Patent Number: 051075300
Section: summary

The present invention relates to the control of the shutter of an x-ray diffractometer and more specifically relates to an x-ray diffractometer with a non-magnetic shutter position sensor and indicator. BACKGROUND OF THE INVENTION X-ray diffractometers are known in the art and are used for applications such as directing an x-ray beam toward a crystal to obtain reflection angles of the beam from the crystal for use in studying the crystal. The analysis of a crystal using an x-ray diffractometer can require a significant amount of time with eight hours to three days not being unusual. During the period of experimentation with a crystal, the shutter of the x-ray diffractometer may be opened and closed 10,000 to 15,000times in a 24-hour period. In a conventional x-ray diffractometer, such as a Model No. AFC6, RU200B Series x-ray diffractometer from Rigaku Corporation of Japan, the shutter comprises a rotary controlled shutter element which is rotated between a first closed position and a second open position. When the shutter is open, a path is provided between a radiation source and a target, such as a crystal. In this apparatus, a solenoid is rotated to rotate a shaft which in turn rotate the shutter. A bar magnet is supported on the shaft and is shifted as the shaft rotates between a first position, corresponding to the closed position of the shutter, and a second position, corresponding to the open position of the shutter. As the shaft rotates between the respective first and second positions, reed switches at these positions are activated to provide a shutter position indicating signal. In operation, a computer controller of the Rigaku device causes the solenoid to shift the shutter to a desired position, such as to the second or open position. The computer then receives a position indicating signal from one of the reed switches and compares this signal with the expected position corresponding to the position to which the shutter has been operated by the solenoid in response to the controller. If the expected position does not correspond to the detected position determined from the signals from the reed switches, a shutter error position signal is generated. In the case of a shutter error, the solenoid is operated to close the shutter and the system shuts down. Because of the large number of shutter operations normally required during the analysis of a crystal or during other uses of the x-ray diffractometer, the reed switches tend to wear, with frequent component replacement being required. Also, proper alignment of the replacement reed switches is difficult to attain. Furthermore, as the parts deteriorate through use, false shutter position indicating signals are generated and result in the erroneous shutdown of the equipment. This results in a substantial loss of many hours of experimentation time, particularly when x-ray diffractometers are set up for the automatic running of an experiment overnight or on a weekend with a researcher returning and learning that the experiment has stopped midstream. In addition, sometimes valuable sample crystals are lost due to the instability of these crystals and the fact that these crystals lack the stability simply to restart an experiment which has erroneously been terminated. The inventor has found that the Rigaku system as described above frequently provided false shutter position errors, with errors occurring at least once every three or four days over many periods of operation of the x-ray diffractometer. This problem with accurately controlling and detecting the presence of a shutter under the adverse operating conditions required by an x-ray diffractometer have been present for a number of years. That is, since the Rigaku x-ray diffractometer mentioned above was introduced, the inventor understands that this problem of generating false shutter position signals has plagued users of this device without being solved. The assignee of the present invention first obtained this model of Rigaku x-ray diffractometer in November of 1987. Therefore, a need exists for an improved shutter control mechanism for an x-ray diffractometer designed to overcome these and other problems of the prior art. SUMMARY OF THE INVENTION An x-ray diffractometer directs an x-ray beam toward a target through a shutter. The x-ray diffractometer has a controller for causing the shifting of the shutter between a first closed position, in which the passage of the x-ray beam through the shutter is blocked, and a second open position, in which the passage of the x-ray beam through the shutter is not blocked by the shutter. The controller has an input for receiving a shutter position indicating signal. The controller compares the shutter position corresponding to the shutter position indicating signal with the expected shutter position corresponding to the shutter position to which shutting of the shutter has been caused by the controller. The shutter is caused to be shifted to a closed position in the event the shutter position corresponding to the shutter position indicating signal does not also correspond to or match the expected shutter position. In accordance with the invention, an optical shutter position sensor or detector is mounted to a support in proximity to the shutter for sensing the position of the shutter and for producing the shutter position indicating signal. Such a shutter position sensor is a non-mechanical sensor in that it does not rely upon a mechanical switch, that is one in which mechanical components included in an electrical circuit path move to mechanically open and close the circuit utilized in sensing the shutter position. This type of shutter position sensor electronically produces the shutter position indicating signal which corresponds to the position of the shutter. Although various optical detectors or sensors may be used, including the reflecting optical beam type sensor, a preferred form of the sensor is an interrupting optical beam type sensors. In this specific form of the invention, a mechanism is provided for interrupting a first optical beam when the shutter is in the first closed position and for interrupting a second optical beam when the shutter is in the second open position. The position indicating signal corresponds to the optical beam which is interrupted and thereby indicates the position of the shutter. This mechanism may comprise first and second optical beam breaking elements coupled to the shutter for respectively interrupting the first and second optical beams depending upon the shutter position. Alternatively, the mechanism may comprise a single element, such as an arm mounted to or otherwise coupled to the shutter and movable with the shutter between first and second positions to interrupt the respective first and second optical beams as the shutter is shifted between the first closed position and second open position. This arm may be L-shaped with a flag portion which is disposed between a transmitter and receiver of an optical detector to break a beam being transmitted between the receiver and detector to thereby indicate the shutter position. More specifically, the first and second optical detectors may comprise respective optical isolators each having an optical beam source and an optical beam receiver. The shutter may be of any convenient form and may comprise a rotary shutter operated in response to control signals from a conventional controller. These control signals may be delivered to a rotary solenoid for shifting the shutter between the first closed position and the second open position. It is accordingly one object of the present invention to provide an x-ray diffractometer with an improved controller and more specifically with an improved shutter position indicating mechanism. Another object of the present invention is to provide an x-ray diffractometer which is capable of operating for substantial periods of time without falsely indicating the shutter position, which can cause the termination of an experiment and significant lost time. Still another object of the present invention is to provide an x-ray diffractometer shutter position sensor which is extremely durable. These and other objects, features and advantages of the present invention will become apparent with reference to the following description and drawings.