This paper presents new type of thermal flow converter with the pulse frequency output. The integrating properties of the temperature sensor have been used, which allowed for realization of pulse frequency modulator with thermal feedback loop, stabilizing temperature of sensor placed in the flowing medium. The system assures balancing of heat amount supplied in impulses to the sensor and heat given up by the sensor in a continuous way to the flowing medium. Therefore the frequency of output impulses is proportional to the heat transfer coefficient from sensor to environment. According to the King’s law, the frequency of those impulses is a function of medium flow velocity around the sensor. The special feature of presented solution is total integration of thermal sensor with the measurement signal conditioning system. Sensor and conditioning system are not the separate elements of the measurement circuit, but constitute a whole in form of thermal heat-balance mode flow-to-frequency converter. The advantage of such system is easiness of converting the frequency signal to the digital form, without using any additional analogue-to-digital converters. The frequency signal from the converter may be directly connected to the microprocessor input, which with use of standard built-in counters may convert the frequency into numerical value of high precision. Moreover, the frequency signal has higher resistance to interference than the voltage signal and may be transmitted to remote locations without the information loss.
Frequency of an output signal from a Light-to-Frequency Converter (LFC) is proportional to light intensity. Under
dynamic conditions, instantaneous frequency values represent instantaneous values of light intensity. In order to
precisely determine frequency of the pulse signal in short time it is required to measure its successive periods. But if the
light intensity changes, time between successive pulses of the output signal from LFC changes too, which prevents from
obtaining the results of light measurement at regular time intervals. This work presents an algorithm for digital
processing of a pulse frequency signal from LFC to obtain instantaneous values of light intensity at regular time
intervals. Appropriate analytical dependences and examples of measurement results are also presented. Measurement
circuit was built using DAQ-Card PCI-6602 and LabVIEW package of National Instruments.
The FFT makes a powerful tool for analyzing signals in numerous measurement applications. The present paper deals with several problems concerning the application of the Fourier series analysis in the case when a fluctuating value of a measureand is converted into a frequency signal i.e. when instantaneous frequency of a pulse signal represents an instantaneous value of the measured quantity.