Page 312 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

304 CHAPTER 12 Sensor Theory
• Bandwidth: Sensor bandwidth, sometimes referred to as “frequency response,” is the sensor’s ability to respond to instantaneous changes in the physical conditions being measured, i.e., how the sensor responds at different frequencies. The spread between the higher and the lower frequency limits of the frequency components (for significant amplitudes in the spectrum over which the gain remains reasonably constant) is called the “bandwidth” of the signal and is expressed in hertz.
The typical electronic sensor starts up with the following logical steps:
1. Power is applied to the sensor.
2. The firmware within the system reads the instructions to start up the sensor.
3. As part of the startup sequence, a sensor measurement is taken in analog format.
4. The reading is put into a digital format (normally in ASCII format in either a proprietary
protocol or an industry standard format—e.g., National Marine Electronics Association standard
NMEA 0183).
5. The output is placed into serial format (e.g., 8-N-1 or eight (8) data bits, no (N) parity bit, and
one (1) stop bit) at a specified rate (a default value of 4800 b/p/s for a typical GPS signal and transmitted down a standard communications line—e.g., RS-232 cabling).
As an example of a common industry sensor data transfer protocol, the NMEA 0183 standard
specifies a series of output “sentences” used by common marine sensors to output sensor data by one sensor (the “talker”) to a single or multiple receiver(s) (the “listener(s)”) in a format acceptable to sensor manufacturers all the way to end users. The standard data output is in ASCII characters in serial communication protocol. The sentence begins with the “$” character and then defines the sentence type via the next few ASCII characters with data fields separated (delimited) by comma separators (“,”). NMEA sentences are structured with the following rules:
a. Each sentence starts with the “$” character (position 1).
b. Positions 2 and 3 identify the transmitting device (see below for sample list).
c. Positions 4, 5, and 6 identify the type of message.
d. All fields are comma delimited.
e. Where data is unavailable, the field contains NUL bytes between the comma delimiters
(i.e.,“,,”).
f. ,CR. ,LF. ends the sentence.
g. The last character following the last data field is an “” if a checksum is present and is followed by a two-character checksum.
So, consider a GPS outputting a GGA sentence:
$GPGGA, [and then the comma-delimited data . . .]
A short sample list of output device (“talker”) designations (at positions 2 and 3) is:
• GP—Global Positioning System (GPS)
• HC—Heading—Magnetic Compass
• HE—Heading—North Seeking Gyro
• HN—Heading—Non North Seeking Gyro
• SD—Sounder, Depth