Visibility Sensor Model JVS-1200A
Functional Description

The Visibility Sensor emits eye-safe, invisible infrared pulses at preprogrammed intervals when it is in operation. An optical detector on the sensor measures the fraction of emitted light which is scattered back into the sensor field of view. A train of backscatter signals is processed by the sensor processor and an alarm is sent when persistent hazardous driving conditions are detected. The criteria is determined by the number and amplitude threshold of the backscatter pulses, which sets a poor visibility alarm. Both of these variables are programmable. Once the poor visibility alarm is set, it stays set until visibility improves beyond a programmable threshold and time period. This avoids repetitious or frequent alarms when visibility changes rapidly over time. The measurement of the backscattered light can be related to visibility regardless of whether the poor visibility is the result of fog, smog, dust, rain, snow, or a combination of these. The volume sampled by the sensor for visibility is in excess of 5 cubic meters. The dynamic range of the instrument is designed to cover visibility ranges of interest to highway applications (approximately 50 to 2,000 feet). A one-time calibration procedure in clear air compensates for objects in the field of view after installation. The calibration is stable over temperature and power input variations.

Software

All necessary operating software is delivered with the Visibility Sensor. This software includes two different modes of operation. The first mode provides complete autonomous/standalone operation. The Sensor powers up in this mode and runs self-contained software algorithms for detection of low visibility conditions. Communication is via TTL status flags.

A second mode of operation is provided which places the Sensor in Command mode. In this mode the Sensor provides two-way communication via the RS-232 interface with a diagnostic and maintenance program running on a personal computer under DOS. This mode of operation can be used for direct communication with the Visibility Sensor for the purpose of local and/or remote data collection, installation, maintenance, and troubleshooting functions.

Test and Diagnostic Capabilities

The Sensor continuously monitors operating parameters such as electronic health, temperature, and power supply voltage. Should any of these parameters fall outside default values the Sensor responds in three ways:

1. A TTL status alarm is set to zero volts (low true).
   
2. The Status Field in the logging software reports FAIL.
   
3. The Sensor goes into "idle" mode. In this mode the Sensor does nothing for a programmable length of time (factory default is 15 minutes). At the end of that time the Sensor will "wake up" and recheck parameters. If the fault condition has been removed, the Sensor will return to normal operation without intervention. If the fault condition remains, the Sensor will return to "idle" mode for the programmed time, as before.

The Visibility Sensor is completely autonomous with regard to calibration. Threshold levels are preprogrammed into the microprocessor's memory with a simple software procedure to be performed by installation or maintenance personnel. This procedure consists of selecting the Calibration Command from the Command Menu of the maintenance software.