1 ohm to 2 ohm measurement

Sunflower

I am considering wrapping a nickel-chromium wire around a post for position feedback on the azimuth tracking of a solar dish. The variable resistance would range between something like 1 ohm to 2 ohms. How should I measure this?

Newzed

Connect a 4.7 K resistor in series with your nichrome resistor.· Carefully measure the actual value of the 4.7K resistor.· Connect 5VDC across the series resistors.· Measure the 5VDC accurately.

Assuming 5VDC = 4.97 volts
Assuming 4.7K = 4790 ohms

Measure the voltage across the 4.7K.· Let's say it is 4.968.

4.968/4790 = .001037 amps

Measure the voltage drop across the nichrome.· Let's say it is .002 volts.

You know there is .001037 amps flowing in the circuit, so........

.002/.001037 = 1.98 ohms.

There measurements are imiginary but you get the idea.






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Sid Weaver
Do you have a Stamp Tester?

http://hometown.aol.com/newzed/index.html
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Sunflower

I am worried ambient temperature variation will affect measurements.
Are there wires with much greater resistance or should I start over with a new idea such as incorporating a pot or memic 2125 tilted by a wire?

Robert Kubichek

What if you mounted a pot, and measured the resistance of the pot as it moves or better yet, 2 pots rotated 180 degrees with common shaft??
Also, the same idea can be used for elevation with a pendulum weight mounted on the shaft of the pot(s).

Martin King

I'm suspect you might be better off using a digital encoder of some sort and counting pulses? Resistors can suffer from both short term drift (temperature) and long term drift (wear, corrosion) both of which would be a particular problem over such a small resistance range. What sort of angular resolution do you need? and how does that relate to actuator shaft movement·and motor revolutions?

Martin

Sunflower

Resolution is not critical. This function is for safety, the dish projects a 10 kW spot of energy that can raise havoc with bystanders, structures, and plants. So pointing at the sun through the clouds and tracking error feedback will be necessary via DS1307, EEPROM (memory of previous sunny positions with time), and current azimuth position. The dish azimuth can be moved against the clutch by hand or by heavy wind. This function must also be reliable after reset so shaft counting or encoding may be a problem. Accurate solar tracking via 4 shadow band photo resistors does not need position feedback. I am just looking at approximate tracking feedback for safety. Good ideas! I am leaning towards a voltage divider with a pot and ADC to compensate for temperature drift. It did not occur to me that a pot is less than 360 degrees. Perhaps I could use a tensioned wire pot wheel 3 times the diameter of the post, or perhaps a lever. I worry about cost and complexity, also about dirt and light leaks affecting a variable intensity optical reflector wrapped around the post. I have trashed the one ohm wire sensor. Mulling and culling ideas... DW

SPENCE

Amateur radio and others who track sun moon and stars and
satalites use kepliarian elements which are updated continously and available. Amateur radio for years have used tracking system which are controlled by computer (using the keps) which feeds azmuth elevation to the az-el rotors. These systems are in use world wide currently.
73
spence
k4kep