Electromagnetic power supply device

Abstract

To provide an electromagnet power supply device capable of controlling a current to be supplied to an electric magnet with higher accuracy even when a current pattern is changed at a high speed.SOLUTION: An electromagnet power supply device comprises: a main circuit part that is connected with an electric magnet, and supplies a current according to the current pattern changed step by step to a magnet; a current detection device that detects the current flowing through the electric magnet; and a control part that controls an operation of the main circuit part on the basis of the current pattern and a detection result of the current detection device. The main circuit part includes: a forcing circuit that is operated during a period of changing the current pattern step by step; and a flat maintaining circuit which is connected to the forcing circuit in serial, and in which the current pattern is operated in a constant period. The flat maintaining circuit includes four switching elements connected in full-bridge. The control part performs the second quadrant control on the flat maintaining circuit.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] SUMMARY OF THE INVENTION An embodiment of the present invention relates to an electromagnetic power supply. [Background technology]

[0002] Electromagnet power supply devices for voltage-source power conversion systems are known that supply an electromagnet with a current corresponding to a current pattern that changes stepwise. In such electromagnet power supply devices, two chopper circuits are connected in series. One chopper circuit operates during a period in which the current command value changes stepwise. This chopper circuit is called, for example, a forcing circuit. The other chopper circuit operates during a period in which the current command value is constant. This chopper circuit is called, for example, a flat-maintaining circuit.

[0003] In this way, by providing two chopper circuits connected in series, it is possible to obtain high responsiveness of the load current to changes in the current command value, and to control the load current with high precision for a constant current command value. Such an electromagnet power supply device is used, for example, in medical scanning electromagnets.

[0004] The forcing circuit and flat maintenance circuit are, for example, full-bridge converters with four switching elements connected in a full bridge configuration. The forcing circuit and flat maintenance circuit can supply positive and negative polarity currents to the electromagnets by switching each switching element. The negative polarity current flows in the opposite direction to the positive polarity current.

[0005] To control the switching of each switching element of the flat maintenance circuit, for example, so-called PWM (Pulse Width Modulation) control is used, which periodically changes the on and off states of each switching element. The flat maintenance circuit changes the ratio (duty ratio) of the on and off states of each switching element to keep the current supplied to the electromagnet at a constant magnitude according to the current command value.

[0006] In the above-described electromagnetic power supply device, relatively high-speed operation is sometimes required, such as switching between the forcing circuit and the flat-maintenance circuit at a frequency of several tens of kilohertz. In other words, relatively high-speed changes in the current pattern are sometimes required. In such high-speed operation, it is particularly important to quickly and stably transition from the forcing circuit to the flat-maintenance circuit and to highly accurately control the current in the flat-maintenance circuit. Therefore, in order to achieve high-speed operation, it is necessary to increase the switching frequency of each switching element in the flat-maintenance circuit.

[0007] However, when performing PWM control, if the switching frequency of each switching element in the flatness maintaining circuit is increased, the influence of the minimum on-time and dead time of each switching element increases, which may result in a decrease in the controllability of the output current. For example, there may be no margin for control near the maximum positive and negative values ​​of the output current, which may result in an inability to properly supply a current of a magnitude corresponding to the current command value to the electromagnet.

[0008] For this reason, it is desirable for the electromagnet power supply device to be able to control the current supplied to the electromagnet with higher precision even when the current pattern changes at high speed. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 1-279311 Summary of the Invention [Problem to be solved by the invention]

[0010] SUMMARY OF THE INVENTION Embodiments of the present invention provide an electromagnet power supply device that can more accurately control the current supplied to the electromagnet even when the current pattern changes rapidly. [Means for solving the problem]

[0011] According to an embodiment of the present invention, there is provided a main circuit unit connected to an electromagnet and supplying a current to the electromagnet according to a current pattern that changes stepwise, a current detector that detects the current flowing through the electromagnet, and a control unit that receives an input of the current pattern and controls the operation of the main circuit unit based on the input current pattern and the detection result of the current detector, wherein the current pattern is a periodic pattern that repeats step-like changes between a maximum value and a minimum value of a current to be supplied to the electromagnet, the main circuit unit has a forcing circuit that operates during a period in which the current pattern changes stepwise, and a flat maintenance circuit that is connected in series with the forcing circuit and operates during a fixed period in which the current pattern changes stepwise, the flat maintenance circuit has four switching elements that are connected in a full bridge configuration, and two of the four switching elements are capable of supplying a current of positive polarity to the electromagnet, and the remaining two of the four switching elements are capable of supplying a current of negative polarity to the electromagnet, and the control unit ... When a positive polarity current is supplied to the electromagnet from the flat maintenance circuit, the on state and off state of the two switching elements for supplying a positive polarity current to the electromagnet are switched in a predetermined cycle, the ratio between the on state and the off state is changed based on the detection result of the current detector and the current pattern, and the two switching elements for supplying a negative polarity current to the electromagnet are set to the off state for the entire period of the predetermined cycle, so that a constant positive polarity current according to the current pattern is supplied to the electromagnet; when a negative polarity current is supplied to the electromagnet from the flat maintenance circuit, the on state and off state of the two switching elements for supplying a negative polarity current to the electromagnet are switched in a predetermined cycle, the ratio between the on state and the off state is changed based on the detection result of the current detector and the current pattern, and the two switching elements for supplying a positive polarity current to the electromagnet are set to the off state for the entire period of the predetermined cycle,There is provided an electromagnet power supply device that performs two-quadrant control so that a constant negative polarity current according to the current pattern is supplied to the electromagnet. [Effects of the Invention]

[0012] According to an embodiment of the present invention, an electromagnet power supply device is provided that can control the current supplied to the electromagnet with higher precision even when the current pattern changes at high speed. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a block diagram schematically illustrating an electromagnetic power supply device according to an embodiment. [Figure 2] FIG. 2 is a circuit diagram schematically illustrating a main circuit section. [Figure 3] 3(a) and 3(b) are graphs that schematically show an example of the operation of the control unit. [Figure 4] 10 is a graph schematically illustrating an example of the operation of the control unit. [Figure 5] 5(a) to 5(d) are timing charts that schematically show an example of the operation of the main circuit unit and the control unit. [Figure 6] 6(a) to 6(d) are timing charts that schematically show reference operations of the main circuit section and the control section.

[0014] Each embodiment will be described below with reference to the drawings. The drawings are schematic or conceptual, and the relationship between the thickness and width of each part, the size ratio between parts, etc. are not necessarily the same as those in reality. Furthermore, even when the same part is shown, the dimensions and ratios may be different depending on the drawing. In the present specification and the drawings, elements similar to those described above with reference to the previous drawings are designated by the same reference numerals, and detailed descriptions thereof will be omitted as appropriate.

[0015] FIG. 1 is a block diagram that schematically illustrates an electromagnetic power supply device according to an embodiment. As shown in FIG. 1, the electromagnet power supply device 10 includes a main circuit unit 12 and a control unit 14. The main circuit unit 12 is connected to an electromagnet 2, which is a load, and supplies the electromagnet 2 with a current according to a current pattern that changes stepwise. The main circuit unit 12 operates the electromagnet 2 by supplying the current. The main circuit unit 12 is, for example, a voltage-type power conversion device. The control unit 14 controls the operation of the main circuit unit 12. More specifically, the control unit 14 controls the operation of the main circuit unit 12 to supply current to the electromagnet 2.

[0016] The main circuit section 12 has a forcing circuit 16 and a flat maintenance circuit 18. The flat maintenance circuit 18 is connected in series with the forcing circuit 16. The forcing circuit 16 operates during a period when the current pattern (current command value) changes stepwise. The flat maintenance circuit 18 operates during a period when the current pattern (current command value) is constant.

[0017] For example, reactors 21 and 22 are provided on the output side of the forcing circuit 16. For example, reactors 23 and 24 are provided on the output side of the flat maintenance circuit 18. The reactors 21 to 24 are, for example, reactors for smoothing ripples. The flat maintenance circuit 18 is connected in series with the forcing circuit 16 via, for example, reactors 22 and 23. The forcing circuit 16 and the flat maintenance circuit 18 (main circuit section 12) are connected to the electromagnet 2 via the reactors 21 and 24.

[0018] The electromagnet power supply device 10 further includes, for example, a DC power supply 30 and a DC power supply 40. The forcing circuit 16 is connected to the electromagnet 2, which is a load, and is also connected to the DC power supply 30. The forcing circuit 16 operates to change the current supplied to the electromagnet 2 in a step-like manner by switching between supplying and stopping the DC voltage supplied from the DC power supply 30 to the electromagnet 2. The current supplied from the main circuit unit 12 to the electromagnet 2 changes in a step-like manner when the forcing circuit 16 is outputting a DC voltage.

[0019] The DC power supply 30 includes, for example, a transformer 31, a rectifier 32, a reactor 33, and a capacitor 34. The rectifier 32 is connected to the AC power supply 4 via the transformer 31. The rectifier 32 rectifies the AC power supplied from the AC power supply 4 and converts it into rectified power. The reactor 33 and the capacitor 34 smooth the rectified power converted by the rectifier 32 and convert it into DC power. In this way, the DC power supply 30 supplies the DC power to the forcing circuit 16.

[0020] The flat maintenance circuit 18 is connected to the electromagnet 2, which is a load, and is also connected to a DC power supply 40. The flat maintenance circuit 18 controls the current supplied to the electromagnet 2 to be a constant current according to a current command value represented by a current pattern by switching between supplying and stopping the supply of DC voltage supplied from the DC power supply 40 to the electromagnet 2.

[0021] The DC power supply 40 includes, for example, a transformer 41, a rectifier 42, a reactor 43, and a capacitor 44. The configuration of the DC power supply 40 is similar to that of the DC power supply 30, and therefore a detailed description thereof will be omitted. As a result, the DC power supply 40 supplies DC power to the flat maintenance circuit 18.

[0022] The configuration of the DC power supplies 30, 40 is not limited to the above, and any configuration may be used that is capable of supplying DC power to the forcing circuit 16 and the flat maintenance circuit 18. Furthermore, the electromagnet power supply device 10 does not necessarily have to include the DC power supplies 30, 40. The forcing circuit 16 and the flat maintenance circuit 18 may receive DC power from an external DC power supply.

[0023] FIG. 2 is a circuit diagram that schematically illustrates the main circuit section. As shown in FIG. 2, the forcing circuit 16 has four full-bridge connected switching elements 50a to 50d and four rectifying elements 52a to 52d connected in anti-parallel to the four switching elements 50a to 50d, respectively.

[0024] Similarly, the flat maintenance circuit 18 has four full-bridge connected switching elements 54a to 54d, and four rectifying elements 56a to 56d connected in anti-parallel to the four switching elements 54a to 54d, respectively.

[0025] In other words, the forcing circuit 16 and the flat maintaining circuit 18 are chopper circuits. In other words, the forcing circuit 16 is a first chopper circuit, and the flat maintaining circuit 18 is a second chopper circuit.

[0026] In the forcing circuit 16, the connection point between the upper (high-side) switching element 50a and the lower (low-side) switching element 50c, and the connection point between the upper switching element 50b and the lower switching element 50d, respectively, form a pair of output points 16a and 16b. Similarly, in the flat maintaining circuit 18, the connection point between the upper switching element 54a and the lower switching element 54c, and the connection point between the upper switching element 54b and the lower switching element 54d, respectively, form a pair of output points 18a and 18b.

[0027] Output point 18a of flat maintenance circuit 18 is connected to output point 16b of forcing circuit 16 via reactors 22 and 23. As a result, the output side of flat maintenance circuit 18 is connected in series with the output side of forcing circuit 16. Output point 16a of forcing circuit 16 is connected to one end of electromagnet 2 via reactor 21. Output point 18b of flat maintenance circuit 18 is connected to the other end of electromagnet 2 via reactor 24. As a result, the output sides of forcing circuit 16 and flat maintenance circuit 18, which are connected in series, are connected to electromagnet 2, and power output from forcing circuit 16 and flat maintenance circuit 18 is supplied to electromagnet 2.

[0028] Each of the switching elements 50a-50d, 54a-54d has a pair of main terminals and a control terminal. Depending on the voltage input to the control terminal, each of the switching elements 50a-50d, 54a-54d switches between an ON state in which current flows between the pair of main terminals and an OFF state in which current is blocked between the pair of main terminals. The OFF state does not necessarily have to be a state in which current flow between the main terminals is completely blocked, but may be a state in which a weak current flows between the main terminals to the extent that it does not affect the operation of the main circuit unit 12. The ON state is, in other words, a first state in which current flows between the main terminals, and the OFF state is, in other words, a second state in which current flows between the main terminals in a smaller amount than in the first state.

[0029] Each of the switching elements 50a to 50d, 54a to 54d may be a self-excited semiconductor switching element such as an IGBT (Insulated Gate Bipolar Transistor), but the switching elements 50a to 50d, 54a to 54d are not limited to this and may be any switching element that can be controlled to switch between an on state and an off state.

[0030] In the forcing circuit 16 and flat maintenance circuit 18 of the full-bridge circuit, the direction of the current supplied to the electromagnet 2 can be controlled by switching the respective switching elements 50a to 50d and 54a to 54d. The forcing circuit 16 and flat maintenance circuit 18 can supply a current of positive polarity and a current of negative polarity to the electromagnet 2 by switching the respective switching elements 50a to 50d and 54a to 54d.

[0031] In the flat maintaining circuit 18, the switching element 54a is, in other words, the first upper switching element. The switching element 54b is, in other words, the first lower switching element. The switching element 54c is, in other words, the second upper switching element. The switching element 54d is, in other words, the second lower switching element. For example, the switching element 54a may be called the U-phase switching element, the switching element 54b the X-phase switching element, the switching element 54c the V-phase switching element, and the switching element 54d the Y-phase switching element.

[0032] One main terminal of the switching element 54a is connected to the high-potential terminal of the DC power supply 40 (capacitor 44). The other main terminal of the switching element 54a is connected to one main terminal of the switching element 54b. The other main terminal of the switching element 54b is connected to the low-potential terminal of the DC power supply 40 (capacitor 44). One main terminal of the switching element 54c is connected to the high-potential terminal of the DC power supply 40 (capacitor 44). The other main terminal of the switching element 54c is connected to one main terminal of the switching element 54d. The other main terminal of the switching element 54d is connected to the low-potential terminal of the DC power supply 40 (capacitor 44).

[0033] The switching elements 54a and 54b are connected in series to form one leg of a full-bridge circuit, while the switching elements 54c and 54d are connected in series and in parallel with the switching elements 54a and 54b to form the other leg of the full-bridge circuit.

[0034] As a result, in the flat maintenance circuit 18, by turning on the switching elements 54a and 54d and turning off the switching elements 54b and 54c, a current of positive polarity can be supplied to the electromagnet 2. By turning on the switching elements 54b and 54c and turning off the switching elements 54a and 54d, a current of negative polarity can be supplied to the electromagnet 2.

[0035] In this way, the flat maintenance circuit 18 can supply a positive polarity current to the electromagnet 2 using two switching elements 54a, 54d of the four full-bridge connected switching elements 54a to 54d, and can supply a negative polarity current to the electromagnet 2 using the remaining two switching elements 54b, 54c of the four switching elements 54a to 54d.

[0036] The configuration of the forcing circuit 16 is the same as that of the flat maintenance circuit 18. However, the configurations of the forcing circuit 16 and the flat maintenance circuit 18 are not limited to the above and may be any configuration that has at least four full-bridge-connected switching elements and is capable of appropriately supplying a current to the electromagnet 2 according to a current pattern that changes stepwise. The forcing circuit 16 and the flat maintenance circuit 18 may be able to handle high voltages and large currents by, for example, configuring each arm with multiple switching elements connected in series or in parallel. The configuration of the forcing circuit 16 does not necessarily have to be the same as that of the flat maintenance circuit 18.

[0037] The control unit 14 is connected to the control terminals of, for example, each of the switching elements 50a to 50d, 54a to 54d, and controls the operation of the forcing circuit 16 and the flat maintaining circuit 18 by controlling the switching between the on state and the off state of each of the switching elements 50a to 50d, 54a to 54d.

[0038] The control unit 14 controls the switching between the on and off states of each of the switching elements 50a to 50d of the forcing circuit 16, thereby controlling the operation of changing the current supplied to the electromagnet 2 in a stepwise manner.

[0039] Furthermore, control unit 14 causes forcing circuit 16 to perform an output operation that outputs a voltage during a period in which the current pattern changes stepwise, and also causes forcing circuit 16 to perform a stop operation that makes the output side conductive (bypass) and stops the output of voltage during a period in which the current pattern is constant. The output operation is, in other words, a forcing operation.

[0040] In this example, the pair of output points 16a, 16b can be brought into a conductive state by turning on the upper switching elements 50a, 50c and turning off the lower switching elements 50b, 50d, or conversely, by turning off the upper switching elements 50a, 50c and turning on the lower switching elements 50b, 50d.

[0041] The control unit 14 controls the switching between the on and off states of each of the switching elements 54a to 54d of the flat maintenance circuit 18, thereby controlling the current supplied to the electromagnet 2 to be a constant current according to the current pattern when the current pattern is in a constant period. The control unit 14 controls the current supplied to the electromagnet 2 to be a constant current according to the current pattern, for example, by quickly switching between the on and off states of each of the switching elements 54a to 54d. The control unit 14 controls the current supplied to the electromagnet 2 to be a constant current according to the current pattern, for example, by performing ACR (Auto Current Regulator) control on the flat maintenance circuit 18.

[0042] 1, the electromagnet power supply device 10 further includes, for example, a current detector 60. The current detector 60 detects the current flowing through the electromagnet 2 and inputs the detection result to the control unit 14.

[0043] 3(a) and 3(b) are graphs that schematically show an example of the operation of the control unit. Fig. 3(a) schematically shows an example of a current pattern input to the control unit 14 and an example of an output current supplied from the main circuit unit 12 to the electromagnet 2. Fig. 3(b) shows an enlarged view of the area surrounded by the dashed line in Fig. 3(a).

[0044] As shown in FIGS. 3(a) and 3(b), a current pattern that changes stepwise is input to the control unit 14. The current pattern is a periodic pattern that repeats a stepwise change between the maximum and minimum values ​​of the current supplied to the electromagnet 2. For example, the current pattern repeats a stepwise change from the minimum value of the current supplied to the electromagnet 2 to the maximum value, and then a stepwise change from the maximum value to the minimum value. Furthermore, the current pattern changes the direction of the current supplied to the electromagnet 2 between the positive and negative sides, for example. The maximum value of the current supplied to the electromagnet 2 is, in other words, the maximum value on the positive side, and the minimum value of the current supplied to the electromagnet 2 is, in other words, the maximum value on the negative side. The frequency of the current pattern is, for example, several kHz (1 kHz or more). In other words, the frequency of the current pattern is the reciprocal of the period from the maximum value to the next maximum value.

[0045] The current pattern is input to the control unit 14 as needed, for example, from a higher-level controller via communication. In other words, the control unit 14 receives the current pattern input from an external device such as a higher-level controller by communicating with the external device. The current pattern is, in other words, a current command value that indicates the magnitude of the current to be supplied to the electromagnet 2. In other words, the control unit 14 receives the current command value input from the external device by communicating with the external device.

[0046] The current pattern (current command value) may be input to the control unit 14 in advance, for example, and stored in a storage unit of the control unit 14 or another storage unit connected to the control unit 14. The method for inputting the current pattern to the control unit 14 is not limited to the above, and any method that can appropriately input the current pattern to the control unit 14 may be used.

[0047] The control unit 14 controls the operation of the main circuit unit 12 (the forcing circuit 16 and the flat maintaining circuit 18) based on the input current pattern and the detection result of the current detector 60.

[0048] The control unit 14 causes the forcing circuit 16 to perform a stop operation to make the output side conductive during a period when the current pattern is constant, and also controls the operation of the flat maintenance circuit 18 based on the detection results of the current detector 60 so that the current supplied to the electromagnet 2 becomes a constant current according to the current pattern.

[0049] As shown in Figure 3(a), when changing the polarity of the current supplied to the electromagnet 2, the control unit 14 changes the polarity of the voltage output from the flat maintenance circuit 18 depending on whether a current of positive polarity is supplied to the electromagnet 2 or a current of negative polarity is supplied to the electromagnet 2.

[0050] FIG. 4 is a graph schematically illustrating an example of the operation of the control unit. As shown in FIGS. 3( a), 3(b), and 4, the control unit 14 causes the forcing circuit 16 to perform an output operation to output a voltage when the current pattern changes stepwise. The control unit 14 changes the polarity of the voltage output from the forcing circuit 16 depending on whether the current supplied to the electromagnet 2 is lower than the current pattern or higher than the current pattern. For example, when the current supplied to the electromagnet 2 is lower than the current pattern and the current supplied to the electromagnet 2 is to be increased toward a target current value, the control unit 14 causes the forcing circuit 16 to output a voltage with a positive polarity. On the other hand, when the current supplied to the electromagnet 2 is higher than the current pattern and the current supplied to the electromagnet 2 is to be decreased toward a target current value, the control unit 14 causes the forcing circuit 16 to output a voltage with a negative polarity.

[0051] When the forcing circuit 16 is caused to perform an output operation, the flat maintenance circuit 18 may continue to control the current supplied to the electromagnet 2 to be constant, or may perform a stop operation to make the output side conductive.

[0052] 3(b) and 4, the forcing-on period during which the forcing circuit 16 performs an output operation is represented by the period indicated by arrow (1), and the flat operation period during which the flat maintenance circuit 18 performs an operation to control the current to a constant level is represented by the period indicated by arrow (2). Note that in Fig. 3(b), in consideration of the response delay between the timing of a change in the current pattern and the timing of operating the forcing circuit 16, the timing of the start of the forcing-on period is shown slightly delayed from the timing of a change in the current pattern.

[0053] After causing forcing circuit 16 to perform an output operation, control unit 14 switches forcing circuit 16 from output operation to stop operation at the timing when a current deviation representing the difference between a target current value represented by the current pattern and the current output current value detected by current detector 60 reaches a predetermined threshold. In other words, control unit 14 switches from a forcing-on period to a flat operation period at the timing when the current deviation reaches the predetermined threshold. In other words, control unit 14 switches forcing circuit 16 from output operation to stop operation at the timing when the current supplied to electromagnet 2 reaches the target current value.

[0054] The threshold value may be set to zero deviation according to the target current value, or may be set to switch to a stopping operation earlier, taking into account overshoot (undershoot) of the current deviation due to a delay in the operation of the forcing circuit 16, etc. For example, as shown in FIG. 4, when switching the forcing circuit 16 from output operation to stopping operation, it may be required that the current deviation be within a predetermined range. The predetermined range is, for example, within ±0.1% of the rated current value. The rated current value is the current supplied to the electromagnet 2 when the forcing circuit 16 performs an output operation. In such a case, the threshold value may be set appropriately, taking into account overshoot of the current deviation, etc., so that the current deviation falls within the predetermined range.

[0055] 5(a) to 5(d) are timing charts that schematically show an example of the operation of the main circuit unit and the control unit. FIG. 5(a) shows an example of the operation of the switching element 54a of the flat maintenance circuit 18. FIG. 5(b) shows an example of the operation of the switching element 54b of the flat maintenance circuit 18. FIG. 5(c) shows an example of the operation of the switching element 54c of the flat maintenance circuit 18. FIG. 5(d) shows an example of the operation of the switching element 54d of the flat maintenance circuit 18.

[0056] 5(a) to 5(d), the on state of each of the switching elements 54a to 54d is shown as a high-voltage state, and the off state of each of the switching elements 54a to 54d is shown as a low-voltage state. In other words, FIGS. 5(a) to 5(d) schematically show an example of a control signal input from the control unit 14 to the control terminal of each of the switching elements 54a to 54d. Each of the switching elements 54a to 54d is in the on state when the voltage of the control signal input to the control terminal is high, and in the off state when the voltage of the control signal input to the control terminal is low.

[0057] 5(a) to 5(d) schematically show an example of operation when a positive current is supplied from the flat maintenance circuit 18 to the electromagnet 2. As shown in FIGS. 5(a) to 5(d), when a positive current is supplied from the flat maintenance circuit 18 to the electromagnet 2, the control unit 14 switches the on and off states of two switching elements 54a and 54d for supplying a positive current to the electromagnet 2 at a predetermined cycle T, and changes the ratio (duty ratio) between the on and off states based on the detection result of the current detector 60 and the current pattern, so that a constant positive current according to the current pattern is supplied to the electromagnet 2. Furthermore, when a positive current is supplied from the flat maintenance circuit 18 to the electromagnet 2, the control unit 14 sets the two switching elements 54b and 54c for supplying a negative current to the electromagnet 2 to the off state throughout the entire cycle T.

[0058] When a negative polarity current is supplied from the flat maintenance circuit 18 to the electromagnet 2, the control unit 14, in contrast to the above, switches between the on and off states of the two switching elements 54b, 54c for supplying a negative polarity current to the electromagnet 2 at a predetermined cycle T, and changes the ratio of the on and off states based on the detection results of the current detector 60 and the current pattern, so that a constant negative polarity current according to the current pattern is supplied to the electromagnet 2. When a negative polarity current is supplied from the flat maintenance circuit 18 to the electromagnet 2, the control unit 14 sets the two switching elements 54a, 54d for supplying a positive polarity current to the electromagnet 2 to the off state for the entire period of the predetermined cycle T.

[0059] In this way, the control unit 14 switches between the on and off states of two of the four full-bridge-connected switching elements 54a to 54d of the flattening circuit 18, and leaves the remaining two switching elements in the off state, depending on the polarity of the current supplied to the electromagnet 2. This type of control is sometimes called, for example, a two-quadrant control method (two-quadrant gate method). The control unit 14 performs two-quadrant control on the flattening circuit 18.

[0060] In the control of the flat maintenance circuit 18 described above, the switching frequency of each of the switching elements 54a to 54d is higher than the frequency of the current pattern. For example, if the frequency of the current pattern is several kHz, the switching frequency of each of the switching elements 54a to 54d is set to several tens of kHz. The switching frequency of each of the switching elements 54a to 54d is set to, for example, 10 times or more the frequency of the current pattern. This allows the operation of the flat maintenance circuit 18 to be controlled so that, by switching each of the switching elements 54a to 54d, a current of a substantially constant magnitude corresponding to the current pattern is supplied to the electromagnet 2.

[0061] 6(a) to 6(d) are timing charts that schematically show reference operations of the main circuit section and the control section. The contents of FIGS. 6(a) to 6(d) are the same as those of FIGS. 5(a) to 5(d), and therefore detailed description thereof will be omitted.

[0062] 6(a) to 6(d), in the reference operation, when a positive polarity current is supplied from the flat maintenance circuit 18 to the electromagnet 2, the on and off states of the switching elements 54a and 54d for supplying a positive polarity current to the electromagnet 2 are switched at a predetermined cycle T, and the ratio of the on and off states is changed based on the detection result of the current detector 60 and the current pattern, so that a constant positive polarity current according to the current pattern is supplied to the electromagnet 2. Also, in the reference operation, when a positive polarity current is supplied from the flat maintenance circuit 18 to the electromagnet 2, the on and off states of the switching elements 54b and 54c for supplying a negative polarity current to the electromagnet 2 are also switched at a predetermined cycle T.

[0063] In this case, in the reference operation, the switching elements 54b and 54c are turned on while the switching elements 54a and 54d are turned off, and the switching elements 54a and 54d are turned on while the switching elements 54b and 54c are turned off, thereby preventing a short circuit between the upper and lower arms. In the reference operation, when a negative current is supplied from the flat maintenance circuit 18 to the electromagnet 2, the opposite operation to the above is performed.

[0064] In this manner, in the reference operation, the four full-bridge-connected switching elements 54a to 54d of the flat maintenance circuit 18 are switched between on and off states, and the duty ratio of each of the switching elements 54a to 54d is changed, thereby supplying a constant magnitude of current according to the current pattern to the electromagnet 2. This type of control is called, for example, PWM control.

[0065] The electromagnet power supply device 10 may be required to operate at a relatively high speed, such as by switching between the forcing circuit 16 and the flat maintenance circuit 18 at a frequency of several tens of kilohertz. In other words, a relatively high-speed change in the current pattern may be required. In such high-speed operation, it is particularly important to quickly and stably transition from the forcing circuit 16 to the flat maintenance circuit 18 and to achieve highly accurate current control in the flat maintenance circuit 18. Therefore, in order to achieve high-speed operation, it is necessary to increase the switching frequency of the switching elements 54a to 54d of the flat maintenance circuit 18.

[0066] 6(a) to 6(d), if the switching frequency of each of the switching elements 54a to 54d of the flat maintenance circuit 18 is increased, the influence of the minimum on-time (periods t1 and t3 in FIG. 6) and the dead time (period t2 in FIG. 6) of each of the switching elements 54a to 54d increases, which may result in a decrease in the controllability of the output current. For example, there may be no margin for control near the maximum value on the positive side of the output current and near the maximum value on the negative side of the output current, which may result in an inability to properly supply a current of a magnitude corresponding to the current command value to the electromagnet 2.

[0067] The minimum on-time is, more specifically, the minimum time required to properly turn on each of the switching elements 54a to 54d. The dead time is, more specifically, the time set from the timing when the upper switching element switches from on to off until the timing when the lower switching element switches from off to on, and from the timing when the lower switching element switches from on to off until the timing when the upper switching element switches from off to on, in order to prevent a short circuit between the upper and lower arms.

[0068] In PWM control, it is not permitted to set each of the switching elements 54a to 54d to the ON state for the entire period of the predetermined cycle T, and each of the switching elements 54a to 54d must be in the OFF state for at least the minimum OFF time within the predetermined cycle T. For this reason, when the frequency of each of the switching elements 54a to 54d increases, the influence of the minimum ON time and dead time on the predetermined cycle T increases, which may cause a decrease in controllability of the output current.

[0069] For example, if the switching frequency of each of the switching elements 54a to 54d is 40 kHz (period T is 25 μs), and the minimum on-times t1, t3 and dead time t2 are each 1.5 μs, the maximum control amount of each of the switching elements 54a to 54d is (25 μs - 1.5 μs - 1.5 μs - 1.5 μs) / 25 μs = 0.82.

[0070] In contrast, in the electromagnet power supply device 10 of this embodiment, the control unit 14 switches between the on and off states of two of the four full-bridge-connected switching elements 54a to 54d of the flat maintenance circuit 18 depending on the polarity of the current supplied to the electromagnet 2, and leaves the remaining two switching elements in the off state.

[0071] In the electromagnetic power supply device 10 according to this embodiment, as shown in FIG. 5, it is possible to eliminate the need to set a dead time associated with switching to the on state of the upper switching element and the switching element opposite the lower switching element.

[0072] For example, in the electromagnetic power supply device 10 according to this embodiment, if the switching frequency of each of the switching elements 54a to 54d is 40 kHz and the minimum off time of each of the switching elements 54a to 54d is 1.5 μs, the maximum control amount of each of the switching elements 54a to 54d can be (25 μs - 1.5 μs) / 25 μs = 0.94.

[0073] In this way, the electromagnet power supply device 10 according to this embodiment can control the current supplied to the electromagnet 2 with higher precision than when PWM control is performed, even when the current pattern changes at high speed.

[0074] For example, the control unit 14 performs two-quadrant control on the flat maintenance circuit 18 during the entire period of the current pattern. For example, the control unit 14 may perform two-quadrant control on the flat maintenance circuit 18 during the period when the absolute value of the current pattern is equal to or greater than a predetermined value, and may perform PWM control on the flat maintenance circuit 18 during the period when the absolute value of the current pattern is less than the predetermined value.

[0075] For example, in a period when the absolute value of the current pattern is 90% or more, the control unit 14 performs two-quadrant control on the flat maintenance circuit 18. In other words, the control unit 14 performs two-quadrant control on the flat maintenance circuit 18 near the maximum value on the positive side and near the maximum value on the negative side of the current pattern.

[0076] In this way, by performing PWM control to switch the on and off states of each of the switching elements 54a to 54d during a period in which the absolute value of the current pattern is less than a predetermined value, it is possible to improve the controllability of the output current during a period in which the absolute value of the current pattern is low, for example.

[0077] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims. [Explanation of symbols]

[0078] 2...electromagnet, 4...AC power supply, 10...electromagnet power supply device, 12...main circuit section, 14...control section, 16...forcing circuit, 18...flat maintenance circuit, 21-24...reactor, 30, 40...DC power supply, 31, 41...transformer, 32, 42...rectifier, 33, 43...reactor, 34, 44...capacitor, 50a-50d, 54a-54d...switching element, 52a-52d, 56a-56d...rectifying element, 60...current detector

Claims

1. a main circuit section connected to the electromagnet and supplying a current to the electromagnet according to a current pattern that changes stepwise; a current detector for detecting a current flowing through the electromagnet; a control unit that receives the current pattern and controls the operation of the main circuit unit based on the input current pattern and the detection result of the current detector; Equipped with the current pattern is a periodic pattern in which the current supplied to the electromagnet repeatedly changes stepwise between a maximum value and a minimum value, The main circuit section includes: a forcing circuit that operates during a period in which the current pattern changes stepwise; a flat-maintaining circuit connected in series with the forcing circuit, the flat-maintaining circuit operating for a fixed period of time; and the flat maintenance circuit has four switching elements connected in a full bridge configuration, and two of the four switching elements are capable of supplying a current of positive polarity to the electromagnet, and the remaining two of the four switching elements are capable of supplying a current of negative polarity to the electromagnet; The control unit switches between the on state and the off state of the two switching elements for supplying a current of negative polarity to the electromagnet at a predetermined cycle when a current of positive polarity is supplied from the flat maintenance circuit to the electromagnet, changes the ratio between the on state and the off state based on the detection result of the current detector and the current pattern, and sets the two switching elements for supplying a current of negative polarity to the electromagnet to the off state for the entire period of the predetermined cycle, so that a current of constant positive polarity according to the current pattern is supplied to the electromagnet; and when a current of negative polarity is supplied to the electromagnet from the flat maintenance circuit, switches between the on state and the off state of the two switching elements for supplying a current of negative polarity to the electromagnet at a predetermined cycle, changes the ratio between the on state and the off state based on the detection result of the current detector and the current pattern, and sets the two switching elements for supplying a current of positive polarity to the electromagnet to the off state for the entire period of the predetermined cycle, so that a current of constant negative polarity according to the current pattern is supplied to the electromagnet.

2. 2. The electromagnetic power supply device according to claim 1, wherein the control unit performs the two-quadrant control on the flat maintenance circuit during all periods of the current pattern.

3. 2. The electromagnetic power supply device according to claim 1, wherein the control unit performs the two-quadrant control on the flat maintenance circuit during a period in which the absolute value of the current pattern is equal to or greater than a predetermined value, and performs PWM control on the flat maintenance circuit by switching the on and off states of each of the four switching elements at a predetermined cycle during a period in which the absolute value of the current pattern is less than the predetermined value.

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