Method for determining the value of a process variable in a fluid power system

Abstract

Method for determining the value of a process variable (P) for process control in a process line of a fluid power plant (10) in which a process medium is conveyed, wherein a first actuator (22) and a second actuator (42) are located in the process line and are fluidically connected, wherein a first operating variable (B1) is assigned to the first actuator (22) and a second operating variable (B2) is assigned to the second actuator (42), and a first characteristic curve (K1) based on the first operating variable (B1) is assigned to the first actuator (22) and a second characteristic curve (K2) based on the second operating variable (B2) is assigned to the second actuator (42), characterized in that the value of the process variable (P), which differs from the operating variables (B1, B2), is determined using the values ​​of the first operating variable (B1) and the second operating variable (B2) and a calculation model (M) comprising the first characteristic curve (K1) and the second characteristic curve (K2).

Description

[0001] The invention relates to a method for determining a value of a process variable in a process line of a fluid power plant in which a process medium is conveyed, wherein at least two different actuators are located in the process line, each of which is assigned at least one operating variable and each of which is assigned a characteristic map based on the at least one operating variable.

[0002] The actuators in the process chain are regularly controlled based on process variables, the value of which is either detected by sensors or calculated.

[0003] However, the use of sensors can be correspondingly expensive, and the use of a calculated process variable using only an actuator characteristic map can be too inaccurate or impossible.

[0004] It is therefore an object of the invention to provide a method by which a process variable in the process stream of a process engineering plant can be determined with corresponding accuracy without having to detect it by sensors.

[0005] In a known manner, the values ​​of a process variable for process control or regulation of a fluid power system are recorded in a process line of a fluid power system in which a process medium is conveyed. The process line contains a first actuator and a second actuator, with the first actuator assigned one operating variable and the second actuator assigned a second operating variable. The first actuator is assigned a first characteristic curve based on the first operating variable, and the second actuator is assigned a second characteristic curve based on the second operating variable.

[0006] According to the invention, the value of the process variable, which differs from the operating variable, is determined using the values ​​of the first operating variable and the second operating variable and a calculation model that includes the first characteristic map and the second characteristic map.

[0007] By taking into account the characteristic curves of at least two actuators in the calculation model, more process variables can now be calculated, or a process variable can be calculated more accurately.

[0008] An operating variable is a variable that is necessary for the operation of the actuator.

[0009] By using the information from two characteristic curves of different actuators that are arranged in a process stream and therefore influence each other, the value of a process variable can be determined with sufficient accuracy and the value of the process variable determined in this way can be used to regulate or control the fluid power system.

[0010] A regulation or control system exists when the behavior of at least one actuator in the fluid power system is influenced.

[0011] Preferably, the value of a first derived variable can be determined indirectly from the value of a first operating variable and the characteristic curve of the first actuator. Similarly, the value of a second derived variable can be determined from the value of the second operating variable and its characteristic curve, since the characteristic curve reveals the relationship between the values ​​of the operating variables and the derived variables. The value of the process variable is then determined from the values ​​of the first and second derived variables.

[0012] In particular, the first and second derived variables correspond to the process variables. This means that two values ​​are available for the process variable, determined based on different criteria. The value of the process variable can then be calculated from these two values.

[0013] According to a preferred embodiment, if two values ​​of the process variable are available by choosing two derivative variables, the value of the process variable can be determined by averaging the two values, e.g. by calculating the arithmetic mean, the geometric mean, or a weighted equal mean.

[0014] According to a further refinement of the method, the values ​​of the two process variables can be intelligently compared to obtain improved quality of the process variable value. This could be done, for example, by taking into account known deviations of the actuator.

[0015] In this way, a higher accuracy can be achieved for the value of a process variable, so that the value of the process variable determined in this way can be used for process control or regulation and a separate measurement using a sensor for measuring the process variable can be omitted.

[0016] Furthermore, the first derived variable can differ from the second derived variable. For example, the first derived variable can correspond to the process variable, and the second derived variable can correspond to an operating variable that influences the process variable. This allows the value of the first derived variable to be corrected by the value of the second derived variable when determining the value of the process variable. In this way, the value of a process variable can be determined at a specific point in the process flow without having to introduce an additional sensor.

[0017] According to an advantageous embodiment of the invention, the values ​​of the operating variables are detected by a sensor.

[0018] Preferably, a first actuator is a pump and a first operating variable is the rotational speed or the power consumption. The characteristic curve can preferably represent the flow rate as a function of the rotational speed.

[0019] Both the rotational speed and the power consumption can also be represented as two operating variables. For example, an n-dimensional characteristic map can also include the variables volume of the pump noise and / or flow rate and / or frequency analysis of the noise after Fourier transformation.

[0020] In particular, a second actuator can be a control valve. The operating variable of the control valve is the valve position. The characteristic curve specifically represents the flow coefficient as a function of the valve position.

[0021] The valve position can be measured directly by a sensor. Alternatively, the operating variable could be the actuator pressure. The valve position can then be determined by incorporating the characteristic map that maps the valve position against the actuator pressure. The flow coefficient is then calculated from the valve position.

[0022] According to a further aspect of the invention, this relates to a process plant comprising a process line in which fluid is conveyed, wherein a first actuator is integrated into the process line and a first characteristic curve via a first operating variable is assigned to the first actuator, which is stored on a first storage unit. Furthermore, the plant comprises a sensor that detects the first operating variable of the first actuator, wherein the detected value of the first operating variable is transmitted to a first communication unit. The plant also comprises a first processing unit that determines the value of a first derivative variable from the characteristic curve and the value of the first operating variable.

[0023] Furthermore, a second actuator is integrated into the process chain, with a second characteristic curve assigned to this actuator via a second operating variable, which is stored on a second memory unit. The system also includes a second sensor that detects the second operating variable of the second actuator, and the detected value of this variable is transmitted to a second acquisition unit. Finally, the system includes a second processing unit that calculates the value of a second derivative variable (delivery head) from the characteristic curve and the value of the second operating variable.

[0024] Furthermore, a third computing unit is provided, which forms the value of a process variable from the first value of the first derivative variable and the value of the second derivative variable.

[0025] The first derivative variable and the second derivative variable correspond to the process variable.

[0026] The first and second derivative variables can be different variables. The process variable can then be calculated from the first and second derivative variables.

[0027] In an arrangement of a pump and a subsequent valve in a process line, for example, the pressure before the valve can be determined as the first derivation variable and the opening width of the valve as the second variable, whereby the pressure after the valve can be determined as the process variable.

[0028] The first computing unit, the second computing unit and the third computing unit preferably form a common computing unit, which is housed in particular in the higher-level control unit / process control.

[0029] According to an alternative configuration, a first computing unit can be located at the first actuator, with the value of the derivative variable being passed to the third computing unit, which is located in the control unit / process control. This configuration can also be applied analogously to the second computing unit.

[0030] Preferably, the method can be applied to a process line comprising a pump and a control valve. According to the procedure for determining the value of the process variable P, the first operating variable is the pump speed, and the first characteristic curve represents the flow rate as a function of the speed. This allows the volumetric flow rate generated by the pump to be calculated. For a more precise calculation, the pump's power consumption can also be taken into account.

[0031] The second actuator is a pneumatically operated control valve. The second operating variable represents the actuator pressure in the control valve's actuator. The second characteristic curve represents the actuator pressure in the unloaded state as a function of the valve position. Using the actuator pressure measurement, the valve position measurement, and the characteristic curve "flow coefficient / valve position," in combination with the characteristic curves "actuator pressure (unloaded state / valve position)" and the throttle body-specific characteristic curve "differential pressure correction factor / valve position," the second value of the volumetric flow rate is determined by also taking into account the material properties of the process medium (density, viscosity), the differential pressure across the throttle body, and the temperature T1.

[0032] The data of the medium can be stored in the control valve in the form of a table or function and read out from there alongside the characteristic curve.

[0033] For example, for an incompressible medium, the parameters density and viscosity are stored, each as a function of temperature. Temperature measurement can be performed either directly in contact with the medium or without contact by applying a temperature sensor to the valve body (optionally to the frame or even within the positioner) with a suitable transfer function.

[0034] The first value of the volume flow rate and the second value of the volume flow rate are evaluated in a third processing step to form a value of the process variable "volume flow rate".

[0035] This is achieved in particular by either calculating an average value or generating a multidimensional characteristic map "process variable deviation (typical error curves of the individual field devices) / valve position / speed".

[0036] In its simplest form, the value of the process variable with the smallest deviation is output. This variant is applicable, for example, when the highest accuracies are not required and no module calibration is available.

[0037] A second option is to generate a resulting process variable correction factor based on the multidimensional characteristic map. This correction factor can be applied either by calibrating individual field devices or the entire module.

[0038] In this way, the volume flow rate in the process line can be calculated with sufficient accuracy from the drive pressure of the control valve, the speed of the pump and the characteristic curves of the pump and the control valve.

[0039] Furthermore, the invention relates to a process plant in which a fluid is conveyed, within which the aforementioned method is applied. The process plant comprises a first actuator, comprising a first actuator communication unit for communication with another actuator and / or with a higher-level control unit and / or a sensor. The plant also comprises a first storage unit on which a first characteristic map of the first actuator, representing a dependency on a first operating variable, is stored.

[0040] The process engineering system also includes a second actuator, comprising a second actuator communication unit for communication with another actuator and / or with a higher-level control unit and / or a sensor, and a second storage unit on which the characteristic map of the second actuator, which represents a dependency on a second operating variable, is stored.

[0041] In the process engineering plant, the first actuator and the second actuator are fluidically connected in a process stream, in particular via a pipeline.

[0042] The process plant also includes a first sensor in the process line for detecting a first operating variable and a first sensor communication unit for communication with another actuator and / or with a higher-level control unit and / or a sensor.

[0043] In addition, a second sensor for detecting a second operating variable and a second sensor communication unit for communication with another actuator and / or with a higher-level control unit and / or a sensor are provided in the process stream.

[0044] The process engineering system also includes a first computing unit for calculating the value of a first derivative variable using the first characteristic map and the value of the first operating variable, as well as a second computing unit for calculating the value of a second derivative variable using the second characteristic map and the value of the second operating variable.

[0045] Furthermore, a third computing unit is provided for calculating the value of the process variable from the value of the first derivative variable and the value of the second derivative variable.

[0046] In particular, the system has a higher-level control unit for controlling and / or regulating at least two actuators, comprising a control communication unit that is connected to the sensors and the actuators.

[0047] Preferably, the first computing unit, the second computing unit and the third computing unit can be implemented in a common computing unit, wherein the common computing unit is preferably arranged in the higher-level control unit.

[0048] Alternatively, the first computing unit can be arranged at the actuator, whereby the first derivative variable determined by the first computing unit can be transmitted using the actuator communication unit.

[0049] In particular, the first actuator and the first sensor form a first field device and / or the second actuator and the second sensor form a second field device.

[0050] In this case, the first actuator communication unit and the first sensor communication unit are implemented in a single, shared first field device communication unit. This allows both the control data for the actuator and the measurement data from the sensor to be transmitted via the field device communication unit to a higher-level control unit or another field device.

[0051] The transmission can be wired or wireless.

[0052] The same can also apply to a second actuator communication unit and the second sensor communication unit, which are implemented in a common second field device communication unit.

[0053] The first actuator can preferably include a memory for the first characteristic map, where the characteristic map specifically represents the actuator's main function. The actuator's main function is, for example, the Kv value as a function of the valve position. In the case of a pump, for example, the flow rate as a function of the speed.

[0054] The actuator can thus access the characteristic curve locally. However, if the actuator is connected to a higher-level control unit, the characteristic curve can also be read from the actuator's memory via a data connection, which may be part of the actuator communication unit. In this way, the actuator, or any field device integrated into a process flow, carries its own characteristic curve, the values ​​of which can be used to calculate further process variables. Alternatively, the characteristic curve can also be stored decentrally on a computer connected via a network. This represents a so-called cloud solution. The higher-level control unit or the actuator communication unit provides the necessary communication interfaces for this.

[0055] Similarly, the second actuator can also include the second memory for the second characteristic map.

[0056] Preferably, the first, second, and third processing units comprise a CPU, an FPGA, a DSP, or a microcontroller. This allows the processing unit to be designed according to the specific requirements and the advantages of the chosen solution.

[0057] According to a further advantageous embodiment, the communication units can be configured to be connected via a mesh network, in particular a radio network. This enables, among other things, communication between two field devices and also between the field devices and a higher-level control unit.

[0058] Further advantages, features and application possibilities of the present invention will become apparent from the following description in conjunction with the exemplary embodiments shown in the drawings.

[0059] The description, claims, and drawing use the terms and associated reference numerals listed below. In the drawing, this means: Fig. 1 a schematic view of the method according to the invention Fig. 2 a detailed illustration of the procedure; Fig. 3 an embodiment of the method with a pump and a control valve; Fig. 4 a functional arrangement according to the invention in a process stream; Fig. 5 a further functional arrangement according to the invention in a process stream; Fig. 6 a further functional arrangement according to the invention in a process stream, and Fig. 7 a further functional arrangement according to the invention in a process stream.

[0060] Fig. Figure 1 shows a schematic representation of the inventive method for determining the value of a process variable P. In the present example, two actuators are provided in a process stream, as in the Fig. As described in sections 4 ff., each actuator is assigned a characteristic curve K1, K2, which represents a relationship via an operating variable B1, B2. The operating variables B1, B2 are primarily measured using sensors. The operating variables B1, B2, as well as the characteristic curves K1 and K2, are incorporated into the computational model M. The computational model M is designed such that, depending on the value of the operating variables B1, B2, the value of a process variable P is determined. The process variable P is different from the operating variables B1, B2.

[0061] Fig. Figure 2 shows a detailed illustration of the procedure, where the computational model M comprises three processing steps. In processing step V1, a derived variable A1 is generated from the characteristic map K1 and the operating variable B1. In processing step V2, a derived variable A2 is generated from the characteristic map K2 and the operating variable B2. In a third processing step V3, the process variable P is formed from the two derived variables A1 and A2.

[0062] This can be achieved by having the derivative variables A1 and A2 represent the process variable, for example, the volumetric flow rate in the process stream. The process variable is determined using the characteristic curves K1 and K2 and the operating variables B1 and B2. The value of the process variable P is obtained, for example, by calculating the arithmetic mean of the two values ​​of the derivative variables A1 and A2, although other combinations of the two derivative variables also exist. A corresponding implementation is shown in Fig. 3 described in more detail.

[0063] Fig. Figure 3 shows an embodiment in which a pump PU and a control valve are fluidically connected in a process line. According to the method for determining the value of the process variable P, the operating variable B1 is the speed N of the pump PU, and the characteristic curve K1 represents the flow rate as a function of the speed N. The presumed volumetric flow rate Q1 generated by the pump PU is known from the flow rate. For a more precise calculation, the power consumption of the pump PU can also be taken into account in V1.

[0064] The second actuator is designed as a pneumatically operated control valve ST. The operating variable B2 represents the drive pressure PS in the diaphragm actuator of the control valve. The characteristic curve K2 represents the drive pressure in the unloaded state via the valve position.

[0065] The volume flow rate Q2 is determined by the second processing unit V2 using the drive pressure measurement and the measurement of the valve position and the characteristic map "flow coefficient / valve position" in combination with the characteristic maps K3 "drive pressure (unloaded state / valve position)" and the throttle body-specific characteristic map K4 "differential pressure correction factor / valve position", by also taking into account the material properties of the process medium (density ρ, viscosity µ) as well as the differential pressure across the throttle body and the temperature T1.

[0066] The characteristic curve K5 can also be "flow coefficient / valve position".

[0067] The differential pressure PD can also be included in the calculation via the control valve.

[0068] The medium can be stored in the control valve as a table or function and read from there, in addition to the characteristic curve. For example, for an incompressible medium, the parameters density and viscosity are stored, each depending on the temperature. Temperature measurement can be performed either directly in contact with the medium or without contact by applying a temperature sensor to the valve body (optionally to the frame or even within the positioner) with a suitable transfer function.

[0069] The first volume flow value Q1 and the second volume flow value Q2 are evaluated in a third processing step V3 to form a volume flow value Q.

[0070] This is achieved in particular by generating a multidimensional characteristic map K6 “Process variable deviation (typical error curves of the individual field devices) / valve position / speed”.

[0071] In its simplest form, the value of the process variable with the smallest deviation is output. This variant is applicable, for example, when the highest accuracies are not required and no module calibration is available.

[0072] A second option is to generate a resulting process variable correction factor based on the multidimensional characteristic map. This correction factor can be applied either by calibrating individual field devices or the entire module.

[0073] Fig. Figure 4 shows a functional schematic view of a first arrangement 10 according to the invention in a process stream, comprising a first field device 20 and a second field device 40. The first field device 20 comprises a first actuator 22, a first computing unit 24, a first sensor 26 for detecting the value of an operating variable B1 of the actuator 22, a first memory 28 on which the characteristic curve K1 of the actuator 22 with respect to the operating variable B1 is stored, and an actuator communication unit 30 for communication with a higher-level process controller 60, which also includes a communication unit 62. The actuator communication unit 30 can, for example, both read the operating variable B1 from the higher-level process controller 60 and receive a control signal for the actuator 22.The actuator communication unit can also be designed as a field device communication unit and can also supply the sensor value to the higher-level control unit 60, as is the case, for example, in a design according to . Fig. 6 is the case. This also applies to the second actuator communication unit 50. The processing unit 24 is designed such that it reads a characteristic map K1 from the memory 28 and receives a value of the operating variable B1 from the sensor 26. Using the characteristic map K1 and the operating variable B1, the first processing unit 24 calculates an initial value of a derivative variable A1 and transmits it via the actuator communication unit 30 to the higher-level process control 60.

[0074] According to the present embodiment, the second field device 40 is constructed similarly to the first field device 20. The second field device 40 also includes a second actuator 42, which has a different function than the first actuator 22. The actuators 22 and 42 are connected to each other via a fluidic connection, in particular via a pipe 12. The second field device 40 includes a second processing unit 44, a second sensor 46 for detecting the value of an operating variable B2 of the second actuator 42, a second memory 48 on which the characteristic curve K2 of the actuator 42 with respect to the operating variable B2 is stored, and a second actuator communication unit 50 for communication with the higher-level process controller 60. The second actuator communication unit 50 can, for example, both read the operating variable B2 from the higher-level process controller 60 and receive a control signal for the actuator 22.The processing unit 44 is configured to read a characteristic map K2 from the memory 48 and receive a value of the operating variable B2 from the sensor 46. Using the characteristic map K2 and the operating variable B2, the second processing unit 44 calculates a value of a second derivative variable A2 and transmits it to the higher-level process control 60 via the actuator communication unit 50.

[0075] The process control 60 includes a third computing unit 64, which determines the value of a process variable P from the derivative variables A1, A2 transmitted by the first computing unit 30 and the second computing unit 50.

[0076] The conversion of the operating variables B1, B2 using the characteristic maps K1, K2 is represented in a calculation model that is implemented using the first calculation unit 30, the second calculation unit 50 and the third calculation unit 64, where each calculation unit 24, 44, 64 represents a subtask of the calculation model M.

[0077] In this way, the characteristic curves K1, K2 and the corresponding values ​​of the operating variables can be evaluated directly at the respective field device 20, 40 in a decentralized manner, with the overall result being evaluated in a central process control 60.

[0078] Fig. Figure 5 shows a further embodiment in a schematic view. Similar functional units, as seen in Fig. Four occurrences are designated the same way. Thus, as well as according to... Fig. As already described in section 4, two field devices 20 and 40 are provided, which are connected to each other via a pipeline 12. According to the design of the in Fig. In the embodiment described in section 5, the first field device 20 comprises a first actuator 22, a first sensor 26, a first memory 28, and a first actuator communication unit 30 for transmitting the value of the first operating variable B1 to the higher-level process control 60. Furthermore, the communication unit is designed such that the process control 60 can read the characteristic map K1 stored in the first memory 28 or determine a value of a derivative variable A1 corresponding to the value of the detected operating variable B1 using the characteristic map K1. For this purpose, the first computing unit 24 is arranged in the central process control 60. The same applies analogously to the embodiment of the second field device 40.The second computing unit 44, also in conjunction with the second field device 40, is housed in the higher-level process control 60 and determines the second derived variable A2 by querying the values ​​of the operating variable B2 and the second characteristic curve K2 from the second field device 40. The field devices 20 and 40 can thus be supplied with a characteristic curve K1, K2, and a sensor for recording the operating variables, ensuring that all necessary information is available when using a corresponding field device. The third computing unit 62 is, as per [reference], Fig. 4 described in the higher-level process control 60 arranged and determines the value of a process variable P from the two derived values ​​according to a given calculation model M.

[0079] In the process control 60, all computing units can be designed, for example, in the form of a PC, which also makes it possible to perform calculations that require high computing power, whereby the computing model M is implemented in particular as a computer-implemented method on the PC.

[0080] This allows the use of field devices whose computing unit is either unavailable or not sufficiently dimensioned for the task at hand.

[0081] Fig. Figure 6 shows another variation of the Fig. 5, in contrast to the Fig. The first storage unit 28 for the first characteristic map K1 and the second storage unit 48 for the second characteristic map K2 are also located in the higher-level process control 60. In this configuration, the field devices only need to query the operating variables B1 and B2, which are then transmitted to the higher-level process control 60. The first computing unit 24 and the second computing unit 44 are also located in the process control 60. The computing units can evaluate the characteristic map K1 and K2 stored in the assigned memory with the value of the corresponding operating variables B1 and B2, and based on the generated derived variables A1 and A2, the third computing unit 64 can then determine the value of the process variable P.

[0082] This arrangement is particularly suitable for retrofitting an existing process line 10, since the characteristic curves do not need to be stored on a memory of the field device, but can simply be stored centrally in the process control 60.

[0083] Fig. Figure 7 shows a further embodiment of a process stream according to the invention. Four field devices 70, 72, 74, 76 are provided in the process stream. The first field device 70 and the third field device 74 each comprise an actuator 22, 42 and an actuator communication device 82, 84. The second field device 72 and the fourth field device 76 each comprise a sensor 26, 46 and a sensor communication device 78, 80.

[0084] In the aforementioned case, the first actuator 22 is controlled by the higher-level process control 60 depending on the operating variable detected by sensor 26. The second actuator 24 is also controlled by the higher-level process control 60 depending on the operating variable detected by sensor 46.

[0085] According to the invention, the actuator 22, 24 and the respective associated sensor 26, 46 do not have to be housed in a common field device, but can also be arranged distributed over the process line.

[0086] Furthermore, a higher-level process control 60 is required according to Fig. 6. The higher-level process control 60 has a communication unit 62. The communication unit 62 is connected to the actuator communication devices 82, 84 and the sensor communication units 78, 80. In this way, the higher-level process control 60 can query the sensor data that record the operating variables for the assigned actuators 22, 24 and evaluate the characteristic maps stored in the memory unit 28, 48. Based on the read-out characteristic maps, as described in Fig. As described in section 6, a desired process parameter can be reliably determined. Reference symbol list 10 Arrangement 20 first field device 22 first actuator 24 first computing unit 26 first sensor 28 first memory 30 Field device communication unit 40 second field device 42 second actuator 44 second computing unit 46 second sensor 48 second memory 50 Field device communication unit 52 Sensor communication unit 64 third computing unit 60 higher-level process control 62 Communication unit 70 first field device 72 second field device 74 third field device 76 fourth field device 78 Sensor communication unit 80 Sensor communication unit 82 Actuator communication unit 84 Actuator communication unit B1 Operating variable B2 Operating Variable P process variable M computational model B2 Operating Variable K1 map K2 map K3 map K4 map K5 map K6 map V1 Processing Step V2 Processing Step V3 Processing Step A1 Derivative variable A2 derivative variable Q volume flow rate Q1 Volume flow rate Q2 Volume flow rate PS Drive pressure PU pump ST control valve N speed L power

Claims

[1] Method for determining the value of a process variable (P) for process control in a process line of a fluid power plant (10) in which a process medium is conveyed, wherein a first actuator (22) and a second actuator (42) are located in the process line and are fluidically connected, wherein a first operating variable (B1) is assigned to the first actuator (22) and a second operating variable (B2) is assigned to the second actuator (42), and a first characteristic curve (K1) based on the first operating variable (B1) is assigned to the first actuator (22) and a second characteristic curve (K2) based on the second operating variable (B2) is assigned to the second actuator (42). characterized by , that by using the values ​​of the first operating variable (B1) and the second operating variable (B2) and a calculation model (M) that includes the first characteristic map (K1) and the second characteristic map (K2), the value of the process variable (P) that differs from the operating variables (B1, B2) is determined. [2] Method according to claim 1, characterized by , that in the calculation model (M) a value of a first derivative variable (A1) is determined from the value of a first operating variable (B1) and the first characteristic map (K1), and a value of a second derivative variable (A2) is determined from the value of the second operating variable (B2) and the second characteristic map (K2), and the value of the process variable (P) is determined from the first derivative variable (A1) and the second derivative variable (A2). [3] Method according to claim 2, characterized by , that the first derivative variable (A1) and the second derivative variable (A2) correspond to the process variable (P). [4] Method according to claim 3, characterized by , that according to the calculation model (M) the value of the first derivative variable (A1) and the value of the second derivative variable (A2) are averaged to determine the value of the process variable (P). [5] Method according to claim 3, characterized by, that according to the computational model (M) the value of the first derivative variable (A1) and the value of the second derivative variable (A2) are intelligently compared to determine the value of the process variable (P). [6] Method according to claim 5, characterized by , that the intelligent comparison takes into account an error curve of the actuator (22, 42). [7] Method according to any of the preceding claims, characterized by , that the value of the operating variables (B1, B2) is recorded by a sensor (26, 46) each. [8] Method according to any of the preceding claims, characterized by , that the first actor (22) and the second actor (24) are different types of actors. [9] Method according to any of the preceding claims, characterized by , that the computational model (M) takes into account fluid parameters such as density and viscosity of the pumped process medium. [10] Method according to any of the preceding claims, characterized by, that the computational model (M) takes into account the effects of the pipelines between the actuators (22, 42) in the process stream. [11] Method according to any of the preceding claims, characterized by , that the characteristic map (K1) of the first actuator (22) is read from a storage unit (28) of the first actuator (22). [12] Method according to any of the preceding claims, characterized by , that the characteristic map (K2) of the second actuator (42) is read from a storage unit (48) of the second actuator (42). [13] Method according to any of the preceding claims, characterized by , that the first actuator (22) is a pump (PU), the operating variable (B1) is the speed (N) of the pump (PU) and the characteristic map (K2) maps the flow rate (Q1) over the speed (N). [14] Method according to any of the preceding claims, characterized by, that the second actuator (42) is a fluidically operated control valve (ST), the operating variable (B2) is the control pressure (PA) and the characteristic map (K2) includes the control pressure, the flow rate (Q2) and / or the noise generation.) [15] Method according to any one of claims 3 to 12, characterized by , that the process variable (P) is the flow rate (Q), that the first actuator is a pump (PU), the operating variable (B1) is the speed (N) of the pump (PU) and the characteristic map (K1) maps the flow rate (Q1) over the speed (N), that the second actuator (42) is a fluidically operated control valve (ST), the operating variable (B2) is the control pressure (PS) and the characteristic map (K1) includes the control pressure (PS), the flow rate (Q2) and the noise generation, furthermore the differential pressure across the control valve is recorded and included in the calculation model (M). [16] Method according to claim 15, characterized by, that the computational model (M) further includes a multidimensional characteristic map (typical error curves of the individual actuators) / valve position / speed for correcting the flow rate value. [17] Method according to claim 15, characterized by , that the calculation model (M) includes the power consumption (L) of the pump (PU). [18] Method according to claim 15, characterized by , that the calculation model (M) includes the efficiency of the pump (PU). [19] Method according to any of the preceding claims, characterized by , that the temperature of the process medium (T1) is measured and included in the calculation model (M). [20] Method according to any of the preceding claims, characterized by , that the density values ​​and viscosity values ​​of the process medium are stored in a memory of the control unit (60) and can be read out. [21] Process plant (10) in which a process medium is conveyed, comprising: - a first actuator (22) comprising a first actuator communication unit (30) for communication with another actuator (42) and / or with a higher-level control unit (60) and / or a sensor (26, 46) - a characteristic map (K1) of the actuator (22) stored on a first storage unit (28) - a second actuator (42) comprising a second actuator communication unit (50) for communication with another actuator (22) and / or with a higher-level control unit (60) and / or a sensor (26, 46) - and a second storage unit (48) on which the characteristic map (K2) of the second actuator (42) is stored, - wherein the first actuator (22) and the second actuator (42) are fluidically connected, in particular via a pipe (12), - a first sensor (26) for detecting a first operating variable (B1) and a first sensor communication unit (32) for communication with another actuator (22, 42) and / or with a higher-level control unit (60) and / or a sensor - a second sensor (26) for detecting a second operating variable (B2) and a second sensor communication unit (80) for communication with another actuator and / or with a higher-level control unit (60) and / or a sensor - a first computing unit (24) for calculating the value of a first derivative variable (A1) using the first characteristic map (K1) and the value of the first operating variable (B1) - a second computing unit (44) for calculating the value of a second derivative variable (A2) using the second characteristic map (K2) and the value of the second operating variable (B2) - a third computing unit (64) for calculating the value of the process variable (P) from the value of the first derivative variable (A1) and the value of the second derivative variable (A2). [22] Plant according to claim 21 characterized by , that the system (10) has a higher-level control unit (60) for controlling and / or regulating at least two actuators (22, 42), comprising a control unit communication unit (62) which is connected to the sensors (26, 46) and the actuators (22, 42). [23] Plant according to claim 22, characterized by , that the first computing unit (24), the second computing unit (44) and the third computing unit (64) are implemented in a common computing unit. [24] Plant according to any one of claims 21 to 23, characterized by , that the first actuator (22) and the first sensor (26, 46) form a first field device (20). [25] Plant according to claim 24, characterized by, that the first actuator communication unit and the first sensor communication unit are represented in a common first field device communication unit (30). [26] Plant according to claim 24 or 25, characterized by , that the first actuator (22) includes a memory (28) for the first characteristic curve (K1). [27] Plant according to any one of claims 21 to 24 to, characterized by , that the second actuator (42) and the second sensor (46) form a second field device (40). [28] Plant according to claim 26 characterized by , that the second actuator communication unit and the second sensor communication unit are represented in a common second field device communication unit (50). [29] Plant according to claim 27 or 28, characterized by , that the second actuator (46) includes the second memory (48) for the second characteristic curve (K2). [30] Plant according to claims 21 to 29, characterized by, that the first computing unit (24), the second computing unit (44) and the third computing unit (46) comprise a CPU, an FPGA, a DSP or a microcontroller. [31] Plant according to any one of the preceding claims 21 to 30, characterized by , that the communication units (30, 50, 78, 80, 82, 84) are set up to be connected via a meshed network, in particular a radio network.

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