Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 Valve converter/positioner with remote feedback and
memory
BACKGROUND OF THE I NVENTION
The present invention relates in general to appara-
tus for effecting accurate positioning of valves, andin particular to such apparatus which provide remotely
available indication of true valve position.
In virtually all process control systems, some
types of valves are used to regulate the flow of pro-
10 cess fluids. Proper operation of these valves is animportant factor in achieving the formulation of a
product within specifications, as well as in an ef~i-
cient manner. Conceivably in potentially explosive
processes, it also could mean the difference between a
safe and an unsafe operating condition.
To facilitate accurate operation, a variety of
valve positioning mechanisms have been used extensively
in the past. Their basic operating principle is to
maintain an energizing signal into a valve actuator
20 until feedback provided from the valve itself, indica-
ting that it has attained the desired position, termi-
nates the energizing signal. Valve positioning
mechanisms are of two basic types: the positioner, in
which the feedback is provided by a mechanical linkage
25 to the valve stem, and the converter, in which the
feedback information is conveyed by either the pneuma-
tic pressure or the electric current present at the
valve actuator, depending on the type of actuator usedO
Both of these mechanisms are intended to ensure that
30 the valve reaches the appropriate setting, despite
friction in the actuator or in the valve stem packing.
Nevertheless there are certain shortcomings in the
previously known types of positioners and converters.
`~
S~
1 Specifically, the communication between a remotely-
located controller and the valve positioning mechanism
is typically one-way. That is, although the controller
emits a command signal to the positioning mechanism,
there is no confirmation at the controller that the
valve has assumed the desired setting. Although local
feedback exists between the valve and its actuator, as
described above, there has not been available an effi-
cient means for providing feedback to the remotely
located controller. However, in certain processes, the
human operator may need positive indication of actual
valve position to reliably and safely regulate the
process. Often, the only way the operator is made
aware of an improperly set valve is by the response of
the process itself. For example, if a cooling water
valve must be opened more fully to reduce the tempera-
ture of a particular reaction, the only confirmation
that the valve has opened adequately may be the even-
tual reduction in temperature. However, because of the
typically long lag time in a temperature loop, an
indication of failure may not become evident until
after an unacceptably long period of time.
It is known to use certain auxiliary devices for
indicating valve position. One such device is a po-
tentiometer, in which the wiper arm is attached to thevalve stem. A constant voltage input is maintained
across the total resistance of the potentiometer, while
the movements of the wiper arm change the output signal
in proportion to the valve position. However, such an
ar~angement involves three extra signal lines and
additional circuitry.
In particularly critical applications, operators
have been known to install direct observation schemes,
,
;
1~3l~5~3
l such as television monitors, for viewing the position
of the valve.
There also exists a need for the valve positioning
mechanism to have a memory, i.e., to be able to hold a
5 valve in a preset position in the event of loss of
power to the positioning mechanism, and yet do so at
sufficiently low energy levels to avoid ignition of
potentially explosive atmospheres. Memory schemes
operating within the framework of a conventional elec-
10 trically~based positioning mechanism typically exceedintrinsically safe operating limits and therefore are
unusable in hazardous environments.
Therefore, in view of the above, it is an object
of the present invention to achieve a valve positioning
15 mechanism having an integral remote-feedback system to
provide an affirmative indication, at a remotely
located control room, of the actual response of the
valve to a control signal emanating from that control
room.
It is another object of the present invention to
provide the remote feedback along the same transmission
lines on which the control signal is transmitted from
the control room to the valve positioning mechanism.
It is a further object of the invention to achieve
25 the above result with a system that is intrinsically
safe for use in explosive atmospheres.
SUMMARY OF THE INVENTlON
The present invention operates in the context of a
valve positioning mechanism of the type in which a
30 valve actuator located within a process environment
moYes the val~e stem in accordance with a command
signal generated by a process controller within a
remotely-located control station. Such a mechanism
5~
--4--
1 generally has a local feedback scheme for terminating
the operation of said actuator when the valve attains
a final position as dictated by the command signal.
In accordance with a specific embodiment of the present
5 invention a remote feedback system generates an output
signal for transmission to the remotely-located control
station, the frequency of the output signal being
indicative of the status of the valve. The output
signal is communicated to the control station via a
10 two-wire transmission line, commonly used for process
control instrumentation communications ? which also
serves to transmit the command signal from the control
station to the valve actuator.
The feedback system includes an oscillator whose
15 output frequency changes in accordance with the induc-
tance value of a variable inductor responsive to
changes in the status of the valve.
Also in this embodiment, the command signal initi-
ates a pneumatic control signal to the actuator by
20 means of a low-power, electropneumatic switch, which
operates at voltage and power levels well within the
intrinsically safe limits established for hazardous
environments.
The configuration of the electropneumatic switches
25 achieves a memory function, in that a pree~isting valve
position will be maintained even in the case of an
interruption of the command signal from the controller,
for ~xample, as in the case of a power failure.
In an alternate embodiment, the invention functions
30 in a fail-safe mode in case of power failure. Upon
loss of signal the memory is disengaged and the valve
is automatically set to a fail-safe or other predeter-
mined condition. Optionally the return to a fail-safe
:
12~)~59~
-- 5 --
setting can be delayed for a preset period of time after the
signal interruption.
DESCRIPTION OF THB DRAWINGS
The novel aspects and distinct advantages of the present
invention will be made clear bv the following detailed descrip-
tion, in conjunction with the accompanying drawings, in which:
Figure 1 is a schematic diagram of a first embodiment
of a valve converter mechanism in accordance with the present
invention;
Figure 2 is a partial schematic of an embodiment of a
valve positioner mechanism in~accordance with the present inven-
tion;
Figures 3A through 3E are detailed schematics of the
circuitry of the INTERFACE CARD portion of Figure l;
Figure 4 is a detailed schematic of the circuitry of
the FIELD ELECTRONICS portion of Figure l;
Figures 5A and 5B are detailed views of the variable
inductor of the pneumatic transducer of Figure l;
Figure 6, appearing in the same sheet as Figure 2, is a
graph depicting the relationship between oscillator frequency and
variable inductor armature position; and
Figure 7 is a schematic diagram of a second embodiment
of a valve converter mechanism in accordance with the present
invention.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
Throughout the following description and the accompanying
drawings, the same reference numerals are used to indicate like
components.
~,
~ " lZO(~S~O
Referring now to Figure 1, there is depicted a process
control system 10 employing a novel scheme for adjusting the
setting of a valve 11, to vary the rate of passage of process
fluids therethrough. A conven-
~3~5~
--6~
1 tional controller mechanism 13, a device whose con-
struction and operation are well known to those skilled
in the process control arts, is the source of command
signals which initiate valve action. In a known
manner, the controller compares an incoming measurement
5 signal indicating the value of the particular process
variable being controlled, with a predetermined set-
point signal representing the desired value of that
variable, and generates an appropriate command signal
over a line 15. This command signal is intended to
10 effect a change in the setting of the valve, so as to
drive the measured value of the process toward the
desired level.
The valve as shown in FIG. 1 is pneumatically-
actuated, although the teachings of the present inven-
15 tion are equally applicable to a valve operated by anelectrical or other conventional mechanism. A pneuma-
tic signal supplied via an air line 17 is applied to
any one of a variety of pneumatically-powered drive
mechanisms, represented generally by reference numeral
20 19, which is coupled to a stem 21 of the valve. In the
illustrated embodiment, raising of the valve stem
further opens the valve, while lowering the stem closes
the valve, although oppositely functioning configura-
tions are possible.
The command signal from the controller 13, in this
case in the form of a d.c. voltage in the range of 0
to 10 volts, is supplied to an electronics interface
card 22, whose structure and function will be described
more fully hereinafter. The interface card and the
30 controller are located at a control station remote from
the controlled process, and communicate via a two-wire
transmission line 23 to a field electronics assembly
~20~5`9~
24, located in the general vicinity of the valve. Such a typical
two-wire transmission line, which is intended to carry two-way
transmission of power and information signals, has found wides~
pread application for long distance communication in the process
control industry. A typical use of such a transmission line is
described in United States patent No. 4,118,977, which was issued
on October 13, 1976 to The Foxboro Company.
The circuitry of the field electronics assembly 24, also
to be described hereinafter, accepts the electrical signal, usually
in the form of a current, from the two-wire transmission line 23,
and directs the signal to either of a pair of electropneumatic
switches 25, 26. These switches initiate the flowof pneumatic
signals for control purposes. In this embodiment the top switch
25 effects a decrease in the pneumatic pressure ultimately supplied
to the valve actuator 19, while the bottom switch 26 increases the
pressure.
Each of the switches 25, 26 includes an outer housing
27 in which there are an upper air chamber 29 and a lower air
chamber 33. The bottom of the upper chamber is defined by a
flexible diaphragm 35. A nozzle 36 permits communication of air
between the upper chamber and the exterior of the housing through
a line 37. A flapper 38 normally rests against the nozzle
opening, sealing off the nozzle against passage of air and permit-
ting pressurization of the upper chamber. An outlet port 39 is in
fluid communication with the lower chamber 33.
Disposed beneath the diaphram 35 is a plate 42 supported
upon and pivotable about a second flexible diaphragm 43. A
pedestal 44, fastened to the underside
s~
--8--
1 of this plate, receives an upward bias from a helical
spring 45 seated within the lower air chamber 33, so
as to maintain the plate in intimate contact with the
diaphragm 35.
Instrument air from a supply 46, typically at 20
psi, is introduced into the upper chamber 29 via a
channel 47 and a restrictor 48. As long as the flapper
38 is sealed against the nozzle 36, the pressure de-
veloped within the upper chamber forces the diaphragm
10 35 downwardly, causing a gasket 49 located in the
underside of the pedestal 44 to seat firmly against a
second nozzle 50, despite the upward bias provided by
the spring 45. This condition occurs with the electro-
pneumatic switch in the "off" position.
A solenoid assembly 51 is located atop each elec-
tropneumatic switch, adjacent the flapper 38. In the
absence of an actuating signal to the solenoid, the
spring force of the flapper keeps the flapper tightly
pressed against the nozzle 36. However, when a current
20 signal is applied to the solenoid~ an electromagnet 53
attracts the flapper away from contact with the nozzle.
The pressure in the upper chamber 29 decreases, al-
lowing the diaphragm 35 to rise, in turn lessenîng the
force applied against the plate 42. The spring 45
lifts the pedestal and gasket 49 away from the second
nozzle 50, allowing the flow of air between the outer
port 39 and an inlet line 55, via the lower chamber 33
and the nozzle 50. This represents the "on" condition.
In the case of the top switch 25, the outlet port
~ 39 is vented to the atmosphere, so that excess pressure
coupled to the inlet line 55 from elsewhere in the
pneumatic network can be released. Thus this switch
acts to decrease the pressure within the pneumatic
s~
_9_
lines leading ultimately to the valve actuator. For the bottom
switch 26/ the outlet port is connected to the 20 psi air supply
~6. When this switch is turned on, 20 psi air is supplied to the
pneumatic control lines, in effect increasing the pressure. As
will be described later, the interface card 22 and field elec-
tronics assembly 24 acting in concert selectively actuate either
switch 25 or switch 26, depending on whether the process conditions
call for line pressure to the valve to be decreased or increased.
It should be pointed out that although the electropneu-
matic switches, which usually are located within the processenvironments, are electrically triggerable, they have a very low
power consumption. In fact, since the considerable motive power
which effects the movement of the valve is supplied by the pneu-
matic pressure, only minimal electrical energy is required to
activate the switches, to appropriately route the flow of air.
Thus the valve control system according to the present invention
can operate effectively at current and voltage levels which are
within the intrinsically safe limits typically required for safe
operation within hazardous or explosive atmospheres~ Such levels
are defined in the publication, "Intrinsically Safe Apparatus for
Use in Division 1 Hazardous Locations 1978," published by The
National Fire Protection Association. The SPEC 200 family o-f
electronic controllers manufactured by The Foxboro Company, Foxboro,
Massachusetts generates electrical control signals within these
intrinsically safe operating limits, and is compa-tible with the
operating requirements of the present invention. Since the
electropneumatic switches
.~
il 2~
--10-
1 utilize a solenoid, which is an inductive, energy-
storing device, a shunt protective component of some
sort must be coupled to the solenoid to suppress
transient voltages or currents above the safe limits.
However, such components, also discussed in the above-
referenced publication, are well known to
those skilled in the field of intrinsic safety, and
will not be discussed further herein.
The pneumatic signal, whether from switch 25 or
switch 26, is supplied over a line 56 to a pneumatic
transmitter 57. The transmitter, together with a
standard pneumatic relay 59 (such as the Model 40 Relay
also manufactured by The Foxboro Company) comprise a
pneumatic transducer assembly 61.
Within the pneumatic transmitter 57, an expandable
receiver bellows 63 is attached at its lower end to a
mounting base 65, and at its upper end to one end of a
lever arm 67. The lever arm is able to pivot about a
fulcrum 69 on a balance bar 71, to cause repositioning
of a flapper 73 relative to a nozzle 75. The nozzle
is connected via a pneumatic line 77 to the standard
relay 59 and to the air supply 46. In a manner well
known to those skilled in the pneumatic arts, the
spacing of the flapper relative to the nozzle deter-
mines the magnitude of an amplified output signal pro-
duced by the relay on the output line 17, and in turn
i supplied to 'he valve actuator 19. Whe~ air entering
from the line 56 causes the bellows~ to expand, the
flapper is pivoted closer to the nozzl~, resulting in
~ an increase ~n the output pressure of the relay. When
the bellows~contracts, the flapper is withdrawn from
the nozzle, decreasing the relay output.
Whether the bellows 63 expands or contracts depends
~V~5~
1 on which of the electropneumatic switches 25, 26 is
activated at any given time. If the "decrease" switch
25 is activated, then a fl~ow path is created from the
interior of the bellowsb to the outside ~tmosphere,
allowing excess ~ressure within the bellows~to escape,
and the bellows~to contract. On the other hand if the
"increase" switch 26 i3 activated, supply pressure is
applied to the bellows~, causing it to expand.
While the position of the flapper 73 is altered by
10 operation of the receiver bellows 63, the position of
the noz~le 75 itself can be adjusted as well. In the
embodiment of FIG. 1, a feedback bellows 81 is inter-
posed between the base 65 and the balance bar 71. A
spring 83 provides a downward bias on the left-hand end
of the bar. Thus this balance bar similarly can be
pivoted about a flexure 84 to relocate the nozzle
relative to the flapper.
The pneumatic signal into the feedback bellows 81
comes from the same output signal of the relay 59 as
is supplied to the valve actuator 19. ~hus, changes
in the actuation pressure to the valve are reflected
by an increase or decrease in the internal pressure of
the feedback bellows 81, which causes the bellows~to
expand or contract accordingly. The movement of the
25 bellows repositions the nozzle 75 relative to the
flapper 73 until a new equilibrium position is reestab-
lished, at which point no further changes in the relay
output occur. T~us, in short, the action of the
receiver bellows prompts a pneumatic drive signal to
30 the valve actuator until it is counterbalanced by the
corIesponding action of the feedback bellows.
Due to the fact that the feedback between the valve
actuator 19 and the pneumatic transmitter 57 is accom-
_12-
1 plished by means of a pressure signal to the feedback
bellows Bl, the type of valve positioning mechanism
depicted in FIG. 1 is known as a valve converter.
However, as shown in FIG. 2, the feedback also can be
provided by a direct mechanical linkage 85 between the
valve stem 21 and the balance bar 71. In this arrange-
ment, the valve positioning mechanism is more accurate-
ly known as a valve positioner.
Referring again to FIG. 1, there is attached to the
top sur~ace of the balance bar 71 an armature 87 in the
shape of a truncated triangular wedge, which forms part
of a variable inductor assembly 89. The inductor is a
component within an oscillator circuit encompassed
within the circuitry of the field electronics assembly
24 (see FIG. 4). The oscillator can be any conven-
tional circuit whose output frequency is dependent on
the value of the variable inductor, and as such its
details will not be further discussed herein. The
upward and downward movement of the balance bar9 in
response to feedback signals supplied to the pneumatic
transmitter, repositions the armature within the air
~ gap 91 of ~ magne~ic assembly 93 included within the
- ~ variable in ductor, as seen more clearly in FIGS. 5A
and 5B. The specially tapered geometry of the armature
and its manner of movement within the gap cause the
inductance to change in such a way that the output
frequency of the oscillator varies in a linear fashion
with respect to armature position. FIG. 6 demonstrates
the linear relationship between oscillator frequency
and armature position over the normal operating range
of the oscillator.
Since each position of the armature ~7 can be
equated to a corresponding unique position of the valve
s~
-13-
1 11, the oscillator output frequency itself is uniquely
related to the true valve position, in the case of a
valve positioner, or to the valve position actuator
pressure, in the case of a valve converter. This
frequency signal is fed back along the same two-way
transmission line 23 to the interface card 22, to be
processed in a manner described below.
With reference now to FIGS. 3A through 3E, a more
detailed description of the circuitry of the interface
card 22 and its operation can be given. The feedback
signal from the oscillator within the field electronics
assembly 24, entering via the two-wire transmission
line 23, is processed through a band-pass amplifier 97
and an opto-isolator circuit 99 into a conventional
frequency-to-voltage converter 101. The converter
senses the frequency of the oscillating feedback signal
and transforms it to a corresponding voltage signal
whose magnitude is proportional to the frequency.
Therefore, this feedback voltage is now indicative of
the valve position. Such frequency-to-voltage conver-
ter circuits are well known to those skilled in the
electronics arts. After passing through a two-pole
filter stage 103, the feedback voltage signal is pro-
cessed by a sample track and hold circuit 105. This
circuit counteracts the effects of severe line tran-
sients produced whenever the electropneumatic switches
25, 26 are activated. Finally, the ~eedback voltage
signal is fed into a deviation amplifier 106.
The command signal from the process controller 13,
in the form of a O-to-lOV d.c~ electrical signal, also
is provided to the deviation amplifier 106. The devi-
ation amplifier amplifies the difference between the
cor7troller command signal and the feedback voltage
5~0
-14-
1 signal, and inputs the resulting error voltage into
both a deadband comparator stage 107 and a deviation
band comparator stage 109. Depending on the magnitude
of the difference between the controller signal and the
5 feedback signal, one or the other of these stages
supplies a current trigger signal to the appropriate
electropneumatic switch 25, 26 so as to properly repo-
sition the valve 13. If the error voltage is less than
a previously selected deadband threshold voltage VRl
10 which is supplied from an adjustable external voltage
source (not shown), neither stage is activated and no
current signal will be provided to either of the elec-
tropneumatic current switches. In the absence of an
activated comparator stage, a current source 111
15 generates only a quiescent current level for powering
the variable frequency oscillator in the field elec-
tronics assembly 24. If the amplified error voltage
signal is greater than the deadband threshold voltage
VRl but less than a deviation band threshold voltage
20 VR2 (also supplied rrom an external source), the dead-
band comparator 107 is activated. This in turn causes
the switched current source 111 to operate in a pulsed
mode, the pulse width and duty cycle of the pulse train
having been previously determined to yield efficient
25 repositioning of the valve. The pulsed mode in effect
offers a fine-tuning type adjustment. The polarity of
the error voltage determines the sense of the output
current delivered to the field electronics assembly 24.
Referring now to FIG~ 4, the field electronics
30 assembly 24 receives from the interface card 22 either
the +I or -I current signal, and actuates either
"increase" switch 26 or ~decrease" switch 25 respec-
tively.
5~
1 If the magnitude of the error voltage is consider-
ably greater, and in fact exceeds the deviation band
threshold, the deviation band comparator stage 109
takes over. This comparator drives the switched
5 current source 111 in a "full-on" mode. Again, the
polarity of the error voltage determines the current
sense, and therefore which electropneumatic switch is
actuated.
In summary, the switched current source lll acting
10 under the control of either the deadband comparator
stage 107 or the deviation comparator stage 109 (de-
pending on the magnitude of the error voltage) contin-
ues to supply current signals to either of the two
electropneumatic switches 25, 26 until the error volt~
15 age signal is reduced below the deadband threshold
voltage. At this point in time, the current sources
will supply only a quiescent current level to the
oscillator, and both electropneumatic switches will be
in the "off" position.
It should be noted that there is a built-in memory
feature in the particular embodiment described above.
In the absence of any command signals to actuate the
electropneumatic switches 25, 26, each of the switches
remains in the "off" condition. Thus, the status quo
25 with regard to the position of the valve is maintained.
In the case of an electrical power failure, or other
interruption of the current signals from the interface
card 22, the electropneumatic switches merely remain
disabled, and the valve stays in its previous position.
30 Once electrical power is resumed, an initialization
circuit (not shown) senses the feedback voltage from
the frequency-to-voltage converter 101 (see FIG. 3B)
~hich indicates valve position, and resets the con-
lX(~S~
1 troller so as to achieve a ~bumpless" transfer.
Referring now to FIG. 7, an alternate embodimentof a valve converter in accordance with the present
invention is achieved by substituting a slightly modi-
5 fied electropneumatic switch 113 for the top switch 25(see FIG. 1). This modified switch functions in
basically the same manner as the electropneumatic
switches 25 and 26 discussed in detail above. The only
difference is that the nozzle 36' is located on the
10 opposite side of the flapper 38, so that in the "off"
condition, i.e., with the solenoid 51 deactivated, the
flapper is not in contact with the nozzle. So, whereas
the switches 25, 26 are "normally closed" in the ab-
sence of a control signal to the solenoid, switch 113
is "normally open," to achieve the memory function
described above with reference to the embodiment of
FIG. 1. Clearly, appropriate modifications must be
made to the electronic circuitry of the field elec-
tronics assembly 24 and/or the interface card 22.
20 Whereas in the previous embodiment the absence of
electrical power to both of the switches 25, 26 would
maintain the status quo, now power must be maintained
to the modified switch 113 to reach the same result.
As long as electrical power above a predetermined
25 threshold value is maintained to the electropneumatic
switch 113 along a line 125 from the field electronics
assembly 24, the switch remains off. However, once the
electrical power drops below the predetermined level,
the switch 113 turns on and permits venting of any
excess pressure within the receiver bellows 63 of the
pn~umatic transducer 61. ~his in turn causes the valve
to go to a fail-safe position, as determined by the
characteristics of the process being controlled. It
5~3~
1 is also possible to incorporate within the pneumatic
system a conventionally known pneumatic delay device,
to forestall the movement of the valve to the failsafe
position until after passage of a predetermined amount
5 of timeO If, prior to the expiration of the delay
period, power is restored to the switch 113, a bump-
less resumption of control again can be reestablished.
Although the present invention has been described
in terms of the illustrated embodiments, certain modi-
fications may become apparent to those skilled in theart. For example, alternate constructions of an elec-
trically triggerable, yet pneumatîcally powered switch
may be envisioned, which will operate within the
context of the present invention in an intrinsically
15 safe manner. Nevertheless, it is intended that such
modifications be included within the scope of the
following claims.
