Note: Descriptions are shown in the official language in which they were submitted.
~2~
APPARATUS AND ME~HOD FOR DISPENSING FLUID MATERIA~S
USING POSITION-DEPENDENT VELOCITY FEEDBACK
Field of the Invention
_ _
The present invention relates to a system
for controlling the flow of fluids. More particu-
larly, the invention-relates to an apparatus and
method for dispensing fluid materials at a controlled
rate.
Background of the Disclo~sure
In many applications, it is desirable to
control the flow of a fluid precisely and to have the
ability to change flow control parameters rapidly.
Such control may be desired either to maintain a given
flow rate in the face of perturbations such as changes
in the fluid's flow characteristics or supply pressure
or, to effect similarly rapid changes in the flow rate
: 15 such as may be required to account for changes in the
relative speed between the dispenser and a workpiPce
onto which the fluid is being dispensed.
When dispensing viscous fluids such as
certain lubricants, adhesives, sealants and the like,
it is often necessary to apply the material to the
;
-2- ~z~ 3
surface of a workpiece in a bead containing a desired
amount of material per unit length. In high produc-
tion processes or where the bead of material must be
positioned with accuracy, robot arms are often used to
apply the material by rapidly guiding a dispensing
nozzle in a programmed pattern over the surface of the
workpiece. Depending on the application, the fluid
being dispensed may either be projected some distance
from the nozzle in a high velocity stream or e~truded
~rom the nozzle at lower velocity with the nozzle
located closer to the workpiece. In either case, the
amount of material applied per unit of lineal distance
along the bead will vary according to both the flow
rate of material discharged from the dispensing nozzle
and the speed of the nozzle with respect to the
workpiece.
In the automotive industry, such a process
is used to apply a unifoxm bead of sealant around the
periphery of the inside surface of automobile doors
before joining the inside panel to the door. Along
long, straight portions of the pattern, a robot arm
can move the nozzle quickly. However, where the
desired bead pattern changes c;irection abruptly, such
as around the corners of a door panel, the robot arm
must be slowed down to achieve a required bead posi-
tioning accuracy. It can be appreciatecl that if the
flow rate of the dispensed f1uid material is held
-2--
~æ92~
--3--
fixed, the amount of material in the applied bead will
increase as the robot arm is decelerated to negotiate
changes in direction and will decrease as the robot
arm is accelerated. Likewise, changes in the fluid
supply pressure or changes in the viscosity of the
fluid material will tend to disrupt control over the
size of the bead.
An apparatus and method which effectively
addresses these difficulties is fully described in
applicant's co-pending Canadian application Serial
No. 546,806, filed September 14, 1987. That
application discloses, inter alla, a fluid dispensing
method and apparatus wherein a servo actuatox drives a
variable fluid metering valve located in close prox-
imity to a fluid discharge nozzle. The valve is aneedle valve comprising a valve seat and a stem
moveable relative the seat to vary the flow through
the seat. A pressure sensor at the nozzle generates a
signal correlated to the instantaneous 10w rate of
the dispensed fluid. Control over flow is achieved by
connecting the dispenser in a closed-loop system in
which the actuator is driven by a control current
derived in accordance with the difference between the
flow rate signal and a driving signal representing a
desired flow rate~ In robotic applications, the
drivin~ signal is preferably related to a toolspeed
.~
-4~
slgnal emanating from the controller of the robot
carrying the dispenser so that ~he control current
will vary as required to maintain a uniform bead of
fluid material even during relatively rapid changes in
the relative speed between the dispenser and the
workpiece onto which material is dispense~.
The stem-and seat needle valve arrangement
used in the device disclosed in the above referenced
patent application falls within a class of valves
which may be described as variable-area flow
restrictors. Other valves in this class include gate
valves and shutter valves. Valves in this class
modulate flow by varying an area through which flow
may take place such as the area between a valve stem
and its seat.
It is an inherent characteristic of such
valves that, as the valve closes, its sensitivity
increases. That is, for a given percentage of actu-
ation, the corresponding percentage change in the flow
through the valve will be greater when the valve is
nearly closed than when it is more fully open. This
is due to geometrical Eactors in that a given amount
of actuation results in a greater change in flow area
when the valve operates near the "closed" end of its
range than when the valve operates more toward the
"open" end of its range.
~5~- ~2~53
When variable-area flow restrictor type a
valve is employed as a metering valve in a closed-loop
dispensing system which must operate accurately and
rapidly over a range of flow rates, including flow
rates where the metering valve is nearly closed, the
above characteristic limits system performance. Since
the valve is quite sensitive when nearly closed,
system stability is decreased when the metering valve
is so positioned. This limits the maximum gain at
which the system can operate which, in turn, limits
system response time so that when the valve is oper-
ating at the more open end of its range, response time
is sIower than required to maintain stability when the
valve is operating at the more closed end of its
range.
Summary of the Invention
In view of the foregoing, it is an objective
of the present invention to provide a method and
apparatus for dispensing fluld materials which pro-
vide for stable operation over the full operatingrange of flow rates and wherein the response time when
the valve is more Eully open is not limited by
requiring the loop~ gain to be set low in order to
- ~ maintain stability at times when the valve is nearly
closed.
It is a further objective of the invention
to provide such a method and apparatus wherein stabil-
ity is maintained over the full operating range of the
--5--
-6- ~ 53
valve by increasing the amount of feedback as the
valve closes and rapid response is maintained by
decreasing the amount of feedback as the valve opens.
It is still a further object of the present
invention to increase the amount of feedback as the
valve closes and decrease the amount of feedback as
the valve opens by providing a position-dependent
velocity signal correlated to both the veloci-ty at
which the valve opens and closes and the position of
the valve.
It is a further object of the present
invention to provide such a method and apparatus
wherein the magnitude of the position-dependent
velocity signal is zero when the valve is not ~oving
and increases as the speed at which the valve opens or
closes increases and is further such that, for any
given velocity, the magnitude of said position-
dependent velocity signal is greater when the valve is
operating near the "closed" end of its range than when
it is more fully open.
It is a further object of the invention to
provide a position-dependent velocity signal which
varies in polarity according to 'he direction of
travel of the valve and which can be used to tend to
oppose movement of the valve in both directions.
It is yet another object of the present
invention to provide such a method and apparatus
_7- ~ 53
whereby the position-dependent velocity feedback
siynal is generated using a magnet moveable with
respect to a coil to influence the coil so that the
magnet and coil move toward one another as the valve
closes and away from one another as the valve opens.
To these ends, a preferred embodiment of the
invention includes a dispenser for viscous fluids
connected in a feedback control loop. The dispenser
has an electro-pneumatic servo act.uator connected to a
fluid metering valve. The valve has an inlet connect-
able to a supply o fluid to be dispensed and an
outlet connected to a dispensing nozzle. Interposed
between the inlet and the outlet is a valve seat
having a central orifice and a valve stem mateable
with the seat and axially moveable with respect
thereto. The valve stem is connected to the servo
actuator such that the stem is moved according to the
electricaI signal applied to the servo actuator~
Movement of the stem toward or away from the seat ~
varies the effective flow area through the orifice to
decrease or increase, respectively, the flow through
the orifice. The actual rate of flow is measured by a
flow rate sensor located at the nozzle. The flow rate
sensor generates a flow rate signal which is used as a
irst feedback signal in the feedbac~ control loop.
The first feedback signal is added to a second feed-
back signal, to be described, and their sum is
--7--
12~ i3
compared with a driving signal representing a desired
flow rate in order to derive the control signal
supplied to the servo actuator.
The invention contemplates the use of a
second feedback signal in the form of a position-
dependent velocity signal to be added to the first
reedback or flow rate signal as noted above. This
signal is correlated to both the velocity and the
position o-F the valve stem with respect to the seat as
to mitigate the adverse impac~ of the high sensitivity
of the metering valve on system stability when the
valve stem is operating close to its seat while at the
same time not slowing system response ti~e when the
valve is operating at more fully open positions and
hence, is less sensitive.
This is accomplished by using the position
dependent velocity signal to increase the total
feedback when the metering valve stem is moving in
close proximity to its seat. The position-dependent
velocity signal;is such that its magnitude is zero
when the stem is not moving relative the seat. Its
magnitude increases with increasing stem velocity and
decreases with decreasing stem velocity. Further, for
any given velocity, the magnitude of the position-
dependent velocity signal increases as the distancebetween the stem and seat decreases. Conversely, for
any given velocity, the magnitude of the signal
-8-
g ~ L53
decreases as -the distance between the stem and seat
increases. This permits the design of a dispensing
system which avoids any undue limitation on system
response time when the valve is less sensitive and the
system inherently more stable. The polarity of the
position-dependent velocity signal reverses as the
direction of travel of the stem relative the seat
reverse and is applied as to always oppose motion of
the valve, whether it is opening or closing.
A position-dependent velocity signal embody-
ing the above characteristics is generatea by mounting
a magnet in a position fixed relative to the valve
stem of the metering valve and a coil in a position
fixed relative to its valve seat so that the magnet
and coil move relative to one another in the same
manner that the stem and seat move relative to one
another. As the flux field of the moving magnet is
cut by the coil, a voltage will appear across the
coil.~ The polarity of this voltage will vary
according to the relative direction of travel.
Further, the magnitude of the voltage will vary as a
function of both velocity and position. When there is
no relative movement between magnet and coil, the
magnitude of the signal will be zero since there will
be no magnetic flu~ being cut by the coil. When
movement takes place, the magnitude of the signal
varies directly with velocity in accordance with
changes in the rate at which the magnetic flux is
_g_
~2~
cut by the coil. Thus, the signal will increase when
the relative velocity between magnet and coil
increases and decrease when it decreases. The
magnitude of the signal also varies inversely with the
distance between the stem and seat. When the stem is
close to the seat, the magnet is close to the coil.
Because the flux lines of the magnet are more closely
spaced in close proximity to the magnet and less
closely spaced further away, the magnitude of the
signal, for any given velocity, will be greater when
the seat is close to the stem than when it is further
away~ Thus, the voltage across the coil is dependent
on both the velocity and distance between the stem and
seat in precisely the manner described above.
Brief Description of the Drawings
Fig. 1 is a schematic cross sectional view
illustrating a preferred embodiment of a dispensing
apparatus constructed according to the invention.
Fig. 2 is a block diagram illustrating a
preferred embodiment of a system for dispensing fluid
materials using position-dependent velocity feedback
according to the invention.
Detailed Description of the Invention
Referring now to Fig. 1 a preferred embodi-
~5 ment of a dispensing gun 10 constructed according to
the invention is shown. Gun 10 includes a C-shaped
frame 11 having a mounting plate 12 adapted to be
--10--
L2~ 3
secured to the tool mounting face 13 of a robot arm
(not shown) by means of one or more cap screws 14 and
alignment pins 15. Frame 11 is preferably constructed
of a rigid, light~weight material such as aluminum
alloy and further includes, extending outwardly from
mounting plate 12, an upper portion 16, and an opposed
lower portion 17. The upper portion 16 of frame 11
carries an electro-pneumatic sexvo actuator 20 which
may eonsist of any of a number of types of compaet,
light weight linear aetuators oEfering rapid response.
Preferably, actuator 20 comprises a double-aeting air
cylinder 22 having a reeiproeable piston rod 23 whose
extension is eontrolled by an eleetrieally aetuated
pneumatie servovalve 24 aeeording to the pressure
balanee aeross a piston 25 affixed to piston rod 23.
Servovalve 24 is disposed to the side of air cylinder
22 (as shown). The lower portion 17 of frame 11
earries a metering valve assembly 26 having a needle
valve 27 loeated between a fluid inlet 28 and a
dispensing nozzle 29 which ineludes a nozzle end 30
threadably eonneeted thereto. Nozzle end 30 has an
outlet 31.
For best control, needle valve 27 is located
as elose to nozzle 29 as is praetical. Valve 27
ineludes a valve stem 32 having a generally conically
tapered end 33 and a valve seat 34 having a matingly
tapered upper shoulder 35. Valve stem 32 is conneeted
-12- ~z~53
to piston rod 23 so that the position of its conical
end 33 relative to valve seat 34 and hence, the flow
rate of fluid discharged from nozzl~ 29 t is controlled
in accordance with the electrical input of electro-
pneumatic servovalve 24.
A flow rate sensor 36 generates an elec-
trical flow rate signal 37 correlated to the rate of
flow of fluid discharged from nozzle 29. Preferably,
flow rate sensor 36 camprises a pressure sensor
located just downstream of needle valve 27 in the wall
of nozzle 29. As will be described in further detail
below, flow rate signal 37 is preferably used as a
first ~eedback signal in a feedback loop in which gun
10 is connected to control the rate of flow of fluid
dispensed from nozzle 29 in accordance with a driving
signal representing a desired flow rate. In robotic
applications, a driving signal which varies with the
relative speed between nozzle 29 and the workpiece 39
and which is supplied by the robot controller can be
used to accurately control the amount of fluid material
per unit length.contained in the bead deposited on the
surface of the workpiece 39 by dispenser 10.
Linear actuator 20 may incorporaie any of a
number~of suitable types of fast-responding, electri-
cally-actuated servovalves including jetpipe, nozzle-
and-flapper, or spool types. The present invention
does not reside in the details of the construction of
-12-
-13- ~Z~3
servovalve 24 such being ~ithin the purview of those
skilled in the art. The servovalve is not described
in complete detail.
In the preferred embodiment illustrated in
Fig. 1, actuator 20 comprises a jet-pipe electro-
pneumatic servovalve 24 which operates double-acting
air cylinder 22. Servovalve 24 includes a housing 42
which supports a threaded, electrical connector 43
secured thereto by screws 44. Wired to connector 43
by way of leads 45 are a pair of series-connected
coils 46 which surround opposing ends 49 of an arma-
ture 50 which is mounted to pivot about a pivot point
51. A hollow, inverted U-shaped jet pipe 52 has one
leg connected to a regulated air supply (not shown) of
about 100 PSI nominal pressure through a threaded
inlet 53 in air cylinder 22 by way of a removeable
particulate trapping screen or Eilter 54. The oppo-
site leg of jet~pipe 52 is secured near its center to
armature 50 so that when armature 50 is pivoted
clockwise by energizing coils 46 at one polarity, the
flow emanating from jet pipe 52 is diverted toward a
first port 60 which communicates with the space above
the piston. Similarly, when coils 46 are energized in
the opposite polarity, armature 50 pivots counter-
clockwlse to direct the flow from jet pipe 52 toward asecond port 61 which opens to the space beneath the
-13-
-14~ S3
piston 25 of air cylinder 22. In elther polarity, the
degree of the deflection of jet pipe 52 and hence, the
pressure in ports 60 and 61 is proportional to the
magnitude of the control current flowing in coils 46.
Armature S0 is spring centered and magnetically biased
such that when coils 46 are in a de-energized state,
jet pipe 52 is centered in a neutral position as shown
so that the pressures in ports 60 and 61 tend to be
equally balanced. Magnetic bias is provided by a pair
of permanent magnets 63 polarized as shown. Each
maqnet 63 which communicates with the armature field
by way of a flux across air gaps 65. This flux is
conducted to gaps 65 hy way of four magnetically
permeable members 66 arranged as shown.
Double acting air cylinder 22 includes an
aluminum alloy cylinder body 70, having a lower flange
72 which is used to secure the body 70 of air cylinder
22 to the upper portion 16 of frame 11 using cap
screws 73. Cylinder body 70 includes first and second
ports 60, 61, threaded air supply inlet 53, filter 54
as well as an axial cylinder bore 75. Received within
bore 75 and connected to piston rod 23 is a piston 25
fitted with a pair of opposed cup seals 78. The space
within bore 75 located above piston 25 communicates
with first port 60 while the space beneath piston 25
is connected to second port 61. The direction and
speed at which piston 25 drives needle valve 27
-14-
-15~ S3
depends upon the differential pressure between ports
60 and 61 which appears across piston 25. As explained
above, this pressure differential is determined by the
deflection of jet pipe 52 due to the control current
flowing in coils 46.
Piston 25 is retained within cylinder bore
75 at the lower end thereof by a cap 80 through which
passes the lower portion 23a of piston rod 23. To
prevent air leakage, cap 80 is provided with an
internal cup seal 81 in the area of piston rod 23 and
an external O-ring seal 82 between the outside surface
of cap 80 and cylinder bore 75. Cup seal 81 is
retained within cap 80 by a snap ring 83. Cap 80 i5
itself retained in the end of cylinder bore 75 by a
snap-ring 84 which engages a groove 84a cut in the
lower portion of the wall of cylinder body 70. The
structure disposed at the upper portion of cylinder
body 70 which is of key importance to the present
invention wil} be described later.
Metering valve assembly 26 includes a rigid,
non-resilient valve body 85 constructed as shown in
Fig. 1, preferably of metal. The lower end of valve
body 85 contains a nozzle passage 86 whose lower end
is threaded to accept a flow restricting nozzle end 30
of a desired configuration and having a discharge
outlet 31. Passage 86 is intersected radially by one
or more threaded holes, one of which receives flow
-15-
-16~ 3
rate sensor 36 and the others of which are sealed by
means of plugs 90. Located immediately upstream of
no~zle 29 and as closely adjacent thereto as practi~
cable, valve body 85 houses needle valve 27 which, as
previously noted, comprises a valve seat 34 and a
valve stem 32 whose conical end 33 is mateable there-
with. Valve stem 32, by virtue of its being connected
to piston rod 23 in the manner to be described as
moveable axially toward end away from valve seat 34
under the control of servo-actuator 20. For long
life, both valve stem 32and valve seat 34 are prefer-
ably fabricated of a hard material such as sintered
tungsten carbide. A fluid supply inlet 28 enters
valve body 85 upstream of needle valve 27. Inlet 28
is threaded so that a hose can be attached to a
pressurized supply lnot shown) of the fluid material
to be dispensed.
Valve body 85 threads onto the lower end of
a bonnet 97 and is sealed with respect thereto by
means of an O-ring seal 98. Bonnet 97 includes an
internal packing gland 93 which holds a plurality of
annular PTFE pacXing seals 100. Seals 100 are
retained in sealing but nonbinding compression about
valve stem 32 by means of an adjustable gland nut 101
threaded into the top of bonnet 97. To attach metering
valve assembly 26 to frame 11, bonnet 97 is received
by the extending lower portion 17 of frame 11 and
-16-
-17~ 53
secured thereto at a desired angular orientation by
means of a locknut 102. This feature is useful where,
for example, it is found that the fluid supply hose
(not shown) which connects to fluid inlet 28 inter-
S feres with some structure in a given application.
Locknut 102 can then be loosened and valve body 85
rotated to another angular position to avoid the
interference, if possible.
Metering valve assembly 26 is connected to
the piston rod 23 of actuator 20 by means oE a coupling
105 having a threaded axial bore 106 into which the
lower end of piston rod 23 is threaded. The lower end
of coupling 105 carries a threaded recess 107 which
receives a bushing 108 which has an axial bore 109
into which the upper end of valve stem 32 is received
and secured by means of a press fit. Coupling 105 is
prevented from unthreading from piston rod 23 by an
Allen head type locking screw 110 which is threaded
into bore 106 into secure pressure engagement with the
end face of piston rod 23 as shown.
To provide for fail-safe operation, gun 10
is provided with a compression spring 115 operable to
close valve 27 in the event the control signal or air
supply to actuator 20 is interrupted in order to avoid
an uncontrolled flow of fluid from gun 10. Spring 115
is compressed between cap 80 and an annular shoulder
116 provided on coupling 105.
-17-
-18- ~2~53
Flow rate sensor 36 may comprise any suit-
able sensor capable of generating a flow rate signal
37 indicative of the rate of flow of ths fluid dis-
pensed from noz~le 29. The flow of a ~iscous
newtonian fluid at low Reynolds numbers is substan-
tially linearly proportional to the pressure drop
across a nozzle or tubular restriction placed in the
flow path. Accordingly, sensor 36 may comprise a
strain gauge pressure transducer operably disposed to
sense the instantaneous fluid pressure at a location
inside passage 86 immediately downstream of needle
valve 27 to sense the pressure drop across nozzle 29.
Cne pressure transducer suitable for this purpose is
model A20S manufactured by Sensotec of Columbus, Ohio.
According to the invention, dispensing gun
10 also includes a transducer assembly 119 for gener-
ating a position-dependent velocity sig~al 120.
Transducer assembly 119 includes an upper cap 121 made
of soft steel or other magnetically permeable material
disposed atop cylinder body 70 as shown.
Upper cap 121 includes an axial bore 122
through which the upper portion of piston rod 23
passes. Further, upper cap 121 carries a O-ring 123
in an outer annular groove 124 for sealing bore 75
~rom air leakage. Air leakage about piston rod 23 is
avoided by means of a cup seal 126 disposed in a
recess 127 in cap 121 and retained therein by a snap
-18-
-19~ z~53
ring 128. Cap 121 is held inside cylinder body 70 by
means of a snap ring 129 which is installed above a
small outwardly proiecting flange 130 on cap 121.
Flange 130 is received within a stepped recess 132 in
cylinder body 70 to prevent cap 121 from slipping
downward into bore 75. The upper side of cap 121
includes a cylindrical well 133 whose longitudinal
axis is aligned with that of bore 75 and piston rod
23. Between well 133 and recess 127, cap 121 includes
an inward projection 131 having a sufficiently large
cross-sectional area to provide a good magnetic path.
The air gap between projection 131 and piston rod 23
should be kept to a few thousandths of an inch or
less.
Disposed within well 133 is a coil 134 wound
on bobbin 135 made of plastic material such as DELRIN
a reinforced nylon made by the Dupont Company of
Wilmington Delaware.
Bobbin 135 has an axial bore 136 which is
also axially aligned with piston rod 23. Bobbin 135
has sufficient lubricity and bore 136 has sufficient
clearance, that piston rod 23 is free to reciprocate
axially therethrough W.7 thout significant frictional
drag. Coil 134 consists of lO00 turns of ~38 A.W.G.
magnet wire wound in a single continuous length upon
bobbin 135. The end leads 137, 138 of coil are routed
away fxom bobbin 135 through a small reservoir 139 in
--19--
-20- ~2~ 3
cap 121. Reservoir 139 is filled with electrical
potting compound for mechanical protection of leads
137 and 138 which are electrically connected by
soldering or other suitable means to the terminals of
a two piece plug-type electrical connector 140.
Connector 140 is mounted in the wall of a protective
metallic housing 141 which is secured to cylinder body
70 by screws or other con~entional means (not shown).
Threaded onto the upper end of piston rod 23
is a ring shaped armature 145 made of soft steel or
other magnetically permeable material. Armature 145
is prevented from loosening by means of a lock-nut 146
which is threaded onto piston rod 23 behind armature
145 as shown. A permanent ring shaped magnet 150 of
Alnico, ceramic or ceramic plastic magnetic material
is attached to armature 145 on the side of armature
145 which faces coil 134. Magnet 150 is polarized as
shown in Fig. 1 and has a magnetic flux density of
about 2,200 gauss. Preferablyj armature 145 includes
a circumferential step 151 about its periphery mate-
able with the inner diameter of magnet 150. Magnet
150 is secured to armature 145 at step 151 using a
thin layer of epoxy or other suitable adhesi~e (not
shown). For mechanical protection, the leads 137, 138
of coil 134 are separated from reciprocable components
such as magnet 150, armature 145, locknut 146 and
piston rod 23 by interposing a cylindrical shroud 16
-20-
~1 ~2~ .53
o~ insulating materlal between them. Shroud 160 need
not be rigidly secured to any surrounding structure so
long as it is retained within the confines OL housing
141 as shown in such a way that separation of leads
137, 138 from these moving components is assured, in
order to avoid pinching or shorting of leads 137, 138.
In light of the foregoing, it can be appre-
ciated that coil 134 remains at all times fixed in its
position with respect to valve seat 34. Likewise,
magnet 150 remains at all ti~.es fixed in position with
respect to the conical end 33 of valve stem 32 by
virtue of its rigid mechanical linkage thereto. It
can be further appreciated that as the piston 25 of
actuator 20 causes valve stem 32 to move a~ially with
respect to valve seat 34, the same rela-tive a~ial
movement will take place between magnet 150 and coil
134. In the present embodiment, the full range of
travel of stem 32 and magnet 150 is about 5/16 of an
inch. When valve 27 is fully closed with the conical
end 33 of valve stem 32 seated flush against the upper
shoulder 35 of valve seat 34, an air gap 162 of less
than 10 thousandths of an inch and preferably only 1
to 2 thousandths of an inch is present bet~leen the
upper face 121a of upper cap 121 and ma~net 150. This
gap is provided to insure that valve 27 can close
fully without magnet 150 first bottoming ou-t on end
cap 121 and should be kept as small as production
-21-
-22~ 2~53
tolerances in the various components of gun 10 will
permit. E~cept for the thin layer of epoxy between
magnet 150 and armature 145 and the air yap 162, whose
width varies according to the relative distance
S between stem 32 and seat 34, the flux associated with
magnet 150 is provided with an otherwise substantially
closed path through magnetically permeable materials.
Flux from magnet 150 flows through armature 145 to
piston rod 23, crosses the small air gap to the
projection 131 of cap 121 and travels though the body
of cap 12~. The flux then crosses variable air gap
162 to return to magnet 150.
In operation, the relative axial movement
between coil 134 and magnet 150, which corresponds to
the relative axial movement between valve seat 34 and
conical end 33 of valve stem 27 will induce a voltage
across coil 134 in the form of a position-dependent
velocity signal 120. Signal 120 has one polarity when
valve 27 is moving closed (i.e. the distance between
the stem 32 and seat 34 is decreasing.) and the
opposite polarity when valve 27 is moving open (i.e.
the distance between stem 32 and seat 34 is in-
creasing). The magnitude of the position-dependent
velocity signal 120 varies directly according to the
relative velocity between stem 32 and seat 34. For
any given valve position, signal 120 increases as
velocity increases and decreases as velocity
-22
-23- ~z~53
decreases. The magnitude of the position-depended
velocity signal 120 also varies inversely according to
the distance between stem 32 and seat 34 such that, at
any given velocity, its magnitude decreases as this
distance increases and, conversely, increases as it
decreases. Position-dependen~ velocity signal 120
also has the characteristic that when the relative
velocity between stem 32 and seat 34 is zero, this
magnitude is also zero irrespective of valve position.
Further according to the invention, gun 10
can be connected to an electronic controller 200 to
form a fast responding closed-loop servo control
system which utilizes flow rate signal 37 as a first
feedback signal and position-dependent velocity signal
120 as a second feedback signal to control the rate of
fluid dispensed from nozzle 29 as will be described
now with reference to Fig. 2.
Dispensing gun 10 is carried by the tool
mounting surface 13 of a robot having a robot con-
tro]ler (not shown) programmed to guide nozzle 29 oYerthe surface of a workpiece 39 to dispense a bead of
fluid thereon in a desired pattern. The metering
valve assemb~y 26 of gun 10 is connected at its fluid
inlet 28 with a continuous pressurized supply of fluid
~not shown) to be dispensed onto workpiece 39. ~s
previously described, flow rate sensor 36 continuously
senses the pressure drop across nozzle 29 to generate
-23-
-24- ~2~3
a flow rate signal 37 correlated to the rate of flow
of fluid discharged from the outlet 31 of nozzle end
30. Signal 37 is received and amplified by a preamp
210 to generate amplified flow rate signal 211. Leads
137 and 138 of the coil 134 of ~transducer 119 are each
connected in series with a blocking capacitor 170 to
filter out any D.C. componer.t of the voltage appearing
across coil 134 and then connected to the plus input
172 and minus input 173 respectively of a differential
amplifier 174 to generate a single-ended position-
dependent velocity signal 120a. Amplifier 174 has a
fixed gain of 10 and a high input impedance as not to
significantly load coil 134. Flow rate signal 211
is then fed into a minus input 212 of a summing
lS junction 213 as a first feedback signal and position
dependent velocity signal 120a algebraically summed
therewith by being fed into another minus input 212a
of the same summing junction 213 as a second feedback
signal~
Flow rate signal 211 is also received at the
minus input 212 of a summing junction 213 as well as
at a firs-t input 214 of a comparator 215 whose second
input 216 recelves a fixed, selectable voltage refer-
ence, ~REF1 and whose output 217 generates a digital
PRESSURE OVERRANGE signal 218 which is received hy the
robot controller. Ir the magnitude of oukput signal
211 exceeds VREF1, digital PRESSURE OVERRANGE signal
-24-
-25- ~29~
assumes a logical 1 value. ~his can occur for example
if needle valve 27 opens too far. In such event, the
robot controller can be programmed to present a fault
indication, shut down the system or take other appro-
priate action.
Summing junction 213 also includes a plus
input 219 which receives a driving signal 222. In the
embodiment of Fig. 2, dr,ving signal 222 is generated
by an amplifier 227 in accordance with a toolspeed
signal 228 from the robot. Toolspeed signal 228 is a
signal available from the robot controller and varies
according to the speed of travel of gun 10 relative to
workpiece 3~. ~hrough the robot controller, the gain
of signal 228 can be adjusted by way of a toolspeed
multiplier selected to provicle a desired flow rate as
a function of speed of travel.
Amplifier 227 is an operational amplifier
whose gain is selected to properly scale toolspeed
signal 228 so that driving signal 222 will be within a
range compatible with the rest of the circuit.
AmpIifier 227 is preferably connected as a precision
limiter such that or inputs between zero volts and an
adjustable threshold voltage, the voltage o driving
signal executes a decisive step in a direction proper
to close needle valve 27. Typically, the threshold
voltage would be adjusted so that when toolspeed
signal 228 is about 50 mV or less, needle valve 27 is
-25-
-26~ S3
driven positively closed. This prevents needle valve
27 from leaking by providing a negative bias current
to servovalve 24 effective to drive needle valve 27
positively closed at times when toolspeed signal 228
is either not present or is quite small.
Summing junction 213 produces an analog
error signal 230 whose magnitude and polarity is equal
to the algebraic difference between driving signal 222
and the sum of, flow rate signal 211 and position-
dependent velocity signal 120a. Error signal 230 is
received by an amplifier 231 whose gain is adjusted
for optimum system stiffness. The output signal 232
from amplifier 231 is received by a compensation
network 234 having an output signal 239. Compensation
network 234 is designed and adjusted according tostandard control techniques to stabilize closed-loop
system response and maximize response speed with
minimum overshoot. Signal 239 is received by a
current driver 240 as well as by the first input 241
of a comparator 242.
Comparator 242 includes a second input 243
which receives a fixed, selectable voltage reference,
VREF2. And an output 244 which generates a digital
VALVE OVERRANGE SIGNAL 245. In the event the magni-
tude of signal 239 exceeds VREF2, digital VALVEOVERRANGE signal assumes a logical 1 state. Such a
condition may arise for example if the supply of fluid
-26-
-27- ~%~5~
to dispensing gun 10 is cutoff or if supply pressure
is inadequate to meet the demand imposed by dr~ving
signal 222. Like PRESSIJRE OVE~ANGE signal 218, VALVE
OVERRANGE signal 245 is directed to th~ robot con-
troller which may be programmed to generate a faultindication, shut the system down or otherwise initiate
corrective action.
Current driver 240 generates an analog
control current signal 246 which is applied to the
coils 46 of servovalve 24. This causes jet pipe 52 to
~e diverted toward first port 60 or second port 61,
depending on the magnitude and polarity of control
current signal 246, to move the piston 25 of air
cylinder 22 either downward or upward, respectively.
Downward movement of piston 25 tends to close needle
valve 27 of metering valve assembly 26 thereby
reducing the flow of fluid while upward movement of
piston 25 tends to open needle valve 27 thereby
increasing the flow of fluid.
The polarity of flow rate signal 37 is
selected such that as the magnitude of signal 37
increases, it will tend to cause control signal 246 to
close the valve 27. When signal 37 decreases, its
polarity is such that valve 27 tends to be driven more
closed by control signal 246. In a similar fashion,
flo~ rate signal 120 is applied to amplifier 174 in
such a polarity as to always tend to oppose movement
-28 ~2~ 3
of valve stem 32. That is, as stem 32 moves away from
seat 34, the polarity of position-dependent flow rate
signal 120a should be such as to tend to cause control
signal 246 to move stem 32 toward seat 34. Conversely,
as valve 27 tends to close, so that stem 32 moves
further from seat 34, the polarity of position-
dependent flow rate signal should be such that it
tends to cause control signal 246 to open valve 270
In operation, the system functions as a
closed loop servo system responsive to the pressure
drop across nozzle 29 as sensed by flow rate sensor 36
as well as to the position and velocity of stem 32
relative to seat 34 as indicated by position-dependent
velocity signal 120a. With needle valve 27 initially
closed, no flow occurs and the pressure drop across
nozzle 29 is zero. Assuming toolspeed signal 228 is
less than the threshold voltage associated with
amplifier 227, amplifier 227 generates a driving
signal 222 of the proper polarity and of suficient
magnitude to generate a control current 246 to deflect
jet pipe 52 toward -first port 60. This holds piston
25 down so that needle valve 27 is held closed under
force thereby pre~enting leakage. ~his condition is
maintained until toolspeed signal 22~ rises above the
threshold voltage of amplifier 227 indicating that
flow should commence. When this occurs, driving
signal 222 reverses polarity. Since there is
- -28-
-29- ~2~
initially no flow, flow rate signal 211 is at its zero
value. Since there is initially no relative movement
between stem 32 and seat 34, position-dependent
velocity signal 120a is also zero. Accordingly, an
error signal 230 whose magnitude is determined by the
difference between driving signal 22 and the sum of
flow rate signal 211 and position~dependent velocity
signal 120a will cause a control current 246 to be
applied to coils 46 in such a polarity as to cause jet
pipe 52 to deflect toward second port 61. In response,
piston 25 begins to move upward causing needle valve
27 to open by l~fting the conical end 33 of valve stem
32 away from valve seat 34. When this movement
occurs, position-dependent velocity signal 120a
assumes a non-zero value of a polarity tending to
resist movement of stem 32. As the pressure signal 37
generated by pressure transducer 36 increases in
response to the opening of valve 27, error signal 230
and control current 246 both tend to decrease and jet
pipe 52 moves toward its null position and the magni-
tude of position-dependent velocity signal 120a
declines toward zero. As the pressure drop across
nozzle 29 approaches a stead~ state value corre-
sponding to a desired flow rate, jet pipe 52 causes
valve 27 to remain open by an amount just sufficient
to maintain the pressure drop across nozzle 29 at that
value.
-29-
-30- ~2~2153
When driving signal 222 changes due to a
change in the relative speed between dispenser 10 and
workpiece 39 or when the flow rate signal 211 changes
for some other reason such as a change in viscosity of
the 1uid or a change in the fluid supply pressure,
position-dependent velocity signal 120a operates to
maintain system stability. It does so by increasing
the total amount of feedback, i.e., the sum of signals
211 and 120a, by an amount sufficient to insure
stability without requiring a decrease in the overall
loop gain. Since the amount of additional feedback
depends on both the position of valve 27 and its
velocity, more feedback is provided when valve 27
tends to be less stable, that is, when it operates at
higher velocity and/or near the "closed" end of its
range. Conversely~ relatively less feedback and hence
faster response time is provided when valve 27 tends
to be more stable, that is, when it operates slowly
andjor at more fully open positions. This permits the
20~ system to operate stablely over the entire range of
valve 27 at a~significantly higher loop gain than
would otherwise be possi~le. For example, it has been
found that the system of Fig. 2 can operate stablely
over its full range with the gain of amplifier 231 set
5 times higher than the same system with position-
dependent velocity signal 120a disconnected. Depending
on the particular flow characteristics of the fluid
-30-
~Z~i3
~31-
being dispensed, operation at even higher gain has
been found to be possible, especially with fluids
which are not highly viscous or shear sensitive.
These and other advantages will become apparent to one
skilled in the art having the benefit oE the present
disclosure~
While the above description constitutes a
preferred embodiment of the apparatus and method of
the invention, it is to be understood that the ir.ven-
tion is not limited thereby and that in light of the
present disclosure of the invention various alterna-
tive 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.
,1, ~
