1. Field of the Invention
This invention generally relates to a method of, and an arrangement for, measuring impedances, particularly capacitors having individual non-linear characteristic low capacitance values over a range, in a measuring circuit particularly useful in radiosondes for measuring various physical parameters such as pressure, temperature and humidity.
2. Description of the Prior Art
It has been proposed in the prior art to use an impedance as an electrical sensor for measuring various physical quantities. The operation of such impedance sensors is based on the fact that the impedance value of the sensors is a function of the physical quantity to be measured, such as pressure, temperature, humidity, the position of an observed item, force, etc. Examples of such impedance sensors are strain gauges, pressure-sensitive and/or temperature-sensitive resistors, and capacitors having condenser plates separated by a dielectric. Such capacitors can sense pressure or temperature as a function of the mutual position of the condenser plates, or can sense relative humidity as a function of the variation in the dielectric constant. The manufacturing tolerances of the conventional impedance sensors render the same individual, that is, each exhibits a different non-linear response characteristic of impedance values over the range of measurement.
A measuring circuit which utilizes capacitive sensors was described in U.S. Pat. Nos. 4,295,090 and 4,295,091, which respectively correspond to Finnish Pat. Nos. 54,664 and 57,319, and in U.S. patent application Ser. No. 739,326, filed May 30, 1985 now abandoned. These patents and application describe a method of, and an arrangement for, measuring low capacitance values, and the entire contents of said patents and application are incorporated herein by reference.
The low capacitance values of these capacitive sensors range from a few picofarads to about twenty, to forty picofarads or, at most, a hundred picofarads. The measurement of such low capacitance values entails problems due to, among other factors, stray capacitance, input voltage variations, electrical interference, and other causes.
When measuring pressure, temperature, humidity, or other physical quantities with electrical or electromechanical sensors, it has been proposed in the prior art to connect one, or more, reference impedances of a stable, known impedance value to the measuring circuit so that inaccuracies of the sensor and/or in the measuring circuit could be compensated for. Thus, it is known to connect a reference capacitor having a known capacitance value to the measuring circuit, and to alternately connect a capacitive sensor having a capacitive value to be measured to the measuring circuit in place of the reference capacitor. More particularly, the reference capacitor and the capacitive sensor are alternately connected, each in its turn, to an input of an RC-oscillator whose output frequency is a function of the capacitance connected to the oscillator at that time. The measuring circuit can be suitably adjusted so that an output signal derived from the reference capacitor may, for each measurement, be set to a correct level.
Some measuring circuits, particularly those using bridge networks, utilize one reference impedance, in which case, the measurement of the sensor impedance value is accurate only if the reference impedance is close to the sensor impedance value, which occurs when the bridge network is balanced. The greater the deviation between the sensor impedance value and the reference impedance value, the greater will be the measurement errors derived, for example, from variations in the measurement dynamics of the measuring circuit. Of course, the one-reference measuring circuit has the advantages of simplicity in construction and in calculation.
Other measuring circuits utilize two or more references in order to increase their measuring accuracy over a wider range. However, the two- or multi-reference measuring circuits have the disadvantages of complexity in construction and of calculation.