A Research On High Resolution Measurement Technologies Of Resonant Sensor Output Signals

Compared with laboratory equipments, downhole probes for testing reservoirlimit should have the advantages of simple structure, small size, high resolution andexcellent high temperature performance.Resonator-based sensors are utilized in a wide range of applications in fieldinstruments. The resonant quartz crystal sensor is one of the most widely used in theoil field for testing reservoir limit.Resonator-based sensors output includes initial frequency and variation of thefrequency caused by the change of the measured parameter. The range of initialfrequency is several thousand to several hundred thousand Hz. The variation of thefrequency is much smaller than the initial frequency. The uncertainty ofmeasurement can never be smaller than its resolution. Measuring small frequencyincrements with high resolution remains a problem. In order to solve the problem,the measuring method of small frequency increments, the input and outputcharacteristics of logic mixers, the measuring method and processing algorithm offrequency are studied in this paper. The main contributions and innovation points areas follows:Firstly, a high resolution measuring method on small frequency increments forfield instruments was proposed.Secondly, the input and output characteristics of a D flip-flop mixer werestudied in theory. Its application conditions were given. The main reasons resultingin its output measurement errors were analyzed. The electric connection mode of theinput signal with D flip-flop and the range of reference signal value were putforward to reduce the measurement error. High measuring resolution has beenobtained through proper selection of measurement signals.Thirdly, the frequency mixing characteristics of logic gate mixers were studiedin theory and mixers were classified according to their mixing characteristics. TheXOR and XNOR mixers have better performances than the others. A new frequencysubtracting circuit based on XOR gates with a first-order RC filter was presentedand validated in experiments..Fourthly, based on the theoretical analysis and experimental validation, a highresolution frequency measurement method and algorithm, Continuous Counting andPeriod Marking with its shortened form Marking Method, was introduced.Finally, using the real sensor output, a comparison among several methods mentioned in this thesis was carried out. A D-type flip-flop mixer which receivestwo input signals of different frequencies forms an output signal whose frequency isequal to the frequency difference between these two input signals. Counting thenumber of rising or falling edges of a reference frequency within each cycle of the Dflip-flop mixer output, a measurement array is obtained. By selecting valid data fromarray and calculating, the frequency of the unknown signal can be obtained withhigh resolution. This method is very suitable for field instruments usingresonator-based sensor systems. The resolution of Marking Method is much higherthan other methods. With some further improvements; this method will be ahigh-resolution frequency measurement method with very broad applicationprospects.Marking method is the main creative contribution of this thesis. It is superior tomethod based of period measurement since the period measuring method paysattention only to the total number of pulses counted during certain period of time,where as the marking method takes into consideration also the numbers of pulsescounted during each individual short interval. Marking method is superior toregression method since the later one is based on calculation the average pulse rateper unit of time where as the marking method takes into account all the details ofpulse rate variations within the whole measuring time as well. The advantage ofthe marking method becomes even more significant when there are multiple loopsinvolved in the counting process.