Dual Modulation Digital PWM for RF Laser Power Control
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Solution Overview
Problem
Existing pulse width modulation (PWM) methods for CO2 gas discharge lasers powered by RF power supplies lack the accuracy to control average power output at higher pulse repetition frequencies, such as above 1 kHz, due to limitations in clock frequency resolution, which is not practical for commercial lasers.
Innovation Solution
The dual modulus digital pulse width modulation (DMDPWM) method allows for incrementally variable pulse durations within a train of digital pulses, enabling precise control of average power by varying the duration of individual pulses, rather than all pulses equally, thereby improving resolution without requiring higher clock frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the pulse repetition frequency (PRF) is increased to achieve higher productivity, then the laser processing speed increases, but the power control resolution deteriorates due to the fixed clock frequency limitations
Solution Approach 1:
The pulse train is segmented into multiple individual pulses, and the duration of selected pulses is independently varied using an N-modulo counter. This allows the total aggregate duration to be precisely controlled by adjusting individual pulse widths rather than treating the pulse train as a single unit, thereby maintaining high resolution at high PRF.
Solution Approach 2:
The pulse duration is made dynamically variable within the pulse train. The system transitions from static uniform pulse widths to dynamic variable pulse widths, where the duration of individual pulses can be incrementally adjusted based on the N-modulo counter state, enabling fine-grained power control at high repetition frequencies.
2Measurement precision
If the clock frequency is increased to improve power control resolution, then the accuracy of average RF power control increases, but the device complexity and cost increase due to requiring faster circuit components
Solution Approach 1:
The system transitions from controlling power resolution through the time dimension (higher clock frequency) to controlling it through the pulse train structure dimension (varying individual pulse durations within a train). The N-modulo counter introduces a new degree of freedom by allowing selective duration adjustment of individual pulses, achieving high resolution without requiring proportionally higher clock frequencies.
Solution Approach 2:
The system changes the parameter being controlled from uniform pulse width to variable pulse width within a train. By introducing the parameter N (number of pulses to stretch) and using an N-modulo counter, the system achieves fine-grained control of aggregate duration through parameter variation rather than relying solely on increased clock frequency.
3Ease of operation
If the pulse duration is uniformly increased to control average power, then the power control is simplified, but the resolution of power control deteriorates at high pulse repetition frequencies
Solution Approach 1:
Instead of uniformly adjusting all pulses in the train, the system applies local quality variation by selectively stretching the duration of specific individual pulses based on the N-modulo counter state. This localized adjustment allows precise control of the aggregate duration while maintaining simplicity in the control logic.
Data Source
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AI summary
A pulse width modulation method for controlling the output power of a pulsed gas discharge laser powered by a pulsed RF power supply comprises delivering a train of digital pulses to the RF power supply. Each pulse in the train has an incrementally variable duration. The power supply is arranged to deliver a train of RF pulses corresponding in number and duration to the train of digital pulses received. The average power in the RF-pulse train can be varied by incrementally varying the duration of one or more of the digital pulses in the digital pulse train. The train of RF pulses is used to power a gas discharge laser. The gas discharge laser outputs a pulse train corresponding to the RF pulse train.