Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF INVE~TION
Field of Invention:
This invention relates generally to a telemetering
system in which a direct-current signal yielded by a transmitting
device responsive to a variable of interest is conveyed
over a two-wire line to a receiving station having a DC
power supply whose output i.s supplied to the transmitting
device over the same line to provide operating power therefGr,
and more particularly to a system of this type capable
of supplying a relatively large amount of power to the
transmitting device.
Status of Prior Art:
A two-wire telemetering system is particularly
useful in an industrial process control loop in which a
value sensed at a transmitting device by a thermocouple
or other sensor of the process variable being metered is
converted into a direct-current signal that is conveyed
over a two-wire line to a remote receiving station for
operating indicators, recorders, controllers or other instruments
in the process control loop. Systems of this type are
disclosed in the Herzl et al. patent 4,084,155, the Shauger
patent 4,158,765, and the Sterling et al. patent 4,692,328.
One important advantage of a two-wire telemetering
system is that the same wires serve not only to convey
the current signal from the transmitting device to the
station but also to conduct operating power from the receiving
station to the transmitting device, thereby obviating the
need for extra wires in remote control applications. Also,
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a current output minimizes susceptibility of the system
to voltage noise spikes and eliminates line drop problems.
For a process control telemetering system, American
National Standard ANSI-MC 12.1 - 1975 and ISA-S 50.1, "Com-
patibility of Analog Signals for Electronic Industrial
process Instruments" specify that the standard output signal
(of a transmitting device) shall be a current a range of
4 mA to 20 mADC [Section 3.2 of the Standard], and that
the standard voltage signal (of the receiver) shall be
1 volt to 5 volt dc [Section 3.3.2 of the Standard]. These
standards are generally accepted and practiced by the
industrial process control industry.
It has been found that known telemetering systems
of this type fail in some instances to supply adequate
operating power for transmitting devices. For example,
if the transmitting device is a differential-pressure (D-P)
transducer operating in conjunction with a square root
extractor, the power demands of the D-P transducer and
the associated square root extractor are not satisfied
at low input levels when the device operates in the usual
4 to 20 mAdc range. And if one wishes to include a micro-
processor in a transmitting device, because of the existing
constraints in power availability, this may not be possible.
It is known to effect linear regula~ion of the
power supply voltage for the transmitting device in a two-wire
telemetering system. But such linear regulators act to
restrict power consumption of the transmitter control circuitry
and to reduce its drive capability. Typically, a two-wire
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transmitter is specified with a minimum power supply operating
voltage with stated conditions, a linear relationship being
established between operating voltage and the resistance
load drive capability.
I For example, operation at 12.5 volts and zero ohms
may be specified as well as operation at 24 volts and 500
ohms. The internal transmitter control electronics is
necessarily limited to a current consumption of less than
j', 3.8 mA, for the operating range of the system is 4 to 20
~ mA. Hence the higher the minimum operating voltage, the
lower the load resistance must be.
By reason of such power limitations, it may become
necessary in many instances to operate the telemetering
system in a four-wire configuration rather than with a
¦ two-wire line. Thus additional wires are required to convey
¦ adequate operating power to the transmitting device, thereby
sacrificing the important benefits of a two-wire system.
I For any two-wire loop, the transmitter control
¦ electronics power consumption is limited by the external
l power supply voltage (the source of all power consumed
¦ in the loop) and the minimum operational current in the
~ loop (4 ma). For example, the power consumption of the
I ¦ control electronics of all standard two-wire transmitters
ll is limited to 4 ma~ V power supply.
1 If the control electronics inside the transmitter
requires a voltage equal to the power supply voltage, then
4 ma is the maximum current which can be consumed. However,
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in a situation where the control electronics requires less
than the power supply voltage, then a higher current consumption
can be achieved. The invention is addressed to this situation
by providing more power to the transmitter control electronics
S when the control electronics is implemented with circuitry
requiring relatively low voltage with respect to the available
voltage, rather than limiting the power consumption to
4 ma~ V low.
SUMMARY OF INVENTION
In view of the foregoing, the main object of this
invention is to provide a two-wire telemetering system
wherein the same line conducts a direct-current signal
from a field station transmitting device to a receiving
~ station which supplies operating power at a substantially
constant level to the transmitting device, the amount of
operating power supplied being sufficient even at low signal
levels to energize relatively complex transmitting devices
such as magnetic flowmeters, field-mounted multiplexers
and microprocessor-based transmitting transmitting devices.
1~1 1032
More particularly, an object o~ the present invention
is to provide a two-wire telemetering system o~ the above
type in which the power supplied to the transmitting device
is regulated by a switching-type power regulator that delivers
to the power input terminals of the device substantially
constant power under high voltage-low current as well as
under low voltage-high current conditions.
Also an object of the invention is to provide a
two-wire telemetering system of the above type which is
of relatively simple, low cost construction, yet operates
efficiently and reliably.
Briefly stated, these objects are attained in a
telemetering system in which a DC powered transmitting
device responsive to a process or other variable yields
a direct-current signal in accordance with the variable
that is supplied over a two-wire line to a receiving station
in which a DC voltage supply is connected to the line through
a load resistor. Developed across this resistor is an
output signal in a predetermined current range for operating
an indicator or other instrument. The same line supplies
operating power to the power input terminals of the transmitting
devi-ce through a switching-type step down power regulator
that yields constant power under high voltage-low current
as well as under low voltage-high current conditions, thereby
making more power available to the transmitting device
and increasing its load drive capability.
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60538-1023
More particularly, the present invention provides a
two-wire tel~meterirlg system comprising: (a) a DC powered
-t.ransmitting device operating at a predetermined voltage and
having power input terminals connected to an internal
electronic circuit, said device being responsive to a process
variable metered by a sensor to yield at its output a direct-
current signal in accordance with said variable in a
predetermined current range; (b) a two-wire line, one end of
which is connected to the output of said transmittiny device to
convey said signal; (c) a receiving station remote from the
transmitting device provided with a DC power supply having a
voltage at least twice as high as said predetermined voltage
and a load resistor in series therewith connected to the other
end of the line to receive said signal which is applied to a
receiver and to at the same time supply power from said DC
supply to said transmitting devi.ce; and (d) a switching-type
step-down power regulator interposed between the power input
terminals of the transmitting device and said one end of the
line, the output voltage of the power regulator always being
lower than the voltage applied thereto from the power supply
through the line, the power regulator yielding constant power
under high voltage-low current as well as under low voltage-
high cuxrent conditions, thereby making more power available to
the transmitking device and increasing its load drive
capability.
5b
~31 ~32
BRIEF DESCRIPTION OF DRAWI~GS
For a better understanding of the invention as
well as other objects and further features thereof, reference
is made to the following detailed description to be read
in conjunction with the accompanying drawings, wherein:
Fig. 1 is a schematic circuit diagram of a two-wire
telemetering system in accordance with the invention;
Fig. 2 is a schematic circuit diagram of the basic
elements of a step-down switching-type regulator included
~ 10 in the system; and
; Fig. 3 is a simplified circuit representing the
general case in regard to the amount of power which is
available when linear and when switching-type regulation
is used in the system.
DETAILED DESCRIPTION OF INVENTION
The Telemeterinq SYstem:
Referring now to Fig. 1, there is shown in this
figure the basic components of a two-wire telemetering
system in accordance with the invention which includes
a DC-powered transmitting device responsive to a process
or other variable whose value is metered by a sensor 10.
The-transmitting device is linked by a two-wire line Wl
and W2 to a remote receiving station in which a DC power
supply represented by battery 11 i- connected in series
with a load resistor 12 through which flows an output signal
in the standard current range (i.e., 4 to 20 mAdc). The
voltage developed across load resistor 12 is applied to
a receiver 13 which may be a process variable indicator,
a recorder, a controller and any other device appropriate
to process control. The two wire line Wl and W2 are connected
to the transmitter device at terminals T4 and T5.
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I Sensor 10 may be a thermocouple, a differential-pressure
transducer or any other field-mounted device for metering
the process variable. This variable may be fluid flow
rate, temperature or pressure to produce an analog signal
proportional thereto. In the example shown, this analog
signal is applied to the microprocessor-controlled circuit
14.
The output yielded by microprocessor-controlled
circuit 14 which appears at terminal Tl is applied to one
10 , input of a differential amplifier lS to whose other input
is applied a feedback voltage developed across a feedback
resistor RFB. ~he output of amplifier 15 modulates an
i output transistor 16 connected in series with resistor
RFB between lines Wl and W2.
I Microprocessor-controlled circuit 14 is provided
with power input terminals T2 and T3, terminal T3 being
connected to line W2 through resistor RF~ at terminal T5.
Terminal T2 is connected to line Wl through a switching-type
,I power regulator 17 at terminal T4. Regulator 17 serves
;~ 20 ¦~ to supply relatively high power to the microprocessor-controlled
,I circuit 14 at a constant level, which remains substantially
unchanged under high voltage-low current as well as low
¦ voltage-high current conditions.
ll The voltage between terminals 2 and 3 is V2_3,
~' while that between terminals 4 and 5 is V4_5. The current
going into regulator 17 is designated Ips, and the current
from regulator 17 into the microprocessor-controlled circuit
14 is designated IdC. Voltage V2_3 is determined by the
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~31 1032
60538-1023
power supply sp~clfications of the componen~s used in
microprocessor-controlled circuit 14. Typically, voltage V2 3
is 5Vdc.
The aim of the invention is to obtain an IdC current
which is greater than I current at the regulated 5Vdc voltage
in order to satisfy the current requirements of the
microprocessor-controlled circuit 14. This goal is achieved by
switching regulator 17 when V2 3 is lower than V4 5 as will
later be explained.
Switching-type power regulators suitable for this
purpose are disclosed in the following publications:
A. 1987 "Switchmode (A Designers Guide for Switching Power
Supply Circuits and Components)" published by Motorola
Corporation.
8. "Applications Handbook (1987-1988)" published by Unirode
Corporation.
C. "Linear and Interface Circuit Applica~ions 1985" (Section
C - Switching Power Design) published by Texas Instruments.
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131 1032
Regulator 17 includes a transistor switch 18 which
switches on and off at a predetermined frequency. During
the interval switch 18 is on, the input voltage is applied
to the input of an LC filter formed by an inductor 19 and
a capacitor 20, thereby causing the current to increase.
When the swi~ch is off, the energy stored in inductor lg
maintains current flow to the load, circulating through
a "catch" diode 21.
The regulator is monitored and controlled by a
control circuit generally represented by block 22. This
control circuit includes an oscillator driving a pulse
width modulator, an error amplifier and a precision voltage
reference. The error amplifier compares the input reference
voltage with a sample of the voltage from the filter circuit.
As the load increases, the output voltage drops. The error
amplifier senses this drop and causes the pulse-width modulator
to remain on for a longer period of time, delivering wider
control pulses to transistor switch 18.
~he width of the pulse determines how long the
transistor switch permits current to flow and therefore
how much current is yielded at the output. If the load
decreases, narrower control pulses are delivered to the
switching transistor until the output voltage remains at
; a constant value.
The SteP-Down Power Requlator:
Switch-type power regulators are available in three
basic configurations:
(1) A step-down or "buck" regulator
(2) A step-up or "boost" regulator
(3) an inverting regulator
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The present invention makes use of a step-down
power regulator whose operation will now be explained in
connection with Fig. 2. It will be seen that transistor
switch 18 is placed in series with inductor 19 between
the DC input Vl and the DC output V0, diode 21 being connected
to the input side of the inductor and capacitor 20 to the
output side thereof.
Transistor switch 18 in the buck circuit interrupts
the DC input voltage to supply a variable width pulse to
the simple averaging filter formed by inductor 21 and capacitor
2n. When switch 18 is closed, the input voltage is applied
across this filter and current flows through inductor 21
to the load. When switch 18 is open, the energy stored
in the field of the inductor maintains the current through
the load.
In this buck circuit, peak switching current is
proportional to the load current. The output voltage V0
is equal to the input voltage Vl times the duty cycle.
Hence the output voltage is always less than the input
voltage in the step-down switching regulator.
Comparison of Regulators:
- A linear series or shunt regulator, because it
functions in a continuous mode, will dissipate relatively
large amounts of power. Typically, the efficiency of a
linear regulator is less than 50%. And when the input-to-output
voltage differential is large, the resultant efficiency
then falls to well below 40~. In contradistinction, a
switching-type power regulator, which uses the on-off cycle
of a transistor switch to regulate power, has typical
efficiencies running well over 60%.
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There are three reasons why a switching-type regulator
achieves much higher efficiencies than a linear regulator.
First, since the power transistor switch is turned either
off or on, this results in either low current or low voltage
during most of its operation. S~cond, good regulation
is attainable over a wide range of input voltages, and
third, high efficiency can be achieved over wide ranges
in load current.
To further explain these distinctions, reference
is now made to Fig. 3 in which element 23 represents the
transmitter which is supplied with a voltage VT by battery
11 (24 volts) through a two-wire line Wl and W2 in series
with a load resistor R.
The power dissipated (PD) by transmitter 23 is
expressed by the following equations:
PD = 24 I - RI~ = V~I (Power Dissipated by Transmitter)
@ I = 4ma, PD = 96mW - R (16 uW/ohm)
= 0 _ R L 6K ohm
~ I = 20ma PD = 480 mW - R (400 uW/ohm)
= oL R~ 1.2K ohm
= VT = 24 -20 (10 3 ) R
- - For linear regulation the maximum power (PE) available
for the electronics in the transmitter is:
PE = 4ma- VT ~ 20ma
= 4ma [24 - 20 (10-3)-R ]
= 96 mW - (80 uW/ohm)~R
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For a switching-type regulator, the maximum power
(PE) available for the electronics in the transmitter is:
PE ~ PDMIN = ~ L96 MW -R (16 uW/ohm~;
where C~ is the efficiency of the switching regulator
PE = .75 ~96 mW - R (16 uW/ohm)~ @~ = 75%
In one practical embodiment, the required operating
voltage for the electronic circuits of the transmitter
included in the two-wire telemetering system is the +5Vdc.
As shown in Fig. 1, the current drawn from power supply
11 is represented as Ips, the current going through microprocessor
14 supplied with power by regulator 17 is represented as
IDC, the current passing through signal output transistor
16 is represented as ICTL~ and the 4 to 20 mA current passing
through resistor RFB is represented as IouT.
We shall now assume that instead of a switch-type
regulator, use is made in Fig. 1 of a linear regulator.
With linear regulation applied to this system, the major
c~nsideration is then that IDC, the current dissipated
~y the +5Vdc microprocessor 14 and the electronics associated
therewith is always equal to or slightly less than Ips,
the power supply current. In addition, since IpS + ICTL = IoUT
- (the transmitter output current which is in the 4 to 20
mA range), the intensity of IDC~ the curxent going through
the electronic circuits in the transmitter cannot be permitted
to exceed 4mA.
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Typical design values for the linear regulation
scheme are as follows:
IouT (minimum) = 3.8 ma ~ Input Conditions
V+/- (minimum) = 12.5 volts~
IDC (maximum~ - 3.8 ma @ +5vdc
Power available for control electronics = 19 MW
Maximum loop load resistance @ 24vdc and 20.8 ma
(24 - 12.5) = 553 ohms
20.8 ma
When however the system is in accordance with
the invention and includes a switching-type regulator 17
as shown in Fig. 1 to supply constant power to the electronic
circuitry of the transmitter the major consideration ls
that minimal power be dissipated by the regulator so that
the power supplied to the electronic circuitry is only
slightly less than the power supplied by the power supply.
As a consequence the current IDC consumed by the transmitter
electronics can be greater than the power supply current
Ips.
Typical design values for the switching regulator
scheme are as follows:
IoUT (minimum) = 3.8 ma
Regulator Efficiency (minimum) = 75~ - Input Conditions
Loop Resistance (maximum) = 550 ohms
V +/- (minimum~ = 6.5 volts
Power available for electronic circui~ry of transmitter:
a) IUT = 3.8 ma
DC = 75 24-[3.8ma-(550~ + RFB)] 3.8 ma = 12.27 ma
5vDC
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RFB = 100 ohms,,V ~ input of power supply = 21.53 volts
a) ~ IOUT = 20-8 ma
~C 75 ~24-[20.8ma (550n+ RFB)]~20.8ma = 32.70 ma
RFB = 100 ohms, V input of power supply =
10.48 volts
Thus a comparison of linear regulation with switching-type
power regulation in accordance with the invention leads
; to the following conclusions:
I. The switching-type regulator in the two-wire
telemetering system affords significantly more power for
the electronic circuits of the transmitter with 550 ohms
of load impedance powered by a 24Vdc power supply and operating
over a current range of 3.8 to 20.8 mA.
The linear regulator produces 19 MW of power, whereas
the switching-type regulator yields 61.35 MW, resulting
in a 222% increase in available power.
, II. The switching-type regulator is capable of
driving a significantly higher load resistance when the
two-wire telemetering system is powered by a 24 volt power
supply. A linear regulator with a minimum V+/- of 12.5V
, equals a 552 ohm load. A switching-type regulator with a minimum
' V+l/- of ~.5 V equals an 841 ohm load. This represents
, a 50~ increase in drive capability.
' While there has been shown and described a preferred
-'' 25 embodiment of a two-wire telemetering system including
power regulated transmitting device in accordance with
the invention, it will be appreciated that many changes
, and modifications may be made therein without, however,
departing from the essential spirit thereof.
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