Abstract:
A system that is adapted for determining the delay time between an initiating event and a response to that event. The system includes an optical sensor which is preferably a photo resistor having a resistance inversely proportional to the intensity of a warning light in its proximity. The output of the photo resistor is an analog signal that can be fed to an analog input device such as a strip chart recorder for display versus time. The initiating event signal can also be displayed so that, by comparing the signals, the delay between the initiating event and the changing of the warning light from off to on can be measured and compared to a specified value.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to measurement of time delays between an events and a response to the event. In particular, the present invention relates to the detection of the response of a system to an initiating event. 
     BACKGROUND OF THE INVENTION 
     Throughout industry, processes depend on a sequence of events where one action is taken in response to an initiating event: a tank fills, a valve closes once the tank fills, an agitator begins to agitate when the tank valve is closed. Sometimes the action is in response to an unexpected event that has safety consequences. For example, at a nuclear power plant, if a control rod drops into the nuclear core by accident, it may adversely affect the power distribution in other areas of the core. The response might include notification of the operator and a check on the power to the control rod actuator, for example. 
     Especially under circumstances where a response needs to occur within a short time after an initiating event for safety reasons, it is important to know that a response will in fact occur and will occur within a prescribed time. Because it is important, these systems may be audited to see if they respond as they are designed to do. Their response time is measured against a simulated initiating event and compared to the specified response time. 
     At nuclear power plants, some responses must take place very quickly after initiating events in order to prevent damage to equipment or perhaps the release of radioactive material. Measuring short time intervals to verify that response times are within specifications is a significant problem. 
     See for example, the patents issued to Palusamy et al., U.S. Pat. No. 4,908,775; Miller et al., U.S. Pat. No. 4,752,869; Sawyer et al., U.S. Pat. No. 4,640,812; and Dennis et al., U.S. Pat. No. 4,517,154, for examples of patents that relate generally to measurement of responses of systems at nuclear power plants. 
     Nonetheless there remains a need for a simple system that will measure when a response occurs following an initiating event, particularly, when the response is the lighting of a warning light to alert an operator that an initiating event has occurred. 
     SUMMARY OF THE INVENTION 
     The invention is an apparatus for determining the interval of time between an initiating event and a visual response to that event. Typically, the present apparatus is useful when verifying that short and consistent intervals are being obtained between the initiating event and response events, such as in the response system at nuclear power plants. These systems are to be tested periodically to see if the response times are within specification. The present invention makes testing much easier and more accurate. 
     In a preferred embodiment, the present apparatus includes an optical sensor mounted in close proximity to a warning light and connected electrically to electrical circuitry that processes the electrical output of the sensor so that an analog signal results. The analog signal can be forwarded to an output device to display changes in the signal that result from changes in the intensity of the warning light, i.e., the light has gone on or off. If the signal that caused the warning light to go on is simultaneously sent to the same output device, then a comparison of the times when the two signals (initiating and response) are received reveals the response time. 
     An important feature of the present invention is that the sensor can be mounted easily near the warning signal light. No electrical connection need be made with the light and its distance from the warning light need only be such that a direct exposure is available and the interference from other sources of light be relatively less than the visual signal from the warning light. 
     Another important feature of the present invention is the use of an optical signal from the optical sensor. Use of an optical signal avoids the need to calibrate the signal to determine on and off intensities of different lamps and different levels of ambient light. 
     Still another feature of the invention is the use of an optical sensor rather than other types where the signal speed between the warning lamp and sensor might be significant with respect to the response time. With optical sensors, the signal speed is virtually instantaneous. 
     Other features and their advantages will be apparent to those skilled in the art of instrumentation from a careful reading of the Detailed Description of Preferred Embodiment, accompanied by the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, 
     FIG. 1 is a schematic illustration of an apparatus according to a preferred embodiment of the present invention, and its relationship to the response system at a facility; 
     FIG. 2 is a schematic illustration of a detail of the present apparatus according to a preferred embodiment of the present invention; and 
     FIG. 3 is a schematic illustration of a detail of the present apparatus according to an alternative preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is an apparatus for measuring the response time of a response system to an initiating event. It will be illustrated in the context of a nuclear power plant but it will be clear that the invention is not limited to that context but may be readily adapted to other settings. 
     Essentially, the present apparatus is installed temporarily or permanently in the setting where it will be used occasionally to verify that the response time for a safety or other system is within specifications. The context therefore must include a response system  10  that responds to an initiating event  12  by causing a warning light  14  to light. The response system is not part of the present apparatus, which is generally indicated by reference number  20  directed to a collection of components tied electrically and optically with each other and to response system  10 . 
     The present invention therefore includes an event signal simulator  22  that generates an event signal simulating the initiating event, a sensor  24  that detects a change in the intensity of warning light  14  which responds to the initiating event, and to an audit system  26  that receives a signal from sensor  24  and from simulator  22  and allows their display and analysis. 
     FIG. 2 illustrates schematically the present invention in more detail. Warning light  14  changes in intensity in response to a signal from response system  10 . The light from warning light  14  reaches optical sensor  24 , which is preferably a photo resistor  30 . Photo resistor  30  is preferably a cadmium sulfide photo resistor and is connected to ground on a first side  32  and on a second side  34  to a pull up resistor  36 . The output from second side  34  is an analog signal that is inversely proportional to the intensity of the light from warning light  14 . Pull up resistor  36  divides the voltage in accordance with the resistance of resistor  36  so that a low voltage signal is output from photo resistor  30  at second side  34 . The output from second side  34  is fed into an analog input such as the input of a strip chart  38 . This arrangement reduces the impact of electrical noise that otherwise affects the operation of photo resistor  30 . Also, the use of an analog signal rather than a digital signal allows a programmed computer to calibrate the data to compensate it for different lamp intensities and ambient light conditions. 
     Alternatively, as illustrated schematically in FIG. 3, the output from second side  34  can be converted to a digital signal by an analog-to-digital converter  40  and displayed by a computer  42 , for example. 
     The junction voltage is recorded by strip chart  38  as an indication of the status of warning light  14 . Any change in lamp intensity is recorded and can be later interpreted. 
     This apparatus is noninvasive, easy to install and remove, and allows for highly accurate high speed detection of the status of warning light  14 . It is useful for response time testing, and the status of warning light  14 , which indicates whether warning light  14  is on or off, would be recorded so that the initiating event, or “trip”, and its corresponding reset points can be easily determined by seeing what the voltage on strip chart recorder  38  was when warning light  14  changed state from “off” to “on”. This application would allow trip checks to be done in a matter of seconds rather than the current 10-15 minutes, which vastly reduces the time a system is undergoing testing. 
     The raw data carried by the signals from optical sensor  24  may be forwarded to either strip chart recorder  38  or computer  42  where they are time tagged, loaded into a database and analyzed to determine a change in the signal and the corresponding time of the change. Sophisticated spreadsheet software aided by macro files can be used to record the data and determine when the “on” and the “off” occurs. A precision timing card in the computer generates a timing signal that is recorded in a database along with data from each sensor. The data in the database can be analyzed and plotted to show the response time for each signal, and perhaps compare each response time to a prescribed maximum response time to verify compliance with standards or specifications. 
     The apparatus can be activated manually by component  22  using a hand switch or automatically in response to a digital signal from, say, a relay opening or closing. That signal is simultaneously fed to each recorder  38  or computer  42 . Upon receipt of the initiating signal, all the recorders  38  or computers  42  begin collecting raw data from optical sensors  24  connected to each of them and associating that signal with a time. The data from each sensor is maintained by its recorder  38  or computer  42  as a separate chart or as a file that can be off-loaded via floppy disk or direct cable connection to a central computer. 
     A timing program in a timer  50  produces a 1/1000 of a second time tick by counting backwards from an arbitrary but high maximum count, preferably 10,000 minutes. Timer  50  itself is composed of cascaded registers: one for dividing a time card&#39;s 10 MHz clock signal to 1/1000 of a second, another register keeps track of 60,000 one thousandths or one minutes worth of “ticks”. The third register keeps track of the minutes. 
     When the initiating signal is received by strip chart  38  or computer  42 , it begins to record the incoming analog or digital input from optical sensor  24  to a datafile along with a time from timer  50  and then waits until either (1) the timer shows 1/1000th of a second has elapsed or (2) a stop signal has been received. If the timer shows 1/1000th of a second has elapsed, the data are recorded, and again they are time tagged. If a stop signal has been received, the process stops. The macros review the accumulated data to determine when a change in data values is indicative of a response event and compares the time associated with that data entry with the time of the initiating event to determine the response time. 
     Timer  50  plus strip chart recorder  38  are part of the audit system  26  of FIG. 1, or, alternatively, timer  50  plus analog-to-digital converter  40  and computer  42 , for an alternative audit system  26 ′. 
     If there are dozens of optical sensors  24  monitoring dozens of warning lights  14 , some of which are keyed to the responses of other devices, the recorders  38  or computers  42  can sort out all the response times and display them for convenient comparison with standards or specifications and determine if any do not meet standards or are out of specification. If the sensors are coupled to 10 different recorders or computers and they monitor responses to a single initiating signal—such as the tripping of a relay or the throwing of a manual switch—the computers and recorders can all receive the initiating signal by telephone wire and thus share a common starting point for recording elapsed time. 
     The apparatus includes one or more strip chart recorders  38  or computers  42 , optical sensors  24  and wiring to connect the sensors to the recorders or computers. If more than one recorder  38  or computer  42  is used, the additional ones may be in different locations and connected via telephone wire (such as a RJ114 conductor wire or dedicated telephone line). An optical sensor  24  or more than one optical sensor may be mounted to a light, and send a signal to its local computer for collection and analysis. 
     In protective systems at, for example, nuclear power plants, indicating lights are used to indicate the state of the protective system bi-stables. The present system can record the time at which the input signal is given for a particular channel and the response time of all bi-stables connected to that protective system channel. A ramp input signal can be injected, in either the up or down direction, and recorded along with the time the bi-stable indicating light changes. Using the digital recording system of the present invention, the trip point and reset point for each bi-stable can be precisely determined by comparing the input signal to the point in time where the bistable light is illuminated or is extinguished. This approach can save a great deal of time in testing and vastly reduce the time that a protective channel is unavailable for service because of testing. 
     It will be apparent to those skilled in the art of instrumentation that many changes and substitutions may be made to the foregoing preferred embodiment without departing from the spirit and scope of the present invention, defined by the appended claims.