Fluid handling – With indicator – register – recorder – alarm or inspection means – Position or extent of motion indicator
Reexamination Certificate
2001-01-04
2001-11-13
Chambers, A. Michael (Department: 3753)
Fluid handling
With indicator, register, recorder, alarm or inspection means
Position or extent of motion indicator
C137S552000, C324S207150
Reexamination Certificate
active
06314994
ABSTRACT:
This invention relates to devices for verifying the correct operation of a valve positioner in a process control system, and in particular, to a signal transmitter having adjustable limit switches that move in response to movement of a valve positioner.
BACKGROUND
In a feedback control system, a controller obtains the value of a controlled variable, compares that value with a setpoint, and adjusts the value of a manipulated variable in order to drive the controlled variable toward the setpoint. In the context of a process control system, adjustment of the manipulated variable generally involves adjusting a valve. For example, if the controlled variable is the level of fluid in a tank having an intake valve and an outlet valve, the manipulated variable can be the volume rate of flow into or out of the tank. Both of these variables are ultimately manipulated by adjusting the position of a valve. A valve for controlling the flow of fluid is thus a critical component in the control of a processing plant.
To control a valve, the controller sends a signal to a positioner, which is a mechanical device intimately associated with the valve that moves in response to the signal. When the positioner moves, it changes the position of the valve and hence, the value of the manipulated variable controlled by that valve. This change in the manipulated variable results in a corresponding change in the controlled variable. The controller then measures the value of the controlled variable and, if necessary, sends another signal to the positioner to correct the value of the manipulated variable. This process of measurement, followed by correction on the basis of the measurement, is at the heart of a feedback control system.
Unfortunately, it is possible for the controller to send the positioner a signal and for the positioner to do nothing, to move an incorrect amount, or, in the worst case, to move in the wrong direction. The failure of a positioner can, of course, be detected by measuring the value of the controlled variable and observing whether that value is inconsistent with the expected value of the manipulated variable. However, in many processes, there may be significant lag time or dead time. In such processes, it may be some time before the controller realizes that the controlled variable is not changing as expected. During this lag time, significant damage may occur. For example, if the valve controls the flow of coolant in a nuclear power plant, by the time the temperature of the coolant rises, the core temperature may already be dangerously high.
It is therefore desirable to detect the failure of a positioner as soon as possible. Because the positioner is typically hidden from view, this is most readily accomplished by having the positioner transmit a signal verifying that it has, indeed, moved to the location specified by the controller. This generally requires a signal transmitter mechanically coupled to the positioner such that when the positioner is in the desired position, an electrical signal is transmitted to the controller, to an alarm panel, or to some other appropriate location. In a typical signal transmitter of this type, a protruding signal flag coupled to the positioner moves into engagement with an electromagnetic switch when the positioner reaches a desired position.
A disadvantage of known signal transmitters is the difficulty encountered in adjusting the location of the signal flag to accommodate variations in valve positioners. In known signal transmitters, adjustment of the flag location generally requires access to the top and sides of the signal transmitter. In addition, when the flags are loosened for adjustment, they move relatively freely and are therefore difficult to adjust independently of each other with precision.
Because of the difficulty in adjusting the signal flags with precision, the mechanical motion of the positioner needs to be amplified so that small errors in positioning the flags do not result in large errors in the perceived position of the valve. This, in turn, requires that a system of gears having a gear ratio selected to amplify the mechanical motion of the positioner be interposed between the positioner itself and the signal transmitter. This gear system provides yet another source of possible failure, adds to the cost of the signal transmitter, and, because the mechanical resolution of the system is limited by the spacing between the gear teeth, decreases the overall resolution of the signal transmitter.
An additional disadvantage of known signal transmitters is that the signal flags are mounted in a manner susceptible to vibration. Exposure to such vibrations can eventually cause the signal flags to become misaligned. As a result, such signal transmitters require frequent maintenance.
A position indicating apparatus according to the preamble of claim
1
is known from GB 2 265 204 A.
It is thus an object of the invention to provide a signal transmitter in which the signal flags can be adjusted independently of each other with sufficient precision to eliminate the need for an amplifying gear between the positioner and the signal transmitter.
It is a further object of the invention to significantly reduce the sensitivity of the signal flags to vibrations.
SUMMARY
These objects are achieved by an apparatus according to claim
1
.
Further developments of the invention are given in the dependent claims.
A signal transmitter incorporating includes a signal flag mounted to a shaft that rotates in azimuth in a manner indicative of the valve position. The signal flag is mounted in either an operating state, in which the signal flag rotates only when the shaft rotates and a calibration state in which the signal flag can be rotated independently of the shaft and any other signal flags mounted thereon.
The signal flag has a switch-engaging portion, which engages a switch when the signal flag is rotated to a selected azimuth angle, thereby causing the switch to generate a signal indicative of the azimuth angle of the switch-engaging portion. This azimuth angle is, in turn, indicative of a particular valve position
In the preferred embodiment, the signal flag is an annular disk that is coaxial with the shaft and held between first and second surfaces by a variable compressive force. The annular disk has an inner rim with teeth adapted to engage a gear formed thereon and an outer rim having a protrusion extending radially outward from the switch-engaging portion.
To specify what valve position is to be associated with a particular azimuth angle, an apparatus embodying the invention includes a signal-flag adjuster fixedly mounted to the shaft and coupled to the signal flag. When the signal flag is mounted in its calibration state, the signal-flag adjuster selectively rotates the signal flag independently of the rotation of the shaft and independently of the rotation of any other signal flags mounted to the shaft.
In the preferred embodiment, the signal-flag adjuster includes a rotatable gear for engaging the teeth on the inner rim of the annular disk forming the signal flag. When the signal flag is mounted in its calibration state, rotation of this gear rotates the switching engaging portion of the signal flag independently of the shaft. Typically, the rotating gear is a radially extending portion of the shank of a screw extending parallel to the shaft axis and having a screw head accessible from the outside. Maintenance personnel can therefore perform the necessary calibration without the need to significantly dismantle the signal transmitter.
The diameter of the gear extending from the screw is typically smaller than the inner diameter of the annular disk on which the gear teeth are formed. Consequently, a full rotation of the screw (and hence of the gear) results in only a small change in the azimuth angle of the signal engaging portion of the signal flag. This allows the azimuth angle of the signal flag to be adjusted with great precision.
To switch between the calibration state and the operating state, an apparatus embodying the invention include
Chambers A. Michael
Foley Hoag & Eliot LLP
Liepmann W. Hugo
Oliver Kevin A.
The Foxboro Company
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