2.4.4

Cable Tightening ToolFigure 11 Cable Tightening Tool

WMT700 is shipped with a cable tightening tool (SP. When you insert a cable in the cable tightening tool, it is easier to grip and rotate. After tightening, you can leave the cable tightening tool in place.

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29

3. Functional Description

Operating PrincipleWMT700 uses the Vaisala WINDCAP ultrasonic sensor technology in wind measurement.

The sensor has an onboard microcontroller that captures and processes data and communicates over serial interfaces.

The wind sensor has an array of three equally spaced ultrasonic transducers on a horizontal plane. Wind speed (WS) and wind direction (WD) are determined by measuring the time it takes the ultrasound to travel from each transducer to the other two.

The wind sensor measures the transit time (in both directions) along the three paths established by the array of transducers. The transit time depends on wind speed along the ultrasonic path. For zero wind speed, both the forward and reverse transit times are the same. With wind along the sound path, the upwind direction transit time increases and the downwind transit time decreases.

The following figure shows how the time shift of the ultrasonic signals is measured and how tailwind and forward wind affect the measurement.

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Figure 12 Ultrasonic Measurement Principle

1

Ultrasonic measurement with zero wind

2

Impact of tailwind

on ultrasonic measurement3

Impact of headwind on ultrasonic measurement

The microcontroller calculates WS from the measured transit times using the following formula:

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V

W

=0.5·L·(1/t ft r

)

V

W

Wind velocity

L

Distance between two transducers t

f

Transit time in the forward direction t

r

Transit

time in the reverse directionMeasuring the six transit times allows

V

w to be computed for each of the three ultrasonic paths. Using

V

w values of two array paths is enough to compute WS and WD.

The different paths of WMT700 and the vectors provided by the wind sensor:

Figure 13 Measurement Paths of WMT700 Measurement paths 1 to 6 of WMT700

L

a

, Lb, L

c

Distance between two transducers

The vectors are calculated as follows:

V

a

=0.5·L

a

·(1/A

1

-1/A

2

)

V

b

=0.5·L

b

·(1/A

3

-1/A

4

)

V

c

=0.5·L

c

·(1/A

5

-1/A

6

)

Chapter 3 – Functional Description

33

The equation depends on the accurate distance of the measurement path (L. The computed wind speeds are independent of altitude, temperature,

and humidity, which are canceled out when the transit times are measured in both directions, although the individual transit times depend on these parameters Coordinate Systems Vector and Polar

Calculations

The triangular geometry of the sensor is converted to orthogonal coordinates to achieve the x and y components. Then the sensor converts the wind vectors into polar coordinates.

The measurement results are reported as follows:

• WMT700 reports WS (x, y) as two scalar speeds, one parallel to the NS direction (x)

and the other (y) parallel to the WE direction.

x = -WS × cos (WD)

y = -WS × sin (WD)

• WMT700 reports polar wind speed as a scalar speed in selected units (ms, kt, mph,

km/h).

Polar wind direction is expressed in degrees (°). WMT700 indicates the direction that the wind comes from. North is represented as 0°, East as 90°, South as 180°, and West as 270°.

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