U.S. patent application number 10/038491 was filed with the patent office on 2003-06-26 for fast-recovery viscometer.
Invention is credited to Kasameyer, Robert E., Warren, Wayne.
Application Number | 20030115936 10/038491 |
Document ID | / |
Family ID | 21900256 |
Filed Date | 2003-06-26 |
United States Patent
Application |
20030115936 |
Kind Code |
A1 |
Kasameyer, Robert E. ; et
al. |
June 26, 2003 |
FAST-RECOVERY VISCOMETER
Abstract
Electrical current driven through one or the other of two coils
(20 and 22) draws a ferromagnetic bob (28) along a chamber (26)
containing a liquid whose viscosity is to be measured. The current
that flows through the coil includes an AC component, and the
resultant magnetic field causes in the other coil an AC voltage
whose magnitude depends on the bob's position. A position detector
(38, 40) monitors the electromotive force thus induced and
concludes that the ferromagnetic bob has reached a predetermined
end-of-travel position when the magnitude of the electromotive
force has fallen to a predetermined fraction the maximum value that
it had attained during the stroke, and a coil driver (36, 38)
switches current drive from one coil to the other so as to begin
driving the bob in the opposite direction. If the position detector
fails to detect the bob's reaching the end-of-travel position
within a predetermined timeout interval, the coil driver reverses
coil drive despite the absence of such detection. The predetermined
timeout interval's duration is ordinarily determined as a function
of the bob-stroke duration that recent valid end-of-travel
detections have defined. When the first timeout occurs, though, the
time-out-interval duration is the same, relatively short value for
each of the plurality of strokes in a clean-out period, after which
the timeout-interval duration is immediately increased to a
relatively high value.
Inventors: |
Kasameyer, Robert E.;
(Cohasset, MA) ; Warren, Wayne; (Lexington,
MA) |
Correspondence
Address: |
CESARI AND MCKENNA, LLP
88 BLACK FALCON AVENUE
BOSTON
MA
02210
US
|
Family ID: |
21900256 |
Appl. No.: |
10/038491 |
Filed: |
December 21, 2001 |
Current U.S.
Class: |
73/54.23 ;
73/54.18 |
Current CPC
Class: |
G01N 11/162
20130101 |
Class at
Publication: |
73/54.23 ;
73/54.18 |
International
Class: |
G01N 011/10; G01N
011/00 |
Claims
What is claimed is:
1. A viscometer system including: A) a bob; B) bob guide that
guides the bob along a bob path in which a fluid whose viscosity is
to be measured can be disposed; C) first and second coils so
positioned with respect to the path that current flowing through
them produces respective magnetic fields that tend to drive the bob
along the bob path in respective opposite directions; D) a position
detector responsive to the coils' inductance to produce a detector
signal that indicates when the bob has reached end-of-travel
positions if the bob speed does not exceed a bob-speed maximum; E)
a coil driver that: i) alternately drives the first and second
coils in respective drive strokes and is responsive to the position
detector to switch between the first and second coils upon the
earlier of: a) the time at which the detector signal indicates when
the bob has reached one of the end-of-travel positions; and b) the
end of a predetermined timeout interval; ii) sets the duration of
the predetermined timeout interval employed during a drive stroke
to a value that it determines as a function of the duration of at
least one previous drive stroke; and iii) responds to a stroke's
first lasting to the end of a timeout interval by keeping the
duration of the timeout interval at a fixed, cleanout-stroke value
for a clean-out period comprising a plurality of strokes at least
if the detector signal does not in the interim indicate that the
bob has reached one of the end-of-travel positions; and F) an
output generator responsive to the drive strokes' durations to
generate a viscosity output representative of the viscosity that
the strokes' durations indicate.
2. A viscometer as defined in claim 1 wherein the coil driver
further sets the duration of the predetermined timeout interval for
the first stroke after the clean-out period to a value at least six
times the clean-out-stroke value.
3. A viscometer as defined in claim 2 wherein the value to which
the coil driver sets the duration of the predetermined timeout
interval in at least some instances is the sum of the last stroke
duration and a safety-margin value.
4. A viscometer as defined in claim 2 wherein the value to which
the coil driver sets the duration of the predetermined timeout
interval is the sum of the last stroke duration and a safety-margin
value only if that sum falls within predetermined timeout-value
limits.
5. A viscometer as defined in claim 2 wherein the value to which
the coil driver sets the duration of the predetermined timeout
interval is the sum of the last stroke duration and a safety-margin
value only if the detector signal indicated that the bob had
reached one of the end-of-travel positions during the previous
stroke before the end of the timeout period.
6. A viscometer as defined in claim 5 wherein the value to which
the coil driver sets the duration of the predetermined timeout
interval is the sum of the last stroke duration and a safety-margin
value only if the detector signal indicated that the bob had
reached one of the end-of-travel positions, during the stroke
before the previous one, before the end of the timeout period.
7. A viscometer as defined in claim 5 wherein the value to which
the coil driver sets the duration of the predetermined timeout
interval is the sum of the last stroke duration and a safety-margin
value only if that sum falls within predetermined timeout-value
limits.
8. A viscometer as defined in claim 2 wherein fixed,
clean-out-stroke value is independent of previous stroke
durations.
9. A method of measuring viscosity that includes: A) providing a
bob guide that includes: i) a bob; ii) bob guide that guides the
bob along a bob path in which a fluid whose viscosity is to be
measured can be disposed; iii) first and second coils so positioned
with respect to the path that current flowing through them produces
respective magnetic fields that tend to drive the bob along the bob
path in respective opposite directions; and iv) a position detector
responsive to the coils' inductance to produce a detector signal
that indicates when the bob has reached end-of-travel positions if
the bob speed does not exceed a bob-speed maximum; B) alternately
driving the first and second coils in respective drive strokes and
is responsive to the position detector to switch between the first
and second coils upon the earlier of: i) the time at which the
detector signal indicates when the bob has reached one of the
end-of-travel positions; and ii) the end of a predetermined timeout
interval; C) setting the duration of the predetermined timeout
interval employed during a drive stroke to a value that it
determined as a function of the duration of at least one previous
drive stroke; D) responding to a stroke's first lasting to the end
of a timeout interval by keeping the duration of the timeout
interval at a fixed, clean-out-stroke value for a clean-out period
comprising a plurality of strokes at least if the detector signal
does not in the interim indicate that the bob has reached one of
the end-of-travel positions; and E) in response to the drive
strokes' durations, generating a viscosity output representative of
the viscosity that the strokes' durations indicate.
10. A method as defined in claim 9 further comprising setting the
duration of the predetermined timeout interval for the first stroke
after the clean-out period to a value at least six times the
clean-out-stroke value.
11. A method as defined in claim 9 wherein the value to which the
duration of the predetermined timeout interval is set in at least
some instances is the sum of the last stroke duration and a
safety-margin value.
12. A method as defined in claim 9 wherein the value to which the
duration of the predetermined timeout interval is set is the sum of
the last stroke duration and a safety-margin value only if that sum
falls within predetermined timeout-value limits.
13. A method as defined in claim 9 wherein the value to which the
duration of the predetermined timeout interval is set is the sum of
the last stroke duration and a safety-margin value only if the
detector signal indicated that the bob had reached one of the
end-of-travel positions during the previous stroke before the end
of the timeout period.
14. A method as defined in claim 13 wherein the value to which the
duration of the predetermined timeout interval is set is the sum of
the last stroke duration and a safety-margin value only if the
detector signal indicated that the bob had reached one of the
end-of-travel positions, during the stroke before the previous one,
before the end of the time-out period.
15. A method as defined in claim 13 wherein the value to which the
duration of the predetermined timeout interval is set is the sum of
the last stroke duration and a safety-margin value only if that sum
falls within predetermined timeout-value limits.
16. A method as defined in claim 10 wherein fixed, clean-out-stroke
value is independent of previous stroke durations.
Description
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
[0001] The present invention concerns viscometers. It is directed
particularly to the type that drives a bob in alternating
directions through the liquid to be measured and infers the
liquid's viscosity from the duration of a bob stroke.
BACKGROUND INFORMATION
[0002] U.S. Pat. No. 4,864,849 to Hubert A. Wright, which is hereby
incorporated by reference, describes a type of viscometer that is
particularly simple mechanically. A bob containing ferromagnetic
material is disposed in a channel that a liquid to be measured can
enter. A coil is so positioned that the magnetic field caused when
current flows through it tends to draw the bob in one direction
along the channel. A second coil is so positioned as to draw the
bob along the channel in the opposite direction. Driving first one
coil and then the other applies an alternating magnetic force to
the ferromagnetic-material-including bob, and the viscosity of the
liquid through which the bob is thus driven can be inferred from a
speed at which it travels through the liquid in response to these
magnetic forces.
[0003] This use of coils to drive the bob is advantageous because
the same coils can also be used for the bob-position sensing that
inferring viscosity from bob speed requires. The Wright patent
mentioned above describes a convenient approach to using the coils
for such sensing. A small AC signal is superimposed on the DC level
used to drive the coil that attracts the bob, and the
magnetic-field component resulting from the driven coil's AC
current causes an AC voltage in the non-driven coil. The non-driven
coil is coupled to a filter, which, among other things, increases
the system's signal-to-noise ratio. Because the bob includes
ferromagnetic material, coil inductance varies with bob position.
In the Wright arrangement, the variation is such that the resultant
filter-output amplitude increases to a maximum when the position of
the bob's ferromagnetic material is approximately symmetrical with
respect to the coils, and the amplitude decreases thereafter. The
Wright arrangement concludes that the bob has reached the end of
its travel when that output's magnitude falls to some predetermined
percentage of the maximum that it had attained during the bob
stroke. The current drive is then switched from one coil to the
other, and the liquid's viscosity is inferred from the time that
elapses between end-of-travel detections.
[0004] The approach that the Wright patent describes is quite
effective, but it has to include provisions that compensate for the
effects of delays that result from the need to enhance the system's
signal-to-noise ratio by filtering the non-driven coil's output. In
a given installation, the viscometer may be intended for use in
measuring the viscosity of a relatively viscous liquid, but that
liquid's flow through a conduit that the viscometer monitors may be
interrupted from time to time by flow of very-low-viscosity liquid.
An example occurs in printing-industry installations when an
ink-color change takes place and a low-viscosity solvent is used to
flush the previous ink color out of the ink lines. The bob travel
through the low-viscosity solvent can be too fast that for
detector's filter to follow variations in the non-driven coil's
output with any precision. As a result, the filter output does not
vary enough to meet the criterion that the system employs to
recognize the bob's having reached its predetermined end-of-travel
position. The system would therefore fail to switch coil drive in
the absence of some contrary provision.
[0005] Systems that have employed the Wright approach have
therefore included provisions for switching coil drive if the
system fails to detect the end-of-travel position within a timeout
period whose duration exceeds a stroke duration corresponding to
the highest expected viscosity. But suppose that the stroke
duration corresponding to the highest viscosity intended to be
measured is a full minute. That means that system flushing with a
very-low-viscosity solvent would cause a delay of at least a minute
before the viscosity of a subsequent, higher-viscosity liquid can
be measured.
[0006] To reduce this delay, some users have made the
timeout-interval duration adjustable, setting it to the sum of some
safety margin and the most-recent valid stroke-duration
measurement. When the unit times out, they gradually increase the
timeout duration until there is a valid end-of-stroke detection
before the timeout period ends, presumably because the next,
higher-viscosity fluid has begun to flow. When the viscosity of the
previous liquid is significantly less than the high end of the
intended viscosity range, the shortened timeout period results in
less delay.
SUMMARY OF THE INVENTION
[0007] We have developed a way reducing the delay even further. In
accordance with our invention, the timeout-interval duration that
prevails after a timeout has occurred is kept constant through
subsequent cycles until a predetermined time period has elapsed, at
least if no valid detection occurs in the interim. When that
timeout period ends, the time-out-interval duration will typically
be increased immediately to a high value.
[0008] We have recognized that such an approach has the potential
to make the viscometer respond more quickly to in response to
viscosity transients of the type mentioned above. Timeouts usually
are the result of the viscometer's encountering a solvent or some
other low-viscosity liquid, as was mentioned above, and, in most
environments, the approximate duration of the solvent's flow is
known ahead of time. The predetermined time period for which the
timeout-interval duration is kept constant will usually be chosen
to approximate the expected time of solvent flow, so a valid
measurement can usually be based on the first stroke after the
timeout interval is raised again. And, if the constant
timeout-interval duration is relatively low, the resultant rapid
bob reciprocation fills the viscometer's bob chamber more rapidly
with the next, higher-viscosity liquid that it can measure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention description below refers to the accompanying
drawings, of which:
[0010] FIG. 1 is a cross-sectional view of the mechanical part of a
viscometer that employs the present invention's teachings;
[0011] FIG. 2 is a block diagram of the viscometer's electronics;
and
[0012] FIGS. 3A and 3B together form a flow chart depicting the
manner in which the viscometer adjusts its timeout period.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
[0013] FIG. 1 depicts in cross section the mechanical part 10 of a
viscometer that embodies the present invention's teachings. A
generally cylindrical shell 12 forms a flange 14 that mates with a
joint flange 16 that a pipe 18 forms. The liquid whose viscosity is
to be measured flows in the pipe.
[0014] The shell forms an interior chamber 19 in which two coils 20
and 22 are mounted. The coils are coaxial with a generally
cylindrical bob guide 24 that extends through the coils' central
voids and forms a bob chamber 26 in which a bob 28 is slidably
disposed. The bob chamber contains liquid from the pipe, and the
liquid's viscosity can be inferred from the speed at which a given
force causes the bob to move through the liquid.
[0015] Surrounding the coils is a coil housing 30 that is generally
cylindrical but has disk-shaped ends forming openings though which
the bob guide 24 extends. Unlike the shell 12, the coil housing 30
is made of ferromagnetic material, as is a disk-shaped divider 32
disposed between the coils 20 and 22. The bob, too, includes
ferromagnetic material, so it can be made to reciprocate by driving
the two coils alternately with current from a cable 34. The bob 28
can be made solely of ferromagnetic material, or it can, say,
include a non-ferromagnetic, corrosion-resistant envelope enclosing
ferromagnetic material.
[0016] Since the bob includes ferromagnetic material, its movement
changes coil inductance, and bob position can therefore be inferred
from measurements of inductance-dependent quantities. Although the
illustrated embodiment measures the amplitude of the voltage signal
that magnetic coupling from the driven coil causes in the
non-driven coil, and although this quantity is a particular
function of both mutual and self inductance, quantities that are
different functions of position-dependent coil inductance can be
measured instead. By thus measuring the time that the bob takes to
move between two positions, the viscometer determines the viscosity
of the liquid in the bob chamber.
[0017] As the bob 28 reciprocates, it tends to refresh the contents
of the bob chamber 26. Specifically, the bob's movement away from
the bob chamber entrance tends to drive fluid from the bob chamber
into the pipe, and the bob's movement toward the entrance tends to
draw fluid from the pipe into the bob chamber.
[0018] FIG. 2 depicts circuitry to which cable 34 of FIG. 1
connects the viscometer's mechanical part. A drive circuit 36
cooperates with a microcomputer 38 to act as a coil driver that
alternately drives coils 20 and 22. A_ON and B_ON signals generated
by a microcomputer 38 indicate which coil the drive circuit is to
drive. In determining which coil to select, the microcomputer 38
relies on a DETECT signal, which detection circuitry 40 generates
to indicate when the bob 26 has reached either of two predetermined
positions in its travel. As will be explained in more detail
presently, the detection circuitry cooperates with the
microcomputer 38 to act as a position detector that monitors the AC
signals on coils 20 and 22 for this purpose. The microcomputer 38
also uses the time between successive DETECT signals in determining
the liquid's viscosity: it bases the value of a VISCOSITY output
that it generates on bob 28's round-trip travel time.
[0019] The drive circuitry 36 includes a clock 42 whose output is a
square wave having a DC level. A low-pass filter 44 removes the
higher-frequency components from the square wave to produce an
approximately sinusoidal AC component. In the filter's output, this
AC component is superimposed on a DC level set by a
digital-to-analog converter 46's output. The microcomputer 38 uses
the digital-to-analog converter 46's input signal, GAIN, to control
that DC component. The DC component provides the coils' main drive
current. The current that the AC component causes to flow in one
coil causes an AC voltage whose amplitude is a function of
position-dependent inductances. The detection circuit 40 monitors
that voltage.
[0020] Switches 48 and 49 respond to the A_ON and B_ON signals by
forwarding filter 44's output selectively to respective driver
circuits 50 and 51, which respectively drive coils 20 and 22. The
drivers 50 and 51 are high-output-impedance circuits: they produce
currents whose magnitudes are determined by drive-level signals
from the microcomputer and are not greatly affected by
coil-impedance changes. The A_ON and B_ON signals are so timed that
the coils are driven alternately: when coil 20 is being driven,
coil 22 is not, and vice versa.
[0021] Taking its state from the value of the A_ON signal, the
detection circuit 40's switch 52 forwards the non-driven coil's
voltage to a band-pass filter 54. That filter's output amplitude
depends on bob position. For one thing, the magnetic coupling
between the coils depends on that position. More important in the
illustrated embodiment, that filter's center frequency depends on
the inductance of the coil 20 or 22 to which the switch 52 connects
it. That in turn depends on bob position in such a manner that the
center frequency equals clock 42's fixed fundamental frequency,
i.e., the excitation frequency, when the bob is in the middle of
its travel. Filter 54 feeds its output to the remainder of the
detection circuitry 40. As was mentioned above, that circuit's
purpose is to determine when the bob 26 has reached a predetermined
point in each stroke.
[0022] For the sake of discussion, we will assume that the
viscometer's mechanical part 10 is oriented vertically, as FIG. 1
indicates, although orientation is largely irrelevant. When the bob
26 begins its top-to-bottom stroke, most of the bob 28's
ferromagnetic material is initially disposed between the coil
housing 30's top end and ferromagnetic divider 32, with the result
that coil 20's inductance is relatively low. As the bob moves down,
that coil's inductance falls, moving the filter's center frequency
closer to the excitation frequency until the bob's ferromagnetic
material is positioned more or less symmetrically with respect to
the coils. As the filter's center frequency thus approaches the
excitation frequency, the filter output's amplitude increases.
After that, further downward travel places most of the bob's
ferromagnetic material between the ferromagnetic divider 32 and the
coil housing's lower end. The resultant further reduction in coil
20's inductance now moves the filter's center frequency past the
excitation frequency, so the filter output's amplitude falls below
the mid-stroke peak. By determining when the amplitude has fallen
to a predetermined percentage of its peak value, the detection
circuitry determines when the bob 26 has reached a predetermined
position toward the end of its downward travel.
[0023] Specifically, the filter 54 applies its output to a peak
detector 56. The peak detector retains as its output the highest
instantaneous voltage that it has received from filter 54 since a
transition in the microcomputer's RESET output last reset it, at
the beginning of the stroke. From that peak voltage, a voltage
divider 58 produces an output that is, say, 90% of the peak
detector's output. A comparator 60 subtracts this 90%-peak signal
from the filter output and thereby produces a square wave so long
as the peaks of the filter output exceed 90% of their highest
previous level during the current stroke. That is, the comparator
output takes the form of a square wave while the amplitude
increases with downward travel, and it continues to be a square
wave until the amplitude falls back to 90% of the peak. At that
point, the square wave ceases, indicating that the predetermined
position has been reached.
[0024] The comparator sends its output to a one-shot circuit, a
retriggerable monostable multivibrator 62. The one-shot's purpose
is to generate a high output so long as the square wave is present:
its output must stay high between triggerings by the comparator
output's low-to-high transitions, but it must eventually go low
when the square wave ceases. So the one-shot 62's characteristic
delay is greater than the clock period and thus greater than the
period of comparator 60's square-wave output. Preferably, that
characteristic delay is actually several clock periods, because
this makes the detection circuitry relatively immune to noise that
might suppress one of comparator 60's output pulses. When the bob
26 reaches the predetermined position and the comparator 60's
output square wave therefore ceases, the one-shot 62 stops being
triggered, and its output, the DETECT signal, goes low after the
one-shot's characteristic delay.
[0025] Normally, the microcomputer 38 treats this high-to-low
DETECT-signal transition as an end-of-stroke indication. In
response to this indication, it reads and resets a stroke-duration
counter that it has incremented periodically since the last such
resetting, it changes the states of switches 48 and 49 to start
driving the bob 26 in the opposite direction, it operates switch 52
to the state in which the detection circuit 40 receives the voltage
of the coil from which coil drive has just been removed, and it
resets the peak detector 56. It then adds the just-read stroke
duration to the previous stroke's duration and computes from the
result the viscosity of the liquid through which the bob traveled,
and it generates a VISCOSITY output, which represents the value
thereby computed.
[0026] There are occasions when the microcomputer changes switch
states and thereby begins another stroke without waiting for the
DETECT-signal transition. To understand why, remember that the
filter 54 must have a bandwidth narrow enough to provide the
required degree of noise suppression. This means that it can
respond to input changes only slowly. A consequence of its slowness
to respond is that, if the bob travels too fast, the filter
output's peak will not exceed its other values by enough for the
output of divider 88 ever to exceed the filter output's
instantaneous peaks. So comparator 60's output will keep on
triggering monostable multivibrator 62 even after the bob reaches
the end of its travel, and the DETECT signal will not make the
high-to-low transition that the microcomputer treats as the
end-of-travel indication.
[0027] In the absence of a provision to deal with this, the same
coil-say, coil 22--would continue to be driven indefinitely,
although a stop 64 (FIG. 1) provided for that purpose would end bob
travel. So bob time-out routines have been employed to deal with
this possibility. If the stroke-duration counter reaches a
predetermined timeout value, viscosity is not computed from it, and
the coil drive is switched even though the circuit has not detected
that the bob has reached the end of its travel. This prevents the
viscometer system from "hanging up."
[0028] A way of employing such a time-out routine is to make the
time-out duration some fixed value equal to, say, 20% higher than
the highest expected bob-stroke duration. According to the present
invention, though, that timeout duration varies in accordance with
previously measured stroke durations, as will now be explained.
[0029] FIGS. 3A and 3B (together, "FIG. 3") form a flow chart of
one of typically several routines that the microcomputer 38 enters
at the end of a bob stroke. As was just explained, the circuitry of
FIG. 2 ordinarily considers a stroke to end when detection of the
bob's passing through one of the predetermined positions causes the
DETECT signal to make a high-to-low transition. Block 66 represents
the occurrence of this transition or the end of the timeout
interval, and such an event is the trigger for the microcomputer 38
to perform the remainder of FIG. 3's operations, typically after it
performs some other similarly triggered tasks.
[0030] As will presently be explained in more detail, the timeout
interval's duration is variable in accordance with the present
invention, but systems that employ the present invention's
teachings may additionally provide a mode in which the timeout
duration does not vary, and block 68 represents determining whether
the system is in the mode in which its timeout duration is indeed
variable. If not, the system simply skips the remainder of FIG. 3's
operations, as block 70 indicates.
[0031] Also, most such systems will provide a calibration mode, in
which the present invention's teachings would not be employed,
either. Block 72 represents therefore determining whether the
system is in that mode. If not, the system employs the present
invention's teachings of making the timeout interval's duration
depend on recent stroke-interval durations. Although embodiments of
the present invention may employ any one of a wide variety of
relationships between recent stroke-interval durations and timeout
duration, the illustrated embodiment employs a timeout duration
that simply equals the sum of the just-measured stroke duration and
a safety margin, but only if the resultant timeout duration is
within predetermined limits and only of the reliability of the
most-recent stroke-interval duration's measurement is indicated by
the fact the end-of-travel detections occurred before the end of
the timeout interval for both of the previous two bob strokes.
[0032] Block 74 represents imposing the latter condition: it is
only if the condition is met that the routine performs block 76's
step of making the timeout duration equal to the last measured
stroke duration plus some safety margin. Blocks 78, 80, 82, and 84
represent keeping the timeout duration within predetermined
limits.
[0033] If a valid end-of-travel detection did indeed occur, then
the determination made in the step represented by block 86 is
negative: the timeout interval did not expire without a valid
detection. In that case, the timeout interval has been set
appropriately, and the microcomputer ends its performance of the
FIG. 3 routine, as block 70 indicates. If that routine was instead
entered as a result of the timeout interval's having ended before a
valid end-of-travel detection, on the other hand, the illustrated
embodiment begins a clean-out operation, which will continue for
several strokes.
[0034] To understand why the illustrated embodiment performs the
clean-out operation, it helps first to consider the likely reasons
why the timeout interval has elapsed. One possible reason is that
the liquid being measured is so viscous that the stroke duration
exceeded the timeout-interval maximum imposed in FIG. 3's block 84.
In most instances, though, that maximum is set to a value greater
then any viscosity likely to be encountered in the viscometer's
environment, so this is not the normal reason why the timeout
interval will have elapsed.
[0035] More typically, the timeout interval has elapsed because the
viscosity has fallen below the normal measurement range. Such a
situation may arise in printing environments, for example. The
liquid whose viscosity is to be measured may be a relatively
viscous ink, but the lines carrying the ink may be flushed out from
time to time with a solvent whose viscosity is well below the
expected measurement range. When the lines thus contain solvent,
the bob moves too fast for the detection circuitry, and a valid
end-of-travel detection does not occur. When this happens, it is
desirable for the bob to reciprocate rapidly for some time. The
rapid reciprocation enables the solvent to remove ink of the
previous color from the bob chamber quickly. Then, when the new ink
begins to flow, it rapidly draws ink of the new color into that
chamber.
[0036] In the absence of the present invention's timeout-duration
adjustment, though, the time that would elapse before the system
reacts to the solvent flow by timing out would be relatively long;
it would exceed the length of time that a valid stroke would take
for a liquid whose viscosity is at the high end of the range. In
contrast, the present invention enables the system to respond more
quickly whenever the previous liquid's viscosity was not at the
high end of the range.
[0037] It responds by entering a period of rapid reciprocation. To
this end, the routine of FIG. 3 includes a step, represented by
block 88, of starting a clean-out timer, which is intended to count
up to a value representing the intended duration of a clean-out
period, during which the bob reciprocates rapidly to refresh the
bob-chamber contents. That clean-out duration will depend on the
particular environment in which the viscometer is employed. In the
case of the multi-color-ink environment just mentioned, the
duration may approximately equal the expected length of time
required for solvent to flush the lines and for flow of a new-color
ink to begin. Such an interval will span many bob reciprocations,
so the result of the block-90 test will be negative during an
execution of the FIG. 3 routine in which the block-88 operation
initializes the clean-out timer. As block-92 indicates, therefore,
the timeout duration that will be employed for the next stroke is a
relatively short, cleanout-stroke duration. As block 70 indicates,
the FIG. 3 routine returns each time after thus setting the timeout
duration to the clean-out stroke duration, and the system awaits
the end of the next stroke.
[0038] The clean-out-stroke duration is so short that the stroke
timer next times out before any valid detection can occur. The
routine will therefore be entered the next time as a result of a
timeout rather than as a result of a valid end-of-stroke detection.
So there will again be a positive result when the routine reaches
the step that block 86 represents. The block-88 and block-92 steps
of incrementing the clean-out timer and setting the is timeout
duration to the short, clean-out-stroke duration will be repeated,
so a timeout rather than a valid end-of-stroke detection will again
be what causes the routine to be entered the next time.
[0039] The timeout-interval duration used in the illustrated
embodiment during the clean-out period is a preset value that is
independent of events that occur during viscometer operation. But
some embodiments may instead give it a value that depends on the
duration of one or more previous, valid strokes. Once the clean-out
period starts, though, that duration remains the same, constant
clean-out-stroke duration.
[0040] After a number of clean-out-period strokes, block 88's
incrementing the clean-out timer causes it to reach the intended
clean-out duration. When that happens, the block-90 test yields a
positive result, and the timeout duration, which had been the
short, clean-out-stroke duration, is immediately set to the stroke
duration corresponding to the high end of the viscosity scale. What
that duration is will vary from installation to installation, but
it will be relatively long, equal to at least six times the
clean-out-stroke duration, and typically considerably longer. In
one system in which we have employed a clean-out-stroke duration of
two seconds, for example, the timeout duration is immediately
increased to thirty-two seconds at the end of the clean-out
time.
[0041] Now, the clean-out time will have been so set as normally to
extend through the solvent-flush interval and into the beginning of
the new ink's flow. And the rapid reciprocation that resulted from
the short timeout duration will rapidly have filled the bob chamber
with the new ink. The fluid in the bob chamber will therefore cause
the bob to travel slowly enough to permit a valid end-of-stroke
detection. So such a detection is what will cause the routine to be
entered the next time, and the viscometer will be able to make a
valid measurement of the new ink's viscosity.
[0042] In short, the rapid reciprocation that in accordance with
the present invention occurs during the clean-out interval enables
the viscometer to respond rapidly to the ink change, and the change
in timeout duration from the short, clean-out-stroke duration to
the maximum stroke duration enables that first stroke after the
clean-out interval ordinarily to be used in making a valid
viscosity measurement. So the present invention enables the
viscometer to respond more quickly than conventional arrangements
and therefore constitutes a significant advance in the art.
* * * * *