistler Instrumente AG by calling +41 52 224 11 11, emailing [email protected], or visiting www.kistler.com
poled after the sintering process. Ceramics are obtained by sintering a fi nely ground powder mixture, which has been previously compressed. Today, PZT (lead-zirconate-titanate) is the most common piezoceramic available on the market.
Advantages of PZT include its low manufacturing costs and its high sensitivity (useful when measuring very small forces). However, disadvantages predominate, including ageing (sensitivity decreases over time as a result of depolarization), huge pyroelectric effect (charge generation due to temperature change, which cannot be distinguished from a mechanical load), poor linearity (which means low measurement accuracy) and low electric resistance (only dynamic measurements possible).
These downsides lead to inaccurate measurements with serious errors that change with time and temperature. The use of ceramics is limited to applications where accuracy and reliability are not decisive. Unlike ceramics, quartz is not affected by these negative phenomena.
Piezopolymers are polymeric fi lms and are capable of converting heat or pressure into electricity. Unlike quartz or piezoceramics, they are fl exible and can be used on uneven structures. However, they show the same defi ciencies as piezoceramics.
Strain gauge technology belongs to the category of resistive principles. It is used in loadcells for general force applications and also for static and low-speed vehicle weighing. The strain gauge is a device used
PROTECTION FOR THE ROADWhen it comes to monitoring road traffi c loads, attention is now increasingly focused not only on gross vehicle weight, but also on axle and wheel load. Road authorities, therefore, have an appropriate interest in detecting vehicles with excessive axle loads.
Kistler’s Lineas WIM sensor is at home when measuring wheel and axle loads and for determining the gross vehicle weight under rolling conditions.
Figure 5: Lineas is a rugged and reliable sensor, distinguished by its accuracy, stability and long life
TRAFFIC TECHNOLOGY INTERNATIONAL AUGUST/SEPTEMBER 200780
