JP4334212B2 - X-ray computed tomography system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray computed tomography apparatus that rotationally drives a rotary frame using a brushless motor.
[0002]
[Prior art]
In the X-ray computed tomography apparatus, since a large current flows when X-rays are exposed, the power supply voltage fluctuates due to a voltage drop caused by line impedance existing in the power supply system. In Patent Document 1, in an X-ray fluoroscopic apparatus including a compression unit that compresses a specific part of a subject, a single-phase induction motor is provided to drive the compression unit, and a conduction phase angle of an AC power control element is provided. A technique for controlling the power supply voltage to the single-phase induction motor to be constant by controlling the power is disclosed.
[0003]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-206756 [0004]
[Problems to be solved by the invention]
In recent years, a brushless motor capable of high-efficiency and high-precision control has been used as a motor that rotationally drives a rotary frame to which an X-ray tube is attached. This brushless motor drive device performs speed control based on a position signal indicating the position of the rotor. For this reason, even if the power supply voltage decreases to some extent, it is possible to control the rotational speed to the command rotational speed by increasing the PWM modulation rate and the PWM duty ratio, for example. However, when the power supply voltage further decreases, the rotation speed cannot be maintained due to insufficient voltage.
[0005]
In response to this, when designing a brushless motor, it is sufficient to have a design that allows for the power supply voltage drop at the time of the X-ray exposure, but it becomes over-spec during normal operation, which increases the size and cost of the motor. Inevitable increase. In addition, it is conceivable to separately provide a step-up transformer that compensates for a drop in the power supply voltage and a stabilized power supply device that stabilizes the power supply voltage. However, there are still problems of increasing the size of the equipment and increasing the cost.
[0006]
The present invention has been made in view of the above circumstances, and an object of the present invention is to perform a predetermined rotation of a rotating frame against a decrease in power supply voltage due to X-ray exposure or the like without increasing the scale of the apparatus with respect to a conventional apparatus. An object of the present invention is to provide an X-ray computed tomography apparatus capable of maintaining a speed.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, an X-ray computed tomography apparatus according to the present invention includes a fixed frame, a rotary frame supported rotatably with respect to the fixed frame, and an X-ray tube attached to the rotary frame. An X-ray detector that detects X-rays emitted from the X-ray tube and transmitted through the subject, rotation driving means that rotationally drives the rotary frame, output of the X-ray detector, and rotation of the rotary frame In the X-ray computed tomography apparatus having a computer unit that forms tomographic image data of the subject based on the rotational position of the X-ray tube, the rotation driving means includes a rotor and a winding provided with a permanent magnet A brushless motor having a stator wound thereon, power conversion means for driving the brushless motor, voltage detection means for detecting a power supply voltage input to the power conversion means, and when X-rays are not exposed Power supply When the threshold value is a voltage value set lower than the voltage and higher than the power supply voltage at the time of exposure, and the power supply voltage detection value detected by the voltage detection means is lower than the threshold value And a control means for controlling the current of the stator winding so as to be in a leading phase with respect to the induced voltage of the brushless motor .
[0008]
According to this configuration, the control unit controls the power conversion unit based on the power supply voltage detection value detected by the voltage detection unit, and controls the current phase of the stator winding with respect to the induced voltage of the brushless motor. The phase between the voltage (terminal voltage) output from the conversion means to the brushless motor and the induced voltage of the brushless motor changes. By this phase control, the characteristics of the terminal voltage-generated torque-rotation speed of the brushless motor change, and it is possible to rotate at a lower terminal voltage for the same rotation speed. Since this means can be realized by a slight addition of a voltage detection means or the like, the apparatus scale is not increased.
[0009]
In an X-ray computed tomography apparatus, the power supply voltage input to the power conversion means may temporarily decrease due to X-ray exposure or the like, but this is after the rotation speed of the rotating frame has stabilized at a constant speed. Moreover, the moment of inertia of the rotating frame is very large. For this reason, at the time of acceleration / deceleration of the rotating frame that requires a large torque, there is no X-ray exposure, and the power supply voltage is hardly lowered, and the torque generated by the brushless motor is reduced due to the phase angle control. There is nothing. In addition, after the rotational speed is stabilized, the brushless motor can be generated even when the rotational speed is maintained and controlled by temporarily changing the phase angle in accordance with the X-ray exposure timing. Although the maximum torque is reduced, the influence on the rotational speed is very small due to the moment of inertia.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of an X-ray computed tomography apparatus according to the present invention will be described with reference to the drawings.
FIG. 2 is a side view of the X-ray computed tomography apparatus, and FIG. 3 is a front view as seen from the direction of arrow A in FIG. The X-ray computed tomography apparatus 1 (hereinafter referred to as the X-ray CT apparatus 1) employs a rotation / rotation method in which the X-ray tube 2 and the X-ray detector 3 are integrally rotated around the subject P. It is an apparatus that constructs one tomographic image from projection data sets for one round (about 360 °) around the subject P.
[0011]
The X-ray CT apparatus 1 includes a gantry unit 4 and a bed 5 on which a subject P lies. The gantry unit 4 includes a fixed frame 6 and a substantially annular rotating frame 7 that is rotatably supported by the fixed frame 6. Among these, the rotary frame 7 includes, for example, a cone beam type X-ray tube 2 and a multi-slice type X-ray detector 3 sandwiching the subject P on the bed 5. It is mounted in a positional relationship facing each other.
[0012]
A rotor 8 (see FIG. 1) having a permanent magnet and an annular yoke coil is attached to the rotating frame 7, and windings 9u, 9v, and 9w (see FIG. 1) are attached to the fixed frame 6. The provided stator is attached. The rotor 8 and the stator constitute a direct drive brushless motor 10 (see FIG. 1) that rotationally drives the rotating frame 7.
[0013]
Further, the rotary frame 7 includes an IV converter that converts a current signal output from the X-ray detector 3 into a voltage signal, an integrator that periodically integrates the voltage signal, and an output signal of the integrator. A box that houses a data acquisition system consisting of an amplifier to be amplified, an A / D converter that converts the output signal of this amplifier into a digital signal, etc., a slip ring mechanism for power supply to the X-ray tube 2 and signal transmission / reception is rotationally balanced Are arranged.
[0014]
In order to perform energization control and speed control of the brushless motor 10, stop position control of the rotating frame 7, tomographic image configuration processing, etc., it is necessary to detect the rotation angle of the rotating frame 7 (that is, the rotational position of the X-ray tube 2). It is. For this reason, an annular detection gear 11 made of a magnetic material (for example, iron) is attached to the front end surface of the rotary frame 7, and the fixed frame 6 is separated from the tooth tip of the detection gear 11 by a predetermined distance from the magnetic resistance. A type sensor 12 is attached. An unillustrated image construction processor (corresponding to a computer unit) constructs tomographic image data of the subject P based on the output signal of the X-ray detector 3 and the rotational position of the X-ray tube 2 due to the rotation of the rotary frame 7. It is like that.
[0015]
FIG. 1 is an electrical configuration diagram showing a driving device of the brushless motor 10. This driving device 13 (corresponding to a rotation driving means) receives power from the commercial power supply 14 and outputs a sinusoidal AC voltage to the windings 9u, 9v, 9w of the brushless motor 10. The main circuit portion (corresponding to power conversion means) of the drive device 13 is composed of a converter 15 and an inverter 16. Here, the converter 15 includes a diode bridge circuit 17 including diodes 17a, 17b, 17c, and 17d and a smoothing capacitor 18, and the inverter 16 includes three transistors 16up, 16un, 16vp, 16vn, 16wp, and 16wn. It is configured with phase bridge connection.
[0016]
The voltage detection unit 19 (corresponding to voltage detection means) detects the voltage of the commercial power supply 14 at the input terminal of the converter 15 (hereinafter referred to as power supply voltage VAC). For example, an instrument transformer, a full-wave rectifier circuit, It is configured by combining.
[0017]
A control unit 23 (corresponding to a control unit) including a lead angle determination unit 20, a phase determination unit 21, and a voltage signal formation unit 22 is processed by software by a processor such as a DSP (Digital Signal Processor). Of these, the advance angle determination unit 20 determines the advance angle θp with respect to the detected power supply voltage VAC. For this purpose, the advance angle determination unit 20 includes table data (see FIG. 6) indicating the relationship between the power supply voltage VAC and the advance angle θp. Yes. Here, the lead angle θp refers to the current phase of the windings 9u, 9v, 9w with respect to the induced voltage em of the brushless motor 10.
[0018]
The signal output from the magnetoresistive sensor 12 described above is input to the phase determination unit 21 as a position signal Sh through an encoder (not shown). This position signal Sh is a signal indicating the tooth position of the detection gear 11, that is, the rotation angle θr of the rotary frame 7 (rotor 8), and has a resolution of, for example, 432 pulses per rotation. The phase determination unit 21 determines the phase θv of the terminal voltages Vu, Vv, Vw of the brushless motor 10 so that the current phase with respect to the rotation angle θr becomes the advance angle θp.
[0019]
The voltage signal forming unit 22 generates a three-phase sine wave signal based on the voltage phase θv and sine wave data previously prepared as table data, and compares the three-phase sine wave signal with the carrier signal to obtain a sine wave. Voltage signals Su, Sv, Sw having a PWM waveform are generated. Although not shown, the control unit 23 includes a speed feedback control loop, and the amplitude of the three-phase sine wave signal is determined according to the speed deviation.
[0020]
The drive signal forming unit 24 receives the voltage signals Su, Sv, and Sw, and supplies the drive signals Sup, Sun, Svp, and Svn to the transistors 16up, 16un, 16vp, 16vn, 16wp, and 16wn constituting the inverter 16, respectively. , Swp, Swn are output.
[0021]
Although the description of the configuration of the image configuration processor is omitted, the position signal used when forming the tomographic image data is output from the magnetoresistive sensor 12 instead of the above-described position signal Sh of 432 pulses / revolution. This is a high-resolution pulse signal obtained by multiplying the signal (for example, 10800 pulses per rotation).
[0022]
Next, the operation of the driving device 13 will be described.
Since the control unit 23 performs speed control, the rotational speed N of the rotary frame 7 (rotor 8) is set to coincide with the command rotational speed Nr (for example, 2 rpm) within a range where the power supply voltage VAC has a margin. The magnitudes of the terminal voltages Vu, Vv, Vw, that is, the PWM modulation rate are controlled. However, in a state where the power supply voltage VAC is reduced and the PWM modulation rate reaches the upper limit, the speed control no longer functions.
[0023]
FIG. 4 shows the relationship between the generated torque T and the rotational speed N of the brushless motor 10 when the terminal voltages Vu, Vv, Vw and the lead angle θp are changed in the state where the PWM modulation rate has reached the upper limit (TN characteristic). ). In FIG. 4, straight lines 1, 2, and 3 are characteristics under the following conditions, respectively.
[0024]
(1) Straight line 1 ... when advance angle θp = 0 ° and power supply voltage VAC = V1 (2) Straight line 2 ... when advance angle θp = 0 ° and power supply voltage VAC = V2 (<V1) (3) Line 3 Lead angle θp> 0 °, power supply voltage VAC = V2
According to FIG. 4, the maximum rotational speed Nmax that can be operated in a state where the PWM modulation rate reaches the upper limit is substantially proportional to the power supply voltage VAC. For example, in FIG. 4, if the torque required to rotate the rotating frame 7 at the command rotational speed Nr (2 rpm) is Tr, the motor rotates at the command rotational speed Nr when the power supply voltage VAC is V1 (straight line 1). Although it is possible, when the power supply voltage VAC decreases to V2 (straight line (2)), the rotation speed Nr cannot be maintained due to insufficient voltage.
[0026]
On the other hand, when lead phase control (so-called lead angle control) is performed, the torque T-rotational speed N characteristic changes (straight line (3)), so that the power supply voltage VAC drops to V2 and the terminal voltages Vu, Vv, Even when Vw is lower than the induced voltage of the brushless motor 10, a positive torque can be generated. Therefore, in the above-described example, even if the power supply voltage VAC is reduced to V2, it can be rotated at the command rotational speed Nr. FIG. 5 shows a winding current waveform and an induced voltage waveform of the brushless motor 10 when the advance phase control is performed. When the advance phase control is performed, the upper limit torque Tmax that can be generated decreases. For this reason, it is not preferable to perform advance phase control at the time of acceleration / deceleration of the rotating frame 7 that requires a large torque.
[0027]
Therefore, the drive device 13 of the present embodiment performs the following control.
FIG. 6 shows the relationship between the power supply voltage VAC that the advance phase determination unit 20 has as table data and the advance angle θp. When the power supply voltage VAC detected by the voltage detector 19 is equal to or higher than the voltage Vt, the advance angle θp is set to 0 °, and when the power supply voltage VAC is lower than the voltage Vt, the power supply voltage VAC is The advance angle θp is set to increase in proportion to the decrease. Since a large current flows when X-rays are exposed by the X-ray tube 2, the power supply voltage VAC temporarily decreases due to the influence of the line impedance existing in the system of the commercial power supply 14. The voltage Vt is set to a value lower than the power supply voltage VAC during non-exposure of X-rays and higher than the power supply voltage VAC during exposure.
[0028]
By controlling in this way, the power supply voltage VAC is not greatly reduced (that is, lower than the voltage Vt) during the acceleration / deceleration of the rotating frame 7 where no X-ray exposure is performed. Therefore, the advance phase control is not performed, and it is possible to generate high acceleration / deceleration torque with high power factor and high efficiency (straight line (1) shown in FIG. 4).
[0029]
On the other hand, during constant speed rotation where X-ray exposure is performed, phase control is performed when the power supply voltage VAC drops below the voltage Vt due to X-ray exposure, so the rotation speed is commanded. The rotation speed Nr can be followed (straight line (3) shown in FIG. 4). Further, since the required torque is small during the constant speed rotation, the torque does not become insufficient due to the advance phase control.
[0030]
As described above, according to the present embodiment, the voltage detection unit 19 detects the power supply voltage VAC, and when the power supply voltage VAC is lower than the voltage Vt, the brushless motor 10 that drives the rotary frame 7 is advanced and phased. Control. By this advance phase control, the TN characteristic of the brushless motor 10 is changed, and it is possible to rotate with the lower terminal voltages Vu, Vv, and Vw with respect to the same rotation speed. As a result, the rotational speed of the brushless motor 10 can be made to follow the command rotational speed Nr even if the power supply voltage VAC decreases due to X-ray exposure or the like.
[0031]
The X-ray CT apparatus 1 adds a voltage detection unit 19 to the conventional configuration, changes a part of the program executed by the processor, and writes table data indicating the relationship between the power supply voltage VAC and the advance angle θp to the memory. This can be realized without increasing the scale of the apparatus or increasing the size of the brushless motor 10 and with only a slight increase in cost.
[0032]
Furthermore, in the X-ray CT apparatus 1, the power supply voltage VAC input to the drive circuit 13 may temporarily decrease due to X-ray exposure or the like, but this is because the rotation speed of the rotating frame 7 is stabilized at a constant speed. There is a situation that the moment of inertia of the rotating frame 7 is very large. For this reason, at the time of acceleration / deceleration of the rotating frame 7 that requires a large torque, there is no X-ray exposure, and the power supply voltage VAC is unlikely to decrease, and torque does not fall short due to advance phase control. Further, even when the rotational speed is stabilized and the phase control is temporarily advanced in accordance with the X-ray exposure timing, the torque generated by the brushless motor 10 is reduced, but the rotation is caused by the inertia moment. The impact on speed is very small.
[0033]
In the present embodiment, the advance angle θp is set to increase as the power supply voltage VAC decreases, so that the motor speed and the generated torque are maintained as high as possible while maintaining the rotational speed according to the decrease in the power supply voltage VAC. Control is performed. Further, in the normal state where the power supply voltage VAC is equal to or higher than Vt, the advance angle θp is controlled to 0 °, so that high-efficiency driving is possible.
[0034]
The present invention is not limited to the embodiment described above and shown in the drawings. For example, the present invention can be modified or expanded as follows.
In the above-described embodiment, the main circuit composed of the converter 15 and the inverter 16 is the power conversion means referred to in the present invention, and the voltage detection unit 19 detects the power supply voltage VAC input to the power conversion means. Instead of this, the inverter 16 may be considered as power conversion means, and the DC voltage VDC input to the inverter 16 may be detected as the power supply voltage.
[0035]
When the power supply voltage VAC is equal to or higher than the voltage Vt, the advance angle θp does not have to be exactly 0 °. It is also possible to monotonically increase the lead angle θp corresponding to the decrease of the supply voltage VAC. Furthermore, when the power supply voltage VAC is lower than the voltage Vt, the advance angle θp may be a positive constant value or may be increased stepwise.
[0036]
【The invention's effect】
As is apparent from the above description, the X-ray computed tomography apparatus of the present invention detects the power supply voltage input to the power conversion means for driving the brushless motor when controlling the brushless motor for driving the rotating frame. In addition, since the current phase of the stator winding with respect to the induced voltage of the brushless motor is controlled by controlling the power conversion means based on the detected power supply voltage value, the power conversion means can be obtained by X-ray exposure or the like. Even when the power supply voltage input to the power supply voltage temporarily decreases, it is possible to prevent a decrease in the rotation speed due to insufficient voltage. In addition, the present invention can be realized without increasing the scale of the apparatus.
[Brief description of the drawings]
FIG. 1 is an electrical configuration diagram of a driving apparatus showing an embodiment of the present invention. FIG. 2 is a side view of an X-ray computed tomography apparatus. FIG. 3 is a front view of an X-ray computed tomography apparatus. FIG. 5 is a diagram showing the relationship between the generated torque T and the rotational speed N of the brushless motor when the modulation rate reaches the upper limit. FIG. 5 shows the winding current waveform and the induced voltage waveform of the brushless motor when the advance phase control is performed. FIG. 6 is a diagram showing the relationship between the power supply voltage VAC and the advance angle θp as table data.
1 is an X-ray CT apparatus (X-ray computed tomography apparatus), 2 is an X-ray tube, 3 is an X-ray detector, 6 is a fixed frame, 7 is a rotating frame, 8 is a rotor, 9u, 9v and 9w are wound Lines (stator windings), 10 is a brushless motor, 13 is a driving device (rotation driving means), 19 is a voltage detection unit (voltage detection means), and 23 is a control unit (control means).

Claims (2)

A fixed frame, a rotary frame supported rotatably with respect to the fixed frame, an X-ray tube attached to the rotary frame, and an X-ray that is emitted from the X-ray tube and passes through the subject. A tomographic image data of the subject based on a line detector, a rotation driving means for rotating the rotating frame, an output of the X-ray detector and a rotation position of the X-ray tube by rotation of the rotating frame; In an X-ray computed tomography apparatus having a computer part comprising:
The rotation driving means includes
A brushless motor having a rotor with permanent magnets and a stator wound with windings;
Power conversion means for driving the brushless motor;
Voltage detection means for detecting a power supply voltage input to the power conversion means;
A power supply voltage detection value detected by the voltage detection means having a voltage value set to a value lower than the power supply voltage during non-exposure of X-rays and higher than the power supply voltage during exposure. And a control means for controlling the current of the stator winding so as to be in a leading phase with respect to the induced voltage of the brushless motor when X is lower than the threshold value. Line computed tomography equipment. 2. The X-ray computed tomography apparatus according to claim 1, wherein the control means controls the advance phase to increase as the power supply voltage detection value decreases .

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