Electrical transmission or interconnection systems – With nonswitching means responsive to external nonelectrical... – Responsive to approach or passage of an object
Patent
1996-01-30
1998-04-14
Tran, Toan
Electrical transmission or interconnection systems
With nonswitching means responsive to external nonelectrical...
Responsive to approach or passage of an object
307517, H03K 17945
Patent
active
057395985
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a selfadjusting capacitive level switch for a non-contact or contact sensing of media or objects also in the case when a layer of electrically conductive material sticks to a sensor belonging to the switch and it also concerns a selfadjusting capacitive level switch, which is turned on or off when the medium surface level varies, always within a narrow interval of medium surface levels, regardless of a possibly present electrically conductive layer sticking to the level switch sensor.
Level switches of this kind are applicable to detect whether the surface level of the medium in a vessel has reached the sensor. The vessels may be silos, storage tanks or reservoirs, which may be open or closed, exposed to atmospheric pressure, to a variable or a steady pressure. Materials contained therein, however, may be quite diverse: liquid, granular or powdery materials of different density, viscosity and adherence, pure or mixed with rigid particles of different size, with air bubbles, with a surface foam, homogeneous or inhomogeneous, with steady or variable electric properties.
The medium surface level sensing is based on different principles. There are known mechanical, electromechanical and hydrostatic level switches, ultrasonic level switches, capacitive and conductive level switches, microwave level switches, optical and radiometric level switches.
Owing to a float or a vibrating member, mechanical and electromechanical level switches are very sensitive to material being deposited thereon and to turbulences in liquid media.
Capacitive level switches are more universal. The capacitance of the measuring capacitor within a sensor varies according to the surface level displacement of the material contained in a tank since the relative permittivity value of the material differs from 1.
There has been known a capacitive level switch VEGA FTC 968 which has been described already in the prior art section of the patent SI-92 00 073.
There are also known non-contact (proximity) capacitive level switches which detect a body approaching to the level switch sensor. Between capacitor plates an electric field sensitive to any permittivity change within its range is created by an oscillator. Non-contact capacitive level switches are very sensitive to moisture and to a contamination of the sensor plates, which have to be maintained and cleaned regularly.
Inductive non-contact level switches are not sensitive to contamination, however, they sense just metal objects. Therefore in silos or storage tanks containing loose and powdery material capacitive level switches cannot be replaced by inductive noncontact level switches when the former fail due to moisture condensing on their sensor, due to the sensor being spattered or due to an electrically conductive layer sticking to the sensor.
It is well known that the turning on and turning off of the level switch within a narrow interval of material surface levels can also be achieved by vibrational level switches "Soliphant" and "Liquiphant" of the firm Endress & Hauser for powdery or granular and liquid materials, respectively. As soon as the level switch sensor consisting of a tuning-fork excited by a piezoelectric crystal is touched by a material, the damping of the tuning-fork is changed and a relay indicating the presence of the material is turned on. The mentioned level switches, however, are very sensitive to deposits gathering on their sensor. Moreover, when using the first mentioned level switch, the range of grain sizes should not exceed a half-distance between the tuning-fork prongs and when using the second mentioned level switch, the liquid should not contain any solid inclusions.
The solution as proposed by the invention is closest resembled by the third embodiment of a selfadjusting capacitive level switch which is disclosed in the patent SI-92 00 073 of the present applicants and in numerous patent applications originating therefrom, e.g. EP-A-0 568 973 representing a prior right. This selfadjusting capacitive level switch comprises a sensor, in th
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patent: 5532527 (1996-07-01), Zatler et al.
Eferl Franc
Zatler Andrej
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