Title: Process control loop signal converter
Abstract: A device for use in a process control system includes a first pair of electrical connections configured to couple to a two-wire process control loop. The loop includes a two-wire process variable transmitter. A second pair of electrical connections is configured to couple to an input channel of a process device having multiple input channels. An electrical component is connected in series in the loop for use in digital communication with the two-wire process variable transmitter.
Patent Number: 7,016,741 Issued on 03/21/2006 to Arntson
| Inventors:
|
Arntson; Douglas W. (Maple Grove, MN)
|
| Assignee:
|
Rosemount Inc. (Eden Prairie, MN)
|
| Appl. No.:
|
685167 |
| Filed:
|
October 14, 2003 |
| Current U.S. Class: |
700/19; 700/9; 370/410 |
| Current Intern'l Class: |
G05B 11/01 (20060101) |
| Field of Search: |
700/1,9,11,12,19,37,282,39,90,2
370/410
340/825,825.01
|
References Cited [Referenced By]
U.S. Patent Documents
| 4122719 | Oct., 1978 | Carlson et al.
| |
| 4243931 | Jan., 1981 | Dela Cruz.
| |
| 4413314 | Nov., 1983 | Slater et al.
| |
| 4678937 | Jul., 1987 | Price.
| |
| 4910658 | Mar., 1990 | Dudash et al.
| |
| 4936690 | Jun., 1990 | Goetzinger.
| |
| 5307346 | Apr., 1994 | Fieldhouse.
| |
| 5495769 | Mar., 1996 | Broden et al.
| |
| 5551053 | Aug., 1996 | Nadolski et al.
| |
| 5706007 | Jan., 1998 | Fragnito et al.
| |
| 5713668 | Feb., 1998 | Lunghofer et al.
| |
| 5737543 | Apr., 1998 | Gavin et al.
| |
| 5803604 | Sep., 1998 | Pompei.
| |
| 5825664 | Oct., 1998 | Warrior et al.
| |
| 5876122 | Mar., 1999 | Eryurek.
| |
| 5980078 | Nov., 1999 | Krivoshein et al.
| |
| 6014612 | Jan., 2000 | Larson et al.
| |
| 6016523 | Jan., 2000 | Zimmerman et al.
| |
| 6026352 | Feb., 2000 | Burns et al.
| |
| 6035240 | Mar., 2000 | Moorehead et al.
| |
| 6047222 | Apr., 2000 | Burns et al.
| |
| 6052655 | Apr., 2000 | Kobayashi et al.
| |
| 6094600 | Jul., 2000 | Sharpe, Jr. et al.
| |
| 6104875 | Aug., 2000 | Gallagher et al.
| |
| 6192281 | Feb., 2001 | Brown et al.
| |
| 6574515 | Jun., 2003 | Kirkpatrick et al.
| |
| 6711446 | Mar., 2004 | Kirkpatrick et al.
| |
| 2002/0010518 | Jan., 2002 | Reid et al.
| |
| 2004/0190592 | Sep., 2004 | Lojen.
| |
| Foreign Patent Documents |
| 297 20 492 | Mar., 1998 | DE.
| |
| 0 601 344 | May., 1994 | EP.
| |
| 0 666 631 | Jan., 1995 | EP.
| |
| 2 329 039 | Mar., 1999 | GB.
| |
| 52-108194 | Sep., 1977 | JP.
| |
| 07162345 | Jun., 1995 | JP.
| |
Other References
"Advanced Systems Simplify Control", Machine Design, Penton, Inc., vol.
68, No. 12, pp. 118, 120, Jul. 11, 1996.
"PLCs: Looking Around for More Work to Do", by M. Bab, Control Engineering,
pp. 59, 60 and 62, Oct. 1996.
"D5000 Series Users Manual", 23 pages, Revised Jan. 1, 1998.
"Model 848T Eight Input Temperature Transmitter With FOUNDATION™ Fieldbus",
Product Data Sheet 00813-0100-4697, Mar. 2003.
"ACE pc," Arcom Control Systems, 1 page dated Feb. 23, 2000, downloaded from http://www.arcom.co.uk/products/iep/systems/ace/default.htm.
"Smart Transmitter (HART Protocol) Interface Products," 1770 Communication Products,
2 pages dated Apr. 26, 1999, downloaded from http://www.ab.com/catalogs/ html/b112/io/smart.html.
"Smart Head and Rail Mount Temperature Transmitters," Models 644H and 644R, FISHER-ROSEMOUNT
Managing the Process Better, pp. 37-52 (1998).
"Smart Temperature Transmitter," Models 3144 and 3244MV, FISHER-ROSEMOUNT Managing
the Process. Better, pp. 19-36 (1998).
"Transducer Interfacing Handbook," A Guide to Analog Signal Conditioning, by
Daniel H. Sheingold, 5 pages (1980).
Instruction Manual FD0-BI-Ex12.PA, German language document, and apparent English
equivalent Part No.: 107591, Jan. 24, 2001.
Universal temperature multiplexer for Foundation Fieldbus, Universal converter,
analogue, F2D0-TI-Ex8.FF, 2003.
Valve Coupler for Foundation Fieldbus, Manual FD0-VC-Ex4.FF, Nov. 22, 2000.
PROFIBUS-PA Valve Coupler Filed Box, 2002 IS Catalog, 2002.
PROFIBUS-PA Sensor Interface Field Box, 2002 IS Catalog, 2002.
EC-Type Examination Certificate for FD0-VC-Ex4.Pa dated Dec. 18, 1998 (German
Language document and apparent English translation).
Supplement to EC-Type Examination Certificate for FD0-VC-Ex.Pa dated Sep. 18,
2000 (German Language document and apparent English translation).
Supplement to EC-Type Examination Certificate for FD0-VC-Ex4.Pa dated Sep. 21,
2000 (German Language document and apparent English translation).
1st Amendment to EC-Type Examination Certificate for FD0-VC-Ex4.Pa
dated Dec. 22, 1999 (German Language document and apparent English translation).
Office Action from the Chinese Patent Office in related Chinese application.
Observations by Third Party filed in related European Application including Translation.
|
Primary Examiner: Picard; Leo
Assistant Examiner: Kosowski; Alexander
Attorney, Agent or Firm: Westman, Champlin & Kelly, P.A.
Claims
What is claimed is:
1. A signal conversion device for use in a process control system, comprising:
a first pair of electrical connections configured to couple to a two-wire process
control current loop which includes a two-wire process variable transmitter which
provides an analog current level on the two-wire process control current loop related
to a sensed process variable;
a second pair of electrical connections configured to couple to an analog voltage
input channel of a process device; and
a first electrical component electrically connected to a first electrical connection
of the first pair of electrical connections and a first electrical connection of
the second pair of electrical connections, the first electrical component further
configured to couple to a digital communicator to provide a connection for digital
communication between the digital communicator and the two-wire process variable transmitter;
a second electrical component connected between the first and a second electrical
connection of the second pair of electrical connections to provide a connection
for communication with the voltage input channel of the process device with an
analog voltage related to the analog current level on the two-wire process control
loop; and
a switch connected in parallel with the first electrical component between the
first electrical connector of the first pair of electrical connections and the
first electrical connector of the second pair of electrical connectors, the switch
configured to selectively allow digital communication by the digital communicator
through the first electrical component with the two-wire process variable transmitter.
2. The apparatus of claim 1 wherein the first electrical component is in series
between the first electrical connection of the first pair of electrical connections
and a first electrical connection.
3. The apparatus of claim 1 wherein the first electrical component comprises
a resistor.
4. The apparatus of claim 3 wherein the resistor has a resistance of between
about 230 and about 600 ohms.
5. The apparatus of claim 1 wherein the second electrical component is in series
between the first and the second electrical connections of the second pair of electrical connections.
6. The apparatus of claim 1 wherein the second electrical component comprises
a resistor.
7. The apparatus of claim 6 wherein the second electrical component comprises
a resistor.
8. The apparatus of claim 6 wherein the resistance of the resistor has a resistance
of 5 ohms.
9. The apparatus of claim 1 wherein a current through the two-wire process control
current loop ranges between about 4 mA and 20 mA.
10. The apparatus of claim 1 wherein a voltage between the second pair of electrical
connections ranges between about 20 mVolts and about 100 mVolts.
11. The apparatus of claim 1 including a power supply.
12. The apparatus of claim 11 wherein the power supply provide a DC output of
between about 10 V and about 50 V and is coupled in series with the two-wire process
control current loop.
13. The apparatus of claim 1 including a output indicative of an active power
supply on the two-wire process control current loop.
14. The apparatus of claim 13 wherein the output comprises an optical output.
15. The apparatus of claim 1 wherein the process device includes multiple input channels.
16. The apparatus of claim 1 wherein the first pair of electrical connections
is configured for HART® communication.
17. A signal conversion device for use in a process control system, comprising:
a first pair of electrical connections configured to couple to a two-wire process
control current loop which includes a two-wire process variable transmitter which
provides an analog current level on the two-wire process control current loop related
to a sensed process variable;
a second pair of electrical connections configured to couple to an analog voltage
input channel of a process device; and
digital communication coupling means electrically coupled between a first electrical
connector of the first pair of electrical connections and a first electrical connector
of the second pair of electrical connectors for coupling a digital communication
signal between a digital communicator and the two-wire process variable transmitter
through the first pair of electrical connections;
switch means connected in parallel with the digital communication coupling means
between the first electrical connector of the first pair of electrical connections
and the first electrical connector of the second pair of electrical connectors
for selectively bypassing the digital communication coupling means; and
electrical component means connected between the first and a second electrical
connectors of the second pair of electrical connectors for communicating with the
voltage input channel of the process device an analog voltage related to the analog
current level on the two-wire process control loop.
18. The apparatus of claim 17 wherein the digital communication coupling means
comprises a resistor.
19. The apparatus of claim 17 wherein the electrical component means comprises
a resistor.
20. A method for use in a process control system, comprising:
providing a process control current loop for coupling to a two-wire process variable
transmitter which provides an analog current level on the two-wire process control
current loop related to a sensed process variable;
providing a first pair of electrical connections on the two-wire process control
current loop for coupling to a digital communicator;
providing a second pair of electrical connections for coupling to an analog voltage
input channel of a process device;
providing a first electrical component between a first electrical connector of
the first pair of electrical connectors and a first electrical connector of the
second pair of electrical connections configured to couple to the digital communicator
for digital communication between the digital communicator and the two-wire process
variable transmitter;
providing a second electrical component between the first and a second electrical
connection of the second pair of electrical connections for communication with
the voltage input channel of the process device using an analog voltage related
to the analog current level on the tow-wire process control loop;
providing a switch in parallel with the first electrical component between the
first electrical connector of the first pair of electrical connectors and the first
electrical connections of the second pair of electrical connections;
opening the switch and digitally communicating with the transmitter through the
first electrical component; and
closing the switch and bypassing the first electrical component.
21. The method of claim 20 wherein the first electrical component comprises a resistor.
22. The method of claim 20 wherein the second electrical component comprises
a resistor.
23. The method of claim 20 wherein the voltage drop across the second pair of
electrical connections is between about 20 mVolts and about 100 mVolts.
24. The method of claim 20 wherein a two-wire process control current loop carries
an electrical current between about 4 mA and 20 mA.
25. The method of claim 20 including digitally communicating with the two-wire
process variable transmitter.
26. The method of claim 25 wherein digital communicating comprises communicating
in accordance with the HART® standard.
Description
BACKGROUND OF THE INVENTION
The present invention relates to process devices. More specifically, the present
invention relates to field-mounted process control and measurement devices.
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.
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.
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.
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.
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®) Protocol,
FOUNDATION™ 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.
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.
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.
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.
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.
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. One two-wire field mountable process device
having multiple channels for coupling to a process interface element is shown and
described in U.S. Pat. No. 6,574,515 entitled TWO-WIRE FIELD-MOUNTED PROCESS DEVICE
by William R. Kirkpatrick et al. which issued on Jun. 3, 2003 to Rosemount Inc.
of Eden Prairie Minn. and is incorporated herein in its entirety.
SUMMARY OF THE INVENTION
A device for use in a process control system includes a first pair of electrical
connections configured to couple to a two-wire process control loop which includes
a two-wire process variable transmitter. A second pair of electrical connections
is configured to couple to an input channel of a process device having multiple
input channels. An electrical component is connected in series between a first
electrical connection of the first pair of electrical connections and a first electrical
connection of the second pair of electrical connections. The component is used
for digital communication with the two-wire process variable transmitter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view of a process control system employing a two-wire
field mounted process device.
FIG. 2 is a system block diagram of the process device shown in FIG. 1.
FIG. 3 is a diagram of electrical circuitry for coupling to inputs of the device
of FIG. 1.
FIG. 4 is a schematic diagram of electrical circuitry for coupling to the process
device of FIG. 1.
FIG. 5 is a schematic diagram of electrical circuitry for coupling to the process
device of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
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.
The present invention provides a method, apparatus and system in which a process
variable transmitter is coupled to a two-wire process device which communicates
with the control room
12. FIG. 1 is a diagram showing a field mountable
process device
16 coupled to process variable transmitters
24 and
30 through signal conversion devices
25 and
31, respectively.
Operation of signal conversion devices
25 and
31 is described below
in greater detail.
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.
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® protocol, the FOUNDATION™
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.
In this embodiment, process device
16 includes cover
17 and base
19 which are preferably constructed from a suitable plastic material. Device
16 is preferably adapted to operate solely upon electrical power received
through loop
14, and is adapted for field-mounting. 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.
In this embodiment, process device
16 is coupled to sensors
20,
22,
26 and
28, process control transmitters
24 and
30, actuators
32 and
34. Sensors
20 and
22 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 and
28 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.
Process variable transmitters
24 and
30 are coupled to device
16 through signal translation devices
25 and
31. Transmitters
24 and
30 are configured to sense process variables using sensors
23 and
29, respectively, which couple to process fluid carried in
process piping
27 and
33, respectively. Operation of transmitters
24 and
30 and signal translation devices
25 and
31
is described below in greater detail.
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™ 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.
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.
In one embodiment, 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.
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.
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).
In this embodiment, 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, transmitters, 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.
Channel
48 is similar to channel
42, 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
56 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.
As discussed above, in some instances it is desirable to connect a process variable
transmitter, such as transmitters
24 or
30, to inputs of device
16.
The inputs of device
16 are configured to receive a voltage input, for example,
an input which ranges between 20 and 100 mVolts DC. However, the output of a process
variable transmitter is typically in accordance with a different standard, for
example a 4-20 mA standard in which a process variable is represented by an electrical
current in a process control loop. A 4 mA signal can represent a low value of the
process variable while a 20 mA signal can represent a high value of the process
variable, or other conditions such as an alarm condition.
FIG. 3 is a diagram of electrical circuitry
120 for use in coupling process
variable transmitter
24 to inputs of device
16. The process variable
transmitter
24 is connected in series with a DC source
122 and a
resistor
124 or other impedance to form a process control loop
126.
The values of source
122 and resistor
124 can be chosen as appropriate,
for example 24 volts and 5 ohms, respectively. In this configuration, the appropriate
voltage level will appear across resistor
124, for example ranging between
20 mVolts and 100 mVolts, as the current level through loop
126 is controlled
by transmitter
24. This voltage is applied to inputs of device
16
as discussed above. The power supply
122 provides the power for operation
of transmitter
24 and process control loop
126.
One problem with the circuitry
120 shown in FIG. 3 occurs when digital
communication is attempted with transmitter
24. For example, in accordance
with the HART® standard, a digital signal can be superimposed on the DC current
in the process control loop
126. The digital signal can be used to transmit
data to transmitter
24 or receive data from transmitter
24. However,
the elements in circuitry
126 do not provide a sufficiently large impedance
for typical digital communication devices used in the process control industry
to communicate with transmitter
24. For example, some digital communication
devices used with process control loops require a connection across an electrical
component having an impedance of between about 230 and about 600 ohms.
FIG. 4 is a schematic diagram of one embodiment of electrical circuitry
140
similar to the circuitry
120 shown in FIG. 3 which includes a series resistance
142 coupled in series between transmitter
24 and resistor
124.
This series resistance
142 can be of about 250 ohms and can be used to provide
sufficient voltage drop for digital communication with transmitter
24. For
example, resistor
142 can comprise a resistor of between about 230 ohms
and 600 ohms. For example, HART® communication requires between about 230
and 600 ohms of impedance. A HART® communication unit can be coupled across
resistor
142 or transmitter
24, for example, and used for digital
communication with transmitter
24. More specifically, a digital communicator
can be coupled across terminals
1 and
2 across terminals
2
and
3 as shown in FIG. 4. This configuration provides a device
146
for use in a process control system which is used to couple the transmitter
24
on two-wire process control loop
126 to an input channel of a process device
having multiple input channels. One example process device is process device
16
discussed above. More specifically, a first pair of electrical connections is configured
to couple to the two-wire process control loop
126 which includes the two-wire
process variable transmitter
24. The first pair of electrical connections
can comprise any two of the terminals
1,
2 and
3 shown in
FIG. 4. A second pair of terminals
148 provided by terminals
5 and
6 is configured to couple to an input of the process device
16. At
least one electrical component is connected in series between one of the connections
of the first or second pair of electrical connections and is configured for use
in digital communication with the two-wire process variable transmitter
24.
In the specific example shown in FIG. 4, the electrical component comprises a resistance,
such as resistance
142. However, any individual electrical component or
group of electrical components can be used. The component can be active or passive
and can have any electrical characteristic selected as desired.
FIG. 5 is a simplified diagram of electrical circuitry
160 which includes
a process device
162 in accordance with another example embodiment of the
present invention. Process device
162 shown in FIG. 5 is similar to process
device
146 shown in FIG. 4. Device
162 includes a switch
164
connected in parallel with resistor
142. When switch
164 is closed,
resistor
142 is electrically short circuited and is effectively removed
from the series connection with process control loop
126. Additionally,
electrical circuitry
160 shows connections
8 and
9 configured
to couple device
162 to process control loop
126. The circuitry
162
operates in a manner similar to that discussed above with regard to FIGS. 3 and
4 and allows the process transmitter
24 to be connected to an input of process
device
16. Further, the device
162 is configured to allow a digital
communicator
166 to be coupled to process control loop
126 for communication
with transmitter
24. The communication device
166 can comprise, for
example, a hand-held communicator such as a Rosemount 275 hand-held communicator.
Switch
164 can be selectively closed when device
162 is used in configurations
which do not require digital communication with transmitter
24. Alternatively,
switch
164 can be closed when device
162 is used in configuration
in which an alternative impedance is provided across other components which are
not shown in FIG. 5 in process control loop
26. For example, the impedance
provided by such other elements can be used for the digital communication with
transmitter
24. The particular configuration and components shown for device
162 can be configured as desired. The device
162 can be used for
connection of a transmitter
24 of the type which communicates through a
current base process control loop with a process device which requires a voltage
base input. In another example, an indicator such as an LED
170 provided
in device
162 and used to indicate that the power supply
122 is active
on loop
126. The power supply
122 can be an integral component of
process device
162 if desired.
The process device
162 can couple to any type of process variable transmitter
of the type used to sense a process variable. The device
162 allows for
digital communication with the process variable transmitter
24 such that
digital data can be received from transmitter
24 or sent to transmitter
24. This allows the transmitter
24 to be configured or otherwise
monitored using appropriate process devices such as a hand-held communicator.
Although the present invention has been described with reference to preferred
embodiments, 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.
Although the electrical component is illustrated as a resistor coupling between
the electrical connections, digital communications can also be provided by placing
the component on the other side (the negative side) of the loop. Similarly, it
is within the scope of the present invention to place the electrical component
in other configurations. Although a 24 volt power supply is illustrated, the actual
supply voltage can be selected as desired, for example, between about 10 volts
DC and about 50 volts DC.
*
