Communications: electrical – Continuously variable indicating – With particular transmitter
Reexamination Certificate
2000-05-15
2003-01-07
Horabik, Michael (Department: 2635)
Communications: electrical
Continuously variable indicating
With particular transmitter
C702S057000, C702S085000, C702S104000, C702S116000
Reexamination Certificate
active
06504489
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to process control transmitters used to measure process variables in industrial processing plants. More particularly, the present invention relates to a process control transmitter having an externally accessible DC circuit common.
Process control transmitters are used in industrial processing plants to monitor process variables and control industrial processes. Process control transmitters are generally remotely located from a control room and are coupled to process control circuitry in the control room by a process control loop. The process control loop can be a 4-20 mA current loop that powers the process control transmitter and provides a communication link between the process control transmitter and the process control circuitry. Typically, the transmitter senses a characteristic or process variable, such as pressure, temperature, flow, pH, turbidity, level, or the process variables, and transmits an output that is proportional to the process variable being sensed to a remote location over a plant communication bus. The plant communication bus can use a 4-20 mA analog current loop or a digitally encoded serial protocol such as HART® or FOUNDATION™ fieldbus protocols, for example.
Referring now to
FIG. 1
, a simplified block diagram of a process control transmitter as can be found in the prior art is shown. Here, process control transmitter
10
includes housing
12
, circuitry
14
, and first and second terminals
16
A and
16
B. Housing
12
is not permanently hermetically sealed and generally includes lower housing member
12
A and removable cap
12
B. A seal (not shown) is typically sandwiched between lower housing member
12
A and cap
12
B to seal housing
12
. Process control loop
18
can couple process control transmitter
10
to control room
20
at first and second terminals
16
A and
16
B. Circuitry
14
is configured to receive a sensor input
22
relating to a process variable and communicate the process variable information to control room
20
over process control loop
18
.
Circuitry
14
generally communicates with control room
20
over process control loop
18
by adjusting loop current I
T
flowing through process control loop
18
and first and second terminal
16
A and
16
B. Circuitry
14
senses loop current I
T
with feedback output FB, which relates to the voltage at node
24
with respect to DC common
26
or the voltage drop across sense resistor R
SENSE
. Feedback output FB is communicated to circuitry
14
through conductor
28
which includes series resistor R
SERIES
which allows a negligible amount of current to flow through conductor
28
between node
24
and circuitry
14
. Circuitry
14
uses feedback output FB to adjust loop current I
T
in accordance with the sensor input
22
.
The voltage drop across sense resistor R
SENSE
, second terminal
16
B has a voltage that is offset from DC circuit common
26
by the voltage drop across R
SENSE
. Additionally, the voltage difference between second terminal
16
B and DC circuit common
26
will vary as loop current I
T
is varied by circuitry
14
. As a result, communication signals produced by circuitry
14
, which are regulated with respect to DC circuit common
26
, cannot be conveniently communicated to processing circuitry that is external to process control transmitter
10
without performing a level shift in the voltage of the communication signals to compensate for the voltage drop across sense resistor R
SENSE
. This level-shifting requirement would result in increased cost and complexity of processing electronics that are to be coupled to transmitter
10
and adapted to communicate with circuitry
14
using signals which are regulated with respect to DC circuit common
26
. Additionally, there is an increase in the potential for error due to mismatched level-shifting or DC circuit common.
SUMMARY OF THE INVENTION
A process control transmitter having an externally accessible DC circuit common is provided that eliminates the need to perform level shifting of signals communicated between the transmitter and external processing electronics. The process control transmitter includes first, second and third externally accessible terminals, a series regulator, circuitry, a shunt, and a shunt current regulator. The first and second terminals are coupleable to a process control loop and are adapted to conduct a loop current through the transmitter. The circuitry is energized by a load current and is generally adapted to manage process variable and transmitter-related information and provide a digital signal to the third terminal that is regulated relative to a DC circuit common. The DC circuit common is electrically coupled to the second terminal and the digital signal is externally accessible between the second and third terminals. The series regulator is coupled to the first terminal and is adapted to conduct the load current and provide a first feedback output that is representative of the load current. The shunt is adapted to conduct a shunt current and provide a second feedback output that is representative of the shunt current. The loop current is substantially a summation of the load current and the shunt current. The shunt current regulator carries the shunt current and controls the loop current as a function of the first and second feedback outputs.
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Nelson Richard L.
Roper Weston
Westfield Brian L.
Horabik Michael
Rosemount Inc.
Westman Champlin & Kelly
Wong Albert K.
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