Patent Number: 047117540
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for impacting a, surface with a controlled desired impact energy. More particularly, the present invention relates to a method and apparatus for impacting a surface with a small desired and controllable impact energy, whereby the sensitivity of impact detectors mounted on the surface, and in particular a surface of a nuclear reactor coolant system, can be tested. 2. Description of the Prior Art In the operation of pressurized water power generating plants, it is desireable to have a system which will enable the early detection of failure of primary system mechanical components. The failure of such mechanical components characteristically results in metal debris which concentrate in the steam generator input plenum and the bottom plenum of the reactor vessel. Moreover, metal debris in the form of objects left in the system during the construction phase are sometimes encountered. Such metal debris, when left undetected, have caused extensive damage to various components of the coolant system. During normal operation of the reactor system, the metal debris are transported to collection points by the normal flow of the primary coolant and, during their travel, are propelled against the metal walls enclosing the primary system coolant paths. Accordingly, surveillance of the energy imparted to the metal walls as a result of the impacts will provide both an indication of primary system component failure, and an indication of the presence of undesirable metallic debris which could cause subsequent failures. To detect the presence of metallic debris in the nuclear reactor coolant system, various systems for detecting the impact energy have been utilized. One such system is disclosed, for example, in U.S. Pat. No. 3,860,481 issued Jan. 14th, 1975 to R. Gopal et al and assigned to Westinghouse Electric Corporation. According to this system, a number of impact sensors, e.g. accelerometers, are disposed at strategic positions on the nuclear reactor coolant system, e.g. at the reactor vessel upper and lower plenums and the input plenum of each steam generator of the reactor coolant system, and the output signals from the impact sensors are detected and analyzed. According to current regulations issued by the Nuclear Regulatory Commission, each impact sensor must have a sensitivity capable of detecting an impact energy of 0.5 foot-pounds (0.68 joules) within three feet (0.91 meters) of an impact sensor. Accordingly, in order to test the sensitivity of the impact sensors, it becomes necessary to periodically impart an external impact with an energy corresponding to the desired sensitivity to a surface of the reactor coolant system adjacent a particular impact sensor. Moreover, in order to stress the design limits of the various detection algorithms, researchers often desire to measure and control desired impact energies of even a smaller value than the required sensitivity for the system, e.g. less than 0.1 foot-pounds. A number of methods and apparatus for imparting such external impact energies to a surface are known. Such devices may be, for example, a spring loaded mass which renders impact energy proportional to the spring constant, a pendulum device for providing an impact energy corresponding to the mass of the pendulum and the vertical height from which it is dropped to provide an impact, or a manually operated force hammer, including a transducer which produces an output which is a force versus time function, which is struck against the desired surface and the kinetic energy calculated from the area under the force versus time function. All of the known devices suffer from the disadvantage that there is always a degree of uncertainty of the true impact energy. This results for example, because the first two devices include frictional forces and/or spring constants which may vary considerably, and because all of the devices involve some manual manipulation of the impact imparting device, thus rendering it difficult, to repeatedly apply an impact of a desired energy. Moreover, as a result of the manual manipulation involved, and the increased time required to determine or calculate the impact energy and, if necessary, vary the impact force and repeat the impact to provide the desired impact energy, the time of exposure to radiation by the personnel operating the impact device is undesirably increased. Finally, the known devices suffer from the problem that they are difficult, if not impossible, to use on surfaces with particular orientation or location. This latter problem is of particular significance when attempting to impact the bottom plenum of the reactor vessel which can only be approached from the bottom, and not from the side, and thus the impact must be applied in an upward vertical direction. This is not possible with a pendulum and difficult with the other devices. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method and apparatus for impacting a surface with a desired impact energy which is fully automatic in operation and does not require any manual manipulation of the impact device by the operator. It is another object of the present invention to provide a method and apparatus which can automatically and repeatedly provide a desired impact energy to a surface in a short period of time, and which will immediately indicate the impact energy to the operator. It is a further object of this invention to provide an apparatus for impacting a surface with a desired impact energy which is simple to use, and which is substantially unaffected by the shape or location of the surface to be impacted, or by the orientation of the impacting device. The above objects are achieved according to the basic concept of the present invention by a method comprising the steps of: (a) providing an impacting device which can apply a variable impact energy corresponding to the magnitude of an input control signal; (b) placing the impact device adjacent the surface to be impacted; (c) thereafter applying an input control signal of a present magnitude to said impact device to initiate an impact; (d) determining the kinetic energy of the impact; (e) comparing the value of the determined kinetic energy with a value corresponding to the desired kinetic impact energy; (f) indicating the results of the comparison; (g) adjusting the present magnitude of the signal used for said control signal to reduce any difference as a result of the comparison; and (h) thereafter repeating steps (c) to (g) until a repeatable impact of the desired impact energy is determined and indicated. According to the preferred embodiment of the invention, the impacting device is a solenoid whose plunger of a known mass provides the impact, the input control signal is an input voltage applied across the solenoid coil, and the kinetic impact energy is determined by measuring the velocity of the plunger just prior to impact. Preferably this final velocity is measured by: detecting the movement of the plunger and producing an output signal whose magnitude is proportional to the distance moved by the plunger after application of said input voltage; sampling the magnitude of the output signal at uniform time increments; comparing the sampled values from successive sampling times; and, upon detecting a zero difference between two compared values (indicating that impact has occurred), utilizing the difference between the last two different sampling values as a measure of the velocity of the plunger just prior to impact, and thus of the kinetic impact energy. According to a feature of the invention, compensation for initial adhesion forces which may exist in the solenoid is provided by: initially slowly increasing the magnitude of a voltage signal applied to the solenoid coil, sensing the position of the plunger, and applying the input control voltage of present magnitude to the solenoid coil to initiate an impact only upon sensing an initial change in the position of the plunger. Although the method according to the invention can theoretically be carried out utilizing analog data processing, according to the preferred and disclosed embodiment of the invention, the method is carried out under control of a programmed microprocessor. Moreover, according the preferred embodiment of the invention, the position of the solenoid plunger is detected by means of a linear variable differential transformer whose moveable core follows the movement of the plunger and whose output signal magnitude is thus a measure of the plunger position.