Capacitive level meter

 

(57) Abstract:

The inventive device includes a multivibrator 1, differentiator 2,3, operational amplifier with 4 link 8 negative feedback out phase-sensitive rectifier 5, the voltage divider 6, a stable source of 7 bipolar power supply, two-electrode capacitive sensor 9. 4 Il.

Capacitive level meter is designed to measure the level of the dielectric liquid such as oil, and a signal proportional to the liquid level, and is designed for operation primarily on transport machines.

For level measurement of dielectric liquids are widely used capacitive sensors, which are based on the change in electrical capacitance of the sensing element (capacitive sensor) when changing the level of the liquid in the capacitive sensor. Along with the significant advantages of capacitive sensors in relation to other types of sensors: simple design, high manufacturability and reliability they have small values of electrical capacitance of the sensing element, comparable in magnitude with the capacity of communication lines (cables, etc is and transporting machines for remote measurement of the oil level, the increment value of capacitance of the sensing element when changing the oil level can be several times less than the capacity of the communication lines. In such circumstances, obtaining information about the capacitance is quite a challenge, and its solution is used, for example, the differential inclusion of two capacitive sensors for bridge and other circuits or high-q resonant circuits in the circuits of the signal processing of the capacitive sensors.

Small values of capacitance of the sensing element lead to dependence on its size is only a problem when high-frequency signals, therefore, in the design and manufacture of sensor is required to take into account the specific features of the production technology of high-frequency electronic technology to ensure the transmission and processing of high-frequency signals and, as a rule, to apply a high frequency generator of the reference voltage. In addition, the use of high-frequency signals for receiving and transmitting information about the capacitance of the capacitive sensor leads to a large sensitivity of the circuit selection signal to electromagnetic radiation of industrial machines and plants, such as ele is excavator or a crane with a hydraulic system, equipped with a level gauge.

Therefore, ensuring high precision measuring changes in the capacitance of the sensing element during operation by the action of the transmitter wide range of electromagnetic radiation with larger changes in the ambient temperature very difficult task. It is difficult enough to ensure the stability characteristics of the level from sample to sample in the process of production.

Known discrete capacitive level meter on author.St. N 1076763, CL G 01 F 23/26 with, to improve measurement accuracy, two sensor element of a capacitive sensor, which led to the complication of measuring and increasing its size and cost.

The closest in technical essence to the invention is a capacitive level meter on author.St. N 1201686, CL G 01 F 23/26, 1982. In this device, high sensitivity and accuracy of measurement is achieved by applying a high-frequency generator and use high-frequency reference signal AC. This places high demands on technology devices and communication lines, reduces noise and prevents removal of the capacitive sensor Glowiak significant changes in ambient temperature, for example, from +50 to -50aboutTo maintain high accuracy of liquid level measurement with this device.

The aim of the present invention is to improve the accuracy of distance measurement of liquid level.

This objective is achieved in that the capacitive level meter, containing two-electrode sensor, a second electrode which is connected to the housing, and a capacitance-to-voltage made in the form of an operational amplifier with the level of negative feedback for not inverting input of which is connected to the first electrode of the sensor, put out phase-sensitive rectifier, a voltage divider and connected in series stable source bipolar power supply, asymmetrical multivibrator, the first differentiator and a second differentiator, the output of which is connected to the inputs of the operational amplifier, the output of which is connected to the input of out phase-sensitive rectifier, to the control input of which is connected to the output of the first differentiator, the unbalanced output of the multivibrator is connected also to the input of the stabilized source of the bipolar power supply, a first outlet through which the voltage divider is connected to the second outputs stable power supply bipolar power is being supplied to the power input of the operational amplifier.

Significant features of the proposed technical solutions that contribute to solving technical challenges are:

the use of asymmetrical multivibrator as the generator of the reference voltage, and the multivibrator not only produces capacitive sensor probe pulse, but also sets the interval between voltage pulses such length that transients in the power circuit elements to the moment of generation of the next pulse has ended and did not create electromagnetic radiation, preventing the selection and processing of the useful signal. Double negation of the output voltage of the multivibrator during a small part of the oscillation period ensures low noise level at the end of a longer period of preparation for the measurement (detection);

a relatively low pulse repetition rate and detection (selection) using out phase-sensitive detector with the filling of the useful signal during a small part of the period, when the ratio of signal to noise ratio is most favorable for separation of the useful signal. Out phase-sensitive rectifier with memorization allows you to keep useful signal for sravnitelnoi, corresponding to the duration of the probe pulse;

decreasing the influence of the instability of the parameters of the elements of the multivibrator during operation and in the manufacture of the properties of the input control signal out phase-sensitive rectifier and maintaining optimal performance of this signal by suppressing the DC component of the signal of the multivibrator to the introduction of the first differentiator. Increasing demands on the stability of the DC component of the control signal out phase-sensitive rectifier stability workspace based on the need to ensure high performance out phase-sensitive rectifier when a signal memory for the duration of the corresponding very short probe pulse;

the offset of the period of detection of the useful signal in the time interval in which the output of the operational amplifier are not evident noise (interference) switching output transistors of the multivibrator by introducing a second differentiator having a time constant greater than the time constants of the filters the low frequencies that are implemented at the operational amplifier, the connection is not is delayed relative to the signal at the control input out phase-sensitive rectifier, and remembering signal of the operational amplifier occurs at precisely the time when the noise level switching is minimal, and the level of the useful signal maximum;

using the error voltage of the power inherent in the simple voltage regulators, to compensate for the change of the output signal of the sensor when changing the duty cycle signal of the multivibrator and the power supply voltage of the operational amplifier due to the introduction of a voltage divider connected between the output of the stabilized source of the bipolar power supply and the input of the operational amplifier. In the proposed device the output signal of the capacitance-to-voltage depends on the duty cycle of the reference voltage generated by the multivibrator, and on the value of the stabilized voltage supplied to the operational amplifier. In addition, the stabilized voltage bipolar power supply depends on the duty cycle signal of the multivibrator, for example, changing the duty cycle will change and the negative supply voltage (this is due to the drop in voltage on a real circuit elements of the power supply). Therefore, to compensate for the influence of ismens the voltage divider to a stable power source, moreover, the mid-point of the voltage divider is connected to the chassis (ground potential) of the device, and the gain from the input of the voltage divider to the output of the operational amplifier, which implements the capacitance, is selected by the change gear ratio of the divider in order to compensate for the effects of changes in duty cycle. The selection of the gear ratio of the voltage divider is useful during Assembly of the device, ensuring the constancy of the output signal of the device when changing the duty cycle signal of the multivibrator. This significantly reduces the requirements for the deviations of the parameter values of the elements from which we collect the device from the nominal values and provides high accuracy measurements during operation in case of change of parameters of elements of the device, for example, when the temperature of the environment.

In Fig. 1 is a block diagram of the device of Fig.2 scheme of the stabilized power supply of Fig.scheme 3 out phase-sensitive rectifier of Fig.4 stresses.

The proposed device (Fig. 1) consists of a series of multivibrator 1, the first differentiator 2, the second differentiator 3, omnica 7 bipolar power supply, both the output of which is connected to the operational amplifier 4, in addition, one output is connected to the multivibrator 1 and the other through the voltage divider 6 is also connected to the input of the operational amplifier 4, is included between the output and inverting input of the operational amplifier 4 link 8 negative feedback and connected between reinvestiruet the input and the case electrode of the capacitive sensor 9, and the output of the multivibrator 1 is connected to the input of the stabilized power source 7.

A stable source of bipolar power supply (Fig.2) can be implemented, for example, by means of the transistor 1, the Zener diode D1, diodes D2-D4, capacitors1-C3and resistors R1-R4. On a stable source of bipolar supply unregulated supply voltage, for example, the power excavator UPeteand rectangular unipolar voltage pulses from the output of the multivibrator UmVand remove the positive and negative supply voltage from pins a and B. A positive voltage is set using a voltage divider implemented on series-connected resistor R1. Flowing through the resistor R3at the base of the transistor T of the rectangular voltage pulses periodically open and close the transistor T, connecting the positive electrode of the capacitor C2or to the housing through the open transistor, or to supply the power through the resistor R2moreover , the maximum level of positive voltage is limited through the diode D2stabilized by the Zener diode D1the tension. Since the negative electrode of the capacitor C2connected to the housing through the diode D4on this electrode in the asymmetric overcharging of the capacitor will accumulate negative potential, limited modulo the maximum level of the positive voltage applied to the capacitor. This negative potential through the diode D3arrives at the filter capacitor C3and the stabilized voltage of negative polarity can be removed from the contact b stable source bipolar power supply.

Out phase-sensitive rectifier (Fig.3) can be implemented, for example, using a field-effect transistor T,Italia comes the control of the switching voltage Uy, the plot of which is shown in Fig.4,and at the main switching (detected) as an input signal the voltage output from the operational amplifier UOS, the plot of which is shown in Fig.4 in.

When a negative control voltage Uythis voltage passes through the diode D, is supplied to the gate of the transistor T and locks it, while on the capacitor remains available on it to lock the voltage. When a positive control voltage Uyat the gate of the transistor T through the resistor R goes to zero potential and the transistor is open. The capacitor C through the resistor R2charging voltage UOSc output of the operational amplifier. The time constant out phase-sensitive rectifier, equal to the product of the resistance value of the resistor R2on the capacitance of the capacitor C is selected to provide a sufficiently fast tracking voltage amplitude, and on the other hand, to provide suppression of high frequency noise. The diode D prevents saturation of the transistor T the carriers of positive charge, which reduces the switching time of the transistor from open to closed.

Capacitive level meter in a is a rectangular voltage pulses having a fixed repetition rate. This signal is fed to the input of the first differentiator 2, where the constant component of the signal of the multivibrator 1 is suppressed and the output of the first differentiator 2 passes only the variable component of the signal of the multivibrator 1. Plot of this signal is shown in Fig.4,and. Negative control signal out phase-sensitive rectifier 5 is locked. Upon receipt of a positive pulse through the first differentiator 2 from the multivibrator 1 out phase-sensitive rectifier is opened, and it is memorization switching voltage.

The voltage output from the first differentiator 2 is fed to the input of the second differentiator 3, where the constant component of the signal is suppressed, and the variable goes almost without distortion. From the output of the second differentiator 3 pulsed AC voltage is supplied to the inverting and non-inverting inputs of the operational amplifier 4 connected to the link 8 negative feedback and two-electrode capacitive sensor 9. This inclusion of the operational amplifier 4 provides a low pass filter of the first order as inverting and not inverting input, and the time constant in the first sluchayah sensor 9. The output of the operational amplifier 4 signal proportional to the difference of works equal input signals on the transfer function for inverting and not inverting channels. If a transfer function equal, i.e. equal to the capacity of the link 8 negative feedback and two-electrode capacitive sensor 9, the output signal of the operational amplifier 4 will be zero. As the capacity of the two-electrode capacitive sensor 9 depends on the liquid level, and the amplitude of the output signal of the operational amplifier 4 will change when the liquid level. The amplitude values of a signal of the operational amplifier 4 are stored and averaged on out phase-sensitive rectifier 5.

At the output of an ideal low pass filter of the first order signal would have to be in accordance with Fig.4,b. In this case, switching the sign out phase-sensitive rectifier 5 at the time of memorization would very quickly changing the voltage output from the operational amplifier 4, which significantly hampers the selection of the useful signal against the background noise. In the proposed device through the use of and accounting for lag signal when passing through the real operational userentity 3 plot of the signal at the output of the operational amplifier 4 has the form, presented on Fig.4,C. As follows from Fig.4, the memory signal of the operational amplifier 4 is in the time period when the value of the useful signal reaches its maximum and the speed of its change is minimal. In addition, the impact of the transition of the multivibrator 1 from a static to a dynamic state on the output signal of the operational amplifier 4 is not affected during the storing of the signal and the noise generated by the multivibrator in a static state of minimal and certainly less than that of the high-frequency generator. At this point, the ratio of signal to noise ratio is most favorable for separation of the useful signal.

Translation in the static state of the multivibrator (power electronic part of the device) for a time sufficient to complete the transition process in other elements of the device, allows the memorization of the useful signal with minimum noise level. A little while memorizing supply the multivibrator short, i.e., high frequency pulse, a sufficiently high frequency so that when passing through the low pass filter on the basis of the two-electrode capacitive sensor having small capacity, the properties of this filter as filter neapolitana asymmetrical multivibrator can significantly improve the signal-to-noise in the measurement process accordingly, to simplify the circuit of the device, abandoning the use of high-q resonant filters to highlight useful component of the signal, to reduce the sensitivity of the device to changes in the parameters of the components of the device during manufacture and operation.

A feature of this device is the dependence of the output signal of the capacitance-to-voltage duty cycle reference voltage generated by the multivibrator, and on the value of the stabilized voltage supplied to the operational amplifier. In addition, the feature of the device is and interdependence stable supply voltage and the duty cycle signal of the multivibrator, for example, changing the duty cycle will change and the negative supply voltage (this is due to the drop in voltage on a real circuit elements of the power supply: the transistor T, the diodes D2-D4). Therefore, to compensate for the effect of changes in duty cycle and voltage in the proposed device (see Fig.1) input of the operational amplifier 4 is connected through the voltage divider 6 to the stabilized power source 7, and the midpoint of the voltage divider 6 to the output of the operational amplifier, where implemented capacitance, is selected by the change gear ratio of the divider in order to compensate for the effects of changes in duty cycle. The selection of the gear ratio of the voltage divider is useful during Assembly of the device, ensuring the constancy of the output signal of the device when changing the duty cycle signal of the multivibrator. This significantly reduces the requirements to the variation of parameters of elements from which the unit is going, and ensures high measurement accuracy in operation when changing the parameters of the elements of the device, for example, when the temperature of the environment.

CAPACITIVE LEVEL METER, containing two-electrode sensor, a second electrode which is connected to the housing, and a capacitance-to-voltage made in the form of an operational amplifier with the level of negative feedback, not inverting input of which is connected to the first electrode of the sensor, characterized in that it introduced out phase-sensitive rectifier, a voltage divider and connected in series stable source bipolar power supply, asymmetrical multivibrator, the first differentiator and the second is Ohm out phase-sensitive rectifier, to the control input of which is connected to the output of the first differentiator, and the unbalanced output of the multivibrator is connected also to the input of the stabilized source of the bipolar power supply, a first outlet through which the voltage divider is connected to the input of the operational amplifier, and a common point of the voltage divider is connected to the housing, and first and second outputs stable source bipolar power supply connected to the power input of the operational amplifier.

 

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1 dwg

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