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A portion of the disclosure of this patent document contains material which
is subject to copyright protection. The copyright owner has no objection
to the facsimile reproduction by anyone of the patent document or the
patent disclosure, as it appears in the Patent and Trademark Office patent
file or records, but otherwise reserves all copyright rights whatsoever.
BACKGROUND OF THE INVENTION
This invention relates to a valve positioner for controlling a valve which
affects a process variable and more particularly, to such valve
positioners having microprocessors.
There is a desire to improve the accuracy, dynamic performance and
stability with which valve positioners operate, and to provide real-time
diagnostics to a control room, for quality auditing requirements and so
that maintenance and plant down-time can be predicted rather than suffer
emergency shutdowns or unnecessary planned valve maintenance.
Various types of positioners are used in the process control industry. Some
positioners are mechanically coupled to an actuator while some incorporate
the actuator within the positioner. The actuator provides means for
physically positioning the valve and may be electric, hydraulic or
pneumatic. Electric actuators have a current signal which drives a motor
which positions the valve. Hydraulic actuators have oil-filled means for
positioning the valve. By far the most common in the process control
industry, a pneumatic actuator has a piston or a combination of a spring
and diaphragm. Depending on the application and the level of control
integration, positioners receive several types of input from a controller
which are representative of the desired valve position. One type is a
current input having a 4-20 mA or 10-50 mA magnitude, a second is a
digital signal superimposed on the current signal and a third is a fully
digital input such as Fieldbus or Modbus.RTM.. Alternatively, the
positioner may receive a 3-15 pound per square inch (PSI) pneumatic input
representative of the desired valve position. Depending on the level of
integration and the application as well, positioners have different types
of outputs. Some positioners provide an output current to a motor, while
still others have a fast responding hydraulic output. The most common type
of positioner output is a 0-200 PSI pneumatic output. Positioners, as the
word is used in this application, includes all these field mounted
instruments, including the various inputs and outputs, and their
respective means for positioning valves, if applicable.
In the most common case of a spring and diaphragm actuator, the diaphragm
deflects with the pressure delivered by the positioner, thereby exerting a
force or torque on a control valve stem or rotary member, respectively, so
as to change the position of the valve. Almost all positioners have a
mechanical or an electronic position sensor to provide a position signal
which is fed back into a microprocessor-based control section of the
positioner. No matter what the specific means are for delivering force to
position a valve, positioners having microprocessor based control
algorithms are known. Existing positioners improve the loop dynamic
response, but have a limited bandwidth so that their usage is limited to
slow control loops such as one which controls level in a tank or
temperature in a reactor.
One obstacle to ideal valve dynamic position control is that the valve
characteristic (defined in this application as the relationship between
flow and stem position or angle) deviates from the published valve
characteristics by as much as five percent. Such non-ideality typifies all
three major types of control valve characteristics: linear, equal
percentage and quick opening. Furthermore, rotary and sliding stem valves
may exhibit a nonlinear relationship between the actuator force provided
to the valve and the flow through the valve, which is difficult for the
inherently linear positioner to control even with the present valve stem
position feedback. In fact, rotary valves have a non-monotonic torque vs.
flow function as a result of the flow induced dynamic torque on the
ball/butterfly in the valve. Everyday wear on valve components contributes
to non-ideality in the control loop as well. In practice newly installed
loops are "detuned", or purposefully assigned non-ideal control constants,
to compensate for wear so that the loop remains amble over a wide variety
of conditions. Compounding these issues of static and dynamic control
accuracy, valve-related loop shutdowns are undesirable and expensive for
industry.
The Electric Power Research Institute estimates that electric power
utilities would save $400 million U.S. dollars if each control valve
operated only one week longer each year. Most plants schedule regular
maintenance shutdowns to monitor and repair valves, replace worn packing
and worn out valve components so as to avoid even more costly and
hazardous emergency shutdowns. Diagnostic systems which monitor valve
integrity are known, but are generally configured to diagnose problems in
valves disconnected from a process. One real-time control valve has
limited diagnostics capability.
A positioner, control valve and actuator are assembled and properly
configured for installation in a time consuming process called
bench-setting. During benchset, an operator manually sets the valve's
maximum travel position (called the stroke position), the minimum travel
position (called the zero), limit stops and stiffness parameters. The
process is iterative because the settings are not independent.
Thus, there is a need for a microprocessor-based valve positioner easily
configurable at benchset, with increased bandwidth and improved dynamic
positioning accuracy, which also has real-time diagnostics to provide
valve and actuator integrity information.
SUMMARY OF THE INVENTION
In this invention, a valve positioner provides a control pressure to a
valve actuator mechanically coupled to a valve as a function of a signal
representative of the position of the valve, a desired position setpoint
received from a controller and the time derivative of the sensed control
pressure. The positioner includes receiving means connected to a current
loop for receiving the setpoint, sensing means for sensing the valve
position and the control pressure and transducer means for converting a
supply of pneumatic air to the control pressure as a function of a command
output received from a control circuit within the positioner. In another
embodiment of the invention, a valve positioner has a control circuit with
position feedback includes a sensing circuit for sensing a set of state
variables related to the valve performance. The positioner includes a
diagnostic circuit for storing an attribute of the valve and provides an
output as a function of the stored valve attribute and a selected one of
the state variables. Examples of stored valve attributes are position
versus flow, torque versus position and torque versus flow curves. In
another embodiment of the invention, the positioner includes a benchset
control circuit which ramps the control pressure between an initial
control pressure and a final control pressure and back to the initial
control pressure, while sampling specific control pressures and their
corresponding positions, in order to provide an output indicating the
proper spring preload force on an actuator spring. In another embodiment
of the invention, a valve positioner has a control circuit having position
feedback providing a command output to a transducer circuit which provides
a control pressure as a function of the command output. The positioner
includes a sensing circuit for sensing a set of state variables related to
the valve performance. The positioner includes a correction circuit which
stores a valve attribute affected by one of the physical parameters and
dynamically compensates the command output as a function of the sensed
physical parameter and the stored valve attribute.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a control flow chart of a control loop including a prior art
valve positioner.
FIG. 2 is a block drawing of a valve positioner according to the present
invention, connected to a master and an actuator mechanically coupled to a
valve.
FIG. 3 is a block drawing of a valve positioner according to the present
invention, connected to a master and an actuator mechanically coupled to a
valve.
FIG. 4 is a plot of stem position as a function of flow for quick opening,
linear and equal percentage valves.
FIG. 5A is a plot of unit torque as a function of angular position for a
butterfly valve; FIG. 5B is plan drawing of the butterfly valve in a pipe.
FIG. 6 is a block drawing of a valve positioner according to the present
invention, connected to a master and an actuator mechanically coupled to a
valve.
FIG. 7 is a plot of position versus flow as a function of valve seat wear.
FIG. 8 is a plot of actuator torque versus angular distance travelled, as a
function of valve seat wear.
FIG. 9 is a block drawing of a valve position according to the present
invention, communicating with a hand-held communicator and an actuator
mechanically coupled to a valve.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Positioners are generally used in slower process control loops, such as
level or temperature, to improve loop performance. A control diagram of a
typical prior-art electropneumatic positioner is shown in FIG. 1, where an
inner cascade loop 20 comprises an error generator 22 for generating a
difference between a controller setpoint 24 and a position sensor feedback
26, a current-to-pressure converter 28, an actuator connected to a valve
30 and a position sensor 32. An outer loop, shown generally at 34,
includes an error generator 36 for differencing a desired setpoint 38 and
a measurement 40 representative of the state of the process 42, and a
controller 44 in series with cascade loop 20 and process 42. The overall
system shown in FIG. 1 is usually stable if the bandwidth of the
positioner, shown as the cascade loop 20, is at least a factor of four
times larger than the bandwidth of outer loop 34. For loops which are
difficult to tune, the factor should be increased. Prior art loops are
purposefully detuned, or tuned non-ideally, to provide stability over a
wide range of operating conditions. In addition, it is desirable to
minimize overshoot. For second order systems with proportional feedback in
a typically underdamped system, however, the overshoot increases when
bandwidth is increased.
In a valve positioner 50 made according to the present invention and shown
in FIG. 2, a derivative of the pressure feedback signal 52 provides the
rate feedback required to decrease overshoot without affecting bandwidth.
In other words, the amount of overshoot, which is well controlled by the
amount of damping in the process loop, is reduced without decreasing the
bandwidth of positioner 50, unlike the loop shown in FIG. 1. In FIG. 2, a
process loop shown generally at 60 includes a master 62 located in a
control room sending a desired valve position signal to valve positioner
50 over a two wire current loop, although other comunications loops, such
as three and four wire current loops may be used. Positioner 50 receives a
supply of pneumatic air 61 and provides a control pressure 64 as a
function of the desired setpoint from master 62 and two variables: the
derivative of the control pressure signal 52 and a sensed position signal
68. Control pressure 64 provides pressurized air to an actuator 70
mechanically connected to a linear stem control valve 72, although rotary
valves are adaptable for use with the present invention. Actuator 70 has a
diaphragm 71 which deflects when the pressurized control pressure air
pushes against it, so as to urge stem 76 downwards. Stem 76 is fastened to
a valve plug 78 which stops the flow between a first passageway 80 and a
second passageway 82 when plug 78 is fully seated. Valve 72 is connected
via flanges 84 to a pipe 86 carrying the flow Q. A transmitter 88 measures
a flow Q and transmits a signal representative of the flow to master 62.
Within positioner 50, a receiving circuit 92 receives a 4-20 mA signal
from master 62, but may also receive the signal from a hand-held
communicator. The magnitude of the current is representative of the
desired valve position, but digital information including sensor selection
commands and data may be superimposed on the current according to a
protocol such as HART.RTM., or with digital protocols such as DE,
BRAIN.RTM., Infinity or Modbus.RTM.. For critical control, position signal
68 is temperature compensated within a microprocessor. Alternatively,
master 62 uses a fully digital protocol such as Fieldbus to communicate
with positioner 50. This feature provides added flexibility and less
wiring complexity over other schemes since the master need not recognize
the need for the variable, request the process variable and subsequently
transmit it to the field device which requires such variable. This direct
communication of a process variable between transmitter 88 and positioner
50 significantly reduces delay in loop 60, making positioner 50 adapted
for use in faster control loops such as ones which control flow.
A control circuit 94 provides a command output 97 as a function of the
desired setpoint from circuit 92, the position signal 68 and pressure
signal 52. A time derivative circuit 96 within circuit 94 provides a rate
feedback signal, or in other words, a derivative of pressure signal 52
with respect to time for the control algorithm within circuit 94. It is
preferable to use the pressure signal as the rate feedback signal because
it is available as a diagnostic and/or dynamic error correction signal in
other embodiments of the present invention, but a force or torque signal
suffices. Control circuit 94 is preferably implemented in a low power CMOS
microprocessor, or another appropriate technology offering improved power
and bandwidth, using an adaptive control algorithm which makes use of
available sensed signals such as pressure, position, force, packing and
seat wear to fine tune PID constants, and thereby obviate loop detuning.
Power consumption is a concern in all embodiments of the present
invention, since positioner 50 operates wholly on the 4-20 mA at 10-15 VDC
(9 mA at 9 V for Fieldbus) received from master 62. For this reason,
capacitance and frequency at which digital logic in the positioner
operates must be minimized. Even aside from capacitance and frequency
concerns, positioner 50 minimizes power in that it incorporates a current
to pressure transducer and a pneumatic positioner, both of which are 4-20
mA instruments. Therefore, valve control which previously consumed a
maximum of 40 mA now consumes a maximum of 20 mA. A transducer and
pneumatics circuit 100 receives a 0-200 PSI supply of air 61 and provides
control pressure 64 as a function of the control signal from circuit 94,
using a co-linear magnetic actuator and a deflected jet pilot stage as in
Rosemount's Current to Pressure Transducer 3311 disclosed in U.S. Pat. No.
4,325,399 to Frick, owned by the same assignee as the present invention.
Sensing means 102 senses signals from a pressure sensor 54 and a
mechanical position sensor 55, digitizes the signals and provides both to
control circuit 94.
In addition to the primary benefit of decreasing the overshoot without
affecting the undamped natural frequency (and therefore the bandwidth),
rate feedback has other advantages. Actuators have varying internal load
volumes, shown generally at 98, which have a wide range of pneumatic
compliances. Those actuators used with low flow valves with a relatively
small diameter have a smaller compliance than do actuators used with
larger control valves. In prior art positioners, the gain in the control
algorithm had to be manually adjusted to accommodate varying load volumes
to assure stability. However, the present invention, which is initially
tuned to accommodate large actuator compliance, requires no gain
adjustment for small compliances because the magnitude of the rate
feedback is necessarily smaller for a small actuator. When the positioner
is connected to an actuator with a small load volume, the rate of change
of pressure is large, so that the effective positioner loop gain is
reduced during transients to prevent excessive overshoot, ringing and
limit cycling. When the positioner is connected to an actuator with a
large load volume, the rate of change of pressure is small, so that the
effective positioner loop gain remains high during transients. By properly
balancing the amount of pressure rate feedback with the proportional gain
and integral action of the control algorithm, a large range of actuator
load volumes are accommodated while maintaining minimal overshoot and
minimizing bandwidth.
In FIG. 3, a control loop 200 controls the flow Q in a pipe 202. A
transmitter 204 senses the flow and transmits a signal to a master
controller 206 over a pair of twisted wires. Controller 206 sends a signal
over another pair of twisted-wires 208 to a valve positioner 210.
Positioner 210 provides a control pressure 212 to a valve 214 through an
actuator 216. A diaphragm 220 in actuator 216 deflects with the control
pressure and exerts a spring force on a sliding stem 222 fastened to a
valve plug 224 located in flow Q, thereby urging plug 224 to further
obstruct and therefore lessen flow Q. In order to increase the flow, the
control pressure is exhausted in order to allow the spring force to
re-position plug 224 upwards.
Positioner 210 comprises a receiving circuit 228, a control circuit 230, a
transducer circuit and pneumatics 232, a sensing circuit 234 and a
correction circuit 236. Sensing circuit 234 is connected to a pressure
sensor 238 for sensing the control pressure, a mechanical member 240
connected to stem 222 for sensing valve position, and a load cell 242 for
sensing force or torque as appropriate. However, force or torque is
preferably sensed by dividing the pressure sensor 238 output by the
actuator diaphragm area, so as to reduce the cost, power consumption and
complexity associated with load cell 242. For applications requiring fine
control, the sensed force signal is modified by the air spring effect from
the volume of air between the diaphragm and the case. For all embodiments
of the present invention, a non-contact position sensor with a continuous
output but without moving linkages, such as LVDT sensors, RVDT sensors,
and Hall Effect sensors are most appropriate. A multiplexer circuit 246
selects which of the sensor inputs is supplied to correction circuit 236,
as a function of a command received from receiving circuit 228.
Receiving circuit 228 receives a 4-20 mA signal from master 206, but may
also receive the signal from a hand-held communicator. Circuit 228
operates in substantially the same way as circuit 92. Control circuit 230
receives a digital signal from circuit 228 representative of the desired
valve position and a sensed position signal 229 representative of the
valve position and provides an electrical control signal 231 as a function
of appropriate PID constants set in circuit 230. Transducer and pneumatics
circuit 232 receives a 0-200 PSI supply of air and uses standard
current-to-pressure technology, as exemplified in Rosemount
Current-to-Pressure Converter 3311 to provide control pressure 212 at the
positioner nozzle.
Correction circuit 236 is preferably embodied in a low power CMOS
microprocessor and includes a non-volatile storage 250 for storing an
attribute of valve 214. In a first mode, generic information specific to
valve 214 is stored in storage 250, such as its fully opened and fully
closed positions, or its maximum and minimum acceptable pressures for
control pressure 212. The former data provides for correction of
overdriven or underdriven valves; the latter data provides for correction
of excessive over or under pressurization. In a second mode, laboratory
tested flow and torque measurements are collected for valve 214 and
downloaded to storage 250 from master 206 through receiving circuit 228.
Alternatively, the measured attribute may be stored in a non-volatile
memory such as EEPROM and subsequently installed in positioner 210.
Positioning is thereby tailored to the particular non-linearities of a
valve to be used in the process. In a third mode of operation used for
very precise positioning control, flow and torque attributes are initially
stored in storage 250 and then dynamically updated while the valve is in
operation. In this mode, a measured attribute is downloaded into storage
250 and then updated, point by point, as data is sampled at each point of
operation. For all these modes, correction circuit 236 compares the stored
attribute to the actual sensed physical parameter from sensing means 234
and compensates command output 231 accordingly. The stored attributes are
updated dynamically during valve operation.
One such stored attribute is the flow through valve 214 as a function of
position. The flow is given by:
##EQU1##
where Q is the flow, C.sub.v is the valve coefficient, DP is the
differential pressure across the valve and SG is the specific gravity of
the fluid in the pipe. FIG. 4 shows three types of general flow versus
position characteristics for quick opening, linear and equal percentage
valves, labelled respectively at A, B and C. A set of curves as a function
of specific gravity are stored in storage 250. Correction circuit 236
receives a signal representative of the sensed flow from transmitter 204
and compares the stored position corresponding to the sensed flow to the
sensed position. Correction circuit compensates command output 231 for the
deviation between the actual sensed position and the predicted position
based on the sensed flow, using op-amp summing junction techniques. The
effective bandwidth of the positioner may be lessened in this mode if the
time required to request and receive the process variable is long compared
to the response time for the positioner pneumatics. For implementations
which impose significant transfer delays, such as a delay of 600 mS, the
positioner bandwidth is necessarily lessened. However, when a
communications protocol such as Fieldbus, which has a 1 mS request and
retrieve time is employed, the target positioner bandwidth of 12 to 20 Hz
is preserved.
A second stored attribute is the torque versus position attribute of valve
214. As a positioner is an inherently non-linear device, it has difficulty
controlling valve position in a non-linear part of the torque vs. position
curve. For some rotary valves, torque vs. position is not only non-linear
but non-monotonic, FIG. 5A shows a pair of torque versus angular travel
attributes for a rotary valve 400 in a pipe 402, as shown in FIG. 5B.
Torque as a function of angular travel for opening the valve is shown by
curve 404, while valve closing attributes are shown by curve 406. The
accuracy provided by this feature is especially useful for control valves
which pivot about a central operating point, since they continuously
switch between disjointed operating attributes and have special problems
associated with their control. In this mode, correction circuit 236
receives a torque signal required to move the valve through a unit
distance (for a stem valve), or unit angle (for a rotary valve) and
compares it to the stored force required at the current sensed position or
angle. Correction circuit 236 compensates command output 231 for the
deviation between the actual sensed position and the predicted position
based on the sensed force. A third stored attribute is the measured torque
versus flow attribute of valve 214. In this mode, correction circuit 236
compares the sensed torque, as reported from load cell 242, to the stored
torque at the desired flow Q, and compensates command output 231 for the
difference.
Over time, the valve packing (shown at 244) degrades, and the seat (shown
at 246) starts to leak, both of which change valve flow as a function of
position. In this mode, the initial flow vs. position curve is stored in
storage 250 as discussed earlier, but is dynamically updated. For example,
when a position versus flow curve is selected, the sensed flow as reported
from transmitter 204, at each new operating position is stored, so as to
replace a previous operating point on the characteristic. Standard
interpolation algorithms are used to interpolate between large
discontinuities in the updated curve. As the flow and the corresponding
sensed position output is stored over time, a new curve is constructed
which reflects the dynamic flow versus position attributes of the
positioner. Modification of these attribute curves over time makes dynamic
correction, combined with real-time updating of the stored torque
attribute, essential to accurate static positioning. Although the previous
example shows a loop controlling flow, appropriate alterations to the same
scheme are apparent for control of other physical variables such as
temperature, pH, upstream and downstream process pressure and valve
position at operating limits (e.g. limit switches).
In FIG. 6, a control loop 300 including a transmitter 304, a master 306, a
positioner 310, an actuator 314 and a valve 316 controls the flow Q in a
pipe 302 in substantially the same way as discussed regarding FIG. 3.
Positioner 310 comprises a receiving circuit 330, a transmit circuit 358,
a control circuit 332, a transducer circuit and pneumatics 334, a sensing
circuit 336 and a diagnostic circuit 338. Receiving circuit 330
communicates in substantially the same way as receiving circuit 228.
Circuit 330 provides an output to a storage 354 for downloading valve
attributes and another output to a mux 352 for selecting which of the
sensed signals is selected for use in diagnostic circuit 338. Control
circuit 332 receives both a position signal 333 representative of the
valve position and a desired valve position signal from circuit 330 to
provide an electrical command output 335 as a function of PID constants
set in circuit 332. Transducer and pneumatics circuit 334 receives a 0-200
PSI supply of air and uses standard current-to-pressure technology to
provide control pressure 3 12 at the positioner nozzle. Sensing circuit
336 is connected to a pressure sensor 340 for sensing a control pressure
312 at a nozzle output on positioner 310, a mechanical member 342
connected to a valve stem 344 for sensing the valve position, a load cell
346 for sensing force, an acoustic sensor 348 for sensing cavitation and
valve packing noises and a fugitive emission sensor 350 for sensing vapors
from organic chemicals in piping 302. Other sensors which sense physical
parameters related to valve performance can be added, such as ones for
sensing upstream and downstream temperature, process pressure and limit
switches for sensing position at extreme open and closed positions or
sensors which provide process variables to cascaded control loops. A
multiplexer circuit 352 selects which of the sensor inputs is supplied to
diagnostic circuit 338, as a function of a command received from circuit
330. A transmit circuit 358 transmits alarms and diagnostic data to master
306.
Diagnostic circuit 338 is preferably embodied in a CMOS low power
microprocessor and includes non-volatile storage circuit 354 for storing
physical parameters related to the valve. As appropriate, the
characteristics are in the form of a range of acceptable values or a
single expected value representative of a maximum limit. The expected
values are downloaded to storage means 354 from master 306 over a two wire
loop 308. Master 306 typically is a loop controller located in a control
room, but may also be a hand-held communicator communicating
communications protocols such as HART.RTM. or Fieldbus. A comparator 356
compares the expected physical parameter with the sensed physical
parameter and provides a diagnostic output to transmit circuit 358. The
diagnostic output may be an alarm or alert transmitted through circuit 358
to master 306 for immediate action, as when valve 314 is improperly
positioned in a critical control loop, but may also be a value transmitted
to master 306 on a regular basis, or available upon polling, so as to
assess when maintenance is required.
Seat wear is also important in planning maintenance as it contributes to
valve leakage. For example, valve seat leakage is particularly critical in
quick opening valves or such valves which provide a significant change in
flow for a small adjustment to valve position. Curve A on FIG. 4 shows a
quick opening valve characteristic, which is translated upwards by a
constant representative of the amount of leakage (see dashed curve D).
Seat leakage occurs when fluid flows between passageways 353a,b when plug
360 is fully seated in seat 360. One way to assess leakage is to store a
position value corresponding to a fully-seated valve as manufactured, or
alternatively the seated position value at last maintenance. As valve seat
360 wears, plug 356 seats at progressively lower positions. The diagnostic
circuit compares the sensed position value when the valve is seated to the
stored seated position value. When the difference exceeds a stored limit,
a valve seat wear value is transmitted to master 306. Another way to
assess leakage is to compare a valve characteristic of valve 316 as
originally manufactured, to another valve characteristic collected after
wear has induced leakage. In FIG. 7, a position versus flow characteristic
is shown at A for valve 316 as originally manufactured, or alternatively,
as collected at a previous maintenance. After use, the dashed curve B
represents the same characteristic collected at a later time. The
characteristic is collected dynamically and constructed point by point at
each position at which the valve is operated. The difference between the
x-axis intercepts is representative of the leakage, which is reported to
master 306 through circuit 358.
Diagnosis of valve packing-related failures is also critical .to proper
valve maintenance. In this diagnostic mode, a value is stored in storage
354 representative of the cumulative distance at which the packing must be
re-packed. The stored value is compared to the cumulative distance
travelled, (degrees of travel for rotary valves) so that when the
cumulative distance travelled exceeds the distance at which re-packing is
required, circuit 358 transmits the diagnostic output to master 306.
Another measure of packing and seat erosion is the degradation, over time,
of the force required to unseat the valve. In FIG. 8, curve A represents
the actuator torque versus angular distance of a valve as originally
manufactured, or at a previous maintenance, and curve B represents the
same characteristic at a later date. The difference in the x-axis
intercept represents the difference in force required to unseat the valve.
When the difference is greater than a stored limit, the actuator force
value is transmitted master 306.
Determining when the valve trim (i.e. valve stem and cage assemblies) is
galling is also critical for planning maintenance. In this mode, the force
signal is selected for use in diagnostic circuit 338 and compared to a
value representative of an excessive amount of force. When the sensed
force signal exceeds the stored force value, the force value is reported
to master 306 through circuit 358. Another stored attribute is related to
solenoid valves, which are either fully open or shut. They are common in
critical control applications, where they bring the loop to a safe state.
Solenoid valves are prone to undesirable sticking after long periods of
non-use. In this mode, control circuit 332 sends out alternating open and
shut position commands at a rate faster than that which the process
responds, so tha | | |
