Speed control system in dental radiographic apparatus

#1. Field of the Invention

#This invention relates to an X-ray film feed speed control system in a dental radiographic apparatus for photographing the entire jaws.

#2. Prior Art

#In the drawings:

#FIG. 1 is a schematic view of the mechanical component parts of the apparatus of the invention;

#FIG. 2 is a plan view showing a relation between the center of the rotary arm and the cam plate;

#In FIGS. 1 and 2, the numeral 1 designates a base shaft suspended from a support base 2, and a horizontal rotary arm (to be referred to as a rotary arm) 4 is horizontally rotatably suspended by the base shaft 1 through a bearing 3. The rotary arm is provided with an X-ray generator 5 and an X-ray film cassette holder 6 which are respectively held at one end of the rotary arm 4 and at the other end thereof in an opposed relation with each other in an angular phase difference of 180.degree.. The arm is so constructed that when an X-ray photograph is taken, the arm is rotatingly moved within the same area of plane as that within which the X-ray generator 5 and X-ray film cassette holder 6 having an object P disposed therebetween encircle the object P and an X-ray film (not shown) is fed within the X-ray film cassette holder 6 in synchronism with the travelling speed of the arm. In order to rotate the arm 4, an induction motor IM is integrally mounted on the arm and a pinion 8 is fixedly mounted on an output shaft 7 of the motor IM. On the other hand, a receiving plate 9 is fixedly mounted on the base shaft 1 and a rack 10 is provided circumferentially of the circle described around the base shaft 1 on the underside of this receiving plate 9 and the rack 10 is in mesh with the pinion 8. Forced rotating force of the rotary arm 4 is induced by the pinion 8 being rotated on the rack 10 in meshing with each other by the rotation of the induction motor IM. Also, the arm 4 is provided with a pulse motor PM for feeding the X-ray film. Both motors IM and PM are electrically connected to each other by means G for converting the number of rotations of the arm drive motor IM into an electrical signal as well as by means of a rectifier circuit 18, DC voltage-pulse converter circuit 16 and a film feed motor drive circuit 17 as particularly shown in the circuitry in FIG. 3, and accordingly, if a variable resistor VR1 is fixedly mounted, the motor PM is synchronously changed in speed in proportion to the speed of rotation of the motor IM, namely the rotatingly travelling speed of the rotary arm 4 and of the X-ray generator 5, thereby controlling the feed speed of X-ray film.

#But the invention is designed to vary a variable resistor VR1 automatically in the process of rotation of the rotating arm 4 and detect the X-ray irradiation position as a change in the resistance value of the variable resistor VR1 to thereby control the DC voltage-pulse converter circuit 16. Namely, the numeral 11 designates a cam plate detachably mounted on the top of a receiving plate 9 and the cam plate 11 is designed to variably control a transfer constant of a film feed motor drive control circuit A. Namely, the cam plate 11 has a circumferential cam face 12, and expansion and contraction of the plunger 13 in accordance with the rotation of the cam plate 11 successively change the resistance value of the variable resistor VR1 and cause the transfer constant of the circuit A to be changed, in turn, by the change in resistance value.

#In FIG. 3, a means for converting the number of rotations of the arm drive motor IM into an electrical signal is a tachogenerator connected directly to the output shaft 7 of the motor IM, the tachogenerator including AC voltage in proportion to the number of rotations of motor IM. The character RFD designates a rectifier for changing AC voltage into a pulse. The variable resistor VR1 connected directly to the rectifier RFD is variable in accordance with the rotation of the cam plate 11, and accordingly varies the feed speed of X-ray film in accordance with the number of rotations (speed of rotation) of motor IM, namely the travelling speed of the generator 5. The circuit including condensers C1 and C2, resistors R1 and R2 and an amplifier A1 composes low-pass filter circuit 15 and is intended to shut off high-frequency component contained in the above pulse. Also, the numeral 16 designates a DC voltage-pulse converter circuit which includes a Miller integration circuit, a comparator circuit and a buffer amplifier A4 and which produces a pulse voltage in a repetitive period proportional to an input voltage from a low-pass filter circuit 15 and which functions to operate flip-flop FF1 as a trigger pulse. Transistors Q2 and Q3 compose a motor drive circuit 17 for pulse motor PM, the resistors R7 and R8 designating base resistors for the transistors Q2 and Q3. Also a resistance series connector of resistor R3b and variable resistor VR2b are selected by switch SW1 in the circuit 16. This selection is intended to select a conversion factor of the circuit 16, and stated more concretely, the time constant of a Miller integration circuit constructed to include amplifier A2 is determined by CR circuit between this selected resistance connector and condenser C3. The selection of this resistance series connector is effected before starting of X-ray photographing. The comparator circuit compares a reference voltage source E1 with this integration output by a conmparator A3 and includes resistors R4 and R5 and condenser C4.