Passive and active roles

Both types of sensor carry benefits, but may be most effective together

Active Sensor Article December 30, 2003
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Should we send the salt trucks out tonight? For those of us
who are responsible for maintaining safe roads while minimizing costs, this
question is one  we often ask.

Knowing what is happening at the pavement surface is
essential to making the best decision. Today's most comprehensive
pavement surface information is provided by a large number of roadway weather
information systems (RWIS) and their embedded pavement sensors. The collected
data enables users to make timely and informed decisions regarding road
treatment timing and strategy. From a historical standpoint, the collected
pavement data provides a solid base for optimizing future treatment programs.

The most important benefit of the pavement sensor is its
ability to gather data remotely so it is not necessary to be everywhere during
hazardous weather. A second benefit is to obtain non-visual information we
cannot obtain by driving to the specific location, such as pavement temperature
and the freeze point of water/chemical solution on the road.

Although several different sensors are available, all are
based on one of two basic pavement sensor technologies: passive or active. The
basic principles of the two technologies are very different. Passive sensors do
not alter the state of the substance on the pavement. However, the active
pavement sensor uses cooling and heating to alter the state of the material it
is sensing. The following sections provide an introduction to each sensor
technology and its value to our roads and bridges.

Passive pavement sensor

The passive pavement sensor is a single solid-state sensor
constructed of materials with thermal characteristics similar to commonly used
pavement materials. The color and surface texture of the sensor will often
match that of the surrounding pavement to allow for comparable heat absorption
and similar liquid flow. The sensor head is installed with an epoxy material
that bonds with the pavement surface and sensor head to provide a waterproof
installation. The top of the sensor is installed flush with the surrounding
pavement surface. Climatic conditions, ice control chemicals or vehicle impacts
do not affect the sensor. The size of the sensor may vary, but a typical sensor
head is 5.25 in. in diam. and 1.75 in. high.

To measure pavement temperature, a small thermistor is
located near the surface of the sensor head. Electronics inside the sensor head
provide the intelligence and physical connection to surface detection points by
way of four graphite pins in a small liquid collection well and four graphite
pins on the top surface of the sensor. (The location, material and number of
exposed detection points on the sensor's surface may vary from sensor to
sensor). Power to and communication with the sensor head is accomplished
through a wired connection to a field processor known as the remote processing
unit (RPU).

The passive pavement sensor uses conductance and capacitance
along with atmospheric data from air temperature, relative humidity and
precipitation sensors to determine pavement condition and other important
pavement data. Chemical analysis is achieved based on the principle that a
given chemical solution has a corresponding conductivity. With the RPU
calibrated for a single known chemical, it is possible to convert a
conductivity measurement from the surface pins into information about chemical
concentration, freeze point and ice percentage. It's important to
emphasize that each RPU will support only one type of chemical at a time, and
the chemical algorithm will be applied to all sensors connected to that RPU.
Chemical algorithms exist for most common roadway chemicals.

Data points and conditions available to the user may vary,
but in general the passive pavement sensor will report some or all of the
following pavement information:

* Pavement surface temperature;

* Dry pavement;

* Wet pavement;

* Chemical wet (below 0°C [32°F]) with enough
chemical to keep the moisture from freezing);

* Snow/ice warning (below 0°C [32°F]) with
insufficient chemical to keep the moisture from freezing);

* Snow/ice watch (moisture not associated with
precipitation at or below 0°C [32°F]);

* Freezing-point temperature of the
moisture/ice-control chemical solution present on the surface of the pavement
sensor;

* Depth of the moisture/ice-control chemical solution
present on the surface of the pavement sensor from a depth of 0.01 in. to 0.50
in.; and

* Percentage of ice particles present in the
moisture/ice-control chemical solution resident on the surface of the pavement
sensor.

The typical operational limits for the passive pavement
sensor include a temperature range of -60°F to 176°F and a wired
connection length between the sensor head and the RPU of approximately 3,000
ft.

Active pavement sensor

At the core of the active sensor is a two-sided
thermoelectric element (Peltier element). This small pill-shaped element is
mounted in the top of the sensor. The top of the element is exposed in the
bottom of a small collection well at the surface of the sensor. The material
used for the sensor housing is similar to that of the passive sensor. A wired
connection provides power and a communication link between the thermoelectric
element and a processor board located in the RPU. Field installation of the active
sensor head is identical to that of the passive sensor. The size of the active
sensor may vary, but a typical sensor head will be 1.5 in. in diam. and style="mso-spacerun: yes">  less than 2 in. high.

The active sensor is able to measure the freeze point of
water or a chemical solution because of a common but interesting
characteristic. When the liquid is cooled, the temperature drops below its
natural freeze temperature. This behavior is called super-cooling. Then,
suddenly, a small amount of material freezes to form ice. This quick formation
of ice raises the temperature to exactly the freeze point. This temperature,
following the dip, is the reported freeze point.

The Peltier thermoelectric element can be used to heat as
well as cool the solution on the pavement. The actual heating and cooling of a
substance is achieved by continuously reversing the current fed through the
thermoelectric element. Depending on the direction of the current, the top and
bottom of the element will either be heating or cooling. The active sensor goes
through several heating and cooling cycles before reporting the detected freeze
point of a substance.

The active sensor delivers a freeze-point temperature and a
moisture presence indication when in its range of operation. With its active
element, the sensor provides an accurate freeze-point temperature of any
substance found on its surface, without regard to the specific chemical
mixture.

The typical operational limits for the active pavement
sensor include a temperature range of -4°F to 32°F and a wired connection
length between the sensor head and the RPU of approximately 2,000 ft.

It takes two?

RWIS systems and their passive and active pavement sensors
can be found in many locations across North America. The most common areas of
deployment for both sensor types include bridges, highways, city streets,
intersections, ramp structures, parking areas and airport runways. Typically,
each location has three or four pavement sensors strategically placed in areas
providing a clear indication of the general pavement condition in the
surrounding vicinity.

The decision to deploy a passive or active technology is
highly dependent on the local application. The passive sensor provides accurate
pavement temperature and an extended temperature range of operation for all-year
applications, but is limited to the detection of one type of chemical at a
time. The active sensor depends on an external pavement temperature reading and
is limited in its operating range, but is capable of detecting an accurate
freeze point independent of the chemical mixture on the pavement.

Despite the fact the two technologies were developed with
the same purpose in mind, the passive and active sensors are emerging as
complementary devices. Even though each technology provides significant value when
deployed by itself, the installation of the two technologies in tandem is
becoming a popular approach because together they provide maximum flexibility
in determining the pavement conditions. This complete pavement information
enables the safest pavement for the least cost.

About the author: 
Christiansen is director of project/business development for Surface Systems Inc., a Quixote Co., St. Louis.
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