Level detector

A system for monitoring the level of material in a container comprising:

• (a) signal generation means including:
• (i) reference oscillator means for providing a reference oscillatory signal, and including first sensitivity control means for selectively adjusting the sensitivity of said reference oscillator means, and frequency adjustment means for selectively adjusting the frequency of said reference oscillatory signal;
• (ii) antenna oscillator means for providing a second oscillatory signal, and including second sensitivity control means for selectively adjusting the sensitivity of said antenna oscillator means;
• and
• (iii) antenna probe means for positioning within said container and for receiving said second oscillatory signal whereby the frequency of said second oscillatory signal is varied in accordance with the modification of impedance of said antenna probe means due to the exposure of said antenna probe means to material within said container;
• and
• (b) processing means, including differencer means for receiving both said reference oscillatory signal and said second oscillatory signal and for producing at least one output signal of frequency equal to the difference between the frequencies of said two oscillatory signals;
• and
• (c) test means, as part of said processing means, for receiving such output signal and providing test output indicative of the value of said differencer means output signal frequency, including
• (i) counter means for receiving said output signal and for providing, as said test output, counter signals, initiated in sequence, of frequencies that are, progressively, smaller fractional multiples of the frequency of the output signal, wherein a zero output signal frequency results in no oscillatory counter signals being provided;
• (ii) indicator means for separately signaling the production of each such counter signal;
• and
• (iii) timing means for periodically providing a reset signal to said counter means to define a time span during which said counter means may so produce said counter signals in sequence, said counter means thereafter initiating a subsequent time span for producing counter signals.

A frequency-modulated continuous level detector including switch-adjusted sensitivity control means and test circuit means for use in both adjusting the sensitivity of, and zeroing, said detector, wherein said test circuit means includes:

• (a) counter means for receiving an output signal of the detector and for providing, as test output, counter signals, initiated in sequence, of frequencies that are, progressively, smaller fractional multiples of the frequency of the detector output signal, wherein a zero output signal frequency results in no oscillatory counter signals being provided;
• (b) indicator means for separately signaling the production of each such counter signal;
• and
• (c) timing means for periodically providing a reset signal to said counter means to define a time span during which said counter means may so produce said counter signals in sequence, said counter means thereafter initiating a subsequent time span for producing counter signals.

A method of monitoring the quantity of material at a location comprising the following steps:

• (a) providing a first oscillator and an antenna probe for positioning at the location and such that the frequency of the output signal produced by the first oscillator varies from a predtermined base frequency as the impedance of the antenna probe is altered by exposure of the antenna probe to material;
• (b) providing a reference oscillator with adjustable frequency control whereby the frequency of the output signal generated by the reference oscillator may be selectively adjusted;
• (c) providing an adjustable sensitivity control in the circuit of each of the two oscillators;
• (d) providing a signal processor including a differencer circuit for comparing the output signals of both oscillators and for producing an output signal whose frequency is equal to the difference in frequencies of the output signals of the two oscillators;
• (e) providing a test circuit as part of the signal processor including a counter for receiving the differencer circuit output signal and producing, initiated in sequence, counter output signals dependent on the frequency of the differencer circuit output signal, the production of such counter output signals being limited by the periodic occurrence of a reset signal from a timer such that the number of such counter output signals that may be produced during the time between successive reset signals increases as the frequency of the differencer output signal increases, no such counter output signals being produced for a differencer output signal of zero frequency;
• (f) with the antenna probe free from exposure to such material, adjusting the sensitivity control of the first oscillator in accordance with the extent of effect material to be monitored is anticipated to have on the antenna probe impedance;
• (g) adjusting the sensitivity control of the reference oscillator to match the sensitivity of the reference oscillator to that of the first oscillator whose sensitivity control has been so adjusted by determining the number of counter output signals being produced at a time as the sensitivity control of the reference oscillator is being adjusted;
• and
• (h) adjusting the frequency control of the reference oscillator, whose sensitivity has been so adjusted, to match the frequency of the signal generated by the reference oscillator to the base frequency of the first oscillator whose sensitivity has been so adjusted by determining the number of counter output signals being produced at a time as the frequency control of the reference oscillator is being adjusted.