BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to a digital converter apparatus for improving the
output of a two-wire transmitter sensing a process variable.
[0002] US-A-4250490 (Dahlke) discloses a two-wire current transmitter which provides a current
signal representative of a parameter measured by a variable reactance sensor. The
sensor may be located in a hostile environment remote from the signal conditioning
and transmitter electronics. Circuitry provides power to the sensor which provides
the signal conditioning and transmitter electronics with a signal representing the
parameter measured. The signal controls the current control to modify the total current
through the same two wires to carry power to the signal conditioning and transmitting
electronics so that the total current is representative of the measured parameter.
"Span" and "zero" temperature compensation networks are provided and include potentiometers
which may be set to provide the required compensation.
[0003] US-A-4520488 (Houvig et al) discloses a communication system for communicating analog
and digital data with a process variable transmitter, eg. a pressure transmitter,
over the two wires which supply power to the transmitter apparatus. Normally, the
process variable being monitored by the transmitter produces an analog signal by means
of direct current in the communication loop in a predetermined range, eg. 4-20mA,
representing process variable analog values. The digital data communication is accomplished
by forcing the loop current to change rapidly between the preset limits, eg. 4mA and
20mA. This change of the loop current carries the serial digital bit information.
The system is therefore capable of selective alternate digital and analog communications.
SUMMARY OF THE INVENTION
[0004] According to a first aspect of the present invention there is provided a method for
adding a capability to a parameter value transmitter for receiving input information
where such a transmitter initially has, through a set of transmitting circuits therein,
a capability for transmitting, along a two-wire current carrying loop adapted for
electrical connection to first and second terminals of the transmitter, output information
formed by those values measured by a sensing means in the transmitter of a parameter
the values of which depend on conditions in a structure to which the transmitter is
affixed, the method comprising:
disconnecting and removing at least a portion of the set of transmitting circuits
which are initially electrically connected between the sensing means and the first
and second terminals in the transmitter affixed to the structure; and
providing in the transmitter affixed to the structure a set of transmitting and
receiving circuits electrically connected between the sensing means and the first
and second terminals, the set of transmitting and receiving circuits being capable
of enabling the then-modified transmitter simultaneously to transmit the output information
and to receive the input information, the input information being used for correcting
those values measured by the sensing means.
[0005] An existing analog two-wire transmitter comprises a sensor module means coupled to
a process variable for sensing and for providing a sensor output as a function of
the process variable. The existing transmitter further comprises excitation means
coupled to the sensor module means for providing excitation thereto. The existing
transmitter further comprises analog detector means for providing analog conversion
of the sensor signal to a two-wire transmitter output representative of the second
process variable. The existing transmitter is modified such that the transmitter's
output is improved. The analog detector means are removed from the transmitter and
replacement apparatus which digitally calculates the transmitter's output are disposed
in the transmitter. The replacement apparatus receive the sensor output and provide
linearity of other correction to the output. In a preferred embodiment, the existing
excitation means are removed and the apparatus comprise replacement excitation means
which are disposed in the transmitter and coupled to the sensor module means for providing
excitation thereto. In a further preferred embodiment the sensor module means comprises
at least one capacitive sensor for sensing the process variable, rectification means
coupled to the sensor output for providing rectification thereto, and analog correction
means for providing analog corrections to the sensor.
[0006] In yet a further preferred embodiment, the apparatus comprises a microprocessor calculating
output correction, spain, and zero adjustments.
[0007] The existing transmitter can thus have its output improved while the transmitter
remains in situ and coupled to the process variable and the loop. A transmitter with
a digitally corrected output is thus provided without replacement of the existing
sensor module or decoupling of the transmitter from process lines or the two-wire
loop.
[0008] According to a second aspect of the present invention there is provided a parameter
value transmitter for transmitting, along a two-wire current carrying loop adapted
for electrical connection to first and second terminals of the transmitter, output
information formed by those values measured by a sensing means in the transmitter
of a parameter the values of which depend on conditions in a structure to which the
transmitter is affixed and for receiving input information, the transmitter including
a set of transmitting and receiving circuits electrically connected between the sensing
means and the first and second terminals, the set of transmitting and receiving circuits
being capable of enabling the transmitter simultaneously to transmit the output information
and to receive the input information, the input information being used for correcting
those values measured by the sensing means.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009]
FIG. 1 is a drawing of a PRIOR ART analog transmitter showing a sectional view of
an upper housing and a lower housing with parts broken away;
FIG. 2 is a drawing of a transmitter according to this invention showing a sectional
view of an upper housing and a lower housing with parts broken away;
FIG. 3 is a block diagram of a first preferred embodiment of a transmitter according
to this invention;
FIG. 4 is a block diagram of a second preferred embodiment of a transmitter according
to this invention; and
FIGS. 5A, 5B and 5C together provide a schematic diagram of a transmitter according
to this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0010] In FIG. 1, a PRIOR ART process variable transmitter 10 is shown bolted to flange
adapter unions 12 which couple fluids to the transmitter 10. Transmitter 10 senses
pressure of the fluids at flange adapter unions 12 and provides an output current
representative of the sensed pressure to a two-wire loop 14. The transmitter 10 is
energized by an external power supply 14A coupled to the two-wire loop and the output
current such as a 4-20 milliampere signal is provided to an external load 14B which
is also coupled to the two-wire loop 14. The transmitter 10 comprises a housing 16
which has three internal compartments 18, 22 and 24 which are sealed from one another.
The transmitter 10 further comprises a capacitive pressure sensor 26 disposed in the
compartment 18 for sensing a process variable such as differential, gauge or absolute
pressure. Sensor 26 is electrically coupled via wires to a circuit board assembly
28 in compartment 18 which comprises diodes or rectifiers 30 for rectifying the sensor's
output and analog correction components 32 for correcting the sensor's output. The
corrections include temperature compensation. A cable 34 passes through a seal 36
between compartments 18 and 22 and electrically couples the circuit board 28 to a
connector board 38 in compartment 22. The connector board 38 comprises additional
analog correction or compensation circuitry 40 which provides temperature correction
of characteristics of the sensor 26. Connector board 38 mates with a multipin connector
42 providing connection to further transmitter circuitry.
[0011] In operation, the sensor 26, the circuit board assembly 28, the cable 34, and the
connector board 38 together comprise a sensor module 35 in the transmitter 10 which
senses the process variable and provides a sensor output to connector 42 which includes
analog correction for temperature.
[0012] A terminal strip 44 in sealed compartment 24 provides connection to the two-wire
loop 14 in a conduit 14C and has sealed electrical feedthroughs 46 coupling the two-wire
circuit from compartment 24 to the compartment 22. Compartment 22 in the housing 16
is configured to accept an analog converter and excitation circuit assembly 23 which
utilizes analog circuitry to excite the sensor and convert the sensor signal to a
4-20 milliampere output. Assembly 23 comprises a printed circuit board 23A comprising
analog converter and excitation electronics coupled to connector 42 and printed circuit
board 23B comprising span and zero adjustment circuitry coupled to board 23A. A pair
of sealed adjustment screws 50 extend from the interior of compartment 22 to the exterior
of the transmitter housing 16. The adjustment screws 50 provide adjustment of span
and zero potentiometers 23C, 23D on the circuit board 23B. The analog converter provides
a reliable, low cost means of providing the output, and the setting of span and zero
is accomplished using potentiometers 23C, 23D. The mechanical adjustment of the potentiometers
23C, 23D may be subject to mechanical vibration which may alter the settings of span
and zero. The potentiometers 23C, 23D can adjust the span and zero settings of the
output to the adjustment and resolution capabilities of the potentiometers. Linearity
of the 4-20 milliampere output of transmitter 10 as a function of the sensed pressure
is improved by the analog correction circuitry of transmitter 10. While adjustments
are provided in the analog converter, excitation or sensor module for such nonlinearity,
the linearity achieved can be further improved by use of a digital circuit.
[0013] The investment in assembling and installing the transmitter 10 in a process plant
is considerable, and the entire cost of such an installation of transmitter 10 would
be lost if transmitter 10 were removed and replaced with a higher accuracy transmitter
such as one using digital circuitry. When transmitter 10 is installed in a process
plant, such as a chemical, petroleum or pulp and paper plant, complete replacement
of transmitter 10 is a costly and time consuming process. If separate shut-off valves
have not been provided in the pressure lines to flange adapter unions 12, at least
a portion of the plant may need to be shut down to depressurize lines so that process
fluid does not spill out from the flange adapter unions 12 when they are unbolted
from the transmitter. If shut off valves are provided, they may need to be shut off
while the transmitter is being replaced. Flange adapter unions 12 must be unbolted
from the transmitter 10 and rebolted to a replacement transmitter. Seals between the
flange adapter unions 12 must be inspected because there is a possibility of leaks
when the flange adapter unions 12 are bolted to a replacement transmitter. After replacement
of transmitter 10, a replacement transmitter frequently must have its pressure lines
bled to remove air which has entered the line during replacement. Complete replacement
of transmitter 10 requires disconnection of loop 14 from terminals 44 in the transmitter
10 and disconnection of conduit 14C from the transmitter 10. A replacement transmitter
must next be recoupled to the loop 14 and conduit 14A. In some cases, it is not practical
to shut down a portion of the process plant to replace a transmitter, and replacement
of transmitter 10 is therefore delayed until a scheduled shutdown for maintenance.
The process pipes coupling to flange adapter union 12 and the conduit 14C may be weakened
by age, corrosion, or vibration; handling these lines during a complete replacement
may damage some of these parts. If the transmitter is not completely replaced, but
is instead upgraded with a digital converter replacing analog electronics in the transmitter
10, the cost and time of complete replacement can be avoided. When a digital replacement
converter is used, it is possible to avoid disturbing the process itself, the pressure
lines leading to the transmitter, the flange adapter union 12, the loop wiring 14,
the terminal strip 44 and the conduit 14C. A digital converter, moreover, can comprise
span and zero adjustment that is electrical; the use of potentiometers which may be
sensitive to vibration is thus avoided. Electrical adjustments of span and zero can
be accomplished with a higher resolution than the resolution of potentiometers, and
can provide more accurate span and zero settings. A digital converter can further
comprise digital linearity corrections that provide for a transmitter output which
can have better linearity over a wider range. The major portions of transmitter 10
such as the housing, sensor module, including temperature compensation components,
terminal strips and connections to the loop are adequate for use with a digital circuitry
and the transmitter's performance can be improved with a high accuracy system. Accordingly,
the analog converter 23 can be removed from transmitter 10 and the transmitter 10
can be improved to a desired level by installation of apparatus comprising a digital
converter. Upgrading the transmitter with a digital converter avoids wasting the labor
and materials invested in the original materials, assembly labor, analog compensation
and installation in a process line. Span, zero and other adjustments are made to the
digital converter without the use of potentiometers, and high stability and setability
are thus achieved.
[0014] In FIG. 2, an exemplary transmitter 11 is shown which comprises such a digital converter.
In FIG. 2, reference numerals which are the same as those in FIG. 1 identify corresponding
features. In FIG. 2, a digital converter 52 is installed in transmitter 11, the assembly
23 having been previously removed from transmitter 11. The transmitter 11 thus has
an output to the loop 14 which has an improved accuracy for interfacing with a control
system via the loop. Converter 52 provides a second compensation or correction in
addition to the analog correction done in the sensor module such that the transmitter
output is improved to a desired level while avoiding the time, cost and inconvenience
of replacing the entire transmitter.
[0015] The apparatus 52 is installed in the chamber 22 of the transmitter 11 and is thus
sealed in the transmitter. The circuitry of apparatus 52 can be configured to control
energy storage such that the intrinsic safety features of the transmitter are preserved.
[0016] FIG. 3 is a block diagram of a first embodiment of a transmitter 500 made according
to this invention. The transmitter 500 couples to a process variable along a line
514. The process variable on line 514 can comprise absolute, gauge or differential
pressure, temperature, pH, flow, conductivity or the like. The transmitter 500 senses
the process variable on line 514 and provides an output as a function of the process
variable. The transmitter 500 further comprises output terminals 502, 504 coupled
to a two-wire loop 506 along lines 503 and 505 respectively. An energization source
508 couples in series with loop 506 between lines 503 and 507 and provides energization
to transmitter 500. Transmitter 500 comprises a current control 536 coupled along
line 520 to output terminal 502 and coupled along line 540 through a resistance 542
to output terminal 504. Current control 536 controls current I in loop 506 as a function
of the sensed process variable and hence current I is a transmitter output. Current
I is preferably a low frequency 4-20 milliampere current which is linearly proportional
to the sensed process variable. Current control 536 is preferably also coupled along
line 544 to output terminal 504 for sensing a potential developed across resistance
542. The potential thus developed is representative of loop current I. Current control
536 can thus monitor loop current I and provide closed loop control of loop current
I. A resistance 510 is coupled between lines 505 and 507 in loop 506. The loop current
I flows through resistance 510. A utilization device 512 coupled to resistance 510
uses a potential developed across resistance 510. Utilization device 512 can comprise
a control computer, loop controller, chart recorder, meter or other indicating, recording
or control apparatus.
[0017] The current control 536 can also generate a first communication output. The first
communication output is preferably a high frequency, frequency-shift-keyed (FKS),
serial signal. The keying or modulation frequency of the first communication output
is preferably selected to be spaced from the low frequency of loop current I such
that the first communication output can be superimposed on the loop current I without
substantially interfering with the operation of utilization device 512. The first
communication output comprises data representative of transmitter operation or installation
parameters such as span and zero settings, serial number of the transmitter, identification
of the process variable sensed, current magnitudes of the process variable and the
like. The first communication output is coupled from current control 536 along lines
520, 540 to output terminals 502, 504 respectively. The first communication output
is coupled from output terminals 502, 504 to lines 503, 505 respectively of loop 506.
A communications means 516 is coupled along lines 546, 548 to lines 503, 505 respectively.
Communication means 516 receive the first communication output from current control
536 along lines 520, 503, 546, 505 and 540. Communication means 516 thus receive the
data comprised in the first communication output and provides such data to a user
at a location which can be remote from the transmitter. Communication means 516 are
preferably capacitively coupled to the loop 506 so that the low frequency loop current
I does not flow through communication means 516. While the embodiment described in
connection with FIG. 3 sends and receives communication signals over the loop, it
will be understood by those skilled in the art that such communication signals can
be alternately coupled to the transmitter over a line or bus which is separate from
the loop.
[0018] Transmitter 10 further comprises a regulator 518 coupled to line 520 for receiving
a portion of loop current I and for energizing further transmitter circuitry with
controlled levels of energization. Regulator 518 couples energization along line 522
to excitation means 526 and couples energization along a line 524 to calculating means
532. The portion of loop current coupled to regulator 518 is returned to the loop
along line 550 coupled between the regulator and line 540, and along line 552 coupled
between the calculating means 532 and line 540.
[0019] The excitation means 526 generate an excitation output which is coupled along line
527 to a sensor module 528. The sensor module is coupled along line 514 to the process
variable for sensing the process variable. The excitation output on line 527 excites
the sensor module 528 and the sensor module 528 couples a sensor output along line
530 which is a function of the sensed process variable. The sensor module 528 further
comprises an analog circuit 529 providing a correction to the sensor output on line
530. The correction provided by analog circuit 529 corrects for a response of the
sensor output which deviates from a desired response of the sensor output to the sensed
parameter. The correction provided by the analog circuit 529 can comprise a correction
to the linearity of the sensor output as a function of the process variable; a temperature
correction of a sensor output representing pressure, flow, conductivity; cold junction
compensation for a thermocouple, or the like. In a preferred embodiment, sensor module
528 further comprises rectification means for rectifying the sensor output on line
530.
[0020] The sensor output on line 530 is coupled to calculating means 532. Calculating means
532 calculate a calculated output as a function of the sensor output. The calculated
output is representative of a desired output such as the amplitude of current I in
loop 506 and is a function of the sensed parameter. A constant 533 is stored in the
calculating means. Constant 533 is representative of a digital correction to the transmitter
output which improves the transmitter output beyond the correction provided by analog
circuit 529. Constant 533 can comprise a linearity correction, a span correction,
a zero correction or other correction which improves a characteristic of the transmitter's
output. In a preferred embodiment, constant 533 comprises multiple corrections of
linearity, span and zero settings. The calculated output is coupled along line 534
to current control 536. In a preferred embodiment, current control 536 compares the
calculated output on line 534 to the sensed or actual current I sensed at line 544
and controls current on line 520 so that the actual current I is substantially equal
to the calculated current I as represented by the calculated output on line 534. The
transmitter's output is thus improved by both an analog and a digital correction.
The current received by utilization device 512 is a better representation of the sensed
parameter because a digital correction has been made in transmitter 500.
[0021] In a preferred embodiment, the calculating means 532 also generates an output representative
of the first communication output which is coupled along line 534 (along with the
calculated output) to the current control 536. The current control 536 thus superimposes
a current which is the first communication output on the loop current.
[0022] In a further preferred embodiment, the communications means 516 receives data representative
of correction constants from a user. The communication means 516 couples a second
communications output comprising correction constants on lines 546, 548 to lines 503,
505 respectively. The second communication signal is coupled along lines 503, 505
to output terminals 502, 504 respectively. In the transmitter 500, the second communication
output is coupled from terminals 502, 504 through resistance 542 and along lines 522
and 520 to calculating means 532. Calculating means 532 receives the second communication
signal and stores data contained therein as constant 533. Transmitter 500 can thus
be provided with correction constant 533 from a remote location and it is not necessary
to locate or open transmitter 500 to adjust the correction constants 533. The transmitter
500 in FIG. 3 utilizes the existing sensor module 528 in a transmitter and the replacement
converter comprises calculating means 532, current control 536, regulator 518 and
resistor 542. Replacement excitation means 526 can also be provided.
[0023] In FIG. 4, a block diagram of a second preferred embodiment of the circuitry in transmitter
10 is shown coupled to a two-wire, 4-20 milliampere loop 14. The transmitter 10 is
coupled to the loop at terminals 60, 62 in transmitter 10. An energization source
64 such as a battery or power supply is coupled along line 15 in series with a loop
load represented by resistance 66. The loop load can comprise a control computer,
a chart recorder, or current meter for example. A loop current flows from source 64
along line 64A into the transmitter at terminal 60 and out of the transmitter at terminal
62 along line 62A to resistance 66A, thus energizing transmitter 10 from the loop.
A diode 59 in transmitter 10 provides reverse polarity protection to the transmitter
10. The amplitude of the low frequency loop current is controlled by current control
66 coupled to terminals 60, 62 such that the amplitude of the loop current is a function
of the process variable sensed by the transmitter. A first regulator 68 is coupled
to terminal 60 and provides a first regulated potential on line 70 in the transmitter
10. A second regulator 72 is coupled to line 70 and provides a second regulated potential
to a line 74. Current flowing through the transmitter is returned to circuit common
conductor 76 in the transmitter 10 and the common conductor is coupled to terminal
62 through a resistor 78. The potential developed across resistor 78 is representative
of the actual loop current and this potential is coupled along line 80 back to a digital-to-analog
converter (DAC) 82 to provide closed loop control of the transmitter output current.
An excitation means 84 is energized from line 70, 74 and provides excitation along
a line 86 to a sensor module 88. The sensor module 88 can comprise a capacitive pressure
sensor, analog linearity and temperature compensating components, and rectification
circuitry.
[0024] The sensor module 88 couples a sensor output as a function of the sensed parameter
on line 90 to an integrator 92. Temperature compensation using analog techniques are
performed in the sensor module 88. An interface circuit 94 is coupled to the integrator
92 along lines 91, 93 and interfaces the integrator circuit 92 to an integrator timer
96 and a microcomputer 98. The integrator 92 is energized from lines 70 and 76 and
operates at higher potentials than timer 96 and microcomputer 98 which are energized
from lines 74 and 76. Because of the difference in potential, the interface circuit
provides level shifting to ensure compatible signal levels. The integrator 92, the
interface circuit 94, and the integrator timer 96 operate in conjunction with the
microcomputer 98 to form a dual slope type A-to-D converter 99. The dual slope type
converter 99 performs an analog-to-digital conversion of the corrected analog sensor
output from sensor module 88. The dual slope converter 99 thus presents a digital
signal to microcomputer 98 which is representative of the sensor output corrected
for temperature. The microcomputer 98 is preferably a single chip microcomputer having
microprocessor, program memory and random access memory all on one integrated circuit
to provide preferred low power consumption and small size. In another embodiment,
microcomputer 98 can alternatively comprise separate microprocessor, program ROM and
RAM if space and power specifications are compatible with the design. In one preferred
embodiment, a "watchdog" timer 102 is coupled to the microcomputer 98 and senses when
the microcomputer 98 fails to perform a selected task in a time limit set by the watchdog
timer 102. Failure to perform the task in the time limit is an indication of malfunction
of the microcomputer 98, and the watchdog timer resets the microcomputer when such
failures occur. A non-volatile memory 104 coupled to microcomputer 98 has been loaded
with constants which are representative of digital linearity corrections for the transmitter.
The improved transmitter thus can provide digital corrections to the transmitter output
in addition to the analog corrections which were made in the sensor module 88 when
the transmitter was originally manufactured. The microcomputer 98 calculates a transmitter
output based on the digital correction words stored in memory 104 and the calculated
output is improved in accuracy over the accuracy of the original analog transmitter
output. The calculated transmitter output is coupled along line 106 to the digital-to-analog
(DAC) circuit 82. The DAC 82 compares the calculated output to the signal on line
80 which is representative of the actual loop current. The DAC 82 couples a signal
along line 108 to the current control 66 so that the current in the loop is equal
to the desired calculated transmitter output. A communication circuit 112 coupled
to microcomputer 98 provides means for receiving digital words from the loop such
as correction constants and span and zero settings for the transmitter. The communication
circuit 112 in the transmitter is coupled along lines 126, 128, 62A, 64A for two-way
communication circuit with a second communication circuit 114 which can be a part
of a digital control system or can be a separate device coupled to the loop at a remote
point. Data is entered into the second communication circuit 114 which represents
span, zero and linearity corrections. The second communication circuit 114 couples
a high frequency signal over loop conductors 62A, 64 and lines 76, 126 in the transmitter
to the communication circuit 112. The high frequency signal is detected by the communication
circuit 112 in the transmitter and a "carrier detect" signal is coupled from the communication
circuit 112 to the microcomputer 98 along line 116. When the carrier detect signal
is sensed, the microcomputer 98 couples a signal on line 118 to the communication
circuit 112 which closes switch 122 and energizes a modem 124 in communication circuit
112. Modem 124 performs two-way communication with the second communication circuit
114 along lines 126, 128, 76, 62A, 64A. Correction constants are received by the modem
124 and are transferred to the memory 104 by microcomputer 98. Span and zero constants
are likewise received and stored in the memory 104. The modem 124 transmits to the
second communication circuit 114 data representative of the status of constants stored
in memory 104 which may include parameters controlling transmitter function, serial
numbers and maintenance history as well as data representative of the process variable.
[0025] The combined energization currents for the circuitry can exceed the 4 milliampere
energization level available from the loop. The excitation circuit 84 and the microcomputer
98 are coupled in series so that the same current flows through both and total energization
current from the loop is effectively controlled. A charge pump 132 can be coupled
between conductors 70, 74 and 76 to further reduce the excitation current at the loop
terminals. The charge pump transfers charge between the series loads so that the current
requirements of the two series energization circuits are better balanced. This further
reduces energization current at the transmitter terminals. Switch 122 is open during
normal operation of the transmitter so that the modem does not operate, further reducing
energization requirements. The energization current to the transmitter from the loop
can thus be kept below 4 milliamperes and hence the transmitter can be operated from
the 4-20 mA loop 14. During periods of communication between modem 124 and circuit
114, however, excitation current consumption may temporarily exceed 4 mA.
[0026] In FIG. 5A, a first portion of circuitry of a transmitter is shown. A sensor module
88 is shown enclosed in a dashed line and comprises a capacitive pressure sensor 140
coupled through fixed capacitors 142, 144 to an array of rectification diodes 146.
The rectification diodes 146 are coupled to an excitation circuit 84 which provides
excitation to the capacitive pressure sensor 140 through the rectification diodes
146. The sensor module 88 further comprises selected fixed resistances 148, 150, 152,
154, 156, 158 and thermistors 162, 164 which are compensation components coupled together
with sensor 140 and fixed capacitors 142, 144 to provide analog temperature compensation
of the sensor 140. The sensor module 88 further comprises a correction capacitor 166
which was used with the former analog converter but which need not be connected to
the digital converter and is not used.
[0027] The excitation means 84 comprises resistors 168, 170, 172, 174, 176, 178, capacitors
180, 182, 184, 186, 188, 190, 192, amplifiers 194, 196, transistor 198, and transformer
200 which has five windings coupled together for providing excitation. The operation
of the excitation circuit in cooperation with the sensor module is substantially as
described in U.S. Patent 3,646,538 to Roger L. Frick.
[0028] The sensor module 88 couples a sensor current "Is" representative of the sensed pressure
along line 202 to an integrator circuit 92. The sensor module 88 also couples an analog
temperature compensation current "It" along line 204 to the integrator circuit 92.
The sensor current "Is" and the temperature compensation current "It" are summed at
node 206 of an amplifier stage comprising amplifier 208, resistors 210, 212, 214,
216 and capacitor 218. This amplifier stage provides a potential on line 218 which
is representative of the sum of the currents (Is + It) and is thus representative
of the sensor output corrected with the analog compensation circuitry of the sensor
module. The line 218 is coupled through a switch (field effect transistor) 220 to
an integrator stage 222. A substantially fixed reference potential is present on line
224 and is coupled through switch (field effect transistor) 226 to the integrator
stage 222. The integrator stage 222 comprises an amplifier 228, a capacitor 230, and
a resistor 232 coupled together as shown in FIG. 5A. The switches 220 and 226 are
actuated alternately so that the integrator stage 22 alternately integrates the sensor
potential and the fixed potential. The integrator stage 222 has an output on line
234 which is the time integral of the potentials applied by the switches 220, 226.
The integrator stage output is coupled along line 234 to comparator 236 which compares
the integrator output to a substantially fixed potential on line 238. The comparator
output is coupled out on line 240 to circuitry in FIG. 5B which is explained later.
[0029] A portion of the supply circuitry, second regulator 72, is coupled between conductors
70 and 74 and generates intermediate supply potentials on lines 242 and 238 which
supply reference potentials to the excitation and integrator circuits and temperature
compensation circuitry in sensor module 88. The second regulator comprises resistors
244, 246, 248, 250, 252, and adjustable reference 254 and capacitors 256 and 258 coupled
together as shown in FIG. 5A.
[0030] A connector indicated as "J2" in FIG. 5A mates with a connector likewise labeled
"J2" in FIG. 5B.
[0031] In FIG. 5B, NAND gate 246 and 248 are coupled together to comprise a flip-flop circuit
250. The comparator output (FIG. 5A) is coupled along line 240 through connector J2
to a "set" input of flip-flop 250. A first output Q of flip-flop 250 is coupled along
line 244 through connector J2 to the gate input of switch 226 (FIG. 5A). A second
output Q of the flip-flop 250 is coupled along line 242 through connector J2 to the
gate input of switch 220 (FIG. 3). Excitation potentials are coupled along lines 70,
74 and 76 through connector J2. A timer 96 provides a low level timer output on line
252 to a level shifting buffer 254 which provides a high level timer output to inverter
256. Timer 96 preferably comprises a part number CD 4536B manufactured by RCA Corporation.
Inverter 256 couples the high level timer output to a reset input of the flip-flop
250 along line 258. The Q output of flip-flop 250 is coupled through buffers 260 to
a reset input of the timer 96. The Q output of flip-flop 250 is coupled through inverter
262 to an input of microcomputer 98. Microcomputer 98 is preferably a part number
80C59 manufactured by OKI Semiconductor. The microcomputer 98 provides a clock signal
along line 264 to the timer 96. The flip-flop 250, the timer 96 and the integrator
92 function together as a dual slope integrator circuit. The Q output of flip-flop
250 has a pulse width that is representative of the combined current (Is + It) and
hence the signal coupled to the microcomputer 98 is representative of the sensed parameter,
including the analog correction made in the sensor module 88. The microcomputer 98
counts its own clock pulses during this pulse width from inverter 262 to complete
the analog-to-digital conversion of the sensor output (Is + It).
[0032] Watchdog timer 102 comprises inverters 268, 270, capacitors 272, 274, 276, resistors
278, 280, transistor 282 and diode 284 coupled together as shown in FIG. 5B. During
the normal operation of microcomputer 98, the microcomputer 98 periodically provides
a pulse on line 290 to the watchdog timer 102. The pulse on line 290 resets the watchdog
timer 102 and prevents triggering the watchdog output on line 292. If, however, the
microcomputer 98 malfunctions and fails to present a pulse on line 290 for a selected
time interval set by the watchdog timer, the watchdog timer output on line 292 is
triggered and resets the microcomputer 98 so that normal operation can be resumed.
The selected time interval is a function of the resistances of resistors 280, 278
and the capacitances of capacitors 274, 276.
[0033] An electrically-eraseable-read-only-memory (EEROM) 104 is coupled to microcomputer
98 and stores digital words representative of digital corrections, span, zero and
the like as explained in connection with FIG. 2. The microprocessor reads the correction
constants stored in memory 104 and calculates corrections for the output as a function
of the constants.
[0034] A crystal 292 is coupled to the microcomputer 98 to provide a stable clock or time
reference.
[0035] While the operation of the transmitter is described with reference to a separate
non-volatile memory 104, it will be understood by those skilled in the art that a
portion of RAM in the microcomputer 98 may be energized by a battery to provide non-volatile
storage of correction constants and the like. Lines 70, 74 and 76 are coupled to level
shifter 254 to provide energization to it.
[0036] A connector labelled "J3" in FIG. 5B couples lines from the microcomputer 98 to circuitry
shown in FIG. 5C. Supply lines 70, 74, 76 are also coupled through connector "J3"
to circuitry in FIG. 5C.
[0037] In FIG. 5C, a connector labelled "J3" couples to the connector labelled "J3" in FIG.
5B and supply lines 70, 74, 76 are coupled through connectors J3 to the circuitry
in FIG. 5B. The transmitter is coupled to the loop 14 through terminals 60, 62 in
FIG. 5C. Current from loop 14 flows into the transmitter at terminal 60. Terminal
60 is coupled to a line 126 through a polarity protection diode 59. Meter terminals
61, 63 are coupled to diode 59 providing for connection of an optional indicating
meter 65 in the wiring compartment 24 (shown in FIG. 2). A first regulator 68 is coupled
to line 126 for receiving an excitation portion of the loop current from line 126.
Regulator 68 supplies a first regulated potential to the line 70. The first regulator
comprises resistors 300, 302, 304, 306, 308, 310, 312, capacitors 314, 316, 318, amplifier
320, transistor 322, 324, diodes 326, 328, and zener diodes 330, 332, 334 and 336
coupled together as shown in FIG. 5C for generating regulated potentials.
[0038] A current control circuit 66 is coupled between line 126 and terminal 62 for controlling
the magnitude of current in the loop. The current control circuit 66 comprises an
amplifier 350, resistors 78, 352, 354, 356, transistors 356, 360, capacitor 362 and
zener diodes 364, 366 coupled together as shown in FIG. 5C for controlling current
flow from line 126 to terminal 62. The amplifier receives a control input on line
368 and couples a current through resistor 354 to transistors 358, 360 which are arranged
in a Darlington configuration. A portion of the loop current flows from line 126 through
zener diode 364, transistors 358, 360 and resistor 356 to line 76. Current from further
portions of transmitter circuitry flows into line 76 which is the circuit common line.
Substantially all of the loop current thus flows from line 76 through resistor 78
to terminal 62 and back to the loop. A potential developed across resistor 78 is coupled
along line 370 to DAC 82. The DAC 82 preferably comprises a part number AD7543 manufactured
by Analog Devices. The DAC 82 compares the potential on line 370 to a calculated output
signal received by the DAC from bus 372. Bus 372 is coupled from the DAC through connectors
J3 to microcomputer 98 (shown in FIG. 5B).
[0039] A communication circuit 112 couples a communication output along line 128 to the
current control for providing the first communication signal to the loop as explained
in connection with FIG. 3. A second communication output is coupled from the loop
at terminal 60 along line 126 to the communication circuit 112. The communication
circuit 112 receives the second communication signal from line 126 and demodulates
the second communication signal. The demodulated second communication signal is coupled
along bus 374 through connector J3 to the microcomputer 98 (in FIG. 5B). The communication
circuit 112 comprises a filter 376 for filtering and amplifying communication signals
received from the loop. Filter 376 is coupled to a detector circuit 378 which detects
presence of a carrier, and to a MODEM 124 which modulates and demodulates communication
signals. The MODEM 124 preferably comprises a part number TCM3105 manufactured by
Texas Instruments. The carrier detector 378 is coupled along line 116 through connectors
J3 to microcomputer 98 (FIG. 5B). When a carrier is detected, the microcomputer 98
couples a signal along line 118 to a switch 122 which energizes MODEM 124.
[0040] A charge pump 132 is coupled between lines 70, 74 and 76. The charge pump preferably
comprises a capacitor 390 coupled to a charge pump integrated circuit 392. Charge
pump integrated circuit 392 preferably comprises a part number 7660 manufactured by
Intensil. The capacitor 390 is charged from the lines 70, 74 and then discharged into
lines 74, 76 such that current is balanced.
[0041] The apparatus can thus be configured to provided desired digital corrections to the
output while the transmitter remains in place in the process plant. The cost of replacing
the entire transmitter can be avoided while still acheiving an output which is digitally
calculated to provide digital linearity correction. The transmitter can be fitted
with the apparatus of this invention while the transmitter remains in place in the
process installation. While the embodiments herein described have linear outputs,
it will be understood by those skilled in the art that this invention can likewise
be used with non-linear outputs such as square root outputs or with reverse acting
outputs.
1. A method for adding a capability to a parameter value transmitter (10) for receiving
input information where such a transmitter (10) initially has, through a set of transmitting
circuits (23) therein, a capability for transmitting, along a two-wire current carrying
loop (14) adapted for electrical connection to first and second terminals (60,62)
of the transmitter (10), output information formed by those values measured by a sensing
means (26,88) in the transmitter (10) of a parameter the values of which depend on
conditions in a structure (12) to which the transmitter (10) is affixed, the method
comprising:
disconnecting and removing at least a portion of the set of transmitting circuits
(23) which are initially electrically connected between the sensing means (26,88)
and the first and second terminals (60,62) in the transmitter (10) affixed to the
structure (12); and
providing in the transmitter (10) affixed to the structure (12) a set of transmitting
and receiving circuits (52,10 in Fig. 4) electrically connected between the sensing
means (26,88) and the first and second terminals (60,62), the set of transmitting
and receiving circuits (52,10 in Fig. 4) being capable of enabling the then-modified
transmitter (11) simultaneously to transmit the output information and to receive
the input information, the input information being used for correcting those values
measured by the sensing means.
2. The method of Claim 1 wherein the transmission of the output information by the modified
transmitter (11) occurs through use of transmitted signals having a frequency content
in a first frequency range, and wherein the reception of the input information by
the modified transmitter (11) occurs through receiving signals having a frequency
content in a second frequency range separated from the first frequency range.
3. The method of Claim 1 wherein the sensing means (26,88) comprises a sensor (140) and
sensor output signal correction circuitry (148,150,152,154,156,158,162,164), the sensor
and the sensor output correction circuitry being sealed from that space (22) in the
transmitter (10) in which the set of transmitting circuits (23) were located before
removal and from that space in the modified transmitter (11) in which the set of transmitting
and receiving circuits (52,10 in Fig. 4) are provided.
4. A parameter value transmitter (11) for transmitting, along a two-wire current carrying
loop (14) adapted for electrical connection to first and second terminals (60, 62)
of the transmitter (11), output information formed by those values measured by a sensing
means (26,88) in the transmitter (11) of a parameter the values of which depend on
conditions in a structure (12) to which the transmitter (10) is affixed and for receiving
input information the transmitter including a set of transmitting and receiving circuits
(52,10 in Fig. 4) electrically connected between the sensing means (26,88) and the
first and second terminals (60,62), the set of transmitting and receiving circuits
(52, 10 in Fig. 4) being capable of enabling the transmitter (11) simultaneously to
transmit the output information and to receive the input information, the input information
being used for correcting those values measured by the sensing means.
5. A transmitter according to Claim 4 wherein the transmitting and receiving circuit
(52,10 in Fig. 4) are arranged to transmit the output information through use of transmitted
signals having a frequency content in a first frequency range and to receive the input
information through receiving signals having a frequency content in a second frequency
range separated from the first frequency range.
6. A transmitter according to Claim 4 wherein the sensing means (26,88) comprises a sensor
(140) and sensor output signal connection circuitry (148,150,152,154, 156,158,162,164),
the sensor and the sensor output connection circuitry being sealed from that space
in the transmitter (11) in which the set of transmitting and receiving circuits (52,10
in Fig. 4) are provided.
1. Verfahren zum Nachrüsten eines Parameterwertgebers (10) für den Empfang von Eingabeinformationen,
wobei ein solcher Geber (10) ursprünglich durch eine darin enthaltene Gruppe von Sendeschaltungen
(23) die Fähigkeit aufweist, über eine stromführende Zweidrahtschleife (14), die für
eine elektrische Verbindung mit einem ersten und einem zweiten Anschluß (60, 62) des
Gebers (10) eingerichtet ist, Ausgabeinformationen zu übertragen, die von den durch
eine Meßeinrichtung (26, 88) im Geber (10) gemessenen Werten eines Parameters gebildet
werden, dessen Werte von Bedingungen in einer Struktur (12) abhängen, an welcher der
Geber (10) befestigt ist, wobei das Verfahren aufweist:
Abtrennen und Ausbau zumindest eines Teils der Gruppe von Sendeschaltungen (23)
die ursprünglich in dem an der Struktur (12) befestigten Geber (10) zwischen der Meßeinrichtung
(26, 88) und dem ersten und zweiten Anschluß (60, 62) elektrisch geschaltet sind;
und
Ausrüsten des an der Struktur (12) befestigten Gebers (10) mit einer Gruppe von
Sende- und Empfangsschaltungen (52, 10 in Fig. 4), die elektrisch zwischen der Meßeinrichtung
(26, 88) und dem ersten und zweiten Anschluß (60, 62) geschaltet sind, wobei die Gruppe
von Sende- und Empfangsschaltungen (52, 10 in Fig. 4) den dann modifizierten Geber
(11) in die Lage versetzen kann, gleichzeitig die Ausgabeinformation zu übertragen
und die Eingabeinformation zu empfangen, wobei die Eingabeinformation für die Korrektur
der von der Meßeinrichtung gemessenen Werte verwendet wird.
2. Verfahren nach Anspruch 1, wobei die Übertragung der Ausgabeinformation durch den
modifizierten Geber (11) unter Verwendung von Sendesignalen erfolgt, deren Frequenzgehalt
in einem ersten Frequenzbereich liegt, und wobei der Empfang der Eingabeinformation
durch den modifizierten Geber (11) über Empfangssignale erfolgt, deren Frequenzgehalt
in einem zweiten, vom ersten Frequenzbereich getrennten Frequenzbereich liegt.
3. Verfahren nach Anspruch 1, wobei die Meßeinrichtung (26, 88) einen Sensor (140) und
eine Sensorausgangssignal-Korrekturschaltung (148, 150, 152, 154, 156, 158, 162, 164)
aufweist, wobei der Sensor und die Sensorausgangssignal-Korrekturschaltung gegen den
Raum (22) im Geber (10), in dem die Gruppe von Sendeschaltungen vor dem Ausbau untergebracht
war, sowie gegen den Raum im modifizierten Geber (11) abgedichtet sind, in dem die
Gruppe von Sende- und Empfangsschaltungen (52, 10 in Fig. 4) untergebracht ist.
4. Parameterwertgeber (11) zum Übertragen von Ausgabeinformation , die von den durch
eine Meßeinrichtung (26, 88) im Geber (11) gemessenen Werten eines Parameters gebildet
wird, dessen Werte von Bedingungen in einer Struktur (12) abhängen, an welcher der
Geber (10) befestigt ist, über eine stromführende Zweidrahtschleife (14), die für
eine elektrische Verbindung mit einem ersten und einem zweiten Anschluß (60, 62) des
Gebers (11) eingerichtet ist, und zum Empfang von Eingabeinformation, wobei der Geber
eine Gruppe von Sende- und Empfangsschaltungen (52, 10 in Fig. 4) aufweist, die elektrisch
zwischen der Meßeinrichtung (26, 88) und dem ersten und zweiten Anschluß (60, 62)
geschaltet sind, wobei die Gruppe von Sende- und Empfangsschaltungen (52, 10 in Fig.
4) den Geber (11) in die Lage versetzen kann, gleichzeitig die Ausgabeinformation
zu übertragen und die Eingabeinformation zu empfangen, wobei die Eingabeinformation
zur Korrektur der durch die Meßeinrichtung gemessenen Werte verwendet wird.
5. Geber nach Anspruch 4, wobei die Sende- und Empfangsschaltungen (52, 10 in Fig. 4)
so eingerichtet sind, daß sie die Ausgabeinformation unter Verwendung von Sendesignalen
übertragen, deren Frequenzgehalt in einem ersten Frequenzbereich liegt, und die Eingabeinformationen
über Empfangssignale empfangen, deren Frequenzgehalt in einem zweiten, vom ersten
Frequenzbereich getrennten Frequenzbereich liegt.
6. Geber nach Anspruch 4, wobei die Meßeinrichtung (26, 88) einen Sensor (140) und eine
Sensorausgangssignal-Korrekturschaltung (148, 150, 152, 154, 156, 158, 162, 164) aufweist,
wobei der Sensor und die Sensorausgangssignal-Korrekturschaltung gegen den Raum im
Geber (11) abgedichtet sind, in dem die Gruppe von Sende- und Empfangsschaltungen
(52, 10 in Fig. 4) untergebracht ist.
1. Procédé pour conférer, à un transmetteur de valeur de paramètre (10), la capacité
de recevoir des informations en entrée, ledit transmetteur (10) ayant initialement,
au moyen d'une série de circuits internes de transmission (23), la capacité d'émettre,
le long d'une boucle de transport de courant (14) à deux fils agencée de façon à être
connectée électriquement à des première et seconde bornes (60, 62) du transmetteur
(10), des informations de sortie constituées par les valeurs, mesurées par des moyens
sensibles (26, 88) dans le transmetteur (10), d'un paramètre dont les valeurs dépendent
de conditions dans une structure (12) à laquelle le transmetteur (10) est fixé, ledit
procédé comprenant les opérations consistant
à déconnecter et enlever une partie au moins de la série de circuits de transmission
(23) qui sont initialement connectés électriquement entre les moyens sensibles (26,
88) et les première et seconde bornes (60, 62) dans le transmetteur (10) fixé à la
structure (12); et
à disposer, dans le transmetteur (10) fixé à la structure (12), une série de circuits
d'émission et de réception (52, 10 sur la fig. 4), et à les connecter électriquement
entre les moyens sensibles (26, 88) et les première et seconde bornes (60, 62), la
série de circuits d'émission et de réception (52, 10 sur la fig. 4) étant capable
de mettre le transmetteur ainsi modifié (11) en mesure simultanément d'émettre les
informations de sortie et de revevoir les informations en entrée, les informations
en entrée étant utilisées pour corriger les valeurs mesurées par les moyens sensibles.
2. Procédé selon la revendication 1, dans lequel l'émission des informations de sortie
par le transmetteur modifié (11) se produit par l'utilisation de signaux émis qui
ont un contenu en fréquences dans une première gamme de fréquences, et dans lequel
la réception des informations en entrée par le transmetteur modifié (11) se produit
par la réception de signaux ayant un contenu en fréquences dans une seconde gamme
de fréquences, distincte de la première gamme de fréquences.
3. Procédé selon la revendication 1, dans lequel les moyens sensibles (26, 88) comprennent
un détecteur (140) et un circuit (148, 150, 152, 154, 156, 158, 162, 164) de correction
des signaux de sortie du détecteur, le détecteur et le circuit de correction des signaux
de sortie du détecteur étant isolés hermétiquement de l'espace (22) dans lequel, dans
le transmetteur (10), la série de circuits d'émission (23) était placée avant d'être
enlevée, ainsi que de l'espace dans lequel, dans le transmetteur modifié (11), la
série de circuits d'émission et de réception (52, 10 sur la fig. 4) est disposée.
4. Transmetteur de valeur de paramètre (11) pour transmettre, le long d'une boucle de
transport de courant (14) à deux fils agencée de façon à être connectée électriquement
à des première et seconde bornes (60, 62) du transmetteur, des informations de sortie
constituées par les valeurs, mesurées par des moyens sensibles (26, 88) dans le transmetteur
(11), d'un paramètre dont les valeurs dépendent de conditions dans une structure (12)
à laquelle le transmetteur (11) est fixé, et pour recevoir des informations en entrée,
le transmetteur comprenant une série de circuits d'émission et de réception (52, 10
sur la fig. 4) connectés électriquement entre les moyens sensibles (26, 88) et les
première et seconde bornes (60, 62), la série de circuits d'émission et de réception
(52, 10 sur la fig. 4) étant capable de mettre le transmetteur (11) en mesure simultanément
d'émettre les informations de sortie et de revevoir les informations en entrée, les
informations en entrée étant utilisées pour corriger les valeurs mesurées par les
moyens sensibles.
5. Transmetteur selon la revendication 4, dans lequel les circuits d'émission et de réception
(52, 10 sur la fig. 4) sont agencés de façon à émettre les informations de sortie
par l'utilisation de signaux émis qui ont un contenu en fréquences dans une première
gamme de fréquences, et de façon à recevoir les informations en entrée par la réception
de signaux ayant un contenu en fréquences dans une seconde gamme de fréquences, distincte
de la première gamme de fréquences.
6. Transmetteur selon la revendication 4, dans lequel les moyens sensibles (26, 88) comprennent
un détecteur (140) et un circuit (148, 150, 152, 154, 156, 158, 162, 164) de correction
des signaux de sortie du détecteur, le détecteur et le circuit de correction des signaux
de sortie du détecteur étant isolés hermétiquement de l'espace dans lequel, dans le
transmetteur (11), la série de circuits d'émission et de réception (52, 10 sur la
fig. 4) est disposée.