Method for Controlling the Delivery Pressure of a Supply Pump

Abstract

Methods for controlling supply pump delivery pressure in a system having a supply pump, a metering pump, a delivery line extending between them where the supply pump delivers material through the delivery line to a metering pump inlet; and a pretensioned pressure buffer of variable buffer volume is connected upstream of the metering pump inlet, the buffer volume capable of receiving material from the delivery line when delivery line pressure near the pressure buffer is >pretensioning pressure P of the pressure buffer and capable of dispensing material to the delivery line when delivery line pressure near the pressure buffer is <the pretensioning pressure P; the system may also include a pressure sensor and a control unit, connected to the sensor, supply pump, and metering pump, which transmits a value to the supply pump based on metering pump status and delivery line actual pressure deviation from target pressure.

Claims

1. A method for controlling delivery pressure of a supply pump in a system for delivering and metering a viscous material, wherein the system has a supply pump, a metering pump, and a delivery line which extends between the supply pump and the metering pump, and wherein the supply pump delivers the viscous material from a material store to an inlet of the metering pump through the delivery line using a delivery pressure D, wherein, furthermore, a pressure buffer which is pretensioned to a pretensioning pressure P and has a variable buffer volume is connected upstream of the inlet of the metering pump, wherein the buffer volume can receive viscous material from the delivery line when the delivery pressure in the delivery line in the region of the pressure buffer is higher than the pretensioning pressure P of the pressure buffer, and viscous material can be dispensed into the delivery line from the buffer volume when the delivery pressure in the delivery line in the region of the pressure buffer is lower than the pretensioning pressure P of the pressure buffer, wherein the system additionally has a pressure sensor, connected upstream of the inlet of the metering pump for detecting an actual pressure value in the delivery line, and a control unit, wherein the control unit is connected to the pressure sensor, the supply pump, and the metering pump and receives a signal from one or more of them, wherein the control unit transmits a specification value to the supply pump based on a deviation between a specified target pressure value and the actual pressure value in the delivery line as well as operating state of the metering pump.

2. The method according to claim 1, wherein the specification value transmitted from the control unit to the supply pump is provided with a correction value, depending upon the operating state of the metering pump.

3. The method according to claim 1, wherein the pretensioning pressure P of the pressure buffer is applied by at least one spring designed as a wave spring.

4. The method according to claim 1, wherein the pressure buffer comprises a rolling membrane for hermetically separating the variable buffer volume.

5. The method according to claim 1, wherein the variable buffer volume comprises a volume of 500 to 5,000 mm.sup.3.

6. The method according to claim 1, wherein, in addition to the signal from the pressure sensor and the signal from the metering pump regarding its operating state, information regarding the fill level in the buffer volume is also included in the control.

7. The method according to claim 6, wherein the pressure buffer comprises a plunger which protrudes from a housing of the pressure buffer and via which a fill level of the buffer volume can be determined.

8. The method according to claim 1, wherein the pressure sensor and/or a pressure-safeguarding element is accommodated in a housing of the pressure buffer.

9. The method according to claim 8, wherein a time-dependent viscosity profile of the viscous material is stored in the control unit, and the time-dependent change in the viscosity of the viscous material is taken into account when determining the specification value transmitted to the supply pump.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1 shows a system for delivering and metering a viscous material as used in the method according to the invention, in a schematic representation;

[0034] FIG. 2 shows an exemplary embodiment of a pressure buffer which is used in the method according to the invention, in a sectional view.

DETAILED DESCRIPTION

[0035] FIG. 1 shows a schematic representation of a system, designated as a whole by 1, for delivering and metering a viscous materialfor example, a plastic component for a sealing material. The system 1 comprises a supply pump 2, designed as a pneumatic scoop piston pump, which pumps the viscous material from a material supply 5 into a delivery line 4. Due to the delivery pressure built up by the supply pump 2, the viscous material is delivered through the delivery line 4 to the inlet 6 of a metering pump 3. The viscous material is discharged to a mixing head 10 via the metering pump 3. In the mixing head 10, the viscous material is mixed with one or more other components, not specified here, before the mixture, indicated by the arrow 11, is discharged from the mixing head 10for example, in the form of a sealing bead. As an alternative to the mixing head 10 shown here, a component valve (not shown) can also be used if the viscous material is not to be mixed with another component, but instead discharged directly.

[0036] The supply pump 2 and the metering pump 3 are controlled via a control unit 9 which includes a PID controller. Since the supply pump 2 is a pneumatic piston pump, the system 1 has a proportional pressure control valve 12, arranged between the control unit 9 and the supply pump 2, which converts an electrical signal from the control unit 9 into a pneumatic pressure that acts upon the supply pump 2.

[0037] To detect an actual pressure value in the delivery line 4, the system 1 comprises a pressure sensor 8 which is accommodated in a housing 13 of a pressure buffer 7 and will be described in detail later. The pressure sensor 8 is connected to an input of the control unit 9 via a signal line 14. A given specified target pressure value is stored in the control unit 9, with which the actual pressure value is compared. Depending upon the result of this comparison, a corresponding specification value is transmitted to the supply pump 2 or to the proportional pressure control valve 12 via a signal line 15.

[0038] The metering pump 3 is controlled by the control unit 9 via a further signal line 16. In addition to the signal from the pressure sensor, information regarding the operating state of the metering pump 3 is also taken into account by the control unit 9 when determining the specification value for the supply pump 2. In other words, in addition to the pressure value detected by the pressure sensor 8, the control unit 9 also processes information regarding a start or stop of the metering of the metering pump and takes this information into account when determining the specification value transmitted to the supply pump 2. In this way, it is possible to react very quickly to changes in the operating state of the metering pump 3, and a strong increase or decrease in the pressure at the inlet 6 of the metering pump 3 can be prevented. In practice, the operating state of the metering pump 3 is taken into account when regulating the delivery pressure of the supply pump 2 in such a way that the specification value transmitted from the control unit 9 to the supply pump 2 or the proportional pressure control valve 12 is corrected, depending upon the operating state of the metering pump 3. As soon as the metering is stopped, the value of the I component of the PID control is multiplied by a factor of 0.5, compared to the last value of the I component during the previous metering process. As a result, the specification value transmitted to supply pump 2 suddenly drops to a significantly lower value when metering is stopped. As soon as metering starts again, the value of the I component is reset to the last value of the I component during the previous metering process. As a result, the specification value transmitted to the supply pump 2 again increases suddenly, and a strong pressure drop on the inlet side of the metering pump 3 when the metering starts again can be prevented.

[0039] In order to influence the pressure in the delivery line 4, in particular at the inlet 6 of the metering pump 3, the system 1 additionally has a pressure buffer 7 connected upstream of the metering pump 3. The pressure buffer 7 comprises a variable buffer volume which can absorb viscous material from the delivery line 4, or from which viscous material can be discharged into the delivery line 4.

[0040] The structure and functioning of the pressure buffer 7 will be explained in more detail with reference to FIG. 2. The pressure buffer 7 is connected to the delivery line 4, through which viscous material is delivered in the direction indicated by the arrows from the supply pump 2 to the metering pump 3. The pressure buffer 7 comprises a housing 13 and a plunger 17 which is movable in plain bushings 23 in a direction indicated by the arrow S. The plunger 17 protrudes at least partially from the housing 13. At the end, facing the delivery line 4, of the plunger 17, a rolling diaphragm 18 is arranged, by which a variable buffer volume 19 acting as a compensation volume is limited. The buffer volume 19 is hermetically sealed adjacent to the plunger 17 by the rolling diaphragm 18, while it is open adjacent to the delivery line 4 and is in contact therewith. The rolling membrane 18 enables the pressure buffer 7 to operate practically friction-free.

[0041] The pressure buffer 7 is pretensioned to a pretensioning pressure P which depends upon the fill level of the buffer volume 19. The pretensioning pressure P is applied by a steel spring, designed as a wave spring 20, which is arranged between a shoulder 21 of the plunger 17 and the inner wall of the housing 13. As soon as the delivery pressure in the delivery line 4 in the region of the pressure buffer 7 and upstream of the metering pump 3 is higher than the pretensioning pressure P of the pressure buffer 7, viscous material from the delivery line 4 enters the buffer volume 19 and presses on the plunger 17 via the rolling diaphragm 18, such that the latter moves upwards away from the delivery line 4, and the wave springs 20 are compressed. The variable buffer volume 19 accordingly increases, and the pretensioning pressure P of the pressure buffer 7 increases. This process continues until a pressure equilibrium is established or until the plunger 17 strikes the housing 13 with its shoulder 22, as FIG. 2 shows. If the delivery pressure in the delivery line 4 drops again below the now prevailing pretensioning pressure P of the pressure buffer, the restoring force of the wave springs 20 causes a movement of the plunger 17 in the opposite direction towards the delivery line 4, whereby the buffer volume 19 is reduced, and viscous material is discharged from the buffer volume 19 into the delivery line 4 to supply the metering pump 3. In this way, the pressure buffer 7 can mitigate pressure fluctuations in the delivery line 7 and thereby contribute to a more constant pressure on the inlet side of the metering pump 3. By using wave springs 20, the pressure buffer 7 has an almost linear pressure-volume behavior. In order to prevent overloading the pressure buffer 7, the variable buffer volume 19 can expand in a pressure range of approximately 2 to 20 bar. If these values are exceeded or fallen below, the movement of the plunger 17 is limited by a mechanical stop.

[0042] By providing a variable buffer volume on the one hand, and taking into account the operating state of the metering pump 3 on the other, the method according to the invention makes it possible to regulate the delivery pressure of the supply pump 2 such that a substantially constant supply pressure can be provided on the inlet side of the metering pump 3, regardless of any downtimes of the metering pump 3.

[0043] In a variant of the method not shown in the figures, additional information regarding the fill level of the buffer volume 19 can be included in the control. For this purpose, the fill level of the buffer volume 19 can be determined indirectly based upon the length of the portion of the plunger 17 protruding from the housing 13 of the pressure buffer 7. For this purpose, a preferably optical sensor can be provided which detects the position of the plunger 17 and sends a corresponding signal to the control unit 9 for further processing.