Digital PWM (Pulse Width Modulation) control method for gradient amplifier

Abstract

The invention discloses a gradient amplifier digital PWM (Pulse Width Modulation) control method, which comprises the following steps of: receiving input control and output current feedback signals by a PWM control module, processing data through a PID (Proportion Integration Differentiation) algorithm after data processing, generating real-time error data, correspondingly generating multiple paths of PWM signals from the real-time error data through a specific PWM control mechanism, and outputting the multiple paths of PWM signals. Correspondingly controlling the on-off and duration time of each power electronic power device; the power output module receives multiple paths of PWM signals, drives corresponding power electronic power devices, and converts the high-voltage direct-current signals into high-voltage pulse signals corresponding to the PWM signals; and the output filtering module filters the received high-voltage pulse signal into an expected direct-current voltage signal and loads the direct-current voltage to two ends of a load of the gradient amplifier to form a required precise current signal, and meanwhile, a current feedback signal is detected and output through a current sensor. According to the invention, high current change rate and low steady-state current fluctuation output by the gradient amplifier can be realized.

Description

Technical Field

[0001] The invention belongs to the technical field of nuclear magnetic resonance imaging, and in particular relates to a digital PWM control method for a gradient amplifier. Background Art

[0002] Gradient amplifiers are an important part of magnetic resonance imaging systems and are usually used to provide current to the gradient coils in magnetic resonance imaging systems. Their function is to generate magnetic field gradients in the X-axis, Y-axis, and Z-axis directions of the magnetic resonance imaging space area and use magnetic field gradients for spatial encoding for imaging.

[0003] The gradient amplifier drives the gradient coil to provide accurate current with fidelity that meets the requirements of magnetic resonance imaging, so that the gradient coil generates a linearly changing magnetic field in the imaging space, namely the gradient magnetic field. For the gradient amplifier, the stability of its output current, high current change rate, low steady-state current fluctuation and waveform tracking ability determine the quality of the gradient magnetic field performance, and the performance of the gradient magnetic field directly affects the quality of imaging and the diagnosis results of the human body.

[0004] As the gradient amplifier current change rate required by magnetic resonance imaging systems becomes higher and higher, the output voltage of the gradient amplifier needs to be higher and higher when the parameters such as the gradient coil inductance remain relatively unchanged. However, due to the technical parameter limitations of the currently available power electronic power devices, the switching frequency of high-current and high-voltage power devices is very low, and the hard switching frequency is generally less than 10KHz, while the voltage level of high-current power devices with higher switching frequency is relatively low, generally not exceeding 1200V. In order to achieve high current change rate and low steady-state current fluctuation of the gradient amplifier output, the design solutions of domestic and foreign equipment manufacturers mostly choose a multi-level series structure. Some manufacturers choose low-frequency high-voltage tube switches to achieve high current change rate, and superimpose multiple low-frequency high-voltage tube phase-shift switches to achieve low steady-state current fluctuation.

[0005] Disadvantages of the prior art: Gradient amplifiers with a multi-level series structure often require a large number of power supplies and have a relatively complex structure. In addition, when the power is sheared, the output voltage of the gradient amplifier will have a voltage jitter at the moment of shearing that is difficult to avoid. This jitter is reflected in the current output to the gradient coil, and there will be a slight current change. This slight current change will cause image distortion, such as artifacts, in the advanced imaging of the magnetic resonance imaging system, thereby affecting image diagnosis.

[0006] Gradient amplifiers that use a multi-path phase-shifted parallel connection of low-frequency, high-voltage tube switches are often extremely large in size, have large switching losses in power electronic devices, low power efficiency, and very high costs. Summary of the invention

[0007] In order to solve the defects and shortcomings in the above-mentioned prior art, the present invention provides a digital PWM control method for a gradient amplifier that controls power electronic power devices of corresponding circuit structures through a specific PWM control and regulation mechanism, adjusts the output high-voltage pulse duty cycle, and realizes a gradient amplifier design with high voltage output and low ripple current using power electronic power devices of low voltage level and high switching frequency, and can realize a high current change rate and low steady-state current fluctuation of the gradient amplifier output.

[0008] The technical solution of the present invention is a gradient amplifier digital PWM control method, comprising:

[0009] The PWM control module receives input control and output current feedback signals, processes the data through the PID algorithm, generates real-time error data, and generates multiple PWM signals corresponding to the real-time error data through a specific PWM control mechanism, which controls the switch and duration of each power electronic power device, and outputs the multiple PWM signals to the power output module;

[0010] The power output module receives multiple PWM signals, drives the corresponding power electronic power devices, and converts the high-voltage DC signal into a high-voltage pulse signal corresponding to the PWM signal. The high-voltage pulse signal consists of a voltage V1 with a certain duty cycle and a voltage V2 with a certain duty cycle. The high-voltage pulse signal is output to the output filter module;

[0011] The output filter module filters the received high-voltage pulse signal into the expected DC voltage signal, and loads the DC voltage to both ends of the gradient amplifier load to form the required precise current signal. At the same time, the output current feedback signal is detected by the current sensor.

[0012] Preferably, the power output module is composed of no less than four half-bridge circuit structures, and every two half-bridge circuits form a group, wherein the working power supply voltage V1 of the upper half bridge and the working power supply voltage V2 of the lower half bridge, the negative end of the upper half bridge working power supply voltage V1 is connected to the positive end of the lower half bridge working power supply voltage V2, the middle output node of the upper half bridge circuit is short-circuited with the corresponding middle output node of the lower half bridge circuit, and this short-circuit point is connected to the output filter module, and no less than two groups of the above circuits with the same configuration constitute the power output module.

[0013] Preferably, the PWM control module generates multiple PWM signals, each group of four PWM signals, corresponding to the four driving ends of the power electronic power devices of each group of two half-bridges, and determines the voltage and duty cycle output by the two half-bridges of the driven group based on the PWM signal generated by the real-time error data.

[0014] Preferably, the multi-channel PWM signals output by the PWM control module drive the high-voltage pulse signal formed by the power output module to orderly adjust the output voltage V1 pulse duty cycle or the output voltage V2 pulse duty cycle according to the PWM control logic, or simultaneously adjust the output voltage V1 pulse duty cycle and the output voltage V2 pulse duty cycle.

[0015] Preferably, the PWM control method generates multiple PWM signals of corresponding duty cycles according to real-time error data, adjusts the phases between the PWM signals, and enables the effective output duty cycle to be achieved four times within one switching cycle without increasing the switching frequency of the power electronic device, thereby increasing the effective switching frequency of the dynamic output current and output voltage and reducing the dynamic output ripple current output to the gradient coil.

[0016] Preferably, no less than two groups of the above-mentioned circuits with the same configuration form a power output module. According to actual needs, the power output module can be composed of two groups, four groups, six groups or multiples of eight groups of the above-mentioned circuits. At the same time, under the control of the multi-channel PWM signals output by the specific PWM control module, a certain phase difference can be generated between the outputs of the above-mentioned circuits in the same functional group, thereby further improving the effective switching frequency of the output current and the output voltage and reducing the output ripple current output to the gradient coil.

[0017] Preferably, the PWM control method optimizes the duty cycle of the PWM control signal near the zero current output of the gradient amplifier load, thereby effectively reducing the output ripple current, while reducing the power loss of the output filter module and improving the power utilization efficiency of the gradient amplifier.

[0018] Preferably, the PWM control method, through a specific PWM control mechanism, can alternately adjust the duty cycle of each PWM signal when generating multiple PWM signals according to real-time error data, balance the switching and conduction losses of each power electronic device, and improve the reliability of the power output module.

[0019] Preferably, the working power supply voltage V1 of the upper half bridge and the working power supply voltage V2 of the lower half bridge can be equal (V1=V2) or unequal. In practical applications, it is more conducive to PWM control and achieving the best comprehensive performance if the two power supply voltages are basically equal.

[0020] The present invention controls the power electronic power devices of the corresponding circuit structure through a specific PWM control and regulation mechanism, adjusts the output high-voltage pulse duty cycle, and realizes a gradient amplifier design with high voltage output and low ripple current using power electronic power devices of low voltage level and high switching frequency, which can achieve a high current change rate and low steady-state current fluctuation in the output of the gradient amplifier. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 12 is a schematic diagram of the principle of a gradient amplifier according to an embodiment of the present invention; Figure 2 It is a schematic diagram of the working mechanism of the PWM control module according to an embodiment of the present invention; Figure 3 Schematic diagram of output waveforms of four half-bridge output nodes P and N of a power output module according to an embodiment of the present invention; Figure 4 2 is a schematic diagram of an eight-half-bridge structure of a power output module according to an embodiment of the present invention; Figure 5 Schematic diagram of output waveforms of eight half-bridge output nodes P1P2 and N1N2 of the power output module according to an embodiment of the present invention; Figure 6 Schematic diagram of output waveforms of eight half-bridge output nodes P1P2 and N1N2 with a phase shift of 90° in a power output module according to an embodiment of the present invention; Figure 7 This is a schematic diagram of PWM output of the PWM control module in an embodiment of the present invention when the duty cycle is near 0%; Figure 8 It is a schematic diagram of PWM output of the PWM control module in an embodiment of the present invention when the duty cycle is around 25%. DETAILED DESCRIPTION

[0029] The technical solution proposed by the present invention will be described in more detail below in conjunction with specific embodiments. Obviously, the described embodiments are only part of the embodiments described in the present invention and cannot represent the required technical methods described in the present invention. Technical variations made based on the present invention are all within the technical protection scope of the present invention.

[0030] The embodiment of the present invention discloses a digital PWM control method applied to a gradient amplifier, which is applied to the gradient amplifier and includes (eg Figure 1 ):

[0031] The PWM control module 100 receives input control and output current feedback signals, processes the data through a specific PID algorithm, generates real-time error data, and generates multiple PWM signals corresponding to the real-time error data through a specific PWM control mechanism, correspondingly controls the switch and duration of each power electronic power device, and outputs the multiple PWM signals to the power output module 200;

[0032] The power output module 200 is composed of at least four half-bridge circuit structures, and each two half-bridge circuits form a group, wherein the working power supply voltage V1 of the upper half bridge and the working power supply voltage V2 of the lower half bridge are connected, the negative end of the working power supply voltage V1 of the upper half bridge is connected to the positive end of the working power supply voltage V2 of the lower half bridge, the middle output node of the upper half bridge circuit is short-circuited with the corresponding middle output node of the lower half bridge circuit, and the short-circuit point is connected to the output filter module, and at least two groups of the above circuits with the same configuration constitute the power output module 200. The power output module receives multiple PWM signals, drives the corresponding power electronic power devices, and converts the high-voltage DC signal into a high-voltage pulse signal corresponding to the PWM signal. The high-voltage pulse signal consists of a voltage V1 with a certain duty cycle and a voltage V2 with a certain duty cycle, and the high-voltage pulse signal is output to the output filter module 300.

[0033] The output filter module 300 filters the received high-voltage pulse signal into an expected DC voltage signal, and loads the DC voltage to both ends of the load coil of the gradient amplifier to form the required precise current signal. At the same time, the output current feedback signal is detected by the current sensor.

[0034] The PWM control module 100 is as follows: Figure 2, receiving the input control signal 101 and the output current feedback signal 102. The input control signal 101 is input to the input data processing module 103, and the input control signal is processed according to the actual needs, such as AD conversion, filtering, delay adjustment, gain adjustment, applicable adjustment and limitation, and the processed data enters the PID module 105; the output current feedback signal 102 is input to the feedback data processing module 104, and the current feedback signal is processed according to the actual needs, such as AD conversion, filtering, delay adjustment, gain adjustment, limit protection, and the like, and the processed data enters the PID module 105; the PID module 105 generates real-time error data through PID algorithm processing and outputs it to the PWM generation module 106, and the PWM generation module 106 calculates the total duty cycle parameter according to the error data and the accumulated error; and outputs this parameter to the PWM control mechanism module 107; the PWM control mechanism module 107, through a specific PWM control mechanism, converts the P of a certain duty cycle corresponding to the error data into a P value. The WM signal is converted into a PWM signal of each half-bridge circuit, and according to the optimization setting of actual use, a PWM signal based on the optimized initial pulse width is formed, and further according to the optimization, the effective duty cycle, phase and phase shift processing of the PWM signal between each half-bridge circuit are optionally performed to form a corresponding multi-channel PWM output to the PWM distribution module 108; the PWM distribution module 108 distributes the multi-channel PWM signals PWM1-PWMn to the corresponding switching points of each half-bridge circuit according to the pre-setting, and optionally, the switching state of the same switching point can be adjusted according to the predicted loss or actual loss state, and the switching and duration of each power electronic power device can be correspondingly controlled.

[0035] The power output module 200 is as follows: Figure 1 As shown, the basic unit is composed of two groups of four half-bridge circuits, the negative end of the working power supply voltage V1 is connected to the positive end of the working power supply voltage V2, which is recorded as 0 potential, the working power supply voltage V1 of the half-bridge circuit K1 (upper half-bridge switch K101, lower half-bridge switch K102, and the same below), the working power supply voltage V2 of the half-bridge circuit K2, and the intermediate output node of the half-bridge circuit K1 are connected to the intermediate output node of the half-bridge circuit K2, which is recorded as P, and connected to the filter module 300; the working power supply voltage V1 of the half-bridge circuit K3, the working power supply voltage V2 of the half-bridge circuit K4, and the intermediate output node of the half-bridge circuit K3 are connected to the intermediate output node of the half-bridge circuit K4, which is recorded as N, and connected to the filter module 300. The high-voltage pulse signal formed by the multi-channel PWM signal output by the PWM control module 100 drives the power output module 200, and according to the PWM control logic, realizes orderly adjustment of the pulse duty ratio of the output voltage V1 or the pulse duty ratio of the output voltage V2, or simultaneously adjusts the pulse duty ratio of the output voltage V1 and the pulse duty ratio of the output voltage V2. Corresponding to the output P node and N node, the typical output waveform example is as follows Figure 3 shown.

[0036] At least two groups of the above circuits with the same configuration form the power output module 200. According to actual needs, the power output module 200 can be composed of two groups, four groups, six groups, eight groups, or multiples of two groups of the above circuits. Figure 4 As shown, it is composed of four groups of eight half-bridge circuits in actual typical applications, and the corresponding four output nodes are P1, P2 and N1, N2. The typical output waveform example is as follows Figure 5 As shown, the application is more flexible and the power expansion space is large. At the same time, under the control of the multi-channel PWM signal output by the specific PWM control module, a certain phase difference can be generated between the outputs of the above circuits in the same functional group, further improving the effective switching frequency of the output current and output voltage, and reducing the output ripple current output to the gradient coil. The typical output waveform example is as follows Figure 6 As shown, the output nodes are P1, P2 and N1, N2 as an example output waveform with a phase shift of 90°.

[0037] A specific PWM control method generates multiple PWM signals with corresponding duty cycles based on real-time error data, adjusts the phases between the PWM signals, and enables the effective output duty cycle to be achieved four times within one switching cycle without increasing the switching frequency of the power electronic device. Figure 3 , Figure 5 , Figure 6 As shown, combined with the output filter module, it can be designed to be a valid output in the XOR case of P and N, so that four valid output pulses can be achieved in one switching cycle, thereby increasing the effective switching frequency of the dynamic output current and output voltage, and reducing the dynamic output ripple current output to the gradient coil.

[0038] A specific PWM control method optimizes the duty cycle of the PWM control signal near the zero current output of the gradient amplifier load, thereby effectively reducing the output ripple current, while reducing the power loss of the output filter module and improving the power efficiency of the gradient amplifier. Figure 7 As shown, Figure 7 This is a schematic diagram assuming that the switching period is 40 and the duty cycle is around 0%. In the figure, when the duty cycle is 0%, the initial single pulse width is set to 4. This is just a schematic diagram. The required initial single pulse width and the ratio to the switching period can be set according to actual design needs. Figure 7 At the same time, the switching states of the power electronic devices of each half-bridge circuit corresponding to this situation are exemplified. As described in the function of the PWM control module 100, in the PWM control mechanism module 107 and the PWM distribution module 108, the phase shift of the switching states of the power electronic devices of each half-bridge circuit can be adjusted as needed, and the switching states of the same switching points can be adjusted, for example Figure 7The switch logic of K102 and K201 can be interchanged, or they can be completed by a single switch K102 or K201, or K102 completes one part and K201 completes the other part. The same is true for K302 and K401. The switch logic can be flexibly set according to actual design needs, such as power or loss balance considerations.

[0039] The specific PWM control method can alternately and evenly adjust the duty cycle of each PWM signal when generating multiple PWM signals according to real-time error data through a specific PWM control mechanism. Figure 8 The output nodes P and N are shown when the duty cycle is about 25%, and the states of the switch points of the four half bridges are schematically shown. As described in the function of the PWM control module 100, in the PWM control mechanism module 107 and the PWM distribution module 108, in order to increase the effective output duty cycle of the P direction, through a specific PWM control mechanism, only the pulse width of the switch tube in the V1 direction can be increased, or only the pulse width of the switch tube in the V2 direction can be reduced, or the pulse width of the switch tube in the V1 direction and the pulse width of the switch tube in the V2 direction can be adjusted at the same time. In practical applications, alternately and evenly adjusting the duty cycle of each PWM signal is more conducive to balancing the switching and conduction losses of each power electronic device and improving the reliability of the power output module.

[0040] The working power supply voltage V1 and the working power supply voltage V2 of the half-bridge circuit can be equal, V1=V2, or unequal. In practical applications, it is more conducive to PWM control and achieving the best comprehensive performance if the two power supply voltages are basically equal.

[0041] It should be noted that the embodiments mentioned above illustrate rather than limit the present invention, and those skilled in the art will be able to design alternative embodiments without departing from the scope of the claims. Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention, so the scope of protection of the present invention shall be subject to the scope defined by the claims.

Claims

1. A digital PWM control method for a gradient amplifier, characterized in that: include The PWM control module receives input control and output current feedback signals, processes the data through the PID algorithm, generates real-time error data, and generates multiple PWM signals corresponding to the real-time error data through a specific PWM control mechanism, which controls the switch and duration of each power electronic power device, and outputs the multiple PWM signals to the power output module; The power output module receives multiple PWM signals, drives the corresponding power electronic power devices, and converts the high-voltage DC signal into a high-voltage pulse signal corresponding to the PWM signal. The high-voltage pulse signal consists of a voltage V1 with a certain duty cycle and a voltage V2 with a certain duty cycle. The high-voltage pulse signal is output to the output filter module; The output filter module filters the received high-voltage pulse signal into the expected DC voltage signal, and loads the DC voltage to both ends of the gradient amplifier load to form the required precise current signal. At the same time, the output current feedback signal is detected by the current sensor.

2. The gradient amplifier digital PWM control method according to claim 1, characterized in that: The power output module is composed of no less than four half-bridge circuit structures, and every two half-bridge circuits form a group, wherein the working power supply voltage V1 of the upper half bridge and the working power supply voltage V2 of the lower half bridge, the negative end of the working power supply voltage V1 of the upper half bridge are connected to the positive end of the working power supply voltage V2 of the lower half bridge, the middle output node of the upper half bridge circuit is short-circuited with the corresponding middle output node of the lower half bridge circuit, and this short-circuit point is connected to the output filter module, and no less than two groups of the above circuits with the same configuration constitute the power output module.

3. The gradient amplifier digital PWM control method according to claim 1, characterized in that: The PWM control module generates multiple PWM signals, each group of four PWM signals, corresponding to the four driving ends of the power electronic power devices of each group of two half-bridges. The PWM signals generated by the real-time error data determine the voltage and duty cycle output by the two half-bridges of the driven group.

4. The gradient amplifier digital PWM control method according to claim 1, characterized in that: The multi-channel PWM signals output by the PWM control module drive the high-voltage pulse signal formed by the power output module, and according to the PWM control logic, orderly adjust the output voltage V1 pulse duty cycle or the output voltage V2 pulse duty cycle, or simultaneously adjust the output voltage V1 pulse duty cycle and the output voltage V2 pulse duty cycle.

5. The gradient amplifier digital PWM control method according to claim 1, characterized in that: The PWM control method generates multiple PWM signals with corresponding duty cycles according to real-time error data, adjusts the phases between the PWM signals, and enables the effective output duty cycle to be realized four times within one switching cycle without increasing the switching frequency of the power electronic device, thereby increasing the effective switching frequency of the dynamic output current and output voltage and reducing the dynamic output ripple current output to the gradient coil.

6. The gradient amplifier digital PWM control method according to claim 2, characterized in that: No less than two groups of the above circuits with the same configuration form a power output module. According to actual needs, the power output module can be composed of two groups, four groups, six groups or multiples of eight groups of the above circuits. At the same time, under the control of the multi-channel PWM signals output by the specific PWM control module, a certain phase difference can be generated between the outputs of the above circuits in the same functional group, further improving the effective switching frequency of the output current and the output voltage, and reducing the output ripple current output to the gradient coil.

7. The gradient amplifier digital PWM control method according to claim 1, characterized in that: The PWM control method optimizes the duty cycle of the PWM control signal near the zero current output of the gradient amplifier load, thereby effectively reducing the output ripple current, while reducing the power loss of the output filter module and improving the power utilization efficiency of the gradient amplifier.

8. The gradient amplifier digital PWM control method according to claim 1, characterized in that: The PWM control method, through a specific PWM control mechanism, can alternately adjust the duty cycle of each PWM signal when generating multiple PWM signals according to real-time error data, balance the switching and conduction losses of each power electronic device, and improve the reliability of the power output module.

9. The gradient amplifier digital PWM control method according to claim 1, characterized in that: The working power supply voltage V1 of the upper half bridge and the working power supply voltage V2 of the lower half bridge may be equal, that is, V1=V2, or may be unequal.

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