TECHNICAL FIELD

This invention relates to a method and system for controlling the flow of liquid coating material and, in particular, to a hybrid control method and system for controlling the flow of liquid coating material by controlling a flow regulator in response to a reference signal.

BACKGROUND ART In full-time closed loop fluid control systems the desired flow rate is compared with the actual flow rate and, based on a control algorithm which-oftentimes includes gain factors _r the. output command signal is corrected to achieve the desired flow rate. Such a closed loop system is oftentimes executed on a periodic basis to bring the actual flow rate in line with the commanded flow rate.

One problem associated with such full-time closed loop systems is that such a control method does not work well with systems exhibiting poor dynamic response characteristics. This is because the gain in such a closed loop system is set prohibitively low to ensure system stability. Such a low gain, however, requires that the control system takes a prohibitively long time to reach the desired set point.

Another approach oftentimes used in fluid control systems is the open loop system wherein calibration data associates a given output command

with an actual flow rate. Such a system requires calibration through successive iterations of manually setting the output command (i.e. flow rate set-point) and then beakering the flow for a given period of time. This is done until a command is found that produces the desired flow rate. In such an open loop system, if a range of flow rates are to be controlled and the system is non-linear, multiple calibration points are typically required.

One problem with such open loop control systems is that the calibration data is only as accurate as the manually derived data points. Human error can result in bad data and, depending how manually intensive the process is, much fluid can be wasted during the calibration process. Furthermore, the process requires that the open loop system be off line during calibration which equates to lost production time.

Also, such open loop fluid control systems do not automatically accommodate changes in fluid temperature which directly affects fluid viscosity and, ultimately, fluid flow rate. A significant change in temperature means that the manual calibration procedure would need to be repeated at different temperatures for accurate fluid flow control.

Yet another problem with such open loop fluid control systems is that such systems do not accommodate system hysteresis. In other words, the output or reference command required to achieve a predetermined flow rate when the system is operating

at a flow rate lower than the predetermined flow rate, can be different from the output command required to achieve the same predetermined flow rate

> when the system is operating at a flow rate higher than the predetermined flow rate.

A number of other problems are associated with fluid control systems of both types including: (1) the lack of ability to automatically tune the gain of the system for each fluid being employed; (2) the lack of ability to automatically adjust flow meter calibration or K factor based on temperature change; (3) lack of set point variability for varying humidity; and (4) the lack of ability to accommodate differences in system response time based on command direction.

The U.S. Patent to Hirano et al 4,724,865 discloses a control circuit for proportional electro- hydraulic fluid control valves. The control circuit includes a control loop changeover mechanism which changes the control mode from a closed loop control, in accordance with an error signal, to an open loop control, in accordance with a preset input signal, when the error signal is greater than a predetermined level.

The U.S. Patent to Geisel et al 4,787,332 discloses an adhesive dispensing pump control system for use with a robot, wherein the flow rate of the adhesive material being dispensed on a workpiece is automatically changed. The system maintains a substantially constant inlet and outlet pressure difference across the dispensing pump, thereby making

the system relatively insensitive to changes in viscosity of adhesive being dispensed.

The U.S. Patent to Ewing 4,679,585 discloses a fast response flow meter having electrical circuitry which enhances responses to low level signals and cooperatively induces electromechanical oscillations offsetting hysteresis of the valve armature suspension.

The U.S. Patent to Vetter 4,838,311 discloses a control system for a programmed spraying device mounted on a painter robot. The paint is switched on and off by an automatically controlled pilot needle valve at predetermined times as a function of the relative position of the robot and the motor vehicle body to be painted.

The U.S. Patent to Rossetti 4,705,083 discloses a computerized method for calibrating the amount of coloring agent dispensed from solenoid valves by calculating the viscosity and delay time for opening and closing the valves so that coloring agents are dispensed with a minimum of less than plus or minus one percent error.

The U.S. Patent to Gaiotto et al 4,726,528 discloses a pressure equalizer for use with a robot, including a pressure sensing element for sensing the pressure at which enamel or paint is fed to a spray gun mounted on the robot.

Other patents of lesser relevance to the invention of the present application include U.S.

Patent NOS. 3,653,393, 3,989,935, 4,162,689,

4,420,812, 4,484,120, 4,546,795, 4,556,032, 4,785,760 and 4,807,561.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an improved method and system for controlling a flow regulator initially in an open loop mode and then controlling the flow regulator in a closed loop mode after flow of liquid through the regulator stabilizes.

Another object of the present invention is to provide an improved hybrid method and system for controlling a flow regulator wherein an initial command used in an open loop mode is determined from calibration data and wherein the calibration data is updated based on the actual command required to achieve a desired flow rate.

Yet, still another object of the present invention, is to provide an improved method and system wherein calibration data utilized in an open loop mode of the system is acquired and stored with respect to a particular fluid and wherein the calibration data is dynamically updated to compensate for varying system operating conditions, such as fluid temperature, fluid viscosity and mechanical/pneumatic system changes. In this way, the calibration data is automatically adjusted as required to achieve a desired flow rate.

In carrying out the above objects and other

objects of the present invention, a method for controlling a flow regulator and a transducer for operating the flow regulator in response to electrical control signals is provided. The flow rate of liquid coating material regulated by the flow regulator is controlled in response to a reference signal. The method has a closed loop mode and an open loop mode. The method includes the steps of storing a set of calibration data representing at least one expected flow rate for the liquid coating material. Also, the method includes the steps of correlating the reference signal with the calibration data to generate a set signal as a function of the reference signal and generating a first electrical control signal in accordance with the set signal in the open loop mode so that the flow regulator regulates the flow of liquid coating material at the expected flow rate. The method further includes the steps of generating a feedback signal as a function of the actual flow of the regulated liquid coating material and generating an error signal as a function of the difference between the set signal and the feedback signal. Also included is the step of generating a second electrical control signal as a function of the error signal in a closed loop mode. The error signal is representative of the desired amount of liquid coating material flow change. Finally, the open loop mode is changed to the closed loop mode after a predetermined period of time has elapsed after the step of generating the first electrical control signal.

Preferably, the method also includes the step of modifying the set of calibration data when

the actual flow rate is within a predetermined range of the flow rate represented by the second electrical control signal in the closed loop mode to obtain a modified set of calibration data representing a desired flow rate to compensate for varying system operating conditions.

A method is also provided for spray-coating an article in a coating zone wherein the method utilizes a robot having an arm provided with a support head movable about a plurality of control axes. An atomizing device is attached to the support head. A flow regulator controls the flow of liquid coating material from a source of liquid to the atomizing device. The method for spray-coating includes each of the above-noted steps, including the steps of moving the atomizing device about the plurality of control axes to different positions along a programmed path at a predetermined distance from the article in the coating zone for coating the surface of the article with the atomized liquid coating material and automatically coordinating operation of the flow regulator and the robot to control the spray coating of the article in the coating zone.

A system for carrying out each of the above- noted methods is also provided in order to carry out the above objects and other objects of the present invention.

Also, preferably, a temperature sensor is employed for generating a temperature signal as a function of the temperature of the regulated liquid

coating material, which temperature signal may be utilized to modify the set of calibration data as a function of the temperature signal.

A humidity sensor may also be employed in much the same fashion as the temperature sensor.

The advantages accruing to the use of the method and system of the present invention are numerous. For example, significant reductions in manual labor, waste of material and lost production time can be realized. In addition, the method and system of the present invention are readily adapted for use for robotically operated spray equipment, as illustrated in the preferred embodiment of the present invention herein.

The advantages of the present invention can be readily appreciated when the same becomes better understood by reference to the following detailed description when taken in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIGURE 1 is a schematic view, illustrating the method and system of the present invention; and FIGURE 2 is a schematic view illustrating the controller of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawing figures, there is illustrated a paint spray control method and

system incorporating various features of the present invention for coating parts, such as automotive parts.

The paint spray system generally includes an atomizing device, such as a spray gun 10. The spray gun 10 is preferably mounted on a support head 12 of an arm 14 which, in turn, is part of a robot 16. The robot 16 provides programmed multi-axes spray gun movement and is generally commercially available from the assignee of the present application. The spray gun is available from the SAMES Company.

While the preferred embodiment of the invention is utilized with the robot 16, it is to be understood that the method and system are equally applicable for use in hand-held or automatic systems, as well as robot systems.

The spray gun 10 is typically mounted on the support head 12 in a paint spray booth, in which a part or article is to be coated. The part is often conveyed through a coating zone in the paint spray booth by a conveyor.

The robot 16 is controlled by a programmable control means or controller 24. Under control of the robot controller 24, the robot 16 and, consequently the spray head 12, is capable of moving the atomizing device 10 about a plurality of control axes to different positions along a programmed path, a predetermined distance from the article in the coating zone of the paint spray booth.

The air spray gun 10 includes a liquid coating passage (not shown) which is in fluid communication with a paint supply line 26, which supplies paint from a paint or liquid source 28. The liquid source 28 can take the form of either a paint pressure tank or a paint recirculation line. The paint is supplied in a controlled fashion to the paint supply line 26 by a controlled flow regulator 30. The flow regulator is available from the SAMES Company and preferably includes a manifold with an integral paint regulator.

In turn, the flow regulator 30 is controlled by air control signals from a converter means or transducer 32. Such a transducer is available from Proportion Air Company.

The air control signals developed by the transducer 32 are developed under control of a control system or fluid flow controller, generally indicated at 34, from a source of pressurized air or air source. The air source may also provide air pressure to the liquid source 28 if the liquid source 28 should take the form of a paint pressure tank. The air source may also be fluidly connected to an atomizing air passage (not shown) of the spray gun 10 if the atomizing device takes the form of an air spray gun.

The control system 34 is characterized as a hybrid control system in that it has an open loop mode and a closed loop mode for controlling the flow regulator 30 through the transducer 32. Under control of the control system 34, the flow regulator

30 regulates the flow of liquid coating material in response to a reference signal, *A, from the robot controller 24 appearing on line 38.

In a calibration mode, calibration data is determined off line by a feedback means or mechanism. The feedback mechanism includes a flow transducer or sensor 40 which, preferably, provides a frequency signal, *F, for measuring the actual flow of the regulated liquid coating material through the regulator 30 to obtain an electrical flow signal. The flow signal includes a number of pulses, the number of pulses being a function of the actual flow of the regulated liquid coating material through the regulator 30. The sensor 40 is available from the Kupper Company and is also referred to as a gear meter.

The feedback mechanism includes a counter means or counter within a block 42 of the control system 34 for counting the number of pulses in the flow signal. The count within the counter thereby represents the actual flow of the regulated liquid coating material through the regulator 30.

Mode control switches 44 and 46 are under control of a keyboard (not shown) coupled to the system 34 or an I/O interface 42 which is coupled to the switches 48 by a serial communication line 48. The I/O interface 42 may comprise a programmable logic controller.

In the uppermost position of the switches 44 and 46, the signal from the I/O interface 42 is

directly coupled to the transducer 32 in a transparent mode.

In a second, horizontal position of the switches 44 and 46 the system 34 enters an open loop mode.

Finally in a third, lowermost position of the switches 44 and 46 the system 34 enters a closed loop adaptive mode. The system 34 is capable, however, of entering the open loop mode from the closed loop mode if there is a change in a set signal of more than a predetermined amount as illustrated at block 50.

In the calibration mode, all calibration data is entered into a calibration data table 52 for a user selected fluid over a user selected operating range. Typically, user selectable sample points are entered in engineering units after being scaled by a scaling unit 53 which takes a current signal in the range 4-20 mA and converts same into engineering units. This data is used to calculate slope and Y- intercept tables for future use by an interpolation unit 54 which calculates the initial (i.e. open loop) flow command.

The robot controller 24 typically initiates the calibration cycle through the I/O interface 42.

However, it is to be understood that the calibration cycle may be initiated locally at the control system

34 as well, for example, under operator control.

It is also to be understood that the

calibration data is typically stored in table format in a memory device such as a semiconductor memory device. The calibration data represents at least one expected flow rate for the liquid coating material.

Once the decision has been made to go to the table 52 to pick a corresponding value in the open loop mode (i.e. typically when there is a relatively large change in the input signal) , the block 57 is entered. In block 57 a delay period is set wherein the system stays in the open loop mode for a period of time to allow the flow point to stabilize. The amount of time is set during calibration and is dependent upon system response time and the amount of change in the input signal over the entire range of input signals.

Preferably, the table 52 includes a set of points (for example, 10) over the usable flow range for each type or color of paint.

The interpolation unit 54 is required to allow set values to fall between the points in the table 52. Preferably, the interpolation unit 54 utilizes straight line interpolation.

As previously mentioned, the flow sensor 40 measures the flow of liquid coating material through the regulator 30 and outputs a flow signal, the frequency of which represents the amount of flow through the flow regulator 30.

The robot controller 24 sends flow command or reference signals to the I/O interface 42 which,

in turn, sends the signals along the line 38 to the control system 34 wherein the magnitude of each flow command signal is directly proportional to the desired flow rate. In general, when the control system 34 receives a new command signal, the appropriate calibration data is accessed to calculate the initial or open loop flow command by the interpolation unit 54. The initial open loop flow command signal appears on a line 56 and a first electrical control signal is output at switch 46 in the open loop mode. The first electrical control signal is sent to the transducer 32 along a line 58. In turn, the transducer 32 converts the control signal to a signal to control the flow regulator 30 which, in turn, regulates the flow of liquid coating material from the liquid flow source to the atomizing device 10 at the expected flow rate, as represented by the calibration data contained within the table 52.

The junction 62 compares the feedback signal generated by the flow sensor 40 and the 42 with a set point signal. In other words, the junction 62 compares the actual flow rate with the set point chosen.

At block 64, if the change in flow rate is less than a predetermined amount (i.e. 2%) the correction delay is tested to determine if the delay period is zero. If not, then no change is made. If the delay period is zero, then a correction algorithm is entered at block 66.

The correction algorithm 66 implements a PI

law wherein the new output value equals the old output value plus a quantity equal to a gain factor multiplied by the difference between the set value and the actual value of fluid flow plus an integral term (i.e. (Output Value) new = (Output Value) old + Gain (Set Value-Actual Value) + I) .

At block 68, the difference between the actual flow rate and the correct flow rate (i.e. the error) is tested to see if it is greater or less than a predetermined tolerance.

At block 70, if the error is greater than the tolerance, an output command (i.e. a second electrical control signal) is output via switch 46 and out onto line 58. Block 70 includes converting the digital value of the output signal to its analogue representation (having a range of 4-20 mA) .

If the error signal is less than the predefined tolerance, a calibration table correction table loop is entered at block 72. At block 72, the error is tested to see if it is within a predetermined shift band of a particular calibration point. If it is, block 74 is immediately entered wherein the calibration table is updated. In this way, a particular calibration point is corrected.

However, if the error is outside the predetermined shift band, interpolation is performed at block 76 and at block 78, the whole table of points is "tilted" around the user settable origin. The correction is typically larger at larger flow rates. The curve which represents the table of

points, consequently, is treated as a first degree polynominal with a changing multiplier: y = kx + constant wherein k is a changing multiplier or "tilter".

Finally, after block 78, the table of points is again updated or corrected at block 74. In this way, the modified calibration data represents the desired flow rate, thereby compensating for varying system operating conditions, such as fluid temperature, fluid viscosity andmechanical/pneumatic changes which may have occurred over time in the system.

After the table is updated at block 74, block 80 indicates to the I/O interface 42 that the set point is reached.

In order that the control system 34 may more quickly adapt to temperature changes of the liquid coating material, a temperature sensor 82 may be provided to sense the temperature of the liquid coating material flowing through the flow regulator

30. Such a temperature sensor 82 typically provides an analog temperature signal as a function of the temperature of the liquid coating material to an A to

D converter 84 contained within the control system

34. In turn, the output of the A to D converter 84 is monitored by the robot controller 34 through the I/O interface 42 along line 78. The output of the A to D converter 84 may also be directly utilized by the control system 34 to modify the set of calibration data as a function of the temperature

signal .

Additionally, a humidity sensor (not shown) can be utilized so that the robot controller 24 can modify its signal to the I/O interface 42 or the feedback signal can be used to directly modify the set of calibration data.

A selector means or liquid set circuit (not shown) which can be either manually set at the control system 34 or automatically set from the robot controller 24 to indicate which gain factor is to be applied by the control system 34 and, consequently, what liquid coating material is being controlled.

The method and system of the present invention provide numerous advantages. For example, once a fluid has been calibrated, anytime the system is used, be it a full automated painting cycle or a manual mode, the calibration data is automatically adjusted as required to achieve the desired flow rate.

Also, the ability to measure the actual flow rate of the liquid coating material allows the control system 34 to accommodate system hysteresis. In other words, the initial open loop set point may be adjusted as required, depending on whether the new command is greater or less than the present liquid coating material flow rate. This results in the actual fluid flow to more accurately approach the desired flow rate with the initial open loop command in spite of any system hysteresis.

Also, the control system 34 is capable of

providing diagnostic and data logging capabilities required to minimize manual labor, material wastage and lost production time. For example, diagnostic information may be provided to indicate when a new flow rate set point has been reached, when an error condition has been detected, what type of error has been detected. Data logging information, for example, the quantity of material applied during a given process is available for enhancing production/process control. This information may be useful, for example, in determining the transfer efficiency of a particular paint process. This capability may prove to be invaluable when used to substantiate transfer efficiency performance as directed by regulating agencies.

Other advantages and features of the method and system of the present invention allow for a manual flow test cycle which may be initiated locally to select and then verify a desired flow rate. Also, the method and system provide the capability to access/modify calibration data, either locally or remotely in a serial fashion.

The method and system are adapted to be configured in a full time closed loop mode, hybrid (part time) closed loop mode, open loop calibrated mode and a transparent (pass through) mode.

Also, the control system allows that the output command be held, for example, during pushout cycle to maintain the desired flow rate until all ^' of the fluid is flushed out of the fluid line. Such a signal may be used during trigger off events to

prevent the control system from increasing the output command during the period of time it needs to determine that the atomizing device 10 is disabled. This may be especially important during trigger- off/on events with no accompanying flow rate change.

Also, the transparent mode may be utilized during fill time to max out the fluid command to minimize fill time. This may be used any time complete fluid control by the robot controller 24 is desired.

Finally, the system may be utilized to maintain the accumulated flow total during events when totalizing is not desired. For example, during a fluid change cycle, this may be used until the totalized value can be accessed serially by the robot controller 24.

The invention has been described in an illustrative manner and it is to be understood that the terminology which is used is intended to be in the nature of words of description rather than of limitation.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example, instead of a robot controller, the process controller may comprise a system controller or cell controller. Also, the control system 34 may be analog or digital in nature and, consequently, the transducer 32 may take analog or digital control signals from the control system 34 to provide corresponding control signals to the flow

regulator 30. Also, the temperature sensor 82 may provide an analog or digital signal, depending on whether the control system is analog or digital in nature.

Finally, the flow sensor 40 may provide any type of flow feedback signal whether digital or analog in nature.

It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.