Page 316 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

308 CHAPTER 12 Sensor Theory
Galvanometer
FIGURE 12.6
A Wheatstone bridge circuit.
Current
capture/acquisition device). Also, capacitive measurement circuits are more adaptive to digital circuits due to their reactance to clock speeds of rapidly oscillating circuits.
12.1.4.3 Inductance measurement circuits
Inductance circuits and resistance circuits are very similar in measurement techniques. These cir- cuits typically measure voltage induced into reference coils located within the sensor. These types of sensors have limited usage due to their expense and bulk. An example of an inductance-type sen- sor used in an ROV environment is a cable tracker for sensing cable power frequency through the induced current through sensor coils from the cable’s AC power conductors (Figure 12.7). These types of inductance sensors are very effective at discriminating power frequency and are used in subsea power cable and pipeline tracking (when a reference “tone” (i.e., nominal or “reference” current passed through the cable/pipeline at some discrete frequency) placed on either the cable/ pipeline or nominal grid power is applied at 50/60 Hz grid frequency).
The types of sensors measuring each category of physical parameters vary greatly. Each type of sensor has its own set of characteristics. Each must be examined for its performance characteristics given the sensing range and sensor application. As an example, a list of differing sensor types for temperature measurement includes:
• Thermocouple
• Thermistor
• RTD
• Pyrometer
• Langmuir probe (for plasma electron temperature)
• IR
Known resistance (R1)
Unknown resistance (RU)
Known resistance (R2)
Variable resistance (RV)