Strumenti RON

Wavec

Principle of wave direction and height measurement
The Wavec buoy follows the movements of the water surface and measures its own vertical acceleration, pitch and roll angles and the magnetic field components. The elevation of the water surface and the slope of the water surface in North and West direction are derived from these measurements. The co- and quadrature spectra of the heave and slope signals are calculated by the microprocessor in the Wavec receiver (Direc). For obtaining directional data, the slope of the water surface should be known besides the heave signal. The signals which are transmitted from the buoy are:

Heave = double integrated vertical acceleration
Hx = magnetic field component in x direction
Hy = magnetic field component in y direction
Hz = magnetic field component perpendicular x and y (downwards)
Pitch = sine of angle between neg. x axis and horizontal plane (p)
Roll = sine of angle between y axis and horizontal plane (r)

The calculations are performed by the micro computer in the Wavec receiver (Direc). All these signals are available on line at the output ports of the Wavec receiver.

Data flow at receiver side Receiver

1.The received bits are collected in a block of 128 bits containing the sampled values of roll, pitch, heave Hx, Hy and Hz.
2.Error detection c.q. correction.
3.Calculation from data of slope North, slope West, and compass angle of Wavec buoy and inclination and absolute value of earth magnetic field.
4.Storage of 256 samples of received heave and calculated slope data (= 200 seconds of data). The first 32 samples and last 32 samples are tapered.
5. Calculation of Fourier components cn = an + ibn from the collected heave and slope data.
6.Formation of the co- and quadrature spectra from the Fourier COD

C(vv) C(nn) C(ww) v = heave
C(vn) C(vw) C(nw) n = slope North
Q(vn) Q(vw) Q(nw) w = slope West
The spectra obtained from max. 9 consecutive time series of 200 seconds are averaged and corrected for the transfer of the heave signal and the anti-aliasing filters.
Output messages of Wavec receiver Data as received from buoy:
Status characters Heave, Hx, Hy, Hz, pitch, roll and spare channel (batteries voltage or battery voltage and temperature information).

Processed data:
All co- and quadrature spectra of the heave and slope signals.
Possible derived outputs:Administration data Data quality report Significant wave height Power spectrum Plots of mean direction as function of frequency Beam width as function of frequency The frequency resolution is 0.005 Hz.
Wavec transmitter Characteristics:
Power : 0.4 Watt + 0.1 Watt
Frequency range : 27 MHz to 40 MHz
Antenna : One quarter wave omnidirectional
Polarisation : Vertical

Hippy 120 sensor
Introduction
The Hippy measures pitch, roll and heave. Reference plane for the pitch roll measurements is a gravity stabilised platform with a natural period time of 120 sec. An accelerometer is mounted on this stabilised platform. Principle of operation Acceleration measurement: The deflection of the tip of a cantilever is a measure for the acceleration (vertical acceleration since the accelerometer is mounted on a gravity stabilised platform). As the cantilever is placed in an electric field, the potential of the cantilever is a measure for this deflection (no static friction nor hysteresis).

Pitch-roll measurement:
An alternating magnetic field H1 is generated parallel to pitch axis and another field H2 at different frequency parallel to roll axis (by means of coils A and B, fixed to housing and perpendicular to each other). A pick up coil C, mounted on the stabilised platform (horizontal plane) measures the vertical components of H1 and H2. The induced voltage in coil C is amplified, phase sensitive demodulated and amplified again. The pick up coil C is placed in series with the output of the accelerometer.

High pass filter for heave measurement The heave is obtained by double integrating the acceleration. Temporary offset of the platform caused by horizontal acceleration leads to false acceleration outputs; in order to minimise the resulting false heave outputs a high pass filter is used. Platform orientation The pitch/roll measurement is independent of the platform orientations as the platform with pick up coil is symmetrical for rotation around a vertical axis.
Specifications:
General Temperature range : -5 / +35 °C (+35 / +50 °C, short term, weeks)
Storage temperature range : -5 / +40 °C (+ 40 / +50 °C short term, weeks)
Supply : 10- 30 V dc, 180 mW Vibration maximum 4-16
Hz: 1 mm peak, > 16 Hz: 1 g peak.

Pitch/roll
Scale: Output is proportional to the sine of angle c between axis and horizontal plane.
Bandwidth : Low pass third order filter
up to 1 Hz: amplitude transfer within 1% linear phase shift equal to 0.06 sec.
up to 5 Hz: within 6 dB. Linearity + scale error: < 0.05° up to 5° < 0.15° up to 30° < 0.6° up to 60°
Zero offset: After quick rotation around vertical < 1°. With time over one year < 1°.
Noise : Below 0.05° peak.

Heave

Scale: 0.5 V/m, maximum range +- 20 m. Output positive going for upward movement of the sensor.
Position of accelerometer : Approx. 30 cm above bottom plate in centre of cylinder.
Accuracy : Better than 2% of measured value.
Zero offset : Below 30 cm (150 mV), typically 3 cm.
Noise : Below 1 cm peak.
Frequency response : See table below and descr. of high pass filter.

Scheda Tecnica

Period	Ampl. transfer	Phase shift
30 sec	-28.6%	+ 98°
20 sec	-8.5%	+ 64.5°
15 sec	-3.0%	+ 47°
10 sec	-0.7%	+ 30.5°
5 sec		+ 1'5°
2.5 sec		+ 7.5°

The wave amplitudes with frequencies between 0.067 and 0.033 Hz can be corrected within 3% of the true value from the frequency response, table above.

Acceleration
Scale: 0.5 V/m/sec2, max. range +- 10 m/sec2. Output is positive for upward acceleration.
Bandwidth : Low pass third order filter;
up to 1 Hz: linear phase shift equal to 0.06 sec
up to 5 Hz: within 6 dB
Accuracy: Better than 2% of measured value.
Towing If towing is intended keep in mind that speed is limited. Max. speed depends on sea state but will not be larger than 2 m/sec. Possible towing configuration.