DE3934007C2 - Circuit arrangement for the transmission of measured value signals from a transmitting station to a receiving station - Google Patents

Description

The invention is based on a circuit arrangement for transmission of measured value signals from a transmitter station to a receiver station over two lines according to the preamble of claim 1 specified features.

In such a, generally as a telemetry arrangement operated circuit arrangement is the measured value signal informa  tion in the form of a direct current signal from the transmitting station transmit the two-wire line to the receiving station, which the Has DC voltage supply source. The electrical The transmitter is supplied from the supply source also via the two-wire line.

A corresponding two-wire telemetry arrangement is suitable especially for use in a control loop for indu strial processes in which one in the transmitter by one Thermocouple or another sensor for the one to be measured Process variable detected value converted into a DC signal is that over the two-wire line to a remote Emp interception station for controlling displays, recording devices ten, regulators or other instruments in the process rule loop is transmitted.

A major advantage of such a two-wire telemetry arrangement voltage can be seen in the fact that one and the same line pair not only for the transmission of the current signal from the transmitting station to the receiving station, but also for the electrical supply the transmitting station from an electrical source of reception station serves, whereby separate lines for the Stromver supply of the transmitter stations is eliminated.

For example, for a process control telemetry arrangement national US standard ANSI-MC 12.1-1975 and ISA-A 50.1, "Compatibi lity of Analogue Signals for Electronic Industrial Process Instruments "indicates that the standard output signal (a sendta tion) a direct current in the range from 4 mA to 20 mA (section 3.2 of the standard) and the standard voltage signal (of the receiver) a DC voltage of 1 to 5 V (section 3.3.2 of the standard) should be. These standards are commonly used by the industry industrial process regulations adopted and practiced.

A common problem with two-wire telemetry systems is the electrical operating power of the Receiving station made of electrically supplied components of the  Transmitting station ready for reliable operation even then to be able to put when the signal current level of the two-wire cable for example within the usual range of 4 mA is small up to 20 mA. It is therefore difficult to always be confident ensure reliable operation of a two-wire telemetry system len, if the ver from the receiving station electrically care components of the transmitter a comparatively have large electrical operating power requirements like this For example, the case is when the sending station has a diffe contains profitable pressure transducers and / or a microprocessor.

If a linear regulator is used to regulate the supply supply voltage of the transmitting station, this will result in the offer of electrical power for the operation of the components of the Sending station limited, so that the driver ability of Driver components or control circuits in the transmitter station is reduced. A transmitter station in a two-wire telemetry an arrangement of the type considered here usually requires one minimal operating voltage, with a linear relationship between the operating voltage and the ability to have a cons to drive standing load exists. For example, a company at 12.5 V and 0 Ω or an operation of 24 V and 500 Ω be specified. The internal transmitter control electronics is necessarily on a power consumption of less than 3.8 mA limited when the two-wire telemetry arrangement is in the current range of 4 mA to 20 mA is operated. The smaller the minimum operating voltage, the smaller the load must be stood up.

Due to such power limitations, high elec trical power requirements of the transmitting station become necessary a four-wire system to access the electrical ver ensure the supply of the transmitter components. Go through this however, the main advantages of the line saving ver loren.

A circuit arrangement (two-wire telemetry arrangement)  type mentioned is known from DE 36 15 452 A1.

The Transmitting station of the known circuit arrangement has one Transmitter with a transducer and a transducer cut put that over a voltage regulator with the two wire line to the receiver station is connected. The voltage regulator provides regardless of any voltage fluctuations on the Two-wire line a constant operating voltage for the measurement converter ready. The power supply lines between Voltage regulators and transducers through which the supply equals current of the transducer flows are switched by a Branch branch bridged with a current limiter. Of the Current flow in the secondary acting as an additional consumer closing branch is made in accordance with one of the measurement concerned value-dependent output signal of the transducer on and off switches to a pulse modulated direct current flow through the Provide shunt branch. The pulse modulated direct current is via the voltage regulator and the two-wire line from the Voltage source removed in the receiving station. The as additional shunt branch acting consumer is therefore pulse connected to the voltage source. On the two wire line is the pulse-modulated direct current as measuring current the constant DC supply current for the transducer stores.

The prior art regarding two-wire telemetry arrangements will also refer to US Pat. No. 4,084,155 and US Pat. No. 4,158,765 referred.

The state of the art in terms of structure and effectiveness example of voltage switching regulators can for example on that Technical book: "Semiconductor circuit technology", U. Tietze - Oh. Schenk, Springer-Verlag Berlin-Heidelberg-New York, 4th Edition, 1978, and on the specialist book: "Electronics without ballast", O. Limann - H. Pelka, Franzis-Verlag GmbH, Munich, 7th edition, 1987.

The power consumption is the same for every two-wire loop  Transmitter control electronics through the external supply voltage (the source of the total power consumed in the loop) and the minimum operating current in the loop (e.g. 4 mA) limited. For example, the power consumption of the transmitter control electronics all standard two-wire transmitters to 4 mA Ver supply voltage V limited.

The transmitter control electronics requires one of the supply same voltage, 4 mA can be used as the maximum current be consumed. However, the transmitter control electronics are required a smaller voltage than the supply voltage, so a higher power consumption can be achieved.

The invention has for its object in a circuit arrangement of the type mentioned the transmitter station with a to supply essentially constant power, being this Power is sufficient to even at low signal levels a relatively complex broadcasting station, for example a Microprocessor contains supply.

This task is based on the in the preamble of claim 1 specified features solved by that the voltage regulator is a voltage step-down switching regulator, which converts the current supplied to it into a stronger current delt.

The voltage down switching regulator leads the transmitter station, whose specified operating voltage is less than the ex ternal supply voltage, a substantially constant Operating power too, even with small DC signal level is sufficient for a relatively complex transmitter station that such as a magnetic flow meter, a multiplexer and / or may contain a microprocessor. Of the Voltage step-down switching regulator delivers in the invention Circuit arrangement both under the condition of high voltage and small current as well as under the condition of small span voltage and high current are essential for the reliable function of the Sending station sufficient operating performance. The fiction  according circuit arrangement is relatively simple and nevertheless works efficiently and reliably.

Appropriate developments of the invention are in the sub claims specified.

An embodiment of the invention is described below Reference to the figures explained in more detail. It shows:

Fig. 1 is a circuit diagram of a voltage Schaltungsanord according to the invention,

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

Fig. 3 is a simplified circuit diagram to explain the available power when a linear control or when a switching control is carried out in the circuit arrangement.

Fig. 1 shows the basic elements of a circuit arrangement hereinafter known as the two-wire Fernmeßanordnung shows according to the invention, which driven a by a process variable or other variable that contains DC voltage supplied sending station, wherein said variable is 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 supply direct voltage source formed by a battery 11 is connected in series with a load resistor 12 , via which an output signal in the standard current range (for example 4 to 20 mA direct current) flows. The dropping at the load resistor 12 voltage is fed into a receiver 13 , which can be an indicator for the process variable, a recording device, a controller or another device for process control. The two-wire line W₁ and W₂ is connected to the transmitting station at terminals T₄ and T₅.

The sensor 10 can be a thermocouple, a differential pressure transducer or another device for measuring the process variables. This variable can be the flow rate, temperature or pressure, from which a proportional analog signal is generated. In the embodiment shown, this analog signal is fed into a circuit 14 controlled by a microprocessor.

The device 14 supplied by the microprocessor-controlled circuit device 14 output signal is fed into an input of a differential amplifier 15 , in the other input of which a drop-back at a feedback resistor R _FB feedback voltage is fed. The output signal of the amplifier 15 modulates an output transistor 16 , which lies between the lines W 1 and W 2 in series with the resistor R _FB .

The circuit 14 controlled by a microprocessor has supply input connections T₂ and T₃, the connection T₃ being connected to the line W₂ via the resistor R _FB at a connection T₅. The terminal T₂ is connected via a voltage down-switching regulator 17 at a terminal T₄ to the line W₁. The controller 17 is used to supply a relatively large supply power 14 to the circuit 14 controlled by a microprocessor with a constant level, which remains essentially unchanged both at high voltage and low current and at low voltage and high current.

The voltage between the terminals T₂ and T₃ is V _2-3 , while the voltage between the terminals T₄ and T₅ is V _4-5 . The current flowing into the controller 17 is designated I _ps , while the current flowing from the controller 17 into the circuit 14 controlled by a microprocessor is designated I _dc . The voltage V _2-3 is determined by supply size specifications of the components used in the circuit 14 controlled by a microprocessor. 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 current requirement of the circuit 14 controlled by a microprocessor. This is realized by the voltage step-down switching regulator 17 when the voltage V _{2-3 is} less than the voltage V _4-5 , as will be explained in the following.

Suitable switching regulators for this purpose are shown in the following explained in the publications:

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

The controller 17 contains a switching transistor 18 which is switched through and blocked at a predetermined frequency. During the interval when the switch 18 is turned on, the input voltage is fed into the input of an LC filter formed by an inductor 19 and a capacitor 20 , as a result of which the current increases. If the switch is blocked, the energy stored in the inductance 19 maintains the current flow to the load via a “catch” diode 21 .

The controller 17 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 input reference voltage with a sample of the voltage from the filter. The output voltage drops as the load increases. The error amplifier senses this drop and keeps the pulse width modulator effective for a longer period of time, thereby delivering wider control pulses to the switching transistor 18 .

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

Switching regulators are available in three basic designs available:

• (1) Downward or "counteraction" controller
• (2) Up or "boost" slider
• (3) inverting controller

According to the invention, a voltage step-down regulator is used, the function of which is described below with reference to FIG. 2. The transistor switch 18 is in series with the inductance 19 between an input DC voltage V₁ and an output DC voltage V₀, the diode 21 on the input side of the inductor and the capacitance 20 on the output side.

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

In this counteraction circuit, the peak switching current is proportional to the load current. The output voltage V₀ is equal to the product of the input voltage V₁ and the Tastver ratio. The output span is therefore in the voltage step-down switching regulator voltage always lower than the input voltage.

In a linear series or shunt regulator due to the continuous operation the power loss relatively large. Typically the efficiency is one linear regulator less than 50%. Is the difference between input and output voltage large, so the resulting efficiency significantly below 40%. in the In contrast, a switching regulator has a continuous switched 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. Since the switching transi stor is either turned on or off, this will result first, during most of the operation either a small current or a small voltage. Secondly is a good one in a wide range of input voltages  Control behavior realizable, thirdly in broad Areas of the 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 lying 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 - RI² = 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 ϑ 1.2 k ohms
= V _T = 24 - 20 (10 ^-3 ) R

For a linear control, that for the electronics of the Transmitter available maximum power (PE) by the following Given equation:

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

For a voltage step-down switching regulator, that for the electronics of the Maximum power (PE) available through the transmitter given 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)} with α = 75% applies.

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

It is now assumed that a linear regulator is used instead of a switching regulator in FIG. 1. In the case of linear control, it is essential in the arrangement that the current I _DC , ie the leakage current in the circuit 14 fed with a direct voltage of + 5V, is always the same or slightly less than the supply current I _PS . Since I _PS + I _CTL = I _OUT (the transmitter output current in the range from 4 to 20 mA) also applies, the size of the current I _DC flowing through the electronics in the transmitter must not exceed 4 mA.

Typical values for a linear controller are:

I _OUT (minimum) = 3.8 mA (input variable)
V +/- (minimum) = 12.5 V (input variable)
I _DC (maximum) = 3.8 mA at + 5V DC voltage

Power available for control electronics = 19 mW.

Maximum loop load resistance at 24 V DC and 20.8 mA

However, the arrangement according to the invention contains a scarf controller 17 according to FIG. 1 in order to supply the electronics of the transmitter with a constant supply power, so the main aspect is that the minimum power loss of the controller is such that the supply power supplied to the electronics is only slightly less than is the supply service provided by the supply source.

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

I _OUT (minimum) = 3.8 mA (input variable)
Controller efficiency (minimum) = 75% (input variable)
Loop resistance (maximum) = 550 ohms (input variable)
V +/- (minimum) = 6.5 V.

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

a) I _OUT = 3.8 mA

R _FB = 100 ohms, V at input supply voltage = 21.53 V.

a) I _OUT = 20.8 mA

R _FB = 100 ohms, V input supply voltage = 10.48 V.

A comparison of a linear control with a switching rain lung according to the invention results in the following:

• I. The switching regulator in the two-wire telemetry system ensures the electronics of the transmitter with a load impedance of 550 ohms, which is fed with a DC supply voltage of 24 V and works in a current range from 3.8 to 20.8 mA, a substantial Lich greater performance.
  The linear regulator produces 19 mW of power, while the switching regulator delivers 61.35 mW of power, which increases the available power by 222%.
• II. The switching regulator can withstand a far greater load stood to feed when the two-wire telemetry arrangement fed from a 24 V supply voltage source becomes. A linear regulator with a minimal voltage V +/- of 12.5 V equals a load of 552 ohms. On Switching regulator with a minimum voltage V +/- of 6.5 V. is equal to a load of 841 ohms. This represents one 50% increase in the possibility of control or driver capability.

Claims (6)

1. Circuit arrangement for the transmission of measured value signals from a transmitting station to a receiving station via two lines (W 1, W 2),

• - In which the transmitting station contains an electronic circuit ( 14 ) which has a signal input connected to a measuring sensor ( 10 ) and one which emits the measured value signals, the signal output (T 1) and one with a line (W 1) via a voltage regulator ( 17 ) connected supply input (T₂) and a supply input (T₂) connected to the other line (W₂), the voltage regulator ( 17 ) keeping a voltage between the supply inputs (T₂, T₂) essentially constant,
• - In which the receiving station has a line (W₁, W₂) interconnecting series connection of a supply source ( 11 ) and a load resistor ( 12 ),
• - In which the signal output (T₁) of the electronic circuit ( 14 ) to components ( 15 , 16 , 18 ) is connected, which from the supply source ( 11 ) to the lines (W₁, W₂) supplied current (J _OUT ) according to the measured value signals modulate, and
• - in which a correspondingly modulated voltage drop on the load resistor ( 12 ) is fed to signal inputs of a receiver ( 13 ),

characterized by
that the voltage regulator ( 17 ) is a voltage step-down switching regulator ( 17 ) which converts the current (I _PS ) supplied to it into a stronger current (I _dc ). 2. Circuit arrangement according to claim 1, characterized in that the switching regulator ( 17 ) contains a switching transistor ( 18 ), the duty cycle with decrease in the electronic circuit ( 14 ) supplied current (I _dc ) from a control circuit ( 22 ) increases becomes. 3. Circuit arrangement according to claim 2, characterized in that between the switching transistor ( 18 ) and the electronic circuit ( 14 ) on the one hand and between the supply inputs (T₂, T₃) of the electronic circuit ( 14 ) on the other hand, an LC element ( 19 , 20th ) with an inductor ( 19 ) as a longitudinal element, a capacitance ( 20 ) as a transverse element and an upstream, inductor ( 19 ) permeable diode ( 21 ) as a transverse element. 4. Circuit arrangement according to one of the preceding claims, characterized in that the components ( 15 , 16 , 18 ) connected to the signal output (T₁) of the electronic circuit ( 14 ) contain a control transistor ( 16 ) and that the lines (W₁, W₂ ) supplied current (I _OUT ) is divided into two partial currents (I _PS , I _CTL ), of which one partial current (I _PS ) is supplied to the switching regulator ( 17 ) and the other partial current (I _CTL ) from the control transistor ( 16 ) is modulated. 5. Circuit arrangement according to claim 4, characterized in that the control transistor ( 16 ) is controlled by a differential amplifier ( 15 ), one input of which is connected to the signal output (T₁) of the electronic circuit ( 14 ) and the other input of which the load resistor ( 12 ) leading line (W₂) downstream of a in this line (W₂) lying resistor (R _FB ) is connected.