FI74543B - SAETTING OVER THE ELECTRICAL TRUCKING. - Google Patents

Description

1 74543

Method and apparatus for electronic pressure measurement - Sätt och anläggning vid elektrisk tryckmätning

The invention relates to a method for electrical pressure measurement, in which an electrical signal corresponding to the process pressure from a sensor is converted in the measuring transmitter into an electronic message suitable for process actuators and the like.

Prior art sensors and measuring transmitters are designed to be suitable only for either static pressure measurements and signal generation, or in particular for dynamic pressure measurements and signal generation. In the foregoing, as well as in the text below, static pressure refers to an electrical message from a sensor used in process control, and dynamic pressure refers to a disturbance to this static message caused by vibration of process equipment such as pumps and piping. Typically, the static pressure is slowly changing from 15 to the dynamic pressure, which is higher in frequency. If the sensor and measuring device are designed for static pressure measurements, the dynamic pressure is a disturbance to the process being monitored and is often filtered out during amplification. Because dynamic pressure fluctuations are characterized by a high frequency and a low signal level, it is difficult to distinguish them from an unfiltered message of a conventional static pressure transmitter. For dynamic pressure measurements separately, dynamically suitable sensors and measuring elements are available for this purpose.

It is known per se that dynamic pressure signals are generated by the vibration of process actuators such as pumps and other components such as pipelines. These dynamic pressure signals can be used to monitor the condition of process equipment. If these dynamic pressure signals are suitably amplified, the variations in them can be used to anticipate the repair needs of the process equipment, such as damage to the pump impellers and fatigue damage to the piping.

It is an object of the present invention to provide a method for electronic pressure measurement which, using one and the same sensor, can first modify the electrical signals required for both actuator control and process device condition monitoring in a measuring transmitter and secondly separate the signals supplied to the receiver for the above purposes. use.

To achieve this object, the method according to the invention is mainly characterized in that 10 - the measuring transmitter emphasizes the dynamic part of the amplified signal from the sensor, the section is used in a manner known per se for controlling the actuators and the dynamic section is used for monitoring the condition of the process devices.

The invention thus offers a considerable economic advantage in a situation where it is desired to measure both static pressure and dynamic pressure at the same time, which contain higher-frequency changes with a substantially lower static pressure level. In this case, the acquisition, installation and cabling of a separate sensor and measuring transmitter for measuring the dynamic pressure can be avoided. Highlighting the high frequency portion in the measurement transmitter facilitates the acquisition of a dynamic pressure signal at the receiving member because the relative proportion of interference is attenuated. This is particularly useful when the transmission distances of the messages are long. In addition, the installation and cabling work of the equipment used in the application of the method is simple. Between the measuring transmitter and the receiving element

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3 7454 3 signal transmission can be implemented using a bipolar communication cable.

The invention also relates to an apparatus with which the method can be applied. The apparatus according to the invention mainly comprises a measuring transmitter and a receiving element, which are connected by a communication cable. The apparatus according to the invention is mainly characterized in that the measuring transmitter has an amplifier circuit by means of which an electrical signal formed by the sensor to the measuring transmitter containing a combination of static and dynamic Fig. 15a is in electronic communication via a communication cable with a measuring transmitter and is intended for generating messages for use in controlling actuators in accordance with signals from the measuring transmitter; a second circuit isolated from the first circuit, the messages from which are used to monitor the condition of the process equipment, and means arranged between the first and second circuits for separating a dynamic, high frequency signal from the first circuit to the second circuit.

The sensor of the apparatus according to the invention is dimensioned according to the static pressure measuring range, so that its dynamic pressure signal degree avoids overload problems. In addition, the structure of the apparatus allows the sensitivity number of the dynamic pressure signal to be determined by the static signal processing section, which in turn can be calibrated by conventional methods.

The fact that the circuits of the receiving member are insulated allows the equipment to be used with different types of condition monitoring devices.

4,74543

Preferred embodiments of the apparatus according to the invention are set out in the appended claims.

In the following description, with reference to the accompanying drawing, the method according to the invention and the apparatus 5 are illustrated by means of a schematically illustrated embodiment.

Referring to the figure, the sensor 2 connected to the process and the process medium 1 converts the pressure, both static and dynamic, into an electrical signal which is preamplified by a preamplifier 3. The amplified signal is applied to the actual amplifier circuit 4, the operation of which will be described later. After the amplifier circuit, the measuring transmitter has a control circuit 5. Furthermore, an electrical message is passed from the amplifier circuit 4 via a bipolar cable K2 to the receiving member 6, where the dynamic signal is highlighted. The first circuit 7 is in electrical communication via a cable K2 to the measuring transmitter 2, 3, 4. The second circuit 8 is isolated from the first circuit. Between the first circuit 7 and the second circuit 8 there is a transformer 9, whereby a message (KV) used for monitoring the condition of the process equipment is separated from the signal flowing in the first circuit to the second circuit 8. The first circuit 7 functions to generate messages (TO) for controlling the actuators.

Amplifier circuit 4 includes an inverting operational amplifier A1. Its input impedance is formed by a resistor and a capacitor C 25 and a resistor R2 connected in series. The resistor R 1 forms a feedback resistor connected between the input impedance circuit and the output impedance of the operational amplifier A1.

The electrical operation of the amplifier circuit 4 can be explained as follows with respect to the connection shown in the drawing. The amplifier circuit 30 4 emphasizes the high frequencies of the composite signal. The signal voltages varying as a function of time and U2 can also be expressed as Fourier transforms U1 (w) and U2 (w), which describe

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74543 5 distribution of the amplitude of the voltages to components with different angular frequencies w. The gain of the operational amplifier A1 of the amplifier circuit 4 shown is very large, e.g., 10 ^, and the input impedance is large compared to the resistors R. ^ 5 r2 of the amplifier circuit, and the current flowing through its input terminals is insignificant. Then the relationship between the Fourier changes of the voltages is: A R3 \ U 2 (w) = f- + -K (w) \ R1 z2 / 2 where Z2 = R2 + ^ wC = (1 + iWCR2 ^^^ wC ^ '3 = -1

At low frequencies (w << l / £ (Rn + R0) CQ) the amplifier has a strong J. c.

10 coefficient of inclination K = R -. / R ... At high frequencies (w >> 1 / (R ~ C)) J 1 i i Δ has a gain of K = R2 * (/ R ^ + / R2) · When R2 <a clear frequency-emphasizing effect is obtained with an amplifier circuit.

After the amplifier circuit 4, the operational amplifier A2 of the control circuit in the measuring transmitter controls the current flowing through the current measuring resistor 10 by means of the terminating transistor T1 15 to proportional to the voltage U2.

Accordingly, the separation of the signal by the galvanic principle in the receiving member can be explained as follows.

The signal proportional to the pressure variation is preferably transmitted by the transformer 9 from the first circuit 7 of the transmitter to the isolated second circuit 8. The operation of the transformer, diagram A, is best described by means of a reverse circuit, diagram B.

6 1 τ 74543 nl Lh2r | R -TR © —p TW '-y—

Jl) N1 p l N2 y \ U2m ^ I Lp U RLr | u2r

GRAPH A GRAPH B

The following notations are used in these diagrams = number of turns of the primary winding N2 = number of turns of the secondary winding

Lp = primary inductance of the primary winding ^ L ^ = stray inductance of the primary winding L ^ 2 = stray inductance of the secondary winding

Rl = secondary load resistance

In mating secondary quantities have been reduced primary from:

Lh2r - <VN2) 2'Lh2 10 RLr = tNl / N2> 2-RL

U2r *

The frequency response of the transformer is

(jw) =] w Lp_. R

xl (jw) RLr + ^ w Lh2r + - 'wLp ^ = jwLp 2 1 + DW (L., + Lp); j = -1 (j = imag. unit) / RLr

The ratio (L ^ 2r + Lp ^ RLr = T is the time constant of the coupling and determines the lower limit frequency w = 1 / T of the transformer, at which smaller angular frequencies 15 '* · ^ the signal transmission to the secondary decreases as the angular frequency decreases, i.e. when w << w, then 3 74543 7 U2r (jw) a * jwLp * I1 (jw)

DC (w = 0) does not pass through the transformer. At frequencies well above the lower limit frequency, the signal shift is frequency independent: U2r <j “> ~ -L,, - ^ Lp - · <jw) = h2r = w iLp * I, (jw); w >> w d 1 a

One suitable transformer is the Schaffner IT332 pulse-5 transformer, which is available as a finished component, with a main inductance Lp of 200 mH and a speed ratio N 2 / N 2 = 2 x 25 x 25 x 25 mm 1.

In the experiments, the lower limit frequency has been found to be about 6 Hz when the external load resistance was 0.47 ohms. (The own resistance of the secondary winding is 0.5 ohms, R = 0.97 ohms).

The receiving member 6 comprises a current measuring resistor 12, a capacitor 13 and a direct current source 14 in the first circuit 7. The capacitor 13 performs pre-filtering of the high-frequency part of the composite signal of the first circuit 7.

Claims (4)

1. In the case of electrical pressure measurement, in which the coming from the sensing means (2), the pressure of the process corresponding to the electrical signal is processed in the measuring transmitter (2, 3, 4, 5) for electrical information, which 5 fits the functional means of the process or the like, characterized thereof, in the measurement transmitter (2, 3, 4, 5), the dynamic part of the coming amplified (3) signal coming from the sensing means (2) is emphasized, the signal comprising the combination formed by the static part and the stressed dynamic part. the high frequency part, is led to a receiving means (6) and in the receiving means (6) the dynamic high frequency part is separated from the combination signal, the static part being used in a manner known per se for controlling functional means and wherein the dynamic part is used to control the state of process devices. 2. An electrical pressure measurement system, said system belonging to a measuring transmitter (2, 3, 4) and a receiving means (6) connected by a signal cable (1), characterized in that the measuring transmitter (2, 3, 4), 5) includes a gain circuit (4) by which the dynamic high-frequency portion of the sensing member (2) is emphasized to the measuring transmitter's forming electrical signal, which comprises the combination formed by the