WO2005056198A1

WO2005056198A1 - A gas conduit system, in particular in a powder spray apparatus

Info

Publication number: WO2005056198A1
Application number: PCT/IB2004/004042
Authority: WO
Inventors: Gerald Haas
Original assignee: Itw Gema Ag
Priority date: 2003-12-10
Filing date: 2004-12-09
Publication date: 2005-06-23
Also published as: TW200528196A; DE10357814A1

Abstract

A gas conduit system, in particular a powder spraycoating apparatus containing such a gas conduit system. One or more throttles (12, 16, 20, 24) are adjusted as a function of a desired flow (Itr/min) and of a throttle intake pressure measured by a pressure sensor (8) in a manner that an approximately constant actual flow is attained at the throttle even when the intake pressure changes.

Description

A GAS CONDUIT SYSTEM, IN PARTICULAR IN A POWDER SPRAY APPARATUS

The present invention relates to a gas conduit system defined in the preamble of claim 1 and in particular to a powder spraycoating apparatus containing such a system. In the context herein, "gas" denotes compressed gas, in particular compressed air at a pressure higher than atmospheric. Powder spraycoating apparatus fitted with air conduit systems are known for instance from the following documents: WO 00/10725, WO 00/10726, WO 00/10727, EP 0 636 420 A2, US 4,284,032; US 5,131 ,350; DE 44 09 493 A1 , US 3,625,404; GB 2 029 271 A, DE 42 39 496 A1. The invention may be applied to all such powder spraycoating apparatus of the state of the art, provided a controlled throttle be used therein in lieu of their disclosed pressure regulators. The accuracy of the compressed air flow (volumetric stream per unit time) in a throttle, for instance in a powder spraycoating apparatus disclosed in WO 00/10725, WO 00/10726 and WO 00/10727, substantially depends on the air flow constancy upstream of the throttle (throttle intake pressure). In order to attain the airflow accuracy required by the powder spraycoating apparatus, present-day practice uses precise mechanical pressure regulators such as are illustratively described in EP 0 636 420 A2, US 4,284,032 and US 5,131 ,350. Such pressure regulators are comparatively expensive and also occupy much bulk in the control unit required to control them. The air conduit systems fitted with controlled throttles disclosed in said three WO documents each may be considered being a system implementing Inearization with an unknown (the adjusted angle of rotation of the throttle as a function of air flow). The airflow also depends on the flow impedance downstream of the throttle. However this flow impedance may be calculated for each gas conduit system and in most instance it is constant. The throttle setting required for a given gas flow may be ascertained empirically. The gas or air flow may be defined as Itr/min (liter per minute) or as Nm ³ /h (standard cubic meter per hour) or in other units. The objective of the present invention is to reduce or to preclude gas flow fluctuations in a gas conduit system, in particular in a powder spraycoating apparatus, whenever the gas pressure varies in a gas source, without entailing a pressure regulator to attain this goal. This problem is solved by the features of claim 1 of the present invention. As stated therein, the invention relates to a gas conduit system containing a controlled throttle in at least one gas conduit, further an adjustment drive to set the throttle in a manner that a given gas intake pressure results in a given gas flow; a control unit to actuate the throttle adjustment drive as a function of an adjusted or adjustable reference gas flow; a pressure sensor upstream of the throttle to measure the actual intake pressure at the throttle and to transmit signals corresponding to the actual intake pressure to the control unit; a memory in the control unit storing — for a predetermined flow impedance downstream of the throttle ~ a mutual operational relationship between adjusted throttle position, throttle intake pressures and throttle gas flows; a processor in the control unit to generate by means of the stored mutual operational relationship an adjustment signal for the adjustment drive as a function of the particular adjusted reference gas flow set at the control unit and simultaneously as a function of the pressure sensor signals corresponding to the particular actual throttle intake pressure, said adjustment signal actuating the adjustment drive to adjust the throttle in a way that even if the intake pressure were to vary, the actual gas flow shall correspond at least approximately to the reference gas flow. In particular the present invention relates to a powder spraycoating apparatus containing such a gas conduit system to pneumatically convey coating powder or to fluidize coating powder or to pneumatically convey and fluidize coating powder by means of a compressed gas, preferably air. Further features of the present invention are stated in the dependent claims. Advantages of the Invention In the powder spraycoating of the state of the art WO 00/10725, the linearization of throttle operation is implemented by an unknown (the throttle's angle of rotation as a function of gas flow). The present invention on the other hand offers linearization by two unknowns, the particular adjusted position (for instance the angle of rotation) of the throttle being a function of the desired (reference) gas flow and of the throttle intake pressure. The invention offers another advantage in that all the gas pressure at the gas source, for instance a compressed air source, is available for spraycoating. When using a mechanical pressure regulator of the state of the art, the pressure is restricted to a value for instance of 5 bars which at most may be as high as the lowest pressure of the compressed air supply. As a result the maximally feasible expulsion of coating powder from a spray apparatus also is restricted to the pressure regulator setting. Ordinarily the coating powder is moved through an injector acting as a feed pump to the spray apparatus. In the process, a compressed air flow of about 5 bar in the injector sucks coating powder out of a powder bin. In order that the pressure regulator be able to operate in all pressure ranges, it must be adjusted to a pressure which at most shall be as high as the lowest pressure from the compressed air supply. In the invention, on the other hand, the particular highest extant pressure of the compressed air supply can be used (for instance up to 10 bars) and be transmitted to the spray apparatus. As a result the powder output (conveyed volume of powder per unit time) may be increased significantly. The invention is elucidated below in relation to drawings and an illustrative embodiment mode. Fig. 1 is a plot stored in a read memory of a control unit of a gas conduit system, the horizontal axis representing adjustment positions, for instance angles of rotation steps of a stepping motor, the vertical axis showing throttle flows of compressed gas, for instance Itr/min and furthermore a parametric family of at least two curves of various throttle intake gas pressures, for instance one curve one curve each for 5 bars, 6 bars, 7 bars, 8 bars, 9 bars and 10 bars. Fig. 2 schematically shows a powder spraycoating apparatus of the invention. Fig. 3 schematically shows an axial section of the front end segment of a spray apparatus that is shown only on a much smaller scale in Fig. 2. Fig. 1 is a plot showing mutual operational dependency of the three parameters of a throttle: the flow in Itr/min (or in Nm ³/h) on an illustratively vertical axis; several (at least two) parametric throttle Intake pressure curves ; and possible (analog or digital) throttle adjustment positions, for instance in the form of adjustment steps, illustratively angles of rotation steps. A desired flow for instance of 100 Itr/min at an instantaneous throttle intake pressure for instance of 8 bars may be attained by setting the throttle at a given adjustment position, for instance at an angular step #96 of a stepping motor which acts as the throttle adjustment drive. If the throttle intake pressure for instance changes from 8 to 10 bars, the same flow of 100 Itr/min shall be retained if the throttle automatically is adjusted to a matching adjustment position, for instance an angular step #80. If on the other hand the intake pressure drops for instance from 8 bars to 6 bars, the desired adjustment position of 100 Itr/min shall be attained by the throttle being automatically set into a matching adjustment position, for instance an angular step #116. In the light of these illustrations, the required throttle adjustment position shall also be attained for other desired flows as a function of throttle intake pressure. Fig. 2 illustrates the invention by means of one of many possible embodiment modes of a powder spraycoating apparatus. The data of the plot of Fig. 1 are stored in a memory 2 of a control unit 4. The desired flow in Itr/min or in Nm ³/h (or in other units) is preset in fixed manner at an adjusting element 8 of the control unit 4 as a function of the control unit's embodiment mode and of the powder spraycoating apparatus' application, or else said flow may be manually and variably set or be adjusted by an overriding control unit as a function of other criteria such as the particular kind of powder. By means of a reference flow set at the adjustment element 8 and as a function of the measured throttle intake pressure measured by a pressure sensor 8 and communicated to the control unit 4, a processor 6 of the control unit 4 determines, from the plot of Fig. 1 , the throttle adjustment position required to attain the set reference flow. The control unit 4 generates an adjustment position corresponding to the required set position whereby the throttle is adjusted to the required set position. Typically the gas shall be compressed air, though it also may be another compressed gas. Powder spraycoating apparatus typically use compressed air and therefore the air mode shall be exclusively considered hereafter. The throttle may be configured in the air conveying conduit 10 as a throttle 16 and/or it may be a throttle 16 in a supplemental-air conduit 14 and/or throttle 20 situated in the fluidization conduit 18 and/or it may be a throttle 24 in an electrode air- flushing conduit 22. A fixed-setting throttle may be mounted instead of a controlled throttle in one or another conduit as discussed above, for instance it may be the throttle 24 of the electrode air-flushing conduit 22, or a conduit such as the fluidizing air conduit 18 may lack a throttle. However, in the preferred embodiment of the present invention, both the conveyed air conduit 10 contains a controlled throttle 12 of the invention and the supplemental-air conduit 14 contains a throttle 16 controlled in the manner of the invention. A supplemental-air conduit 14 is absent from another, omitted embodiment mode, because only compressed air is required in the air-conveying conduit 10 for the basic operation of an injector 26. In the injector 26, the conveying air produces a partial vacuum and therefore will evacuate - through a suction conduit 28 - coating powder 30 from a powder bin 32 and thereby will pneumatically move the aspirated powder through a spray apparatus 34 , said powder being electrostatically charged by one or more electrodes 36 and sprayed onto an omitted object to be coated. The coating deposited on said object is then molten onto the said object in an oven. The compressed air is tapped from a compressed air supply 40 and is fed through controlled valves 42, 44, 46 of the air-conveying conduit to the supplemental air conduit 14, to the electrode air-flushing conduit 22 and to the fluidizing air conduit 18. The minimum of one electrode 36 is connected to a high voltage source 48 which may be configured within or outside the spray apparatus 34. The fluidizing air conduit 18 may be connected at its downstream end to a fluidizing duct 52 which, jointly with the suction pipe 28 and the injector 26, constitutes one construction unit and which at its lower end comprises a fluidizing air outlet 54 that is adjacent to the suction intake 56 of the suction pipe 28. The fluidizing compressed air flowing out of the fluidizing duct 52 fluidizes the powder 30 in the powder bin 32 in the vicinity of the suction intake 56 of the suction pipe 28, as a result of which it may be easily moved pneumatically from there. In an alternative or in addition to the fluidizing air duct 52, the powder bin 32 may be fitted with an air- permeable intermediate bottom and the fluidizing air conduit 18 may be connected to the intermediate bottom space in order that the fluidizing air be able to pass through the intermediate bottom into the powder bin 32 and shall fluidize the coating powder 30 in the manner known in the state of the art. The adjustment drives 60 of the throttles 12, 16, 20 and 24 are only schematically indicated in Fig. 2 as arrows and preferably contain a stepping motor as adjustment drive motor. However other known drives are appropriate as adjustment drives, for instance electromagnetic, pneumatic or hydraulic adjustment drives. In the plot of Fig. 1 , a small number of steps indicates a small throttle opening cross-section. The larger the number of steps shown - or adjustment positions ~ the larger shall be the throttle's opening cross-section through which flows the compressed air. Preferably the control unit 4 is one equipment unit, the position of the injector 26 being independent of said unit. In Fig. 1 , the shown intake pressure curves of 5 to 10 bars are merely didactic. At least two such intake pressure curves are required to counteract the throttle's intake pressure fluctuations. The more intake pressure curves are stored in memory, the more accurate shall be the throttle setting. In other words, the more intake pressure curves have been stored, the more accurately the actual flow through the throttle will correspond to the flow adjusted at the control unit. The plot of Fig. 1 may be stored in analog or digital form. In a special embodiment mode of the present invention, the memory 2 of the control unit 4 stores NOT a plot such as shown in Fig. 1 but an algorithm by means of which a processor 6, on the basis of the reference gas flow set at the adjustment element 8 and of the throttle intake pressure measured by the pressure sensor 8, shall calculate the adjustment signal for the adjustment drive 60 to set a corresponding throttle adjustment position. The desired value therefore is the throttle adjustment position, so that, when using an electrical stepping motor, the angular step (angular [position) at which the flow (m) desired by a user shall be set at the adjustment element 8. The intake pressure (p) is measured by the pressure sensor 8. This procedure may be formulated mathematically as s = f(m, p). There are two ways to determine the proper angular step (angle of rotation) of the stepping motor by the processor 6:

1. The said plot of Fig. 1 or a similar plot is stored as a matrix. Thereupon, on the basis of the intake pressure determined by the pressure sensor 8 and of the desired reference flow which may be adjusted by a user or by an overriding main control at the adjustment element 8 or which is permanently preset, the processor 6 will determine the proper the stepping motor's appropriate angle of rotation. This procedure incurs the drawback that much storage space is required for said plot in the memory 2. On the other hand, this procedure offers the advantage of very high throttle adjustment accuracy

2. The plot of Fig. 1 or a similar plot is approximately defined mathematically by x-order polynomials. The following expression applies generally:

where n, I: order of polynomial; ay : polynomial coefficients; p: intake pressure; m: flow; s: angular step (angle of rotation). The advantage of calculating the appropriate angle of rotation, i.e. the angular steps using such an algorithm is that its implementation requires only little space in the read memory 2. On the other hand such an algorithm incurs the drawback of slightly lower accuracy than when using a plot such as in Fig. 1 , and that much computational capacity is required. The said coefficients depend on throttle geometry. The use of one or more plots as shown in Fig. 1 , or calculating the throttle setting by means of a stored algorithm, are all applicable to all said throttles 12, 16, 20 and 24. In all embodiment modes of the present invention, the setting (for instance the angle of rotation) of the throttle will track any changes in the throttle intake pressure, and in the event of an increase in intake pressure, the throttle shall be automatically reset to a smaller flow cross-section, whereas in the event of a decreasing intake pressure, the throttle will be automatically reset to a larger flow cross-section. In other words, the throttle setting is made to automatically adapt to changes in intake pressure. Such adaptation may be carried out in analog or preferably digital manner, continuously or step-wise. It is clear to the expert that the said throttle settings only result in the desired gas or compressed air flow when the gas flow impedance remains at least approximately constant downstream of the particular throttle. Several plots or several algorithms may be stored, each being associated with a different flow impedance downstream of the throttle. The memory 2 is readable. It is preferably also writable. When using a write memory 2, the plot or the data and curves of the plot of Fig. 1 or the algorithm, data or the algorithm, may be changed and in this manner be adapted to changing operational conditions, for instance different spraycoating methods, different kinds of objects to be coated and different kinds of powders,

Claims

PATENT CLAIMS

1. Gas-carrying system,' including a controllable throttle (12, 16, 20, 24) in at least one gas line (10, 14, 18, 22); an actuating drive (60) for setting the throttle, so a defined input pressure of the gas results in a defined actual gas flow rate; a control device (4) for setting the actuating drive (60) of the throttle (12, 16,, 20, 24) as a function of a desired gas flow rate which is or can be set at the control device (4); a pressure sensor (8) upstream of the throttle (12, 14, 20, 24) for measuring the actual input pressure of the throttle and for transmitting signals which correspond to the actual input pressure to the control device (4); a memory (2) in the control device (4), in which a functional mutual dependency of setting positions of the throttle, input pressures of the throttle and gas, flow rates through the throttle is stored for a predetermined flow, resistance downstream of the throttle (12, 14, 20, 24); a processor (6), in the control device (4), which, on the basis of the stored functional mutual dependency, generates, as a function, of the desired gas flow rate which is in each, case set at the control device (4) and at a same time as a function of the signals from the pressure sensor (8) which correspond to the prevailing actual input pressure of the throttle, an actuating signal for the actuating drive (60), by means of which the actuating drive (60), sets the throttle (12, 16, 20, 24), so that the actual gas flow rate at least approximately corresponds to the desired gas flow rate even in the event of a fluctuating input pressure.

2. Gas-carrying system according to Claim 1 , characterized in that the functional mutual dependency is stored in the form of a diagram which includes the functionally mutually dependent three parameters setting positions of the throttle, input pressures of the throttle and desired gas flow rates through the throttle.

3. Gas-carrying system according to Claim 1 , characterized in that a plurality of input pressure diagram curves, a multiplicity of setting positions of the throttle are stored on one diagram axis, and a multiplicity of desired gas flow rates are stored on another axis of the diagram.

4. Gas-carrying' system according to Claim 1 , characterized in that the functional mutual dependency is stored in the form of an algorithm, on the basis of which the processor (2) calculates, as a function of the desired gas flow rate which is in each case set and the input pressure of the throttle which is in each case measured by the pressure sensor, the actuating signal for setting a corresponding setting position for the actuating drive (60), of the throttle (12, 16, 20, 24).

5. Gas-carrying system according to any one of the preceding claims, characterized in that the actuating drive (60) includes a stepper motor for setting the throttle (12, 16,20, 24) in steps.

6. Gas-carrying system according to at least one of the preceding claims, characterized in that the gas is compressed air.

7. Powder spray-coating device, which includes a gas-carrying system according to any one of the preceding claims for pneumatically conveying coating powder.

8. Powder spray-coating device according to Claim 7, characterized in that the gas line or one of the gas lines is a delivery-air line (10), the, downstream end of which is connected to a delivery-air inlet of an injector (26), which is designed as a pump for pneumatically conveying coating powder.

9. Powder spray-coating device according to Claim 7 or 8, characterized in that the gas line or one of the gas lines is an additional-air line (14), the downstream end of which is connected to a powder flow path for pneumatically conveyed powder.

10. Powder spray-coating device according to one of Claims 6 to 9, characterized in that the gas line or one of the gas lines is a fluidizing-air line (18), the downstream end (54) of which opens out into a powder vessel for coating powder or can be immersed in a powder vessel for the purpose of fluidizing coating powder which is present therein by means of compressed air.