MXPA96006088A - Transmitter with electric circuit to inhibit the discharge of energy storage - Google Patents

Abstract

The present invention relates to a transmitter (10) coupled with the control circuit (16) carrying a current of the circuit detects a process variable that is transmitted through the circuit (16). The circuit in the transmitter has an effective Ceff capacitance to the control circuit (16). The transmitter (10) includes the isolation network coupled between the circuit (14) and the control circuit (18) which prevents the discharge of the energy stored in the Ceff capacitance towards the control circuit (16).

Description

"TRANSMITTER WITH ELECTRICAL CIRCUIT TO INHIBIT THE DISCHARGE OF STORED ENERGY" BACKGROUND OF THE INVENTION The present invention relates to transmitters for use in process control circuits. More specifically, the present invention relates to isolating the transmitting circuit of the control circuit. Process control systems are used in manufacturing plants to supervise the operation of a process. A transmitter is placed in the field and monitors a process variable, for example, pressure, temperature or flow. The transmitter is coupled to a control circuit and transmits the information through the control circuit to a controller that supervises the operation of the process. The control circuit is a two-wire circuit that carries a current that provides power for the operation or operation of the transmitter. The communication is through a field busbar standard that is a digital communications standard where more than one transmitter can be coupled with a single control circuit to transmit the detected process variable to the control room. This The standard is described in ISA 50.02-1992 Section 11. HART (R) is another standard that allows digital communication through a process variable signal of 4 to 20 mA. The circuit in the transmitter presents an effective Ceff capacitance to the control circuit. The load can accumulate in this capacitance causing the transmitter to store energy through its connections to the circuit. This energy can be discharged through the circuit. In the field bus protocol, multiple transmitters are attached to the same circuit so that the combined Ceff can be particularly large. Attempts have been made to reduce the size of the Ceff and thus reduce the amount of potential energy storage capacity of the transmitter. In addition, the Ceff varies between transmitters. Therefore, the exchange of transmitters is limited. It would be desirable to isolate the capacitance of the transmitter from the control circuit. This would prevent the discharge of energy into the circuit and would provide greater interchangeability between the transmitters regardless of their internal Ceff.

COMPENDIUM OF THE INVENTION A two-wire transmitter is coupled to a control circuit carrying a circuit current. The transmitter detects a process variable and provides an electrical representation of it to the circuit. The circuit in the transmitter has an effective Ceff capacitance that is connected directly to the control circuit. The transmitter includes a capacitance isolation network coupled to the circuit and the control circuit, and inhibits the discharge of stored energy by the Ceff capacitance effective to the control circuit. In one embodiment, the isolation network comprises at least three diodes connected in series in the direction of the current flow of the circuit current.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a simplified functional diagram of a transmitter according to the invention coupled with a control circuit. FIGURE 2 shows a modality of a capacitance isolator network in accordance with the present invention.

FIGURE 3 shows a modality of a capacitance isolator network in accordance with the present invention. FIGURE 4 shows a modality of a capacitance isolator network in accordance with the present invention. FIGURE 5 shows a modality of a capacitance isolator network in accordance with the present invention. FIGURE 6 is a cross-sectional view of the transmitter of FIGURE 1. For reasons of convenience, the articles in the figures having the same reference symbol are the same or serve an equal function or a similar function.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES FIGURE 1 is a simplified functional diagram of the transmitter 10 in accordance with the present invention. The transmitter 10 includes a sensor 12 and electronics 14 of the sensor that is coupled to the control circuit 16. The electronics 14 includes the measurement circuit 18 and the capacitance isolation circuit 20. The circuit 18 The measuring circuit includes the processing circuit 22 and the interface circuit 24. The transmitter 10 is energized through the electric current 1 which is received from the control circuit 16 which causes a voltage drop across the transmitter 10. The voltage drop is typically between 9 and 35 volts. In one embodiment, the control circuit 16 operates under the protocol of the field bus and carries digital information. The sensor 12 detects a process variable that is related to a process parameter, such as temperature, pressure or flow. The processing circuit 22 processes the process variable. For example, the processing circuit 22 can correct errors in the sensor or carry out other calculations in the process variable. The processing circuit 22 is coupled to the interface circuit 24 and controls the interface circuit 24 to transmit the process variable through the control circuit 16. In one embodiment, the interface circuit 24 receives the digital instructions through the circuit 16 that control the operation or operation of the processing circuit 22. In addition, the interface circuit 24 includes a power supply output that energizes the processing circuit 22 and another circuit in the transmitter 10.

The current I of the circuit flows through the interface circuit 24. Electronics 14 includes components that have resistance, capacitance and inductance. These components present an effective eff capacitance to the circuit 16. FIGURE 1 shows the effective Ceff capacitance through the measurement circuit 18. The Ceff is presented to the capacitance isolator circuit 20 according to the invention. During operation, the interface 24 transmits the digital information related to an output from the sensor 12 through the circuit 16, in response to a transmission data request whereby a parameter detected by the sensor 12 or other data is transmitted through the circuit 16. In the prior art systems, the energy stored in the Ceff capacitance could be discharged through the circuit 16. In the present invention, the capacitance isolator circuit 12 isolates the capacitance Ceff of the circuit 16 to prevent this discharge. However, the capacitance isolator circuit 20 does not interfere with the operation of the measurement circuit 18, allowing the current I of the circuit to flow and the digital information to be exchanged. FIGURES 2 to 5 show the different configurations of the capacitance isolator circuit 20. In FIGURES 2 to 5, the effective capacitance and the The resistance of the measurement circuit 18 is modeled as a capacitor Ceff and a resistor Refff respectively. In FIGURE 2, the isolation circuit 20 includes the diodes 30, 32 and 34. The diodes 30 to 34 are connected in the flow direction 1 of the current and are placed in series with the circuit 16. The diodes 30 to 34 they prevent a discharge of the energy stored in capacitance Ceff through circuit 16 by blocking the flow of current through circuit 16 against the flow of current I. The use of three diodes 30 to 34 provides three levels of redundancy. Any two of the diodes 30 to 34 may fail and the remaining diode will prevent the discharge of power. FIGURE 3 shows another embodiment of the isolation circuit 20. In FIGURE 3, the isolation circuit 20 includes the diodes 40, 42 and 44 connected in series with the circuit 16 in the direction of the current flow, as the current I enters the transmitter 10. The circuit 20 it also includes diodes 46, 48 and 50 connected in series in the direction of current flow as current I exits from transmitter 10. In FIGURE 3, isolation circuit 20 also includes filter capacitors 52 and 54 of RF and capacitively couple the current circuit with the electrical ground. Typically, capacitors 52 and 54 are placed on a central wall 56 of the transmitter 10 and have a value of approximately 2,000 pF. During operation or operation, the capacitors 52 and 54 provide an electrical ground short circuit for the high frequency RF signals that interfere with the operation of the measurement circuit 18. The insulation circuit 20 includes two sets of insulation diodes. This avoids the discharge of the energy in the Ceff capacitance to the circuit 16 through a path through the electrical ground and the capacitor 52 or the capacitor 54, in situations where a wire of the circuit 16 is connected to ground. Again, three levels of redundancy are provided using two sets of three diodes each (40, 42 and 44 or 46, 48 and 50), a set for any path. The discharge could occur if either capacitor 52 or 54 fails and will provide an electrical path to ground that is typically the chassis of the transmitter, (not shown in FIGURE 3). FIGURE 4 shows another modality of the circuit capacitance isolator that includes only three diodes 60, 62 and 64 to achieve the desired isolation. The diodes 60, 62 and 64 are connected in series in the direction of flow 1 of the circuit current along the input path of the current of the circuit. circuit. The central wall 56 carries capacitors 52 and 54 of RF filter. The resistors 66 and 68 have been added to the input and output paths, respectively, of the current I of the circuit. Typically, the resistance value of the resistor 66 is equal to the resistance value of the resistor 68 and has a value of 5.5 ohms. The resistors 66 and 68 are selected to have a value large enough to limit any potential energy storage in the capacitors 52 and 54 against any rapid discharge to the circuit 16. In addn, the resistors 66 and 68 and the capacitors 52 and 54 must be large enough to reduce the failure. In comparison with the circuit of FIGURE 3, the circuit of FIGURE 4 includes only three diode voltage drops while providing three levels of redundancy. FIGURE 5 shows another modality of the circuit . FIGURE 5 shows the isolation circuit 20 where the central wall 56 carries the capacitors 52 and 54. The circuit 20 also includes a full wave bridge rectifier 70 having the diodes 72, 74, 76 and 78. The output of the bridge 70 is connected to diode 80. Circuit 20 of FIGURE 5 operates in a manner similar to circuit 20 of FIGURE 4. However, bridge 70 allows connection between transmitter 10 and control circuit 16 so that is reversed without affecting the operation or operation of the transmitter 10. In addn, the bridge 70 and the diode 80 ensure that the path of the current I through the transmitter 10 always flows through at least three diodes thus isolating the measurement circuit 18 and providing three levels of redundancy. Using the example of the current flow 1 in the direction indicated in FIGURE 5, the current I of the circuit passes through the diode 72 of the bridge 70, the diode 80, the measurement circuit 18 and the diode 76 of the bridge circuit 70 . Again, if any two diodes in the current path fail, the third remaining diode prevents unwanted discharge of the energy stored in circuit 18 through circuit 16. FIGURE 6 is a cross-sectional view of transmitter 10 showing the relative posn of circuit 22 and circuit 24 and central wall 56. FIGURE 6 shows the body 100 of the transmitter which is divided into a first chamber 101a and a second chamber 101b via the central wall 56. The first chamber 101a is sealed by the end cap 102, and the second chamber 101b is sealed by the end cap 104. The first chamber 101a carries the processing and interface circuit 22/24. The RF filter capacitors 52 and 54 are placed on the central wall 56 and coupled with a circuit board carrying the bridge rectifier 70. A circuit board 110 is carried in the second chamber 101b and includes terminals that engage the current circuit 16 and enter the housing 100 through the conduit 108. The circuit board 110 includes the resistors 66 and 68. The housing 100 of the transmitter is fixed to a sensor housing 106 that carries the sensor 12 (not shown in FIGURE 6). As shown in FIGURE 6, the capacitors 52 and 54 allow the current I to enter the circuit in the first chamber 101a but filter the high frequency RF interference. The bridge 70 and the diode 80 prevent the discharge of the energy stored in the circuit 22/24 back to the current circuit 16, as explained above. The specific transmitter 10 shown in FIGURE 6 is for measuring a differential pressure. The diodes must be selected to provide a low direct voltage drop, for example the Schottky barrier diodes. It is desirable to select diodes for the bridge rectifier 70 that provide low reverse current exhaust to reduce the apparent transmitting current. In a preferred embodiment, the diodes of the bridge rectifier 70 and the diode 80 are Schottky barrier diodes. The diode 80 should be selected to provide linearity through its scale of operation. Because the diode 80 is only one "leg" of the path of current I, non-linearities can cause distortion of the signal. In one embodiment, the diode 80 has a large current capacity (eg, 30 amps) so that the voltage drop is small at low current levels of a two-wire transmitter (4-20mA). However, there is a change between the direct voltage drop and the exhaust current. Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes in form and detail can be made without departing from the spirit and scope of the invention. For example, the invention can be used in a central controller, a field-mounted controller, a configuration device (such as a unit held in the hand), a modem or other device that couples with the current circuit of two. wires In addition, insulation elements other than diodes, such as FETs, could be used.

Claims (18)

CLAIMS:

1. A transmitter for coupling with a two-wire control circuit carrying a circuit current comprising: a sensor for detecting a variable of a process; a measuring circuit coupled with the sensor that provides a transmitter output related to the process variable, the measurement circuit is energized with the circuit current and has an effective Ceff capacitance; and a capacitance isolating network operably coupled between the interface circuit and the control circuit, the isolation network comprises at least three diodes connected in such a way that the circuit current flows through three diodes simultaneously and inhibits the discharge through the control circuit of the energy stored in the Ceff.

The transmitter according to claim 1, wherein the isolation network comprises four diodes connected in a full-wave field rectifier having input terminals connected to the control circuit and an output terminal coupled with a fifth diode that is connected to the measuring circuit.

3. The transmitter according to claim 1, wherein the isolation network includes a component of the RF filter coupled to the control circuit.

The transmitter according to claim 3, wherein the RF filter component is connected through the measurement circuit and the three diodes.

5. The transmitter according to claim 3, wherein the isolation network includes a resistive element connected between the RF filter component and the control circuit.

The transmitter according to claim 3, wherein the RF filter component is placed on a central wall of the transmitter, which separates the measuring circuit from the control circuit.

7. A transmitter for use in a process control system comprising: a sensor for detecting a process variable; a transmitter housing having a first chamber and a second chamber divided by a wall that operably couples the measurement circuit in the first chamber of the housing with a two-wire control circuit and the sensor, the measurement circuit is energized by the circuit current flowing through the circuit and transmits the process variable through the circuit, where the measurement circuit has an energy storage to store the energy; the energy isolating circuit in the first housing chamber is coupled between the measuring circuit and the control circuit including at least three rectifying elements that block the discharge of energy to the control circuit from the measurement circuit, all three Rectifier elements are connected where one of the elements blocks the discharge of energy during a failure of any of the other two elements.

8. The transmitter according to claim 7, wherein the rectifier elements comprise diodes.

The transmitter according to claim 7, wherein the isolation circuit comprises four rectifying elements connected with a full-wave bridge rectifier having input terminals connected to the control circuit and an output terminal coupled with a fifth rectifier element that is connected to the measurement circuit.

The transmitter according to claim 7, wherein the isolation circuit includes an RF filtering component.

The transmitter according to claim 10, wherein the RF filter component is coupled to the control circuit and the three rectifier elements.

The transmitter according to claim 10, wherein the isolation circuit further includes a resistive element connected between the RF filter component and the control circuit.

The transmitter according to claim 10, wherein the RF filter component is placed on the wall of the transmitter.

14. A control circuit device for coupling with a two-wire control circuit carrying a circuit current comprising: a connection for coupling with the two-wire control circuit; a communication circuit to communicate the information through the two-wire control circuit and present a Ceff capacitance effective and a capacitance isolator network operably coupled between the communication circuit and the connection to the two-wire control circuit, the isolation network comprises at least three diodes in such a way that the circuit current flows through the three diodes simultaneously and inhibits the discharge of energy stored in the Ceff.

The control circuit device according to claim 14, wherein the isolation network comprises four diodes connected in a full-wave bridge rectifier having input terminals connected to the control circuit and an output terminal coupled with a fifth diode that connects to the communication circuit.

16. The control circuit device according to claim 14, wherein the isolation network includes an RF filter component coupled to the control circuit.

The control circuit device according to claim 16, wherein the RF filter component is connected through the communication circuit and the three diodes. - lí

18. The control circuit device according to claim 16, wherein the isolation network includes a resistive element connected between the RF filter component and the control circuit.