Metering pump and method for controlling a metering pump

Abstract

A Metering pump includes a displacement element (4), a drive system with an electric drive motor (12) driving the displacement element (4) and a control device (22) controlling the electric drive motor (12). The control device (22) is configured in such a manner that it detects the current position of the displacement element (4), detects the torque (M) of the electric drive motor (12) at several positions of the displacement element (4) and monitors the torque (M) in relation to the position of the displacement element (4), and a method for controlling such metering pump.

Claims

1. A metering pump comprising: a displacement element; a drive system comprising an electric drive motor driving said displacement element and a control device controlling said electric drive motor, wherein said control device is configured to: detect a current position of the displacement element; detect a torque of the electric drive motor or to detect a drive force acting on the displacement element at several positions of the displacement element; monitor the torque or force in relation to the position of the displacement element; and derive the current pressure, acting on the displacement element, from the detected current torque of the drive motor or detected drive force, wherein the control device is configured such that the forces resulting from a deformation of the displacement element, inertial forces acting on the drive and/or forces resulting from a deformation of at least one spring element in the drive are represented by predefined values which are stored in the control device.

2. A metering pump according to claim 1, wherein said displacement element is a membrane or a piston.

3. A metering pump according to claim 1, wherein said drive system further comprises an eccentric drive coupled to the displacement element and driven by the electric drive motor.

4. A metering pump according to claim 1, wherein the electric drive motor is a brushless DC motor or a stepping motor.

5. A metering pump according to claim 1, wherein at least one position sensor detects the position of the displacement element and is connected to said control device.

6. A metering pump according to claim 1, wherein at least one sensor detects a rotational angle of the electric drive motor.

7. A metering pump according to claim 1, wherein the control device is configured to detect the torque or the force along an entire travel of the displacement element.

8. A metering pump according to claim 1, wherein the control device is provided with a log module logging the torque or force or a value derived from the torque or the force over a travel of the displacement element and with an analyzing module analyzing a logged pressure for detecting at least one abnormal condition of the metering pump.

9. A metering pump according to claim 8, wherein the control device comprises a flow detection module detecting an effective stroke length of the displacement element from the logged pressure and calculating the actual flow on basis of the effective stroke length.

10. A metering pump according to claim 1, wherein the control device is configured to derive a current pressure acting on the displacement element from the detected current torque of the drive motor in consideration of friction of the drive, forces resulting from a deformation of the displacement element, inertial forces acting on the drive and/or forces resulting from a deformation of at least one spring element in the drive.

11. A metering pump according to claim 1, wherein the control device is configured such that the control device detects a current friction torque of the entire drive by measuring the torque of the electric drive motor when the displacement element is in a dead-center position.

12. A method for controlling a metering pump comprising a displacement element and a drive system comprising an electric drive motor driving said displacement element and a control device controlling said electric drive motor, wherein said control device is configured to detect a current position of the displacement element, to detect a torque of the electric drive motor or to detect a drive force acting on the displacement element at several positions of the displacement element and to monitor the torque or force in relation to the position of the displacement element, the method comprising the steps of: detecting a current position of the displacement element; detecting a torque of the electric drive motor or a drive force acting on the displacement element at several positions of the displacement element; and monitoring the torque in relation to the position of the displacement element, wherein a current pressure acting on the displacement element is calculated based on the detected torque or the force for at least several points along the travel of the displacement element, wherein a current friction torque is measured when the displacement element is in a dead-center position as a basis for the calculation of the pressure.

13. A method according to claim 12, wherein the detection of position and torque as well as the monitoring of the torque in relation to the position are carried out along an entire travel of the displacement element.

14. A method according to claim 12, wherein the friction torque is monitored during operation of the metering pump for detecting malfunctions based on a detected change of the friction torque.

15. A metering pump comprising: a displacement element; a drive system comprising an electric drive motor driving said displacement element and a control device controlling said electric drive motor, wherein said control device is configured to: detect a current position of the displacement element; detect a torque of the electric drive motor or to detect a drive force acting on the displacement element at several positions of the displacement element; and monitor the torque or force in relation to the position of the displacement element, wherein the control device is configured such that the control device detects a current friction torque of the entire drive by measuring the torque of the electric drive motor when the displacement element is in a dead-center position.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following preferred embodiments of the invention are described with reference to the attached figures. In the figures:

(2) FIG. 1 is a schematic view of a metering pump according to the invention;

(3) FIG. 2 is a schematic view of metering pump according FIG. 1 with the membrane in a first dead-center position being the advanced position of the membrane;

(4) FIG. 3 is a schematic view of a metering pump according FIGS. 1 and 2 with the membrane in the second dead-center position being the retracted position of the membrane; and

(5) FIG. 4 is a graph as an example for an indicator diagram.

DESCRIPTION OF PREFERRED EMBODIMENTS

(6) In the following the metering pump according to the invention and the method according to the invention are described using an example of a diaphragm or membrane pump, respectively. It has to be understood that the invention can be carried out in the same manner with other types metering pumps, for example a metering pump using a piston instead of a membrane. Also a combination of diaphragm or membrane pump, respectively, with a piston pump may be used, for example a piston-diaphragm pump having a hydraulic coupling between a membrane forming a wall of a metering chamber and a piston for compressing a hydraulic fluid for moving the diaphragm.

(7) The membrane pump schematically shown in FIG. 1 has a metering chamber 2 a sidewall of which is formed by a membrane 4. At the lower side of the metering chamber 2 there is arranged a suction valve 6 whereas on the upper side there is arranged a pressure valve 8. During operation liquid is sucked through the suction valve 6 into the metering chamber 2 and pushed out of the metering chamber 2 through the pressure valve 8. The membrane 4 can be moved in an oscillating manner periodically increasing and decreasing the volume of the metering chamber 2. For this the membrane 4 is connected to a piston or connection rod 10, respectively. By movement of the connection rod 10 the membrane 4 is moved forward and backward between an advanced and a retracted position as indicated by the arrows S.sub.1 and S.sub.2 in FIG. 1.

(8) The connection rod 10 is part of a drive system having an eccentric drive 14. The drive system comprises an electric drive motor 12 which in this example is coupled to the eccentric drive 14 via a gear drive 16. Although in this example a gear drive 16 is shown it has to be understood that according to different embodiments it would be possible to directly connect the drive motor 12 with an eccentric drive 14. The eccentric drive 14 contains an eccentricity e. This means the connection rod 10 is pivotally connected to the eccentric drive 14 at a connection point 18 which is distanced from the rotational axis x by the eccentricity e. This causes a linear movement of the connection rod 10 in the direction S if the eccentric drive 14 is rotated in the rotational direction R. In this example furthermore a spring 20 is arranged in the drive. The spring 20 is a compression spring connected to the connecting rod 10 such that the spring 20 is compressed when the connecting rod 10 is moved backwards in direction S.sub.1 moving the membrane 4 in the retracted position. By this the spring 20 can accumulate energy during the suction stroke. This energy is released during the pressure stroke 20 when the connecting rod together with the membrane 4 is moved in the forward, i. e. advanced position in the direction S.sub.2. By this the spring 20 soothes the torque to be applied by the electric drive motor 12 during the entire stroke. It has to be understood that it is also possible to arrange a spring being compressed during the pressure stroke and acting as a return spring. Furthermore, the invention may also be realized with a drive without a spring.

(9) The electric drive motor 12 is controlled by a control device 22. The control device 22 in particular controls the speed of the drive motor 12 to control the flow rate of the metering pump, i. e. the amount of liquid is pumped by the membrane 4 through the metering chamber 2 per unit of time.

(10) According to the invention the control device 22 monitors the position of the membrane 4 as well as the torque to be applied by the drive motor 12. For this the control unit 22 contains a torque detection module 24. The torque detection module 24 may be configured for example such that it derives the torque acting on the drive motor 12 from the motor current applied to the electric drive motor 12. The drive motor 12 in this example preferably is a brushless DC motor. However, in case that a stepping motor should be used it would for example be possible to derive the motor torque on basis of a measured deviation between a desired rotor angle and a current rotor angle measured. For this a sensor or encoder 26 may be attached to or implemented into the electric drive motor 12 to detect the angular position of the rotor of the drive motor 12. The encoder 26 has a signal connection with the control device 22 such that the sensor signals from the encoder 26 are forwarded to the control device 22. Furthermore, in an alternative embodiment, it would also be possible to detect the torque of a stepping motor without use of a sensor, for example as described in DE 10 2011 000 569 A1.

(11) Instead of detecting the motor torque, it would be possible to directly measure the drive force acting on the displacement element 4, for example by a force sensor 23, as indicated in FIG. 1. It has to be understood that the use of a force sensor 23 which directly detects the drive force acting in opposite direction to the force F shown in FIG. 1 would be an alternative solution to the detection of the motor torque. In the following, preferred embodiments or options of the invention are described with reference to the detection of the motor torque. However, they may be realized in a similar manner on basis of the direct detection of the drive force.

(12) Furthermore, on basis of the signal of encoder 26, the control device 22 can detect the current position of the membrane 4 between the advanced position shown in FIG. 2 and the retracted position shown in FIG. 3. This is possible because of the fixed mechanical coupling between membrane 4 and electric drive motor 12 via gear drive 16 and eccentric drive 14. It has to be understood that for detecting the membrane position further or different sensors may be used which signals are received by the control device 22.

(13) The encoder 26 may be an absolute encoder detecting the absolute rotational angle φ. However, it would also be possible to use a relative encoder or transducer detecting the rotational angle or actual position of the membrane 4 along the axis of movement S relatively starting from a reference position detected by reference sensor in the system.

(14) Based on the position signal representing the current position of the membrane 4 and the torque derived from the torque detection module 24 an indicator diagram as shown in FIG. 4 is created by a log module 28 of the control device 22. In such indicator diagram the detected torque or a pressure p acting inside the metering chamber 2 is plotted over the detected position of the membrane 4 forming a displacement element, i. e. over the stroke length. To detect the pressure p the control device 22 may contain a pressure detection module 30 calculating the pressure on basis of the detected torque. On basis of the detected torque the force acting on membrane 4 can be calculated. Then, with knowledge of the size of membrane 4 pressure p acting on the membrane 4 inside the metering chamber can be derived. It has to be understood that the described modules of the control device 22 preferably are provided as software modules. The modules may be implemented into a control device 22 arranged directly on the drive motor 22 for example inside an electronic housing of the drive motor 12. However, it would also be possible to arrange at least parts of the control device 22, e. g. at least one or more of the modules separately to the metering pump and to connect these modules with the control device of the metering pump via a network connection, like the internet. Thus, parts of the control device or modules may be realized by cloud-computing, i.e. in a centralized computing system connected to the metering pump via the internet. In particular the log module 28 may be arranged in a centralized computing system. Furthermore an analyzing module 32 is provided in the control device 22 for analyzing the curves or indicator diagram created by the log module 28. Also this analyzing module 32 may either be arranged in a control device 22 directly integrated into the metering pump, i. e. arranged in an electronic housing of the metering pump, or arranged distanced, preferably in a centralized computer system.

(15) The pressure inside the metering chamber 2 may be calculated by the pressure detection module 30 on basis of the pressure effective motor torque M.sub.pressure provided by the electric drive motor 12 and acting on the eccentric drive 14. Depending on the rotational angle φ (see FIGS. 2 and 3) the eccentricity e provides a lever I between the rotational axis x and the connection point 18 of the connection rod 10. The lever I is responsible for the force F acting on the membrane 4. This force F divided by the size of the membrane 4, i. e. the effective surface A.sub.effective is the resulting pressure p inside the metering chamber 2. The effective surface A.sub.effective influencing the force F acting on the membrane 4 and the connection rod 10 is the area of the membrane surface 4 in a plane perpendicular to the longitudinal axis of the connection rod 10. Thus the pressure can be detected on basis of electrical parameters of the drive motor 12 without the necessity to provide a pressure sensor in the fluid system.

(16) To calculate the pressure effective torque M.sub.pressure the torque in particular components resulting from friction, inertial forces, elasticity of the membrane 4 and the spring 20 should be evaluated and eliminated in the pressure calculation by the torque detection module 24. The inertial forces as well as a spring force provided by the spring 20 and the forces resulting from deformation and elasticity of membrane 4 can be calculated and are preferably stored inside the control module 22 in a table in dependency of the rotational angle φ which is detected by the encoder 26. The detection of the membrane position or stroke position may also be carried out without the encoder 26. For example, an internal sensor of a motor like a brushless DC motor, for example a hall sensor inside the motor, can be used to count the number of rotations carried out, in particular starting from a reference position, which may be detected by a further sensor.

(17) According to the invention the torque component resulting from the friction in the drive system, i. e. the friction torque M.sub.friction is not regarded as being constant but measured in the system. The friction torque M.sub.friction can be detected by the torque detection module 24 close to the dead-center position of the membrane 4 as shown in FIGS. 2 and 3. In FIGS. 2 and 3 the gear system 16 and the drive motor 12 as well as the control device 22 are not shown for simplification. It can be seen that in the dead-center positions and close to the dead-center positions as shown in FIGS. 2 and 3, respectively, the level I is zero. FIG. 2 shows the advanced membrane position for a rotational angle φ 180°, whereas FIG. 3 shows the retracted membrane positon for a rotational angle φ=0°. Since the lever I is zero the pressure p inside the metering chamber 2 and the resulting force F acting on the membrane 4 cannot provide any torque about the rotational axis x anymore. Also the torque provided by the spring force resulting from the spring 20 and the force resulting from the deformation of membrane 4 are depending on the lever I such that in the dead-center positions the torque components M.sub.membrane and M.sub.spring resulting from these forces are also approximately zero. The torque component M.sub.acceleration resulting from the inertial forces at the dead center positions or close to the dead center positions may also be approximately zero. However, even if this torque component does not become zero at the dead center position, it can be eliminated, since it can be calculated in advance and the torque component M.sub.acceleration as calculated may be subtracted from the measured torque at the dead center position, so that the influence of this torque component can be eliminated. This means the only remaining forces in the system resulting in a torque acting on the drive motor 12 are the friction forces. This means that the torque detected by the torque detection module 24 when the membrane 4 is in or close to one of the two dead-center positions corresponds to the friction torque M.sub.friction resulting from the friction in the drive system. Thus it is possible to measure the actual friction torque M.sub.friction in the system which allows a more precise calculation of the pressure relevant torque M.sub.pressure on basis of which the pressure p inside the metering chamber 2 may be derived or calculated.

(18) It is preferred that control device 22 continuously monitors the pressure relevant torque M.sub.pressure and the derived pressure p in relation to the membrane position 4. On basis of this, as described above an indicator-diagram showing the pressure p over the membrane positon can be created (FIG. 4). The analyzing module 32 is configured for analyzing these indicator-diagrams during the entire operation of the metering pump. In particular the analyzing module 32 configured for detecting changes in the pressure curve over time allows to detect different malfunctions or certain operational conditions of the metering pump. According to the invention this can be carried out without the need of a pressure sensor detecting the actual fluid pressure. Instead the fluid pressure can be derived from the motor torque.

(19) FIG. 4 shows an example for an indicator diagram showing a plot of pressure p over the stroke length of a pressure stroke of the membrane 4, i. e. a stroke moving the membrane 4 towards its advanced position decreasing the volume of the metering chamber 2. Instead of plotting the pressure over the stroke length it would also be possible to directly plot the pressure effective torque M.sub.pressure over the stroke length. In FIG. 4 the dotted line shows the normal pressure curve for normal operation of the metering pump without any disturbance. On the other hand the continuous line shows a pressure curve resulting when cavitation occurs inside the pressure chamber. Thus by comparing different torque or pressure curves over the stroke length it is possible to detect certain malfunctions like cavitation of the metering pump. This analyze is carried out by the analyzing module 32 by either directly comparing torque or pressure curves detected over time or by comparing a detected pressure or torque curve with a sample curve stored in a data base connected or integrated with the analyzing module 32.

(20) Furthermore, the analyzing module 32 is configured to detect characterizing points on the pressure curve of the indicated diagram as shown in FIG. 4 on the curve drawn in dotted line and showing a curve of a normal operation. There may be detected for example four characterizing points 1, 2, 3 and 4 referring to the opening and closing of the suction valve 6 and the pressure valve 8. At the point 1, the suction and the discharge valve are closed. At point 2, during the pressure stroke, the discharge valve, i.e. the pressure valve 8 is opened. At the end of the pressure stroke at point 3, the pressure valve 8 is closed. At this point, there begins the suction stroke. At point 4 during the suction stroke, the suction valve 6 is opened. At the end of the suction stroke at point 1, the suction valve is closed again. In particular, points 2 and 4 can be recognized on the pressure curve, since there the pressure curve makes a deflection, which can be detected by the analyzing module 32. The stroke length between points 2 and 3 corresponds to an effective hydraulic discharge stroke, whereas the stroke length between points 1 and 4 corresponds to an effective hydraulic suction stroke. On basis of these effective stroke lengths, it is possible to calculate the effective or actual stroke volume V.sub.H according to the following formula:
V.sub.H=S.sub.h*A.sub.effective,
wherein s.sub.h is the effective stroke length, i.e. the effective hydraulic discharge stroke or the effective hydraulic suction stroke, as described above and A.sub.effective is the effective membrane surface.

(21) On basis of the effective stroke volume V.sub.H, the effective flow may be calculated by multiplying the stroke volume V.sub.H by the frequency of the movement of the displacement element 4. This measurement or detection of the effective flow rate allows a feedback-control by adapting the speed of the drive motor 12 by the control device 22 to achieve a desired flow rate. Furthermore, malfunctions may be detected, if the desired flow rate cannot be achieved. In this case, the control device 22 may signalize an alarm.

LIST OF REFERENCE NUMERALS

(22) 2—metering chamber 4—membrane 6—suction valve 8—pressure valve 10—connection rod 12—drive motor 14—eccentric drive 16—gear drive 18—connection point 20—spring 22—control device 23—force sensor 24—torque detecting module 26—encoder 28—log module 30—pressure detection module 32—analyzing module S—arrows showing membrane movement, direction of membrane motion S.sub.1—backward movement S.sub.2—forward movement e—eccentricity x—rotational axis R—rotational direction p—pressure I—lever φ—rotational angle F—force A—area M—torque