Data processing: generic control systems or specific application – Specific application – apparatus or process – Mechanical control system
Reexamination Certificate
2000-03-31
2003-02-11
Picard, Leo (Department: 2126)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Mechanical control system
C137S487500, C251S129040
Reexamination Certificate
active
06519508
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a valve positioner using digital communication; and more particularly, to an improvement thereof, wherein the current to be allocated to a current-to-pneumatic conversion module can be increased; and wherein, the invention can be applied to convert electrical signals to pneumatic signals.
2. Description of the Prior Art
A valve positioner directly controls the opening of a valve and its feedback signal uses a valve opening signal or a stem position signal. A current-to-pneumatic converter converts an electrical signal, such as, for example, 4 to 20 mA, into a pneumatic signal such as 0.2 to 1.0 [kgf/cm
2
]. An example of a prior valve positioner is disclosed in Japan Unexamined application 9/144,703.
FIG. 1
shows a conventional valve positioner
100
, wherein an operating signal for valve positioner
100
, using an electrical signal, such as for example, 4 to 20 mA, is inputted to terminals T
1
and T
2
. Variable impedance circuit
3
and shunt regulator
4
, connected in series, are connected to input terminals T
1
and T
2
. Internal power voltage V
2
, which drives the internal circuits of the valve positioner
100
, is generated on/the positive side of shunt regulator
4
. The shunt regulator
4
may comprise one or more Zener diodes, integrated circuits, or combinations thereof with their peripheral elements.
Impedance control circuit
1
is connected to input terminals T
1
and T
2
and operates to adjust the impedance of variable impedance circuit
3
to control the voltage between input terminals T
1
and T
2
normally to an approximately constant voltage of 12V or less. The operation maintains the impedance between input terminals T
1
and T
2
in a low state in the DC region of the operating signal. The variable impedance circuit
3
may comprise npn transistors, pnp transistors, or field effect transistors (FET).
DC—DC converter
5
, connected in parallel to shunt regulator
4
, is used to increase the current capacity by stepping down internal power voltage V
2
supplied by shunt regulator
4
. Thus, DC—DC converter
5
supplies operating voltage V
3
to current-to-pneumatic conversion module (called “E/P module”)
14
which consumes high power and micro-controller
9
. Since the valve positioner
100
must be operated so that its minimum operating current is 4 mA at most and normally is 3.6 mA or less because of the limitation of the input signal current, the desired current capacity is achieved by using DC—DC converter
5
. The DC—DC converter
5
may comprise a voltage stepping down DC—DC converter, such as a charge pump type or a switching regulator type.
Current detecting or sensing element
2
and current detector
7
detect a current signal inputted to input terminals T
1
and T
2
and the detected signal is set to A/D converter (ADC)
8
. The current detecting element
2
is a resistor and the current detector
7
is an amplifier using an operational amplifier.
Transmit-and-receive circuits
6
receive a request signal, sent from a corresponding instrument (not shown) and transmit a response signal to the corresponding instrument via digital communication. In this case, the corresponding instrument is connected to input terminals T
1
and T
2
via a two wire transmission line.
Micro-controller
9
, which carries out digital communication with and position control to valve
16
, comprises a microprocessor and peripheral circuits, such as a memory, and stores communication processing programs, such as request signals, and response signals, and control programs, such as PID control and fuzzy control. Digital to analog converter (DAC)
10
converts a digital control output signal of the micro-controller
9
to an analog signal. Driver
13
carries out amplification and impedance conversion of the analog signal, sent from DAC
10
, and transmits the resulting signal to E/P module
14
. Sensor interface
11
processes the signal from the position sensor
12
and sends the resulting signal to analog to digital converter (ADC)
8
. ADC
8
digitizes the input current signal, sent from current detector
7
, and the position signal, from valve
16
, and transmits the digitized results to micro-controller
9
.
The pneumatic system operates as follows. E/P module
14
converts the input drive current to a corresponding pneumatic signal and, for example, controls the air pressure of a nozzle using a torque motor. Control relay
15
amplifies the pneumatic signal and thus, for example, drives valve
16
to be in an open or closed state using the pneumatic signal of 0.2 to 1.0 [kgf/cm
2
]. Since the opening of valve
16
is correlated to changes of its stem position, the stem position is detected by position sensor
12
.
In the
FIG. 1
system, digital communication is provided between the corresponding instrument and the valve positioner by superimposing digital signals according to a predetermined protocol on a two wire transmission line that sends and receives operating signals, such as of 4 to 20 mA value. In addition, for implementing digital communication with the corresponding instrument, it is necessary to keep the impedance between the input terminals T
1
and T
2
at a definite high value in a communication frequency band in order to generate digital communication signals sent from the corresponding instrument between terminals T
1
and T
2
. Accordingly, impedance control circuit
1
controls the impedance of variable impedance circuit
3
to high values of, for example, 230 ohms to 1100 ohms in the communication band.
Valve position control is provided as follows. A position signal of position sensor
12
is sent to micro-controller
9
via sensor interface
11
and ADC
8
, is subjected to control computation in micro-controller
9
and a resulting control output signal is sent to drive circuit
13
via DAC
10
. Valve opening is controlled to a target value by driving valve
16
via the signal route of drive circuit
13
→E/P module
14
→control relay
15
→valve
16
.
Typical operating specifications are as follows. Minimum operating voltage between terminals: 12 V DC (between input terminals T
1
and T
2
). Minimum operating current: 3.6 mA. That is, the digital communication function and valve position control must function within the range of 4 mA supplied to the input terminals T
1
and T
2
. On the other hand, in the case of using a microprocessor for the micro-controller
9
, even though power consumption of electronic devices is decreasing due to energy saving techniques, the current consumption for E/P modules
14
is still limited in efficiency as compared with circuits that do not use a microprocessor. However, since most E/P modules
14
are current operated devices, a problem exists in the prior art in that decreasing the current allocation to the E/P module worsens the valve response or eliminates the stability margin due to disturbances such as due to temperature.
In the microprocessor itself, the control cycle for control computation must be shortened by increasing the clock frequency to obtain stability in valve control. However, disadvantageously, another problem arises, in that current consumption in the microprocessor itself increases when the clock frequency is increased.
Hence, in order to effectively utilize the power provided to a valve positioner as an operating signal, a technique has been tried to achieve a supply current to internal circuits,including E/P modules
14
, using DC—DC converters
5
, which step down the power voltage, such as shown in FIG.
1
. To realize such DC—DC converter
5
, a charge pump type, using a capacitor or voltage stepping down switching regulator using an inductance, has been considered. However, such methods all have a further problem in that the manufacturing cost thereof increases because of the necessity to increase mounting surfaces and/or the number of components. Furthermore, disadvantageously, if the voltage stepping down switching regulator is used, adverse effects on other circuits d
Cabrera Zoila
Kojima Moonray
Picard Leo
Yokogawa Electric Corporation
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