Capacitive level gauge

 

(57) Abstract:

The inventive capacitive sensor contains an asymmetrical multivibrator 1, two differentiator 2, 3, operational amplifier with 4 link 9 negative feedback memory out phase-sensitive rectifier 6, the element offset 7, stable source 8 of the bipolar power dualtronic capacitive sensor 10. 3 Il.

The invention is intended for measuring the level of the dielectric fluid, such as oil, and the formation with the fluid specified level and is designed for operation primarily on transport machines.

Known discrete transmitter (ed. St. N 1076763, CL G 01 F 23/26, 1984), with the aim of improving measurement accuracy two capacitive sensing element of the sensor and, consequently, have a relatively high complexity, size and cost.

The closest in technical essence to the invention is a capacitive level meter. A disadvantage of the known device is high enough accuracy in distance measurement. This is due to the fact that this device is difficult to implement the operation stability and a stable oscillator and the high frequency reference signal AC demands high level of manufacturing technology devices and communication lines, reduces noise and prevents removal of the capacitive sensor from the other elements of the transmitter.

The aim of the invention is to improve the accuracy of remote control of liquid level.

This is achieved in that in the transmitter, containing two-electrode sensor, a second electrode which is connected to the housing of the sensor, the threshold device and the 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, has introduced an element of bias memory out phase-sensitive rectifier 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 memory out phase-sensitive rectifier, a control input connected to the outputs of the first differentiator and item bias, whose input is connected to the first output stable source bipolar power supply, the outputs of which are connected with the power input operating gain the tx2">

Features of the proposed technical solutions that contribute to solving technical challenges are: the use of an asymmetrical multivibrator as the generator of the reference voltage. And not only multivibrator produces a 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; a relatively low pulse repetition frequency; detection (selection) of the useful signal during a small part of the period; the offset of the period of detection of the useful signal in the time interval, in which no apparent noise (interference) switching output transistors of the multivibrator; the implementation of double shifts characters of the output voltage of the multivibrator during a small part of the oscillation period, which ensures a low noise level at the end of a longer period of preparation for the measurement (detection).

In Fig. 1 is a block diagram of the device of Fig.2 scheme of the out phase-sensitive wyboista in accordance (Fig.1) consists of a series of multivibrator 1, the first differentiator 2, the second differentiator 3, the operational amplifier 4, out phase-sensitive rectifier 5 and the threshold device 6, item bias 7, stable source 8 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 element offset 7 to the control input out phase-sensitive rectifier 5, connected between the output and inverting input of the operational amplifier 4 link 9 negative feedback and connected between reinvestiruet the input and the case electrode of the capacitive sensor 10.

In accordance with Fig.2 out phase-sensitive rectifier can be implemented, for example, using a field-effect transistor T, diode D, a capacitor and two resistors R1and R2. On the control input out phase-sensitive rectifier is fed controls the switching voltage Uy, the plot of which is shown in Fig.3A, and the main switching (detected) as an input signal the voltage output from the operational amplifier Uoy, the plot of which is shown in Fig.3V.

When a negative control voltage Uya With 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, 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.

The transmitter operates as follows.

After switching on the output of the multivibrator 1 arise periodically 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 is only the variable component of the signal of the multivibrator 1. This signal is added to the signal element of smites 5 level, providing optimum performance out phase-sensitive rectifier and supplied to the control input out phase-sensitive rectifier (plot of this signal is shown in Fig.3A).

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, to which are respectively connected link negative feedback 9 and two-electrode capacitive sensor 10.

This inclusion of the operational amplifier 4 provides a low pass filter of the first order as inverting and not inverting inputs, and the time constant in the first case is determined by the capacity of the negative link is La 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 level of negative feedback 9 and two-electrode capacitive sensor 10, the signal at the output of the operational amplifier 4 will be zero. As the capacity of the two-electrode capacitive sensor 10 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 remembered for out phase-sensitive rectifier 5 and fed to a threshold device 6, which outputs a signal when reaching the output signal out phase-sensitive rectifier preset level, i.e., when the level is reached by the liquid.

At the output of an ideal low pass filter of the first order signal would have to be in accordance with Fig.3b. 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 would impede 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 a real operational amplifier 4, predstavlyayushie at the output of the operational amplifier 4 has the form, presented on Fig.3V. As follows from Fig.3b, 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 condition of the power electronic part device multivibrator long enough 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 f the way the use of asymmetrical multivibrator allows to improve the signal-to-noise in the measurement process, respectively, to simplify the circuit of the device, abandoning the use of high-q resonant filters to reduce the sensitivity of the device to changes in parameter values of the components of the device during manufacture and operation.

CAPACITIVE SENSOR containing two-electrode sensor, a second electrode which is connected to the housing of the sensor, the threshold device and the 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 an element of bias storage out phase-sensitive rectifier 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 memory out phase-sensitive rectifier, a control input connected to the outputs of the first differentiator and element smeatharpe connected to the power input of the operational amplifier, and the output of memory out phase-sensitive rectifier connected to the input of the threshold device.

 

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