U.S. patent application number 15/651275 was filed with the patent office on 2018-07-26 for loop powered process control instrument with communication bypass circuit.
The applicant listed for this patent is Magnetrol International, Incorporated. Invention is credited to Kevin M. Haynes, Timothy S. Sussman.
Application Number | 20180212648 15/651275 |
Document ID | / |
Family ID | 62874456 |
Filed Date | 2018-07-26 |
United States Patent
Application |
20180212648 |
Kind Code |
A1 |
Haynes; Kevin M. ; et
al. |
July 26, 2018 |
LOOP POWERED PROCESS CONTROL INSTRUMENT WITH COMMUNICATION BYPASS
CIRCUIT
Abstract
A loop powered process instrument comprises a control system
including a control circuit, a modem circuit and a loop output
circuit. The control circuit measures a process variable and
develops a measurement signal representing the process variable and
includes a loop control circuit and a communication circuit. The
modem circuit is operatively connected to the communication circuit
and includes a modulation input port and a modulation output port.
The loop output circuit receives a measurement signal from the loop
control circuit and is connected to the modulation input port. A
two-wire circuit is for connection to a remote power source using a
two-wire process loop. A power supply with isolation is connected
to the two-wire circuit and the loop output circuit to isolate the
two-wire circuit from the control system. The power supply receives
power from the two-wire process loop and supplies power to the
control system and draws loop current on the two-wire process loop
in accordance with the measurement signal and provides the
modulation output on the loop current. A bypass circuit with
isolation is connected between the two-wire circuit and the modem
circuit modulation input port for providing input modulated signals
to the modem circuit bypassing the power supply.
Inventors: |
Haynes; Kevin M.; (Lombard,
IL) ; Sussman; Timothy S.; (Bolingbrook, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Magnetrol International, Incorporated |
Aurora |
IL |
US |
|
|
Family ID: |
62874456 |
Appl. No.: |
15/651275 |
Filed: |
July 17, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62449653 |
Jan 24, 2017 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 3/542 20130101;
H04B 3/56 20130101; G01F 23/284 20130101; H04B 1/40 20130101; H04B
3/548 20130101; G05F 1/46 20130101; H04L 27/04 20130101 |
International
Class: |
H04B 3/54 20060101
H04B003/54; H04B 1/40 20060101 H04B001/40; H04B 3/56 20060101
H04B003/56; H04L 27/04 20060101 H04L027/04 |
Claims
1. A loop powered process control instrument comprising: a control
system comprising a control circuit, a modem circuit and a loop
output circuit, the control circuit measuring a process variable
and developing a measurement signal representing the process
variable and including a loop control circuit and a communication
circuit, the modem circuit operatively connected to the
communication circuit and including a modulation input port and a
modulation output port, the loop output circuit receiving the
measurement signal from the loop control circuit and being
connected to the modulation output port; a two-wire circuit for
connection to a remote power source using a two-wire process loop;
a power supply with isolation, connected to the two-wire circuit
and the loop output circuit, to isolate the two-wire circuit from
the control system, the power supply receiving power from the
two-wire process loop and supplying power to the control system and
drawing loop current on the two-wire process loop in accordance
with the measurement signal and providing the modulation output on
the loop current; and a bypass circuit with isolation connected
between the two-wire circuit and the modem circuit modulation input
port for providing input modulated signals to the modem circuit
bypassing the power supply.
2. The loop powered process instrument of claim 1 wherein the
bypass circuit receives a communication input voltage modulated
signal from the two-wire circuit.
3. The loop powered process instrument of claim 1 wherein the
bypass circuit comprises series connected high voltage isolation
capacitors and maintains a high input impedance.
4. The loop powered process instrument of claim 1 wherein the power
supply comprises a voltage regulator receiving loop power and
developing a regulated output voltage and the bypass circuit
connects to the two-wire circuit before the voltage regulator.
5. The loop powered process instrument of claim 1 wherein the power
supply comprises a transformer.
6. The loop powered process instrument of claim 1 wherein the modem
circuit receives the input modulated signal and generates digital
signals to the control circuit.
7. The loop powered process instrument of claim 1 wherein the modem
circuit receives digital signals from the control circuit and
generates the output modulated signal to cause modulation on the
loop current.
8. The loop powered process instrument of claim 1 wherein the
control circuit comprises a microcontroller.
9. The loop powered process instrument of claim 1 wherein the modem
circuit comprises a modem with Highway Addressable Remote
Transducer (HART) capabilities.
10. The loop powered process instrument of claim 1 wherein the
modem circuit comprises a Fieldbus modem.
11. A two-wire transmitter comprising: a dual compartment housing
defining a wiring compartment and a control compartment; a control
system in the control compartment comprising a control circuit, a
modem circuit and a loop output circuit, the control circuit
measuring a process variable and developing a measurement signal
representing the process variable and including a loop control
circuit and a communication circuit, the modem circuit operatively
connected to the communication circuit and including a modulation
input port and a modulation output port, the loop output circuit
receiving the measurement signal from the loop control circuit and
being connected to the modulation output port; a two-wire circuit
and a power supply in the wiring compartment, the two-wire circuit
for connection to a remote power source using a two-wire process
loop and the power supply, with isolation, connected to the
two-wire circuit and the loop output circuit, to isolate the
two-wire circuit from the control system, the power supply
receiving power from the two-wire process loop and supplying power
to the control system and drawing loop current on the two-wire
process loop in accordance with the measurement signal and
providing a modulation output on the loop current; and a bypass
circuit with isolation in the wiring compartment connected between
the two-wire circuit and the modem circuit modulation input port
for providing input modulated signals to the modem circuit
bypassing the power supply.
12. The two-wire transmitter of claim 11 wherein the bypass circuit
receives a communication input voltage modulated signal from the
two-wire circuit.
13. The two-wire transmitter of claim 11 wherein the bypass circuit
comprises series connected high voltage isolation capacitors and
maintains a high input impedance.
14. The two-wire transmitter of claim 11 wherein the power supply
comprises a voltage regulator receiving loop power and developing a
regulated output voltage and the bypass circuit connects to the
two-wire circuit before the voltage regulator.
15. The two-wire transmitter of claim 11 wherein the modem circuit
receives the input modulated signal and generates digital signals
to the control circuit.
16. The two-wire transmitter of claim 11 wherein the modem circuit
receives digital signals from the control circuit and generates the
output modulated signal to cause modulation on the loop
current.
17. The two-wire transmitter of claim 16 wherein the modem circuit
comprises a modem with Highway Addressable Remote Transducer (HART)
capabilities.
18. The two-wire transmitter of claim 16 wherein the modem circuit
comprises a Fieldbus modem.
19. The two-wire transmitter of aim 11 wherein the control circuit
comprises a microcontroller.
20. The two-wire transmitter of claim 11 wherein the wiring
compartment comprises an explosion proof compartment and the
control compartment comprises an intrinsically safe compartment.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of provisional application
No. 62/449,653, filed Jan. 24, 2017.
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable.
MICROFICHE/COPYRIGHT REFERENCE
[0003] Not Applicable.
FIELD
[0004] This invention relates to process control instruments and,
more particularly, to a loop powered instrument with a
communication bypass circuit.
BACKGROUND
[0005] Process control systems require the accurate measurement of
process variables. Typically, a sensor in the form of a primary
element senses the value of a process variable and a transmitter
develops an output having a value that varies as a function of the
process variable. For example, a level transmitter includes a
primary element for sensing level and a circuit for developing an
electrical signal representing sensed level.
[0006] Knowledge of level in industrial process tanks or vessels
has long been required for safe and cost-effective operation of
plants. Many technologies exist for making level measurements.
These include buoyancy, capacitance, ultrasonic and microwave
radar, to name a few.
[0007] In one form, a through air measurement instrument, such as a
microwave radar level transmitter, launches a radar signal which
reflects off a liquid or other surface and the instrument measures
time of flight between transmission and reception of the radar
signal. Electrical energy is converted to an electromagnetic wave
from a launch element. The wave propagates through free space.
[0008] A two-wire transmitter includes two terminals connected to a
remote power supply. The transmitter loop current, drawn from the
power supply, is proportional to the process variable. A typical
instrument operates off of a 24 volt DC power supply and varies the
signal current in the loop between 4 and 20 milliamps (mA) DC.
Thus, the instrument must operate with current less than 4
milliamps.
[0009] While low power circuits are continuously developed, there
are other increasing demands placed on performance capabilities of
the process control instruments. For example, with a radar level
measurement device, the instrument's performance is enhanced by
more powerful digital signal processing techniques driven by a
microprocessor. In addition to the microprocessor, there are
several other circuits, such as the radar transceiver, which
requires electric power. To be successful, the instrument must use
optimum processing capability and speed. This means making maximum
power from the loop available to the electronics, and using it
efficiently.
[0010] More recently, the loop powered instruments have utilized
digital communications. Typical digital communications rely on
two-way communication signals. The communication into a typical
sensor device is by voltage level modulation. The communication out
of a typical sensor device is by modulation of the current draw of
the unit. In normal operation, the instrument must allow for 4 mA
to 20 mA loop current while still communicating digital signals via
modulation of the supply voltage and loop current. In addition, it
is necessary to maintain high input impedance for digital
communications.
[0011] In applications where a device must satisfy explosion proof
requirements, a galvanic isolation circuit may be provided.
However, such an isolation circuit can cause a problem with
modulation of the supply voltage. Digital communications require a
high input impedance into the level measuring instrument.
Unfortunately, with the galvanic isolation circuitry, the
communication input voltage modulated signals are not reliable once
they are received at the modem through the instrument's traditional
power line connections.
[0012] The present invention is directed to solving one or more of
the problems discussed above in a novel and simple manner.
SUMMARY
[0013] As described herein, a loop powered process control
instrument uses a bypass circuit for a digital communication input
signal which bypasses a power supply.
[0014] Broadly, there is disclosed a loop powered process control
instrument comprising a control system including a control circuit,
a modem circuit and a loop output circuit. The control circuit
measures a process variable and develops a measurement signal
representing the process variable and includes a loop control
circuit and a communication circuit. The modem circuit is
operatively connected to the communication circuit and includes a
modulation input port and a modulation output port. The loop output
circuit receives a measurement signal from the loop control circuit
and is connected to the modulation input port. A two-wire circuit
is for connection to a remote power source using a two-wire process
loop. A power supply with isolation is connected to the two-wire
circuit and the loop output circuit to isolate the two-wire circuit
from the control system. The power supply receives power from the
two-wire process loop and supplies power to the control system and
draws loop current on the two-wire process loop in accordance with
the measurement signal and provides the modulation output on the
loop current. A bypass circuit with isolation is connected between
the two-wire circuit and the modem circuit modulation input port
for providing input modulated signals to the modem circuit
bypassing the power supply.
[0015] It is a feature that the bypass circuit receives a
communication input voltage modulated signal from the two-wire
circuit.
[0016] It is another feature that the bypass circuit comprises a
series connected high voltage isolation capacitors and maintains a
high input impedance.
[0017] It is another feature that the power supply comprises a
voltage regulator receiving loop power and developing a regulated
output voltage. The bypass circuit is connected to the two-wire
circuit before the voltage regulator.
[0018] It is a further feature that the power supply comprises a
transformer.
[0019] It is another feature that the modem circuit receives the
input modulated signal and generates digital signals to the control
circuit.
[0020] It is yet another feature that the modem circuit receives
digital signals from the control circuit and generates the output
modulated signal to cause modulation on the loop current.
[0021] It is an additional feature that the control circuit
comprises a microcontroller.
[0022] It is still another feature that the modem circuit comprises
a modem with highway addressable remote transducer (HART)
capabilities.
[0023] It is still another feature that the modem circuit comprises
a Fieldbus modem.
[0024] There is disclosed in accordance with another aspect a
two-wire transmitter comprising a dual compartment housing defining
a wiring compartment and a control compartment. A control system in
the control compartment comprises a control circuit, a modem
circuit and a loop output circuit. The control circuit measures a
process variable and develops a measurement signal representing the
process variable and including a loop control circuit and a
communication circuit. The modem circuit is operatively connected
to the communication circuit and includes a modulation input port
and a modulation output port. The loop output circuit receives a
measurement signal from the loop control circuit and is connected
to the modulation output port. A two-wire circuit and a power
supply are in the wiring compartment. The two-wire circuit is for
connection to a remote power source using a two-wire process loop.
The power supply, with isolation, is connected to the two-wire
circuit and the loop output circuit to isolate the two-wire circuit
from the control system. The power supply receives power from the
two-wire process loop and supplies power to the control system and
draws loop current on the two-wire process loop in accordance with
the measurement signal and provides a modulation output on the loop
current. A bypass circuit with isolation is in the wiring
compartment connected between the two-wire circuit and the modem
circuit modulation input port for providing input modulated signals
to the modem circuit bypassing the power supply.
[0025] It is a feature that the wiring compartment comprises an
explosion proof compartment and the control compartment comprises
an intrinsically safe compartment.
[0026] Other features and advantages will be apparent from a review
of the entire specification, including the drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a side view of a loop powered process control
instrument including a bypass circuit in accordance with the
invention;
[0028] FIG. 2 is a side view, similar to FIG. 1, with a dual
compartment control housing separate from a primary element;
[0029] FIG. 3 is a side section view of the dual compartment
control housing;
[0030] FIG. 4 is a block diagram illustrating the relationship
between circuit boards in the dual compartment control housing of
FIG. 3; and
[0031] FIG. 5 is a block diagram of the circuitry of the loop
powered process control instrument.
DETAILED DESCRIPTION
[0032] Referring to FIGS. 1 and 2, a loop powered process control
instrument 10, also referred to as a two-wire transmitter,
according to the invention is illustrated. The process control
instrument 10 uses micro power impulse radar (MIR) in conjunction
with equivalent time sampling (ETS) and ultra-wideband (UWB)
transceivers for measuring a level using time domain reflectometry
(TDR). Particularly, the instrument 10 uses through air radar for
sensing level. While the embodiments described herein relate to an
MIR level sensing apparatus, various aspects of the invention may
be used with other types of process control instruments for
measuring various process parameters, as will be apparent to those
skilled in the art.
[0033] The process control instrument 10 includes a control housing
12 and a sensor or primary element 14. In the illustrated
embodiment, the primary element 14 is an antenna.
[0034] The antenna 14 includes a process adapter 16 for connection
to the housing 12. The process adapter 16 is mounted to a process
vessel V, see FIG. 1, using a flange 18. The process adapter 16 may
be threaded or welded to the flange 18. Alternatively, the process
adapter 16 may be threaded directly into an opening in the process
vessel V.
[0035] The instrument 10 uses pulse-burst radar technology with ETS
circuitry. Short bursts of microwave energy are emitted and
subsequently reflected from a surface. The distance is calculated
by the equation.
D=(velocity of EM propagation)*transit time (round trip)/2.
[0036] Level is then calculated by applying a tank height value.
ETS is used to measure the high speed, low power electromagnetic
(EM) energy. The high-speed EM energy (1,000 ft/.mu.s) is difficult
to measure over short distances and at the resolution required in
the process control industry. ETS captures the EM signals in real
time (nanoseconds) and reconstructs them in equivalent time
(milliseconds), which is much easier to measure. ETS is
accomplished by scanning the vessel to collect thousands of
samples. The round-trip event on a 65 ft. tank takes only 133
nanoseconds in real time. After it is reconstructed in equivalent
time it measures 200 milliseconds.
[0037] The through air radar level measurement instrument 10
launches a radar signal which reflects off a liquid or other
surface and measures time of flight between transmission and
reception of the radar signal. Electrical energy is converted to an
electromagnetic wave from the launching element which propagates
through free space. The system operates a signal around 26 GHz.
[0038] Referring to FIG. 3, the control housing 12 comprises a dual
compartment housing including a base 22 defining an explosion proof
wiring compartment 24 and an intrinsically safe control compartment
26 connected via a passage 28. A first cover 30 encloses the wiring
compartment 24. A second cover 32 encloses the control compartment
26. The wiring compartment 24 houses a wiring board 34 and a
galvanic transformer board 35 for connecting to a remote power
source and including necessary interface circuitry. This circuitry
is in communication with a digital PC board 36 and an analog PC
board 38 in the control compartment 26. The digital PC board 36
includes a microprocessor for controlling functionality of the
overall instrument. The analog PC board 38 includes signal
processing circuitry which drives a radio frequency (RF) module 40
and further processes the return signal from the RF module 40. The
RF module 40 is in communication with the antenna 14, as described
below. A display/keypad PC board 42 is connected to the digital PC
board 36 and is viewable through and accessible upon removal of the
second cover 32.
[0039] The form of the housing 12 and the circuits therein are
illustrated and described by way of example only. The invention is
particularly directed to a communication bypass around a galvanic
isolation power supply, as described below.
[0040] The RF module 40 has a printed circuit board 44 with a
conventional launching element. In the illustrated embodiment, the
launching element comprises electro-magnetic radiating elements
which are conductive traces designed on the circuit board 44. The
launching element generates and receives a high frequency signal
for measuring level.
[0041] An air-filled antenna waveguide 46 is sealingly mounted to
the control housing 12 and aligned with the launching element on
the printed circuit board 44. Thus, the launching element works
together with the waveguide 46 and a waveguide cap 47 to generate
the launching signal to the antenna 14, as is known. The air-filled
waveguide 46 is adapted to operate in the K band.
[0042] The antenna waveguide 46 is surrounded by a quick connect
coupler 48 for mating with a corresponding quick connect coupler 49
on the antenna 14, see FIG. 2. This provides a quick
connect/disconnect coupling that allows the vessel V to remain
sealed upon removal of the control housing 12.
[0043] While this application describes the bypass circuit and
galvanic isolation in connection with a through air radar level
transmitter, this circuitry can be used with process control
instruments for measuring other parameters and using other
technologies including, for example, guided wave radar,
capacitance, or the like.
[0044] Referring also to FIG. 4, the display/keypad PC board 42
provides a user interface for entering parameters with a keypad and
displaying user and status information. The digital PC board 36
includes a conventional microcontroller and memory. The memory may
comprise both non-volatile memory for storing programs and
calibration parameters, as well as volatile memory used during
level measurements. The digital PC board is also connected through
the galvanic transformer board 35 to the wiring board 34 for
connecting to a remote and external power source over a two-wire
loop. The two-wire connection is used to communicate level
information, as is well known.
[0045] As described more particularly below, the circuits in the
wiring compartment 24 accept supply voltage at input terminals TB1
from the customer and provide power to the balance of the unit 10
through a galvanically isolated barrier. The galvanic isolation is
important because it allows the unit to operate as explosion-proof
in the wiring compartment 24 and intrinsically safe (IS) in the
control compartment 26, while not requiring a special IS ground
wire. Digital communication signals, such as, for example, HART,
Fieldbus or Profibus, or other, must pass cleanly through the
circuits.
[0046] Referring to FIG. 5, a block diagram illustrates the
circuitry in the wiring compartment 24 which includes a two-wire
circuit 50 and a power supply 52. The two-wire circuit 50 is for
connection to a remote power source using a two-wire process loop,
as is known, for controlling current on the loop in accordance with
a measurement signal from a control system 54 comprising the
circuitry in the intrinsically safe control compartment 26. As will
be apparent, only a portion of the circuitry of the control system
54 is illustrated herein. The power supply 52 has a galvanic
isolation barrier and is connected between the two-wire circuit 50
and the control system 54 to isolate the two-wire circuit 50 from
the control system 54. The power supply 52 receives power from the
two-wire process loop and supplies power to the control system
54.
[0047] The two-wire circuit 50 comprises a two-wire input block 56
and an input filter circuits block 58. The two-wire input block 56
provides the customer input to the unit at the terminal block TB1,
see FIG. 4. This is the user connection to the instrument 10. The
user must provide suitable power and the unit will draw loop
current based on the level in the process as measured by the
control system 54. Typical of most two-wire instruments, this unit
will draw 4 mA to 20 mA based on the measured level in the process.
The digital communication into the two-wire input block 56 may be
by voltage level modulation. The digital communication out is by
modulation of the current draw of the unit.
[0048] The input filter circuits block 58 includes standard filter
circuits that suppress noise from entering deeper into the unit 10
where it could cause damage to the unit 10 or corrupt normal
operation.
[0049] The power supply 52 is on the galvanic transformer board 35,
see FIG. 4. The power supply 52 comprises a linear voltage
regulator 60. In the illustrated embodiment, the voltage is set by
voltage reference Zener diodes and can be shifted by the control
system 54 using a voltage shift control 62 using an optical
coupler. The voltage regulator 60 makes the input impedance appear
high to the digital communication signals. The voltage of the
voltage regulator 60 must be lower than the terminal voltage to the
transformer board 35. However, the voltage must be high enough to
supply sufficient power for the unit 10 to operate properly. The
voltage regulator 60 also helps to eliminate noise at the terminals
TB1 caused by the circuits deeper in the unit 10. The voltage shift
of the regulator voltage is important to maintain high input
impedance over the range of loop current that must pass the
circuit.
[0050] The voltage regulator 60 supplies regulated voltage to EMI
filter circuits 64 which filter against electromagnetic
interference. A switcher circuit block 66 is connected between the
EMI filter circuit 64 and a galvanic isolation block 68 which
includes a DC-DC transformer 70. The switcher circuit 66 is the
switching oscillator of a DC-DC converter circuit. The oscillator
drives the primary of the DC-DC transformer 70. The oscillator is
free running so that whenever power is supplied to the board, the
switcher is oscillating. The frequency of this switching oscillator
must be sufficiently high, such as about 150 kHz, to allow the
lower frequency communication signals to be passed cleanly through
the circuit. Also, the goal of the overall circuit is to have the
secondary current of the transformer 70 to be closely matched by
the primary current. The close match of the current allows the loop
control to be performed at the secondary of the transformer 70 and
yet be tightly coupled to the primary and thus to the user
terminals TB1. The current transfer is the critical parameter that
must be maintained by these circuits. The current loop control, 4
mA to 20 mA, takes place in the secondary circuits via the control
system 54.
[0051] The galvanic isolation block 68 uses the DC-DC transformer
70 as the primary component. To assure proper isolation, this
transformer 70 must meet several specific IS safety requirements.
The transformer must meet high isolation voltage requirements and
assure proper creepage and clearance spacing requirements. The
galvanic isolation circuit 68 must be capable of passing the
current modulated signal, without distortion, to the user terminals
TB1.
[0052] The secondary of the galvanic isolation block 68 is
connected to a rectifier circuit 72 to provide a DC voltage. The
result is a DC supply voltage which is loosely controlled by the
voltage regulator 60 but tightly passes the loop current. An output
filter circuit block 74 receives the rectified DC voltage and
includes a low pass filter to suppress the switcher edges. The
resulting DC voltage must not have switching frequency noise which
could disrupt operation of the unit 10. A safety limiting circuit
76 limits the level of the supply DC voltage to the control system
54.
[0053] The power supply 52 also includes a communication bypass
path block 78 which allows digital communication signals, such as
HART, Fieldbus, Profibus, or the like, to bypass the galvanic
isolation circuit 68.
[0054] The control system 54 comprises circuits in the
intrinsically safe control compartment 26 that includes a block 80,
referred to below as a microcontroller block, that provides
connections to microcontroller circuits, a communication modem
circuits block 82 and a loop output block 84. The microcontroller
block 80 includes conventional circuitry for low voltage power
supply loop control circuits, communication circuits, programmed
logic circuits, user interface and measurement circuits. These
circuits measure the process variable and develop a measurement
signal representing the process variable. This measurement signal
is output from a loop control circuit and is labeled LOOP which
determines level of loop current to be drawn by the unit 10. There
is also communication circuit labeled COMMUNICATION for controlling
digital communications over the two-wire process loop.
[0055] The modem circuits block 82 controls the digital
communications. In the illustrated embodiment, this may comprise a
DS8500 modem circuit used for conventional Highway Addressable
Remote Transducer (HART) communications. However, the invention is
not limited to use with HART communications and may be used with
other forms of digital communications including, for example,
Fieldbus, Profibus, or the like.
[0056] HART communication, as is typical of other digital
communication systems, relies on the two-way communication signals.
The communication into a typical sensor device is by voltage level
modulation. The communication out of a typical sensor device is by
modulation of the current draw of the unit.
[0057] From a HART physical layer perspective when using HART
communications, the power supply galvanic isolation causes a
problem with the HART communication input signal which is a voltage
modulation signal that is sent to the modem circuits block 82 on
the digital board 36 through the two-wire 4/20 mA power lines. HART
communication requires a high input impedance into the level
measuring instrument. With the galvanic isolated circuitry, the
communication input voltage modulated signals are not reliable once
they are received at the HART modem through the instrument's
traditional power line connections.
[0058] As described herein, a bypass circuit path is provided for
the communication input voltage modulated signal to the modem
circuits block 82. This is shown as the communication bypass path
block 78 in FIG. 5. This circuit path, or bypass, makes a
connection in front of the voltage regulator 60 on the wiring board
34. This connection is then isolated from the power supply 52 using
two series connected high voltage isolation capacitors (not shown)
and maintains a high impedance to the power lines which is needed
for the HART physical layer specification. After the capacitors,
the separate circuit path connects to the INPUT VOLTAGE MODULATION
input of the communication modem circuits block 82. The HART
voltage modulated input signals are de-modulated and generate the
digital signals RX-DATA and CARRIER_DETECT which are passed to the
microcontroller block 80 for processing, as is known.
[0059] The microcontroller block 80 controls the loop current DC
level, 4 mA to 20 mA, as an indication of the process level that
the instrument monitors, and responds to the received communication
signals and in turn controls the digital transmit signal, TX_DATA,
and the LOOP signal from the loop control circuit. The modem block
82 modulates the TX_DATA signal to generate the OUTPUT MODULATION
CONTROL output to the loop output block 84 which causes the
modulation on the loop output. The loop output block 84 also
receives the LOOP output from the microcontroller block 80 and uses
this to control the loop current drawn from the safety limiting
circuit 76 while including current modulation responsive to the
OUTPUT MODULATION CONTROL from the modem block 82. Thus, the
returning current modulated signal from the modem block 82 moves
through the galvanic transformer 70 on the wiring board 34, then
through the voltage regulator 60, and finally out of the device on
the two-wire 4/20 mA power lines.
[0060] It will be appreciated by those skilled in the art that
there are many possible modifications to be made to the specific
forms of the features and components of the disclosed embodiments
while keeping within the spirit of the concepts disclosed herein.
Accordingly, no limitations to the specific forms of the
embodiments disclosed herein should be read into the claims unless
expressly recited in the claims. Although a few embodiments have
been described in detail above, other modifications are possible.
Other embodiments may be within the scope of the following
claims.
[0061] The foregoing disclosure of specific embodiments is intended
to be illustrative of the broad concepts comprehended by the
invention.
* * * * *