"A quartz sensor based Weigh-In-Motion System with Doppler radar provides Accurate vehicle weights."

Overview

Transport Data Systems has integrated a Weigh-In-Motion (WIM) capability into its AVC product line. The AVC add-on uses quartz sensors from Kistler to provide the raw pressure inputs from the tire crossings. This information coupled with accurate velocity data from the Doppler radar delivers accurate wheel by wheel weight measurements.

Weigh-In-Motion System Block Diagram


The TDS weigh-in-motion system uses a combination of two Kistler quartz detectors and a Doppler radar to calculate the weight of a vehicle. The system is designed to be added to the TDS automatic vehicle classification system but could be implemented on a stand-alone basis. It is particularly well suited to pre-classification systems as applied to smart card tolling systems. Since the sensor also provide the axle data, a separate axle detector is not required. The AVC system can be delivered in a number of different configurations using a light curtains or an overhead scanner to do the vehicle profiling. Dual tire detection is also an optional add on to the system. TDS also offers a low cost alternative that does not provide any profiling and uses a loop for vehicle separation. Characteristics of the various models are shown in the table below. Brochures and technical descriptons are available in the documentation section at the bottom of this web page.


Model Number Separator Length Width Profile Height Profile Tire Group Weight Dual Tire Detect
310 10 Foot Light Curtain Yes No Yes Yes No
311 10 Foot Light Curtain Yes No Yes Yes Yes
320 Laser Overhead Scanner Yes Yes Yes Yes No
321 Laser Overhead Scanner Yes Yes Yes Yes Yes
330 Loop Yes No No Yes No
340 5 Foot Light Curtain Yes No No Yes No
341 5 Foot Light Curtain Yes No No Yes Yes

Principles of Operation - Weigh in Motion

The Kistler quartz sensor provides a measure of the total pressure that is applied to the sensor during the period that the wheel is actually on the sensor. To determine the actual weight, the unit requires the speed of the vehicle. Classically system designers have included multiple quartz sensors to determine the velocity of the vehicle and loops to provide vehicle separation. Depending on the system design, the accuracy of the velocity can vary considerably. Since the actual weight calculation is directly dependent on velocity, this error translates directly in an error in the weight measurement. Furthermore loops are notoriously noisy and can result in incorrect vehicle separations.

The TDS AVC system uses a profiler and a Doppler radar to provide the ability to separate vehicles and classify them using axle locations along with length and a height profile. This provides a very accurate instantaneous velocity measurement 16 times a second. This velocity plus the quartz sensor outputs can provide accurate and reliable vehicle weight information. It also provides for excellent vehicle separation.

The quartz sensors can also fill the requirement for axle detection, thereby eliminating the need for additional axle detectors.

In order for the quartz sensors to operate properly, the vehicle can not stop on the sensors. Therefore it is imperative that the vehicles be metered through the measurement area. In order to accomplish this, a separate traffic light and/or gate is used to control the entry of the vehicle into the measurement zone. This technique not only guarantees a more accurate weight measurement but also increases the fundamental accuracy of the vehicle classifier.

With the addition of these sensors, the toll road operator can now add weight to the list of discriminants that can be used to calculate the toll. Small trucks carrying cargo can be differentiated easily from small trucks with no cargo and cars. Each class of truck can be classified based on total weight or on axle weight on the rear axles. This allows the toll road operator to assess tolls in such a way as to more accurately correlate with the vehicles that do the most damage to the roadway. Furthermore the toll road operator can choose not to allow overloaded vehicles onto the toll road in order to protect the road surface and substructure. As a result, expensive maintenance work is significantly reduced, traffic tailbacks are avoided and road safety is increased.

The Quartz Sensor

Kistler's LINEAS WIM Type 9195C is a force sensor with quartz elements. The sensor is a modular element that is installed into a slot that is saw-cut across an asphalt or concrete road. When a force is applied to the sensor surface, the quartz disks yield an electric charge proportional to the applied force through the piezoelectric effect. The electric charge is converted by a charge amplifier into a proportional voltage which can then be processed as required. Key characteristics include:

The sensors are available in 0.75 and 1.0 meter lengths. Two or more sensors can be connected together to make a longer single sensor strip. Several sensors that are installed adjacent to one another can be connected electrically in parallel and operated with a single charge amplifier. The output signal then corresponds to the sum of the forces acting simultaneously on all sensors connected.

Kistler-supplied sand/epoxy grout secures the sensor strips into the pavement slots. After curing, the hardened grout and the exposed top surfaces of the sensor modules are ground flush with the surrounding pavement using a belt sander. After grinding, the sensors may be immediately exposed to traffic. An overnight post cure is recommended before calibration and acceptance tests are performed.

Industrial Charge Amplifier

The industrial charge amplifier (Type 5038A2Y43) is a 2 channel amplifier. Each channel converts the charge output of the Lineas sensor to a proportional voltage. The adjusting potentiometers are designed as plug in units to avoid the need for recalibration when an amplifier is replaced. The unit requires an unregulated 15 to 20 volt DC supply. Its key features include:

Lane Controller Interface

The outputs of the sensor amplifier are brought to the lane controller over special cables capable of extending the distance from the sensor amplifier to over 80 feet. The signals are fed into a digital processing card which contains a 12 bit analog to digital converter.

 

BROCHURES - Weigh In Motion Systems

TECHNICAL DESCRIPTIONS - Weigh in Motion Systems

WIM with Lateral Profilers

WIM with Overhead Profilers

WIM without Profiling