U.S. patent application number 10/960130 was filed with the patent office on 2005-04-07 for pwm voltage clamp for driver circuit of an electric fluid dispensing gun and method.
This patent application is currently assigned to Nordson Corporation. Invention is credited to Near, Timothy P..
Application Number | 20050072949 10/960130 |
Document ID | / |
Family ID | 26936588 |
Filed Date | 2005-04-07 |
United States Patent
Application |
20050072949 |
Kind Code |
A1 |
Near, Timothy P. |
April 7, 2005 |
PWM voltage clamp for driver circuit of an electric fluid
dispensing gun and method
Abstract
An electric fluid dispenser for dispensing a fluid onto a
substrate. A power switching circuit is connected to an unregulated
power supply providing a varying voltage. A solenoid connected to
the power switching circuit operates a dispensing valve to move
between open and closed positions. A control circuit is responsive
to the varying voltage from the power supply and provides a drive
signal to the power switching circuit having a time variable
component determined by the varying voltage. The power switching
circuit, in response to the drive signal, provides an output signal
to the solenoid that causes the dispensing valve to move between
the open and closed positions substantially independent of the
varying voltage from the unregulated power supply.
Inventors: |
Near, Timothy P.;
(Alpharetta, GA) |
Correspondence
Address: |
WOOD, HERRON & EVANS, LLP (NORDSON)
2700 CAREW TOWER
441 VINE STREET
CINCINNATI
OH
45202
US
|
Assignee: |
Nordson Corporation
Westlake
OH
|
Family ID: |
26936588 |
Appl. No.: |
10/960130 |
Filed: |
October 7, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10960130 |
Oct 7, 2004 |
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09880649 |
Jun 13, 2001 |
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60244512 |
Oct 31, 2000 |
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Current U.S.
Class: |
251/129.05 |
Current CPC
Class: |
B05C 11/1026 20130101;
F16K 31/0675 20130101 |
Class at
Publication: |
251/129.05 |
International
Class: |
B67D 003/00 |
Claims
What is claimed is:
1. A driver circuit for an electrically operated fluid dispenser
dispensing a fluid onto a substrate, the fluid dispenser having a
dispensing valve movable between open and closed positions and a
solenoid coil operative to cause the dispensing valve to move
between the open and closed positions, the driver circuit
comprising: a power switching circuit operably connected to the
solenoid coil; a power source providing an unregulated voltage to
said power switching circuit; and a power switch control operable
to cause said power switching circuit to provide a stepped current
waveform to the solenoid coil, said stepped waveform comprising an
initial peak current followed by a lesser hold current, said
initial peak current having a rate of current flow represented by a
slope of a leading edge of said initial peak current, said power
switch control operable to maintain the slope of the leading edge
of said initial peak current substantially constant in response to
changes in said unregulated output voltage.
2. The driver circuit of claim 1 wherein said power switch control
comprises a line voltage compensator responsive to said unregulated
output voltage and being operable to cause said power switch
control to maintain the slope of the leading edge of said initial
peak current substantially constant in response to said unregulated
output voltage changing in magnitude.
3. The driver circuit of claim 2 wherein said line voltage
compensator causes the power switching circuit to provide a first
slope of a leading edge of said initial peak current corresponding
to a desired magnitude of said unregulated output voltage.
4. The driver circuit of claim 3 wherein said line voltage
compensator maintains the first slope of the leading edge of said
initial peak current substantially constant in response to said
unregulated output voltage deviating from said desired
magnitude.
5. The driver circuit of claim 3 wherein said first slope of the
leading edge of said initial peak current corresponds to a lowest
expected magnitude of the unregulated output voltage.
6. The driver circuit of claim 2 wherein said line voltage
compensator is operable to cause said power switch control to
modulate the leading edge of said initial peak current with a duty
cycle determined as an inverse function of the unregulated output
voltage, thereby maintaining a time required to move said valve to
an open position substantially constant.
7. The driver circuit of claim I wherein said hold current provides
a rate of current flow represented by a slope of a trailing edge of
said hold current, the slope of the trailing edge being produced in
response to said unregulated output voltage, said line voltage
compensator being operable to cause said power switch control to
modulate the trailing edge of said hold current with a duty cycle
determined as an inverse function of the unregulated output
voltage, thereby maintaining a time required to move said valve to
a closed position substantially constant.
8. The driver circuit of claim 2 wherein said power switch control
further comprises: a waveform generator producing a stepped
waveform representative of said initial peak current followed by
said lesser hold current; a current sensor providing a current
feedback signal representing current flow in the solenoid coil; a
summing node responsive to said stepped waveform from said waveform
generator and said current feedback signal; a hysteresis modulator
connected to an output of said summing node; a pulse width
modulator; and a first logic circuit having inputs connected to an
output of said hysteresis modulator and an output of said line
voltage compensator and causing the leading edge of said initial
peak current to be modulated with a duty cycle determined as an
inverse function of the unregulated output voltage.
9. The driver circuit claim 8 wherein the power switch control
further comprises a second logic circuit having an input connected
to said output of said line voltage compensator and causing the
trailing edge of said hold current to be modulated with a duty
cycle determined as an inverse function of the unregulated output
voltage.
10. The driver circuit of claim 2 wherein said line voltage
compensator comprises a pulse width modulator.
11. The driver circuit of claim 10 wherein said pulse width
modulator is a fixed frequency pulse width modulator.
12. The driver circuit of claim I further comprising a system
control providing a trigger signal to said waveform generator for
initiating a generation of said stepped waveform.
13. A driver circuit for an electrically operated fluid dispenser
dispensing a fluid onto a substrate, the fluid dispenser having a
dispensing valve movable between open and closed positions and a
solenoid coil operative to cause the dispensing valve to move
between the open and closed positions, the driver circuit
comprising: switching means for alternately supplying and
interrupting a flow of current to the solenoid coil; means for
providing an unregulated voltage to said switching means; and means
for providing a stepped current waveform to said switching means,
said stepped waveform comprising an initial peak current followed
by a lesser hold current, said initial peak current having a rate
of current flow represented by a slope of a leading edge of said
initial peak current, said providing means operable to maintain the
slope of the leading edge of said initial peak current
substantially constant in response to changes in said unregulated
output voltage.
14. A method of operating a driver circuit for an electric fluid
dispenser operable to dispense a fluid onto a substrate, the fluid
dispenser having a dispensing valve operatively connected to a
solenoid coil, the solenoid coil being operative to move the
dispensing valve between open and closed positions for controlling
a flow of the fluid from the electric fluid dispenser, the method
comprising: providing a power switching circuit connected to the
solenoid coil; providing a power source supplying an unregulated
output voltage to the power switching circuit; producing with the
power switching circuit a stepped current waveform having an
initial peak current followed by a hold current, the initial peak
current providing a rate of current flow represented by a slope of
a leading edge of the initial peak current, the slope of the
leading edge being produced in response to the unregulated output
voltage; maintaining the slope of the leading edge of the initial
peak current substantially constant in response to the unregulated
output voltage changing in magnitude; and applying the stepped
current waveform to the solenoid coil to operate the solenoid coil
and the dispensing valve with an operational speed substantially
independent of changes in the unregulated output voltage.
15. The method of claim 13 further comprising providing a slope of
a leading edge of said initial peak current corresponding to a
desired magnitude of the unregulated output voltage.
16. The method of claim 14 further comprising maintaining the slope
of the leading edge of said initial peak current substantially
constant in response to the unregulated output voltage deviating
from said desired magnitude.
17. The method of claim 15 wherein said desired magnitude of the
unregulated output voltage is a lowest expected magnitude of the
unregulated output voltage.
18. The method of claim 13 further comprising: modulating the
leading edge of the initial peak current with a duty cycle
determined as an inverse function of the unregulated output voltage
to maintain the slope of the leading edge of the initial peak
current substantially constant; and applying the initial peak
current to the solenoid coil to maintain the actuation time to open
the dispensing valve substantially constant and independent of
changes in the unregulated output voltage.
19. The method of claim 13 wherein the hold current provides a rate
of current flow represented by a slope of a trailing edge of the
hold current, the slope of the trailing edge being produced in
response to the unregulated output voltage, the method further
comprising: maintaining the slope of the trailing edge of the hold
current substantially constant in response to the unregulated
output voltage changing in magnitude; and applying the hold current
to the solenoid coil to operate the solenoid coil and the
dispensing valve with an actuation time to close the dispensing
valve substantially independent of changes in the unregulated
output voltage.
20. The method of claim 18 further comprising modulating the
trailing edge of the hold current with a duty cycle determined as
an inverse function of the unregulated output voltage.
Description
[0001] This application is a Divisional of U.S. application Ser.
No. 09/880,649, filed on Jun. 13, 2001, which claims the benefit of
U.S. Provisional Application No. 60/244,512, filed on Oct. 31,
2000.
FIELD OF THE INVENTION
[0002] The present invention relates generally to fluid dispensing
systems for dispensing flowable material, such as adhesives,
sealants, caulks and the like, onto a substrate and, more
particularly, to a driver circuit for controlling an operation of a
solenoid-actuated valve within an electric dispensing gun.
BACKGROUND OF THE INVENTION
[0003] Electric fluid dispensers have been developed for dispensing
applications requiring a precise placement of a fluid, for example,
an adhesive, onto a moving substrate, for example, packaging or a
woven product. Dispensing guns of this type include a liquid
passage that communicates between a pressurized adhesive supply and
a valve mechanism provided at the end of the liquid passage. The
valve mechanism is typically a movable valve stem positioned to
selectively obstruct a dispensing orifice formed in a valve seat.
The valve stem is extended and retracted relative to the valve seat
in a controlled manner by a solenoid for providing repeatable and
accurate dispense patterns of the liquid onto the moving substrate.
Generally, the solenoid comprises an electromagnetic coil
surrounding an armature that is energized to produce an
electromagnetic field with respect to a magnetic pole, thereby
moving the valve stem. More specifically, the forces of magnetic
attraction between the armature and the magnetic pole move the
armature and valve stem toward the pole, thereby opening the
dispensing valve. At the end of a dispensing cycle, the
electromagnet is de-energized, and a return spring returns the
armature and valve stem to their original positions, thereby
closing the dispensing valve.
[0004] Dispensing systems have been developed that employ driver
circuits to control the operation of the solenoid within the
dispensing gun. To open the valve, the driver circuit applies a
fast pull-in current to the solenoid coil to quickly retract the
valve stem and open the dispensing orifice at the beginning of a
dispensing cycle. The driver circuit maintains a minimal holding
current which holds the valve stem in an open position while
minimizing the amount of heat build-up in the solenoid coil during
the dispensing cycle. Finally, the driver circuit provides a fast
demagnetization of the solenoid so the valve stem is quickly closed
over the orifice at the end of the dispensing cycle.
[0005] Closing of the valve stem is often achieved by a spring
mechanism connected to one end of the valve stem. When the solenoid
is sufficiently demagnetized, the stored energy in the compressed
spring mechanism forces the valve stem to the closed position and
in sealing engagement with the dispensing orifice. One example of
such a dispensing system is set forth in U.S. Pat. No. 5,812,355,
owned by the assignee of the present invention, the disclosure of
which is incorporated herein by reference in its entirety.
[0006] In unregulated gun drivers, current to the electric gun coil
is supplied by a power switching circuit that is connected to an
unregulated power supply. Thus, any variations in line voltage
changes the output voltage from the power supply which is applied
to the power switching circuit. Changing the voltage applied to the
power switching circuit results in a corresponding variation in the
current being supplied to the gun solenoid. The operational speed
of the solenoid is directly related to the magnitude of the applied
voltage; and therefore, as the magnitude of the applied voltage
goes up, the armature and valve stem move faster. Similarly, as the
magnitude of the applied voltage goes down, the armature and valve
stem move slower. Thus, the operational speed of the armature and
valve stem is related to the magnitude of the voltage applied to
the coil and hence, the actuation time or time required to open and
close the electric gun is changed by variations in line voltage
applied to the unregulated power supply.
[0007] Uncontrolled and unpredictable variations in the actuation
time of the dispensing gun adversely impact the adhesive deposition
process. Line voltage variations changing the actuation time of the
dispensing gun also change the starting and stopping locations of
the dispensed adhesive on the substrate. If adhesive is to be
dispensed on a package flap moving past the dispensing gun, an
increase in line voltage causing the gun to switch-on or open
faster than expected may cause adhesive to be dispensed too soon.
Opening the gun too soon may cause adhesive to be dispensed prior
to a leading edge of the flap reaching the dispensing location.
Similarly, a decrease in line voltage causes the gun to switch-off
or close slower than expected. This slower gun operation may cause
adhesive to continue to be dispensed after a trailing edge of the
flap passes the dispensing location. Any unpredicted dispensing of
adhesive onto a surface not intended to receive adhesive,
potentially results in a scrap product. In addition, spurious
adhesive spray that misses the substrate may lead to additional,
time consuming, labor intensive and expensive cleaning and
maintenance of equipment and areas adjacent the adhesive dispensing
gun. Thus, such line voltage variations may result in a less
efficient, less economical and/or lower quality fluid dispensing
operation.
[0008] It is known to use a regulated gun driver, that is, a gun
driver with a regulated power supply. A regulated gun driver
provides a constant voltage to the coil independent of the voltage
variations to the power switching circuit. Thus, with respect to
line voltage variations, the use of a regulated gun driver provides
a more consistent dispensing gun performance. However, regulated
gun drivers are more expensive than unregulated gun drivers and
create more heat which requires more cooling and thus, further adds
to their cost.
[0009] Therefore, there is a need to provide an electric fluid
dispenser that uses an unregulated solenoid gun driver that is
insensitive to variations in the applied line voltage.
SUMMARY OF INVENTION
[0010] The present invention provides an unregulated gun driver for
a fluid dispenser that has an improved performance. The gun driver
of the present invention executes a stable, consistent and high
quality fluid dispensing process independent of line voltage
variations. Further, the gun driver of the present invention has
the advantages of being less expensive, operating more efficiently
with less power loss and requiring less cooling than regulated gun
drivers. In addition, the unregulated gun driver of the present
invention can be readily added to many existing gun driver
circuits. Thus, the fluid dispenser of the present invention is
especially advantageous in those applications where better
performance is required at a lesser cost.
[0011] In accordance with the principles of the present invention
and the described embodiments, the invention in one embodiment
provides an electric fluid dispenser for dispensing a fluid onto a
substrate. The dispenser has a dispensing valve movable between
open and closed positions for controlling a flow of the fluid from
said fluid dispenser. The dispenser further has a source of power
providing a nonconstant voltage to a power switching circuit. A
solenoid is connected to the power switching circuit and is capable
of causing the dispensing valve to move between the open and closed
positions. A control circuit is responsive to the nonconstant
voltage and provides a drive signal having a time variable
component determined by the nonconstant voltage from the source of
power. The power switching circuit, in response to the control
signal, provides an output signal to the solenoid causing said
dispensing valve to move between the open and closed positions
substantially independent of variations in the nonconstant
voltage.
[0012] In one aspect of the invention, the control circuit provides
an initial peak current followed by a hold current to energize said
solenoid, and the control circuit provides the initial peak with an
initial duty cycle varying as an inverse function of the variations
of the nonconstant voltage from the power source.
[0013] In another embodiment of the invention, a method is provided
for operating an electrically operated fluid dispenser for
dispensing a fluid onto a substrate. The fluid dispenser has a
dispensing valve operatively connected to an electrically operated
solenoid, and the dispensing valve is movable between open and
closed positions for controlling a flow of the fluid from the fluid
dispenser. A power switching circuit is connected to a power source
supplying a varying voltage. A drive signal is produced having a
time variable component determined as a function of the varying
voltage of the power source, and the drive signal is applied to the
power switching circuit to operate the solenoid and dispensing
valve substantially independently of the varying voltage of the
power source.
[0014] In one aspect of this invention, the drive signal has an
initial peak current followed by a hold current, and the method
further comprises modulating a leading edge of the initial peak
current at a duty cycle determined as an inverse function of the
varying voltage of the power source.
[0015] Various additional advantages, objects and features of the
invention will become more readily apparent to those of ordinary
skill in the art upon consideration of the following detailed
description of embodiments taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is an schematic block diagram of a dispensing gun
driver for an electric fluid dispenser in accordance with the
principles of the present invention.
[0017] FIG. 2A is a schematic diagram of a current waveform
provided by a prior art driver circuit that does not use the gun
driver of FIG. 1.
[0018] FIG. 2B is a schematic diagram of a current waveform
provided by the gun driver of FIG. 3.
[0019] FIG. 3 is a schematic block diagram of a specific gun driver
that may be used to operate a fluid dispenser in accordance with
the principles of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0020] FIG. 1 illustrates an unregulated gun driver or controller
10 for an electric fluid dispensing gun normally used to dispense
adhesive onto a substrate moving with respect to the gun. As
previously discussed, electric guns are preferred because of the
precision with which they may be controlled during a fluid
dispensing operation. The gun driver 10 has a control circuit 11
operating in response to signals from a system control 12 to
provide a stepped waveform to a power circuit 13. The system
control 12 includes all of the other known dispensing system or
machine controls necessary for the operation of the dispensing
system, for example, a pattern control. The system control 12 also
includes input devices such as a keypad, pushbuttons, etc. and
output devices such as a display, indicator lights, etc. that
provide communication links with a user in a known manner. Within
the power circuit 13, a power switch circuit 18 is connected to an
unregulated power supply 19 and provides a stepped current waveform
to a solenoid coil 14 within an electric dispensing gun 15.
[0021] To turn the gun on, the system control 12 provides a trigger
signal to a waveform generator 16. The waveform generator 16
initially sets the duty cycle of a PWM 17 to a high level, for
example, 100%, thereby operating a power switch circuit 18 at an
equally high rate. The power switch circuit 18 is connected to an
unregulated power supply 19 and applies an initial pull-in current
magnitude I.sub.pk (FIG. 2A) to the coil 14. Upon reaching the
desired current setpoint or value determined by the waveform
generator 16, the PWM 17 operates with a lesser duty cycle, for
example, 60%, to maintain the current at the desired peak current
value I.sub.pk.
[0022] A current sensor 20 is often used to provide a current
feedback signal to a summing junction 72 so that the current in the
coil 14 is maintained at the desired setpoint value provided by the
waveform generator 16. An initial peak current pulse 200 (FIG. 2A)
is maintained for a duration T.sub.pk as determined by the waveform
generator 16. The large initial peak current I.sub.pk is effective
to quickly open the dispensing gun 15.
[0023] After opening the dispensing gun 15, the gun driver 10
supplies a current necessary to hold the dispensing gun 15 open by
overcoming the opposing force of a return spring (not shown). After
the peak current duration T.sub.pk, the waveform generator 16
substantially reduces the duty cycle of operation of the PWM 17,
for example, to 20%. Reducing the duty cycle of the PWM 17 also
reduces the duty cycle of the power switch 18. The reduced duty
cycle causes the power switch circuit 18 to apply a lesser, hold
current magnitude I.sub.h 202 (FIG. 2A) to the coil 14 for the
remaining period of the dispensing cycle on-time T.sub.on. At the
end of the dispensing cycle as determined by a pattern control (not
shown) within the system control 12, the dispensing gun 15 is
turned off or closed. In many dispensing guns, current to the coil
is reduced to zero or a minimal valve; and current induced in the
coil from the collapsing inductive field is dissipated. A return
spring moves an armature and valve stem in an opposite direction to
close the dispensing gun. A zero or minimal current is then
maintained for an off time during the remaining time of the current
waveform period.
[0024] As will be appreciated, the waveforms illustrated in FIG. 2
are for purposes of discussion; and the real waveform consists of
exponential functions that transition the current between levels.
The real time, on time wave shape can look radically different from
the idealized waveforms of FIG. 2, depending on many factors such
as I.sub.pk, I.sub.h, T.sub.pk, T.sub.on, T.sub.p, L.sub.coil14,
R.sub.coil14, etc. The T.sub.on is related to the adhesive pattern
required for a particular product. The inductance and resistance of
the coil are a function of the gun itself, and the I.sub.pk is
normally bounded by various considerations of the fluid dispenser
15 such as magnetic saturation, thermal considerations or force
requirements.
[0025] Initial values of magnitudes of the peak and hold currents
are based on the coil specifications. However, the peak current
magnitude I.sub.pk, the magnitude of the hold current I.sub.h and
the duration of the peak current T.sub.pk are often adjustable by
the user. The user may adjust the current waveform and the
dispensing line rate in order to tune the dispensing operation to
its peak performance.
[0026] While the gun driver 10 of FIG. 1 performs well, there is
one condition which impairs its performance. The gun driver 10 is
designed to provide a desired opening time of the dispensing gun 15
for a given output voltage, for example, 240 V.sub.DC, from the
power supply 19. The rate of current flow through the coil 14 is a
function of the power supply voltage and the inductance of the coil
14. Thus, with the power supply voltage of 240 V.sub.DC and a
constant coil inductance, the rate of current flow through the
solenoid coil 14 is represented by the slope 208 of FIG. 2A.
Further, by design, the slope 208 provides a current flow to the
solenoid coil 14 so that the dispensing gun 15 opens at a desired
speed or within a desired time duration to dispense adhesive onto
the substrate at a desired location.
[0027] However, in many environments, the magnitude of the line
voltage from the source 21 varies, thereby adversely affecting the
actuation time of the dispensing gun 15. Assume that the line
voltage from the supply 21 rises, thereby increasing the output
voltage from the power supply 19, for example, to 300 V.sub.DC. The
increased power supply voltage of 300 V.sub.DC increases the rate
at which current flows to the solenoid coil as shown in phantom by
the slope 210 of FIG. 2A. Increasing the rate at which current is
supplied to the solenoid coil 14 causes the dispensing gun 15 to
open more quickly than with 240 V.sub.DC. Opening the dispensing
gun more quickly, or sooner than desired, causes adhesive to be
dispensed earlier than anticipated; and hence, adhesive is
dispensed onto surfaces not intended to receive adhesive. The
improper placement of adhesive often produces scrap product and
other problems.
[0028] To ameliorate that problem, the control circuit 11 of FIG. 1
has a duty cycle control 22 that functions, at appropriate times,
to clamp the duty cycle of the PWM 17 at a desired value. That
action makes the operation of the power switch circuit 18
independent of changes in the magnitude of the line voltage of the
supply 21 and the unregulated output voltage from the power supply
19. In the example described above, one can observe that it is
desirable that the rise in the initial current pulse have a
constant slope 208 independent of variations in the power supply
voltage V.sub.DC. One method of stabilizing the rate of current
flow at the beginning of the peak current pulse is to reduce the
magnitude of the effective voltage applied to the solenoid coil 14
from the power switch circuit 18 as a function of the increase in
power supply voltage. The effective voltage is the product of the
power supply voltage V.sub.DC and the duty cycle. Limiting the duty
cycle in the coil 14 in inverse proportion to increases in the
power supply voltage causes the effective voltage applied to the
solenoid coil 14 to be held to a constant value. Holding the
effective voltage constant provides a slope substantially
approximating the slope 208 in FIG. 2A.
[0029] Referring to the above example, the slope 208 is a result of
the power switch circuit 18 operating at a 100% duty cycle with a
power supply voltage of 240 V.sub.DC. If the power supply voltage
increases to 300 V.sub.DC, without any other action, current will
flow to the solenoid coil 14 at a rate indicated by the slope 210,
thereby causing the dispensing gun to switch faster. However, if
the duty cycle of the PWM 17 is proportionally reduced and clamped
to a lower value, for example, 80%, the current that is supplied by
the power switch circuit 18 is limited. The current slope provided
by an 80% duty cycle is represented by the slope 212 of FIG. 2B.
Hence, the current slope 212 in the coil 14 at 300 V.sub.DC
substantially approximates the current slope 208 of FIG. 2A
supplied to the coil at 240 V.sub.DC; and the operational speed and
actuation time of the dispensing gun 15 is unchanged. Therefore,
the time required to open the dispensing gun is maintained constant
independent of variations in the line voltage of the supply 21 and
the resulting variations in the output voltage V.sub.DC from the
power supply 19. Thus, problems resulting from variations in the
line voltage are eliminated.
[0030] A specific embodiment of a gun driver is illustrated in FIG.
3, and the common numbers in FIGS. 1 and 3 refer to items that are
similar in function. The gun driver or driver circuit 10 includes a
control circuit 11 and a power circuit 13 for controlling operation
of one or more electric dispensing guns of the type used to
dispense adhesives, sealants, caulking and the like, represented
diagrammatically at 15. The power circuit 13 includes an
unregulated power supply 19 that is connected to a voltage source
21 providing a line voltage.
[0031] The electric dispensing gun 15 includes a solenoid 23 having
a movable armature 24 to regulate the flow of liquid through the
gun 15. The armature 24 is usually biased by a spring mechanism 25
that is connected between one end of the armature 24 and a fixed
reference 26. The armature 24 is connected to a plunger or valve
stem 27 that operatively cooperates with an orifice 28 to form a
dispensing valve 31 within the electric dispensing gun 15.
Retracting the armature 24 against the force of the spring
mechanism 25 opens the dispensing valve 31; and pressurized
adhesive flows through the orifice 28 onto a substrate 29. As is
well known in the art, the armature 24 is actuated by application
of current through the solenoid coil 14. The coil 14 has electrical
properties modeled as a resistance in series with an inductance.
The ends of the coil 14 terminate at first and second terminals 34,
36 that are selectively coupled to the power supply 19 as described
in detail below.
[0032] The unregulated power supply 19 is connected to a source of
power 21. The power supply 19 has an AC to DC converter 38 that is
lowpass filtered by a capacitor 40 coupled across a positive output
42 and a negative output 44. The power supply outputs 42, 44 are
connected to the first and second terminals 34, 36 of the solenoid
23 by switches 48, 50 as described in detail below. The switches
48, 50 may be insulated gate bipolar transistors (IGBT), although
equivalent switches are contemplated.
[0033] A forward current path through the solenoid coil 14 is
generated when the first switch 48 is closed connecting the first
terminal 34 to the positive output 42 and the second switch 50 is
closed connecting the second terminal 36 to the negative output 44.
A discharge current path through the solenoid coil 14 is generated
when the first and second switches 48, 50 are open, thereby
connecting the second terminal 36 to the positive output 42 via a
diode 54 and connecting the first terminal 34 to the negative
output 44 via a diode 56. The current sensor 20 is coupled between
the second terminal 36 and the junction of the second switch 50
with the diode 54. The current sensor 20 provides a current
feedback to the summing node 72 in the control circuit 11 for
closed loop control of the coil current. The current sensor 20 can
be implemented with any one of many current measuring devices and
methods, for example, a simple resistor, a Hall effect device, a
current transformer, etc.
[0034] The control circuit 11 receives a gun ON/OFF signal from the
system control 12. Operation of a forward current circuit 66 of the
control circuit 11 is initiated by a leading edge of the gun ON/OFF
signal. The forward current circuit 66 includes a waveform
generator 16, a summation node 72, a hysteresis modulator 74 and a
first switch driver 76. The hysteresis modulator 74 functions
similarly to a variable frequency PWM. The summation node 72
compares a stored current model stored in the waveform generator 16
to the current feedback from the current sensor 20 and generates an
error signal. The error signal drives the hysteresis modulator 74.
During the on-time T.sub.ON of the current pulse, the switch driver
92 holds the switch 50 closed; and the output of the modulator 74
commands the switch driver 76 to close the switch 48 in response to
the current in the coil 14 being less than the current setpoint.
The switches 48, 50 provide a forward current path through the
solenoid coil 14, and current in the coil 14 increases to the
setpoint value. When the current in the coil 14 exceeds the current
setpoint, the output of the modulator 74 commands the switch driver
76 to open the switch 48. Thereafter, the hysteresis modulator 74
modulates the switch 48 in a known manner to keep the sensed
current near the current setpoints, for example, I.sub.PK and
I.sub.h. Thus, the current in the coil 14 is shown as having a
saw-tooth form as the hysteresis modulator 74 modulates the
operation of the switch 48 so that the current in the coil 14
approximates the desired current model being output from the
waveform generator 16.
[0035] Operation of the line voltage compensation in the gun driver
10 will now be described. As shown in FIG. 2B, the electric gun
driver circuit 10 is initially in a deactivated State 0 wherein the
solenoid 18 has only minimal or no coil current. At State 1, the
control circuit 11 receives a gun ON/OFF signal from the system
control 12 in the form of a rising leading edge of a pulse. A line
voltage compensator 33 includes a duty cycle controller 35 and a
fixed frequency PWM 37. The PWM 37 has an output connected to one
input of an AND gate 39. The other input of the AND gate 39 is
connected to the output of the hysteresis modulator 74. The duty
cycle controller 35 has inputs connected to the gun ON/OFF signal
output from the system control 12 and the output voltage from the
unregulated power supply 19. With the output voltage from the power
supply 19 at its desired nominal value, for example, 240 V.sub.DC,
the duty cycle control holds the duty cycle of the PWM 37 at 100%.
Thus, all of the output from the hysteresis modulator 74 passes
through the AND gate 39; and as earlier described, the initial peak
current will rise to its desired value along the slope 208 of FIG.
2A.
[0036] If the power supply voltage is nonconstant and rises, for
example, to 300 V.sub.DC, without any compensation, as previously
described, the hysteresis modulator 74 would cause current to be
supplied to the coil 14 at a rate having the slope 210 of FIG. 2A.
That increased slope with no other change, increases the
operational speed and decreases the actuation time of the
dispensing valve 31. By opening faster, the dispensing valve 31
dispenses adhesive at a location on the substrate that is not
intended to receive the adhesive.
[0037] However, with the line voltage compensator 33, that problem
is eliminated. The duty cycle controller 35 senses the increase in
voltage from the unregulated power supply 19 and determines a
proportionality constant defined by a fraction. The fraction has a
numerator equal to the desired voltage, in this example, 240
V.sub.DC, and a denominator equal to the current power supply
output voltage, for example, 300 V.sub.DC. Thus, in this example,
the proportionality constant is 0.80, and the duty cycle controller
35 sets the duty cycle of the PWM 37 to 80%. With the output from
the PWM 37 reduced to 80%, only 80% of the output from the
hysteresis modulator 74 passes through the AND gate 39. The switch
driver 76 and power switch 48 are then modulated such that current
is supplied to the coil in accordance with the slope 212 of FIG.
2B. Further, the duty cycle from the duty cycle controller 35
causes the slope 212 to approximate the slope 208 of FIG. 2A. Thus,
the lower 80% duty cycle reduces the effective voltage supplied to
the coil 14 during the duration of the initial peak current to the
desired level, and the dispensing valve 31 moves from its closed
position to its open position in the desired time. In other words,
the operational speed and actuation time of the dispensing valve 31
with a power supply voltage of 300 V.sub.DC is the same as when the
power supply voltage is 240 V.sub.DC. Thus, the time required to
open the dispensing valve is independent of changes in the line
voltage at the power source 21 and resulting changes in the output
voltage of the power supply 19.
[0038] At State 2, at the end of a predetermined pull-in time
T.sub.PK, the switch 48 is opened while the switch 50 remains
closed, thereby disconnecting the terminal 34 from the positive
supply line 42. Current in the coil drops to the desired hold
current setpoint as determined by the waveform generator 16.
Thereafter, the hysteresis modulator 74 again modulates the
operation the switch 48 to maintain the current in the coil 14 at
the hold current setpoint value. At State 3, the falling trailing
edge of the gun ON/OFF signal from the system control 12 causes the
current reference to go to zero.
[0039] Changes in the line voltage and the output voltage from the
unregulated power supply 19 also changes the slope 214 in FIG. 2A
in a similar manner as previously discussed with respect to the
slope 208, thereby changing the speed and actuation time at which
the dispensing valve is closed. Therefore the output of the line
voltage compensator 33 is used in a similar manner as previously
described to modulate the current so that, at 300 V.sub.DC, current
flows at a rate represented by the slope 216 in FIG. 2B. When a
trailing edge of the gun ON/OFF signal is received from the system
control 12, the inverse of the output of the PWM 37 of the line
voltage compensator 33 is applied via AND gate 41, OR gate 43 and
driver 92 to control the operation of the switch 50. In State 3,
when the current is decreasing from the hold current, the switch 48
is opened; and the switch 50 is modulated at a duty cycle
determined as a function of the change in the power supply voltage
in a similar manner as earlier described. Thus, the current
decreases to zero along a slope 216 in FIG. 2B that approximates
the slope 214 in FIG. 2A, thereby keeping the operational speed of
the dispensing valve constant. With a constant operational speed,
the actuation time or time required to close the dispensing valve
31 remains constant and independent of variations in the output
voltage from the power supply 19.
[0040] While the present invention has been illustrated by a
description of various embodiments and while these embodiments have
been described in considerable detail, it is not intended to
restrict or in any way limit the scope of the appended claims to
such detail. Additional advantages and modifications within the
spirit and scope of the invention will readily appear to those
skilled in the art. FIGS. 1 and 3 are only examples of how the
present invention may be implemented, and other circuit
architectures may be used to implement the principles of the
present invention. For example, the gun driver 10 of FIG. 3
utilizes two switches 48, 50 to dissipate current from the coil 14.
Such a switch configuration is commonly known as a 1/2 H-bridge
driver configuration. As will be appreciated, other gun driver
configurations may implement the principles of the claimed
invention. For example, a full H-bridge configuration uses four
switches to provide a reverse current flow through the coil in
order to more quickly close the dispensing valve. As will be
appreciated by those skilled in the art, the claimed invention can
be readily applied to drivers having a full H-bridge configuration,
thereby making operational speed and actuation time of the
dispensing valve independent of changes in the magnitude of the
output voltage from an unregulated power supply.
[0041] In the detailed description, a power supply voltage of 240
V.sub.DC is used as an example of a voltage commanding a 100% duty
cycle of the PWM 37. As will be appreciated, the lowest voltage
expected to be encountered in an application should be used as the
voltage commanding a 100% duty cycle of the PWM 17 or the
hysteresis modulator 74.
[0042] In FIG. 3 and the associated description, a variable
frequency hysteresis modulator 74 is used. Such a modulator is
illustrated only by way of example; and as will be appreciated, the
principles of the claimed invention can be applied to other gun
driver designs using different modulators. As will be further
appreciated, depending on the design and application parameters,
the invention may be implemented using analog, digital or a
combination of digital and analog circuit components in any
configuration that automatically holds the operational speed and
actuation time of the dispensing valve constant and independent of
variations in the output voltage of the unregulated power supply
19.
[0043] Therefore, the invention in its broadest aspects is not
limited to the specific detail shown and described. Consequently,
departures may be made from the details described herein without
departing from the spirit and scope of the claims which follow.
* * * * *