modes for carrying out the invention will be described in detail with reference to the drawings . [ 0039 ] fig1 is a schematic perspective view to show one example of a gantry in an x - ray ct system mounted on rails in a mode for carrying out the invention . as shown in fig1 a gantry 1 has a gantry base part 2 and is mounted on a pair of side rails 3 a and 3 b . the gantry 1 can move along the side rails 3 a , 3 b and while the gantry 1 moves , a scan is performed . moreover , a center rail 4 is disposed at the central position sandwiched between the side rails 3 a and 3 b in the direction along the side rails 3 a and 3 b . the structure of the center rail 4 will be described later . for the sake of convenience in the following description , let &# 39 ; s define a direction of disposition of the side rails 3 a and 3 b , that is , a direction in which the gantry moves , as a z - axis direction , a direction along a floor and perpendicular to the z - axis direction as an x - axis direction , and a direction perpendicular to the floor ( vertical direction ) as a y - axis direction . [ 0042 ] fig2 is a side view of the gantry 1 shown in fig1 . as shown in fig1 wheels rolling on the side rails are provided in the gantry base part 2 ( wheels 5 , 6 rolling on the side rail 3 a are shown in fig2 ). this enables the gantry 1 to move in the z - axis direction . [ 0043 ] fig3 is an illustration to show the relationship between the wheel 5 and the side rail 3 a . as shown in fig3 the side rail 3 a is formed convexly in its cross section and its protruding portion becomes a rail tread surface . moreover , a groove corresponding to the width of the rail tread surface is formed on the wheel tread surface of the wheel 5 along the direction in which the wheel rolls . that is , the wheel tread surface is formed concavely . then , the groove is engaged with the rail &# 39 ; s protruding portion to enable the wheel 5 to move along the rail 3 a to thereby prevent the wheel from coming off the rail . this holds true also for the other wheels . [ 0044 ] fig4 is a top perspective view to show the side rails 3 a and 3 b , the center rail 4 , and the gantry 1 which are shown in fig1 . as described above , the wheels 5 , 6 are set on the side rail 3 a whereas wheels 7 , 8 are set on the other side rail 3 b . the wheels 6 and 8 are connected to each other by a shaft 9 and are driven by a motor 11 via a gear 10 ( both of them are mounted in the gantry base part 2 ). that is , the wheels 6 , 8 are driving wheels and the wheels 5 , 7 are driven wheels . a linear guide rail 41 is mounted on the center rail 4 in the z - axis direction . then , two linear guide blocks 42 , 43 mounted on the bottom surface of the gantry base part 2 are mounted on the linear guide rail 41 so that they can slide on the linear guide rail 41 . fig5 shows one example of a state in which the linear block 42 is mounted on the linear guide rail 41 and fig6 shows a cross - sectional view taken on a line a - a in fig5 . referring to this cross - sectional view makes it clear that the linear guide block 42 is fitted on the linear guide rail 41 with bearings 45 a , 45 b interposed between their side surfaces . such structure enables the linear guide block 42 to slide along the linear guide rail 41 . this holds true also for the linear guide block 43 . moreover , in order to absorb errors in height of the side rails 3 a , 3 b and the center rail 4 , respectively , the linear guide block 42 is mounted on the bottom surface of the gantry base part 2 with a gap of about several millimeters in the y - axis direction so that it can slide thereon . measuring the position of the gantry 1 in the z - axis direction ( also simply referred to as a position detection ) in the embodiment is performed by the use of a linear encoder . first , a linear scale 50 of the linear encoder is mounted on the center rail 4 along the linear guide rail 41 . further , a pickup sensor 51 is mounted on the gantry base part 2 so as to oppose the linear scale 50 . the outputs of this pickup sensor 51 ( a - phase and b - phase encoder signals ) are sent to a control board 100 ( see fig4 ) and the position of the gantry 1 in the z - axis direction can be measured based on this outputs . since the two linear guide blocks 42 , 43 mounted on the bottom surface of the gantry base part 2 are fitted in this manner on the linear guide rail 41 mounted on the center rail 4 so that they can freely slide , the deviation of the gantry 1 in the x - axis direction with respect to the movement of the gantry 1 in the z - axis direction is forcibly limited . therefore , even if the gaps between the grooves of the wheels 5 , 6 , 7 , 8 and the rails 3 a , 3 b apply forces to the gantry 1 in the x - axis direction as the gantry 1 moves or apply a force for changing the direction of the gantry 1 gradually to the gantry 1 as the gantry 1 moves , the gantry 1 is not deviated by the forces but can be moved correctly straight . this can reduce a deviation in the rotational center of scan and a deviation in a scan position in the z - axis direction . further , it is possible to detect the position of the gantry 1 in the z - axis direction with high accuracy without being affected by the machining accuracy such as the size of the wheel and the direction of an axle . moreover , since the linear scale 50 is mounted on a lower portion near the center in the x - direction of the gantry 1 , even if a force of torsion is applied to the gantry base part 2 to deform the gantry base part 2 , it is possible to detect the scan position correctly . next , an operation of moving the gantry 1 will be described . here , referring to fig4 on the right side of paper is provided a table ( not shown ) on which a body to be inspected is placed . in the following description , in the direction of movement of the gantry 1 , that is , in the z - axis direction , let &# 39 ; s call a direction in which the gantry is brought near to the table as an in side and a direction in which the gantry is brought away from the table as an out side . further , in fig4 on the bottom surface of the gantry base part 2 are mounted a first limit switch s 1 and a second limit switch s 2 . moreover , a protrusion 52 for operating the first limit switch s 1 is provided on an outside movement limit position ( out limit position ) as a first check point on the center rail 4 and a protrusion 53 for operating the second limit switch s 2 is provided on a predetermined position sandwiched between the protrusion 52 and the table as a second check point on the center rail 4 . these protrusions 52 , 53 are used in a processing of setting the scan base position that will be described later . [ 0054 ] fig7 is a block diagram to show the constitution of the control board 100 ( see fig4 ) built in the gantry base part 2 . in fig7 a reference numeral 61 denotes a cpu for controlling the movement of the gantry 1 and a reference numeral 62 denotes a rom for storing the operation processing procedure ( program ) of the cpu 61 and a reference character 63 a denotes a ram functioning as a main storage and a reference character 63 b denotes a flash memory functioning as an auxiliary storage . a reference numeral 64 denotes an interface for inputting data from an operator console 200 for outputting information relating to scan to the gantry 1 . reference numerals from 65 to 68 denote an operating panel 20 , the first limit switch s 1 , the second limit switch s 2 , and an interface for inputting data from the pickup sensor 51 . a reference numeral 69 denotes an up / down counter indicting a relative position in the z - axis direction based on the a - phase and b - phase encoder signals from the pickup sensor 51 and a reference numeral 70 denotes a motor driver for performing the driving control of the motor 11 . before performing a scan , for example , after turning on the power , an operation of moving the gantry 1 once on the in side and the out side by a manual operation to make a check that moving the gantry 1 does not cause a danger is performed . the movement of the gantry 1 by this manual operation can be performed by the use of a movement button provided on the operating panel 20 ( see fig1 ) arranged on the gantry 1 . [ 0057 ] fig8 shows one example of the operating panel 20 . a reference numeral 21 denotes a first movement button for moving the gantry 1 on the in side , and a reference numeral 22 denotes a second movement button for moving the gantry 1 on the out side , and a reference numeral 23 denotes a setting button for setting the base position of the scan . moreover , a reference numeral 24 denotes a display part for producing various displays . of course , buttons for other objects can be provided but only the ones necessary for describing the present invention are shown in the drawing . while the operator presses the first movement button 21 ( on ), the gantry 1 is moved on the in side at a predetermined speed . while the operator presses the second movement button 22 ( on ), conversely , the gantry 1 is moved on the out side at a predetermined speed . in both the buttons , when the operator presses off the button , the button is immediately turned off to stop the movement of the gantry 1 . the maximum range of movement of the gantry 1 performed by this manual operation is stored in the ram 63 a . then , the gantry 1 can be moved by a remote control from the operating console 200 and in this case , the gantry 1 is allowed to move only within the maximum movement range stored in the ram 63 a . here , for example , it is also possible to clear the maximum movement range stored in the ram 63 a when any one of operations of moving the gantry 1 to a retracted position ( out - side movement limit position ), moving the table , and a predetermined operation on the operating console 200 is performed . however , in order to move the gantry 1 or to perform a scan by a remote control thereafter , it is necessary to move the gantry 1 again by the manual operation to make a check of safety . it is possible to ensure safety by limiting the movable range of the gantry 1 by the remote control in this manner . next , a processing of setting the scan base position in the embodiment will be described . the scan base position can be set by moving the gantry 1 and pressing the setting button 23 at a desired position . as described above , however , there is presented the problem that even if the scan base position is the same for each scan , the setting operation by the setting button 23 ( manual setting operation ) needs to be performed every time , which makes operability worse . in the case where the manual setting operation is performed every time , a setting error or a setting miss may be caused . thus , in the embodiment , the scan base position is automatically set as far as no abnormality is found in the above - described checking operation performed by moving the gantry 1 by the manual operation after turning on the power . a processing of setting the scan base position in the embodiment will be described in detail by the use of a flow chart shown in fig9 and fig1 . [ 0066 ] fig9 is a flow chart to show a process for setting the positional relations of the first limit switch s 1 , the second limit switch s 2 , and the desired scan base position , respectively . it is preferable to perform this process when the system is adjusted . further , the above - mentioned desired scan base position is usually determined in accordance with the installation environment of the system ( such as size of a room ). first , the operator keeps pressing the second movement button 22 to move the gantry 1 to the out limit position ( step s 1 ). the fact that the gantry is moved to the out limit position can be detected by the fact that the first limit switch s 1 is put into contact with the protrusion 52 to be switched from off to on . at this time , the gantry is forcibly stopped . next , the operator presses the first movement button 21 to move the gantry 1 toward the desired scan base position ( step s 2 ). during the movement of the gantry 1 , a position z 1 where the first limit switch s 1 is switched from on to off ( first check point ) is detected and further a position z 2 where the second limit switch s 2 is put into contact with the protrusion 53 to be switched from off to on ( second check point ) is detected , and the distance ds between z 1 and z 2 is stored in the flash memory 63 b ( step s 3 ) thereafter , when the gantry 1 comes to a desired position as the scan base position , by the manual operation , that is , by setting the setting button 23 , the desired position z 0 is set as the scan base position ( step s 4 ). then , the distance dr between the position z 2 where the second limit switch s 2 is switched from off to on and z 0 is stored in the flash memory 63 b ( step s 5 ). in this manner is finished the process for initially setting the positional relations of the first limit switch s 1 , the second limit switch s 2 , the desired scan base position , respectively , which is preferably performed when the system is adjusted . [ 0072 ] fig1 is a flow chart to show a processing of setting the scan base position which is performed in a checking operation performed by moving the gantry 1 by the manual operation after turning on the power or the like . a program corresponding to this flow chart is stored in the rom 62 and is executed by the cpu 61 after turning on the power . first , at step s 11 , it is judged based on the input of the second movement button 22 whether a movement command to the out side is given or not . when the second movement button 22 is pressed down , the routine advances to step s 12 and the gantry 1 starts to move to the out side . here , in the case where the second movement button 22 is separated from the operator to be turned off , at this time , the present process is finished . at step s 13 , it is monitored whether the first limit switch s 1 is turned on or not during the movement of the gantry 1 . then , when the first limit switch s 1 is turned on , the routine advances to step s 14 where the movement of the gantry 1 is stopped . next , at step s 15 , it is judged based on the input of the first movement button 21 whether a movement command to the in side is given or not . when the first movement button 21 is pressed down , the routine advances to step s 16 and the gantry 1 starts to move to the in side . here , also in the case where the first movement button 21 is separated from the operator to be turned off , at this time , the present process is finished . at step s 17 , it is monitored whether the first limit switch s 1 is switched from on to off or not . then , when the first limit switch s 1 is turned off , at step s 18 , a position z 1 ′ at this time is stored in the ram 63 a . next , at step s 19 , it is monitored whether or not the second limit switch s 2 is switched from off to on during the movement of the gantry 1 . then , when the second limit switch s 2 is turned on , at step s 20 , a position z 2 ′ at this time is stored in the ram 63 a . next , at step s 21 , it is judged whether or not the error between the distance between the position z 1 ′ when the first limit switch s 1 is switched from on to off and the position z 2 ′ when the second limit switch s 2 is switched from off to on and the distance information ds stored in the flash memory 63 b is within a predetermined range . that is , it is judged whether or not the error satisfies the following equation . here , in the case where the above - mentioned equation is not satisfied , a position detection by any one of the first limit switch s 1 , the second limit switch s 2 , and the pickup sensor 51 is judged to be not correct and the routine advances to step s 22 where a predetermined error is displayed on the display part 24 , or an alarm sound may be issued to inform the operator of an error . in any case , in the case of such error , the scan in this state is prohibited . further , in the case where the above - mentioned equation is satisfied , the routine advances to step s 23 where a position obtained by adding the distance information dr stored in the flash memory 63 b to the position z 2 ′ when the second limit switch s 2 is switched from off to on is set as the scan base position and where the obtained base position information is stored in the ram 63 a . in the case where the base position in each scan is the same , as far as no abnormality is found in the checking operation performed by moving the gantry 1 by the manual operation performed after turning on the power , the scan base position is automatically set , so that operability can be drastically improved and the occurrence of a setting error and a setting miss can be prevented . further , in the case where an abnormality is caused in a position detection mechanism provided in the system , the abnormality is informed and the scan in this state is prohibited , so that safety can be ensured . here , while the protrusions 52 , 53 are provided at the predetermined positions in the direction of movement of the gantry 1 and the predetermined positions are detected by the use of the limit switches s 1 , s 2 that are operated when they come in contact with the protrusions 52 , 53 in the embodiment described above , the predetermined positions can be detected also by the other means , for example , a combination of an optical sensor and an optical passing slit , a combination of an optical sensor and an optical reflector , a magnetic position detection sensor , a position detection sensor by an electrostatic capacity , a position detection by an image by means of a ccd camera , and the like . further , while the out limit position is made the first check point and a position where the gantry 1 is moved forward by a predetermined distance from the first check point is made the second check point in the embodiment described above , it is also recommended that a sensor for detecting the first check point and a sensor for detecting the second check point are provided independently from each other and that the first check point and the second check point be provided at the positions that are detected by both the sensors at the same time when the gantry 1 is moved to the predetermined position . further , at step s 5 in fig9 the distance dr from the second check point z 2 to the scan base position z 0 by the manual operation is stored , and at step s 5 in fig1 , the scan base position is set again by using this dr , but it is also recommended that the distance dr ′ from the first check point z 1 to the scan base position z 0 by the manual operation be stored at step s 5 in fig9 and that the scan base position be set again by using this dr ′ at step s 23 in fig1 . still further , while two linear guide blocks 42 , 43 mounted on the bottom surface of the gantry base part 2 are mounted on the linear guide rail 41 in the embodiment described above , as far as they are fixed with sufficient accuracy so that the direction of the gantry 1 is not changed , it is also recommended that only one guide block be used . many widely different embodiments of the invention may be configured without departing from the spirit and the scope of the present invention . it should be understood that the present invention is not limited to the specific embodiments described in the specification , except as defined in the appended claims .
for the purpose of reducing a deviation in the rotational center of scan and a deviation in scan position in the direction of length of a rail , a gantry system including a pair of runway rails and a gantry in an x - ray ct system that can move along the runway rails , is characterized by comprising a linear guide rail arranged in the direction along the runway rails and linear guide blocks mounted on the gantry and slidably fitted on the linear guide rail .