DE3934007A1 - TWO WIRE REMOTE EQUIPMENT - Google Patents

Abstract

A DC powered transmitter, eg for fluid flow rate, temperature or pressure, gives a dc signal, in accordance with the sensed variable, supplied over a two-wire line W1, W2 to a receiver 13 in which a DC voltage supply is connected to the line through load resistor 12. Across resistor 12 is an output signal in a predetermined current range for operating an indicator. The same line supplies operating power to the power input terminals of the transmitter through a switching-type step down power regulator 17 that gives constant power under high voltage-low current as well as under low voltage-high current conditions, thereby making more power available to the transmitter and increasing its load drive capability. <IMAGE>

Description

The present invention relates generally to a telemetry device, in a DC signal passing through one a variable of interest responding transmitting station via a two-wire line to a receiving station with a DC supply source is transmitted, whose Output signal for generating an operating supply for transmit the transmitting station over the same line to this is, and especially an arrangement of this kind, which a relatively large supply for the transmitting station too can deliver.

A two-wire telemetry arrangement is particularly suitable in a control loop for industrial processes in which a in a transmitting station by a thermocouple or a another sensor detected for the process variable to be measured Value is converted into a direct current signal, which has a Two-wire line to a remote receiving station to Control of indicators, recording devices, controllers or other instruments in the process control loop is transmitted. Arrangements of this kind are in the US-PS 40 84 155, 41 58 765 and 46 92 328 described.

A significant advantage of a two-wire Fernmeßanordnung is to be seen in the fact that the same line not only for Transmission of the current signal from the transmitting station to Receiving station, but also for the transmission of the operating supply from the receiving station to the transmitting station, eliminating the need for separate wires in Remote control arrangements deleted. A current output stops as well as the susceptibility of the device to noise surges minimizes and eliminates line failure problems.

For a process control telemeter, the national US Standard ANSI-MC 12.1 - 1975 and ISA-S 50.1, "Compatibility  of Analog Signals for Electronic Industrial Process Instruments " that the standard output signal (a transmitting station) a DC current in the range of 4 mA to 20 mA (section 3.2 the standard) and the standard voltage signal (of the receiver) one DC voltage from 1 to 5 V (section 3.3.2 of the standard) should. These standards are generally used by the industry for adopted and practiced industrial process regulation.

It has also been found that known Fernmeßanordnungen this In some cases, no adequate supply of for transmitting stations. Is for example the Transmitting station a differential pressure transducer (D-P converter), the one with a square root pulling arrangement working together, so are the supply size requirements these components at low input levels not fulfilled, if the arrangement in the usual Gleichstrombereich from 4 to 20 mA works. Should a microprocessor in a transmitting station may be present, this can be for reasons of supply not be feasible. It is known in a two-wire Fernmeßanordnung a linear control of Supply voltage for the transmitting station to perform. Such linear regulators limit power consumption the transmitter control circuit and reduce their driver capabilities. A two-wire transmitter is typically for one minimal power supply with specified conditions designed, with a linear relationship between the Operating voltage and the control possibility of a resistance load is realized. For example, an operation at 12.5 V and 0 ohms or even at 24 V and 500 Ohm be set. The internal transmitter control electronics is necessarily due to a power consumption of less as 3.8 mA in an operating range of the arrangement of 4 to 20 mA limited. The smaller therefore the minimum operating voltage is, the smaller the load resistance must be.  

Because of such power limitations, it can be in many Be necessary, the Fernmeßanordnung not as Two-wire arrangement, but form a four-wire arrangement. Such additional lines are used for feeding the corresponding operating supply to the transmitting station, whereby the main advantages of a two-wire arrangement get lost.

For each two-wire loop, the power consumption of the transmitter control electronics is limited by the external supply voltage (the source of total power consumed in the loop) and the minimum operating current in the loop (4 mA). For example, the power consumption of the control electronics of all standard two-wire transmitters is limited to 4 mA · supply voltage V.

Requires the control electronics in the transmitter one of the supply voltage same voltage, so can use 4 mA as the maximum current consumed. In a situation where the control electronics but a smaller voltage than the supply voltage needed, can be a higher power consumption be achieved.

The present invention has for its object to provide an arrangement of the type in question, in which the transmitter control electronics more power is supplied when this control electronics is realized by a circuit which requires a relatively small voltage with respect to the available voltage, said Power consumption in particular is not limited to a small value of 4 mA · voltage V.

This object is achieved with a two-wire telecommunication device of the aforementioned type according to the invention by the Characteristics of claim 1 solved.  

The invention thus provides a two-wire Fernmeßanordnung before, in which the same power is a DC signal from a transmitting station leads to a receiving station, wherein the Broadcasting an operating supply with essentially constant value is supplied, even at small Sufficient signal levels to relatively complex transmitting stations, like magnetic flux meters, responsive to fields Multiplexer or transmitter to supply microprocessor-based.

According to one embodiment of the invention is in a Two-wire Fermeßanordnung the aforementioned type of the Sendestaton supply supplied by a Switching regulator regulated, the two supply input terminals both in a state of high voltage and small Electricity as well as in a state of low voltage and high current a substantially constant supply size supplies.

The two-wire Fernmeßanordnung invention is here Relatively simple and cheap and still works efficient and reliable.

In the two-wire Fernmeßanordnung invention provides So a supplied with DC transmitting station, which to a process variable or other variable, a DC signal as a function of this variable, the transmitted via a two-wire line to a receiving station is in which a DC voltage source via a Load resistor is connected to the line. At this Resistor will be an output in a given Current range for controlling an indicator or a generated by another instrument. The same line delivers the operating quantity for the supply input terminals of the Transmitter via a down-switching regulator, which both in the state of high voltage and low current as well as in the  State of low voltage and high current a constant Provides operational size, thereby providing more power for the Transmitter is available and their load control capability is increased.

Further embodiments of the invention are the subject of Dependent claims.

The invention is described below with reference to FIGS the drawing illustrated embodiments closer explained. It shows

Fig. 1 is a circuit diagram of a two-wire Fernmeßanordnung according to the invention,

Fig. 2 is a circuit diagram of the basic elements of a down-switching regulator included in the arrangement and

Fig. 3 is a simplified circuit diagram for the general case of the available power when in the arrangement of a linear switching control is performed.

Fig. 1 shows the basic elements of a two-wire remote measuring device according to the invention, which includes a DC-powered transmitting station driven by a process variable or other variable, the variable being measured by a sensor 10 . The transmitting station is connected by a two-wire line W ₁ and W ₂ to a remote receiving station in which a DC supply voltage source formed by a battery 11 is connected in series with a load resistor 12 through which an output in the standard current range (for example, 4 to 20 mA DC ) flows. The voltage dropping across the load resistor 12 is fed to a receiver 13 , which may be an indicator of the process variable, a recorder, a controller, or other process control device. The two-wire line W ₁ and W ₂ is connected to terminals T ₄ and T ₅ with the transmitting station.

The sensor 10 may be a thermocouple, a differential pressure transducer, or other means of measuring process variables. This variable can be the flow rate, the temperature or the pressure, from which a proportional analogue signal is generated. In the illustrated embodiment, this analog signal is fed to a microprocessor controlled circuit 14 .

The supplied through the controlled by a microprocessor circuit 14 output signal at the terminal T ₁ is fed to an input of a differential amplifier 15 , in whose other input a dropping at a feedback resistor R _FB feedback voltage is fed. The output signal of the amplifier 15 modulates an output transistor 16 which is connected between the lines W ₁ and W ₂ in series with the resistor R _FB .

The controlled by a microprocessor circuit 14 has supply input terminals T ₂ and T ₃, wherein the terminal T ₃ is connected via the resistor R _FB at a terminal T ₅ to the line W ₂. The terminal T ₂ is connected via a switching regulator 17 at a terminal T ₄ with the line W ₁. The controller 17 is used to introduce a relatively large supply to the microprocessor-controlled circuit 14 with a constant level, which remains substantially unchanged at both high voltage and low current and at low voltage and high current.

The voltage between the terminals T ₂ and T ₃ is equal to V _2-3 , while the voltage between the terminals T ₄ and T ₅ is equal to V _4-5 . The current flowing in the regulator 17 is denoted by I _ps , while the current flowing from the regulator 17 to the microprocessor controlled circuit 14 is indicated by I _dc . The voltage V _2-3 is determined by supply size specifications of the components used in the microprocessor controlled circuit 14 . Typically, the voltage V _2-3 is a DC voltage of 5 V.

According to the invention, a current I _dc is to be generated which is greater than the current I _ps at the regulated DC voltage of 5 V in order to satisfy the power requirement of the microprocessor-controlled circuit 14 . This is realized by the switching regulator 17 when the voltage V _{2-3 is} smaller than the voltage V _4-5 , as will be explained below.

Switching regulators suitable for this purpose are in the following References:

• A. 1987 "Switchmode (A Designers Guide for Switching Power Supply Circuits and Components) of Motorola Corporation.
• B. "Applications Handbook (1987-1988)" of the company Unirode Corporation.
• C. "Linear and Interface Circuit Applications 1985" (Section C-Switching Power Design) of the company Texas Instrument.

The regulator 17 includes a switching transistor 18 which is turned on and off at a predetermined frequency. During the interval of turn-on of the switch 18 , the input voltage is fed to the input of an LC filter formed by an inductor 19 and a capacitor 20 , whereby the current increases. When the switch is disabled, the energy stored in the inductor 19 maintains the flow of current to the load through a "snubber" diode 21 .

The controller is monitored and controlled by a control circuit generally represented by a block 22 . This control circuit includes an oscillator feeding a pulse width modulator, an error amplifier and a precision voltage reference. The error amplifier compares the self-reference voltage with a sample of the voltage from the filter. As the load increases, the output voltage decreases. The error amplifier samples this drop and maintains the pulse width modulator for a longer period of time, thereby providing wider control pulses to the switching transistor 18 .

The width of the pulse determines how long the transistor switch allows a flow of current, d. H. how much electricity to Output is delivered. If the load decreases, then the Switching transistor fed narrower control pulses until the Output voltage remains at a constant value.

Switching regulators are available in three basic embodiments to disposal:

• (1) down or "counteract" controller,
• (2) up or "up" controls,
• (3) inverting controller.

According to the invention, a buck regulator is used, the function of which will be described below with reference to FIG . The transistor switch 18 is connected in series with the inductance 19 between a DC input voltage V ₁ and a DC output voltage V ₀, wherein the diode 21 is located on the input side of the inductance and the capacitance 20 on the output side thereof.

Transistor switch 18 in the counteracting circuit chops the DC input voltage to feed a variable width pulse into the simple averaging filter formed by inductor 19 and capacitance 20 . When the switch 18 is closed, the input voltage is applied to this filter, and current flows through the inductor 21 to the load. When switch 18 is open, the energy stored in the field of inductance maintains the current flow through the load.

In this reaction circuit, the peak switching current is proportional to the load current. The output voltage V ₀ is equal to the product of input voltage V ₁ and the duty cycle. In the step-down controller, therefore, the output voltage is always lower than the input voltage.

In a linear series or shunt regulator is due to the continuous operation, the power loss relatively large. Typically, the efficiency of a linear regulator less than 50%. Is the difference between input and output voltage large, it falls resulting efficiency significantly below 40%. in the In contrast, has a switching regulator with a through-connected and locked transistor switch for regulation typical efficiencies exceeding 60%.

A switching regulator has a much higher one for three reasons Efficiency as a linear regulator. As the switching transistor either turned on or off, this leads first, during most of the operation either too a small current or a small voltage. Secondly is good over a wide range of input voltages  Control behavior feasible, thirdly in wide Areas of load current achieved high efficiency can be.

These properties are further explained below with reference to FIG. 3, in which an element 23 represents the transmitter, which by a battery 11 (24 V) via the two-wire line W ₁ and W ₂ and the load resistor R in series with a voltage V _{T is} supplied. The power loss (PD) of the transmitter 23 is given by the following equations:

PD = 24 I - RI ² = V _T I (power loss of the transmitter) at I = 4 mA, PD = 96 mW - R (16 μW / ohm) = 0 R 6 K ohms at I = 20 mA PD = 480 mW - R (400 μW / ohm) = 0 R 1.2 k ohms = V _T = 24-20 (10 ^-3 ) R

For linear control, the maximum power (PE) available to the transmitter electronics is given by the following equation:

PE = 4 mA V _T at 20 mA
= 4 mA [24-20 (10 ^-3 ) * R ]
= 96 mW - (80 μW / ohm) · R

For a switching regulator, the maximum power (PE) available to the transmitter electronics is given by the following equation:

PE = α PD _MIN = α {96 MW - R (16 μW / ohm)};

where α means the efficiency of the switching regulator and

PE =. 75 {96 MW - R (16 μW / Ohm)} at α = 75%

applies.

In a practical embodiment, the required for the electronic circuit in the transmitter of the two-wire Fernmeßanordnung operating DC voltage is equal to +5 V. Referring to FIG. 1, the current drawn by the voltage source 11 is equal to I _PS , the current flowing through the circuit 17 circuit 14 is equal to current I _DC , the current flowing through the signal output transistor 16 equal to I _CTL and the current I _OUT flowing through the resistor R _FB equal to 4 to 20 mA.

It is now assumed that instead of a switching regulator in Fig. 1, a linear regulator is used. In linear control is in the arrangement then essential that the current I _DC , ie the leakage current in the fed with a DC voltage of +5 V circuit 14 is always equal to or slightly smaller than the supply current I _PS . Moreover, since I _PS + I + _CTL I applies _OUT (the transmitter output current in the range of 4 to 20 mA), the size of the electronics in the transmitter by flowing current I _DC may not exceed 4 mA.

Typical values for a linear controller are:

input variables
I _OUT (minimum) = 3.8 mA
V +/- (minimum) = 12.5V

I _DC (maximum) -3.8 mA to +5 V DC

Power available for control electronics = 19 mW.

Maximum loop load resistance at 24 V DC and 20.8 mA

However, the inventive arrangement includes a switching regulator 17 as shown in FIG. 1, to supply the electronics of the transmitter a constant supply size, the main point is that the minimum power loss of the controller is such that the electronics supplied supply only slightly smaller than that through the Supply source supplied supply is.

The consumed by the transmitter electronics current I _DC can therefore be greater than the supply current I _PS . Typical values for the switching regulator are the following:

input variables
I _OUT (minimum) = 3.8 mA
Controller efficiency (minimum) = 75%
Loop resistance (maximum) = 550 ohms

V +/- (minimum) = 6.5V

The available for the electronics of the transmitter Supply size is given by the following relationships:

• a) I _OUT = 3.8 mA R _FB = 100 ohms, V at input supply voltage = 21.53 V.
• b) I _OUT = 20.8 mA R _FB = 100 ohms, V at input supply voltage = 10.48 V.

A comparison of a linear control with a switching control according to the invention gives the following:  

• I. The switching regulator in the two-wire remote sensing arrangement provides much greater power to the sensor electronics at a load impedance of 550 ohms, which is fed with a 24 V DC supply voltage and operates in a current range of 3.8 to 20.8 mA ,
  The linear regulator produces a power of 19 mW, while the switching regulator delivers a power of 61.35 mW, resulting in an increase of the available power by 222%.
• II. The switching regulator is able to supply a much greater load resistance when the two-wire remote measuring device is powered by a 24 V supply voltage source. A linear regulator with a minimum voltage V +/- of 12.5 V equals a 552 ohm load. A switching regulator with a minimum voltage V +/- of 6.5 V is equal to a load of 841 ohms. This represents an increase of the control possibilities by 50%.

Claims (8)

1. Two-wire remote measuring device with:

• (a) supplied with direct current transmission station connected to an internal electronic circuit power supply input terminals (T ₂, T ₃) which is driven by a variables of interest to produce as a function of these variables, a Versorgungsgleichstrom- output signal in a predetermined current range,
• (B) a two-wire line (W ₁, W ₂) whose one end is connected to the transmission of the supply current to the output of the transmitting station,
• (C) a receiving station remote from the transmitting station with a DC power source ( 11 ) and a load resistor connected in series ( 12 ), which for receiving the supply current and for simultaneous power supply of the transmitting station with the other end of the line (W ₁, W ₂) are connected,
• (d) one of the power supply input terminals (T ₂, T ₃) of the transmitting station with the one end of the line (W ₁, W ₂) connecting switching regulator ( 17 ) which supplies the internal circuit with a current which is greater than the supply current ,

2. Fernmeßanordnung according to claim 1, characterized in that the controller ( 17 ) is a step-down controller. 3. Fernmeßanordnung according to claim 1 and 2, characterized in that the controller ( 17 ) includes a switching transistor ( 18 ) which is turned on and locked at a predetermined frequency to the DC power source ( 11 ) voltage to an LC filter ( 19 to 21 ). 4. Fernmeßanordnung according to one of claims 1 to 3, characterized in that the variable is a measured by a sensor ( 10 ) process variable. 5. Fernmeßanordnung according to any one of claims 1 to 4, characterized in that the sensor ( 10 ) generates an analog signal which is fed into a controlled by a microprocessor circuit ( 14 ) in the transmitting station whose output supplies the supply current. 6. Fernmeßanordnung according to any one of claims 1 to 5, characterized in that the supply current in one of the one end of the line (W ₁, W ₂) coupled output transistor ( 16 ) is fed. 7. Fernmeßanordnung according to any one of claims 1 to 6, characterized in that the sensor ( 10 ) is a differential pressure transducer. 8. Fernmeßanordnung according to any one of claims 1 to 7, characterized in that the controller ( 17 ) generates a constant supply both in the state of high voltage and low current and in the state of low voltage and high current.