Digital State Space Controller for MRI Gradient Coil Current
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Solution Overview
Problem
Existing MRI gradient magnet systems face accuracy and reproducibility issues due to digital discretization noise and limited processing speed in digital control systems, leading to deviations in generating precise gradient magnet currents.
Innovation Solution
A fully digital control system using a state space controller with high disturbance suppression, eliminating the need for feedforward control, allowing the use of standard digital components and achieving high precision without analog control limitations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Extent of automation
If digital control is used for gradient amplifier, then automation and flexibility are improved, but digital discretization noise and limited processing speed cause accuracy degradation
Solution Approach 1:
The patent implements a digital feedback control system that continuously measures the actual gradient coil current and compares it with the reference current, then adjusts the PWM duty cycle to minimize the error. This closed-loop feedback mechanism compensates for digital discretization noise and processing delays, maintaining high current accuracy while using fully digital control.
Solution Approach 2:
The patent applies feedforward control by pre-calculating the required PWM duty cycle based on the reference current waveform and system characteristics. This preliminary action anticipates the required control adjustments before errors occur, reducing the impact of digital processing limitations and improving current accuracy proactively.
2Measurement precision
If analog control is used for gradient amplifier, then current accuracy is improved, but device complexity and cost increase
Solution Approach 1:
The patent replaces analog control electronics with a fully digital control system implemented on a microcontroller or DSP. The digital controller uses software-based PID control algorithms to achieve the same control functions as complex analog circuitry, thereby reducing hardware complexity while maintaining or improving control accuracy through programmable parameters.
Solution Approach 2:
The patent employs a single digital control unit that integrates multiple control functions including current regulation, waveform generation, and protection logic. This universal digital controller replaces multiple specialized analog circuits, simplifying the overall system architecture while providing flexible and accurate control of the gradient amplifier.
3Measurement precision
If high precision digital control is implemented, then accuracy is improved, but processing speed requirements and cost increase
Solution Approach 1:
The patent implements control at optimized discrete time intervals rather than continuous processing. By updating the PWM duty cycle at appropriate sampling rates that match the gradient coil dynamics, the system achieves accurate current control without requiring excessively high processing speeds, balancing precision with computational efficiency.
Solution Approach 2:
The patent applies digital filtering and smoothing techniques that process control data at moderate speeds while achieving high effective accuracy. By using partial processing with sophisticated algorithms rather than exhaustive real-time computation, the system attains high precision without demanding extreme processing speed capabilities.
Data Source
AI summary
A state space feedback controller operates in the digital domain for the regulation of the current supply to MRI gradient coils from a multiple-bridge PWM power amplifier. The P1-controller includes an integration part (for the integration of the difference between the demand current and the measured gradient coil current) and a subsequent P-controlled system which in turn includes a delay compensator/stabilizer and a plant. The delay compensator/stabilizer includes a multi-path feedback loop by means of which its digital output signal is fed back through delay blocks, on the one hand, and through filter units, on the other hand. The filter units model the transfer functions of a gradient coil output filter for the gradient coil voltage and the output current of the amplifier inverter units, respectively. In the plant, a filter unit, which models the gradient coil transfer function, is connected in series to a delay chain for the delay of the measurement value of the gradient coil current.