Abstract:
An object stress analyzing system includes a scanning head with a detector, a variable gain amplifier and a control unit which controls the scanning head and which acquires stress-related data from the amplifiers. The control unit responds to detector overload conditions by automatically adjusting the gain of the amplifier, rescanning a portion of the object and rescaling data acquired prior to occurrence of the overload condition.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a system for measuring stress in an object. 
     A portion of the disclosure of this patent document contains material to which is subject to a claim of copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all other rights whatsoever. 
     Various non-destructive techniques have been proposed to measure and analyze stresses in a physical object. For example, U.S. Pat. No. 4,378,701 discloses a technique whereby stress is analyzed by detecting thermal radiation produced by a part subject to stress. This technique is utilized in a computer controlled stress analyzing system (SPATE 8000) marketed by Ometron, Inc.. This known system detects the minute temperature changes which arise when an object is cyclically loaded. These temperature changes are correlated with a reference signal which represents the applied load. However, this known system may produce inaccurate results due to overload conditions caused by intermittent thermal disturbances or caused by a strong source of thermal radiation source moving into the field of view of the scanning infra-red detector. 
     If an operator is not present during occurrence of the overload condition, then the known system will retain faulty data caused by the overload. If an operator is present and recognizes an overload situation, the operator can manually adjust the sensitivity to eliminate the overload condition. However, the system will then have acquired two sets of data obtained under different conditions, thus preventing the uniform displaying of such data. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a scanning stress analyzing system which is not adversely effected by object or non-object produced temporary or intermittent thermal disturbances. 
     Another object of the present invention is to provide such a system wherein detector sensitivity is automatically adjusted in response to overloads 
     Another object of the present invention is to provide such a system wherein data collected before and after such a thermal disturbance is automatically recalibrated. 
     A further object is to provide such a system wherein an object may be automatically partially rescanned in response to an overload condition. 
     These and other objects are achieved by the present invention which responds to overload conditions by automatically adjusting the gain of the amplifier, rescanning a portion of the object and rescaling data acquired prior to occurrence of the overload condition so that a consistent set of data for the entire scan is acquired. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic diagram of a stress analyzing system in which the present invention operates. 
     FIGS. 2a-2c are simplified flow charts of the overload-responsive algorithm of the present invention. 
    
    
     DETAILED DESCRIPTION 
     An optical stress analyzing system, shown schematically in FIG. 1, includes a known scanning head unit 10 which operates to optically scan stress-produced thermal radiation emitted by the object to be analyzed 12. Such a scanning head is described in detail in U.S. Pat. No. 4,378,701 issued to Mountain et al. in 1983. The scanning head includes x and y mirror drive motors 14, 16 and an infra-red detector 18. A correlator or lock-in amplifier 20 receives the signal from detector 18. The correlator is preferably an off-the-shelf item, such as the EG &amp; G Models 5206 or 5208 lock-in amplifier. The correlator 20 includes a variable gain amplifier 21, the gain of which may be controlled by control signals coupled thereto. The correlator 20 provides a voltage which represents the amplitude of the detector signal. 
     The amplified detector signal is supplied to a data acquisition and scanning control unit 22 via analog-to-digital converter 24 and I/O Bus 26. The control unit 22 causes the head 10 to scan the object by moving drive motors 14 and 16 in a step-by-step manner via x and y digital-to-analog converters (DAC) 28 and 30, and provides information for display on a monitor 32. At each position during a scan, the correlator 20 receives data from the detector 18 and communicates this data to the control unit 22. 
     A timing control circuit 34 includes a programmable timer which operates to coordinate operation of data acquisition via A-to-D converter 24 and mirror movement via DACs 28 and 30. The control unit 22 includes a digital computer programmed to perform data acquisition loop and scanning control functions. Such control units are included in the SPATE 8000 systems available from Ometron Inc.. Operator control is provided via a keyboard 36. 
     According to the present invention, the control unit is provided with an additional overload condition responsive algorithm which will now be described with reference to FIG. 2a. The algorithm is entered from the main or data acquisition loop at step 100 in response to a system interrupt. Then, in steps 102 and 104, the status of the correlator is examined and an interrupt flag is set equal to 1. 
     If, in the meantime, the overload condition has disappeared, then step 106 causes the algorithm to proceed to step 110 which causes a return to the data acquisition loop without disruption. If the overload condition is due to too large a signal magnitude from detector 18, then step 106 directs the algorithm to step 112 which stops operation of the data acquisition loop, to step 114 which sets a counter value, OVERLD, equal to zero, and then to step 116 which causes the algorithm to pause for approximately 1 second. 
     Then, the correlator status is examined again in step 118. If the overload condition has disappeared at this step, then step 120 directs the algorithm to step 122 which causes the mirror drives 14, 16 to back up a few steps. Then, step 124 returns to and restarts the head scanning and data acquisition loop so that new data will be acquired to replace the data which included the overload condition. 
     If, in step 120, the overload condition still exists, then the algorithm proceeds to steps 126-140. This group of steps operates to count up the number of times the overload condition is present during a certain time period, such as 5 to 10 seconds. During this period, the overload condition is examined periodically, such as every 0.1 seconds, and an OVERLD counter value is incremented for each overload occurrence. After the number of overload conditions have been counted during this time period, step 126 routes the algorithm to step 128 which turns off the delay and cycle associated with this overload counting loop. If less than a certain number of overloads have occurred (about 30, for example), then step 130 directs the algorithm back to the data acquisition loop via step 124. 
     If more than about 30 overloads have occurred, then the algorithm proceeds to step 144 which determines whether the system is displaying on monitor 32 information relating to the data being acquired. If not, the algorithm proceeds to steps 146-150 which operate to automatically adjust the sensitivity of the correlator 20 by one step, to correspondingly adjust the scaling factor relating to the data collected, to cause the mirror drives 14 and 16 to reposition the mirrors in the scanning head to re-execute a small part of the scan just completed so that the data including the overloads can be replaced. Finally, step 152 returns to and restarts the data acquisition loop. 
     Returning to step 144, if data is being displayed on a monitor, then the algorithm proceeds to steps 154-158 which operate to cause a delay of 2 minutes (for example) while the computer waits for a human operator to enter a command via the keyboard 34. If no operator command is entered, then when the 2 minute delay period expires, the algorithm will proceed to step 164 which ends the delay period and then to previously described steps 146-152. If an operator command is entered, then step 160 directs the algorithm to previously described steps 164 and 146-152 if the operator commands that the correlator sensitivity be adjusted. If the operator commands no adjustment, then step 160 directs the algorithm to step 162 which turns off the delay timer and then back to the data acquisition loop via step 152. 
     The following is a program listing of a computer program in the Basic language which operates as just described in connection with the flow charts of FIGS. 2a-2c. ##SPC1## 
     While the invention has been described in conjunction with a specific, embodiment, it is to be understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.