Electrical transducer

An electrical transducer has a sensor outputting a value corresponding to the quantity to be measured, an analog end stage connected downstream of the sensor, a processor circuit, and an analog measurement signal transmission path. The end stage converts the sensor output signal into an impressed output current related to the magnitude of the quantity measured, the electronic transducer being controlled with the processor circuit. The electrical transducer can be scaled by the user, has low inherent power consumption and ensures high response speed because the processor circuit in normal operation is shifted temporarily into a sleep mode, in the analog measurement signal transmission path an analog scaling unit is inserted, the output signal of the sensor and at least one analog setting value are supplied to the analog scaling unit, and the output signal of the analog scaling unit is supplied to the analog end stage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electrical transducer using a two-wire process, with a quality measurement sensor. An analog end stage is connected downstream of the sensor, the sensor and the analog end stage being connected to one another via an analog measurement signal transmission path, and in cooperation with a with a processor circuit, the end stage converts the output signal of the sensor into an impressed output current with a magnitude which is a measurement of the quantity to be measured. The electronic transducer can also be controlled with a processor circuit. In addition, the invention relates to a process for indicating a the measured value with an output current which is proportional to the measured value. The measurement is performed with an electrical transducer, the transducer having a sensor, an analog end stage which is connected downstream of the sensor, and a processor circuit, the analog end stage converting the output signal of the sensor into an impressed output current with a level which is a measure of the quantity to be measured, the electrical transducer capable of being controlled using the processor circuit.

2. Description of Related Art

Electrical transducers using the two-wire process are known, for example, as pressure transmitters. The sensor which has been integrated into the transducer generally has, besides the actual sensor element, a signal conditioning unit. The sensor element can be designed both for contact measurement and also for proximity measurement. Using the sensor element and the signal conditioning unit, which is connected downstream of the sensor element, the quantity to be measured is converted into an electrical output signal which is proportional, and generally linearly proportional, to the quantity to be measured, for example, a DC voltage signal or a direct current signal. In the analog end stage which is connected downstream of the sensor, for example, to a controllable power source, the output signal of the sensor is converted into an impressed output current which represents the output signal of the electrical transducer. Here the magnitude of the output current represents a measure of the quantity to be measured, for example, a pressure which is to be measured.

The output current is generally fixed within the range from 0 to 20 mA or from 4 to 20 mA, with an output current of 0 or 4 mA representing the starting point and an output current of 20 mA representing the end point of the measurement range. If the electrical transducer is, for example, a pressure transmitter with a measurement range from 0 to 100 bar, at a pressure of 0 bar measured by the sensor the pressure transmitter delivers an output current of 0 or 4 mA, while at a pressure of 100 bar measured by the sensor, the output current of the pressure transmitter is 20 mA. The ratio of the measured pressure to the delivered output current is thus linear, so that an output current of 0 or 4 mA corresponds to zero percent of the measurement range and an output current of 20 mA corresponds to one hundred percent of the measurement range.

The advantage of the output current range from 4 to 20 mA is that an output current of less than 4 mA can be detected by a downstream evaluation unit as an error of the transducer or as a broken wire. Of course, it is also possible to choose a different range for the output current, for example, 5 to 20 mA, but an output current range from 4 to 20 mA has prevailed as the industrial standard.

Since modem electrical transducers are generally made as systems-capable intelligent transducers with which both control, and thus error correction of the measured value, as well as communication with an external control and monitoring unit, these electrical transducers usually have a microprocessor as the processor circuit. These processor circuits can process only digital information so that it is necessary for the electrical transducer or the microprocessor to have at least one analog/digital converter and at least one digital/analog converter. The transmission path of these electrical transducers thus consists of an analog sensor, an analog/digital converter, the microprocessor, a digital/analog converter and analog end stage which makes available the output current which is proportional to the quantity to be measured. In these electrical transducers using the two-wire process, the problem is now that, in the least favorable case, only 4 mA is available as a power supply to all electronic components. It follows that conventional, economical microprocessors can be operated only with a short cycle time in order to achieve the required low power consumption of the microprocessor. But this results in that with one such electrical transducer only relatively slow changes of the quantity to be measured can be detected. If fast changes of the quantity to the measured are to be transmitted without significant adulterations, fast and thus power-intensive microprocessors must be used, then the current of 4 mA which is only available in the least favorable case being insufficient.

German Patent DE 16 40 922 C3 discloses the initially described electrical transducer, in which the attempt was made to resolve the contradiction between the requirements for processing speed on the one hand and the power demand of the circuit components on the other, by the transducer having an analog transmission path and a digital transmission path which is located parallel to the latter, which is supplied with the sensor output signal, and into which the processor circuit is inserted. The analog transmission path is used here as the main transmission path for the sensor output signal, while the correction values computed by the processor circuit after conversion into analog signals are combined with the analog output signal of the sensor. In the known electrical transducer, by dividing the transmission path into an analog transmission path and a digital transmission path parallel to it, the speed of response of the transducer to fast changes of the quantity which is to be measured is increased, but in order to accomplish the required low power consumption of the microprocessor, a low clock frequency and thus low processing speed of the processor circuit are necessary.

SUMMARY OF THE INVENTION

An exemplary object of the invention is to make available an electrical transducer of the initially mentioned type which can be scaled by the user, which has low inherent power consumption and still ensures high response speed, and special low-power, and thus expensive, processor circuits can be abandoned.

This exemplary object is essentially achieved so that the processor circuit in normal operation of the transducer is shifted temporarily into a sleep mode. In the analog measurement signal transmission path an analog scaling unit is inserted such that the output signal of the sensor on the one hand, and at least one analog setting value on the other, are supplied to the analog scaling unit, and that the output signal of the analog scaling unit is supplied to the analog end stage.

It was stated above that the electrical transducer is to be scalable, i.e., that the measurement range will be adjustable by the user. If the electrical transducer is, for example, a pressure transmitter and the pressure transmitter is calibrated at the factory to a measurement range of 0 to 100 bar, this means that at the impressed output current of the electrical transducer of 4 to 20 mA, the pressure transmitter at a measured pressure of 100 bar delivers an output current of 4 mA, and at a measured pressure of 100 bar an output current of 20 mA. If at this point a different measurement range is desired by the user, the user can set this by specifying a new starting point and/or a new end point. If, for example, the measurement range is to extend only from 0 to 50 bar, the output signal of the sensor must be multiplied by a factor, in this example the factor 2, so that at a pressure of 50 bar measured by the sensor the output current delivered by the electrical transducer is 20 mA. This follows from the linear relation between the output current IAof the transducer and the output signal UPof the sensor which can be described by the following equation:
IA=f(UP)=k UP+Ck=proportionality factorC=constant

If not only the end point, but also the starting point of the measurement range are to be changed, and the measurement range is to extend, for example, from 20 to 60 bar, the output signal of the sensor must be multiplied not only by a factor, but the output signal must be reduced first by a constant which is proportional to the set starting point, so that at a pressure of 20 bar measured by the sensor the output current of the transducer is 4 mA. The proportionality factor must then be chosen such that at a pressure of 60 bar measured by the sensor a maximum output current of 20 mA flows.

As was stated initially, the power consumption of a microprocessor is generally greater than the current of 4 mA which is available in the least favorable case. To reduce the power consumption of the processor circuit which generally has a microprocessor, in the electrical transducer of the invention, in normal operation, the processor circuit of the transducer is temporarily shifted into the sleep mode. If the activity time of the processor circuit, i.e., the time during which the processor circuit is not in the sleep mode, but in the awake mode, is much shorter than the time in which the processor circuit remains in the sleep mode, the power consumption of the processor circuit can be limited by the selected measure on the average to a fraction of the nonstop consumption.

Due to the above described measure of shifting the processor circuit in normal operation of the transducer temporarily into the sleep mode, the power consumption of the processor circuit can be reduced to the required value, but this measure leads at the same time to the fact that the analog/digital converter connected upstream of the processor circuit or the downstream digital/analog converter cannot be active when the processor circuit is in the sleep mode. In the initially described transmission path of sensor, analog/digital converter, microprocessor, digital/analog converter, analog end stage, this would lead to the electrical transducer not being able to follow the change in the quantity which is to be measured with the desired response speed.

The electrical transducer is therefore further characterized in that an analog scaling unit is inserted in the analog measurement signal transmission path, to which unit the output signal of the sensor and the at least one analog setting value are supplied. This results in that the output signal of the sensor is routed not only past the processor circuit, specifically via the analog measurement signal transmission path, but scaling of the electrical transducer by the analog scaling unit is possible. Applying an analog adjustment value to the analog scaling unit ensures that the analog adjustment value remains unchanged even during the sleep mode of the processor circuit.

To implement the analog scaling unit, electronic potentiometers can be used even during the sleep mode of the processor circuit since they do not change their resistance setting and thus maintain the set scaling. In these electronic potentiometers however, it is disadvantageous that at the desired accuracy requirement they are very costly and in addition enable only a limited resolution. Therefore, the analog scaling unit is advantageously made as an analog arithmetic circuit to which as the analog setting value, at least one dc voltage signal or direct current signal is delivered.

According to one advantageous embodiment of the invention, there is at least one active integrator as the actuator for at least one dc voltage signal or at least one direct current signal, the integrator being connected to the processor circuit and to the scaling unit. Preferably the active integrator is a component of a control circuit with the processor circuit. Based on the storage property of the integrator, the dc voltage which has been set via the processor circuit or the set direct current is kept constant even during the sleep mode of the processor circuit.

Alternatively, to produce the preferably DC voltage signals via the active integrators, the voltages could also be produced via pulse width modulation, with a static state which does not require computing performance and thus which can be kept even during the sleep mode of the processor circuit by pure timer logic. But in this example, one timer is necessary for each voltage so that the required component cost is relatively high, especially when several DC voltage signals are to be produced.

If the active integrators are a component of the control circuit with the processor circuit, possible deviations of the actual voltages on the integrators from the set voltages can be corrected by the processor circuit during its short activity time. The desired possible low increment when the scaling is set, i.e., high resolution of the scaling unit, can be achieved in that for the processor circuit a microprocessor with external or integrated 10 bit analog/digital converters is used. But conventional microprocessors which have only 8 bit converters can be used, and by using the process described in DE 199 22 060 A1, a higher resolution is then achieved.

In order to implement the proportionality factor necessary when the measurement range is set using an analog arithmetic circuit, at least one analog multiplier is used. One such analog multiplier can be implemented by an analog arithmetic circuit with several transistors and several operational amplifiers. The circuit principle of the multiplier is based on the addition of logarithms according to the following equation:

To take the logarithm and raise to the exponent, the current/voltage characteristic of the semiconductor junctions is used, for which the following relationship according to Q. Shockley applies, as is recognized:

I=IS⁡(T)⁢(eKDiffmUT-1)
with:I=diode current in the conduction directionIS=temperature-dependent blocking currentm=correction factor for Shockley diode theory, andUT=voltage equivalent of thermal energy.

Advantageously the analog arithmetic circuit, in addition to the analog multiplier, also has at least one subtractor and/or at least one adder, so that with scaling not only the end point, but in addition the starting point of the measurement range can be changed. How one such analog arithmetic circuit can be built in particular with one multiplier and several subtractors and adders is detailed below in conjunction with the drawings.

It was stated initially that the impressed output current of the electrical transducer is generally 4 to 20 mA. Of course, other values for the impressed output current are also possible. To produce the minimum output current which differs from zero there is preferably one power source.

According to another advantageous embodiment of the electrical transducer, between the scaling unit and the analog end stage an attenuator with a preferably adjustable time constant is connected. Using one such attenuator, very brief fluctuations of the quantity which is to be measured can be suppressed, so that “wobbling” of the output current is prevented. The attenuator can be easily made adjustable by its consisting of different RC elements which can be selectively connected via the processor circuit. If relatively large time constants are to be accomplished with the attenuator, capacitors with a relatively large capacitance value are necessary for this purpose. But with the amount of capacitance of the capacitor the leakage current flowing via the capacitor and the temperature coefficient of the capacitor increase; this leads to an error in the measured value indicated by the impressed output current. Advantageously therefore the attenuator is a component of the control circuit with the processor circuit so that an error at the output of the attenuator is detected by the processor circuit and is compensated using the corresponding correction value.

According to the last advantageous embodiment of the invention which will be briefly mentioned here, on the electrical transducer there is a third input terminal as the third supply terminal which is connected to a detector means so that when a certain power supply voltage is applied to the third input terminal the transducer automatically switches from two-wire operation to three-wire operation. Three-wire operation of the electrical transducer is especially advantageous when the processor circuit is used not only for setting the starting and ending points of the measurement range, i.e., for scaling of the transducer, but when there is to be communication via the processor circuit with an external control and monitoring unit or programming unit. To achieve a sufficient transmission rate, the processor circuit should be permanently in the awake mode in the communications or programming mode of the processor circuit which lasts a longer time. To do this, the detector means is advantageously connected to the processor circuit, by which the latter receives a corresponding information signal when there is a corresponding power supply voltage on the third input terminal so that the processor circuit in three-wire operation of the electrical transducer does not shift into the sleep mode.

In particular, there are a host of possibilities for embodying and developing the electrical transducer as claimed in the invention. To do this reference is made on the one hand to the claims, on the other to the description of embodiments in conjunction with the drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a block diagram of one version of an electrical transducer1which is shown as a whole only schematically inFIG. 7. The electrical transducer1works using the two-wire principle, and has a sensor2for acquiring the quantity which is to be measured, and an analog end stage3connected downstream of the sensor2. The sensor2, besides the actual sensor element which converts the measured quantity into a proportional electrical quantity, has a signal conditioning unit. The signal conditioning unit generally contains a linearizer so that the output signal Upat the output of the sensor2is linearly proportional to the measured quantity, for example, a pressure value. In the analog end stage3which can be implemented, for example, by a power source, the output signal lip of the sensor2is converted into an impressed output current IAwith a magnitude which is indicative of the quantity which is to be measured.

The electrical transducer1has a processor circuit4which is used for programming and control of the electrical transducer1. The processor circuit4has several analog/digital converters5and several push-pull tristate ports6. In the electrical transducer1the processor circuit4is not connected serially between the sensor2and the analog end stage3so that the electrical transducer1has an analog measurement signal transmission path7. In the analog measurement signal transmission path7, there is an analog scaling unit8to which the output signal UPof the sensor2on the one hand and at least one analog setting value U1, U3on the other are supplied. The output signal U4of the analog scaling unit8is supplied to the input of the analog end stage3. The processor circuit4in normal operation of the transducer1is shifted temporarily into a sleep mode, by which the power consumption of the processor circuit4can be limited by the selected measure on the average to a fraction of the nonstop consumption, when the duration of the sleep mode of the processor circuit 4 is much longer than the duration of the awake mode.

Because in the analog measurement signal transmission path7there is an analog scaling unit8with only analog values at its inputs, transmission of the quantity which is to be measured from the sensor2to the analog end stage3and the conversion of the output signal UPof the sensor2into an impressed output current IAcan also take place when the processor circuit4is in the sleep mode.

The analog setting values which are supplied to the analog scaling unit8for scaling of the electrical transducer1are made available via active integrators9. The active integrators9on the one hand being connected to the processor circuit4, and on the other to the analog scaling unit8. Setting of the DC voltage signals using the active integrators9is controlled by the processor circuit4with the active integrators9connected to its push-pull tristate port6. The DC voltage signals can be controlled by return of the output signals of the active integrators9to the analog/digital converters5of the processor circuit4so that deviations of the actual DC voltage signals from the set DC voltage signals can be corrected.

The analog scaling unit8has an analog multiplier10and an analog subtractor11. Thus it is possible for both the starting point and the end point of the measurement range to be set by the user of the electrical transducer1. For this purpose, the output signal UPof the sensor2and the starting value USare sent to the subtractor11. The starting value USis computed by the processor circuit4as a function of the starting point of the measurement range chosen by the user. The difference between the output signal UPof the sensor2and the starting value USthen forms the remainder value U2which is sent as the value which is to be multiplied to the input of the multiplier10. Depending on the chosen starting and end point of the measurement range, a proportionality factor U1is sent to the second input of the multiplier10, so that the output signal of the multiplier10, and thus also of the analog scaling unit8, is a voltage signal U4which is sent to the analog end stage3which then makes available an output current IAwhich corresponds to the measured quantity.

The structure of the analog scaling unit8, especially of the analog multiplier10, will be explained below with reference toFIGS. 3 to 5.

FIG. 3shows a block diagram of a multiplier10based on logarithm addition, in which three log modules12, a subtractor11, an adder13and an e-function generator14are used. In the circuitry of the log module12shown inFIG. 3, at the output of the e-function generator14is the product

FIG. 4ashows a simple circuitry implementation of the log module12, whileFIG. 4bshows a circuit diagram of an e-function generator14. Both the log module12and also the e-function generator14are formed each by a transistor15and an operational amplifier16. The following applies to the output voltage Uaof the log module12:

Ua=-UT⁢ln⁢-UeICS⁢Rl
and the following applies to the output voltage Uaof the e-function generator14:

FIGS. 5aand5beach show a diagram of one version of an analog scaling unit8, especially the multiplier10of the electrical transducer1. The two shown multipliers10are each made as a single-quadrant multiplier which is characterized in that all input voltages must be positive and may not become zero. The multipliers10each have an even number of transistors15, by which temperature-induced deviations of the transistors15can be better compensated. It is especially advantageous if a monolithic transistor array17is used as the multiplier10, by which the voltage equivalents of thermal energy UTand the temperature-dependent blocking currents1s cancel one another, so that the correction factor m becomes “one”. To implement the multiplier10only transistors T1to T4are necessary, while transistors T5and T6are integrated in the transistor array17by the manufacturer and are used for difference amplifier applications, but here are used only as current sink access to the coupled emitters. For the multipliers10shown inFIGS. 5aand5bthe following applies to the currents I1to I4flowing through the transistors T1to T4:

The magnitude of the current flowing through the multiplier10, i.e., the current which is required by the multiplier, is determined here among others by the current I3which is used as the standard to which the other currents I1, I2and I4are referenced. The currents I1to I2are the factors of the multiplier10, the current I1representing the adjustable proportionality factor and the current I2representing the value to be multiplied, i.e., the value which is proportional to the measured quantity. The current I4represents the output quantity of the multiplier10and thus the product.

While the analog scaling unit8shown inFIG. 5aconsists simply of one multiplier10,FIG. 5bshows one preferred development of an analog scaling unit8, with one multiplier10as shown inFIG. 5aand one upstream subtractor11. On the one hand, the output signal UPof the sensor2and on the other a starting value US, which is proportional to the chosen initial value of the measurement range, are applied to the input of the subtractor11. The difference of these two voltage values UP-UScorresponds to the remaining value U2which is present at the input of the multiplier10.

One disadvantage of the single-quadrant multiplier shown inFIG. 5ais that, as has already been stated, multiplication is possible only in the first quadrant, i.e., that all input voltages of the multiplier10must be greater than zero. But since, if the starting point of the measurement range is to be adjustable, the remaining value U2can become negative, specifically when the starting value USis greater than the output signal UPof the sensor2, a constant offset current IQ1is added to the current I2. This offset current IQ1is made available using a reference voltage UREFand a resistance RQ1. This measure expands the definition range of the multiplier10a distance into the second quadrant, i.e., a negative input voltage can also be applied to the multiplier10. But since the offset current IQ1, multiplied by the proportionality factor which is set via the voltage U1, is transmitted to the current on the transistor T4, the offset current IQ1leads to a change of the current I4. To prevent one such change of the current I4, and thus an error, an additional correction current IQ2is supplied to the transistor T4. The correction current IQ2is derived from the voltage U1via the resistor R1. Making available the offset current IQ1and the correction current IQ2in the described manner leads to the two currents IQ1, IQ2with respect to the ratio of the input current I2to the output current I4completely compensating one another as long as the input current I2is positive, i.e., as long as the output signal UPof the sensor2is greater than the starting value US. Compensation takes place independently of the proportionality factor which is set via the voltage U1so that the offset current IQ1and the correction current IQ2are not involved in the measurement result. If the input current I2becomes negative, the offset current IQ1is overcompensated by the correction current IQ2. This leads to widening of the measurement range of the multiplier10into the second quadrant.

To produce a minimum output current IAdifferent from zero, there is a current source which produces a current I04by a voltage U04and resistance R04. Corresponding selection of the voltage U04and of the resistance R04can thus make available an impressed output current IAwith a minimum current IAminwhich is, for example, 4 mA as long as the input current I2is positive. In conjunction with making available the offset current IQ1and the correction current IQ2, a minimum output current IAminless than 4 Ma, for example, 3.6 mA, is possible. By means of the measures described usingFIG. 5b,that is, implementation of an offset current IQ1, a correction current IQ2and a current I04, it is possible to achieve an unambiguous signal output value 0%=4 mA. The unambiguous signal output value 0%=4 mA is achieved by the lower limit of the measurable range being less than the lower limit of the measurement range, i.e., due to the offset current IQ1a minimum output current IAminless than 4 mA, for example 3.6 mA, being possible. In this way, the electrical transducer1generates an output current IAof 4 mA which is associated for the user with the statement 0% of the measurement range only when the quantity to be measured in fact corresponds to the selected lower limit of the measurement range. Only at an output current IAless than, for example, 3.6 mA is there no longer a clear statement. An output current IAof roughly 3 mA or less is interpreted by a downstream evaluation unit as a cable break or defect of the electrical transducer1.

The desired low power consumption of the multiplier10and thus also of the entire electrical transducer1should be overall less than 4 mA, for example, a maximum of 3.6 mA, can be ensured by suitable dimensioning of the individual components of the analog scaling unit8which is shown inFIG. 5b.The maximum current depends essentially on the magnitude of the current13and the maximum proportionality factor which can be set by the voltage U1. So that the output current I4does not become too large, the proportionality factor is limited to a value less than 5, this is sufficient for the possibility of scaling the electrical transducer1by the user. Some values of the components of the scaling unit8which are shown inFIG. 5bare given below by way of example.

If as the reference voltage UREF=U3a value of 2.5 V is applied, at a resistance R3=75 kΩ a current I3of 33.3 μA flows. The proportionality factor should be a maximum 4, and for a proportionality factor of 1 the voltage U1should be 0.5 V. This results in that the resistance R1must be 15 kΩ. This resistance value is also chosen for the resistors R2and R4. At this maximum voltage U2max=2 V, thus the maximum current I2max=133.3 μA arises.

For the offset current IQ2, a magnitude of 1% of the maximum current I2maxis chosen so that the required offset current IQ2at a voltage UQ1=UREF=2.5 V can be set by a resistance RQ1=1.875MΩ. The correction current IQ2necessary at the time and thus the necessary resistance RQ2can be computed from the chosen value for the offset current IQ1as a function of the proportionality factor determined by the voltage U1. At a proportionality factor of 1, which corresponds to U1=0.5 V, the correction current IQ2must correspond to the offset current IQ1, This yields a resistance RQ2=375 kΩ. At the selected values for the individual components of the analog scaling unit8shown inFIG. 5b,for the maximum output current I4of the analog multiplier10there follows:

It can be seen from the block diagram of the electrical transducer1, illustrated inFIG. 1, that an attenuator18is connected downstream of the analog scaling unit8. This attenuator18has an adjustable time constant which is implemented by the attenuator18having several RC elements19. The desired time constant of the attenuator18can be set by the output20of the processor circuit4being selectively connected to one of the RC elements19. To do this, the output20of the processor circuit4is selectively connected to the base point of a capacitor of the RC element19.

The output of the attenuator18is connected on the one hand to the analog end stage3, and on the other to the analog/digital converter5of the processor circuit4so that the error caused by the attenuator18can be compensated via the control circuit with the processor circuit4. To do this, the processor circuit4is connected via another analog/digital converter5to the output signal UPof the sensor2. Using the available output signal UPand using the set parameters, the processor circuit4can thus compute the value which would have to be present at the output of the attenuator18. The processor circuit4compares this computed value to the actual output value and compensates for possible errors via an end stage offset integrator21which is connected to the analog end stage3.

FIGS. 6aand6bshow a block diagram and a circuit diagram of the supply principle of the electric transducer1. In two-wire operation of the electrical transducer1, only the two input terminals22and23are connected, the positive supply voltage UB+being. present at the first input terminal22. The second input terminal23is connected to the output of the analog end stage3so that the impressed output current1Afrom the first input terminal22flows via the electrical transducer1to the second input terminal23. In addition, the electrical transducer1has a third input terminal24, at which the negative supply voltage UB=is present, and a fourth input terminal 25. All four input, terminals22to25are combined in a plug connector26which is connected to the power supply27of the electrical transducer1.

If a negative voltage UP−of a certain magnitude is connected to the third input terminal24, the electrical transducer1automatically switches from two-line operation to three-line operation. To do this, the electrical transducer1has a detector circuit28which detects the flow of current via the third input terminal24. If the third input terminal24is connected to the negative operating voltage UB−, this is ascertained by the detector circuit28, whereupon the detector circuit28delivers a signal to the input of the processor circuit4, by which the processor circuit4remains permanently in the awake mode. The increased current which is required by the processor circuit4in the awake mode is made available via the first input terminal22and the third input terminal24, while the impressed output current IAflows via the second input terminal23.

In two-wire operation of the electrical transducer1, the main current path between the first input terminal22and the second input terminal23consists of a series connection of a Zener diode29and the analog end stage3. The analog end stage3shown inFIG. 6bas the power source regulates the output current IAto a value of 4 to 20 mA. All the electronics are connected in parallel to the Zener diode29, i.e., both the analog scaling unit8and also the processor circuit4are supplied with internal operating voltage by the voltage drop on the Zener diode29. While the analog scaling unit8is directly connected to the anode and the cathode of the Zener diode29, the processor circuit4is connected via its own circuitry to the Zener diode29. This circuitry has a voltage regulator30which is connected to the anode of the Zener diode29via a band-gap diode31and the base-emitter segment of a pnp transistor32. The circuitry of the processor circuit4moreover has another storage capacitor33and a voltage comparator34.

In two-wire operation of the electrical transducer1, the main current path between the first input terminal22and the second input terminal23consists of a series connection of a Zener diode29and the analog end stage3. The analog end stage3shown inFigure 6bas the power source regulates the output current IAto a value of4to20mA. All the electronics are connected in parallel to the Zener diode29, i.e., both the analog scaling unit8and also the processor circuit4are supplied with internal operating voltage by the voltage drop on the Zener diode29. While the analog scaling Unit8is directly connected to the anode and the cathode of the Zener diode29. the processor circuit4is connected via its own circuitry to the Zener diode29. This circuitry has a voltage regulator30which is connected to the anode of the Zener diode29via a hand-gap diode31and the base-emitter segment of a pnp transistor32. The circuitry of the processor Circuit4moreover has another storage capacitor33and a voltage comparator34

In two-wire operation of the electrical transducer1, the power required by the processor circuit4in the awake mode is made available by charging the storage capacitor33which has generous dimensions. The voltage comparator34monitors the charging state of the storage capacitor33and when it falls below the necessary bias, forces the downstream voltage regulator30to set its output voltage Uoutto zero via its shut-down input35and thus to cut off the current in the circuitry of the processor circuit4. The voltage regulator30is only isolated again when the charging voltage of the storage capacitor33rises above the bias set by the voltage comparator and thus makes ready enough current for the following active phase, i.e., following the awake mode, to the processor circuit4.

In three-wire operation of the electrical transducer1, the current flowing from the input terminal22is divided into the impressed output current IAwhich flows via the input terminal23, and the increased operating current which flows via the third input terminal24. If the third input terminal24is connected to the negative voltage UB−, the detector means28bridges the series resistor36upstream of the storage capacitor33in order to prevent an overly large voltage drop on the series resistor36in continuous operation of the processor circuit4as a result of the increased operating current which has then been made available.

FIG. 7shows a schematic of the connection of an electrical transducer1to a programming device37A display device can also be integrated into the programming device37so that not only data can be input via the programming device37into the electrical transducer1, but subsequently data from the electrical transducer1can be read out and displayed on the programming device37. Exchange of data takes place between the programming device37and the electrical transducer1via the fourth input terminal25of the electrical transducer1and the corresponding output terminal of the programming device37.

The fourth input terminal25is connected for this purpose via a serial interface38to the processor circuit4. So that the processor circuit4of the electrical transducer1can remain permanently in the awake mode during the programming and scaling process, the required operating voltage is made available via the programming device37on the first input terminal22and on the third input terminal24of the electrical transducer1.

It is, therefore, apparent that there has been provided, in accordance with the present invention, an electronic transducer using a 2 wire process. While this invention has been described in conjunction with a number of embodiments, it is evident that many alternatives, modifications, and variations would be or are apparent to those of ordinary skill in the applicable arts. Accordingly, the disclosure is intended to embrace all such alternatives, modifications, equivalents and variations that are within the spirit and scope of this invention.