SENSOR FOR MEASURING THE ELECTRICAL CONDUCTIVITY OF A FLUID MEDIUM

Abstract

The invention relates to an inductively operating sensor (1) for measuring the electrical conductivity of a fluid medium (2), having an excitation coil (3), to which an input signal is fed, and a receiver coil (4) coupled with the former via the fluid medium (2), which provides an output signal (IInd), which is a measurement for the conductivity of the fluid medium (2). To make possible the timely detection of damage to the windings of the excitation coil (3) or the receiver coil (4), or respectively a service cable for the sensor (1), which leads to leak currents or short circuits, it is proposed by the invention that the sensor (1) has means (5) for measuring a variable signal at the input of the excitation coil (3). The excitation coil (3) of the sensor is preferably fed by an input voltage (UErr), and the means (5) for measuring the variable signal preferably measure the input current (IErr) at the input of the excitation coil (3).

Description

FIELD OF THE INVENTION

[0001] The present invention relates to an inductively operating sensor for measuring the electrical conductivity of a fluid medium, having an excitation coil, to which an input signal is fed, and a receiver coil coupled with the former via the fluid medium, which provides an output signal, which is a measurement for the conductivity of the fluid medium.

[0002] Such sensors have an excitation coil which is for example designed as a toroid coil and is fed by an a.c. voltage. A ring-shaped magnetic alternating field is generated in the interior of the excitation coil. A receiver coil, which can also be designed as a toroid coil, is arranged at the same level in which the excitation coil also lies. Because of the magnetic alternating field in the excitation coil, mobile ions in the fluid medium generate a ring-shaped current in the fluid medium to be measured, which in turn triggers an output signal in the receiver coil, whose strength is a function of the mobility and concentration of the ions, and therefore of the electrical conductivity of the fluid medium. Customarily the output signal appears as an induction current.

[0003] Sensors of this type are preferably employed in the food or drug industry for monitoring the production processes in production installations for producing food or drugs. The sensors must always provide an accurate and dependable output signal, so that excessive changes in the conductivity of the medium to be measured can be rapidly detected and a correspondingly rapid reaction takes place in order to be able to prevent deterioration of the food or drugs to be produced. The appropriate reactions to a change in conductivity can be triggered either indirectly by the production crews, or directly by the production installations.

[0004] In the course of its employment, the sensor can be exposed to strong mechanical and thermal stresses. Because of this, damage to the windings of the excitation coil or the receiver coil can occur in some cases. Leak currents, or even short circuits, can occur between the damaged windings. The output signal can be distorted because of the leak currents between the windings, and a short circuit between the windings renders the entire sensor unusable.

[0005] Moreover, because of the mechanical or thermal stresses on the sensor, a short circuit or a break in the service cable for the sensor can occur. It is clear that because of this, the output signal can also be distorted, or respectively the entire sensor can become unusable.

[0006] Initially, the distorted signal is not detected as such by the production personnel, or respectively the production installation. Initially, the production personnel, or respectively the production installation assumes that the output signal has detected a changed conductivity of the medium to be measured and reacts accordingly by matching the production processes to the new conductivity values of the medium. Only after some time, or respectively in case of a considerably distorted output signal, will it be possible to detect, for example by means of a probability check, that the output signal is distorted, or respectively that the sensor is defective.

[0007] Normally, production is continued during this time. Changes in the production process can occur because of the distorted output signal, which over time can lead to the production of a defective product. This can lead to the necessity of destroying the entire running production batch for safety reasons in order to dependably preclude any endangerment of the health of the purchasers because of defective food or drugs. This entails considerable costs. In accordance with the prior art it is not possible at all, or respectively possible much too late, to detect damage to the windings of the excitation coil or the receiver coil, or respectively the service cable of the sensor.

OBJECT AND SUMMARY OF THE INVENTION

[0008] The present invention is therefore based on the object of designing and further developing a sensor of the type mentioned at the outset in such a way that it allows an early detection of damage to the windings of the excitation coil or the receiver coil, or respectively of the service cable, which could lead to leak currents or short circuits.

[0009] For the attainment of this object the invention proposes, based on a sensor of the type mentioned at the outset, that the sensor has means for measuring a variable signal at the input of the excitation coil.

[0010] It was noted in accordance with the invention that, in case of the appearance of leak currents or short circuits, damage to the windings of the excitation coil or receiver coil results in a drastic increase in a variable signal at the input of the excitation coil. If the input signal is in the form of a voltage, the input current at the input of the excitation coil will increase as a result of the damage to the sensor. In this case the means provided will measure the input current. If the input signal is in the form of a current, the input voltage at the input of the excitation coil will rise because of the damage. In this case the means provided will measure the input voltage.

[0011] This signal at the input of the excitation coil acts in the same way in case of damage to the service cable of the sensor, which might lead to leak currents or short circuits. The variable signal at the input of the excitation coil thus provides rapid and dependable information regarding the ability of the sensor to function. Damage to the windings of the excitation coil or the receiver coil, or respectively to the service cable of the sensor, which result in leak currents or short circuits, can be detected early and dependably by monitoring this signal at the input of the excitation coil.

[0012] The production crew can react without delay to such a detected sensor defect. For example, production can initially be stopped in order to prevent the production of defective products. The defective sensor can be exchanged for a new one, and production can then be started again. In addition, it would also be possible to perform a measurement check of the conductivity of the medium to be measured in order to check whether the sensor is actually defective. The shut-off and subsequent restart of production can also be performed directly by the control device of the production installation, without the production crew having an input on this.

[0013] It is proposed in accordance with an advantageous further development of the invention that the sensor has a voltage source, which feeds an input voltage to the excitation coil, and that the means for measuring the variable signal pick up the input current at the input of the excitation coil.

[0014] Preferably the means for measuring the input current have a multiplier and measure the voltage dropping across the multiplier. Since the voltage changes proportionally with the input current, it is possible to determine the input current at a sensor designed in this way in a simple manner.

[0015] In accordance with another advantageous further development of the present invention it is proposed that the sensor has a measured value transducer for receiving the output signal, which is connected with the means for measuring the variable signal at the input of the excitation coil, that the means generate a status signal, which is a function of the measured value of the variable signal at the input of the excitation coil, and that the means feed the status signal to the measured value transducer. In this way, during the measurement operation the measured value transducer is always aware of the ability of the sensor to function. The status signal lies within a defined threshold range as long as the sensor functions. However, if the monitored variable signal at the input of the excitation coil steeply increases as a result of damage, the means for measuring the variable signal generate an appropriate status signal which lies outside of the threshold range. The measured value transducer can appropriately react without a time delay to such a status signal, from which it determines the lack of the ability of the sensor to function. As a reaction, the measured value transducer can stop the entire production, for example, so that a production of defective products does not even occur.

[0016] In accordance with a preferred embodiment of the present invention it is proposed that the measured value transducer corrects the output signal as a function of the strength of the status signal. If damage to the windings of the excitation coil or receiver coil, or respectively of the service cable of the sensor, only results in a slight distortion of the output signal, this will also lead to a small change of the variable signal at the output of the excitation coil. The measured value transducer can react to such a change in the variable signal for example with a corresponding correction of the output signal. By means of this it is possible to continue the assurance of the function free of defects of the sensor.

[0017] In accordance with a further preferred embodiment, the measured value transducer causes a signal to be issued if the status signal lies outside of a defined threshold range. This report can be merely used to inform the production crew, which can then react accordingly. However, this report can also have the character of an alarm signal, which automatically triggers defined reactions, or respectively stops the production installation.

[0018] A preferred exemplary embodiment of the present invention will be explained in greater detail in what follows, making reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 shows a sensor in accordance with the invention in a preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] An inductively operating sensor in accordance with the invention is identified as a whole by 1 in FIG. 1. The sensor 1 is used for measuring the electric conductivity of a fluid medium 2. The sensor 1 has an excitation coil 3 designed as a toroid coil, which is fed by an alternating voltage UErr. A ring-shaped alternating magnetic field is generated in the interior of the excitation coil 3. A receiver coil 4 is also arranged on the same level on which the excitation coil 3 is located and is also designed as a toroid coil. A ring-shaped current IMed is generated in the fluid medium 2 to be measured by ions moving in the fluid medium 2 because of the alternating magnetic field in the excitation coil 3, which in turn triggers an induction current IInd in the receiver coil 4. The strength of the induction current IInd is a function of the mobility and concentration of the ions and therefore of the electric conductivity of the fluid medium 2.

[0021] The sensor 1 has means for measuring the input current IErr, which are identified as a whole by the reference numeral 5. The means 5 for measuring the input current IErr have a multiplier R and measure the voltage U dropping across the multiplier R. Damage to the windings of the excitation coil 3 or the receiver coil 4, or respectively to the service cable (not represented) of the sensor 1, which result in leak currents or short circuits, can be detected early and dependably by monitoring the input current IErr of the excitation coil 3.

[0022] It is conceivable for the sensor 1 to have a measured value transducer (not represented) for receiving the induction current IInd, with which the means 5 for measuring the input current IErr are connected. The means 5 for measuring the input current IErr generate a status signal, which is a function of the measured value of the input current IErr and which is supplied by the means to the measured value transducer. Now the measured value transducer can correct, for example, the induction current IInd as a function of the strength of the status signal, so that an error-free function of the sensor 1 is assured in spite of damage to the sensor 1. But the measured value transducer can also cause the issue of an alarm signal, if the status signal lies outside of a defined threshold range.

Claims

1. An inductively operating sensor (1) for measuring the electrical conductivity of a fluid medium (2), having an excitation coil (3), to which an input signal is fed, and a receiver coil (4) coupled with the former via the fluid medium (2), which provides an output signal (IInd), which is a measurement for the conductivity of the fluid medium (2), characterized in that the sensor (1) has means (5) for measuring a variable signal at the input of the excitation coil (3).

2. The sensor (1) in accordance with claim 1, characterized in that the sensor (1) has a voltage source, which feeds an input voltage (UErr) to the excitation coil (3), and that the means (5) for measuring the variable signal pick up the input current (IErr) at the input of the excitation coil (3).

3. The sensor (1) in accordance with claim 2, characterized in that the means (5) for measuring the input current (IErr) have a multiplier (R) and measure the voltage (U) dropping across the multiplier (R).

4. The sensor (1) in accordance with one of claims 1 to 3, characterized in that the sensor (1) has a measured value transducer for receiving the output signal (IInd), which is connected with the means (5) for measuring the variable signal at the input of the excitation coil (3), that the means (5) generate a status signal, which is a function of the measured value of the variable signal at the input of the excitation coil (3), and that the means (5) feed the status signal to the measured value transducer.

5. The sensor (1) in accordance with claim 4, characterized in that the measured value transducer corrects the output signal (IInd) as a function of the strength of the status signal.

6. The sensor (1) in accordance with claim 4 or 5, characterized in that the measured value transducer causes a signal to be issued, if the status signal lies outside of a defined threshold range.