DE4210737A1 - Fuel tank contents measurement - exposing liq. to stray field of capacitor plates mounted on flexible insulating material, and stuck directly on tank. - Google Patents

Abstract

The measurement liquid is exposed to the stray field of a capacitor, i,e the inhomogeneous component of its electrical field, consisting of two adjacent plates. The plates are mounted on flexible, electrically non-conducting material and can be stuck directly onto any shape of tank. A reference capacitance measurement can be made on the tank. This is independent of the contents of the tank but not of its material. USE/ADVANTAGE - Esp. for tanks contg. water, petrol or oil in motor vehicle. Measurement system can be made cheaply in large numbers using integration of electronic components into single IC.

Description

The tank content measurement of fuel tanks in vehicles happens u. a. according to the float principle, being inside the tank must be intervened and thus a considerable constructive effort is to be made.

A significant simplification for series production offers the capacitive measuring principle of stray field measurement, because only two copper-coated foils (Measuring capacitor, reference capacitor) on the plastic tank any shape must be glued.

The measurement signal generation and evaluation takes place directly on Tank (active sensor). I.e. the circuit generates one periodically changing voltage (sine or Triangular voltage) high frequency (approx. 100 KHz) the one Bridge circuit feeds. The AC voltage is converted into a DC voltage converted and amplified. The DC voltage can then be easily transmitted over longer lines.

By integrating the electronic components into an IC, the measuring arrangement becomes very cheap for large quantities.

The design of the sensor (measuring capacitor) is a film corresponding to the height of the tank, on which two parallel copper tracks are coated. The reference capacitor has a similar structure and can have a different area (square or round) depending on the shape of the tank. It is located, for example, on the top of the tank on the inside of which no tank contents can reach for a long time. ( Fig. 1 and 2) The stray field to be measured is shown in Fig. 2. Since the capacitor plates are close together and the tank is usually approx. 5mm thick, the change in the relevant electrical dielectric constant (e ₄ ) of the measuring capacitor is very weak. This means that sensitive electronics are required for exact evaluation.

There are already capacitive bridge circuits. In the present case, however, a special version is required ( Fig. 3).

An _AC sine voltage U _{AC is} generated with a frequency f of 100 kHz, which feeds the measuring capacitor and the reference capacitor.

The voltage drop across the measuring capacitor changes with the change in the capacitive resistance X _c according to the relationship:

U _AC / U _c = (R + X _c ) / X _c ,
U _c = U _{AC *} X _c / (R + X _c ),
X _c = 1 / (2 _* Pi _* f _* C),
C = e _{r *} e₀ _* d / A.

R: ohmic series resistance of the measuring bridge,
U _c : voltage drop across the measuring capacitor,
C: capacitance of the measuring capacitor,
e _r : relative dielectric constant of the measuring capacitor,
e₀: absolute dielectric constant,
d: electrically effective distance between the capacitor plates of the measuring capacitor,
A: Electrically effective area of the measuring capacitor.

The capacitance C changes depending on e _r . This in turn changes depending on the tank content behind the tank wall.

Both voltages, both U _c and the reference alternating voltage U _CR, are rectified, subtracted from one another and amplified. In the example, the rectification is carried out by synchronous sampling using a sample and hold element.

The sine _AC voltage U _AC switches a comparator to a defined phase of U _AC . The switching signal U _s controls the sample and hold element and always sets the memory switch to measurement at the same phase. The AC voltage is therefore always taken from the same voltage value, which is proportional to the amplitude of U _AC .

Since the voltage changes are in the microvolt range, an offset adjustment is required, which also shows the differences of measuring and reference capacitors. Inte This comparison can be achieved by an appropriate IC Electronics to be replaced.

The use of a sine signal for measurement has the Advantage of lower harmonics and is therefore EMC safer.

Claims (5)

1. The measurement of tank contents (especially water, petrol, Oil) according to the capacitive principle, the one to be measured Liquid the stray field (inhomogeneous part of the electrical field) of a capacitor, which consists of two plates lying side by side. 2. The capacitor plates are on a flexible, electrically non-conductive material and can directly on any tank shape can be glued on. 3. By a reference measurement, also on the tank, the Reference capacity determined, regardless of the tank capacity depending on the tank material. 4. The high-frequency signal is evaluated directly on Sensor. 5. The measured value is evaluated via a bridge circuit of measuring and reference capacitor, in which one periodically changing voltage is injected, which is in Dependency of the measuring capacity changes and then after Rectification and screening using synchronous scanning with the equally obtained reference voltage is compared and the difference between them is then amplified.