Cyclic signal processing

A method for processing a cyclically varying signal to determine the time between equivalent points of successive signal cycles, comprising the steps of:

• analyzing the signal to determine its maximum and minimum levels;
• providing a reference voltage at a first predetermined level which is between the minimum and maximum levels of the signal at a time when the signal is below the first predetermined level;
• comparing the signal with the reference voltage;
• starting a timer when the signal level is equal to the reference voltage;
• changing the reference voltage when the timer is started to a second predetermined level, which is less than the minimum level of the signal;
• changing the reference voltage back to the first predetermined level when the signal level decreases below a third predetermined level which is less than the first predetermined level;
• and
• stopping the timer when the signal level is equal to the reference voltage.

In a system for processing a cyclically varying signal to determine the time between equivalent points of successive signal cycles, wherein the system includes a comparator, a timer and a computer, with the computer being coupled to receive the signal and the output of the comparator, the comparator being coupled to receive the signal and to receive a reference voltage provided by the computer, and the timer being coupled to receive the output of the comparator and being effective to time an interval commencing when the comparator first determines that the level of the signal has increased to be equal to the reference voltage and terminating when the comparator next determines that the level of the signal has increased to be equal to the reference voltage, a method of operating the computer to provide the reference voltage, comprising the steps of:

• sampling the signal at a regular rate during a preliminary cycle;
• storing the sampled signal values;
• determining from the stored sampled signal values the minimum level of the signal during the preliminary cycle;
• determining from the stored sampled signal values the maximum level of the signal during the preliminary cycle;
• calculating a first reference voltage level which is between the minimum and maximum signal levels;
• calculating a second reference voltage level which is less than the minimum signal level;
• providing the reference voltage to the comparator at the first reference voltage level at a time when the signal is below the first predetermined level;
• providing the reference voltage to the comparator at the second reference voltage level when the comparator determines that the signal is equal to the reference voltage;
• and
• providing the reference voltage to the comparator at the first reference voltage level when the signal decreases below the first reference voltage level.

An arrangement for non-invasively calibration testing a positive displacement diaphragm gas meter having a non-magnetic housing and an internal cyclically moving ferromagnetic element, comprising;

• a magnet;
• means for supporting said magnet outside said housing and in proximity to said element so that movement of said element causes the strength of the magnetic field of said magnet to cyclically vary;
• magnetic flux detector means for providing a signal representative of the magnitude of magnetic flux passing through said magnetic flux detector means;
• means for mounting said magnetic flux detector means outside said housing and within the magnetic field of said magnet;
• a comparator having a pair of differential input terminals and an output terminal;
• computer means;
• a timer;
• first coupling means for coupling the signal to one of said comparator input terminals;
• second coupling means for coupling the signal to said computer means;
• third coupling means for coupling said computer means to the other of said comparator input terminals to provide a reference voltage thereat;
• fourth coupling means for coupling said comparator output terminal to said timer;
• wherein the comparator is effective to provide a comparator output signal at a first level on its output terminal when the signal level is less than the reference voltage and to provide a comparator output signal at a second level when the signal level is greater than the reference voltage;
• wherein the timer is effective to time an interval commencing when the comparator output signal first changes to its second level and terminating when the comparator output signal next changes to its second level;
• and
• wherein the computer means is effective to sample the signal, to provide the reference voltage at a first predetermined level which is between the minimum and maximum levels of the signal at a time when the signal is below the first predetermined level, to change the reference voltage to a second predetermined level which is less than the minimum level of the signal upon the comparator output signal changing to its second level, and to change the reference voltage back to the first predetermined level when the signal level decreases below the first predetermined level.