Page 311 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0
V Output vs g Input
12.1 Theory 303
FIGURE 12.3
–5 –4 –3 –2 –1 0 1 2 3 4 5 g
Plot of actual versus theoretical sensor output for an accelerometer.
• Noise: All sensors produce output noise as a by-product of the signal generation process.
It is a product of the mechanism that has to be dealt with. The noise could be anything—local magnetic variations inducing noise in the vehicle’s electronics, noise in the electrical generation system, thruster or electrical motor inductance, etc. Noise is most problematic with instrumentation that must have high gain to develop a coherent signal. An example of such an instrument would be the thermocouple, which must scale from millivolts to volts—a theoretical gain of 1000. If any noise is present it could significantly distort the output of the instrument. The higher quality sensors, of course, produce the lowest amount of internally generated noise and/ or the highest signal to noise ratio.
• Resolution: The minimum detectable signal variation (or threshold) defines the sensor resolution. The floor of detection ability of the sensor varies with the technology used as well as the quality of the sensor. As an example, the thermocouple in a turbine jet engine measures a wide dynamic range of temperatures (e.g., 0
1000C), but the minimum resolution of the sensor may only be as low as 50C (low resolution) while a lab instrument measuring the freezing point of water may have a low dynamic range (,10C) but a high resolution of 0.00001C. Electrical noise can be a limiting factor to a sensor’s resolution. The resolution of the sensor must
also apply to the bandwidth of the sensor, which is not always the case. A high-resolution specification that is outside the bandwidth where the measurement will be made may not be useful.
Volts