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Wind direction sensing.

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LAN_403

TheMerovingian

Tue Dec 15 2009, 02:14PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I'm tring to understand how wind dyrection sensor works. Most of commercial ones seems to be relative sensor (must be turned on pointing to north. I was thinking to use hall effect sensors but the wires on the shaft would not allow rotation of the shaft itself. Using magnets of the shaft will give problems because of interaction with earth magnetic field, the only possibility is to use an optical sensor counting the pulses per each rotation but this would not discriminate between clockwise and anticlockwise rotation. Any solutions?

Electroholic

Tue Dec 15 2009, 05:38PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

quadrature optical encorders

Erlend^SE

Tue Dec 15 2009, 05:43PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

Maybe you should use 2 sensors like in computer mice (the old kind).

They use 2 sensors 90 degrees out of phase and can detect direction based on the relative time of each sensor changing level, you may even extract some >100 pulses turn sensors from printers or less pulses/turn from computer mice.

See this Wikipedia article

Sulaiman

Tue Dec 15 2009, 06:36PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

The problem with quadrature/mouse-type encoding is that it's incremental only,
you will also need an optical angular reference, not really a problem but if the vane rotates faster than the encoder can maintain accuracy (no missed pulses etc.) then a recal. is required.
A 'Gray code'

or similar encoder gives absolute position, but due to cost are only 'hobby-friendly' if found surplus or scrap.

Since I doubt that your weather vane is in an 'ideal' location then buildings, trees, hills etc. mean that you don't need a precision sensor, N NNE NE ENE E etc. is 16 positions, 2^4, 4 bits.
For Gray coding that would mean 5 sensors and 5 concentric rings Gray coded on the vane,
or just one magnet on the vane and 16 Hall switches.

I'm sure there are cleverer/simpler methods,
Interesting what members may come up wit?

I guess there must be an optimum number of sensors and magnets for cheapness and ease of construction - i.e. less is more!
Also the encoding could be 'display-friendly'
Assuming that you're going to build a remote display you can go from the 'simplest'
- one magnet, one sensor per 'direction' and wire to remote led- basic (multicore cable)
- precision sensor/processor each end etc. - minimum cable cost (complex)

TheMerovingian

Tue Dec 15 2009, 08:09PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

i was thinking to use two optical sensors and two 180Â° out of phase hole patterns (the quadrature one), kinda relative sensor an a small pic12 microcontroller transmitting via usart the bits the problem is resolution, for 20 sectors the resolutions is a measly 16Â° that may be good for the NW NE SW etc thing, but not for precision things. The two hole patterns out of phase allow determination of the dyrection of the step (for eg if sensor 1 goes high before sensor 2 it is CW else it is CCW). Another way i was thinking is to use 2 hall switches 90Â° relative one to the other and two magnets on the rotor to form a N-S parallel-lines field, two adcs (microcontroller on board ones) and using a ATAN lookup table with interpolation between points. THis sistem has a problem: the magnets will with earth magnetic field like a compass.

mikeselectricstuff

Tue Dec 15 2009, 11:43PM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253

You can also do it with ultrasonics This sensor does wind speed and direction with no moving parts - presumably using doppler shift

Dr. Slack

Wed Dec 16 2009, 08:37AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

A lot depends on the power you have available, must it run from batteries, or have you got a watt or two to burn?

Another "no moving part" way to do it uses a circle of thermistors. Either self-heated, or with a heated resistor in the middle. The temperature change due to wind direction is quite small, but you will always be sensing differentially, which helps. Not for batteries.

If you want to sense the angular poisition of a rotating shaft, then there's all sorts of physics available to help you.

A magnet on the shaft, with crossed Hall sensors for quadrature, but there's a slight torque with the earth's field, and needs ADCs.

You can mount almost anything off-centre on the shaft and detect its position. If it's ferromagnetic, or conducting, then an inductance measurement using several sensors in a circle will do it. You can measure inductance with change of frequency of an oscillator.

With LEDs and a photo detector, you can detect the position of a 45 degree mirror on the bottom of the shaft, or the difference between black and white paint on a cotton-reel.

My favourite, because it can be made to use very low power, has an output that is compatible with a microcontroller or PC sound card for readout, uses a off-centre earthed conductive rotor. Several isolated electrodes surround it, and each is multiplexed in turn by a CMOS 4051 multiplexer to the tuning C of an oscillator, say a simple schmidt relaxation oscillator. The change in capacitance changes the frequency.

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