Measurement and calibration of a nonlinear analog signal using MCS-51 series microcontroller

摘要

Most of the physical parameters when converted to electrical signal, using sensors, for the purpose of measurement, show nonlinearity in between; values of physical parameter and corresponding electrical signals. As an example, the thermoelectric millivolt (mV) sensitivity verses temperature of chromel-alumel thermocouple varies from 0.001mV/℃ to 0.043mV/℃, in its measurement range from -270℃ to 1372℃.The accurate value of physical parameter is generally found, from its nonlinear electrical signal, using two different approaches. In the Hardware methods the nonlinearity in the electrical signal is compensated by setting different gain of the analog amplifier, in the different ranges, such that final out put electrical signal is linear within desired accuracy, in the whole range of the measurement. The output electrical signal of such amplifier represents the value of physical parameter being measured, however after making output electrical signal linear, the calibration of this output signal, in terms of the physical parameter to be measured, has to be done by providing at least two settable resistances, for nullifying the error in offset and gain of the analog amplifier. In Software method, the value of the physical parameter may be deduced from its nonlinear electrical signal either by polynomials curve fitting or by look up table method. In a bigger system like PCs, where processor capability, memory size and speed is not a constraint, polynomials curve fitting and look up table both may be applied for finding out the correct value of physical parameter from its nonlinear electrical signal. The polynomial curve fitting often requires calculation of complex function, which sometime becomes impractical in a system, with limited programmable memory, processor capability and speed. Therefore look up table method is adopted in the proposed system, which is developed using MCS-51 microcontroller (AT89C52), to deduce the temperature value from nonlinear analog voltage signals of chromel-alumel thermocouple.