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August 2020 – TD 316 OPERATING MANUAL – Motus Wave Sensor 5729
CHAPTER 2
Theory of Operation
2.1
Wave Measurement
The wave measurement is based on an IMU with 9-axis accelerometer/gyro/magnetometer, The IMU operates at 1 kHz
internally – 100Hz output for low pass filtering with 4Hz for wave calculation. Mechanical dampening acts as a low pass
filter for the IMU-accelerometer in order to avoid aliasing and reduce low frequency noise. Due to coherent offset
compensation the wave sensor can be installed off rotational origin for the buoy without introducing errors. The user
enters the offset coordinates (x/y/z) for the installation. A user selectable buoy transfer function can be modified and
activated. User selectable compass input. IMU compass, in-run IMU calibration or external compass are also available.
2.2
Sensor Integrated Firmware
The main tasks of the sensor’s integrated firmware are to control the different sensor parts and calculate all parameters.
All the user configurable properties that can be changed for each individual sensor, i.e. calibration coefficients,
parameter outputs and configurations parameters are called sensor properties, see
chapter 1.7
. When the sensor is
connected to an Aanderaa logger the AiCaP mode is normally used. If connected to an Aanderaa logger like
SmartGuard and using AiCaP the properties can be displayed and changed by using the AADI Real-Time Collector
. For a stand-alone sensor the properties may be displayed and changed either via the AADI
Real-time Collector software, see
or using a terminal communication program, refer
RS-232 port. Examples of typical terminal emulation programs are Hyper Terminal and Tera Term.
The sensor will always output a minimum of 3 wave parameters,
Significant Wave Height H
m0
, Wave Peak Direction
and Wave Peak Period.
The rest of the parameters can be switch on in the
Sensor Configuration
menu. If used with
an Aanderaa Logger each parameter can be transmitted in real-time and/or stored in the logger.
In RS-232 mode the Motus sensor will perform a measurement sample and present the result within the first 1.5
seconds after the Motus has been powered up. However some of the parameter needs a number of samples before
they can be calculated.
The higher the number of samples in your
Wave Integration Time
is the better Standard Deviation you will obtain. You
also need a longer
Wave Integration Time
to make sure more repetitions of especially the long period waves. Standard
Wave Integration Time
is normally set to 20 or 30 minutes.
The recording interval might be set to a shorter time than the
Wave Integration Time
. But the output will always be a
calculation using data from the last Wave integration Time. For example if the interval is set to 5 minutes and
Wave
Integration Time
is set to 20 minutes. The sensor will present data every 5 minutes based on data collected the last 20
minutes. After power up the sensor will not calculate any parameters before a full
Wave Integration Time
has passed.
