Browse Prior Art Database

LIQUID LEVEL SENSOR

IP.com Disclosure Number: IPCOM000007015D
Original Publication Date: 1993-Oct-01
Included in the Prior Art Database: 2002-Feb-18
Document File: 2 page(s) / 100K

Publishing Venue

Motorola

Related People

Zbynek A. Capurka: AUTHOR [+2]

Abstract

The problem to be solved by the suggested design is to avoid any moving electrical connections which would be subject to wear. That may happen, e.g. with sliding contacts across a resistor. We use a capaci- tive coupling between the lixed permanent electrodes and a floating device. E.g. two concentric cylinders are arranged according to Figure 1.a. The outer cyl- inder is fixed and has the metalized inner surface split electrically in two halves, and the inner mova- ble floating cylinder has a fully metalized surface. Permanent immobile connections are attached to the outer cylinder parts. Between the cylinders is deposited an isolating dielectric material either on the internal surface of the external electrodes or on the surface of the inner cylinder. This creates two serially connected capacitors CI and C2 having a resulting capacitance C as depicted in Figure 1.b.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 50% of the total text.

Page 1 of 2

MOTOROLA INC. Technical Developments Volume 20 October 1993

LIQUID LEVEL SENSOR

by Zbynek A. Capurka and Walter Czernocki

  The problem to be solved by the suggested design is to avoid any moving electrical connections which would be subject to wear. That may happen, e.g. with sliding contacts across a resistor. We use a capaci- tive coupling between the lixed permanent electrodes and a floating device. E.g. two concentric cylinders are arranged according to Figure 1.a. The outer cyl- inder is fixed and has the metalized inner surface split electrically in two halves, and the inner mova- ble floating cylinder has a fully metalized surface. Permanent immobile connections are attached to the outer cylinder parts. Between the cylinders is deposited an isolating dielectric material either on the internal surface of the external electrodes or on the surface of the inner cylinder. This creates two serially connected capacitors CI and C2 having a resulting capacitance C as depicted in Figure 1.b.

where for one half cylinder the area A is

A = nrh (5) ,,b+a 2 (6)

a.. .inner cylinder radius
b.. .inner radius ofthe external electrodes E .dielectric constant of the material between cylinders
d.. .d = b - a, thickness of the dielectric material between cylinders
h.. .cylinder height overlap

Using the above designations and equations (2) and (4) we may write the capacitance as

C=lE b+a

2 (b - a)h (7)

or we may use equation (3). In both cases the capac- itance is directly proportional to the overlap of the cylindrical electrodes.

  Figure 3 shows schematically a possible arrange- ment for the sensor which will minimize the effect ofsplashing ofthe liquid in a main vessel.

   We can measure the capacitance by any of the available electronic methods. E.g. a method used in a capacitive pressure sensor where a reference oscil- lator and an oscillator controlled by the changing capacitance gives a difference in frequency output. The accuracy of electronics itsel...