A FEEDBACK AIR PRESSURE SENSING SYSTEM

Abstract: A feedback air pressure sensing system is described, for use with spray guns, comprising an air pressure feedback line (1), an air regulator (2) with an air cap (3). Air is supplied from air line (4) via regulator (2) to a dual air-feedback valve (5) operated by trigger (6). Air continues along line (7) to spreader control valve (8) supplying fan air to spray gun cap (3). Air is also fed via line (9) to supply atomising air to air cap (3). The regulator (2) comprises a body (13), an inlet (16) for a main air supply to first (14) and second (15) air chambers, an inlet for feedback air to a third air chamber (26), a diaphragm (19) between outlet chamber (25) and a pilot chamber (27), the diaphragm (19) being supported in the regulator body (13) by a spring-loaded valve (18), and spring loaded adjustment means (24).

Description: A FEEDBACK AIR PRESSURE SENSING SYSTEM This invention relates to a feedback air pressu sensing system and more particularly to spray guns i which the spray of atomised fluid can be controlled t produce a fan to round spray pattern. As a result of increased regulations to control t emission of fluids, e.g. paint from spray guns into t atmosphere there is a need to control the pressure of th air flow in the spray gun's air cap to a maximum of 1 psi (0.7 bar). All known spray guns do not provide fo an automatic control of this pressure e.g. when changin the gun's set-up from one spray pattern to another. To ensure the correct pressure of 10 psi (0.7 bar at the air cap the operator currently checks the pressur in the air cap by either direct measurement using a tes cap, or by tabulations which indicate that the pressur reading on the main air pressure supply gauge wil provide a pressure of 10 psi (0.7 bar) at the air cap. Many conventional air supply systems to spray gun have the disadvantage that they cannot maintain 10 ps (0.7 bar) at the air cap during the change of the spra pattern setting between a fan shaped and a round shaped spray. Control of the pressure is at present made b balancing orifices requiring special baffles or bodies for each air cap and the fluid tip set-up used. In such an arrangement the input pressure has no obvious relationship with the output pressure. An aim of the present invention is to provide a feedback air pressure sensing system with an automatic regulator for controlling the output pressure to overcome the above mentioned disadvantage. According to the present invention there is provided a feedback air pressure sensing system comprising an air supply, an air outlet and means to sense the pressure at the air outlet, the air supply being monitored by an air pressure feedback circuit which connects the air outlet with a regulator so that any change in pressure at the air outlet is sensed and fed back to the regulator to increase or decrease the air supply to maintain the air pressure at the air outlet at a predetermined pressure, wherein the regulator comprises a body, an inlet for a main air supply to first and second air chambers, an inlet for a feedback air supply to a third air chamber, a first diaphragm between an outlet chamber and a pilot chamber, the first diaphragm being supported in the regulator body .by a main spring- loaded valve, a second diaphragm between the third air chamber and a spring loaded adjustment means so that adjustment of the adjustment means controls the main air pressure entering the regulator inlet to a predetermined sensed pressure at the air outlet. Conveniently, the air pressure feedback circuit is fed from the atomising outlet of the spray gun air cap via a trigger-operated air valve/feedback valve to a feedback regulator. The air regulator is preferably connected with a pressure guage in the feedback section to indicate the pressure at the air outlet. An embodiment of a feedback air pressure sensin system, according to the present invention, for use wit a spraygun will now be described, by way of example only with reference to the accompanying diagrammatic drawings in which: Fig. 1 is a diagrammatic view of a feedback ai pressure sensing system according to the invention; Fig. 2 is a detail diagrammatic view of a tim delay valve; Fig. 3 is a diagrammatic cross-section for regulator; and Figs. 4 and 5 are enlarged diagrammatic cross sectional details of the regulator shown in Fig. 3, wit some separation between components for added clarity. Referring to Fig. 1, the spray gun incorporate an air pressure feedback line 1 which enables a main ai regulator 2 , which is normally mounted at a distance fro the spray gun, to sense the spraying, i.e. the atomisin pressure, within an air cap 3 of the spray gun. Air is supplied from an air line 4 via the mai air regulator 2 to a dual air valve/feedback valve operated by a trigger 6. The air continues along line to a spreader control valve 8 to supply fan air to th spray gun air cap 3. The air is also fed via a line to supply atomising air to the air cap. Air pressure sensed in the atomising air outlet 1 of the air cap 3 is fed back to the regulator along feedback line 10 via the dual air valve/feedback valve and a time delay valve 11. Fluid, e.g. paint to be sprayed, is fed from fluid reservoir to the spray gun nozzle, either b pressure, suction, or gravity means (not shown) . When the gun is spraying, the atomised pressur within the air cap 3 is transmitted via the feedbac circuit 1 to the main air regulator 2, via the trigge operated valve 5 and line 1. Variations in the pressure within the feedback circuit 1 cause the main air regulator 2 to automatically readjust the air pressure to the inlet of the spray gun in order to maintain a constant atomising pressure within the air cap 3. A pressure gauge mounted in the feedback section of the main air regulator 2 will indicate the atomising pressure within the air cap 3. As shown in Fig. 3 the feedback regulator 2 comprises a body 13 having first and second air chambers 14 and 15 for a main air supply entering the regulator at 16. The flow of air leaving the regulator at 17 is controlled by a main valve 18, the position of which is determined by the pressure balance across a main diaphragm 19. This pressure balance is maintained by a feedback valve 20 and a feedback relief stem 21 and these valves are opened and closed by the force balance across a feedback diaphragm 22, which is in turn determined by the spring force in spring 23 set by adjustment of screw 24. Under normal set-up conditions the feedback pressure is taken from an outlet chamber 25 and connected directly to feedback inlet 32. In this case any variation in the outlet pressure is sensed in a feedback chamber 26 via inlet 32 under the feedback diaphragm 22 and the deflection of the diaphragm opens either the feedback valve 20 or the feedback relief stem 21. The pressure fed through to the pilot chamber 27 controls the position of the main valve 18. The feedback pressure can be read from a pressure gauge 28 connected to the feedback chamber 26. Equilibrium is restored when the outlet pressure returns to the set value that balances a control spring 23. At this point, both the feedback valve 20 and the feedback relief stem 21 are shut-off to maintain a constant outlet pressure at 17. In a modified construction of the feedba regulator, instead of taking the feedback pressure fr the outlet chamber 25 it is possible to use anoth source downstream of the outlet 17. In this case t regulator acts to maintain the downstream air pressure the predetermined set pressure while the regulator outl pressure is allowed to vary, which variation compensat for any pressure drop dependent on flowrate, such as m be caused by frictional losses in hoses or head loss across restictors between the regulator outlet 17 and t feedback source. Under stable operation the air pressure sensed the spray gun outlet is returned to the feedback inlet and the screw 24 is adjusted to set the requir operating pressure on gauge 28. If the pressure return via the inlet 32 balances the spring force in spring across the feedback diaphragm then the valves 20 and remain closed. If the pressure sensed at the spraygun outlet higher than that which balances diaphragm 22 then t following sequence takes place: a) the feedback to inlet 32 raises the diaphra 22; b) valve stem 21 is caused to lift off from i valve seat in valve 20 allowing excess air pressure escape to atmosphere; c) the resulting reduction in pressure in chambe 27 causes diaphragm 19 to lift under the action of spri 31 and to throttle valve 18 thus reducing the main ai outlet pressure; d) this reduced pressure is sensed by the sprayg and the new pressure is fed via inlet 32 to diaphragm 2 restoring stable operation. In the case where the pressure sensed is lower tha that which balances diaphragm 22 the following sequenc takes place: a) feedback to inlet 32 causes diaphragm 22 to be depressed under the action of spring 23; b) valve stem 21 is depressed against valve 20 which opens allowing air from the main air supply to pass and depress diaphragm 19 thus opening valve 18 and raising the pressure of the outlet air; c) this raised pressure is sensed at the spraygun outlet and the new pressure is fed by inlet 32 to diaphragm 22 restoring stable operation. The air regulator 2 is initially set manually, while the gun is spraying, to give the required level of atomising pressure, typically 10 psi (0.7 bar). With the gun spraying, adjustment of the spreader control valve 8 to change the shape of the spray pattern from a fan shape to a round shape reduces the air flow through the spray gun. Normally this reduction in air flow would result in an increase in the atomising pressure within the air cap 3 but the feedback circuit 1 prevents this by sensing the change in pressure and tramsmitting it to the main air regulator 2 which automatically reduces the air pressure to the spray gun, in order to maintain the atomising pressure at its pre-set level i.e. 10 psi (0.7 bar) maximum. Readjustment of the spreader control valve 8 to change the pattern shape from a round shape back to a fan spray automatically results in the main air regulator 2 increasing the air pressure to the spray gun inlet to maintain the atomising pressure at a constant level. Referring to Fig. 1, if the dual air valve feedback valve 5 is operated rapidly by the trigger 6, a transient pressure signal could be trapped inside the feedback line 1 and the outlet pressure of the regulator would be disturbed. To prevent this the time delay valve 11 can be fitted. As shown in Fig. 2, the time delay valve may comprise an ON/OFF spool valve 34 connected to the main air regulator 2. The spool valv 34 is actuated by a pressure difference across diaphrag 35 that is loaded by a spring 36. The high pressure sid 40 of the diaphragm 35 is connected to the main outle chamber 25 of the regulator 2 and the low pressure sid 37 is connected via a non-return valve 38 and a needl valve 39 to a tapping in outlet port 17 of the regulator. When air flows through the regulator there is a pressur drop between the outlet chamber and the outlet port whic operates the spool valve 34 after a certain time dela set by the setting of the needle valve 39. The time delay prevents any transient pressur signal from reaching the regulator and disturbing the outlet pressure. The valve only transmits steady-state signals. The feedback circuit shown in Fig. 1 therefore enables the atomising pressure within the air cap 3 of the spray gun, once set initially to 10 psi (0.7 bar), to be maintained at a constant level regardless of any adjustment to the pattern of the spray during spraying. Although the principle is described in the feedback regulator circuit to control the atomising air pressure in an air spray gun which is constructed and connected to an air supply in such a way that the air pressure in the air cap of the spraygun it ensed and the sensed pressure is fed back to the regula. r mounted, e.g., on a wall, it may also be used in other air powered tools requiring a predetermined air supply.

Claims: CLAIMS: 1. A feedback air pressure sensing system comprising an air supply, an air outlet (12) and means to sense the pressure at the air outlet, the air supply being monitored by an air pressure feedback circuit which connects the air outlet (12) with a regulator (2) so that any change in pressure at the air outlet is sensed and fed back to the regulator (2) to increase or decrease the air supply to maintain the air pressure at the air outlet (12) at a predetermined pressure, characterised in that the regulator (2) comprises a body (13), an inlet (16) for a main air supply to first (14) and second (15) air chambers, an inlet for a feedback air supply to a third air chamber (26), a first diaphragm (19) between an outlet chamber (25) and a pilot chamber (27), the first diaphragm (19) being supported in the regulator body (13) by a main spring loaded valve (18), a second diaphragm (22) between the third air chamber (26) and a spring loaded adjustment means (24), so that adjustment of the adjustment means (24) controls the main air pressure entering the regulator inlet (16) to a predetermined sensed pressure at the air outlet (12). 2. A feedback air pressure sensing system as claimed in Claim 1, characterised in that the regulator (2) includes a pressure gauge (28) connected to a feedback chamber to indicate the feedback pressure fed back to the regulator (2) from the air outlet (12) 3. A feedback air pressure sensing system as claimed in Claim 2, characterised in that the regulator diaphragms (19, 22) are made of rubber, or fabric reinforced rubber. 4. A feedback air pressure sensing system as claimed in Claims 1 or 2, characterised in that the regulator diaphragms (19, 22) are made of a resilient plastics material. 5. A feedback air pressure sensing system as claimed in any preceding claim, characterised in that the air pressure feedback circuit connects an atomising outlet o the air outlet (12) to a feedback regulator via trigger-operated (6) air valve/feedback valve (5). 6. A feedback air pressure sensing system as claime in any preceding claim, characterised in that a tim delay valve (11) is located in the feedback line (1) between an outlet chamber (25) and an outlet port (17) of the feedback regulator. 7. A feedback air pressure sensing system as claimed in Claim 6, characterised in that the time delay valve (11) is a spool valve (34) operated by a spring loaded diaphragm (35), characterised in that one side of the diaphragm (35) is connected via a non-return valve (38) and an adjustable needle valve (39) with the outlet port (17) of the feedback regulator and the other side of the diaphragm is connected to the outlet chamber (25) of the regulator (2) . 8. A spray gun when used in association with the feedback air pressure sensing system as claimed in any preceding claim.

Inventors: GARLICK PAUL LAWRENCE, SMITH ROWLAND CHARLES, GARLICK, PAUL, LAWRENCE, SMITH, ROWLAND, CHARLES
Applicants: ITW LTD, ITW LIMITED
Application year: 1993
Main classification: B05B7
IPC codes: B05B7