Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
1999-01-22
2001-01-30
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S723000
Reexamination Certificate
active
06181141
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for monitoring in a fail-safe manner potentiometers used in critical control operations. More particularly, the invention relates to monitoring in a fail safe manner the characteristics of potentiometers where the detected control value from a control potentiometer is a critical safety factor in a control system.
2. Description of Prior Art
Determining the absolute position of valves and/or dampers in combustion systems is a critical safety issue. A system such as a non-flow compensating fuel-air ratio control system controlling a large boiler and using potentiometer settings to control the position of valves and dampers is a system where monitoring of the control potentiometers is a critical life safety factor. Control of actuators for a valve and/or damper in such a combustion system in the past has been accomplished with a motor driven mechanical cam. Such a device has mechanical limits to provide for fail-safe control of the valves and dampers. Due to the mechanical linkages and the limits placed on the mechanical linkages, the fuel-air ratios were never permitted to enter a hazardous range of operation.
In today's environment with microprocessor control and electronic sensing of desired position and electronic feedback to control valves, an electronic failure of a control potentiometer can create a fuel-air ratio in a combustion system that is hazardous. Furthermore, because of the harsh operating environment and because of aging, a control potentiometer's resistance may vary. Also, the resistance at the potentiometer's connector terminals may vary, and the wiper resistance on the potentiometer may change. Any one of these changes or all these changes together can create a situation where an incorrect control signal is provided by the potentiometer monitoring circuit to a controlling microprocessor that controls critical elements such as fuel-air valves. What is needed is a fail-safe monitoring system for potentiometers.
SUMMARY OF THE INVENTION
In accordance with this invention the above problems with monitoring control potentiometers have been solved by using a computer processor and a monitor interface circuit connected between the potentiometer and the processor, and then monitoring through the monitor interface circuit the potentiometer, its terminals and its wiper for faults. In addition the monitor interface circuit itself is tested so that a fault in the potentiometer or a fault in the monitor interface circuit is detected to prevent the control potentiometer and monitor interface circuit from producing an incorrect control signal.
In one feature of the invention the monitoring is accomplished by measuring changes in the fill potentiometer resistance from a predetermined value. The changes are compared against boundary limits to detect whether a change is inside or outside the boundary limits. A fault in the control potentiometer is indicated when a change is outside the boundary limits.
In another feature of the invention the monitoring also monitors wiper resistance of the potentiometer. The wiper resistance of the control potentiometer is measured, and the wiper resistance is compared against a fault threshold to detect if the wiper resistance has passed the fault threshold. A fault in the wiper of the control potentiometer is indicated when the wiper resistance has passed the threshold.
In another feature of the invention, the testing of the monitor interface circuit is accomplished by measuring idle signals applied to the full potentiometer and to the wiper when the monitor interface circuit is an idle state. The idle signals are compared against an expected signal to detect if the idle signals are within a predetermined tolerance range of the expected signal. A fault in the monitor interface circuit is indicated if the idle signals are outside the predetermined tolerance range.
Other features, utilities and advantages of the invention will be understood by those of ordinary skill in the art after referring to the complete written description of the preferred embodiments of the invention in conjunction with the following drawings.
REFERENCES:
patent: 3852665 (1974-12-01), Bothner
patent: 4866363 (1989-09-01), Patton et al.
patent: 4987372 (1991-01-01), Tengorang et al.
patent: 5107427 (1992-04-01), Peter et al.
patent: 5289134 (1994-02-01), Kohr
patent: 5812411 (1998-09-01), Calabrese et al.
Bartels James I.
Juntunen Robert Dean
Deb Anjan K
Honeywell Inc.
Merchant & Gould P.C.
Metjahic Safet
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