Hello, I have done a Lot of research and I have Lost faith in a current project maybe You could give me some hints to what I AM doing wrong. Please.
The proyect consist in the following, i have an array of 10 Heating elements which i need to heat to some calculated and around 3 grades centígrades of precision temperature with a PID algorithm. ( I can't use thermistor, thermocoupler or anyother temperature sensor due to the small diameter of My Heating elements.
I plan on giving it power with pwm and a N channel logic MOSFET, so far so good with that part.
I plan on measuring the resistance of the Heating elements as i heat them and using their temperature coefficient of resistance as the temperature goes up to extrapolate the temperature, however i don't know how to do this exactly, cause I have to measure the resistance while the circuit is working, at a precision of around 0.01 ohms, ( i don't care about accuracy) i just want precisión and repeatability, cause i only care about the diferrential between the temperatures i Will be working.
i though of using some voltage divider with the Heating elements and as they are 10, using an analog multiplexer to only use one voltage divider at the output of the mux and then folowed by a op amp buffer followed by an ampliffication stage (i have done the math already) to increase resolution to get the 0.01 omhs precisión i need.
I know i can't use the voltage divider while the circuit is powered on so i plan on turning off the Heating some microseconds, take the measuremts of the Heating elements while the power is off and then the pid gets the data and then power on the circuit again, but how do i do this? How do I switch from reading resistance to power the circuit withouth interfering with each other or destroying the sensing part of the circuit.
Also the multiplexer has some internal resistance and when the resistance of the Heating elements goes in the input to the output of the mux it has added resistance that Will alter the result of the conversion of the voltage divider, thus rendering the sensor measuremts of resistance useless. I wouldnt care of this added resistance if it were constant or stable, as i don't care about accuracy, but i do care about precisión and repeatability,.
Sorry i wrote this much but i really need some advice, because i have thought of everything and I am not very Smart, so if You guys could come with a better solution please.
Maybe it is confusing if You need me to write an schematic i would do it.