EP0854756A1

EP0854756A1 - Liquid control for spray painting applications

Info

Publication number: EP0854756A1
Application number: EP96925948A
Authority: EP
Inventors: Alf Sunde
Original assignee: ABB Flexible Automation AS
Current assignee: ABB Flexible Automation AS
Priority date: 1995-10-20
Filing date: 1996-08-19
Publication date: 1998-07-29
Also published as: SE9503689D0; CA2235338A1; SE9503689L; SE514375C2; WO1997014505A1; AU6628896A

Abstract

Robotic system comprising a fluid supply system for spray painting and coating applications comprising fluid supply means for supply of fluid to spraying equipment (7) and flow control means for control of the volume flow rate of fluid supplied to the spraying equipment where said flow control means comprises a container (2) in the supply line to the spraying equipment, a pressure gas supply means (12) for pressurizing said container, and pressure control means (9, 10, 11) for control of the volume flow by varying the gas pressure.

Description

Liquid Control for Spxay. Painting Applications

TECHNICAL FIELD

The invention relates to a supply system for liquids and suspensions for spray painting and coating applications with dosing and regulation capabilities.

BACKGROUND ART, DISCUSSION OF THE PROBLEM

Spray painting and coating with water based suspensions of anorganic matter as enamel, glazing, metal flake based electrostatic shielding or fiuidized powders are normally performed by using a storage container, pressurized by air. The pressure is used for pumping the fluid to the spraying equipment, the use of pumps with moving parts being avoided due to the abrasive character of these fluids. During the spraying, the fluid level in the container is lowered gradually. A pressure regulator is compensating for this, maintaining constant working pressure.

In order to save fluid and reduce waste volume, it is advantageous to be able to vary the spray volume during the spraying process. A spraying process for an object normally lasts 20 - 60 seconds, and within this time, every painting stroke lasts for about 0.5 - 2 seconds. The fluid volume for each painting stroke is the product of its duration and the volume flow rate. Thus, in principle the fluid volume for each painting stroke can be regulated by both changing its duration and by varying the volume flow rate which depends on the pressure.

With fluid containers which for a normal spray painting application could be in the range of 100-200 1 volume, it is a problem to vary the pressure in the container in order to control the volume flow rate individually for each painting stroke due to the large volumes of air and fluid.

Different attempts have been made to solve this problem:

One solution is to quench the fluid supply by mechanical means with different types of regulators. As many of the fluids dealt with in this type of equipment are abrasive suspensions, this results in strong wear of components which thus have to be exchanged continuously.

Another solution is based on hose pumps in combination with compensators and long pipes aimed for damping and cancellation of the pressure pulsations originated by the hose pump. This solution has the disadvantages of being complicated and having long response time due to the length of the liquid path through the equipment. Also, the hose in the hose pump is worn by abrasive emulsions.

A further problem in spraying fluids as suspensions of solid matter is the supervising of their density. By separation, e g due to gravity, the solid content and thus the liquid density changes, as it does by inclusion of gas bubbles. In both cases, the painting results become unsatisfactory, and thus a fast and reliable detection of density variations is needed. Dynamic methods for density measuring of flowing liquid, e g based on the Coriolis force, have low accuracy and need calibration. Density variations sufficient to result in inferior painting quality can remain undetected by those methods.

SUMMARY OF THE INVENTION

The invention makes it possible both to control the volume flow rate flow individually for each painting stroke and to determine the fluid density accurately. The basic idea of the invention is to place a small container between the large container (or another type of fluid main supply) and the spraying equipment, the small container being sufficiently small to allow for fast individual pressure adjustments for every painting stroke.

This small container is equipped with an inlet valve, a pressure regulator, level control sensors and a weight sensor. Through the pressure regulator the small container is connected to a pressurized air supply.

The dosing of the fluid from the small container to the spaying equipment is performed by controlling the air pressure at the top of the container. The volume flow rate is a monotonously increasing function of that pressure. The actual volume flow rate, assuming a known and constant fluid density, calculated as proportional to the time rate of weight change of the container, measured by the weight sensor. In a closed loop control system this actual volume flow rate is compared with the wanted volume flow rate and continuously adjusted by performing the proper pressure changes. During filling of the container under simultaneous spraying, the flow rate is controlled only by pressure without feedback from the weight measurement. The fluid level in the small container is held between a top and a bottom level.

In addition to the described pressure adjustment it is also necessary to compensate for other contributions to the pressure. The air pressure at the top of the small container is corrected for the varying hydrostatic pressure contribution for different heights of the spraying equipment. This is of special importance for spraying suspension type fluids with low pressure, where the hydrostatic pressure can be a significant fraction of the total pressure, without correction resulting in large volume flow changes and poor painting quality. Also the compressibility of air is taken into account and corrected for.

Separation between the solid and the liquid constituents of the fluid or air entrainment into the fluid can be effectively controlled by the invention. By measuring the weight of the container at its lowest and its highest filling level, the fluid density can be calculated, as the volume difference between highest and lowest filling level is known and constant. If a deviation from the expected value occurs due to e g separation or air entrainment, an alert can be given in order to take proper action such as stopping of the painting process. This density measuring by checking the weight of a known fluid volume is more accurate and reliable than dynamic measurings based on liquid speed and/or acceleration, as e g Coriolis force based methods.

The pressure in the small container may be higher than the fluid supply pressure. In this case, the invention also works as a pressure amplifier, and for safety reason a check valve is installed in the supply line. Typically, the supply pressure is 2 - 4 bar whereas the pressure in the small container may be up to the maximum pressure of the pressurized air, often 10 bar.

To perform these control tasks properly, the small container needs some control volume resulting in typical sizes of one or a few litres.

The invention solves the task of fast and reliable control of the painting fluid to paint spaying equipment handled by robots, especially for fluids which as suspensions are difficult to dose by conventional control means. Thus parameters as speed, quality and degree of automation of painting and coating processes involving these fluids are improved by the invention. BRIEF DESCRIPTION OF THE DRAWING

The principles of operation of the invention are described with reference to Fig. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The fluid, usually a water based suspension or a fiuidized powder, is supplied from the fluid supply 1 which can be a large container pressurized by air or some kind of pump connected to a fluid storage or fluid preparation unit. It passes into a small container 2 with a volume of normally a few litres through the valve 3. The fluid level in the container is measured by the weight sensor 4 which controls the valve 3 maintaining the fluid level 5 between preset maximum and minimum values, sensed by the top level sensor 6a and the bottom level sensor 6b, respectively. By subtracting the output signalε from the weight sensor 4 for the container filled to the top level and the bottom level, respectively, the fluid density can be calculated. The volume flow rate to the paint spraying equipment 7 through the outlet 8 at the bottom of the container is determined by the pressure in the container. It is measured by the pressure sensor 9 which is connected to the control unit 10 which controls the regulator 11 on the pressurized air supply 12. The control unit calculates the actual volume flow rate from the weight change rate measured by the weight sensor 4 and continuously adjusts the pressure for achieving the wanted volume flow rate, compensating for the hydrostatic pressure related to the height of the spraying equipment 7. The control unit also performs corrections for the compressibility of the pressurized air. The container 2 is connected by supporting means 13 to a bearing structure 14. The supporting means 13 allow the movement of the container 2 which is needed for proper function of the weight sensor 4. In the case of using the invention as pressure amplifier, a check valve 15 is placed on the supply line before the valve 3.

The control unit calculates and applies the pressure which is needed for the desired volume flow rate in a painting stroke with compensation for the hydrostatic pressure due to gravity and related to the height of the spraying equipment above the floor.

Claims

1. Robotic system comprising a fluid supply system for spray painting and coating applications comprising fluid supply means for supply of fluid to spraying equipment (7) and flow control means for control of the volume flow rate of fluid supplied to the spraying equipment, c h a r a c t e r i z e d in that said flow control means comprises a container (2) in the supply line to the spraying equipment, a pressure gas supply means (12) for pressurizing said container, and pressure control means (9,10,11) for control of the volume flow by varying the gas pressure.

2. Robotic system comprising a fluid supply system according to claim 1, c h a r a c t e r i z e d in that said pressure control comprises means for correcting for the compressibility of the gas.

3. Robotic system comprising a fluid supply system according to claim 1 or 2, c h a r a c t e r i z e d in that said pressure control comprises means for correcting for hydrostatic pressure changes due to the position of the spraying equipment.

4. Robotic system comprising a fluid supply system according to any of claims 1 to 3, c h a r a c t e r i z e d in that said container is equipped with level control means (6a,6b).

5. Robotic system comprising a fluid supply system according to any of claims 1 to 4, c h a r a c t e r i z e d in that said container is equipped with weight control means (4) .

6. Robotic system comprising a fluid supply system according to any of claims 1 to 5, c h a r a c t e r i z e d in that a check valve (15) is installed upstream of the inlet to said container.

7. Fluid supply system for spray painting and coating applications comprising fluid supply means for supply of fluid to spraying equipment (7) and flow control means for control of the volume flow rate of fluid supplied to the spraying equipment, c h a r a c t e r i z e d in that said flow control means comprises a container (2) in the supply line to the spraying equipment, a pressure gas supply means (12) for pressurizing said container, and pressure control means (9,10,11) for control of the volume flow by varying the gas pressure.

8. Fluid supply system according to claim 7, c h a r a c t e r i z e d in that said pressure control comprises means for correcting for the compressibility of the gas.

9. Fluid supply system according to claim 7 or 8, c h a r a c t e r i z e d in that said pressure control comprises means for correcting for hydrostatic pressure changes due to the position of the spraying equipment.

10. Fluid supply system according to any of claims 7 to 9, c h a r a c t e r i z e d in that said container is equipped with level control means (6a, 6b) .

11. Fluid supply system according to any of claims 7 to 10, c h a r a c t e r i z e d in that said container is equipped with weight control means (4) .

12. Fluid supply system according to any of claims 7 to 11, c h a r a c t e r i z e d in that a check valve (15) is installed upstream of the inlet to said container.