Method of compensating for changes in the flow characteristics of a dispensed fluid to maintain the volume of dispensed fluid at a setpoint

Abstract

Changes in the flow characteristics of a fluid being dispensed from a nozzle under then the control of a metering valve are compensated for in order to maintain the volume of fluid dispensed over a predetermined time interval substantially equal to a selected setpoint. The volume of fluid delivered to the metering valve during a predetermined interval is measured and a correction factor based on the difference between the measured volume and the setpoint is calculated. The correction factor is used to generate a driving signal from which a control signal applied to the metering valve is generated.

Description

This is a division of application Ser. No. 07/243,238, filed Sep. 7, 1988, now U.S. Pat. No. 4,922,852 which, in turn, is a file wrapper continuation of application Ser. No. 06/924,940, filed Oct. cf52 27 to remain open by an amount just sufficient to maintain the pressure drop across nozzle 29 at that value.

In some dispensing applications, the flow characteristics of the fluid supplied to dispensing gun 10 may be subject to change over time. For example if gun 10 is supplied fluid from a drum, the viscosity of the fluid can vary with changes in temperature as the drum sits in a warm production area after having been moved from a cold warehouse. Viscosity may also vary from one drum of fluid to the next or from the top of a given drum to the bottom. Without some means for compensating for such changes, the amount of material dispensed onto a workpiece 39 would be subject to undesirable variations. Also, when dispensing non-newtonian fluids, the overall instantaneous viscosity of the fluid varies with shear rate in a non-linear fashion. Thus, absent correction, shear induced by the geometry of nozzle 29 will result in a non-linear flow rate versus pressure signal 37 flow characteristic. This in turn would render the flow rate venus applied toolspeed signal 128 response nonlinear. According to the invention, these problems are effectively addressed by deriving driving signal 122 in an alternate fashion as described now with additional reference to FIGS. 3 and 4.

FIG. 3 illustrates a second preferred embodiment of the invention which is similar to the embodiment described above except for the manner in which driving signal 122 is generated. As illustrated in FIG. 3, the system of FIG. 2 is modified by adding a positive displacement flow meter 150 to the fluid supply line connected to the inlet 28 of dispensing gun 10. While it is desirable to locate flow meter 150 as dose to gun 10 as possible it is not required to be mounted with the gun 10 on the robot arm. Flow meter 150 includes an incremental encoder 152 which produces an electrical output signal 153 comprising a series of pulses 155. Each pulse 155 represents a predetermined volume of fluid. Signal 153 is input to a pulse counter 156 which counts pulses 155 and is resettable to zero by a reset signal 158 which is generated by a microprocessor based controller 160 which, if desired may be part of the robot controller (not shown). However, to provide maximum system frequency response, controller 160 should run at high speed and is preferably dedicated principally to performing the operations described below. In addition to a microprocessor and associated hardware, controller 160 includes all necessary program and data memory as well as an analog to digital converter (A/D) 163 which receives the toolspeed signal 128 from the robot controller. Pulse counter 156 outputs its pulse count 165 to controller 160. Controller 160 also receives from the robot controller (not shown), a digital cycle status signal 168 and a digital job status signal 170. Cycle status signal assumes a logical 1 value whenever dispensing gun 10 should be operating. Job status signal 170 assumes a logical 1 valve value when a production run is at an end. Controller 160 also communicates by way of an interface 172 with an input/output device 175 such as a keyboard terminal from which control commands and setpoint data are entered. Controller 160 also communicates by way of an output 176 with a digital to analog D/A converter 177 which generates an analog signal 178. Signal 178 is received by amplifier 127 which operates as described above with reference to FIG. 2. Amplifier 127 in turn generates driving signal 122 which is applied to the plus input 119 of summing junction 113 as described above to generate error signal 130. The manner in which driving signal 122 is derived may be further understood with additional reference now to FIG. 4 which illustrates the software program stored in controller 160 responsible for outputting the required data to D/A converter 177.

The program begins running by clearing all data memory and initializing all variables including a set-point representing a desired total volume of fluid to be applied to a single workpiece 39. An appropriate set of pre-programmed flow linearizing factors (FLFs) are also initialized at this point. The FLF's are constants which represent factors by which toolspeed signal 128 must be multiplied in order to linearize system flow response such that when a given percentage of the full scale value of toolspeed signal 128 is applied to summing junction 113, the needle valve 27 of metering valve assembly 26 is positioned so that the same percentage of the full scale flow of fluid is discharged from nozzle outlet 31. FLF's are determined empirically from a measured curve of actual flow from outlet 31 of nozzle 30 versus voltage applied at input 119 of summing junction 113. Since the actual flow curve may vary depending on the geometry of needle valve 27 and nozzle 29 including nozzle end 30 as well as the flow characteristics of the particular type of fluid being dispensed and the supply pressure, the program loads a series of FLF's appropriate to account for a particular set of these conditions.

The program also sets a flow compensation factor (FCF) to an arbitrarily selected initial value. The FCF is a variable which compensates for changes in the flow characteristics which occur over time such as changes in intrinsic viscosity due to changes in temperature or other factors as discussed earlier. The FCF is recomputed once each job cycle that is, once per dispensing operation on a given workpiece 39. The FCF is defined as a factor by which the linearized toolspeed signal must be multiplied so that the total volume of fluid dispensed onto a workpiece 39 is substantially equal to the selected setpoint. Deviation from setpoint cannot be determined at the beginning of the first job cycle because there is no basis for comparison. Accordingly, FCF is preferably initialized at unity. The manner in which FCF is recomputed will be described below.

During initialization, the program resets pulse counter 156 to zero by outputting an appropriate reset signal 158 from controller 160 to counter 158. Next, the program causes controller 160 to read the total pulse count 165. The value of pulse count 165 represents the total volume of fluid dispensed during the previous job cycle. If pulse count is not zero, as will be the case except prior to the first job cycle, the program recomputes the flow compensation factor FCF as a quotient whose dividend is equal to the setpoint and whose divisor is equal to total pulse count 165. After the FCF is recomputed counter 156 is again reset in the manner described above. If pulse count 165 is equal to zero, as it will be at the beginning of the first job cycle, the FCF remains at its initialized valvevalue.

Next, the program enters a loop in which it waits for the robot controller signal that a job cycle is in progress. In the wait loop, the program continuously reads cycle status signal 168 and tests to determine whether it has assumed a logical 1 value. If not, the program stays in the loop. By changing status signal 168 from a logical zero value to a logical 1 value, the robot controller indicates that dispensing should commence. At that point, the program directs controller 160 to read the digital value 180 representing the magnitude of toolspeed signal 128 from the output of A/D converter 163. Based on the magnitude of the digital value, the program selects from a look-up table the corresponding flow linearizing factor FLF from the set of FLF values loaded during initialization. Digital value 180 is then multiplied by the selected FLF value to yield a linearized toolspeed value 181. To adjust driving signal 122 so that the actual volume of fluid to be dispensed during the job cycle conforms to the setpoint despite changes in the flow characteristics of the fluid, such as changes in viscosity, the program next causes the linearized toolspeed value 181 to be multiplied by the flow compensation factor FCF to yield a corrected digital value 182 which is then output to D/A converter 177 whose output 178 is fed to amplifier 127 to generate driving signal 122.

Next, the program again reads cycle status signal 168 to determine whether dispensing should continue. If not, job status signal 168 will not be a logical 1 value, indicating the present cycle has ended. In that case the program causes controller 160 to read job status signal 170 emanating from the robot controller. If job status signal 170 is not a logical 1 value, this indicates that the last workpiece 39 in a given production lot has been finished and the program is stopped. If the production run is not complete, job status signal 170 will remain at a logical 1 value and the program will loop back to the point at which pulse count 165 is read. Although the program described recomputes a flow compensation factor once per job cycle, it should be noted that such periodic adjustments can be made more or less frequently depending on how rapidly the flow characteristics of the dispensed fluid can be expected to undergo significant change.

The advantages realized by the invention are numerous. Most notably, the dispensing systems described provide rapid and precise control of fluid flow rate. Such systems have been found to have an upper 3 dB frequency response cutoff point exceeding 10 hertz.

While the dispensing gun 10 can be directed by any desired means including manually the invention is particularly well adapted for use with robots. Dispensing gun 10 is light weight, compact and easy to maintain. Further, the dispensing systems of the invention provide for automatic flow rate adjustment in accordance with the relative speed between the dispensing gun 10 on the robot arm and the workpiece.

Thus, the invention permits close control over the volume per unit length of the dispensed bead of fluid even during rapid acceleration and deceleration as normally occurs as the robot arm changes its direction of movement. The invention also provides means for periodically compensating for perturbations in the flow characteristic of the fluid being dispensed to insure that the volume of fluid dispensed always conforms closely with a desired setpoint.

While the above descriptions constitute preferred embodiments of the apparatus and method of the invention, it is to be understood that the invention is not limited thereby and that in light of the present disclosure of the invention various alternative embodiments will be apparent to persons skilled in the art. Accordingly, it is to be understood that changes can be made to the embodiments described without departing from the full legal scope of the invention which is particularly pointed out and distinctly claimed in the claims set forth below.

Claims

1. A method of compensating for changes in the flow characteristics of a fluid being dispensed from a nozzle under the control of a metering valve in order to maintain the volume of fluid dispensed over a predetermined time interval at a desired setpoint, said method comprising the steps of:

(a) measuring the volume of fluid delivered to the metering valve during at least one said interval;

(b) calculating a correction factor correlated to the difference between said measured volume and said setpoint,

(c) multiplying a signal by said factor to generate a driving signal, and

(d) controlling said valve in accordance with at least said driving signal to maintain the volume of fluid dispensed at said desired setpoint.

2. The method of claim 1 wherein said controlling step includes the step of applying a signal correlated to said driving signal to a closed-loop feedback system coupled to said metering valve.

3. The method of claim 1 wherein said correction factor comprises a quotient whose dividend is said setpoint and whose divisor is said measured volume of fluid.

4. A method for compensating for changes in the flow characteristics of a fluid being dispensed from a nozzle, said method comprising the steps of:

(a) delivering the fluid under pressure to a metering valve located upstream of the nozzle, said metering valve being operable to modulate the flow of fluid to the nozzle in response to a control signal;

(b) measuring the volume of fluid delivered to said metering valve over an interval of time and generating a corresponding measurement signal, and

(c) adjusting the control signal in accordance with the difference between said measurement signal and a setpoint representing a desired volume of fluid to be dispensed during said interval so that said valve maintains the volume of fluid dispensed at said setpoint.

5. The method of claim 4 wherein said adjusting step comprises the steps of:

calculating a correction factor correlated to the difference between said measurement signal and said setpoint;

multiplying a driving signal by said correction factor, and

generating Said control signal from at least said driving signal.

6. The method of claim 5 wherein said generating step comprises the step of algebraically combining the difference between said driving signal with a signal correlated to the flow rate of the fluid dispensed from the nozzle.

7. The method of claim 6 further comprising the step of:

generating said driving signal in accordance with at least a toolspeed signal of a robot for effecting relative movement between the nozzle and a workpiece.

8. The step of claim 7 wherein said signal correlated to the flow rate of the fluid dispensed from the nozzle comprises a signal representing the pressure drop across said nozzle.

9. The method of claim 4 wherein said interval is a job cycle.

10. The method of claim 4 further comprising the steps of:

locating said valve and said nozzle in sufficiently close proximity to one another that very little fluid pressure drop occurs between said valve and said nozzle;

sensing, at a location between said valve and said nozzle, a parameter correlated to the rate of flow of the fluid discharged from the nozzle and generating a corresponding flow rate signal, and

generating said control signal from at least said flow rate signal and a driving signal.

11. The method of claim 10 further comprising the steps of:

calculating a correction factor correlated to the difference between said measurement signal and said setpoint;

multiplying a driving signal by said correction factor, and

generating said control signal from at least said driving signal.

12. A method of compensating for dynamic flow characteristics of a non-newtonian fluid being dispensed from a nozzle onto a workpiece, the nozzle being in fluid communication with a metering valve responsive to a control signal, and the dynamic flow characteristics representing flow non-linearities introduced by the non-newtonian fluid, said method comprising the steps of:

generating the control signal to produce a desired flow of the fluid through the nozzle, said control signal being correlated to at least a flow rate of the non-newtonian fluid; and

linearizing said control signal to reduce the flow non-linearities introduced by the non-newtonian fluid.

13. The method of claim 12 wherein said step of linearizing said control signal further comprises the steps of:

determining flow linearizing factors based on known flows of the non-newtonian fluid from the nozzle as a function of the control signal for a given set of conditions;

selecting a first flow linearizing factor;

altering said control signal as a function of said first flow linearizing factor. 14. The method of claim 13 wherein the step of determining said flow linearizing factors further comprises the step of determining a series of flow linearizing factors based on known flows of the non-newtonian fluid from the nozzle as a function of said control

signal for given sets of conditions. 15. The method of claim 14 wherein said step of generating said control signal further comprises the steps of:

providing a tool speed signal correlated to relative motion between the nozzle and the workpiece;

selecting said first flow linearizing factor as a function of the tool speed signal; and

multiplying said tool speed signal by said first flow linearizing factor to produce a linearized tool speed value. 16. The method of claim 15 wherein the step of generating said control signal further comprises the steps of:

producing a feedback signal representing a fluid pressure correlated to a flow rate of the non-newtonian fluid; and

producing said control signal as a function of said feedback signal and said linearized tool speed value, thereby compensating said control signal as a function of the flow non-linearities introduced by the non-newtonian fluid and causing the metering valve to dispense the desired flow of the

non-newtonian fluid. 17. A method of compensating for dynamic flow characteristics and intrinsic viscosity changes of a fluid being dispensed from a nozzle onto a workpiece, the nozzle being in fluid communication with a metering valve responsive to a control signal, and wherein the intrinsic viscosity changes are caused by phenomena other than shear effects, and the dynamic flow characteristics representing pressure flow non-linearities introduced by non-newtonian viscosity characteristics in the fluid, the method comprising the steps of:

generating the control signal to provide a desired flow of the fluid through the nozzle, said control signal being correlated to at least a flow rate of the fluid; and

modifying said control signal to reduce the pressure flow non-linearities introduced by the dynamic flow characteristics and to compensate for the

intrinsic viscosity changes of the fluid. 18. The method of claim 17 wherein the step of modifying said control signal further comprises the steps of:

providing a tool speed signal correlated to relative motion between the nozzle and the workpiece;

altering said tool speed signal as a function of the dynamic flow characteristics of the fluid to produce a linearized tool speed value; and

adjusting said linearized tool speed value as a function of the intrinsic viscosity changes of the fluid to produce a driving signal. 19. The method of claim 18 wherein said step of generating the control signal further comprises the steps of:

producing a feedback signal representing a fluid pressure correlated to a flow rate of the fluid; and

producing said control signal as a function of said feedback signal, said driving signal and effects of the dynamic flow characteristics and the intrinsic viscosity changes thereby causing the metering valve to dispense

the desired flow of fluid. 20. The method of claim 18 wherein said step of adjusting said tool speed signal further comprises the steps of:

selecting a flow linearizing factor based on a known flow of fluid from the nozzle as a function of said control signal for a given set of conditions.; and

multiplying said tool speed signal by said flow linearizing factor to produce said linearized tool speed value. 21. The method of claim 20 wherein said step of adjusting said linearized tool speed value further comprises the steps of:

measuring a first volume of fluid delivered to the metering valve during an interval of time;

calculating a flow compensation factor as a function of a difference between the first volume of fluid and a reference; and

multiplying said linearized tool speed value by said flow compensation

factor to produce said driving signal. 22. A method of compensating for intrinsic viscosity changes of a fluid being dispensed from a nozzle onto a workpiece, the nozzle in fluid communication with a metering valve responsive to a control signal, and wherein the intrinsic viscosity changes are caused by phenomena other than shear effects, said method comprising the steps of:

supplying the fluid under a pressure to the metering valve;

providing a tool speed signal representing a varying relative speed between the nozzle and the workpiece;

adjusting said tool speed signal as a function of the intrinsic viscosity changes of the fluid to produce a driving signal;

producing a feedback signal representing a fluid pressure correlated to a flow rate of the fluid; and

producing the control signal as a function of said feedback signal and said driving signal to cause the metering valve to dispense a desired flow of

fluid. 23. The method of claim 22 wherein said step of adjusting the tool speed signal further comprises the steps of:

measuring a first volume of fluid delivered to the metering valve during an interval of time;

calculating a flow compensation factor as a function of a difference between the first volume of fluid and a reference; and

adjusting said tool speed signal as a function of said flow compensation factor. 24. The method of claim 23 wherein said step of adjusting the tool speed signal further comprises the step of multiplying said tool speed signal by said flow compensation factor.

. A method of compensating for effects of pressure flow non-linearities of a fluid being dispensed from a nozzle onto a workpiece, the nozzle in fluid communication with a metering valve responsive to a control signal, said method comprising the steps of:

supplying the fluid under a pressure to the metering valve;

providing a tool speed signal representing a relative speed between the nozzle and the workpiece;

determining a flow factor as a function of the effects of the pressure flow non-linearities of the fluid;

adjusting said tool speed signal as a function of the flow factor to produce a driving signal;

producing a feedback signal representing a fluid pressure correlated to a flow rate of the fluid; and

producing the control signal as a function of said feedback signal and said driving signal to cause the metering valve to dispense a desired flow of fluid.