Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2000-05-24
2002-03-26
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S629000, C324S678000, C073S30400R, C327S517000
Reexamination Certificate
active
06362632
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to proximity detection and level measurement and, more particularly, to electronic circuits used in capacitive sensors that determine the presence or level of an object, fluid or materials.
BACKGROUND OF THE INVENTION
A device or system having the ability to detect and/or measure the presence, level, or quantity of particular materials, commonly referred to as a proximity detector or level sensor, has many uses. For example, proximity detectors may be used to detect or sense the level of grain, aggregate, fluids or other materials in a storage container, or to detect the presence of a metal part on a production line. Capacitive sensors are extensively used for proximity detection and level measurement. A conventional capacitive sensor, which includes one or more conductive plates, is sensitive to changes in the dielectric constants of materials or fluids, and detects the presence, or lack thereof, of material in the vicinity of the plates by measuring the capacitance between the plates, which is proportional to the dielectric constant of the material filling the space between the plates. Similarly, another conventional form of capacitive sensor, which uses a sensing antenna, e.g., a long wire or strip immersed into a tank or storage bin holding a variable level of fluid or material, measures the level of the fluid or material by sensing and measuring the capacitance of the sensing antenna.
Designing and producing proximity sensors and level sensors for use in detecting and/or measuring high dielectric substances, e.g., water, salt water, and certain plastics, is relatively straightforward because the change in capacitance of even modest-sized sensing plates is large. There is a continuing need, however, for a proximity detector or level sensor capable of detecting and/or measuring materials having a low dielectric constant, such as, for example, grain, feed, diesel fuel, and gasoline. Such low dielectric constant materials are difficult to detect or measure using conventional capacitive sensors because the relatively small changes in capacitance due to the material or fluid can become hidden by drift of the sensor electronics due to, for example, variations in power supply voltages. The detection and measurement of these low dielectric materials and fluids requires exceptionally stable circuitry to make such measurements practical. Thus, there is a continuing need for a capacitive sensor that is less susceptible to certain environmental conditions such as temperature variations and variations in the components used in the electronic circuitry associated with the sensor.
Calvin U.S. Pat. No. 4,345,167 illustrates one prior art electronic circuit used in a switched capacitive sensor. Such a circuit is illustrated in prior art
FIG. 1
, in which the electronic switch, oscillates between the reference voltage node V
1
and the node connected to the current-to-voltage circuitry at a particular frequency. Thus, the sensing antenna is repetitively charged to the reference voltage V
1
and then discharged, generating a current proportional to the amount of charge on the antenna. The current-to-voltage circuitry generates an output voltage Vo, that is proportional to the capacitance on the sensing antenna. A variation is illustrated in Philipp U.S. Pat. No. 5,730,165, which discloses a charge integrating capacitor, a simplified version of which is illustrated in prior art FIG.
2
. In this circuit, the switch is cycled a predetermined number of times, injecting charge into a capacitor, which increases the voltage across the capacitor slightly each cycle. The amplifier measures the voltage across the capacitor after the switch has cycled the predetermined number of times. Each of these two prior art circuits operate by alternatively charging and discharging a sensing plate or antenna between two voltages. The electronic charge required to effect this change in voltage is proportional to the capacitance of the plate or antenna and the magnitude of the voltage change according to the equation, q=&Dgr;VC. The capacitance between the two electrodes (e.g., plates) is proportional to the dielectric constant of the materials and/or fluids in the vicinity of the electrodes. The capacitance is based on the geometry of the electrodes and may be represented by, C=
∈r
C
0
, where C
0
is the capacitance between the electrodes with only air present therebetween, and
∈r
is the effective relative dielectric constant of the material and/or fluid between the electrodes. If the material or fluid does not completely fill the space between the electrodes, then the effective dielectric constant will be reduced from that of the material or fluid. The effective dielectric constant for the electrode arrangement, however, does not need to be calculated, only the change in capacitance of the sensor.
The prior art switched capacitor or charge pump sensor circuits measure the change in capacitance by measuring the change in charge required to change the voltage on the sensing electrode, by alternatively connecting the sensing electrode to a reference voltage, V
1
, and then to a second voltage, V
2
(which may be ground). This change in charge may be represented by:
&Dgr;
q=
(
V
1
−V
2
)
∈r
C
0
After the electrode has charged to V
2
, it is immediately re-connected to the reference voltage, V
1
. If this switching sequence is repeated at some frequency ƒ, then the repetitive injection of this charge &Dgr;q represents a current that is proportional to the effective dielectric constant according to:
I
(
∈r
)=(
V
1
−V
2
)
∈r
C
0
ƒ
This prior art sensing technique can be used as a proximity detector by merely detecting when the current exceeds a certain threshold value. Alternatively, if the conventional two plate sensing electrode is replaced by a wire (or pair of wires) immersed into a tank of fluid or granular material, the capacitance of the wire will vary as the level of material changes. This latter arrangement results in a material or fluid level sensor.
When using proximity detectors or level sensors with materials or fluids having high dielectric constants, such as aqueous solutions or pellets of high dielectric constant plastics, the change in capacitance, and therefore the change in current, is very pronounced and relatively easy to detect. For low dielectric constant materials or fluids, such as, for example, grain, feed, diesel fuel, gasoline, and certain low dielectric plastics, the change in capacitance is very slight, for example only a change of a few percent (as compared to doubling or tripling the capacitance in the high dielectric constant applications). This results in a relatively large current without the material present and only a slightly larger current when the material to be detected is present. To make this small change in current more apparent in some devices, a fixed current (which may be approximated by a resistor coupled between the second reference voltage and ground potential) is typically subtracted from the signal current. Thus, if the fixed current is adjusted to be approximately equal to the current when no material is present, then the material will produce a large relative change in net current.
Each of these prior art techniques is dependent on the stability of the reference supply voltage. The sensor current depends directly on the magnitude of the change in voltage. As the reference supply voltage fluctuates, the sensor current will also fluctuate. This dependence on the stability of the reference supply voltages is a limitation of this technique. A need remains, therefore, for a circuit used in capacitive sensors that is less susceptible to fluctuations in the reference supply voltages. Preferably, the circuit will directly and automatically compensate for variations in the reference supply voltages.
SUMMARY OF THE INVENTION
An improved circuit for detecting the change in capacitance of a sensing electrode that is less sensitive to fl
BECS Technology, Inc.
Bryan Cave LLP
Hamdan Wasseem H.
Metjahic Safet
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