Circuit arrangement for monitoring the power supply of a field device, and field device with such a circuit arrangement

Abstract

The present invention describes a circuit arrangement for monitoring an output voltage of a power supply of a self-contained field device, wherein the circuit arrangement comprises a voltage divider and an analog-to-digital converter, and wherein an analog input of the analog-to-digital converter is connected to a center tap of the voltage divider, characterized in that the voltage divider and preferably the analog-to-digital converter can be deactivated by at least one switching element.

Description

[0001] The present invention relates to a circuit arrangement for monitoring a power supply of a field device according to the preamble of patent claim 1 and to a field device with such a circuit arrangement according to patent claim 11.

[0002] Various circuit arrangements for monitoring voltages are known from the prior art. In the circuit arrangements known from the prior art, an analog voltage from the power supply is tapped and digitized using an analog-to-digital converter, so that the digitized voltage value is available as a digital value in the digital domain for further processing.

[0003] If the maximum voltage of a voltage to be measured, particularly of the power supply, is greater than the maximum permissible input voltage at an analog input of the analog-to-digital converter, the voltage must be conditioned, i.e., particularly limited to the maximum permissible value, since otherwise the analog-to-digital converter could be damaged and / or would always output a maximum value once the maximum permissible input voltage is exceeded. According to the present patent application, "power supply" is understood to mean any type of energy source. This includes, in particular, energy storage devices such as batteries, accumulators, and capacitors, power supply circuits, and energy generation circuits such as energy harvesting modules.

[0004] In this case, the voltage to be monitored is reduced by means of a voltage divider in the prior art so that it can be processed by the analog-to-digital converter. Such a circuit arrangement known from the prior art is described in Fig. 1 shown.

[0005] Fig. 1 shows an embodiment of a circuit arrangement for monitoring a voltage U1, for example the output voltage of a battery. Since the maximum output voltage U1 is greater than a permissible maximum input voltage at an analog input 51 of an analog-to-digital converter 5, which is used to convert the voltage from the analog range to the digital range, the voltage U1 is fed to a voltage divider 3. The voltage divider 3 is formed in the present case from two resistors R1 and R2 and thus divides the voltage U1 proportional to the ratio of the resistors R1, R2. The construction of the voltage divider could also be implemented using capacitors, although this is not common practice. Voltage dividers with resistors are described below, but a construction with capacitors is also conceivable. The voltage U2 dropped across the second resistor R2 is applied to a center tap 31 of the voltage divider 3, i.e. at the connection node of the two resistors R1, R2.The analog-to-digital converter 5 is connected with its analog input 51 to the center tap 31, so that the voltage U2 is applied to the analog-to-digital converter 5.

[0006] It is particularly advantageous to monitor voltages in battery-operated devices, so it makes sense to also use this circuit arrangement in stand-alone field devices.

[0007] Self-sufficient field devices can, for example, be designed as self-sufficient fill level or limit level sensors, preferably as radar sensors. To ensure the self-sufficiency of the field devices, they have, in addition to a sensor for acquiring measurement data, a transmission device for the, preferably wireless, transmission of recorded measurement data or measured values, and their own power supply. The transmission device can preferably be a radio module for a narrowband radio technology (LoRa, Sigfox, LTE-M, NB-IOT), which transmits the measurement data or measured values ​​to a cloud, i.e., to a server on the World Wide Web. The power supply is preferably designed as an energy storage device, e.g., a battery or accumulator, and can additionally include an energy harvesting module.

[0008] Typical application scenarios for such field devices include flood protection, inventory management or measurement tasks on mobile containers.

[0009] Monitoring the voltage available at the power supply is essential for such devices in order to detect a failure at an early stage.

[0010] However, the disadvantage of the prior art circuit is that a current constantly flows through the voltage divider, thus constantly consuming or wasting energy. Since the maximum achievable operating time is a relevant factor for self-contained field devices, constant energy consumption is unacceptable.

[0011] If the constant power consumption of the voltage divider is reduced by increasing the number of resistors used, the delay introduced by parasitic capacitances of the resistors and other components during a voltage change increases. This means that a voltage change arrives at the analog-to-digital converter with a delay.

[0012] Furthermore, high resistance values ​​also mean high input impedances for the analog-to-digital converters, although these are usually not designed for this.

[0013] The present application is therefore based on the object of specifying a circuit arrangement of the above type with reduced energy consumption.

[0014] This object is achieved by a circuit arrangement having the features of patent claim 1 and a preferably self-sufficient field device having the features of patent claim 11.

[0015] Advantageous embodiments and variants of the invention emerge from the dependent claims and the following description. The features listed individually in the dependent claims can be combined with each other in any technically reasonable manner, as well as with the features explained in more detail in the following description, and represent other advantageous embodiments of the invention.

[0016] A circuit arrangement according to the invention for monitoring the output voltage of a power supply of a stand-alone measuring device comprises a voltage divider and an analog-to-digital converter, wherein an analog input of the analog-to-digital converter is connected to a center terminal of the voltage divider. The circuit arrangement is characterized in that the voltage divider and preferably the analog-to-digital converter can be deactivated by at least one switching element.

[0017] Because the voltage divider can be deactivated by a switching element, current flow through the voltage divider is prevented, meaning that no energy is consumed by the voltage divider when deactivated. This ensures that the energy storage unit's energy reserves last significantly longer, thus significantly extending the maximum operating time of the self-contained field device.

[0018] In this context, deactivated means that current flow through the voltage divider is prevented.

[0019] Separating the voltage divider and, advantageously, also the analog-to-digital converter from the voltage to be monitored is possible, but it also has disadvantages.

[0020] Among other things, it is disadvantageous from a circuit design perspective if an input voltage is applied to the analog-to-digital converter when it is itself deactivated, as this can lead to leakage currents through the analog-to-digital converter or to damage or destruction of the analog-to-digital converter.

[0021] In one embodiment, a non-inverting amplifier is arranged between the analog input of the analog-to-digital converter and the center terminal of the voltage divider. The non-inverting amplifier conditions the voltage present at the center terminal and thus makes it available to the analog-to-digital converter.

[0022] In a particularly simple embodiment, the non-inverting amplifier can be designed as an impedance converter. An impedance converter has a high input resistance, so the voltage divider is not loaded. This means that the current flowing from the voltage divider into the impedance converter is very small, so the divider ratio of the voltage divider is not affected.

[0023] To prevent a voltage from being applied to the analog-to-digital converter when the voltage divider is deactivated, the non-inverting amplifier is designed in one embodiment so that it can be deactivated together with the voltage divider. Deactivating the non-inverting amplifier, whether by disconnecting the non-inverting amplifier from a supply voltage, deactivating it via a special pin, or by disconnecting the signal input from an input signal, ensures that no input signal is applied to the analog-to-digital converter when the voltage divider is deactivated.

[0024] In this way, leakage currents through the analog-to-digital converter and / or its damage due to an applied voltage can be prevented and the analog-to-digital converter can also be deactivated when the voltage divider is deactivated.

[0025] In a preferred embodiment of the circuit arrangement, the switching element for deactivating the voltage divider is connected in series with elements of the voltage divider. In particular, the switching element can be arranged at a base of the voltage divider, i.e., between the components of the voltage divider and a ground connection of the voltage divider. Arranging the switching element at the base of the voltage divider has the advantage that semiconductor switches (transistors, MOS field-effect transistors) or other switches can be used for this purpose. In particular, the MOS field-effect transistors for the lower part of the voltage divider have advantageous properties, such as better conductivity, control, etc., than those suitable for the head of the voltage divider. A lower on-resistance changes or influences the desired voltage divider less.

[0026] In this design variant, unless further circuitry measures are taken, the analog-to-digital converter must have a tolerance to input voltages that are higher than the supply voltage of the analog-to-digital converter.

[0027] In an alternative embodiment, the switching element can be arranged at the center connection of the voltage divider and preferably be designed such that both the voltage divider and the analog-to-digital converter can be deactivated and / or the impedance converter can be deactivated by the switching element. Simultaneous deactivation of the voltage divider and interruption of the connection between the analog-to-digital converter and the voltage divider has the advantage that the analog-to-digital converter is protected from a voltage being applied to its input, which could cause the negative effects described above. Interrupting the connection between the voltage divider and the analog-to-digital converter can also be achieved by deactivating an impedance converter if it is arranged between the voltage divider and the analog-to-digital converter.This can be done either by a switching element that connects the impedance converter to a voltage supply or by a so-called enable signal.

[0028] In a particularly clever design, an inverted enable input of the impedance converter is connected to a connection node between a foot of the voltage divider and a switching element connected in series with the voltage divider. If the switching element is closed, this node is grounded, and the impedance converter receives a "low" signal, which in this circuit corresponds to activating the impedance converter. If the voltage divider is deactivated by opening the switching element, the potential of this connection node rises, so that the impedance converter receives a "high" signal, which in this circuit corresponds to deactivating the impedance converter.

[0029] A particularly simple and cost-effective design of the switching elements is to implement them as semiconductor switches. In particular, the switching elements can be designed as MOS field-effect transistors.

[0030] To enable operation in potentially explosive environments, it is advantageous if the switching elements and / or the impedance converter and / or the analog-to-digital converter each have an explosion-protection circuit. The explosion-protection circuit is preferably designed as a voltage and / or current limiter. The current limiter can be configured, for example, as a suitably dimensioned resistor, and the voltage limiter as an arrangement with at least one Zener diode.

[0031] Also according to the invention is a preferably self-contained measuring device, wherein the self-contained measuring device has a circuit arrangement for monitoring a power supply of the self-contained measuring device, in particular a battery, wherein the circuit arrangement is designed according to the above description. Examples of such measuring devices are fill level measuring devices, point level measuring devices, and pressure measuring devices with sensors that detect the corresponding process variables: fill level, point level, or pressure.

[0032] To ensure the self-sufficiency of the measuring device, the self-sufficient measuring devices comprise, in addition to a sensor for acquiring measurement data, a transmission device for the preferably wireless transmission of acquired measurement data or measured values, and their own power supply. The transmission device can preferably be a radio module for a narrowband radio technology (LoRa, Sigfox, LTE-M, NB-IOT), which transmits the measurement data or measured values ​​to a cloud, i.e., to a server on the World Wide Web. The power supply is preferably designed as an energy storage device, e.g., a battery or accumulator, and can additionally include an energy harvesting module.

[0033] Typical application scenarios for such measuring devices include inventory management, measuring tasks on mobile containers and in inaccessible or undeveloped locations.

[0034] Self-contained measuring devices are characterized by particularly simple installation without the need for a communication or supply line and thus open up particularly flexible arrangement options, i.e. in particular their installation at the location of the measuring task, e.g. on a container or in a process environment.

[0035] The present invention will be explained in detail below using exemplary embodiments with reference to the accompanying figures. They show: Fig. 1 a circuit arrangement according to the prior art (already discussed) Fig. 2 shows a first embodiment of a circuit arrangement according to the present application, Fig. 3 a further development of the embodiment from Fig. 2, Fig. 4 an alternative embodiment of the circuit arrangement from Fig. 3 and Fig. 5 a second embodiment of a circuit arrangement according to the present application.

[0036] In the figures, unless otherwise stated, the same reference symbols designate the same or corresponding components with the same function.

[0037] Fig. 2 shows a first embodiment of a circuit arrangement 1 according to the present application.

[0038] The circuit arrangement 1 for monitoring an output voltage U1 of a power supply of a standalone field device comprises a voltage divider 3 and an analog-to-digital converter 5. An analog input 51 of the analog-to-digital converter 5 is connected to a center tap 31 of the voltage divider 3.

[0039] In the present embodiment, the voltage divider 3 is formed from a series circuit of a first resistor R1 and a second resistor R2, with the voltage divider 3 being connected between the output voltage U1 of the power supply to be monitored and ground. A connecting node of the resistors R1, R2 forms a center tap 31 of the voltage divider 3. A connecting node of the voltage divider 3 connected to ground is referred to in the present application as a foot 33 of the voltage divider 3.

[0040] The analog-to-digital converter 5 is connected with its analog input 51 to the center tap 31 of the voltage section 3, so that a voltage U2 dropping across the second resistor R2 is supplied to the analog-to-digital converter 5. Knowing the divider ratio of the voltage section 3, the voltage U1 to be monitored can be determined from this second voltage U2. The analog-to-digital converter 5 must be designed for the voltage U1 without being damaged or destroyed.

[0041] In the present embodiment, a switching element 7 is arranged between the base 33 of the voltage divider 3 and ground, which can be opened or closed by means of a switching signal S. In this way, it is possible to deactivate the voltage divider 3 so that no more current flows through the resistors R1, R2, since the voltage divider 3 is disconnected from ground by opening the switching element 7.

[0042] Fig. 3 shows a further development of the embodiment from Fig. 2.

[0043] The Fig. 3 shown further development of the circuit arrangement 1 differs from the embodiment according to Fig. 2, that a non-inverting amplifier 9, which is designed as an impedance converter, is arranged between the center tap 31 of the voltage divider 3 and the analog input 51 of the analog-to-digital converter 5. The arrangement of the impedance converter 9 between the center tap 31 and the analog-to-digital converter 5 prevents the ratio of the voltage divider 3 from being distorted by a current path through the analog-to-digital converter 5 that is parallel to the second resistor R2. An operational amplifier used to implement the impedance converter 9 has a very high input resistance – infinitely high in the idealized equivalent circuit diagram – so that all current flowing through the voltage divider 3 flows through the two resistors R1, R2 and any current flowing into the impedance converter 9 is essentially negligible. The voltage applied to the input side of the impedance converter 9 is nevertheless present at the output of the impedance converter 9, i.e.In this case, the second voltage U2 is applied and can thus be fed to the analog input 51 of the analog-to-digital converter 5.

[0044] In the Fig. In the embodiment shown in Figure 3, the impedance converter 9 can be activated and deactivated via a second switching signal S2, so that when the voltage divider 3 is deactivated, it is possible to also deactivate the path between the center tap 31 and the analog-to-digital converter 5, so that when the voltage divider 3 is deactivated, no voltage is present at the analog input 51 of the analog-to-digital converter 5. In this way, leakage currents and / or damage to the analog-to-digital converter 5 are avoided, since the converter 5 no longer needs to be designed for the voltage U1.

[0045] In a simplified embodiment, the impedance converter 9 can also be replaced by a switching element, in which case the previously described effect that the given ratio of the voltage components 3 is not distorted cannot be achieved.

[0046] In the Fig. In the embodiment shown in Figure 3, it is further provided that both the two switching signals S1, S2 and the connection between the impedance converter 9 and the analog-to-digital converter 5 are protected by an explosion protection circuit 11. The explosion protection circuit 11 can, for example, be formed as a combination of overvoltage protection and overcurrent protection, wherein an overvoltage can be prevented, for example, by a suitable number of Zener diodes and an excessive current can be prevented by suitable resistors.

[0047] Fig. 4 shows an alternative embodiment of the circuit arrangement 1 from Fig. 3.

[0048] In this exemplary embodiment, the impedance converter 9 has an activation circuit which is designed such that the impedance converter 9 can be activated by a "low" signal and is deactivated by a "high" signal. The activation circuit of the impedance converter 9 is connected to the base 33 of the voltage divider 3. The voltage divider 3 is activated by the first switching signal S1 by closing the switching element 7, i.e. if the base 33 of the voltage divider 3 is connected to ground, the potential at the base 33 drops to ground and the impedance converter 9 is activated by this "low" signal. If the voltage divider 3 is deactivated, i.e. if the switching element 7 is opened by a corresponding first switching signal S1, the potential of the base 33 of the voltage divider 3 rises to the applied first voltage U1 and the impedance converter 9 is deactivated by this "high" signal.In this way, activation / deactivation of the impedance converter 9 can be achieved without an additional switching element.

[0049] Also in this embodiment, the first switching signal S1 and the connection between the impedance converter 9 and the analog-to-digital converter 5 are protected by an explosion protection circuit 11.

[0050] In a further training of the Fig. 4, the analog-to-digital converter 5 can also be provided with the activation circuit used in the impedance converter 9, so that the analog-to-digital converter 5 is also automatically deactivated when the voltage divider 3 is deactivated.

[0051] Fig. 5 shows a second embodiment of a circuit arrangement 1 according to the present application.

[0052] In the Fig.In the embodiment shown in Figure 5, the switching element 7 is arranged between the first resistor R1 and the second resistor R2 of the voltage section 3 and connected in series with them. In the present embodiment, the voltage U2 applied to the center tap 31 of the voltage section 3 is supplied via a node arranged in the series circuit after the switching element 7, so that when the voltage converter 3 is deactivated, the potential of the node connected to the analog-to-digital converter 5 drops to ground potential. As an alternative to the prescribed configuration, a double switch can be used as the switching element 7, wherein one switch establishes or breaks a connection between the first resistor R1 and the second resistor R2 and a second switch establishes or breaks the connection from the center tap 31 to the analog-to-digital converter 5.It is also conceivable to always have R2 connected to converter 5, so that only R1 is disconnected from the node. This requires only one switching element. This is roughly equivalent to a voltage divider circuit at the head.

[0053] The switching element 7 and thus the two switches can be controlled by the first switching signal S1.

[0054] In this embodiment, the first switching signal S1 and the connection between the center tap 31 and the analog-to-digital converter 5 can each be protected by an explosion protection circuit 11. List of reference symbols 1 Circuit arrangement 3 voltage dividers 5 analog-to-digital converters 7 Switching element 9 non-inverting amplifier, impedance converter 11 Explosion protection circuit 31 Center tap 33 feet 51 Analog input R1 first resistance R2 second resistor U1 first voltage U2 second voltage S1 first switching signal S2 second switching signal

Claims

[1] Circuit arrangement (1) for monitoring a voltage of a power supply of a field device, wherein the circuit arrangement (1) has a voltage divider (3) and an analog-digital converter (5), and wherein an analog input (51) of the analog-digital converter (5) is connected to a center tap (31) of the voltage divider (3), characterized by that the voltage divider (3) and preferably the analog-digital converter (5) can be deactivated by at least one switching element (7). [2] Circuit arrangement (1) according to claim 1, characterized by that a non-inverting amplifier (9) is arranged between the analog input (51) of the analog-digital converter (5) and the center tap (31) of the voltage divider (3). [3] Circuit arrangement (1) according to claim 2, characterized by that the non-inverting amplifier (9) is designed as an impedance converter (9). [4] Circuit arrangement (1) according to one of claims 2 or 3, characterized by that the non-inverting amplifier (9) is designed to be deactivatable together with the voltage divider (3). [5] Circuit arrangement (1) according to one of the preceding claims, characterized by that the switching element (7) is connected in series with elements of the voltage divider (3). [6] Circuit arrangement (1) according to claim 5, characterized by that the switching element (7) is arranged at the base (33) of the voltage divider (3). [7] Circuit arrangement (1) according to claim 5, characterized by that the switching element (7) is arranged at the center tap (31) of the voltage divider (3) and is preferably designed such that both the voltage divider (3) and the analog-digital converter (5) can be deactivated by the switching element (7). [8] Circuit arrangement (1) according to one of the preceding claims, characterized bythat the switching element (7) is designed as a semiconductor switch, in particular a MOS field-effect transistor. [9] Circuit arrangement (1) according to one of the preceding claims, characterized by that the switching elements (7) and / or the impedance converter (9) and / or the analog-digital converter (5) each have an explosion protection circuit (11). [10] Circuit arrangement (1) according to claim 9, characterized by that the explosion protection circuit (11) is designed as a voltage and / or current limiter. [11] Field device, in particular a level measuring device, wherein the field device has a circuit arrangement (1) for monitoring a power supply of the field device, in particular a battery, characterized by that the circuit arrangement (1) is designed according to one of the preceding claims. [12] Field device according to claim 10, characterized by that the field device is designed as a self-sufficient field device.

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