Communication system design for remote controlled near IR diode laser sensors -
based on a water vapor spectroscopy unit for a large wind tunnel facility

By Jörg Hof
October 1998
 

Cooperation of

University of Applied Sciences Cologne
Electrical and Electronic Engineering

Physical Sciences Inc. Andover
Aeropropulsion Technologies Group
 

Water vapor is a widespread substance in air at earth’s surface. It is easy to generate and using it does not cause any pollution. Therefore, it is used in a wide range of applications like steam engines or wind tunnels for airflow measurement. Some of these processes need to know the exact concentration of water vapor in the environment. Especially in wind tunnel applications the water vapor related dew point is used as an indicator for measurement procedures. Therefore single-point water density sensors may be required. Since process-control collects the information at certain points to interpret the data, the sensor signals must be transmitted to the control unit. These communication lengths vary depending on the process facility. Thus, for long distances a communication system is demanded to transmit the information remotely.
 This diploma thesis deals with the design and evaluation of the long distance communication links for remote operation of diode laser-based sensors for water vapor spectroscopy. It goes through the different steps of design and evaluation of the communication link. The sensor is introduced by a system overview including the physical background, system objectives and the system approach as well as the design engineering. This is followed by an explanation of the initial reason for the signal transmission.  The applied design process describes the attachment of  a communication link and develops the background for transmission-system features. This results in the system selection, the conducted engineering and the testing of the communication link. The testing includes a bench top test and a fully integrated sensor setup. Substantial to the sensor communication, a readout signal transmission is discussed, by mentioning the initial requirements, the design engineering, and testing.
 The conducted work is related to specification for a large US Air Force wind tunnel facility. It is part of a small business innovation research (SBIR) program for commercializing advanced research projects and has been conducted during the summer 1998 at Physical Sciences Inc. (PSI) Andover, MA, USA.

 Based upon the specification for the applied design process and the selected signal transmission system. The communication link for the data transmission for the sensor uses a phased locked loop approach. The transmission successfully demonstrated the capability to transmit the information over a desired length of 3000 ft within a dynamic range of 75,5 dB, an analog bandwidth of 0 to 48 kHz, and an signal input span of 0.9 to 9,8 V. These parameters had been proved in a bench top test series in a laboratory based spectroscopy measurement.
 Additional to the integrated sensor data communication the display information transmission has shown the capability to display the collected data within the required analog bandwidth of 10 Hz over a various transmission distance.