Field device power supply failure detection

Details Field device power supply failure detection Field device power supply failure detection

2001-10-26

2004-08-24

Sircus, Brian (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific voltage responsive fault sensor

Reexamination Certificate

active

06781807

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Under 35 U.S.C. 119, this application claims the benefit of a foreign application filed in EUROPE, and having Serial No. 0012342.8, filed Oct. 27, 2000, which is incorporated by reference in its entirety.
TECHNICAL FIELD
The present invention relates to a field device for monitoring a manufacturing process and for actuating manufacturing process variables, and being able to detect a power supply failure.
BACKGROUND
In general, field devices are used in a manufacturing process to monitor the process and to actuate process variables. Typically, actuators are placed in the manufacturing field to drive different process control elements, such as valves and sensors. Further, transmitters are installed in the manufacturing field to monitor process variables, such as fluid pressure, fluid temperature or fluid flow.
Actuators and transmitters are coupled to a control bus to receive and transmit process information to a centralized controller that monitors the overall operation of the manufacturing process. This control bus may be implemented as a two wire loop carrying a current that provides a power supply for operation of the field devices.
In such control systems, communication is typically executed through a field bus standard, which is a digital communication standard according to which transmitters may be coupled to only a single control bus to transmit sensed process variables to the central controller. Examples for communication standards include ISA 50.02-1992 section 11 and HART®, which overlays communication on a 4-20 mA processor variable signal.
Intrinsic safety is an important aspect of those control systems. When a field device is located in a hazardous area without explosion proof equipment, the electronics in the field device should be intrinsically safe. In general, intrinsic safety means that the electronics must be designed in a way that no sparks and no heat are generated thereby even when one or more electronic component failures occur at the same time.
Usually, intrinsic safety is achieved by having additional protective elements protect the electronics under a failure condition. Design specifications and certifications for the protective elements vary depending on the specific type of application (e.g., the type of explosive gas used within a manufacturing process).
FIG. 1
shows a peripheral part of a manufacturing process control system. As shown in
FIG. 1
, the peripheral part of the control system may comprise a first bus segment
10
of the intrinsic safe type and a second bus segment
12
using, e.g., the RS485 standard for data communication. The intrinsically safe field bus segment
10
and the RS485 bus segment
12
are coupled through a bus coupler
14
. Further, the side of the intrinsically safe field bus segment
10
not attached to the bus coupler
14
is connected to a terminating circuit
16
that helps to avoid reflections on the intrinsically safe field bus segment
10
.
As also shown in
FIG. 1
, to each bus segment
10
,
12
there is connected at least one field device
18
,
20
, and
22
. Each field device is either an actuator, a transmitter or another I/O device receiving/transmitting information. The field devices
20
,
22
attached to the intrinsically safe field bus segment
10
may be powered through an electric current received from the intrinsically safe field bus segment
10
leading to a voltage drop across the field devices
20
,
22
. Typically, the intrinsically safe field bus segment
10
will be operated under a field bus protocol or any other appropriate protocol that allows for exchange of digital information.
As shown in
FIG. 1
, the field devices
20
,
22
coupled to the intrinsically safe field bus segment
10
exchange information through modification of the current flowing into each field device
20
,
22
. For digital communication a basic value of the current of the intrinsically safe field bus segment
10
is modulated to be increased or decreased by predetermined offset value, such as 9 mA for the field bus standard. This modulation of the current flowing into either the field device
20
or the field device
22
leads to a modification of the voltage UB on the intrinsically safe field bus segment
10
, with the voltage modification being used to provide digital communication.
FIG. 2
shows a more detailed schematic circuit diagram of the field devices shown in FIG.
1
. As shown in
FIG. 2
, the intrinsically safe field bus segment
10
may be summarized into an equivalent circuit diagram with an ideal voltage source
24
and a resistor
26
to model AC voltage impedance and to fulfill intrinsic safety requirements for spark protection, current limitation, and power limitation in an hazardous area.
As also shown in
FIG. 2
, each field device is connected to the intrinsically safe field bus segment
10
with two wires
28
,
30
also being connected to a discharge protection unit
32
. At the output of the discharge protection unit
32
there is provided a modulating unit
34
which allows modulation of the operating current flowing into the field device.
The modulating unit
34
is connected in series with a power conversion unit
36
that is adapted to map the operating current flowing over the modulating unit
34
into a suitable power supply signal for a controller unit
38
connected to the output of the power conversion unit
36
and an actuator/sensor unit
40
being controlled by the controller unit
38
.
As also shown in
FIG. 2
, the controller unit
38
is divided into a master controller and a communication controller
44
. While the communication controller
44
controls the operating current modulating unit
34
to achieve a modulation of the operating current and therefore exchange of information between the intrinsically safe field bus segment
10
and the field device, the main control of the field device is carried out by the master controller
42
.
Therefore, the master controller
42
not only controls the communication controller
44
but also controls either actuators
46
,
48
or a sensor
50
in the actuator/sensor
40
. For each actuator
46
,
48
there is provided a dedicated digital/analog converter unit
52
,
54
, while for the sensor
50
there is provided an analog/digital converter
56
.
FIG. 3
shows a more detailed schematic circuit diagram of the power converter unit
36
shown in FIG.
2
. As shown in
FIG. 3
, the power conversion unit
36
comprises a capacitor
58
connected across the input terminals of a DC/DC converter
60
. Operatively, the capacitor
58
achieves a stabilization of the input voltage Ui to the DC/DC converter
60
. The output voltage Uo of the DC/DC converter
60
is then forwarded to the subsequent controller unit
38
.
Operatively, each field device
20
,
22
connected to the intrinsically safe field bus segment
10
receives an operating current from the intrinsically safe field bus segment
10
. When sending information from the field device to the intrinsically safe field bus segment
10
, the current value for the operating current is determined through the modulating unit
34
under control of the communication controller
44
. In other words, according to the control signal supplied from the communication controller
44
to the modulating unit
34
, the operating current supplied to the field device and thus also the voltage of the intrinsically safe field bus segment
10
varies to achieve digital communication.
Further, to receive information in the field device the communication controller
44
maintains the resistance value of the modulating unit
34
constant. Therefore, in case a different field device triggers a change of the voltage on the intrinsically safe field bus segment
10
the remaining field device(s) connected to this intrinsically safe field bus segment
10
may detect this change of the voltage via the connection lines
28
,
30
for further processing thereof in the control unit
38
. This digital communication mechanism is used to provide the master controller
42
in each fie

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