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1 Davis Anemometer and Direction Vane
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An
anemometer is a wind speed measuring device. It usually uses cups
mounted on a vertical shaft that rotates as the wind flows past.
Davis has included a wind direction vane but still refers to the entire
instrument as an anemometer.
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Illustration 1: A complete anemometer before installation.
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The
various parts of the wind measuring device are; the base which mounts
to the user supplied mast, the aluminum arm (tube), the head assembly,
the wind cups, the direction vane, and a pre-wired cable.
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The
anemometer and vane is a passive analog device. It is not
powered. It responds to a brief direction excitation pulse from
the sensor interface module (SIM). Speed pulses are created by a sealed
mechanical reed switch. The head is a sealed unit and not intended to
be serviced or repaired. The head contains the magnetic reed switch on a
circuit board, the direction potentiometer and wires connecting these
to a 4 conductor flat cable. The supplied 40 foot cable has a 6
position, 4 conductor RJ-14 male plug on the end that connects to the
Davis Integrated Sensor Suite (ISS).
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B Wind Speed
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Illustration 2: Anemometer cups can be disassembled for repair.
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This
anemometer uses three cups with their shafts pressed into a plastic
hub. Each cup is replaceable if it is damaged. The three cups are
held in place by a plastic plug pressed on the end of the hub.
The wind cups come assembled, not as shown above.
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The
wind cups are held on to the vertical shaft of the head assembly by a
single 0.050 inch hex drive set screw. The internal vertical shaft is
1/8 inch diameter and 1.6 inch long. The shaft uses one bearing and one
brass bushing. Each are held in place by two spring clips that
fit into channels milled into the steel shaft. The bearing and
bushing measurements are .125 inch bore and .375 inch diameter.
The plastic head assembly is molded for mounting of the bearing/bushing
and also the potentiometer and switch assembly circuit board.
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The
wind cups has a magnet fitted into the plastic hub. Wind causes
the cups to rotate which rotates the magnet across but not touching the
magnetic reed switch. During each pass of the magnet over the reed
switch, the switch closes. The reed switch is normally open but
closes briefly only as the magnet is near the switch. Each
revolution of the wind cups cause one switch closure cycle.
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Link to more information about anemometer/wind cups.
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B Wind Direction
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Wind
direction is sensed by the vane that is mounted above the head
assembly. The vane is fastened on the ¼” shaft of the
potentiometer by a 0.050 inch hex drive set screw. As the wind
direction changes, the vane follows and changes the resistance at the
slider terminal of the potentiometer. The potentiometer is a linear
resistance type that is free to rotate 360º with no mechanical stop. It
is normal that the vane movement has more resistance to rotation than
the wind cups. The reason is that the potentiometer has a
mechanical slider that is in contact with the inner surface of the
potentiometer body. The direction vane needs some drag/dampening to
prevent the vane from excessively overshooting when rotating.
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Notice
that there is an area of roughness near the north or 0º position.
This happens when the slider crosses between the two ends of the pot
resistance strip. The roughness is normal.
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The
manufacturer attaches the vane to the shaft and it will accurately
report direction if the arm is pointed toward true north. If the arm
cannot be pointed to true north due to installation limitations, then
the vane can be re-oriented or aligned. If the vane is removed
from the shaft, then the realigning will have to be done. The
Davis ISS manual has vane adjustment procedures. The direction can
also be aligned by a console adjustment instead of the hardware method.
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Other
than realigning, no other service is needed. The head assembly is
permanently sealed and is not to be opened. Troubleshooting procedures
are in the manual.
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2 Anemometer and Direction Vane Circuit Description
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Refer to the schematic diagram for the description. The flat (not twisted) cable has 4 conductors.
yellow
= direction excitation This is an excitation pulse sent from the
SIM to the anemometer and is applied to the cw terminal of the
potentiometer.
green = direction This is the direction signal
from the anemometer to the SIM. This wire is connected to the
potentiometer slider s terminal of the potentiometer. The slider
returns a portion of the excitation pulse depending on the angular
position of the vane.
red = common or
analog ground This wire is connected to the wind speed and the
wind direction circuits.
black = wind speed signal This wire
provides the wind speed pulses from the magnetic reed switch to the SIM.
This wire connects to the ts terminal on the potentiometer then to the
switch circuit board.
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The
wind speed and wind direction functions has separate circuits but the
red wire is common to both. If you measure the voltage on the
black wire at the anemometer if it is connected to the SIM, it will
probably indicate +3 volts unless the reed switch is closed. This
is because this wire is ‘pulled up’ to that voltage internally in the
SIM. The magnetic reed switch is connected from the red wire to the
black wire. Hence, when the reed switch closes, the black wire voltage
will go from +3 volts to zero while the switch is closed. This +
to – pulse lasts only while the magnet is in the vicinity of the reed
switch. This pulse occurs once per revolution of the anemometer
cups as the device is operating in the wind. Notice in the
schematic there are two 47 Ohm resistors in series with the reed switch.
These resistors protect the reed switch only if someone foolishly
connects power from the red wire to the black wire.
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The
wind direction circuit uses a linear 20K Ohm potentiometer to sense the
position of the vane. A voltage pulse is sent from the SIM to the
pot through the yellow wire. This voltage is applied to one end
of the pot. The mechanical slider in the pot picks a portion of
that voltage depending on the angular position of the vane/slider.
The voltage level of this pulse is determined by the vane/pot slider
position. The pot has 3 contacts, red=common (ccw),
yellow=excitation (cw), green=direction signal (s). One additional
terminal on the potentiometer (ts) is used as a termination point for
the black wire. A 909K Ohm fixed resistor is mounted across the pot
from the yellow to the green wires. This resistor prevents the slider
voltage from ‘floating’ while it is crossing the open end points of the
pot. This resistor causes the indicated direction to remain at north
while the circuit is briefly open. If not for this resistor, the
direction indication may briefly go to a false direction.
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