U.S. patent application number 11/210246 was filed with the patent office on 2006-07-20 for two-wire field-mounted process device.
Invention is credited to Steven J. DiMarco, Robert J. Karschnia, William R. Kirkpatrick, Gary A. Lenz, Marcos Peluso.
Application Number | 20060161271 11/210246 |
Document ID | / |
Family ID | 24278930 |
Filed Date | 2006-07-20 |
United States Patent
Application |
20060161271 |
Kind Code |
A1 |
Kirkpatrick; William R. ; et
al. |
July 20, 2006 |
Two-wire field-mounted process device
Abstract
A two-wire field-mounted process device with multiple isolated
channels includes a channel that can be an input channel or an
output channel. The given input or output channel can couple to
multiple sensors or actuators, respectively. The process device is
wholly powered by the two-wire process control loop. The process
device includes a controller adapted to measure one or more
characteristics of sensors coupled to an input channel and to
control actuators coupled to an output channel. The controller can
be further adapted to execute a user generated control algorithm
relating process input information with process output commands.
The process device also includes a loop communicator that is
adapted to communicate over the two-wire loop.
Inventors: |
Kirkpatrick; William R.;
(Faribault, MN) ; Karschnia; Robert J.; (Chaska,
MN) ; Peluso; Marcos; (Chanhassen, MN) ;
DiMarco; Steven J.; (Chanhassen, MN) ; Lenz; Gary
A.; (Eden Prairie, MN) |
Correspondence
Address: |
WESTMAN CHAMPLIN & KELLY, P.A.
SUITE 1400
900 SECOND AVENUE SOUTH
MINNEAPOLIS
MN
55402-3319
US
|
Family ID: |
24278930 |
Appl. No.: |
11/210246 |
Filed: |
August 23, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10760793 |
Jan 20, 2004 |
6961624 |
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11210246 |
Aug 23, 2005 |
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10400148 |
Mar 26, 2003 |
6711446 |
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10760793 |
Jan 20, 2004 |
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09570268 |
May 12, 2000 |
6574515 |
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10400148 |
Mar 26, 2003 |
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Current U.S.
Class: |
700/27 ;
438/17 |
Current CPC
Class: |
G06F 3/05 20130101; H05K
7/1472 20130101; G05B 19/054 20130101; G05B 2219/14144 20130101;
H05K 7/1484 20130101; G05B 2219/1163 20130101; G06F 1/182 20130101;
G05B 2219/1157 20130101; G05B 2219/1127 20130101; G05B 2219/15091
20130101; G05B 2219/25188 20130101 |
Class at
Publication: |
700/027 ;
438/017 |
International
Class: |
H01L 21/66 20060101
H01L021/66; G05B 11/01 20060101 G05B011/01; G01R 31/26 20060101
G01R031/26 |
Claims
1. A two-wire field-mountable process device comprising: a power
module couplable to a two-wire process control loop and adapted to
wholly power the process device with power received from the
two-wire process control loop; a loop communicator coupled to the
power module, and being couplable to the two-wire loop to
communicate on the two-wire loop in accordance with the Fieldbus or
Profibus protocol, wherein the communicator is configured to
provide an output on the loop; a controller coupled to the loop
communicator and power module; a first input connection operably
coupled to the controller, the first input connection configured to
couple to at least a first sensor and configured to receive a
sensor input; and a second input connection operably coupled to the
controller, the second input connection configured to couple to a
second sensor and receive a sensor input; and wherein the first and
second input connections are substantially electrically isolated
from each other and from the two-wire loop.
2-28. (canceled)
29. The process device of claim 1 wherein the first sensor is of a
first type, the second sensor is of a second type, and wherein the
first and second types are different.
30. The process device of claim 29 wherein the first type is
thermocouples.
31. The process device of claim 29 wherein the first type is
RTDs.
32. The process device of claim 1 and further comprising: a first
analog-to-digital converter operably coupled to the first input
connection and to the controller, the first converter being adapted
to convert an analog signal from the at least one first sensor
input, into at least one digital value at a fist conversion rate,
and to convey the digital signal to the controller; and a second
analog-to-digital converter operably coupled to the second input
connection and to the controller, the second converter being
adapted to convert an analog signal from the at least one second
sensor input, into at least one digital value at a second
conversion rate, and to convey the digital signal to the
controller; and wherein the first and second conversion rates are
different.
33. The process device of claim 32 wherein the first conversion
rate is related to a type of sensor to be coupled to the first
input connection.
34. The process device of claim 33 wherein the second conversion
rate is related to a type of sensor to be coupled to the second
input connection.
35. The process device of claim 1 wherein at least one of the
sensors is a temperature sensor.
36. The process device of claim 1 and further comprising: a
multiplexer coupled to the first input connection; and an
analog-to-digital converter operably coupled to the first input
connection via the multiplexer, the analog-to-digital converter
being operably coupled to the controller, wherein the
analog-to-digital converter is adapted to convert an analog signal
from the at least one first sensor input, into at least one digital
value and to convey the digital signal to the controller.
37. The process device of claim 1 wherein the controller operates
in accordance with a program.
38. The process device of claim 37 wherein the program can be
changed.
39. The process device of claim 38 wherein the program can be
changed by accessing the device or the two-wire loop.
40. The process device of claim 37 wherein the program relates on
input to the output.
41. The process device of claim 39 wherein the controller computes
a process variable based upon a sensor input.
42. The process device of claim 1 wherein the process device is in
accordance with a DIN standard.
43. The process device of claim 1 wherein the process device
complies with an intrinsic safety specification.
44. The process device of claim 1 including a third input
connection operably coupled to the controller, the third input
connection configured to couple to a third sensor and configured to
receive a third sensor input.
45. The process device of claim 44 including a multiplexer
configured to selectively couple the first input to the controller
or the third input to the controller.
46. The process device of claim 45 including an analog to digital
converter coupled to an output of the multiplexer and to the
controller configured to provide a digitized signal to the
controller.
47. The process device of claim 1 wherein the loop communicator
includes isolation circuitry.
48. The process device of claim 1 including a power module which is
configured to power circuitry of the first input connection and
circuitry of the second input connection.
49. A two-wire field-mountable process device comprising: a power
module couplable to a two-wire loop and adapted to wholly power the
process device with power received from the two-wire loop; a loop
communicator coupled to the power module, and being couplable to
the two-wire loop to communicate on the two-wire loop in accordance
with a digital protocol, wherein the communicator is configured to
provide an output of the loop; a controller coupled to the loop
communicator and power module; a first input connection operably
coupled to the controller, the first input connection configured to
couple to at least a first sensor and configured to receive a
sensor input; a second input connection operably coupled to the
controller, the second input connection configured to couple to a
second sensor and receive a sensor input; and wherein the first and
second input connections are substantially electrically isolated
from each other and from the two-wire loop.
50. The process device of claim 49 wherein the digital protocol
comprises the Fieldbus protocol.
51. The process device of claim 49 wherein the digital protocol
comprises the Profibus protocol.
52. The process device of claim 49 including a power module which
is configured to power circuitry of the first input connection and
circuitry of the second input connection.
53. A two-wire field-mountable process device comprising: a power
module couplable to a two-wire loop and adapted to wholly power the
process device with power received from the two-wire loop; a loop
communicator coupled to the power module, and being couplable to
the two-wire loop to communicate on the two-wire loop; a controller
coupled to the loop communicator and power module; a first input
connection operably coupled to the controller, the first input
connection configured to couple to at least a first sensor and
configured to receive a sensor input; a second input connection
operably coupled to the controller, the second input connection
configured to couple to a second sensor and receive a sensor input;
wherein the first and second input connections are substantially
electrically isolated from each other; and wherein circuitry of the
first input connection and circuitry of the second input connection
are powered by the power module.
54. The process device of claim 53 including a third input
connection operably coupled to the controller, the third input
connection configured to couple to a third sensor and configured to
receive a third sensor input.
55. The process device of claim 54 including a multiplexer
configured to selectively couple the first input to the controller
or the third input to the controller.
56. The process device of claim 55 including an analog to digital
converter coupled to an output of the multiplexer and to the
controller configured to provide a digitized signal to the
controller.
57. The process device of claim 54 wherein the power module is
configured to power circuitry of the third input connection.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to process devices. More
specifically, the present invention relates to field-mounted
process control and measurement devices.
[0002] Process devices are used to measure and control industrial
processes such as the refining of petrochemicals, the processing of
food, the generation of electric power, and a number of other
processes. Process measurement devices include process variable
transmitters, which measure a process variable such as pressure or
temperature and communicate the measured variable to a process
controller. Another type of process device is an actuator, such as
a valve controller or the like. Generally, process control is
accomplished using a combination of transmitters, actuators, and a
process controller that communicate across a process control loop
to a controller. Both types of process devices interact with the
physical process through process interface elements. Process
interface elements are devices which relate electrical signals to
physical process conditions, and include devices such as sensors,
limit switches, valve controllers, heaters, motor controllers, and
a number of other devices.
[0003] The process controller is typically a microcomputer located
in a control room away from the process. The process controller can
receive process information from one or more process measurement
devices and apply a suitable control signal to one or more process
control devices to influence the process and thereby control
it.
[0004] In order to couple to the process, transmitters and
actuators are generally mounted near the process in the field. Such
physical proximity can subject the process devices to an array of
environmental challenges. For example, process devices are often
subjected to temperature extremes, vibration, corrosive and/or
flammable environments, and electrical noise. In order to withstand
such conditions, process devices are designed specifically for
"field-mounting." Such field-mounted devices utilize robust
enclosures, which can be designed to be explosion-proof. Further,
field-mounted process devices can also be designed with circuitry
that is said to be "intrinsically safe", which means that even
under fault conditions, the circuitry will generally not contain
enough electrical energy to generate a spark or a surface
temperature that can cause an explosion in the presence of an
hazardous atmosphere. Further still, electrical isolation
techniques are usually employed to reduce the effects of electrical
noise. These are just a few examples of design considerations,
which distinguish field-mounted process devices from other devices,
which measure sensor characteristics and provide data indicative of
such characteristics.
[0005] Aside from the environmental considerations listed above,
another challenge for field-mounted devices is that of wiring.
Since process devices are located near the process far from the
control room, long wire runs are often required to couple such
devices to the control room. These long runs are costly to install
and difficult to maintain.
[0006] One way to reduce the requisite wiring is by using two-wire
process devices. These devices couple to the control room using a
two-wire process control loop. Two-wire devices receive power from
the process control loop, and communicate over the process control
loop in a manner that is generally unaffected by the provision of
power to the process device. Techniques for communicating over
two-wires include 4-20 mA signaling, the Highway Addressable Remote
Transducer (HART.RTM.) Protocol, FOUNDATION.TM. Fieldbus,
Profibus-PA and others. Although two-wire process control systems
provide wiring simplification, such systems provide a limited
amount of electrical power to connected devices. For example, a
device that communicates in accordance with 4-20 mA signaling must
draw no more than 4 mA otherwise the device's current consumption
would affect the process variable. The frugal power budget of
two-wire process devices has traditionally limited the
functionality that could be provided.
[0007] Another way the process control industry has reduced field
wiring is by providing transmitters with two sensor inputs. Such
transmitters reduce the number of transmitters/sensor and thereby
reduce wiring costs as well as overall system costs. One example of
such a transmitter is the Model 3244MV Multivariable Temperature
Transmitter, available from Rosemount Inc., of Eden Prairie,
Minn.
[0008] Although current multivariable transmitters can reduce
wiring costs as well as overall system costs, they have
traditionally been limited to applications involving two sensors.
Thus, in applications with sixteen sensors, for example, eight
multivariable transmitters would still be required. Further, if
different sensor groups are independently grounded, there is, a
possibility that ground loop errors could occur and adversely
affect process measurement.
[0009] Current methods used to overcome the problem of coupling a
large number of sensors to the control room include coupling the
sensors directly to the control room. For example, if a situation
requires a large number of temperature sensors, consumers generally
create "direct run" thermocouple configurations where thermocouple
wire spans the distance between the measurement "point" and the
control room. These direct run configurations are generally less
expensive than the cost of obtaining a number of single or dual
sensor transmitters, however, a significant wiring effort is
required, and process measurement is rendered more susceptible to
electrical noise due to the long runs.
[0010] The process control industry has also reduced the effects of
long wire runs on process control by providing field-mounted
devices that are capable of performing control functions. Thus,
some aspects of process control are transferred into the field,
thereby providing quicker response time, less reliance upon the
main process controller, and greater flexibility. Further
information regarding such control functions in a field-mounted
device can be found in U.S. Pat. No. 5,825,664 to Warrior et al,
entitled FIELD-MOUNTED CONTROL UNIT, assigned to Rosemount
Incorporated.
[0011] Although multivariable transmitters and process devices
implementing control functions have advanced the art of process
control, there is still a need to accommodate applications
requiring a relatively large number of sensors, as well as
applications requiring enhanced control in the field.
SUMMARY
[0012] A two-wire field-mounted process device is provided. In one
embodiment, the process device includes multiple isolated channels
includes a channel that can be an input channel or an output
channel. The given input or output channel can couple to multiple
sensors or actuators, respectively. The process device is wholly
powered by the two-wire process control loop. The process device
includes a controller adapted to measure one or more
characteristics of sensors coupled to an input channel and to
control actuators coupled to an output channel. The process device
also includes a loop communicator that is adapted to communicate
over the two-wire loop. In another embodiment, the two-wire
field-mounted process device includes a controller that is adapted
to execute a user generated control algorithm relating process
input information with process output commands. The process device
of this embodiment also includes a loop communicator that is
adapted to communicate over the two-wire loop.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a diagrammatic view of a process control system
employing a two-wire field mounted process device in accordance
with an embodiment of the present invention.
[0014] FIG. 2 is a system block diagram of the process device shown
in FIG. 1.
[0015] FIG. 3 is a system block diagram of a method of providing a
process variable with a field-mounted process device in accordance
with an embodiment of the present invention.
[0016] FIG. 4 is a system block diagram of a method of operating a
field-mounted process device in accordance with an embodiment of
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] A two-wire field mountable process device 16 shown in FIG. 1
is provided which can be adapted to execute sophisticated user
generated control algorithms, much like those used with traditional
programmable logic controllers. Embodiments can include input
channels, output channels, and any combination of the two.
Generally, each channel is isolated from the remainder of the
process device. Such isolation removes ground loop errors that
currently limit multiple input transmitters. Finally, power
management is such that embodiments of the present invention are
wholly powered by a two-wire process loop 14. These and other
features will become apparent upon review of the figures and the
related description provided below.
[0018] FIG. 1 is a diagrammatic view of process control system 10
which includes control room 12, process control loop 14 and process
device 16. Process control system can comprise a single process
device coupled to control room 12, however system 10 can also
include hundreds of process devices coupled to one or more control
rooms over a number of process control loops.
[0019] Control room 12 is typically a facility located away from
device 16 that includes a microcomputer. A user stationed in
control room 12 uses the microcomputer to interact with various
process devices through process control loop 14 and thus controls
the process(es) from the control room. For clarity, control room 12
is illustrated as a single block. However, in some control system
embodiments, control room 12 may in fact couple process control
loop 14 to a global computer network, such as the internet, so that
users worldwide could access process device 16 from traditional web
browser software.
[0020] Loop 14 is a two-wire process control loop. A number of
two-wire process communication protocols exist for communicating on
loop 14, and any suitable protocol can be used. For example, the
HART.RTM. protocol, the FOUNDATION.TM. Fieldbus protocol, and the
Profibus-PA protocol can be used with embodiments of the present
invention. Loop 14 provides power to connected process devices
while providing communication between the various devices.
[0021] Process device 16 includes cover 17 and base 19 which are
preferably constructed from a suitable plastic material. Base 19 is
adapted to mate with an industry standard DIN rail for mounting. As
will be described in more detail, device 16 is adapted to operate
solely upon electrical power received through loop 14, and is
adapted for field-mounting. Thus, device 16 is configured to
withstand a relatively large temperature range (such as -40 to 85
deg. C.), mechanical vibrations, and relative humidity in excess of
90%. Such environmental resistance is effected primarily through
the selection of robust components, as will be described later in
the specification. Optional enclosure 18 (shown in phantom)
provides added durability and can be any known enclosure such as a
National Electrical Manufacturers Association (NEMA) enclosure, or
an explosion-proof enclosure. The process device embodiment shown
in FIG. 1 has a number of inputs and outputs, and includes suitable
computing circuitry (shown in FIG. 2) to execute a user generated
control algorithm. The algorithm is comprised of a number of logic
statements relating specific input events to outputs controlled by
device 16. The user can change the algorithm either by interfacing
locally with device 16, or by communicating with device 16 over
control loop 14. The algorithm can be generated using conventional
logic generation software such as Relay Ladder Logic and Sequential
Function Charts (SFC's). In this sense, device 16 can be considered
a two-wire field-mountable programmable logic controller. Although
the description will focus upon the embodiment shown in FIGS. 1 and
2, such description is provided for clarity, since embodiments
employing solely inputs, or outputs are expressly contemplated.
Traditionally devices with the computational power of device 16
could not be operated upon two-wire process control loops due to
prohibitive power constraints.
[0022] Process device 16 is coupled to sensors 20, 22, 24, 26, 28
and 30 as well as actuators 32 and 34. Sensors 20, 22 and 24 are
thermocouples, of known type, which are coupled to various process
points to provide voltage signals based upon process variables at
the respective process points. Resistance Temperature Devices
(RTD's) 26, 28 and 30 are also coupled to various process points
and provide a resistance that is based upon process temperature at
the respective process points. RTD 26 is coupled to device 16
through a known three-wire connection and illustrates that various
wiring configurations can be used with embodiments of the present
invention. Actuators 32 and 34 are coupled to process device 16 and
actuate suitable valves, switches and the like based upon control
signals from device 16. As noted above, device 16 can execute a
user generated control algorithm to relate specific input
conditions to specific output commands. For example, device 16 may
sense a process fluid temperature, and cause actuator 32 to engage
a heater coupled to the process fluid in order to maintain the
fluid temperature at a selected level.
[0023] FIG. 2 is a system block diagram of device 16 shown in FIG.
1. Device 16 includes loop communicator 36, power module 38,
controller 40, and channels 42, 44, 46, 48, and memory 52. Loop
communicator 36 is coupled to process control loop 14 and is
adapted for bi-directional data communication over loop 14. Loop
communicator 36 can include a known communication device such as a
traditional FOUNDATION.TM. Fieldbus communication controller or the
like. Additionally, communicator 36 can include suitable isolation
circuitry to facilitate compliance with the intrinsic safety
specification as set forth in the Factory Mutual Approval Standard
entitled "Intrinsically Safe Apparatus and Associated Apparatus for
Use in Class I, II, and III, Division 1 Hazardous (Classified)
Locations," Class Number 3610, published October 1988.
[0024] Power module 38 is coupled to loop 14 such that power module
38 provides power to all components of device 16 based upon power
received from loop 14. Although power module 38 has a single arrow
50 indicating that power module 38 provides power to all
components, it is noted that such power can be provided at multiple
voltages. For example, power module 38 preferably includes a
switching power supply that provides electrical power at a
plurality of voltages. Thus, some components such as the A/D
converters and the isolators can receive a higher voltage such as
4.9 volts, while low-power components such the controller 40,
memory 52 and loop communicator 36 receive a lower voltage such as
3.0 volts. Additionally, power module 38 is preferably programmable
to such an extent that at least one of the voltages provided can be
varied. The selectable nature of power module 38 facilitates power
management, which will be described later in the specification.
[0025] Controller 40 is coupled to memory 52 and executes program
instructions stored therein. Memory 52 is preferably low-power
memory operating on 3.0 volts, such as the model LRS1331, available
from Sharp Electronics. Additionally, memory 52 can be "stacked"
memory in which both flash memory and volatile memory are provided
on a single memory module. The user generated control algorithm, or
"program" executed by controller 40 can be changed by a user either
by coupling to device 16 locally, or by accessing device 16 through
loop 14. In some embodiments the program includes instructions that
relate process event inputs to outputs determined by controller 40.
In this sense, device 16 functions similarly to a programmable
logic controller, which is a device that typically has not been
robust enough for field-mounting, nor able to operate on the low
power levels of two-wire field devices. However, by so providing
the functions of a programmable logic controller, much more
sophisticated process control algorithms can be implemented through
a user friendly interface, such as Relay Ladder Logic or the
like.
[0026] Controller 40 receives power from module 38, and
communicates with loop communicator 36. Controller 40 preferably
includes a low-power microprocessor such as the model MMC 2075
microprocessor available from Motorola Inc. of Schaumburg, Ill.
Additionally, controller 40 preferably has a selectable internal
clock rate such that the clock rate of controller 40, and thus the
computing speed and power consumption, can be selected through
suitable commands sent to device 16 over loop 14. Since higher
clock speeds will cause controller 40 to draw more power, clock
selection of controller 40, and selection of the voltage level
provided by power module 38 to controller 40 are preferably
performed in tandem. In this manner the processing speed and power
consumption of device 16 are selectable and vary together.
[0027] Controller 40 is coupled to the various channels through
interface bus 54, which is preferably a serial bus designed for
high speed data communication such as a Synchronous Peripheral
Interface (SPI). Channels 42, 44, 46 and 48 are coupled to bus 54
through communication isolators 56, 58, 60 and 62, respectively,
which are preferably known optoisolators, but which can be any
suitable isolation devices such as transformers or capacitors. In
some embodiments, channels 42, 44, 46 and 48 provide data in
parallel form, and parallel-serial converters 64 are used to
translate the data between serial and parallel forms. Preferably,
converters 64 are Universal Asynchronous Receiver/Transmitters
(UART's).
[0028] Channel 42 is coupled to controller 40, and includes sensor
terminals 1-n, multiplexer (MUX) 66, analog-to-digital (A/D)
converter 68, communication isolator 56, and power isolator 70. It
is contemplated that communication isolator 56 and power isolator
70 can be combined in a single circuit. Channel 42 is specifically
adapted to measure a specific sensor type such as thermocouples,
resistance temperature devices, strain gauges, pressure sensors, or
other sensor type. Each sensor terminal is adapted to couple a
single sensor, such as a thermocouple, to multiplexer 66.
Multiplexer 66 selectively couples one of the sensors to A/D
converter 68 such that a characteristic of the sensor (voltage for
a thermocouple) is measured and communicated to controller 40
through isolator 56 and UART 64. Power for channel 42 is received
from power module 38 through power isolator 70. Power isolator 70
is preferably a transformer, but can be any suitable device. Those
skilled in the art will appreciate that communication isolator 56
and power isolator 70 cooperate to ensure that channel 42 is
electrically isolated from the rest of device 16.
[0029] Channel 44 is similar to channel 42, and like components are
numbered similarly. Channel 44 can be configured to measure sensors
of a different type than that of channel 42. For example, in one
embodiment, channel 42 is configured to measure the voltage of
thermocouples, and channel 44 is configured to measure the
resistance of RTD's. Each sensor terminal in channel 44 is thus
configured to couple to an RTD in a two, three, or four-wire
(Kelvin) connection. Because channels 42 and 44 are each
electrically isolated from the rest of device 16, coupling a first
independently grounded sensor to channel 42, and a second
independently grounded sensor to channel 44 does not result in the
generation of undesirable ground loop errors. Additionally, since
each channel can be configured for a specific type of sensor, which
can be optimized for a specific application, parameters such as A/D
precision and conversion rate can be tailored for the specific
sensor type. For example, a channel designed for high-precision may
employ an A/D converter of configured to provide a very high
accuracy having a relatively slower conversion time. Conversely, a
channel designed for sensors that measure a process variable that
can changes quickly can employ a lower precision high speed A/D
converter. Essentially, any sensor input can be switched between
operation with resistance-type sensors to operation with
voltage-type sensors based upon configuration information received
from controller 40. Controller 40 can provide the configuration
information based upon information received over loop 14, or
through a local input (not shown). Additionally, controller 40 can
provide configuration information to the channels to adjust
analog-to-digital sampling rates for each channel, or even for each
sensor. This is particularly advantageous where sensor rates of
change are anticipated based upon information known about the
process.
[0030] Channel 46 is similar to channels 42 and 44, however since
channel 46 is configured to receive digital inputs, it does not
include an analog-to-digital converter. As illustrated, inputs 1-n
are coupled to multiplexer 66 which conveys the signal of a
selected input to bus 54 through communication isolator 60 and UART
64. In some digital input embodiments, the input level may be such
that the digital inputs could be provided directly to UART 64
through isolator 60. Digital inputs are generally indicative of
logic-type signals such as contact closure in limit switches as the
like. However, digital inputs 1-n can also be coupled to digital
outputs of other process devices such that the inputs represent
logic signals such as alarms or other Boolean type signals.
[0031] Channel 48 is similar to channel 46, but essentially
operates in reverse compared to channel 46. Thus, serial
information sent to channel 48 through the UART is converted into
parallel form, and conveyed across communication isolator 62 to set
individual actuator outputs. Thus, logic signals are sent to the
terminals labeled ACTUATOR 1-n to cause actuators coupled to such
terminals (not shown) to engage or disengage as desired. Such
actuators can be any suitable device such as valve controllers,
heaters, motor controllers and any other suitable device.
Essentially, any device that is addressable based upon a logic type
output is an actuator.
[0032] FIG. 3 is a system block diagram of a method of providing a
process variable with a field-mounted process device in accordance
with an embodiment of the present invention. The method begins at
block 80 where a field-mountable process device is wholly powered
by a two-wire process control loop. At block 82, the process device
is coupled to a first sensor through a first isolated input
channel. A sensor signal is acquired through the first isolated
input channel, which signal is indicative of a process variable. At
block 84, the process device is coupled to a second sensor through
a second isolated input channel in order to acquire a second sensor
signal. Since the first and second input channels are isolated,
independent grounding of the first and second sensors will not
cause undesirable ground loop errors. At block 86, the process
device computes a process variable based upon one or both of the
sensor signals. Moreover, although the method is described with
respect to two sensors, a number of additional sensors could be
used such that the process variable would be a function of any
number of sensor signals. For example, the process device could
average the values of the sensor, provide their difference,
standard deviation, or any other appropriate function. At block 88,
the computed process device is output. Such output can be in the
form of information sent over the process control loop, a local
display, or a local output effected through an output channel.
[0033] FIG. 4 is a system block diagram of a method of operating a
field-mounted process device in accordance with an embodiment of
the present invention. At block 80, the device is wholly powered by
the two-wire process control loop. At block 92, the device receives
an input. Such input can be in the form of signals received through
input channels, such as the multiple isolated input channels
described above, in the form of process information received
through the two-wire process control loop, in the form of a local
input, or any combination of input signals and information. At
block 94, the device executes user-programmable logic to relate the
input information to one or more process outputs. The
user-programmable logic can be simple or complex algorithms such as
ladder logic, SFC's, fuzzy logic, Adaptive Control, or neural
networks and the like. At block 96, the device provides the output
determined via operation of the user-programmable logic. The output
can be a local output, either digital or analog, or the output can
be sent as information over the two-wire process control loop.
[0034] Although the present invention has been described with
reference to embodiments of two-wire process device having four
channels, workers skilled in the art will recognize that changes
may be made in form and detail without departing from the spirit
and scope of the invention, which are defined by the appended
claims. For example, although various modules have been illustrated
and described separately, it is expressly contemplated that some
such modules can be physically embodied together, such as on an
Application Specific Integrated Circuit. Further, although
controller 40 is described as a single module, its functions can be
distributed upon multiple microprocessors such that one
microprocessor could provide low-level I/O interaction, such as
calibration, linearization and the like, while a second
microprocessor executes the user-generated control algorithm.
Additionally, although the description has focussed upon inputs and
outputs being provided through the disclosed channels, it is
expressly contemplated that some process inputs or process outputs
could be communicated from/to other process devices through the
process control loop.
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