The present invention relates generally to a system which senses electric motor current and/or switch activation signals when testing and monitoring the condition of an actuator.
Related U.S. Pat. No. 4,805,451 (hereinafter the "'451 Patent") discloses a system for evaluating the condition and performance of a motor operated valve ("MOV") comprising a valve and valve actuator or operator combination.
The system of the '451 Patent was primarily intended to be used with MOV's located in nuclear power and processing plants. The valve operator of such MOV's typically comprise an electric motor and a gear linkage between the motor and a stem of the valve. Each MOV further typically include an actuator control circuit including a plurality of torque and limit control switches monitoring and controlling the operation of the electric motor of the actuator through a main power switch. The main power switch, at least in most nuclear plant installations, is located remotely from the MOV, either at a separate location commonly referred to as the motor control center (MCC) or in the control room of the plant. The control switches may be located at the MOV in a housing, commonly referred to as a valve control box, or remotely from the MOV at the motor control center or possibly the plant control room.
The system of the '451 Patent was designed to overcome problems of older MOV monitoring systems by indirectly measuring valve stem forces through a sensor which could be permanently secured to a yoke coupling the valve with the operator. It further provided a system for evaluating the condition and performance of the valve and the valve operator, and for identifying malfunctions or other problems by using the valve stem force measurements obtained indirectly from the valve yoke strain measurements in conjunction with other measurements. These included valve stem motion, motor current, spring pack motion and torque and limit switch indications. In addition to diagnosing existing valve and valve operator problems, the system provided a method for trending changes in overall valve and valve operator conditions in order to predict future valve or valve operator problems.
The system disclosed in the aforesaid '451 Patent optionally included a motor current sensor for detecting the flow of current to the electric motor and generating electric signals proportional to the motor current. It further optionally included torque and limit switch sensors for detecting the state of torque and limit switches provided with the actuator and for generating electrical signals representative of the states of those switches. When provided, these sensor were coupled with an analysis section including a digital computer and associated peripheral hardware. The computer utilized information obtained from the yoke strain sensor in combination with the motor current sensor to permit diagnosis of various motor and operator problems. By combining information obtained from the yoke strain sensor with information obtained from the torque and limit switch indicator means through the computer, one could diagnose various switch problems.
In order to sense the motor current and switch operations in the manner disclosed in the aforesaid '451 Patent, current sensors were temporarily clamped to the outside of selected motor power and control switch leads. Clamp-on sensors of the type noted were found to nip the wire insulation on occasion and, in some cases, were impossible to position on the lead because the lead was too tightly laced with other leads and/or components. In some instances, the clamp-on sensors did not totally close around the lead and did not yield accurate data. Alternatively, temporary clip leads were placed on terminal strips provided extending from each of the pertinent leads. The clip leads were not permanently or positively mounted and tended to slip off the terminal strips when bumped or vibrated during a test of the valve and valve operator. On occasion during such temporary installation of sensors or clips, they could be placed on the wrong leads.
In addition to the foregoing, each time a test had to be performed on the valve or valve operator, the valve control box and/or the motor control center enclosure had to be opened so that the temporary sensor connections could be made. In nuclear power plants, where the system was initially intended for operation, Federal regulations mandated that extensive reports be prepared and maintained each time either type of enclosure was opened. Literally hundreds of such valves and valve operators typically are located in such plants, many in primary and secondary containment areas where workers would be exposed to radiation from the concentrated radioactive material fueling the reactor while installing and removing the sensors.
The present invention overcomes the problems which arose with the original system using temporarily mounted sensors.
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