Wind Speed Problem with Heathkit ID-4001

  I have a problem with my Heathkit ID-4001 Weather Station.  The wind
speed works fine if there is any wind but when there is a calm wind
condition it occasionally starts giving erratic wind speeds.   If it
should be reading zero speed, it reads zero and then jumps to a higher
wind speed, such as 32 and then back down to zero or up to 41.   There
doesn't seem to be any pattern that I can discern.  The actual wind
speed values tend to be random and when they occur appears to be random.  
It can go for some time sitting at zero and then give an erroneous
reading  or several erroneous readings in quick succession and then set
at zero for an another period of time.  As soon the wind picks up it
appears to read the wind speed flawlessly.  

Any ideas on what the problem might be or what to check for?

Thanks.

 

Does this system controller measure the speed by reading a voltage
from the sensor (with voltage being proportion to wind speed) or is it
counting pulses (1, or N pulses per revolution, hence frequency is
proportional to speed)?

If it's counting pulses, then I might suspect either of:

-  A "bouncing" switch in the sensor, which is just barely making
   contact at the point at which the sensor has stopped spinning, and
   is vibrating open and closed.  The cure for this would be a better
   sensor-switch - one with enough hysteresis that it opens or closes
   firmly and doesn't jitter back and forth.
  
-  RF interference from a local transmitter, with the RF signal
   "looking like" contact opens and closures to the control unit.  The
   cure here would be some amount of RF snubbing being added to the
   line to the sensors - possibly wrapping the wires around some
   ferrites, possibly adding a small amount of capacitance across the
   line to shunt out the RF, or possibly both.

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It is indeed counting pulses.



It is not a mechanical switch.   The pulse generator consists of an IR
LED that shines on IR sensor with a plastic disk in between that has a
series of black stripes painted around the edge of it. When the wind
cups spin, the disk spins so the black stripes interrupt the IR path
between transmitter and sensor generating pulses.  The pulses then to go
to a base of a transistor that I assume is a switching transistor that
is either fully on or fully off.  It is an NPN transistor in common
emitter configuration and the output of that is fed directly into the
CPU for counting the pulses in a given time frame for wind speed
determination.


I thought about the possibility of noise pickup and put some a capacitor
across the line where it connects to the weather station and it did not
seem to help.

Thanks for taking the time to reply.

 
jitter back and forth.

If the disk happens to stop partially blocking the IR transmission path, you
might get random pulses...

Probe the signal at both the input and output of the NPN transistor.
That'll tell you if it's the sensor, the transistor, or the CPU which is
causing the issue.

 

Sounds to me as if there may not be enough (any?) hysteresis in the
IR-sensor / transistor-switch system.  If the disk comes to rest in
just the right position (with one of the stripes partially but not
completely blocking the IR path) the amount of IR energy reaching the
sensor, and thus the amount of current going into the switching
transistor base might be right around the transistor's switching
threshold.  Varying amounts of random electrical noise in the
photosensor or transistor could then cause the output to switch state
unpredictably.

If I were going to try to fix this, I'd do one of two things:

-  AC-couple the output of the sensor to the transistor's base, adding
   a biasing network to hold the base a comfortable distance on one side
   or the other of the switching threshold.  That way, only fairly
   substantial (and relatively fast) sensor transitions would dump enough
   current into the base to cause the transistor to switch state.

-  Use a Schmitt trigger, or some other sort of transition-detection
   circuit with explicit hysteresis (e.g. a comparator with a bit of
   positive feedback added).

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