Method for charging energy store of field device and field device for carrying out method

Abstract

The invention relates to a method for charging an energy store of a field device having at least one charging current source and a control unit, the charging current source applying a charging current to the energy store of the field device. The invention also relates to a field device having a rechargeable energy store, at least one charging current source and a control unit, the field device being designed to carry out the method. According to the method and the field device, the service life of the field device can be prolonged.

Description

Technical Field

[0001] The invention relates to a method for charging an energy store of a field device having at least one charging current source and a control unit, wherein the charging current source applies a charging current to the energy store of the field device.

[0002] The invention furthermore relates to a field device having a rechargeable energy store, at least one charging current source and a control unit, wherein the field device is designed to carry out the method. Background Art

[0003] Field devices of the above type are in particular self-contained sensors which are designed for filling level measurement, limit level measurement, interface measurement and flow measurement for liquids and bulk materials, as well as for pressure measurement of liquids and gases. These sensors can meet the relevant safety requirements so that they can be operated in explosion-proof areas. In order to supply power to the measuring unit, the field device has an electrical energy storage device which can be charged by a charging current source which is part of the field device or at least connected to the field device. The possible service life of such a field device depends in particular on the service life of the energy storage device used. Therefore, the object of the present invention is to propose a method and a field device which can increase the service life of a field device. Summary of the invention

[0004] This object is achieved by a method according to claim 1 and a field device according to claim 10. Preferred embodiments of the method and of the field device are given in the dependent claims.

[0005] According to an advantageous embodiment of the invention, the energy store of the field device is a battery. In order to extend its service life, the battery is preferably charged to a maximum of 80% of its full charge capacity, which corresponds to an optimal charge amount.

[0006] Obviously, different values ​​that do not exactly correspond to 80% of the full charge capacity can also be considered as the optimal charge capacity. For example, 75%, 85% or 90% of the full charge capacity can also represent the optimal charge capacity in the sense of the present application.

[0007] In order to ensure that sufficient charging current is provided during the available charging time, according to a preferred embodiment of the present invention, the field device has a plurality of different charging current sources, wherein the control unit specifies which charging current source or a combination of which charging current sources applies the charging current to the energy storage device of the field device taking into account the priorities. Therefore, the charging current sources are designed so that they provide the charging current alone or simultaneously with one or more other charging current sources. Preferably, the field device has at least a battery, a wired interface (e.g., a USB interface) and a solar cell as a charging current source. The battery is preferably replaceable and can be replaced at regular maintenance intervals. When the priorities are sorted by the control unit, at least one or more of the following selection criteria are considered:

[0008] a) A charging current is applied by the battery as a charging current source only if no other charging current source is available. Preferably, the energy extracted from the battery is measured so that the battery can be replaced before the energy is completely exhausted.

[0009] b) When the energy storage device can be charged, the charging current is always applied by the solar cell as a charging current source. Therefore, the solar cell is always selected as a charging current source when the solar cell can generate a suitable charging current, which, however, depends on the fluctuating and unpredictable light conditions. In this case, the energy storage device is preferably charged to 80%, wherein this value can be set variably.

[0010] c) When the wired interface is connected to an external operating device, a charging current can be applied by the wired interface as a charging current source, wherein the energy store is preferably charged to more than 80%. The wired interface is connected to the external operating device, for example, to parameterize the field device, to commission (initialize) the field device, and to test or update. In this case, charging to more than 80% is preferred so that after the external operating device is removed, sufficient energy is still available to complete the startup process and the upcoming measurement routine. This also ensures that the battery is not needed as a charging current source for an additional period of time, thereby extending the operating time of the device. Charging to more than 80% occurs rarely and therefore has little or no effect on the service life of the energy store.

[0011] d) if the selected charging current source cannot provide the required electrical energy to at least fully or optimally charge the energy store of the field device, extracting the maximum possible electrical energy from the selected charging current source to charge the energy store of the field device, and selecting one of the remaining charging current sources to further charge the energy store of the field device.

[0012] As an alternative to automatic prioritization by the control unit, it is also possible to select the charging current sources manually and thus also prioritize them manually. In the simplest case, automatic prioritization can be performed by switching off the charging current sources that are not required (in particular by switching off the IC, opening a MOSFET or a switch). Alternatively, it is also possible to select the charging current source that currently provides the highest charging voltage in each case, for which purpose suitable diodes only allow the higher voltage to pass in order to mitigate the relatively low voltage.

[0013] In another preferred embodiment of the invention, the energy storage device of the field device is charged in time according to the (effective) operating time of the field device, wherein the energy storage device is preferably charged during the period between two (effective) operating times of the field device, wherein the period is preferably utilized as long as possible to charge the energy storage device. In other words, the battery is preferably applied with a charging current as low as possible required to charge the battery during the available time. For example, if the field device is a measuring device and measures at regular intervals of 12h, charging is performed during the 12-hour rest phase of the field device, which allows protective charging of the energy storage device. The charging current is reduced so that the rest phase is optimally utilized to charge the energy storage device.

[0014] In this case, protective charging of the energy storage device is understood to be charging with the lowest possible charging current. Therefore, the charging current is kept as low as possible for protective charging without exceeding the maximum permissible charging current.

[0015] Automatic or manual prioritization also ensures that the battery is charged with the best or maximum possible charging voltage, wherein the charging is carried out in a protective manner as far as possible.

[0016] Since the rest phase may be interrupted by unexpected events such as external triggers (in particular a button pressed by the customer or a signal from another device), it is advantageous to charge the energy storage device as quickly as possible at the beginning of the rest phase. As soon as there is enough energy in the energy storage device for one or more activities of the device, in particular for measurement and / or data transmission, the charging current is reduced in order to charge the accumulator in the most protective manner possible, as described above.

[0017] When charging the energy storage device, the maximum permissible charging current must not be exceeded, wherein the maximum permissible charging current is usually temperature-dependent. The time-dependent curve of the charging current and its current intensity influences the service life of the energy storage device, in particular the number of possible charging cycles. Experience has shown that the charging current should be set as low as possible, but should also be set as high as required. In order not to exceed the maximum permissible charging current, current limiters in the form of fuses, passive resistors or active resistors (OPAMP, semiconductors, MOSFET, etc.) are known in known field devices. The relevant charging current source usually has a constant charging voltage and generates a constant charging current through an inserted variable resistor. If the voltage level of the energy storage device to be charged is low, the voltage difference between the charging current source and the energy storage device sometimes leads to the current limitation being activated, so that the electrical energy, especially in the resistor, is unnecessarily converted into heat, resulting in energy losses. Therefore, according to an advantageous improvement example of the present invention, at least one charging current source is connected to a voltage regulator, which adjusts the charging voltage in a way that the maximum permissible charging current is not exceeded. However, in one embodiment, the charging current can also be set so that it is as close as possible to the maximum permissible charging current, in particular corresponding to the maximum permissible charging current. In this way, the lowest possible load on the charging current source can be achieved. Preferably, the charging voltage is set as a function of the current voltage of the energy store and / or as a function of the current charging current. In this way, an energy-efficient charging of the energy store is achieved, since, by regulating the charging voltage, no additional current limitation is required, or an additional current limitation exists but is not activated because the maximum permissible charging current is not exceeded.

[0018] In this case, a particularly protective charging of the energy store can also be achieved if the charging current is kept as low as possible. This is particularly possible if the duration for charging the energy store (i.e. the duration until the point in time at which the energy store must have a predetermined charge) is known. If this duration is known, in one embodiment the charging current is regulated by regulating the charging voltage in such a way that the charging current is minimized over the entire duration.

[0019] Alternatively, as described above, the predeterminable charging of the energy store can be carried out as quickly as possible and therefore with a charging current as close as possible to the maximum permissible charging current and, starting from the implementation of the predetermined charging, the charging current is reduced to the minimum possible charging current within the duration calculated for optimal charging.

[0020] In a specific embodiment, there is a linear "balance curve" so that the charging voltage in the considered area is greater than the current voltage of the energy storage device by a constant value. For example, if the voltage of the energy storage device is 2.5V, the charging circuit generates an output voltage of, for example, 3V. With each increase in the current voltage of the energy storage device, the output voltage of the charging current source also increases by the same amount until the charging current source reaches the target voltage of 3.9V. If the voltage of the energy storage device reaches the target voltage value of 3.9V, no more charging current flows and the final charging voltage is reached. Usually, there is an approximately known resistance value between the output of the charging circuit and the energy storage device, which resistance value is derived in particular from the resistance value of the fuse, the measuring resistance of the current limiter, etc. Since the voltage of the charging circuit and the voltage of the energy storage device are known, the maximum charging current can be determined. Preferably, a connected semiconductor device is used to limit the voltage, and in an advantageous embodiment, the semiconductor device has a Z diode (Zener diode). In this way, a particularly fast and at the same time energy-efficient charging of the energy storage device is achieved.

[0021] For example, in the above specific embodiment, if the energy storage device is charged to a predetermined voltage according to a predetermined balance curve and when the predetermined voltage is reached, the charging current is set to the minimum possible charging current that can achieve optimal charging or full charging of the energy storage device within the remaining period, a combination of high energy efficiency and protective charging of the energy storage device can be achieved.

[0022] Independently of the voltage regulation, in a preferred embodiment of the invention, there is also a voltage limitation, for which a voltage limiter is provided. Even if the voltage of the energy storage device to be charged is below the maximum voltage to be limited, the voltage limiter consumes energy, for example in the form of leakage current, in the case of a low voltage of the charging current source. According to another advantageous improvement example of the invention, in order to avoid unnecessary energy losses in this case, the voltage limitation is only activated when the voltage of the energy storage device exceeds a minimum voltage, and the voltage limitation is deactivated when the voltage of the energy storage device falls below the minimum voltage. For example, if the maximum voltage allowed at the energy storage device is 4.0 V, the voltage limitation is only activated starting from a voltage of, for example, 3.7 V by a suitable Z diode. Therefore, only when the charging voltage is above 3.7 V will a small leakage current flow through the voltage limiter. By switching on the Z diode as early as possible and with appropriate hysteresis, periodic activation and deactivation (so-called clocking) can be avoided.

[0023] The described voltage limiting circuit is particularly advantageous for batteries as charging current sources, since the energy available here is limited. If the charging voltage is regulated by a voltage regulator so that the current limitation remains deactivated, no unnecessary energy is consumed, which is beneficial to the service life of the field device. However, if the energy storage device is charged by a charging current source in which there is sufficient or non-critical energy (e.g. a wired interface or a solar cell), a relatively higher charging voltage may be useful and the energy loss can be ignored within the scope of the current limitation. Therefore, if a battery is used as the charging current source, it is particularly preferred to use a voltage regulator and switch on the voltage limiter starting from a certain voltage value.

[0024] Finally, when the voltage of the charging current source exceeds a minimum voltage, the voltage regulator is preferably activated. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Specific embodiments of the present invention will be described below with reference to the accompanying drawings.

[0026] Figure 1a and Figure 1b The field devices are shown individually.

[0027] Figure 1c-1f The voltage limiters are shown separately.

[0028] Figure 2 The charging behavior of the energy store is shown in two diagrams. DETAILED DESCRIPTION

[0029] Figure 1a A schematic diagram shows a field device 1 having an electrical consumer in the form of a measuring device 2. Such a measuring device 2 is, for example, a filling level measuring device. In order to supply energy to the measuring device 2, an energy storage device 3 is provided, which in the exemplary embodiment shown is a battery. In order to charge the energy storage device 3, the field device 1 has three charging current sources 4, namely a battery 41, a solar cell 42 and a wired interface 43 in the form of a USB interface. The charging current sources 4 are at least indirectly connected to a control unit 5, which in turn is connected to the energy storage device 3 and receives information about the charging voltage. In addition, the control unit 5 is also connected to a current limiter 6, which transmits the magnitude of the current charging current to the control unit 5. If, for example, the energy storage device 3 needs to be charged due to an upcoming measurement routine, a prioritization of the charging current sources 4 is performed, which is controlled by the control unit 3, wherein a selection is made of which of the charging current sources 4 the charging current is applied according to a predetermined criterion. When the priorities are sorted by the control unit 5, at least one or more of the following selection criteria are taken into account:

[0030] a) The charging current is applied by the battery 41 as the charging current source 4 only when no other charging current source 4 is available.

[0031] b) If the energy store 3 can be charged, the charging current is always applied by the solar cell 42 as charging current source 4 .

[0032] c) If the wired interface 43 is connected to an external operating device, a charging current can be impressed by the wired interface 43 as a charging current source 4 , wherein the energy store 3 is preferably charged to more than 80%.

[0033] d) If the selected charging current source 4 cannot provide the required electrical energy to at least fully or optimally charge the energy storage device 3 of the field device 1, the maximum possible electrical energy is extracted from the selected charging current source 4 to charge the energy storage device 3 of the field device 1, and one of the remaining charging current sources 4 is selected to further charge the energy storage device 3 of the field device 1.

[0034] Therefore, a charging voltage 4 is selected which allows the energy store 3 to be charged in the most efficient and / or protective manner possible.

[0035] In the exemplary embodiment shown, the charging current source 4 is connected to a voltage regulator 71, 72, 73, respectively. In the present exemplary embodiment, the voltage regulator 71, 72, 73 is designed as a voltage generating circuit so that, depending on the current voltage and / or the current charging current of the energy storage device 3, the charging voltage is set such that the charging current does not exceed the maximum charging current even without the control / regulatory intervention of the current limiter 6. The regulation of the charging voltage is preferably temperature-dependent, for which purpose the field device 1 has a temperature sensor 8.

[0036] As additional components, in the exemplary embodiment shown, the field device 1 has a decision maker / comparator 9, which forms a charging protection when the voltage on the energy storage is too low to avoid recharging after a deep discharge. In addition, the field device 1 has one or more additional voltage limiters 10 connected between the current limiter(s) 6 and the energy storage 3, as well as a deep discharge protector 11, which prevents recharging after a deep discharge, but first ensures that a deep discharge situation does not occur.

[0037] Figure 1b Shown with Figure 1a Compared to basically the same embodiment, however, the charging current source 4 is first connected to the decision maker / comparator 9 and then to the voltage regulator 74 .

[0038] Figure 1c-1f Different embodiments of a voltage limiter 10 are shown, which in each case is activated when the voltage of the energy store 3 exceeds a minimum voltage and is deactivated when the voltage of the energy store 3 falls below the minimum voltage.

[0039] according to Figure 1c , a Zener diode 12 and a controllable switch 13 are connected in parallel to the energy storage 3 to limit the voltage, wherein the controllable switch 13 is connected to a voltage comparator 14. For redundancy reasons and to meet any safety requirements, this component combination consisting of a Zener diode 12, a controllable switch 13 and a voltage comparator 13 is also configured twice in the same way (outlined by a dotted line), which can guarantee functionality if one of the above component combinations fails.

[0040] according to Figure 1d , a resistor 15 and a controllable semiconductor 16 are connected in parallel to the energy storage 3 to limit the voltage, wherein the controllable semiconductor 16 is connected to the voltage comparator 14. Here, the component combination is also configured in the same way a total of three times, and they are connected in parallel to each other in order to maintain the function in the event of a fault.

[0041] Figure 1e An embodiment of a voltage limiter 10 is shown, in which different component combinations for voltage limiting are provided. First, a resistor 15 is connected in parallel to the energy storage 3 together with a controllable semiconductor 16, wherein the controllable semiconductor 16 is connected to a voltage comparator 14. In addition, two further structurally identical component combinations are provided, with which a Zener diode 12 and a controllable switch 13 are connected in parallel to the energy storage 3 in each case, wherein the controllable switch is connected to a voltage comparator 14 in each case.

[0042] Figure 1e Shows the functionality of Figure 1c Basically the same exemplary embodiment, wherein each controllable switch 13 is respectively connected to a common Zener diode 12 .

[0043] Figure 2The typical charging behavior of an energy store is shown in two diagrams 21, 22. Figure 21 shows the voltage U(t) across the energy store as a function of the time t, and Figure 22 shows the charging current I(t) as a function of the time t. Assuming a constant charging current I1(t), the voltage across the energy store rises slowly. With a constant charging current I1(t), a higher voltage increases the power consumed by the energy store, so that the voltage U1(t) generally rises more and more slowly. If the charge controller now charges with a constant voltage U2(t), the current will be limited only by the internal resistance of the energy store and the charging current will exceed the maximum permissible charging current. In this case, it is therefore necessary to employ lossy current limiting, which reduces the charging voltage U2(t) until the charging current I1(t) flowing, which is limited by the internal resistance, is low enough. However, for reasons of technical explosion protection, at least one current limiting circuit may still be absolutely necessary and it adds additional resistance to the charging circuit. With the aid of these known resistors or by measurement, it is possible to calculate the charging voltage U3 (t) at the voltage regulator, which is selected so that the desired charging current I1 (t) flows.

[0044] Reference numerals list 1 Field equipment 2 Measurement device 3 Energy storage device 4 Charging current source 41 Battery 42 Solar Cells 43 Wired interface 5 Control unit 6 Current limiter 71 Voltage Regulator 72 Voltage Regulator 73 Voltage Regulator 8 Temperature sensor 9 Decision Maker / Comparator 10 Voltage limiter 11. Deep discharge protector 12 Zener diode 13 Controllable switch 14 Voltage Comparator 15 Resistors 16 Controllable semiconductors 21 Charts 22 Charts I (t) Charging current U(t) Charging voltage t time

Claims

1. A method for charging an energy store (3) of a field device (1), the field device having at least one charging current source (4) and a control unit (5), wherein: The charging current source (4) applies a charging current to the energy store (3) of the field device (1).

2. The method according to claim 1, characterized in that: The energy store (3) of the field device (1) is a battery, wherein the battery is preferably charged to a maximum of 80% of its full charge capacity in order to extend its service life.

3. The method according to claim 1 or 2, characterized in that: The field device (1) has a plurality of different charging current sources (4), wherein the control unit (5) specifies, taking into account priorities, which charging current source (4) or a combination of charging current sources (4) is to apply a charging current to the energy store (3) of the field device (1).

4. The method according to claim 3, characterized in that: The field device (1) has at least a battery (41) as the charging current source (4), a solar cell (42) and a wired interface (43), wherein when the priorities are sorted by the control unit (5), at least one or more of the following selection criteria are taken into account: a) applying a charging current from the battery (41) as the charging current source (4) only when no other charging current source (4) is available; b) when the energy storage device (3) can be charged, the solar cell (42) serving as the charging current source (4) always applies a charging current; c) when the wired interface (43) is connected to an external operating device, a charging current can be applied by the wired interface (43) as the charging current source (4), wherein the energy storage device (3) is preferably charged to more than 80%; d) if the selected charging current source (4) cannot provide the required electrical energy to at least fully or optimally charge the energy storage device (3) of the field device (1), extracting the maximum possible electrical energy from the selected charging current source (4) to charge the energy storage device (3) of the field device (1), and selecting one of the remaining charging current sources (4) to further charge the energy storage device (3) of the field device (1).

5. The method according to any one of claims 1 to 4, characterized in that The energy store (3) of the field device (1) is charged in time as a function of the operating time of the field device (1), wherein the energy store (3) is preferably charged during a period between two operating times of the field device, wherein the period is preferably utilized as long as possible for charging the energy store.

6. The method according to any one of claims 1 to 5, characterized in that To ensure correct operation, the charging current is reduced only after a minimum amount of energy in the energy store has been reached.

7. The method according to any one of claims 1 to 6, characterized in that At least one of the charging current sources (4) is at least indirectly connected to a voltage regulator (71, 72, 73), which regulates the charging voltage so that a maximum allowed charging current is not exceeded.

8. The method according to any one of claims 1 to 7, characterized in that The charging voltage is set according to the current voltage of the energy store (3) and / or according to the current charging current.

9. The method according to any one of claims 1 to 8, characterized in that a) activating voltage limitation by means of a voltage limiter (10) when the voltage of the energy storage device (3) exceeds a minimum voltage, and b) deactivating the voltage limitation by the voltage limiter (10) when the voltage of the energy store (3) falls below a minimum voltage of the charging current source (4).

10. The method according to any one of claims 1 to 9, characterized in that When the voltage of the charging current source (41, 42, 43) exceeds a minimum voltage, the voltage regulator (71, 72, 73) is activated.

11. A field device having a rechargeable energy store (3), at least one charging current source (4) and a control unit (5), wherein: The field device ( 1 ) is designed to carry out the method according to any one of claims 1 to 8.

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