EP1301914B1 - Measuring device for measuring a process variable - Google Patents

Abstract

The invention is directed to a measuring device for measuring an industrial process variable with a predetermined maximum power consumption by the measuring device. More specifically, the invention relates to a measuring device for connection to a current loop, in particular a 4-20 ma current loop, or to a digital communication, comprising devices for regulating the measuring operation of the measuring device in adaptation to the predetermined power consumption, wherein the regulating devices regulate to power consumption by the measuring operation of the measuring device in such fashion that this power consumption is approximated to the predetermined power consumption without the predetermined power consumption being exceeded.

Description

The invention relates to a measuring device for measuring an industrial Process variables for a given maximum power consumption by the Measuring device. More particularly, the invention relates to a measuring device for Connection to a current loop, in particular a 4-20 mA current loop, or to digital communication.

Means for measuring a process variable are used to measure a Process variable to record and the measured values for subsequent Pass on processing. The measured values can be passed on via a current loop happen or via digital communication. In both It is advantageous if the measuring device derives its required power from the from both lines, through which the measured value is passed on.

When the measured values are passed on via a current loop, the current in the Current loop set so that its size is the size of the process variable reflects. A current has prevailed today, the currents between 4 mA and 20 mA, with a current of 4 mA through the Current loop the maximum (or minimum) measured value and a current of 20 mA represents the minimum (or maximum) measured value of the process variable.

This measuring technique proves to be largely insensitive to interference and has large Experience spreading in industrial application.

There is only one measuring device that is supplied by means of a current loop limited power available. This performance depends on the Supply voltage and the current (according to the measured value to be output) set current. Conventional measuring devices are dimensioned that they get along with the minimum available power, i.e. just the power available at minimum current and voltage need. If more power is available, this additional power is a current stage converted into power loss and not in the measuring device for Improvement in measurement used.

Measuring devices that are controlled via digital communication, often have a constant current draw as this is for data transmission necessary is. Here the available performance depends on the applied terminal voltage. Conventional measuring devices are also here designed so that the measuring circuit has a constant power consumption, the corresponds to the power with minimum supply voltage. additionally Offered power with a larger supply voltage is also here Power loss implemented.

From EP 0 687 375 an improvement proposal is known in which a intelligent sensor is equipped with a sensor circuit. The Transducer is operated at a measuring frequency that corresponds to a power consumption which is greater than that at minimum current and minimum Voltage available through the current loop. Is it due to this? a deficit (i.e. the power consumed exceeds the permissible available Power), then the sensor circuit detects this deficit and causes the Execution of the measurement program is suspended until the deficit is no longer consists.

However, this leads, among other problems, to repeated incorrect output Measured values, which is not acceptable.

The object of the invention is a measuring device of the type mentioned to be specified which is capable of displaying the measured value incorrectly, adapt their performance requirements to the available performance.

The total power consumed should be as accurate as possible Fulfillment of the measurement task that, on the one hand, speed and quality of the measurement can be optimized. Theoretically, the entire Power that corresponds to the measured value to be displayed by the correspondingly frequent function of the sensor is used up. In practice but for safety's sake there is still a certain difference between the Available power and used to fulfill the measurement task Performance remains, so there is no performance deficit and therefore no malfunction of the sensor can arise. The excess of power is in the Measuring device converted into power loss (heat). The sum of both The services received must be so large that the total of the Sensor current consumed corresponds to a defined value. That value is for the sensor within a current loop (4 - 20 mA) by the current specified measured value to be output.

In the digitally communicating sensor, for example, the value of corresponds to constant current consumed in connection with the general requirements with the communication protocol used.

According to the invention, the in the independent Characteristic combinations defined claims.

Advantageous configurations are defined in the dependent claims. Basically, in the most preferred embodiments, the Invention the desired adjustment to perform the measurement task power consumed to the power available without it Exceeding enables the current excess of power, the should be converted into power loss, is determined. After investigation of this current excess, the control unit of the sensor is able to through appropriate measures regarding the type and frequency of implementation of the Measuring cycles the power consumption of the measuring device to the predetermined maximum to approximate available power so that the excess is minimized without to fall below a certain predetermined limit for the surplus. (Ideal the excess at this limit is at least approximately zero.)

The current surplus can be determined either by direct Measurement of the excess current or the excess power. But it is also possible indirectly, by measuring current or power consumed for carrying out the measuring task and measuring available performance or knowledge of available Current to determine the current excess via difference. If you choose that By way of indirect surplus determination, one can make a significant simplification achieve with little disadvantage that on single measurements for the determination of current or power is dispensed with and this by suitable Estimates and compliance with larger reserves are replaced.

In addition, it is often possible to identify yourself in order to carry out the Measurement task of power consumed on the power consumption of the Restrict circuit parts that are known to weigh the most fall.

The invention is suitable for any measuring devices for process variables, if these measuring devices have external power consumption, usually one varying maximum power consumption is specified. It is about for example, the specification of the power consumption when using a current loop, because here (varying with the measured value to be displayed) only the maximum amount of power that may be consumed corresponds to the current, that flow in the supply lines to display the correct measured value can.

It is of course conceivable that the limitation of the performance that the Measuring device may consume from other points of view, for example when connecting to digital communication or off completely different reasons.

The invention is particularly suitable for sensors such as, for example Liquid level sensors. The invention will now be described with reference to two Described embodiments which are, on the one hand, a radar level sensor, on the other hand, it is an ultrasonic level sensor. Such sensors are regularly used today via current loops or digital ones Communications (Profibus PA, Fieldbus Foundation, ...) are operated and are therefore exposed to the difficulties to be overcome according to the invention.

A preferred implementation of the invention uses a current stage that generally switched on parallel to the other components of the measuring device becomes. The current stage serves to consume the power ("power loss"), which is left if you look at the total (through the measured value display function) predetermined power the power requirement of the measuring device deducted in measurement mode. This unused excess power is as stated, a measure of the reserve in the system for an increase the measuring performance is still available without it being the same as in the prior art Technology (EP 0 687 375) specified deficit comes.

Such a current stage offers various options for measuring the Excess power, as in the following using exemplary embodiments will be described later.

For this, the current excess power can be measured directly. He can alternatively be predicted. Known data can be used for this the measuring device, for example the relatively large power consumption of individual Components.

It is also not always necessary to take a constant measurement or calculation of yourself always changing performance requirements. There is an easier solution in the total area available, for example 4-20 mA to be divided into sub-areas, each of which has a certain frequency of Measurement per time unit is assigned. It is very easy to achieve that in the sub-area that has the highest predetermined power consumption corresponds, is measured relatively frequently, while in the sub-areas that correspond to lower available services, in principle accordingly is measured less frequently.

It then only has to be monitored in which of these sub-areas the System is currently working, for example when connecting a 4 - 20 mA Current loop depends on which measured value must be output and which current this corresponds to in order to then change the operating mode accordingly choose.

The connection of the measuring device to or from digital communication connected current loop, enables completely analog measures to achieve of the same advantages.

Preferred embodiments of the invention are illustrated below using the example Measuring devices according to the invention described. There is a measuring device always from a generic part that corresponds to FIGS. 1, 2 or 7 corresponds, as well as a connection to the supply according to Figures 3 to 6 or 8 to 13.

A first exemplary embodiment of a measuring arrangement according to the invention is a radar level sensor. The sensor measures the level in one Container. The measured value is either via a current loop with e.g. 4 - 20 mA or via digital communication, e.g. a fieldbus, passed.

Figure 1 shows part of such a radar sensor (101). The is shown generic part that is independent of how the measured value is passed on.

A power supply unit (102) is used to supply energy to the sensor (101) Supply lines (14) and (15) are connected to a current stage.

The sensor is controlled by a microcontroller (106), whose program is in a program memory (107). He uses one for his data EEPROM (109) and RAM (108). The microcontroller controls the HF front end (103), which generates radar signals, sends them to the antenna (114) and the received signals processed. These signals are received by the receiver (104) processed and digitized by means of an A / D converter (105) to the Microcontroller forwarded. The microcontroller determines from the digital signals a measured value. He transfers this after a possible conversion a control line (16) to the current stage (see below), which depending on a current, or to the digital interface that the measured value passes on via digital communication. The control lines (16) and (17) are used as a connection to the digital interface. To reduce the power, the microcontroller has the option of using the HF front end, the receiver or other circuit parts via standby signals in to put an idle state with reduced power consumption, or switch it off completely, as described below. To measure the Current power consumption of the sensor may be used for measuring lines (18) - (20) and an A / D converter (110), which is connected to the microcontroller (106) connected is. The microcontroller has a reduced mode Current consumption. Capacitors (111), (112), and (113) reduce the Current fluctuations that occur when the components are switched on and off.

By changing the duration and frequency with which the microcontroller does each Components in hibernation, it can meet the power requirements of the Affect sensors.

Figure 2 shows a second exemplary embodiment of a similar structure Ultrasonic sensor. The sensor is controlled by a microcontroller (206), whose program is in a program memory (207). He uses an EEPROM (209) and a RAM (208) for its data.

The microcontroller controls the ultrasound transmitter (203), the control signals for provides the transducer (214). The sound transducer (214) thereby generates Sound waves emitted and emitted by a reflective medium be thrown back. The sound converter converts the received signals into electrical signals supplied to the receiver (204). This reinforces and filters the signal before it is sent from the microcontroller by means of an A / D converter (205) (206) is detected. From this, the microcontroller (206) determines a measured value, the after a possible conversion via the control line (16) to the Current stage, which adjusts a current depending on it, or to the digital one Passes on interface that forwards it via digital communication.

A first preferred implementation of the solution according to the invention for the Exemplary embodiments according to FIGS. 1 and 2 are shown in FIG. 3. she serves to measure the excess power needed to optimize the Measuring device operation is available in each case, by means of a current stage (302). The measuring device in Figure 3 is connected with a current loop over the Connections (11) and (12) supplied with power.

The current stage (302) is parallel to the rest of the circuit of the measuring device connected. The current stage monitors the total current via the Voltage drop across a resistor (R301) and keeps it constant. The current is regulated by the current stage so that the total current through the Resistor (R301) remains constant and that through the control line (16) corresponds to the specified value.

The current that flows into the terminals of the measuring device is divided into one Share that flows into the supply line (14) and a share that flows into the Current stage (302) flows. The current through the supply line (14) is from the Measuring device used to work, the current through the current stage not used for supplying the measuring device, it is a measure of the current surplus. The microcontroller measures this excess, in Figure 3 shown as a voltage measurement across a resistor (R302), and adjusts the power consumption of the sensor so that there is always sufficient even if the smallest possible excess is available. The decreases Excess, parts of the measuring device (e.g. the transmitter and Reception area, or the entire signal generation and Processing area) in a power-saving idle state. It is possible, with a corresponding reduction in the surplus temporarily To suspend operation as in the prior art (EP 0 687 375).

By always allowing a small excess to flow, the current stage has the possibility to compensate for short-term fluctuations in the current account, without a deficit. Fluctuations can e.g. a short time increased power consumption or a fluctuation in the supply voltage his.

A more precise measurement of the excess power is obtained if additionally the voltage on the supply line + (14) with the help of the measuring line (19) measures. The result is then obtained directly by multiplying the current and voltage excess performance.

Figure 4 shows alternative ways to build the current stage (402). she is here in line with the supply lines (14, 15). It is a zener diode (403) (alternatively, an electronic circuit that uses a variable Current consumption depending on the voltage). (The electronic circuit is usually preferred.) As above, according to the figure 3, the total current of the complete measuring device over a Resistance (R401) felt and regulated accordingly. The stream divides after the current stage on in a part that is used to supply the measuring device is used (supply line + (14)) and an excess part that is picked up by the Z diode. The excess is measured about the voltage drop across a resistor (R402) because the current through (R402) is a measure of the current power surplus.

The determination of the surplus power becomes more precise if one additionally measures the voltage on the supply line + (14) with the measuring line (18).

FIG. 13 shows an improved circuit compared to FIG. 4. A Current stage (1302) is connected in series to the supply lines. Your is one Circuit (1303) downstream, which consumes excess power. To she feels the voltage on the supply line + (14) and with the help of a line (1304) the voltage before the current stage. The circuit (1303) takes exactly so much current that the voltage drop across the current stage (1302) to Reduction of power loss becomes as small as possible, but remains large enough, so that the current stage can keep the current constant, even if there are fluctuations the supply voltages or the current consumption of the sensor. On The excess power is therefore a measure of the current through the Circuit (1303) which e.g. via the voltage drop at (R1302) using the Measuring line (20) is measured.

The determination of the surplus power becomes more precise if one additionally measures the voltage on the supply line + (14) with the measuring line (18).

FIG. 5 shows a current stage (502) comparable to that in FIG. 3. in the The difference here is not the current power surplus directly measured. The current requirement of the Measuring device determined. From the difference between the known current, which in the current loop flows, and the current demand of the measuring device through (R502) a measure of the excess can be derived. Here too, the excess Performance more precisely through an additional measurement on the supply line + (14) available voltage is determined by measuring line (19) become.

Figure 6 shows a current stage (602), similar to Figure 4. In contrast to However, the measuring device according to FIG. 4 does not directly become the excess measured, but the input power at the terminals of the measuring device and the power consumption that the measuring device requires for supply, certainly. The input power results from the known current, which in the Current loop flows, and the input voltage measured via measuring line (19). The power consumption that the measuring device for supply is required from the current through (R602) and the via measuring line (18) measured supply voltage + (14). The difference between the two Services is a measure of the current surplus of services.

The power consumption of the measuring device (101, 102) is often substantial determined by one or more large consumers. You get information One can make a statement about the power consumption of these components make the power consumption of the measuring device by e.g. for the unknown power consumption of the other components a worst case value accepts. In addition, the available power is determined how e.g. shown in Figures 3 to 6 and from it the excess power certainly. The microcontroller uses the excess power to determine whether Parts of the measuring device must be put into said idle state, to control the power consumption of the measuring device. Figure 7 shows this as a further preferred embodiment of the invention a radar sensor which with the help of a measuring line (715) a statement about the power consumption of the Receivers (704) receives. Whether the sensor uses a current loop or digital communication is irrelevant. At a This is an ultrasonic sensor or a sensor with radar guided on a rope same procedure feasible. The important thing here is just one or more Identify main consumers whose current power needs are determined.

It is possible to simplify the facilities described above. Such Embodiments of the invention will now be explained with reference to Figures 8 and 9.

For a rough statement of how much excess is currently available, it can suffice to determine only the available power. This can be e.g. determine from input current and input voltage. The input current is known because it is from the microcontroller via the control line (16) of the current stage is specified, the input voltage is, as shown in FIGS. 8 and 9, measured by means of a measuring line (18). Depending on the determined to Available power can now be the rest states of each Components used to measure the absorbed power of the sensor to adapt the available power so that always a certain Excess performance remains.

A simplification based on this is the input voltage not to be measured, the measuring line (18) in FIGS. 8 and 9 is then not necessary. Based on the set current, which does not have to be measured, since it is specified by the microcontroller via the control line (16) of the current stage you can make a statement about the available service. At maximum current, e.g. 20 mA, is relative even at minimal voltage a lot of power available, only with relatively small currents, e.g. close to 4 mA, can little power is available. It is therefore sufficient to control the Adjust rest states only depending on the set current and the duration and set the frequency with which the sleep states are activated so that even with minimum input voltage and maximum power consumption individual components do not exceed the available performance becomes.

FIGS. 10 and 11 show further simplifications preferred according to the invention. Here, only the current currently required is measured as a voltage drop across the resistor (R1002) using the measuring line (18) or via (R1102) using the measuring line (20). The microcontroller can regulate this current by controlling the idle states so that it always remains below the current available.
Starting from FIG. 7, it is possible, as a further simplification, to determine only the power requirement of one or more main consumers and, depending on this, to control the idle states of the components without determining the power available.

For measuring devices connected to digital communication, e.g. one Fieldbus, similar demands are placed on the measuring device. The stream that the measuring device may take from the digital bus must be constant, it is usually fixed. Again, there is a need for that Adjust the power consumption of the measuring device to the range of services. The The way in which this is to be implemented corresponds to the previous statements. It should only be noted that the current through the current stage does not come from Measured value depends, but is usually fixed.

Part of such a measuring device is shown as an example in FIG. The Current level (1202) keeps the current at times when there is no communication, constant. To send digital signals, the digital interface (1203) receives the control line (16) from the microcontroller data to it in modulated form the current stage passes on, which changes the current accordingly. The kind of Modulation depends on the specifications of the digital used Communication off. Data is received by the signals on the Supply line + (14) or at the current stage (1202) from the digital Interface (1203) recognized and demodulated via the control line (17) to the Microcontrollers are forwarded. The measurement of the excess is as in Figure 3 already set out, realized by using the voltage drop over (R1202) the measuring line (18) is measured or additionally the voltage at the Supply line + (14) with the measuring line (19). The others are the same previously described method on measuring devices with digital communication applicable.

Claims (11)

1. A measuring device for measuring a process variable with a pre-set maximum power consumption by the measuring device, in particular for connexion to a current loop, such as for example a 4 to 20 mA current loop, or to a digital communication, with a device for regulating the measuring operation of the measuring device in conformity with the pre-set power consumption, in which the regulating device (302, 402, 502, 602, 802, 902, 1002, 1102, 1202, 1302; 403, 603, 903, 1103, 1203, 1303; 106, 206, 706) measures or predicts the power excess by which the pre-set power consumption of the measuring device (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001, 1101, 1201, 1301) exceeds the power consumption for the measuring operation of the measuring device (101, 201, 301, 401, 501, 601, 701, 801, 901, 1001, 1101, 1201, 1301), and regulates the measuring operation in such a way that the latter power consumption is approximated to the pre-set power consumption, without the pre-set power consumption being exceeded.
2. A measuring device according to Claim 1, in which the pre-set power consumption is determined by a pre-set current and/or by a pre-set supply voltage.
3. A measuring device according to Claim 1, in which the regulating device sets the power requirement for the measuring operation of the measuring device in a manner dependent upon the pre-set current, upon the supply voltage or the power determined from both.
4. A measuring device according to Claim 1, in which the regulating device measures or predicts the power requirement for the measuring operation of the complete measuring device or at least one main consumer device (704) of the measuring device (701) and regulates the measuring operation with a view to the result.
5. A measuring device according to Claims 1 to 4, in which the regulating device regulates the measuring operation in such a way that the power excess is minimized.
6. A measuring device according to one of Claims 1 to 5, for connexion to a current loop (11, 12) with a microprocessor (106, 206, 706), a program memory (107, 207, 707) which stores a program to be carried out by the microprocessor, one or more EEPROM and/or RAM modules (108, 208, 708; 109, 209, 709), circuit elements (103, 104; 203, 204; 703, 704) which have an operating mode and a current-saving state of rest, and a current step (302, 402, 502, 602, 802, 902, 1002, 1102, 1302) which is controlled by the microprocessor and which regulates the magnitude of a current flowing in the current loop in such a way that the said magnitude correlates in a pre-set manner with the magnitude of the measurement figure of the process variable, in that it converts excess power exceeding the magnitude of the measurement figure in the current step into loss power, wherein the performance of the measurement program is interrupted by the microprocessor in a manner dependent upon the set current through the current loop and/or in a manner dependent upon the supply voltage.
7. A measuring device according to Claim 6, in which the number of measurement cycles per time interval is set by the microprocessor in a manner dependent upon the set current through the current loop and/or upon the supply voltage.
8. A measuring device according to one of Claims 1 to 5, for connexion to a current loop (11, 12) with a microprocessor (106, 206, 706), a program memory (107, 207, 707) which stores a program to be carried out by the microprocessor, one or more EEPROM and/or RAM modules (108, 208, 708; 109, 209, 709), circuit elements (103, 104; 203, 204; 703, 704) which have an operating mode and a current-saving state of rest, and a current step (302, 402, 502, 1302) which is controlled by the microprocessor and which regulates the current flowing in the current loop in such a way that the said current correlates in a specified pre-set manner with the measurement figure of the process variable, in that it converts excess power in the current step into loss power, wherein the excess power converted in the current step (302, 402, 502, 1302) into loss power is measured and, if the said excess power is above a specified pre-set figure, the number of measurement cycles per time interval is increased by the microprocessor and, if the excess power is below a specified pre-set figure, the number of measurement cycles per time interval is decreased by the microprocessor.
9. A measuring device according to one of Claims 1 to 5, for connexion to a digital communication (8, 9) with a microprocessor (106, 206, 706), a program memory (107, 207, 707) which stores a program to be carried out by the microprocessor, one or more EEPROM and/or RAM modules (108, 208, 708; 109, 209, 709), circuit elements (103, 104; 203, 204; 703, 704) which have an operating mode and a current-saving state of rest, and a current step (1202) which is controlled by the microprocessor, wherein the performance of the measurement program is interrupted by the microprocessor in a manner dependent upon the supply voltage.
10. A measuring device according to Claim 9, in which the number of measurement cycles per time interval is set by the microprocessor in a manner dependent upon the supply voltage.
11. A measuring device according to one of Claims 1 to 5, for connexion to a digital communication (8, 9) with a microprocessor (106, 206, 706), a program memory (107, 207, 707) which stores a program to be carried out by the microprocessor, one or more EEPROM and/or RAM modules (108, 208, 708; 109, 209, 709), circuit elements (103, 104; 203, 204; 703, 704) which have an operating mode and a current-saving state of rest, and a current step (1202) which is controlled by the microprocessor and which converts excess power in the current step into loss power, wherein the excess power converted in the current step (1202) into loss power is measured and, if the said excess power is above a specified pre-set figure, the number of measurement cycles per time interval is increased by the microprocessor and, if the excess power is below a specified pre-set figure, the number of measurement cycles per time interval is decreased by the microprocessor.

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