Method for setting a constant magnetic field strength of a magnetic field and a flowmeter

Abstract

A method for setting a constant magnetic field strength of a magnetic field within a commutation interval using a magnetic-inductive flowmeter having a current controller with which time for setting the constant magnetic field strength of a magnetic field is relatively shorter. Additionally, a first interval having a starting point in time and an ending point in time and a second interval having a starting point in time and an ending point in time are arranged within a commutation interval. A first setpoint current curve for the first interval differs by a difference current curve to effectuate a higher rate of change. A second setpoint current curve is assigned to the constant setpoint current. The current controller is fed the first and second setpoint current curves.

Claims

1. A method for setting a constant magnetic field strength of a magnetic field within a commutation interval, the method comprising: using a magnetic-inductive flowmeter having a measuring tube and an electromagnet to generate a magnetic field, wherein the electromagnet has a current controller and a coil arrangement, the current controller generating a coil current and the generated coil current effectuating the magnetic field in the coil arrangement, the coil current is commutated at the commutation interval, wherein when a constant setpoint current of the current controller is preset, the electromagnet generates the constant magnetic field strength in steady state, a first interval has a first starting point in time and a first ending point in time and a second interval has a second starting point in time and a second ending point in time, the first and second starting points and the first and second ending points are arranged within the commutation interval, and a first setpoint current curve is defined for the first interval, wherein the first setpoint current curve differs by a difference current curve from the constant setpoint current such that the difference current curve, with reference to the constant setpoint current, effectuates a higher rate of change of a magnetic field strength towards the constant magnetic field strength, a second setpoint current curve is defined for the second interval, in that the constant setpoint current is assigned to the second setpoint current curve, and the current controller is fed the first setpoint current curve in the first interval and the second setpoint current curve in the second interval.

2. The method according to claim 1, wherein the current controller is overdriven by the first setpoint current curve at a beginning of the first interval.

3. The method according to claim 1, wherein the first interval is shorter than a settling time constant of the magnetic field.

4. The method according to claim 1, wherein the first setpoint current curve is constant.

5. The method according to claim 1, wherein the coil current is measured such that a point in time is determined in which the measured coil current has reached the first setpoint current curve or the second setpoint current curve, and the determined point in time is adopted as the first ending point in time of the first interval and as the second starting point in time of the second interval.

6. The method according to claim 1, wherein the first setpoint current curve is set so a magnitude of the difference current curve does not exceed 15% of a magnitude of the constant setpoint current.

7. The method according to claim 1, wherein the magnetic field strength, an induction voltage induced in a medium by the magnetic field by a flow of the medium in the measuring tube, the coil current, a coil voltage generated from the coil current in the coil arrangement measured as an indicator quantity, the first interval, and the first setpoint current curve are defined using the indicator quantity.

8. The method according to claim 7, wherein the indicator quantity is measured in a third interval within the second interval.

9. The method according to claim 7, wherein a first measured value and a second measured value of the indicator quantity are measured to form an evaluation quantity, a mean value is formed and subtracted from the first measured value, and the first interval and/or the first setpoint current curve is/are defined using the evaluation quantity.

10. The method according to claim 9, wherein a trend quantity is determined from the evaluation quantity, a change of the evaluation quantity is determined from the second interval of the commutation interval and a second interval of at least one additional commutation interval, and the first interval and/or the first setpoint current curve is/are defined using the trend quantity.

11. The method according to claim 1, wherein a temperature at the magnetic-inductive flowmeter is measured and that the measured temperature is used in determining the first interval and/or the first setpoint current curve.

12. The method according to claim 11, wherein a temperature of the coil arrangement and/or of the medium is also measured.

13. A magnetic-inductive flowmeter, comprising: a measuring tube; an electromagnet for generating a magnetic field; and a control unit, wherein the electromagnet comprises a current controller and a coil arrangement, the current controller generating a coil current and the generated coil current effectuates the magnetic field in the coil arrangement, the electromagnet commutates the coil current at a commutation interval, wherein, when a constant setpoint current of the current controller is preset and the electromagnet generates a constant magnetic field strength in a steady state, the control unit sets the constant magnetic field strength of the magnetic field within the commutation interval, the control unit arranges a first interval having a first starting point in time and a first ending point in time and a second interval having a second starting point in time and a second ending point in time within the commutation interval, and the control unit defines a first setpoint current curve for the first interval, wherein the first setpoint current curve differs by a difference current curve from the constant setpoint current such that the difference current curve with reference to the constant setpoint current effectuates a higher rate of change of the magnetic field strength towards the constant magnetic field strength, the control unit defines a second setpoint current curve for the second interval in that the constant setpoint current is assigned to the second setpoint current curve, and the control unit is fed the first setpoint current curve in the first interval and the second setpoint current curve in the second interval.

14. The magnetic-inductive flowmeter according to claim 13, wherein the electromagnet comprises a commutation unit which commutates the coil current.

15. A magnetic-inductive flowmeter comprising a control unit configured to perform the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an embodiment of a magnetic-inductive flowmeter in accordance with aspects of the present disclosure;

(2) FIG. 2a shows a schematic representation of a setpoint current curve over time in accordance with aspects of the present disclosure;

(3) FIG. 2b shows a schematic representation of a measured commutated coil current over time in accordance with aspects of the present disclosure;

(4) FIG. 2c shows a schematic representation of a measured coil voltage over time in accordance with aspects of the present disclosure; and

(5) FIG. 2d shows a schematic representation of a magnetic field strength of a magnetic field over time in accordance with aspects of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

(6) FIG. 1 shows the magnetic-inductive flowmeter 1 in operation. The magnetic-inductive flowmeter 1 has the measuring tube 2, the electromagnet 3 for generating the magnetic field 4 and the control unit 5. The measuring tube 2 is shown as a cross section. The electromagnet 3 has, in turn, the current controller 6, the coil arrangement 7 and the commutation unit 8.

(7) In this embodiment, the current controller 6 is an analog current controller and has a single controlled current source, which generates the coil current i.sub.s and a control loop. The function of the control loop is described using a setpoint variable, a process variable and a control variable. The control variable controls the controlled current source, with the process variable being the coil current i.sub.s generated from the controlled current source based on the control variable. Additionally, the setpoint variable is the setpoint current fed to the current controller 6, as shown in FIG. 2a. The setpoint current is fed to the current controller 6 by the control unit 5. The control unit 5 generates the signal describing the setpoint current with a microcontroller and a digital-analog converter, and then transmits the signal to the current controller 6 via the first signal path 9.

(8) The current controller 6 evaluates a difference between the setpoint current and the generated coil current i.sub.s as a control deviation. Further, the current controller 6 minimizes the control deviation, whereby the process variable follows the setpoint variable. The tracking of the setpoint variable can thereby be carried out at most with a maximum rate of change of the process variable intrinsic for the current controller 6. A maximum rate of change of the process variable intrinsic for the current controller 6 is based on its control mode. In order to minimize the control deviation in this embodiment, the control loop exhibits proportional as well as integral and derivative control modes. Thus, the current controller 6 is a proportional-integral-derivative (PID) current controller. When the setpoint variable is greater than a setpoint variable that is needed for achieving the slew rate of the process variable, the controller is considered overdriven.

(9) The commutation unit 8 in this embodiment has an H bridge circuit, in which four switches are arranged and controlled such that the coil current i.sub.s is commutated at the commutation interval t.sub.K so that the commutated coil current i.sub.S,k results, as shown in FIG. 2b. The commutated coil current i.sub.S,k differs from the coil current i.sub.s only in that the commutated coil current i.sub.S,k reverses its direction at the commutation interval t.sub.K. Additionally, the commutated coil current i.sub.S,k effectuates the coil voltage u.sub.s, as shown in FIG. 2c, and the magnetic field strength, as shown in FIG. 2d. The coil voltage u.sub.s is represented without a voltage portion caused by an ohmic resistance of the coil 10. In an alternative embodiment, the sole current source directly generates the commutated coil current i.sub.S,k.

(10) The coil arrangement 7 has the coil 10 and the yoke 11. The coil 10 is wound around the yoke 11 in one section of the yoke 11. The yoke 11 consists of a material having a low magnetic resistance, so that the magnetic field 4 generated by the commutated coil current i.sub.S,k in the coil 10 preferably propagates in the yoke 11. The yoke 11 is designed such that it forms a magnetic loop with an air gap, with a measuring tube 2 arranged in the air gap. Only the magnetic field 4 in the air gap is schematically represented in FIG. 1. The magnetic field 4 at least partially interfuses the measuring tube 2 and the medium 12 flowing through the measuring tube 2. Thereby, the direction of the magnetic field 4 is perpendicular to a direction of flow of the medium 12, whereby a potential gradient is induced in the medium 12. The direction of the potential gradient is aligned both perpendicular to the direction of the magnetic field 4 as well as perpendicular to the direction of flow of the medium 12.

(11) The wall of the measuring tube 2 consists of a material that is transparent for magnetic fields, i.e., does not influence the magnetic field 4. Two recesses are located across from one another in the wall of the measuring tube 2, with a first measuring electrode 13 arranged in one recess and a second measuring electrode 14 arranged in the other recess. The first measuring electrode 13 and the second measuring electrode 14 are galvanically in contact with the medium 12. The induction voltage u.sub.I caused by the potential gradient in the medium 12 is tapped between the first measuring electrode 13 and the second measuring electrode 14. The control unit 5 measures the induction voltage u.sub.I over the second signal path 15 and third signal path 16.

(12) In addition to the induction voltage u.sub.I, the control unit 5 also measures the commutated coil current i.sub.S,k over the fourth signal path 17. Further, the control unit 5 also measures the coil voltage u.sub.s over the fifth signal path 18 and the sixth signal path 19.

(13) When the control unit presets a constant setpoint current i.sub.Soll,konst of the current controller 6 via the first signal path 9, the electromagnet 3 generates the constant magnetic field strength H.sub.konst in a steady state. The settling of the magnetic field 4 means that at a point in time starting with the constant setpoint current i.sub.Soll,konst flowing through the coil 10, the constant magnetic field strength H.sub.konst is not yet present in the medium 12. The settling of the magnetic field 4 is essentially caused by material characteristics of the yoke 11. It has been recognized that these material characteristics are similar to the material characteristics that can cause hysteresis in ferromagnetic materials.

(14) In this embodiment, the control unit 5 sets the constant magnetic field strength H.sub.konst within the first commutation interval t.sub.K,1 with a starting point in time t.sub.K,1,A and the ending point in time t.sub.K,1,E. The constant magnetic field strengthH.sub.konst is within the second commutation interval t.sub.K,2 with the starting point in time t.sub.K,2,A and the ending point in time t.sub.K,2,E by carrying out the method as described in the following paragraphs and in conjunction with FIGS. 2a to 2d.

(15) In FIGS. 2a to 2d, the first commutation interval t.sub.K,1 and the second commutation interval t.sub.K,2 are completely shown, where the ending point in time t.sub.K,1,E of the first commutation interval t.sub.K,1 coincides with the starting point in time t.sub.K,2,A of the second commutation interval t.sub.K,2. The commutation intervals differ from one another in that the coil current i.sub.s flows in one direction through the coil 10 during the first commutation interval t.sub.K,1 while the coil current i.sub.s flows in the opposite direction through the coil 10 during the second commutation interval t.sub.K,2. The commutation of the coil current i.sub.s is carried out by the commutation unit 8. In the following explanation, reference is only made to the first commutation interval t.sub.K,1. The explanations for the first commutation interval t.sub.K,1 are also valid for the second commutation interval t.sub.K,2 and are also valid for the commutation intervals preceding the first commutation interval t.sub.K,1 and the commutation intervals following the second commutation interval t.sub.K,2.

(16) In a first step of a method, the control unit 5 arranges the first interval t.sub.1 with the starting point in time t.sub.1,A and the ending point in time t.sub.1,E. Further, the control unit 5 arranges the second interval t.sub.2 with the starting point in time t.sub.2,A and the ending point in time t.sub.2,E within the first commutation interval t.sub.K,1. In this embodiment, the starting point in time t.sub.1,A of the first interval t.sub.1 coincides with the starting point in time t.sub.K,1,A of the first commutation interval t.sub.K,1. Additionally, the ending point in time t.sub.1,E of the first interval t.sub.1 coincides with the starting point in time t.sub.2,A of the second interval t.sub.2 and the ending point in time t.sub.2,E of the second interval t.sub.2 coincides with ending point in time t.sub.K,1,E of the first commutation interval t.sub.K,1.

(17) In the second step of the method, the control unit 5 defines a first setpoint current curve i.sub.Soll,1 for the first interval t.sub.1, where the first setpoint current curve i.sub.Soll,1 differs by a difference current curve i.sub.Soll from the constant setpoint current i.sub.Soll,konst. The difference is such that the difference current curve i.sub.Soll, with reference to the constant setpoint current i.sub.Soll,konst, effectuates a higher rate of change of the magnetic field strength up to the constant magnetic field strength H.sub.konst. The first setpoint current curve i.sub.Soll,1 in this embodiment is constant in the entire first interval t.sub.1. The magnitude of the first setpoint current curve i.sub.Soll,1 is greater than the magnitude of the constant setpoint current i.sub.Soll,konst by the magnitude of the difference current curve i.sub.Soll. Due to the greater magnitude of the first setpoint current curve i.sub.Soll,1, with reference to the magnitude of the constant setpoint current i.sub.Soll,konst, a higher rate of change of the magnetic field strength H up to the constant magnetic field strength H.sub.konst is effectuated. The duration of time from the starting point in time t.sub.1,K,A of the first commutation interval t.sub.K,1 up to the constant magnetic field strength H.sub.konst is called a settling time. Due to the higher rate of change of the magnetic field strength H, the settling time is shortened. Furthermore, the first setpoint current curve i.sub.Soll,1 at the beginning t.sub.1,A of the first interval t.sub.1 causes the current controller 6 to overdrive, whereby the settling time is further decreased.

(18) In the present embodiment, the control unit 5 defines the ending point in time t.sub.1,E of the first interval t.sub.1. This is accomplished in that the control unit 5 measures the commutated coil current i.sub.S,k over a fourth signal path 17, then determines the point in time in which the measured commutated coil current i.sub.S,k reaches the constant setpoint current i.sub.Soll,konst, and then adopts this point in time as ending point in time t.sub.1,E of the first interval t.sub.1. In that, on the one hand, the starting point in time t.sub.1,A of the first interval t.sub.1 coincides with the starting point in time t.sub.K,1,A of the first commutation interval t.sub.K,1 and the ending point in time t.sub.2,E of the second interval t.sub.2 coincides with ending point in time t.sub.K,1,E of the first commutation interval t.sub.K,1. On the other hand, the ending point in time t.sub.1,E of the first interval t.sub.1 coincides with the starting point in time t.sub.2,A of the second interval t.sub.2, in which the commutated coil current i.sub.S,k has reached the constant setpoint current i.sub.Soll,konst, both the first interval t.sub.1 and the second interval t.sub.2 are completely defined.

(19) In a third step of the method, the control unit 5 defines a second setpoint current curve i.sub.Soll,2 for the second interval t.sub.2, in that the constant setpoint current i.sub.Soll,konst is assigned to the second setpoint current curve i.sub.Soll,2.

(20) In a fourth step of the method, the current controller 6 feeds the control unit 5 with the first setpoint current curve i.sub.Soll,1 in the first interval t.sub.1 and the second setpoint current curve i.sub.Soll,2 in the second interval t.sub.2.

(21) Additionally, in this embodiment, the control unit 5 carries out a method for adaptive adjustment of the magnitude of the first setpoint current curve i.sub.Soll,1. The method is comprised of determining an evaluation quantity in at least the first commutation interval t.sub.K,1 and the second commutation interval t.sub.K,2. The determination of the evaluation quantity is the same for all commutation intervals, and therefore is only described for the first commutation interval in the following paragraphs.

(22) First, the control unit 5 measures the coil voltage u.sub.S, illustrated in FIG. 2c. The coil voltage u.sub.S is caused in the coil 10 by the commutated coil current i.sub.S,k, which is illustrated in FIG. 2b, over the fifth signal path 18 and the sixth signal path 19 in the third interval t.sub.3. The third interval t.sub.3 is arranged within the second interval t.sub.2 as an indicator quantity. The adoption of the coil voltage u.sub.S is based on the finding that, when the commutated coil current i.sub.S,k as well as the magnetic field strength H, illustrated in FIG. 2d, are constant, then the coil voltage u.sub.S is also constant. However, when the commutated coil current i.sub.S,k is constant, but the magnetic field strength H is not constant, the coil voltage u.sub.S is also not constant. In this situation, the temporal course of the coil voltage u.sub.S contains information about the temporal course of the magnetic field strength H. The control unit 5 then forms the evaluation quantity from the indicator quantity in that, in this embodiment, one thousand measured values of the indicator quantity are measured. The arithmetic mean value over the thousand measured values is formed and the mean value is then subtracted from the first measured value.

(23) The control unit 5 then determines at least one trend quantity from the evaluation quantity of at least the first commutation interval t.sub.K,1 and the second commutation interval t.sub.K2, in that a change of the evaluation quantity is determined over the commutation intervals. Using the trend quantity, the magnitude of the first setpoint current curve i.sub.Soll,1 is adaptively adjusted.