Abstrict A microcomputer-based programmable control system for desiccant
air dryers monitors the level of moisture saturation of the desiccant
beds and adjusts the regeneration cycle accordingly to optimize
energy efficiency. The moisture sensors have capacitance probes
and adjustable resistors as frequency determining components for
precision oscillators. A multiple counter integrated circuit is
controlled by the microcomputer to establish a predetermined time
interval and to count cycles of the precision oscillators during
the predetermined time interval to obtain final counts for each
probe indicative of the level of moisture saturation in the respective
desiccant beds. The microcomputer compares the final counts to predetermined
threshold number in order to determine whether a regeneration cycle
is required, and also compares the final counts to maximum and minimum
threshold numbers to detect and indicate probe failure. A pair of
high and low pressure sensitive switches is provided for each desiccant
bed, and the microcomputer tests for coincidence of the high switches
and the low switches to monitor full depressurization and repressurization
of the regenerating bed and to analyze and indicate valve malfunction.
The control system directly activates low current, low capacity
solenoid valves which in turn operate high pressure pneumatic relays
to drive the valves in the air dryer.
Claims What is claimed is:
1. A control system for a desiccant gas dryer having a first desiccant
bed and a second desiccant bed, gas to be dried being alternately
directed by valves to pass through one of the beds providing adsorbtion
of vapor while the other desiccant bed is being regenerated or is
in an idle condition in response to at least one bed select signal
and at least one regeneration enable signal, respectively, energy
being required for the regeneration process, the control system
comprising, in combination,
a capacitance probe in each desiccant bed so that the level of
condensate saturation of the desiccant in each bed is indicated
by the capacitance of the respective probe,
at least one oscillator having a frequency setting capacitance
input connected to the probe in the desiccant bed having been used
for adsorbtion of vapor, so that the oscillator frequency is indicative
of the capacitance of the connected probe and thus indicative of
whether the desiccant bed needs to be regenerated,
timer means for establishing a predetermined time interval,
means for counting the cycles of the oscillator frequency over
said predetermined time interval,
means for obtaining a final count from the means for counting at
the end of said predetermined time interval so that the numeric
value of the final count is indicative of the level of condensate
saturation of the respective desiccant bed, and
a microcomputer executing a stored sequence of instructions, said
microcomputer comprising
first means for comparing the final count to a predetermined threshold
count representative of the condensate saturation level for which
regeneration is required, and
means for generating said regeneration enable signal in response
to said means for comparing the final count to the predetermined
threshold indicating that regeneration is required.
2. The control system as claimed in claim 1 wherein the microcomputer
further comprises
means for periodically changing the bed select signal at least
once within a preset minimum time period, so that the vapor load
is shared between the desiccant beds.
3. The control system as claimed in claim 2 wherein the microcomputer
further comprises:
second means for comparing the final count to a predetermined maximum
count,
third means for comparing the final count to a predetermined minimum
count,
means for indicating to a maintenance person that probe failure
is sensed when the second means for comparing indicates that the
final count exceeds the maximum count, and also being activated
when the third means for comparing indicates that the final count
is less than the predetermined minimum count, and
means for activating the regeneration enable signal at least once
after each change of the bed select signal, so that the beds are
periodically regenerated regardless of probe failure.
4. The control system as claimed in claim 1 being operative for
controlling a heaterless gas dryer wherein the beds are regenerated
at a predetermined minimum pressure and the gas is dried at a predetermined
elevated pressure, and further comprising:
for each bed a high pressure sensing switch and a low pressure
sensing switch responsive to the gas pressure in the respective
bed, the threshold of the high pressure switches being substantially
below the elevated gas pressure, and the threshold of the low pressure
switches being substantially below the threshold of the high pressure
switches and substantially above the minimum pressure,
means for generating a high pressure coincidence signal active
when the high pressure switches are in the same states, means for
generating a low pressure coincidence signal active when the low
pressure switches are in the same states, and
wherein the microcomputer comprises first means for sampling both
the high pressure coincidence signal and the low pressure coincidence
signal at least once, when the regeneration enable signal is active,
during each full cycle of the bed change signal, at a time sufficiently
delayed from the times of change in the bed change signal and the
regenerate enable signal so that for the pressures in the beds stabilize
prior to sampling, and
means for indicating the occurrence of a general valve malfunction
to a maintenance person if at least one of the samples of the pressure
coincidence signals sampled by the first means for sampling are
active.
5. The control system as claimed in claim 4
further comprising second means for sampling each pressure coincidence
signal at least once when the regeneration enable signal is not
active, and during each full cycle at a time sufficiently delayed
from the time of change in the bed change signal and the regenerate
enable signal so that the pressures in the beds stabilize prior
to sampling by said second means for sampling, and
means for indicating the occurrence of an exhaust valve malfunction
if at least one of the samples of the pressure coincidence signal,
sampled by the second means for sampling, is not active.
6. The combination as claimed in claim 1 further comprising a
multiple counter integrated circuit external to the microcomputer,
wherein said multiple counter integrated circuit itself comprises
said means for counting, said timer means and said means for obtaining
a final count from the means for counting.
7. The combination as claimed in claim 1 further comprising means
for the microcomputer to activate said valves to alternately direct
the gas to be dried through the beds, wherein said valves are operated
by high pneumatic pressure, said means for the microcomputer to
activate said valves comprising,
low flow rate capacity solenoid valves driven by electrical signals
from the microcomputer, and
pneumatic relays driven by the pneumatic outputs of the solenoid
valves and driving said pneumatically operated valves.
Description TECHNICAL FIELD
This invention relates generally to the field of gas dryers. In
particular, this invention relates to a control for a desiccant
gas dryer of the type having a pair of desiccant beds, wherein one
bed is regenerated as the other bed adsorbs vapor. A control is
required to periodically switch the flow of gas between the bed
adsorbing vapor to the bed being regenerated.
BACKGROUND ART
The desirability of drying air after compression is well known.
A variety of systems have been developed over the years for this
purpose, many of which employ two beds containing an adsorbent desiccant
material such as activated alumina. In such systems the beds alternatively
dry the process stream and then are regenerated either using an
auxiliary source of heat in so-called heat-reactivated systems,
or through conservation and use of the heat of adsorption for effecting
regeneration, in so-called heaterless systems. In heat-reactivated
systems, it has been common to use fixed cycles several hours in
length, such that a bed is on adsorption for a period of time, perhaps
eight hours, after which it is regenerated during the eight hour
adsorption cycle for the other bed. In the heaterless systems relatively
short, fixed cycle times are usually used, typically about four
minutes for drying with four minutes allowed for regeneration while
the other bed is drying the process stream. Mechanical cam timers
operating solenoids which in turn operate the flow control valves
have typically been employed for the control of such fixed cycle
systems.
Typically both heat-reactivated and heaterless systems are sized
for maximum contemplated loading conditions, a condition which may
seldom or never be encountered during actual operation. As a result,
at the end of a fixed drying cycle a bed may have adsorbed only
a fraction of its moisture capacity. In the case of heat-reactivated
dryers, such underutilization of the bed while on adsorption coupled
with a full energy regeneration can seriously affect overall operating
efficiency. Moreover, in the case of heat-reactivated dryers which
employ resistance heaters within the beds, sometimes referred to
as internal heat-reactivated dryers, regeneration with only a partial
moisture load on the bed can result in excessive heating of the
desiccant, which not only tends to degrade the desiccant but can
also be dangerous.
Similarly, off-peak operation of heaterless dryers on fixed cycles
can result in low operation efficiencies. By their nature heaterless
dryers employ approximately 15% of the dry product air for regeneration
purposes such that unnecessarily purging a partially loaded bed
for the full regeneration period is highly inefficient.
Several systems have been suggested in recent years using "demand"
rather than fixed cycling for both heat-reactivated and heaterless
dryers. U.S. Pat. No. 3448561 discloses a system for sensing the
actual moisture content of the on-stream bed and terminating the
adsorption cycle when the bed is saturated. Alternatively, the patent
suggests that the moisture content of the regenerating bed can be
monitored, terminating the purge flow when the moisture content
of the bed has dropped below a predetermined minimum. That patent
discloses the use of a lithium chloride humidity sensor external
to the desiccant bed with gas sampling conduits leading from the
beds to the cell.
Another method which has been suggested for determining the moisture
loading of a bed involves the placing of a capacitor directly into
an adsorbent bed with adsorbent between the plates. As the moisture
loading varies, the dielectric constant of the adsorbent changes,
such that the capacitance provides an electrical indication of the
moisture loading. Such a capacitance sensing arrangement is disclosed
in application Ser. No. 267964 filed May 27 1981.
In one system which has been developed for the use of a capacitance
as disclosed in the co-pending application, a periodic electric
potential is applied to the capacitance probe so that its response
is a function of the capacitance. A frequency to voltage converter
generates a moisture indicating voltage proportional to the capacitance
of the capacitance probe. Typically a differential amplifier compares
the moisture indicating voltage to a threshold voltage to generate
a binary signal indicating that the desiccant is fully regenerated.
The moisture indicating voltage is also compared to high and low
threshold voltages defining a range of moisture content from 0 to
100% saturation of the desiccant. Any voltage out of this range
is assumed to be due to sensor malfunction and hence the "window
comparison" provides a failure signal.
The prior art moisture sensor comprising the capacitance probe
and the electronics for performing a frequency to voltage conversion
and threshold comparison function performs its intended function
of generating a binary signal indicating the end of required regeneration,
but it does so using rather sensitive and "touchy"analog
circuits. The analog circuits require stable reference voltages
and are sensitive to component aging, tolerances and drift. In order
to eliminate undesired noise on the moisture sensing voltage the
moisture sensing voltage analog signal is typically low-pass filtered
at a time constant of approximately ten seconds. This time constant
slows down the response of the sensor and the rather high resistance
and capacitance component values for providing the ten second time
constant are undesirable. The one binary bit of moisture indicating
information provided to the air dryer control system by the moisture
sensor is a rather small portion of the information monitored by
the sensor, but it is just about all of the information that the
prior art control system can use.
As suggested in the co-pending application referenced above a binary
signal indicating that the desiccant is fully regenerated may be
used in conjunction with the flow valve control system with either
heat-regenerated or heaterless dryers to result in "demand"
cycling.
In addition to the deficiencies noted with the previously suggested
capacitance-monitoring system, known controls for valving to alternatively
shift flow between desiccant beds have numerous disadvantages. As
already noted mechanical cam timers have typically been employed
to provide fixed length cycles. Such mechanical timer circuits have
recently been adapted for use with capacitance sensing arrangements
of the type discussed above to provide "demand" cycling
in both heat-regenerated and heaterless dryers by inhibiting certain
of the timing signals. Such mechanical timer based systems tend
to be bulky, offer little or no "reprograming" flexibility
and can be difficult to accurately adjust.
DISCLOSURE OF INVENTION
The general aim of the invention is to provide an intelligent control
system for desiccant gas dryers which monitors at least the level
of vapor saturation of the desiccant beds and adjusts the regeneration
cycle accordingly to optimize energy efficiency.
Another object of the invention is to provide a modular control
system which is easily adapted to various types and sizes of desiccant
gas dryers. A related object is to provide a control system that
is easily specified, installed and maintained.
Yet another object of the invention is to provide a desiccant moisture
sensor which is essentially digital in nature and which supplies
the control system with an increased amount of usable information.
A related object is to provide a desiccant moisture sensor that
has a reduced parts count and which is easier to assemble, calibrate
and maintain, and which is not substantially affected by component
tolerances, aging and drift.
Still another object of the invention is to provide a gas dryer
control system having expanded system diagnostic and condition indicating
functions.
In accordance with the invention, each desiccant bed in a desiccant
gas dryer has a capacitance probe so that the level of condensate
saturation of the desiccant is indicated by the capacitance of the
respective probe. The capacitance of the probe determines the frequency
of an oscillator so that the oscillator output is a binary or digital
signal indicating the level of condensate saturation in the respective
bed. Means are provided for counting the cycles of the oscillator
output during a predetermined time interval established by timer
means. Means are provided for obtaining a final count from the means
for counting at the end of the predetermined time interval so that
the numeric value of the final count is indicative of the level
of condensate saturation of the respective desiccant bed. A microcomputer
executing a stored sequence of instructions accepts the final count
for determining the relative condensate saturation level and accordingly
controls the regeneration of the respective desiccant bed. The microcomputer
comrpises means for comparing the final count to a predetermined
threshold count representative of the condensate saturation level
for which regeneration is required, and further comprises means
for generating a regeneration enable signal in response to that
comparison.
In accordance with further features of the invention, the microcomputer
comprises means for periodically changing the bed selected for drying
and the bed selected for regeneration within a preset minimum period,
so that the vapor load is shared among the desiccant beds.
In order to detect probe failure, preferably the microcomputer
has means for comparing the final count to a predetermined maximum
count and a predetermined minimum count, and also means for indicating
to a maintenance person that probe failure is sensed if the final
count either exceeds the maximum count or is less than the minimum
count. The microcomputer also comprises means for activating the
regeneration of the beds at least once during each bed select cycle
so that the beds are periodically regenerated in the event of the
sensed probe failure.
In an important embodiment of the invention, the control system
is specifically configured for controlling a heaterless gas dryer
wherein the beds are regenerated at a minimum pressure such as atmospheric
pressure, while the gas to be dried is at an elevated pressure.
In order to detect the failure of the valves which pressurize and
depressurize the beds for drying and regeneration, respectively,
for each bed a high pressure sensing switch and a low pressure sensing
switch is provided. The threshold pressure of the high pressure
switches is substantially below the elevated gas pressure, and the
threshold of the low pressure switches is below the threshold of
the high pressure switches and substantially above the minimum pressure.
In other words, each high pressure switch is in its active state
when the respective bed is pressurized to the elevated pressure,
so that the high pressure switch detects substantially full pressurization,
while each low pressure switch is in its corresponding inactive
state only when the bed is approximately at the minimum pressure,
thereby indicating proper valve operation for depressurization.
In order to reduce the number of pressure switch signals to be
sampled by the microcomputer to determine proper valve operation,
means are provided for generating a high pressure coincidence signal
active when the high pressure switches are in the same state, and
means are provided for generating a low pressure coincidence signal
active when the low pressure switches are in the same state. The
microcomputer comprises means for sampling both the high pressure
coincidence signal and the low pressure coincidence signal at least
once during each full cycle of the bed change signal, at times sufficiently
delayed from the time that the beds are switched over from drying
to regeneration and at times sufficiently delayed from the beginning
of regeneration. If the idle bed is depressurized, its switches
are in their inactive states and thus different from the active
states of the pressurized drying bed. Thus if the idle bed is properly
depressurized, the coincidence signals will be inactive. But when
the idle bed is repressurized, its switches will be in their active
states and thus in the same states as the active switches in the
drying bed. Consequently, when the idle bed is properly repressurized
the coincidence signals are in their active states. Means are provided
for indicating the occurrence of a general valve malfunction to
a maintenance person if either one of the pressure coincidence signals
is active when sampled during the time the regeneration enable signal
is active. Similarly, means are provided for indicating the occurrence
of an exhaust valve malfunction when the idle bed fails to be repressurized,
as sensed by an inactive coincidence signal sampled when the regeneration
enable signal is not active.
In a preferred embodiment a single chip microcomputer integrated
circuit is coupled with a multiple counter integrated circuit. The
counter integrated circuit itself embodies the means for counting,
the timer means and the means for obtaining a final count from counting
means.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the invention will become apparent
upon inspection of the drawings, wherein:
FIG. 1 is a perspective view of a heaterless air dryer employing
an exemplary embodiment of the control system for controlling a
heaterless dryer;
FIG. 2 is a simplified schematic diagram of the heaterless dryer,
moisture sensing probes and pressure switches according to the exemplary
embodiment of the invention;
FIG. 3A is a detailed schematic diagram corresponding to the simplied
schematic shown in FIG. 2;
FIG. 3B is an alternative detailed schematic diagram corresponding
to the simplified schematic shown in FIG. 2;
FIG. 3C is a detailed diagrammatic view, in partial section, of
a solenoid valve and pneumatic relay of the type used in FIGS. 3A
and 3B;
FIG. 4 is a block diagram of the exemplary embodiment of the control
system according to the invention;
FIG. 5 is a schematic diagram of a preferred embodiment of the
probe interface;
FIG. 6 is a schematic diagram of a preferred embodiment of the
pressure switch interface for generating high and low pressure coincidence
signals;
FIG. 7 is a schematic diagram of the microcomputer, counter integrated
circuit, power supply and output buffers for driving condition indicating
LEDs and valve control solenoids used in the exemplary embodiment
of the control system;
FIG. 8 is a flow chart for the initialization and fixed control
cycle procedure or sequence of instructions for execution by the
microcomputer;
FIG. 9 is a flow chart of the proble-controlled cycle procedure
for controlling the dryer; and
FIG. 10 is a flow chart for the procedure which initially loads
the counters in the counter integrated circuit, reads the final
counts, and performs the comparison functions on the final count
to sense probe failure and to determine whether the idle desiccant
bed is to be regenerated.
While the invention has been described in connection with a preferred
embodiment it will be understood that the intention is not to be
limited to the particular form of the invention which has been shown,
but the intention is, on the contrary, to cover the various alternative
and equivalent constructions included within the spirit and scope
of the appended claims.
BEST MODE FOR CARRYING OUT THE INVENTION
Turning now to the drawings, there is shown in FIG. 1 a prespective
view of the main components of a heatless air dryer generally designated
20. A corresponding schematic diagram is shown in FIG. 2. The air
dryer 20 has left and right desiccant beds or chambers 21 22 for
drying air received on an inlet pipe 23a from a compressor 23. An
inlet valve assembly generally designated 24 alternately switches
the compressed air to the left or right chamber. Similarly an output
valve assembly generally designated 25 passes the dry compressed
air to an outlet pipe 26. A portion of the dried compressed air
expands to approximately atmospheric pressure through a bleeder
valve 27 (having an associated pressure gauge 43 and orifice 44)
which is alternately switched to the idle desiccant bed for regeneration.
After picking up moisture from the desiccant in the regenerating
bed, the air is vented to atmospheric pressure through an exhaust
valve assembly 29.
The positions for the valves 24 25 28 and 29 are shown for the
left desiccant bed 21 serving to dry the air and the right desiccant
bed 22 being regenerated. The valves are switched to their alternate
positions in order to use the right desiccant bed 22 to dry the
compressed air and to allow the left desiccant bed 21 to be regenerated.
As shown in FIG. 1 the electronic components of the control system
are enclosed in a centrally located housing 30 and the control system
activates the valves 24 25 28 and 29 by pneumatic lines 31. The
control system also has left and right moisture sensing capacitance
probes 32 33 and pressure switches 34 35 36 37 for sensing the
conditions inside the respective left and right desiccant beds 21
22.
A more complete schematic diagram of the mechanical components
and sensors for the heaterless dryer are shown in FIG. 3A. The inlet
valve assembly 24 is comprised of a single three-way inlet valve
40 driven by a differential pressure input. Associated with this
three-way inlet valve is a repressurizing valve 41 which permits
the bed having been previously regenerated to be repressurized without
substantially interrupting the flow of dry air out the outlet pipe
26. In other words, if the inlet valve 40 were merely switched to
repressurize the regenerated bed, the flow of dry air out the outlet
pipe 26 would be disrupted during the time required for the pressure
to build up in the regenerated bed. In the same fashion as the inlet
valve assembly 24 the outlet valve assembly 25 is comprised of
a single three-way outlet valve 42 which is driven by the same differential
pressure that operates the three-way inlet valve 40.
The bleeder valve 27 proportionately divides the outlet pressure
across itself and the orifice 44 so that the flow through the orifice
is regulated by the adjustment of the bleeder valve. The pressure
gauge 43 is responsive to the pressure across the orifice 44 and
thus indicates the air flow to the regenerating desiccant bed. The
air for regeneration passes through the bleeder valve 27 and through
the orifice 44 to a pair of check valves 45 46 which comprise the
valve assembly 28 which directs the dry air to the desiccant bed
being regenerated.
The exhaust valve assembly 29 is comprised of two separate two-way
valves 47 48 for dumping the exhaust from the respective chambers
during regeneration. These two-way dump exhaust valves are actuated
by the difference in pressure in the respective desiccant bed and
respective control pressures. The dump exhaust is passed to a dump
flow restrictor 49 which limits the exhaust air flow to a reasonably
small rate so that the desiccant beads or pellets in the desiccant
bed are not "fluidized" when the respective dump exhaust
valve 47 48 is first opened at the start of regeneration. An exhaust
muffler 50 reduces air dynamic noise associated with the dumping
of the pressurized bed.
The inlet valve 40 repressurizing valve 41 outlet valve 42 and
exhaust valves 47 48 are operated by pneumatic pressures P.sub.C,
P.sub.B, P.sub.E, P.sub.A, P.sub.D, respectively,generated by a
solenoid valve and pneumatic relay assembly generally designated
51 including a set of pneumatic relays 200 operated by electromagnetic
solenoid valves 201. The use of pneumatic relays 200 in addition
to solenoid valves 201 permits the solenoid valves to be operated
by very low relay coil currents of about 250 milliamperes. The pneumatic
relays 200 switch air at the high pressure SP of the dry gas outlet
26 as supplied through a valve 52 and a prefilter 53. The solenoid
valves 201 however, operate at a relatively low regulated pressure
RP of up to 100 PSI above atmospheric pressure, as set by an adjustable
pressure regulator 54 and have a very low flow capacity when they
are open. The solenoid valves 201 are all normally closed (NC),
while the pneumatic relays 200 are either normally open (NO) or
normally closed (NC) as designated in FIG. 3A.
The valve arrangement shown in FIG. 3A is especially suited for
high capacity air dryers (e.g., FIG. 1) having, for example, three
inch diameter pipes and valves in the process air stream (bold path
in FIG. 2). An alternative valve arrangement suitable for lower
capacity air dryers is shown in FIG. 3B. Two separate one-way inlet
valves 40a, 40b are used in lieu of the two-way inlet valve 40 shown
in FIG. 3A, and check valves 42a, 42b are substituted for the two-way
outlet valve 42 shown in FIG. 3A.
A partial cross-sectional view of the solenoid valve and pneumatic
relay assembly 51 is shown in FIG. 3C. The solenoid valve 201 is
a three volt model EV-3M miniature electronic valve manufactured
by Clippard Instrument Laboratory, Inc., 7390 Colerain Road, Cincinnati,
Ohio 45239. The solenoid valves 201 are screwed into a common manifold
202. The pneumatic relays 200 are recessed into vertical bores 203
in the common manifold 202. Vertical bores 203 receive the pneumatic
relays 200 which are model MJV-3C or model MJVO-3C (normally closed
NC or normally open NO, respectively) cartridge valves also manufactured
by Clippard Instrument Laboratory, Inc. End caps 204 retain the
cartridge valves 200 into the bores.
For each solenoid valve 201 and cartridge valve 200 a longitudinal
bore 205 supplies regulated pressure RP to the solenoid valve 201
through a threaded inlet port 206 on the solenoid. The threaded
inlet port 206 also provides the mechanical connection between the
solenoid valve 201 and the common manifold 202. The solenoid valve
201 is activated by electrical current received from lead wires
207. A traverse bore 208 receives the regulated pressure when the
solenoid valve 201 is energized. A piston 209 responds to the switched
regulated pressure and drives the valve stem 210 of the associated
cartridge valve 200. A spring internal to the cartridge valve 200
provides a return force, since a traverse bore 211 sets the opposing
pressure on the piston 209 to atmospheric pressure.
A longitudinal bore 212 conveys the supply pressure SP to the cartridge
valves 200. For each cartridge valve, the respective control pressure
P.sub.A -P.sub.E is passed via a traverse bore 213 to a pneumatic
line 214 secured to the common manifold 202 by a connector 215 screwed
into the traverse bore 213.
A block diagram of the control system according to an exemplary
embodiment of the invention is shown in FIG. 4. The central component
of the control system is a microcomputer integrated circuit 55.
The microcomputer is an Intel Corp. part number D8748 having an
internal crystal oscillator using an external quartz crystal 56
and also having internal random access and fixed or program memory,
a timer, and input and output ports. The most significant data is
derived from the left and right capacitance probes 32 33. Probe
interface circuits 57 57' comprise oscillators having frequency
setting capacitance inputs connected to the respective probes. The
frequencies of oscillation generated by the probe interfaces 57
57' are indicative of the capacitances of the respective probes
and are thus also indicative of whether the respective desiccant
beds need to be regenerated. A multiple counter integrated circuit
58 Intel Corp. part number D8253 has means for counting the frequency
generated by each of the probe interfaces 57 57' and also has a
timer counter establishing a predetermined time interval for enabling
the other counters so that final counts are obtained indicating
the number of cycles generated by each probe interface during the
time interval of the timer counter. The microcomputer 55 controls
the operation of the counter integrated circuit 58 and also exchanges
data with the counter integrated circuit for setting the predetermined
time interval of the timer counter and also for initially setting
the counters counting the frequencies from the probe interfaces.
The microcomputer receives the final counts from the counter integrated
circuit 58 so that the microcomputer senses the level of saturation
of the desiccant in each bed. The microcomputer 55 also receives
pressure data generated by the pressure switches generally designated
59 in FIG. 4 corresponding to the four pressure switches 34-37
shown in FIG. 2. A pressure switch interface 60 processes the pressure
switch signals before they are passed to the microcomputer 55.
The microcomputer 55 feeds data to a solenoid and alarm interface
61 for generating a sufficiently high current level to drive the
solenoid valves 201 and an alarm relay REL. The microcomputer also
feeds an output interface 63 for driving a set of indicator lights
64. These lights indicate whether the left or right desiccant bed
is selected for drying the compressed air, and also indicate probe
and valve malfunctions.
A common schematic for each probe interface 57 57' is shown in
FIG. 5. In order to generate a periodic signal having a frequency
FOUT responsive to the capacitance of the connected probe 32 or
33 a precision oscillator integrated circuit 65 part number 555
is used. The time constant of the oscillator is set by the capacitance
of the probe 32 33 and the resistance of a one megohm resistor
66 in series with an adjustable control 67 Bourns Corp. part number
7105A-108-504. The control 67 comprises a 400K ohm variable resistor
68 in series with a fixed resistance 69 of value 100K ohms. The
control 67 is initially adjusted to calibrate the capacitance probe
32 33 so that it generates a desired output frequency FOUT for
a desired calibration level of saturation of the desiccant. The
integrated circuit oscillator 65 also uses a capacitor 70 of value
0.01 microfarads. Power supply filtering capacitors 71 and 72 of
values 1 microfarad and 0.1 microfarad suppress any noise on the
+5V input lead and the ground GND lead to the probe interface circuit.
A third TEST lead is used so that the microcomputer 55 can determine
whether the proh=interface is used in the system.
The input interface 60 for the pressure switches 34-37 is shown
in FIG. 6. Pressure switches 34 and 36 have a relatively low pressure
threshold while switches 35 and 37 have a relatively high pressure
threshold. The switches are open when the respective pressures exceed
the respective thresholds. The high pressure threshold is substantially
below the pressure of the compressed air, while the threshold of
the low pressure switch is substantially above the atmospheric or
minimum pressure in the system. The high pressure threshold, for
example, is set at 60 PSI in order to be below maximum pressures
of 80 PSI and above. The low pressure threshold, for example, is
set at 1 PSI above atmospheric pressure. Preferably the switches
are Hall effect sensors (e.g., Texas Instruments Corp. part No.
TL172C) indicating the respective positions of magnets mounted on
respective pistons in pressure sensing cylinders.
The signals from the low pressure switches 34 36 are combined
by an exclusive-NOR gate 73 to generate a low pressure coincidence
signal T0. The low pressure coincidence signal T0 is active high
when the low pressure switches 34 36 are in the same state. When
one of the desiccant beds 21 22 is depressurized for regeneration
while the other is pressurized for drying, the low pressure switches
should normally be in different states due to the wide difference
in pressure, and the different states should be indicated by the
low pressure coincidence signal being a logical low. At the end
of regeneration, the regenerated bed is repressurized and hence
at the end of repressurization both of the low pressure switches
34 36 should be in the same state so that the low pressure coincidence
signal T0 should be in a high state. Similarly, an exclusive-NOR
gate 74 combines the signal from the two high pressure switches
35 37 to generate a high pressure coincidence signal T1 which
should be a logical low when one of the desiccant beds is depressurized
for regeneration, and a logical high when the regenerated bed is
repressurized. By generating the coincidence signals from the high
pressure or low pressure switch signals, the number of input lines
to the microcomputer 55 is reduced. Also by using both high pressure
switches and low pressure switches, the microcomputer 55 senses
whether the regenerating cylinder is fully depressurized to below
the low threshold, and can sense whether the regenerated cylinder
is fully repressurized to above the high pressure threshold. Also
shown in FIG. 6 are pressure switch pull-up resistors generally
designated 75 of value 2.2K ohms which are needed to interface the
pressure switches 34-37 to the exclusive-NOR gates 73 74. The exclusive-NOR
gates 73 74 National Semiconductor part No. SN54LS266N, also use
output pull-up resistors 76 77 respectively, for interfacing their
outputs to the inputs of the microcomputer 55.
The schematic for the microcomputer 55 counter integrated circuit
58 solenoid valve and alarm relay interface 61 and indicator light
interface 63 is shown in FIG. 7. The control system is powered by
a 5 volt power supply generally designated 80 having a 5 volt regulator
integrated circuit 81 National Semiconductor Corp. part no. LM7805CT.
The power supply 80 operates off conventional 115 volt alternating
current power lines. The BLACK line is fused by a 1/4 ampere slow-blow
fuse 82 and shunted to the WHITE power line by a varistor 83 General
Electric Corp. part No. Z130LA10A, in order to suppress any transient
spikes on the power lines. A universal 12 volt 1 ampere step-down
transformer 84 feeds full wave rectifier diodes 85 86 part number
1N4002 to generate approximately 10 volts DC across two 1000 microfarad
filter capacitors 87 88. The regulator 81 outputs 5 volts DC across
its 100 microfarad and 0.1 microfarad output capacitors 89 90 respectively.
The counter integrated circuit 58 has an associated 0.1 microfarad
decoupling capacitor 91 and receives the output frequencies FOUT
from the left and right probe interface circuits 57 57'. The counter
integrated circuit 58 has in fact three different counters identified
as CN0 CN1 and CN2. Clock input C1 receives the frequency from
the left probe interface 57 while the clock input C2 receives the
output frequency of the right probe interface circuit 57'. The C0
clock input receives a 122.88 kilohertz periodic signal from the
ALE output on lead 11 of the microcomputer integrated circuit 55.
All three counters have reset inputs R0 R1 R2 respectively, which
receive initial counts from the microcomputer over a bus generally
designated 92. The connections for this bus 92 in terms of the
conventional pin or lead numbers of the respective microcomputer
and counter chips 55 and 58 are as follows: the signal P22 from
the micricomputer 55 on lead 23 is connected to the counter signal
GATE 0 on counter lead 11 microcomputer output P21 on lead 22 is
connected to counter signal A0 on counter lead 19 microcomputer
output signal P20 on lead 21 is connected to counter input A1 on
counter lead 20 microcomputer output RD on lead 8 is connected
to counter input RD on lead 22 microcomputer output WR on lead
10 is connected to counter input WR on lead 23 microcomputer output
ALE on lead 11 is connected to counter input C0 on lead 9 and microcomputer
data bus outputs DB0-DB7 on leads 12-19 respectively, are connected
to counter data inputs D0-D7 on counter leads 8-1 respectively.
The data bus 92 is also used to pass control signals from the microcomputer
55 to the counter inputs GATE0 A0 and A1 for control of the counter
integrated circuit 58 as will be described further in conjunction
with the exemplary procedure executed by the microcomputer.
In general terms, the counters CN0 CN1 and CN2 are first set
to initial values by data written from the microcomputer 55 over
the bus 92 into the counters. Then the first counter CN0 is enabled
to count the frequency on its clock C0 to generate a sampling or
gating signal on its output S0 which is fed to sampling or disabling
inputs S1 S2 on the other counters CN1 CN2 respectively. The
counter CN0 generates an active end of counting or sampling signal
or logic transition on its output S0 a predetermined time after
the counters are enabled for counting, the predetermined time being
a function of the initial count loaded into the first counter CN0.
For the microcomputer procedure described below, the predetermined
time interval is approximately 1/2 of a second. Over this time period,
the second and third counters CN1 CN2 respectively, count the
cycles or transitions generated by the left and right probe interface
circuits 57 and 58. After the predetermined time interval, final
counts are obtained from the second and third counters CN1 CN2
which are transferred across the bus 92 to the microcomputer 55.
These final counts are responsive to the frenquencies FOUT generated
by the left and right probe interface circuits 57 57' and thus
indicate the moisture in the desiccant beds sensed by the probes
32 33.
The microcomputer 55 uses this moisture indicating data to adjust
a controlling sequence for activating the solenoid valves 51 in
FIG. 3 to properly control the operation of the desiccant dryer
20 in FIG. 1. The controlling sequence executed by the microcomputer
55 is also adjusted in accordance with the low pressure coincidence
signal T0 received on the microcomputer's T0 input on lead 1 and
the high pressure coincidence signal T1 received on the microcomputer's
T1 input on lead 39. Moreover, the TEST outputs from the left probe
interface 57 and right probe interface 57' are received by the microcomputer
55 on inputs P23 on lead 24 and INT on lead 6 respectively.
The microcomputer executes its procedure at a precise 1.8432 megahertz
rate set by the crystal 56 which is connected across leads 2 and
3 of the microcomputer integrated circuit. These leads are shunted
to ground by 22 picofarad capacitors 93 and 94 respectively. The
microcomputer procedure also starts executing at an initial location
whenever power to the microcomputer is first turned on, as indicated
by a power on reset signal RESET applied to lead 4 of the microcomputer
by an RC network comprising a 1 microfarad electrolytic capacitor
95 and a 20 megohm resistor 96. The +5V power line to the microcomputer
55 is also decoupled by a 22 microfarad capacitor 97 and a 0.1 microfarad
capacitor 98 to ensure that spikes or noise from the power supply
80 do not disturb the procedure executed by the microcomputer.
The microcomputer 55 generates activating signals to the individual
solenoid valve coils SOL A, SOL B, SOL C, SOL D and SOL E in proper
sequence to control the drying and regeneration performed by the
desiccant beds 21 22. The output interface 61 for driving the solenoids
SOL A-E comprises a set of 2.2K ohm pull-up resistors 99 and an
output buffer 100 part number MC1413P. The microcomputer outputs
P15-P10 on leads 32-27 are connected to the input leads 1-7 of the
output buffer 100 which drive the output leads 16-10 of the output
buffer, which in turn drive the solenoid coils SOL A-E and the alarm
relay REL, respectively. The +5V power line to the output buffer
100 is decoupled by a 0.1 microfarad capacitor 101.
The microcomputer also performs diagnostic and indicating functions
to inform a maintenance person of the status of the air dryer and
control system. The indicator interface 63 has a set of 2.2K ohm
pull-up resistors 102 and an output buffer integrated circuit 103
Motorola Inc. part No. MC1413P. Microcomputer outputs P27-P24 and
P17-P16 feed output buffer leads 1-6 which in turn drive output
buffer output leads 16-11 connected to light emitting diodes 104-109
respectively. These light emitting diodes (LEDs) indicate right
probe malfunction, left probe malfunction, exhaust valve malfunction,
inlet valve malfunction, fixed cycle, and probe cycle operation,
respectively. The microcomputer is also programmed to activate the
alarm relay REL (active when deenergized) whenever one of these
malfunctions occur. The output buffer integrated circuit 103 has
its +5 volt input line shunted to ground by a 0.1 microfarad decoupling
capacitor 103a. The +5 volt power line also drives a "power
on" indicating LED 110. Two additional LEDs 111 and 112 shunt
the solenoid coils SOL A and SOL B, respectively, to indicate whether
the right desiccant bed is drying or whether the left desiccant
bed is drying. As is evident from the fact that the power indicating
LED 110 shunts the +5 volt power line, these LEDs must be rated
at 5 volts or have internal current limiting resistors. A typical
part number is Hewlett-Packard Co. HLMP-3600.
The operation of the exemplary control system as specifically wired
in FIG. 7 is precisely defined by the procedure or set of instructions
stored in the microcomputer's read only memory. An exemplary procedure
or program listing is appended to the specification just before
the claims. Although comments describing the flow of execution are
liberally included in the program, there are three general functions
performed by the program which are of interest in understanding
the overall operation of the control system. These include a fixed
cycle procedure, a probe-controlled cycle procedure, and the procedure
which controls the operation of the counter integrated circuit 58.
A flowchart for the control loop which performs fixed cycle operation
of the air dryer is shown in FIG. 8. FIG. 8 and FIGS. 9 and 10 are
annotated with program labels, identified by suffixed colons, which
reference the corresponding sections in the program listing appended
as part of the specification.
The first step 120 in the fixed cycle control loop of FIG. 8 is
to clear the outputs P10-P17 and P20-P27 of the microcomputer 55.
The alarm relay REL, however, is energized so that loss of power
to the control system will be indicated by the alarm relay.
Once the outputs are cleared, execution jumps into the fixed cycle
loop. In step 121 the right TEST line is read so that in step 122
the microcomputer may determine whether the right probe interface
board is plugged in. If the right probe interface board is plugged
in, then in step 123 the microcomputer reads the left TEST line
to check whether the left probe interface board is also plugged
in. If both boards are plugged in, then in step 124 the microcomputer
determines whether both probe interface circuits 57 57' are working
by actually measuring the left and right output frequencies FOUT.
The manner in which the output frequency FOUT is measured is explained
below in reference to FIG. 10. If the probe interface circuits are
working, then execution of the microcomputer jumps via the off-page
connector A to the probe-controlled cycle shown in FIG. 9.
If the right probe board is not plugged in, step 125 the L/R flag
F0 is set to 0 so that the right desiccant bed is initially selected
for regeneration.
The first half portion (labeled [RIGHT]) of the fixed cycle control
loop starts with step 126 which instructs the microcomputer to turn
on the fixed cycle indicator 107 and solenoid SOL C. When solenoid
SOL C is activated, the input valve 40 in FIG. 3 directs the flow
of air from the compressor 24 to the left desiccant bed 21 and
also the outlet valve 42 directs the flow of dried air from the
left desiccant bed to the output pipe 26. After solenoid SOL C is
activated, the control system in step 127 waits for two seconds
for the pneumatic system of FIG. 3 to respond. In step 128 solenoid
SOL D is turned on to depressurize the right desiccant bed for regeneration.
After a thirty second delay in step 129 to assure that depressurization
is substantially complete, in step 130 the high pressure coincidence
signal T1 is sampled and tested to determine if the right chamber
is depressurized. If the coincidence signal T1 is a logical one,
then the high pressure sensing switches are in the same states and
in step 131 the microcomputer activates the exhaust valve malfunction
indicator 105 the inlet valve malfunction indicator 106 and deenergizes
the alarm relay REL. Both of the indicators are activated since
the failure of the right chamber to depressurize may be due to either
the failure of the exhaust valve 48 to open or the failure of the
pressure inlet valve 40 to close its path to the right desiccant
bed 22.
In step 132 the microcomputer waits for 240 seconds as the right
desiccant chamber is regenerated with the valves and solenoids in
the positions shown in FIG. 3. During this purge of the right chamber,
the microcomputer continually checks the pressure coincidence signals
to verify that the exhaust valve stays open, the inlet valve stays
closed, and that there are no restrictions creating back pressure
in the right chamber.
The microcomputer terminates regeneration of the right chamber
in step 133 by turning off solenoid SOL B which closes the exhaust
valve 48. After a two second delay in step 134 the microcomputer
in step 135 turns on the repressurizing valve 41 to open a connection
between the left and right desiccant chambers. After a thirty second
delay in step 136 the micrcocomputer turns off solenoid SOL E in
step 137 to close the connection. After a two second delay in step
138 the microcomputer in step 139 samples and tests the high pressure
coincidence signal T1 to determine whether the right chamber has
repressurized. If the high pressure coincidence signal T1 is not
a logical one, indicating that the pressures in both the left and
right desiccant beds exceed the high pressure threshold, in step
140 the microcomputer turns on the exhaust valve malfunction indicator
105 and deenergizes the alarm relay REL. Execution then loops back
to step 139 so that the right desiccant bed is again checked for
proper repressurization, and if repressurization occurs, the alarm
relay REL is reenergized and the exhaust valve malfunction indicator
105 is turned off.
If the right desiccant bed has repressurized, then in step 141
the microcomputer turns off solenoid SOL C, complements the L/R
flag F0 in step 142 and waits for two seconds in step 144 for the
pneumatic system to stabilize. This completes the first half of
the fixed cycle wherein the left desiccant bed is drying the air
stream and the right desiccant bed is being regenerated.
In order to regenerate the left desiccant bed and to use the right
desiccant bed for drying, the microcomputer executes the second
half of the fixed cycle [LEFT]. In step 145 solenoid SOL B is turned
on, corresponding to step 126 in the [RIGHT] section, so that the
inlet valve 40 and the outlet valve 42 direct the flow of gas from
the compressor 23 through the right hand desiccant bed 22 towards
the output pipe 26. Note that the means for generating these signals
which alternately energize solenoids SOL B and SOL C in steps 126
141 and 145 are means for periodically changing bed select signals
at least once within a present minimum time period, so that the
vapor load is shared between the desiccant beds. Similarly in the
procedure generally designated 146 the other steps in the [RIGHT]
section are repeated for the left chamber. But the low pressure
coincidence signal T0 is sampled and tested instead of the high
pressure coincidence signal T1 in steps 130 and step 133 and solenoids
SOL A and SOL B are turned on and off instead of solenoids SOL D
and SOL C, respectively.
At the end of these steps for the left desiccant bed being regenerated,
execution by the microcomputer returns to the initial step 120 to
complete the fixed cycle. It should be noted that steps 122-123
are repeated so that during each fixed cycle the microcomputer tries
to put execution into the probe-controlled cycle of FIG. 9.
A flow chart for the probe-controlled cycle is shown in FIG. 9.
It is similar to the fixed cycle of FIG. 8 and the similar steps
are designated with corresponding primed reference numerals. The
probe-controlled cycle differs from the fixed cycle starting with
step 147 in which the microcomputer reads data from the moisture
sensing capacitance probes. The precise method in which data is
read from the probes is described below in conjunction with FIG.
10. The net result of the process for reading the data from the
capacitance probes is a pair of numbers CPR and CPL representing
the level of saturation of the right and left desiccant beds, respectively.
In step 148 the number for the right desiccant bed CPR is tested
to determine whether it is within a maximum and a minimum limit.
If this number is not within these limits, the microcomputer alerts
the operator in step 149 by turning on the right probe malfunction
indicator 104 and execution is transferred to the fixed cycle routine
of FIG. 8. By transferring execution to the fixed cycle routine,
the microcomputer activates regeneration enable signals in steps
128-138 at least once after each changing of the bed selected for
drying, so that the beds are periodically regenerated regardless
of probe failure. If CPR is within the maximum and minimum limits,
then in step 150 the microcomputer compares CPR to a predetermined
threshold count TH representative of the moisture or condensate
saturation level for which regeneration is required. If the count
CPR is less than the threshold TH, then the right desiccant chamber
is relatively dry and does not need to be regenerated during the
current cycle period. Thus the microcomputer will skip the regeneration
sequence by waiting for 270 seconds in step 151 and thirty seconds
in step 152 to bypass steps 128' through 138'. If the count CPR
is not less than the threshold count TH, then the right desiccant
bed must be regenerated and for this purpose solenoid SOL D is turned
on in step 128' to depressurize the right bed as was performed in
the fixed cycle. Note that the signal turning on solenoid SOL D
in step 128' is a regeneration enable signal in response to the
comparison in step 150 of the sample count CPR to the predetermined
threshold TH indicating that regeneration is required.
The steps for reading the capacitance probes and testing the resulting
final counts from the counter integrated circuit 58 in FIG. 7 are
shown in FIG. 10. In step 160 counter CNO is set for a "mode
1 read/load" function. This means that counter CN0 is configured
to receive an initial count from the microcomputer and to establish
a time interval during which the counter CN0 counts clock cycles
on its clock input C0 until a maximum or zero count is reached.
In step 161 counter CN1 is set for a "mode 0 read/load"
function and similarly in step 162 counter CN2 is also set for a
"mode 0 read/load" function. With the counters set in
these modes, counter CN1 and CN2 will count from their initial counts
during the time interval set by the first counter CN0 and will arrive
at a final count at the end of the time interval. In other words,
the first counter CN0 counts from an initial count to a full scale
count, and when the full scale count is reached it generates a sampling
or disabling signal which establishes the time for obtaining final
counts from the other counters CN1 and CN2. The difference between
the initial counts in the counters CN1 and CN2 and their final counts
represents the number of cycles of the respective probe frequencies
FOUT, being applied to the counter clock inputs CN1 and CN2 during
the time interval. Thus the counter integrated circuit 58 performs
a frequency determining function.
In step 163 counter CN0 is loaded with an initial count of 61425.
This initial count generates a predetermined time interval of approximately
1/2 of a second when the counter CN0 is clocked by the 122.88 kilohertz
ALE output of the microcomputer 55 [see FIG. 7]. In steps 164 and
165 the microcomputer loads the counters CN1 and CN2 with maximum
counts to effectively reset these counters. In step 166 the counters
are turned on by passing a "111" binary control code from
the control outputs P22-P20 of the microcomputer 55 to the control
inputs GATE0 A0 and A1 of the counter integrated circuit 58. After
a two second delay in step 167 the microcomputer turns off the
counters in step 168 in preparation for reading final counts from
the counters CN1 and CN2. In step 169 the most significant byte
of the output of counter CN1 is read into the microcomputer's register
1. Thus register 1 contains the number CPL. In step 170 the most
significant byte of the counter CN2 output is read into the microcomputer
register 2 so that register 2 contains the number CPR. In step
173 the microcomputer tests the L/R flag to determine whether the
left or right probe is the pertinent probe for sensing the level
of saturation in the idle desiccant bed. Then in step 174 or step
175 the register having the count from the pertinent probe is moved
to the microcomputer's accumulator ACC.
To determine whether the idle desiccant bed is to be regenerated
or whether there is probe failure, in step 176 the accumulator containing
the pertinent final count is compared to a predetermined theshold
count TH of 60500. If the value of the accumulator is less than
60500 then the measured level of saturation is lower than that
indicating regeneration is required. In step 177 the final count
in the accumulator is compared to 47074 representing a predetermined
minimum count which is always exceeded when the probe and probe
interface circuits are properly functioning. If the accumulator
is less than this predetermined minimum count, then probe failure
is indicated in step 149. If the final count in the accumulator
is not less than the predetermined minimum count, then the regeneration
steps 128' through 138' in the probe-controlled cycle of FIG. 9
are skipped as execution is transferred to an idle or delay sequence
starting with step 151 in FIG. 9. In step 176 if the final count
in the accumulator equals the threshold TH of 60500 then execution
may jump directly to the regeneration steps starting with step 128'
. But if the final count in the accumulator is greater than the
threshold count TH of 60500 the final count in the accumulator
is compared in step 178 to a maximum count of 62620 to determine
whether the final count in the accumulator is excessive. If the
final count in the accumulator is greater than the maximum count,
then probe failure is indicated and execution continues starting
with steps 149. But if the final count in the accumulator is not
greater than the predetermined maximum count, then regeneration
may proceed starting at step 128'. This completes the description
of the software executed by the microcomputer 55. Further details
of the software will become apparent to persons skilled in the art
upon inspection of the program listing and clarifying comments appended
below to the specification.
In view of the above, an intelligent control for a desiccant dryer
has been described which provides improved flexible control and
expanded diagnostic capabilities. Modifications of the control procedure
to accommodate particular models of disiccant dryers may be economically
provided by changes in the programming, without extensive hardware
changes. Moreover, an improved interface for the moisture sensing
probes has been described wherein only a single adjustable resistor
is provided for calibrating each probe. Since the frequency generated
by the probe interface is substantially a function only of the adjustable
resistance and the capacitance of the probe, the output frequency
FOUT is a precise indication of the level of saturation of the respective
desiccant bed. There are no other analog components subject to drift
or aging which lead to significant errors in the sensed moisture.
It should be noted that the output frequency FOUT is very accurately
determined by the microcomputer and counter integrated circuits
since the time base for the frequency counting process is established
by the quartz crystal 56. ##SPC1##
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