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

Abstract

The invention relates to a method for charging an energy store of a field device with at least one charging current source and with a control unit, wherein the charging current source applies a charging current to the energy store of the field device. The invention also relates to a field device having a chargeable energy store, at least one charging current source and a control unit, wherein the field device is designed to carry out the method. By means of the method according to the invention and the field device according to the invention, the service time of the field device can be increased.

Description

[0001] Method for charging an energy storage device of a field device and field device for carrying out the method

[0002] The invention relates to a method for charging an energy storage device of a field device with at least one charging current source and a control unit, wherein the charging current source supplies the energy storage device of the field device with a charging current.

[0003] Furthermore, the invention relates to a field device with a rechargeable energy storage device, at least one charging current source and a control unit, wherein the field device is designed to carry out the method.

[0004] Field devices of the type in question are, in particular, self-contained sensors designed for level measurement, point level measurement, interface measurement, and the measurement of flow rates of liquids and bulk materials, as well as for the pressure measurement of liquids and gases. They can meet the relevant safety requirements for operation in potentially explosive atmospheres. To power a measuring unit, the field device has an electrical energy storage device that can be charged using a charging current source that is part of the field device or at least connected to the field device. The potential service life of such a field device depends in particular on the service life of the energy storage device used. The object of the invention is therefore to propose a method and a field device with which the service life of a field device can be increased.

[0005] This object is achieved by the method according to claim 1 and by the field device according to claim 10. Preferred embodiments of the method and the field device are specified in the subclaims.

[0006] According to an advantageous embodiment of the invention, the energy storage device of the field device is a rechargeable battery. To extend its service life, the rechargeable battery is preferably charged to a maximum of 80% of its full charge capacity, which corresponds to an optimal charge. It goes without saying that a different value that does not exactly correspond to 80% of the full charge capacity can also be considered an optimal charge. For example, 75%, 85%, or 90% of the full charge capacity can also represent an optimal charge within the meaning of the present application.

[0007] In order to ensure the availability of a sufficient charging current during the available charging time, a preferred embodiment of the invention provides that the field device has several different charging current sources, wherein the control unit specifies, taking into account a prioritization, which charging current source or which combination of charging current sources supplies the energy storage device of the field device with a charging current. Accordingly, the charging current sources are designed such that they each provide the charging current alone or in combination with one or more other charging current sources simultaneously. Preferably, the field device has at least one battery, a wired interface, such as a USB interface, and a solar cell as charging current sources. The battery is preferably replaceable and can be replaced at regular maintenance intervals.When prioritizing, the control unit takes into account at least one or more of the following selection criteria: a) The charging current is only supplied by the battery as the charging current source when no other charging current source is available. Preferably, the energy drawn from the battery is measured so that the battery can be replaced before the energy is completely used up. b) The charging current is always supplied by the solar cell as the charging current source when it is possible to charge the energy storage device. The solar cell is therefore always selected as the charging current source when the solar cell generates a suitable charging current, although this depends on the fluctuating and unpredictable lighting conditions. The energy storage device is charged to preferably 80%, although this value can be adjusted.c) The charging current can be supplied by the wired interface as a charging current source if the wired interface is connected to an external control device, whereby the energy storage device is preferably charged to more than 80%. The wired interface is connected to an external control device, for example, for parameterizing the field device, for commissioning (initial setup) of the field device, and for tests or updates. In this case, charging to more than 80% is preferred so that after the external control device is removed, sufficient energy is still available to complete the initiated 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, which increases the device's operating time.The relatively rare process of charging above 80% has little or no impact on the service life of the energy storage device. d) If a selected charging current source cannot provide the required electrical energy to at least adequately or optimally charge the field device's energy storage device, the maximum possible electrical energy is drawn from the selected charging current source to charge the field device's energy storage device, and one of the remaining charging current sources is selected to further charge the field device's energy storage device.

[0008] As an alternative to automatic prioritization by the control unit, the charging current source can also be selected manually, and thus prioritized. Automatic prioritization can be achieved in the simplest case by switching off unused charging current sources (in particular, switching off ICs, opening MOSFETs, or opening switches). Furthermore, the charging current source that currently delivers the highest charging voltage can be selected, with suitable diodes allowing only the higher voltage to pass through to relieve the load on the comparatively lower voltage.

[0009] Within the scope of a further preferred embodiment of the invention, it is provided that the charging of the energy storage device of the field device takes place as a function of time as the (active) operating times of the field device, wherein the charging of the energy storage device preferably takes place during a period that lies between two (active) operating times of the field device, wherein this period is preferably used for as long as possible to charge the energy storage device. In other words, the accumulator is preferably subjected to the lowest possible charging current that is required to charge the accumulator during the available time. If the field device is, for example, a measuring device and measurements are taken at regular intervals of 12 hours, charging takes place during the 12-hour idle phase of the field device, which allows gentle charging of the energy storage device.The charging current is reduced so that the rest phase is optimally used to charge the energy storage device.

[0010] In this context, gentle charging of the energy storage device is understood to mean charging with the lowest possible charging current. Regardless of the fact that a maximum permissible charging current should not be exceeded, the charging current is kept as low as possible to ensure gentle charging.

[0011] Automatic or manual prioritization also ensures that the battery is charged with the optimal or maximum possible charging voltage, with charging being carried out as gently as possible.

[0012] Since unexpected events, such as external triggers, especially a button press by a customer or a signal from another device, can also interrupt the rest phase, 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 sufficient energy in the energy storage device for one or more device activities, especially for measuring and / or sending data, the charging current is reduced so that the battery is charged as gently as possible, as described above.

[0013] When charging an energy storage device, a maximum permissible charging current must not be exceeded. The maximum permissible charging current is generally temperature-dependent. The temporal progression of the charging current and its current intensity influence the service life of an energy storage device, particularly the number of possible charging cycles. Experience has shown that the charging current should be set as low as possible, but as high as necessary. To avoid exceeding the maximum permissible charging current, current limiters in the form of fuses, passive resistors, or active resistors (OPAMPs, semiconductors, MOSFETs, etc.) are common in field devices. The relevant charging current sources typically have a constant charging voltage and generate this at a constant charging current using a connected, variable resistor.At a low voltage level of the energy storage device to be charged, the difference in voltage between the charging current source and the energy storage device sometimes leads to the current limiter being activated, as a result of which electrical energy is unnecessarily converted into heat, particularly in the resistor, and is therefore lost. According to an advantageous development of the invention, it is therefore provided that the at least one charging current source is connected to a voltage regulator which regulates the charging voltage in such a way that the maximum permissible charging current is not exceeded. 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, corresponds to it. In this way, the lowest possible load on the charging current source can be achieved.The charging voltage is preferably adjusted depending on the current voltage of the energy storage device and / or the current charging current. This achieves highly energy-efficient charging of the energy storage device, since the regulation of the charging voltage eliminates the need for additional current limitation, or it is present but not active, since the maximum permissible charging current is not exceeded.

[0014] In this case, too, particularly gentle charging of the energy storage device can be achieved if the charging current is kept as low as possible. This is particularly possible if the time required for charging the energy storage device, i.e., the time until a point in time at which the energy storage device must have a predetermined charge, is known. If this time is known, the charging current is regulated in one embodiment by regulating the charging voltage so that it is minimal over the entire period.

[0015] Alternatively, as described above, it can be provided that a predefined charge of the energy storage device is achieved as quickly as possible and thus with a charging current that is as close as possible to the maximum permissible charging current. Once this predefined charge is reached, the charging current is reduced to the minimum possible charging current for the calculated time period to achieve optimal charging. In a specific embodiment, a linear "adjustment curve" is present, so that the charging voltage in the considered range is a constant value higher than the current voltage of the energy storage device. If, for example, the energy storage device has a voltage of 2.5 V, the charging circuit generates an output voltage of, for example, 3 V. With each increase in the current voltage of the energy storage device, the output voltage of the charging current source is also increased by the same amount until a target voltage of 3.9 V is reached at the charging current source.If the voltage of the energy storage device reaches a target voltage of 3.9 V, no further charging current can flow 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 results in particular from the resistance value of a fuse, a measuring resistor of a 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. Switched semiconductor components are preferably used for voltage limitation; in one advantageous embodiment, these components have Zener diodes. This enables particularly fast and, at the same time, energy-efficient charging of the energy storage device.

[0016] A combination of energy-efficient and gentle charging of the energy storage device can be achieved, for example, with the aforementioned specific embodiment if charging is carried out up to a predetermined voltage of the energy storage device according to the predetermined adjustment curve and, when the predetermined voltage is reached, the charging current is set to the minimum possible charging current with which the optimal charging or a complete charging of the energy storage device can be achieved in the remaining time period.

[0017] Independently of the voltage regulation, a preferred embodiment of the invention also provides voltage limitation, for which purpose a voltage limiter is provided. A voltage limiter consumes energy even at low voltages from the charging current source, for example in the form of leakage currents, even if the voltage at the energy storage device to be charged is below the maximum voltage to be limited. To avoid unnecessary energy loss in this case, a further advantageous development of the invention provides that the voltage limitation is only activated when the voltage of the energy storage device has exceeded a minimum voltage and is deactivated when the voltage of the energy storage device has fallen below a minimum voltage. If, for example, a maximum voltage of 4.0 V is permitted at the energy storage device, the voltage limitation is only activated by means of suitable Z-diodes above a voltage of, for example, 3.7 V.As a result, small leakage currents flow through the voltage limiter only above a charging voltage of 3.7 V. Periodic activation and deactivation (so-called clocking) can be avoided by adding a Zener diode early and applying a suitable hysteresis.

[0018] The described voltage limiting circuit is particularly advantageous for batteries as a charging current source because the available energy is limited. If the charging voltage is regulated by the voltage regulator in such a way that the current limit remains deactivated, no unnecessary energy is consumed, which benefits the service life of the field device. However, if the energy storage device is charged by a charging current source with sufficient or non-critical energy levels, such as a wired interface or a solar cell, a comparatively higher charging voltage can be useful and electrical energy losses due to current limiting can be neglected. Therefore, the use of a voltage regulator and the activation of the voltage limiter above a certain voltage value is particularly preferable when a battery is used as the charging current source.

[0019] Finally, it is preferably provided that the voltage regulator is activated when the voltage of the charging current source has exceeded a minimum voltage.

[0020] Specific embodiments of the invention are explained below with reference to the figures. They show:

[0021] Fig. la, b each show a field device,

[0022] Fig. lc-f each show a voltage limiter and

[0023] Fig. 2 shows the charging behavior of an energy storage device in two diagrams. Fig. 1a shows a schematic representation of a field device 1 which has a consumer in the form of a measuring device 2. Such a measuring device 2 is, for example, a level measuring device. To supply energy to the measuring device 2, an energy storage device 3 is provided, which in the illustrated embodiment is an accumulator. 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. Furthermore, the control unit 5 is also connected to a current limiter 6, which transmits the magnitude of the instantaneous charging current to the control unit 5.If, for example, charging of the energy storage device 3 is necessary due to an upcoming measurement routine, the control unit 3 prioritizes the charging current sources 4, whereby predefined criteria are used to select which of the charging current sources 4 will be supplied with a charging current. During prioritization by the control unit 5, at least one or more of the following selection criteria are taken into account: a) The charging current is only supplied by the battery 41 as the charging current source 4 when no other charging current source 4 is available and ready for use. b) The charging current is always supplied by the solar cell 42 as the charging current source 4 when charging of the energy storage device 3 is possible.c) The charging current can be supplied by the wired interface 43 as charging current source 4 if the wired interface 43 is connected to an external control device, wherein the energy storage device 3 is preferably charged to more than 80%. d) If a selected charging current source 4 cannot provide the required electrical energy to at least sufficiently or optimally charge the energy storage device 3 of the field device 1, the maximum possible electrical energy is taken 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. This allows the charging voltage 4 to be selected that enables the most energy-efficient and / or gentle charging of the energy storage device 3.

[0024] In the illustrated embodiment, the charging current sources 4 are each connected to a voltage regulator 71, 72, 73. The voltage regulators 71, 72, 73 are configured as a voltage generation circuit in such a way that, depending on the instantaneous voltage of the energy storage device 3 and / or the instantaneous charging current, the charging voltage can be adjusted such that the charging current does not exceed the maximum charging current even without the controlling / regulating intervention of the current limiter 6. The charging voltage is preferably regulated in a temperature-dependent manner, for which purpose the field device 1 has a temperature sensor 8.

[0025] As additional components, the field device 1 in the illustrated embodiment has a decision / comparator 9, which provides charging protection when the voltage at the energy storage device is too low to prevent recharging after deep discharge. Furthermore, the field device 1 has one or more additional voltage limiters 10, which are connected between the current limiter(s) 6 and the energy storage device 3, and a deep discharge protection 11, which prevents recharging after deep discharge and ensures that deep discharge does not occur in the first place.

[0026] Fig. lb shows an embodiment which is essentially identical to Fig. 1a, but in which the charging current sources 4 are first connected to the decision / comparator 9 and then to a voltage regulator 74.

[0027] Figs. 1c - f show different embodiments of a voltage limiter 10, which is activated when the voltage of the energy storage device 3 has exceeded a minimum voltage and is deactivated when the voltage of the energy storage device 3 has fallen below a minimum voltage.

[0028] According to Fig. 1c, a Zener diode 12 and a controllable switch 13 are connected in parallel with the energy storage device 3 to limit the voltage, with the controllable switch 13 being connected to a voltage comparator 14. For redundancy reasons and to meet any safety requirements, this component combination of Zener diode 12, controllable switch 13, and voltage comparator 13 is implemented in identical form in duplicate (outlined in dashed lines), which provides functional safety in the event of one of the first-mentioned components failing.

[0029] According to Fig. 1d, a resistor 15 and a controllable semiconductor 16 are connected in parallel with the energy storage device 3 to limit the voltage, with the controllable semiconductor 16 being connected to a voltage comparator 14. Here, too, the aforementioned components are connected in parallel in an identical manner, three times in total, to maintain functionality in the event of a fault.

[0030] Fig. 1e shows an embodiment of a voltage limiter 10 in which different components are provided for voltage limitation. First, a resistor 15 is connected in parallel with the energy storage device 3 together with a controllable semiconductor 16, wherein the controllable semiconductor 16 is connected to a voltage comparator 14. Furthermore, two further identical component combinations are provided, each of which has a Zener diode 12 and a controllable switch 13 connected in parallel with the energy storage device 3, wherein the controllable switch is connected to a voltage comparator 14.

[0031] Fig. 1e shows an embodiment which is essentially functionally identical to Fig. 1c, wherein the controllable switches 13 are each connected to a common Zener diode 12.

[0032] Fig. 2 shows two diagrams 21, 22 typical charging behavior of the energy storage device. Diagram 21 shows the voltage U(t) at the energy storage device as a function of time t, and diagram 22 shows the charging current I(t) as a function of time t. Assuming a constant charging current Ii(t), the voltage at the energy storage device increases slowly. A higher voltage at a constant charging current Ii(t) increases the power absorbed by the energy storage device, which is why the voltage Ui(t) typically increases more and more slowly. If a charge controller now charges with a constant voltage U2(t), current limitation would only occur via the internal resistance of the energy storage device, and the charging current would exceed the maximum permissible charging current. In this case, therefore, a lossy current limitation would be necessary, which reduces the charging voltage Uz(t) until the sufficiently low charging current Ii(t), limited by the internal resistance, flows.However, at least one current-limiting circuit may still be mandatory for reasons of technical explosion protection and introduces an additional resistance into the charging circuit. Using these known resistances or through a measurement, a charging voltage (LhCt) can be calculated at the voltage regulator, which is selected so that the desired charging current Ii(t) flows.

[0033] List of reference symbols

[0034] 1 field device

[0035] 2 measuring device

[0036] 3 Energy storage Charging power source 1 Battery 2 Solar cell

[0037] 43 wired interface 5 control unit

[0038] 6 current limiters

[0039] 71 voltage regulators

[0040] 72 voltage regulators

[0041] 73 voltage regulators

[0042] 8 Temperature sensor

[0043] 9 Decision makers / comparators

[0044] 10 voltage limiters

[0045] 11 Deep discharge protection

[0046] 12 Zener diode

[0047] 13 controllable switches

[0048] 14 voltage comparators

[0049] 15 Resistance

[0050] 16 controllable semiconductors

[0051] 21 Diagram

[0052] 22 Diagram

[0053] Kt) Charging current

[0054] U(t) charging voltage t time

Claims

Patent claims Method for charging an energy storage device (3) of a field device (1) with at least one charging current source (4) and a control unit (5), wherein the charging current source (4) supplies the energy storage device (3) of the field device (1) with a charging current. Method according to claim 1, characterized in that the energy storage device (3) of the field device (1) is an accumulator, wherein the accumulator is preferably charged to a maximum of 80% of its full charging capacity in order to extend its service life. Method according to one of claims 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 a prioritization, which charging current source (4) or which combination of charging current sources (4) supplies the energy storage device (3) of the field device (1) with a charging current.Method according to claim 3, characterized in that the field device (1) has at least one battery (41), one solar cell (42) and a wired interface (43) as charging current sources (4), wherein at least one or more of the following selection criteria are taken into account during prioritization by the control unit (5): a) the charging current is only supplied by the battery (41) as the charging current source (4) when no other charging current source (4) is available and ready for use; b) the charging current is always supplied by the solar cell (42) as the charging current source (4) when charging of the energy storage device (3) is possible; c) the charging current can be supplied by the wired interface (43) as the charging current source (4) when the wired interface (43) is connected to an external operating device, wherein the energy storage device (3) is preferably charged to more than 80%. d) If a selected charging current source (4) cannot provide the required electrical energy to at least adequately or optimally charge the energy storage device (3) of the field device (1), the maximum possible electrical energy is taken 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). Method according to one of claims 1 to 4, characterized in that the charging of the energy storage device (3) of the field device (1) takes place as a function of time as the operating times of the field device (1), wherein the charging of the energy storage device (3) preferably takes place during a period that lies between two operating times of the field device, wherein this period is preferably used for charging the energy storage device for as long as possible.Method according to one of claims 1 to 5, characterized in that, to ensure proper operation, the charging current is only reduced after a minimum energy level in the energy storage device has been reached. Method according to one of claims 1 to 6, characterized in that the at least one charging current source (4) is at least indirectly connected to a voltage regulator (71, 72, 73) which regulates the charging voltage in such a way that the maximum permissible charging current is not exceeded. Method according to one of claims 1 to 7, characterized in that the charging voltage is set as a function of the instantaneous voltage of the energy storage device (3) and / or as a function of the instantaneous charging current. Method according to one of claims 1 to 8, characterized in that a voltage limiter (10) a) activates a voltage limitation when the voltage of the energy storage device (3) has exceeded a minimum voltage, and. b) is deactivated when the voltage of the energy storage device (3) and the charging current source (4) has fallen below a minimum voltage. Method according to one of claims 1 to 9, characterized in that the voltage regulator (71, 72, 73) is activated when the voltage of the charging current source (41, 42, 43) has exceeded a minimum voltage. A field device comprising a rechargeable energy storage device (3), at least one charging current source (4), and a control unit (5), wherein the field device (1) is configured to carry out the method according to one of claims 1 to 8.

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