Electrical transmission or interconnection systems – Plural supply circuits or sources – One source floats across or compensates for other source
Reexamination Certificate
2001-10-26
2004-08-24
Toatley, Jr., Gregory J. (Department: 2836)
Electrical transmission or interconnection systems
Plural supply circuits or sources
One source floats across or compensates for other source
Reexamination Certificate
active
06781256
ABSTRACT:
TECHNICAL FIELD
The present invention relates to controlling a load voltage for a field device in a way that has stable behavior under shut down circumstances.
BACKGROUND
A load voltage controller for a field device may be used in a manufacturing process to monitor the operation of the process and to actuate process variables of the process. Typically, actuators are placed in the manufacturing field to drive different process control elements, such as valves and sensors. Further, transmitters are placed in the manufacturing field to monitor process variables, such as fluid pressure, fluid temperature or fluid flow.
Such transmitters are coupled to a control loop and transmit process information over the control loop to a centralized controller which monitors the overall operation of the manufacturing process. The control loop may be implemented as a two-wire loop carrying a current that provides power for operation of the field devices.
In such control systems, communication is typically through a field bus standard, which is a digital communication standard with which transmitters may be coupled to only a single control loop to transmit sensed process variables to the central controller. Related communication standards are described in ISA 50.02-1992 section 11. Another standard is HART® which allows digital communication to be superimposed on a 4-20 mA process variable signal.
An important aspect with respect to control systems of the type outlined above is intrinsic safety. When field devices are located in a hazardous area without explosion-proof equipment, the electronics in the field device itself should be intrinsically safe. Intrinsic safety means that the electronics must be designed in a way that no sparks and no potential heat from the components may occur even if one or more electronic failures occur at the same time. Intrinsic safety is achieved through additional protection elements designed to protect the electronics under a fault condition. Depending on the specific type of application—e.g., the explosive type of gases used within a manufacturing process—there exist different requirements for a certain design, different specifications for the protection elements, as well as different certifications.
FIG. 1
shows further details of the field device being connected to a field bus. In particular,
FIG. 1
shows elements of a single field device
10
adapted to a field bus
12
.
As shown in
FIG. 1
, the field bus
12
may be represented by an equivalent circuit diagram with an ideal voltage source
14
and a resistor
16
to model AC voltage impedance and to fulfill intrinsic safety requirements for spark protection, current limitation and power limitation in the hazardous area.
As also shown in
FIG. 1
, the field device
10
is connected to the field bus
12
via a first wire
18
and a second wire
20
.
As also shown in
FIG. 1
, the field device
10
divides into a Graetz-diode-bridge
22
, a discharge protection diode
24
, a capacitor
26
, a DC/DC converter
28
and a load
30
. The load
30
is shown as a resistor and describes an actuator operating on, e.g., a valve used in the manufacturing process or any other control element, or a transmitter being adapted to measure manufacturing process variables as outlined above.
As also shown in
FIG. 1
, a connection between the field device
10
and the field bus
12
is achieved by connecting the nodes
32
and
34
of the bridge arm to the first wire
18
and the second wire
20
, respectively. Further, the node
36
connecting the anodes of the two upper diodes in the Graetz-diode-bridge
22
is connected to ground
38
while the node
40
connecting the cathodes of the two lower diodes of the Graetz-diode-bridge
22
is connected to the anode of the discharge protection diode
24
. The cathode of the discharge protection diode
24
is connected to the capacitor
26
which itself is connected across the input terminals of the DC/DC converter
28
.
Operatively, the Graetz-diode-bridge
22
in the field device
10
serves to avoid any polarity sensitivity with respect to the signal of the field bus
12
. Further, the Graetz-diode-bridge
22
is provided to support intrinsic safety together with the discharge protection diode
24
, e.g., when the discharge protection diode
24
fails. In other words, it is not necessary to consider polarity of the voltage on the field bus
12
when connecting the field device
10
to the field bus
12
.
Also, the discharge protection diode
24
is inserted to increase the intrinsic safety of the field device
10
through blocking the discharge of capacitors comprised in the field device in the event that a circuit element of the field device fails. In other words, discharge of the effective capacitance in the field device
10
into the control loop is inhibited by an isolation network consisting of the Graetz-diode-bridge
22
and the discharge protection diode
24
with three levels of redundancy.
Further, operatively the capacitor
26
connected to the cathode of the discharge protection diode
24
serves to stabilize the input voltage Ui to the DC/DC converter
28
. Therefore, if the input voltage Ui to the DC/DC converter
28
breaks down due to lack of energy supply from the field bus
12
, the capacitor
26
is discharged. For this reason, the supply of energy to the DC/DC converter
28
and therefore also the load
22
will be maintained over a certain time through discharge of the capacitor
26
until the voltage across the capacitor
26
is too low to drive the DC/DC converter
28
and therefore to drive the load
30
.
FIG. 2
shows further details of the field device
10
shown in FIG.
1
. In
FIG. 2
, the elements to the left side of the capacitor
26
are summarized into an equivalent current source
42
.
As shown in
FIG. 2
, the DC/DC converter
28
may be realized as a charge pump adapted to map the input voltage Ui, or equivalently the input current, to the DC/DC converter
28
into a suitable output voltage Uo, or equivalently the output current, over n stages.
Further, the circuit diagram shown in
FIG. 2
adds an operational amplifier
44
and a Zener diode
46
to the circuit elements shown in FIG.
1
.
The cathode of the Zener diode
46
is connected to the output of the DC/DC converter
28
while the anode of the Zener diode
46
is connected to ground.
Also, the positive supply of the operational amplifier
44
is the output voltage Uo of the DC/DC converter and the negative supply of the operational amplifier
44
is ground potential. The output terminal of the operational amplifier
44
is connected to the output node of the DC/DC converter
28
. Further, one input terminal of the operational amplifier
44
is also connected to the output of the DC/DC converter
28
while the other input terminal of the operational amplifier
44
receives an externally supplied reference voltage signal Uref.
Operatively, the operational amplifier
44
and the Zener diode
46
are alternatives to limit the output voltage Uo of the DC/DC converter
28
to either the voltage reference Uref of the operational amplifier
44
or to the Zener diode voltage Uz of the Zener diode
46
. When the Zener diode voltage Uz is higher than the voltage reference Uref of the operational amplifier
44
, the Zener diode
46
may operatively achieve protection against higher voltages for overheat protection of circuit elements if the operational amplifier fails. Also, the Zener diode
46
may prevent sparks when capacitors with higher voltages are short-circuited.
As outlined above, operatively the charge pump maps the input voltage Ui or equivalently the input current into a suitable output voltage Uo or equivalently output current over n stages. As is commonly known in the art, the behavior of an ideal charge pump may be described using the following model:
n: numbers of stages
Input side
Output side
Ui:
input voltage
Uo:
output voltage
Ii:
input current
Io:
output current
Pi:
input power
Po:
output power
Ui = n*Uo Ii = Io
Pi = Po
(Eq. 1)
Fro
Fish & Richardson P.C.
Invensys Systems Inc.
Toatley , Jr. Gregory J.
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