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Infant Incubator Board Controller

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haytham

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infant incubator circuit diagram

Hi all
I am designing a control board for some infant incubator.
I have the following situation
1- Need for accurate and precise temperature measuring from 20c to 40c .I selected the RTD DIN 0.385 ohm / C sensor.
2- I will use PIC16f877 MC

My question is,
1- Does any one know a good circuit to interface this temprature sensor with the MC that allows designer to get use of the full range of the voltage range of the A2D module embeded in the PIC MC(0 to 5 volt)
2- If any body could help in supplying me with a PICC code for PID control with self tune or have a prime information on how to build it.

Thanks in advance for your sincere help.
 

lambtron

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how to make infant incubator

Ideally you would force a constant current through the rtd and measure its output voltage, which (after level shifting) would cover the adc input range when operated across your target temperature range. Over such a small temp range, however, this would require an unacceptably huge current.

Even at lower rtd currents you want to watch out for self-heating. So, you can force a current of maybe 500µA through the rtd to avoid self-heating effects and still get a reasonable signal-to-noise ratio. Use an instrumentation amplifier to measure the voltage that develops across the rtd as a result of the constant current excitation. This i-amp will eliminate the common mode voltage and you can inject whatever offset you need to make the i-amp's output 0V at 20°C. Program the i-amp gain so that it outputs 0V at 20°C and it outputs your adc full scale input voltage (5V?) at 40°C.
 

flatulent

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board circuit infant incubator

I would suggest that since this is a life support system that you have two different thermometers for redundancy.
 

haytham

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how to use an incubator with infant

hi lambtron
Thanks for your idea
I want to ask if you have already a circuit diagram for it.


Could any one help me in the PID code with self tune !!

Added after 5 minutes:

Hi flatulent
Thanks for your interset.
Actually redundancy is applied through overheat checking sensor (another sensor that will cause controller to fire alram and close the heater if temperature exceeds certain limit)
There is no much design issues (accuracy/ percision) aboout this sensor because it is used to sense only one temperature. Also its signal conditioning circuit is very simple

Thanks again for your contribution

Haytham
 

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