Page 327 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

Examples of capacitive/inductance sensors used in the ROV industry are touchscreen monitors for human
machine interface (capacitive), label readers (capacitive), position indicators (capacitive or inductive), and proximity switches (capacitive or inductive). Stray capacitance accounts for most of the errors in capacitance measurement. Shielding the capacitance signal and grounding the shield can mitigate the effects of stray capacitance (which, if not guarded, can fluctuate between 10 pF and 10 nF). Further, locating the sensing electronics close to the sensor electrodes will minimize the effects of stray capacitance from local noise.
Capacitive and inductive sensors work best in a vacuum or in air but have limited use in a fluid due to the relatively high attenuation of electromagnetism through water.
12.2.5 Electromagnetic sensors
Electromagnetic sensors make use of perturbances in the electromagnetic field of sensor hardware in order to measure certain changes in the field. Danish physicist Hans Christian Ørsted first dis- covered the relationship between magnetism and electric current when he noticed that a current was induced into a coil when the magnetic field surrounding the coil changed. This opened a whole gamut of applications for this technology. The direct result of this discovery was the follow-on invention of the electric motor and a host of sensor technologies measuring the voltage/current induced by the motion through an electrical field.
The add-on principal enabling sensor development is best captured in Lenz’s Law, which posits that the combined effect of all of the electromagnetic interactions on a field is to resist the modifi- cation to the field.
Any number of usages for electromagnetic sensors can be found. Specifically, in the ROV industry you will find sensors with use of electromagnetic field measurement of:
• inductive proximity sensors (e.g., camera position indicator on a tilt motor such as the linear variable differential transformer (LVDT))
• low-frequency motion (e.g., a geophone for oil and gas seismic measurements)
• magnetism (e.g., magnetometer for sensing ferrous metals or the simple flux gate compass for
measuring vehicle orientation with reference to the earth’s magnetic field)
• tachometer for measuring thruster motor speed
The electromagnetic sensor field is quite in-depth and rapidly evolving. The reader would bene- fit from a study of this field from a basic text on the subject.
12.2.6 Flow/level
The flow sensor is the basic measure of fluid volume passage. As an add-on to the measurement of flow, level indication can be deduced from the measured flow. All fluids—whether air, water, metals, or some combination—flow. The technologies used to measure this flow vary with the char- acteristic being measured. There are different methods peculiar to use in gas as well as in liquid.
Flow rate is measured by the simple velocity measurement of fluid through a conductor (such as a pipe, aqueduct, or other fixed-volume structure) and then multiplying the fluid speed versus time through the fixed-volume conductor to provide a volume quantity.
12.2 Sensor categories 319