Pseudorandom Pulse Sequence for Electric Heater Control
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Solution Overview
Problem
Conventional temperature control methods in manufacturing processes, such as wafer processing, face challenges with long response times and periodic power delivery patterns that result in significant peak deviations from the desired setpoint, making it difficult to achieve precise and dynamic temperature control.
Innovation Solution
A method that employs a digital signal processing microprocessor to generate a pseudorandom pulse sequence for controlling an electric heater, updating the duty cycle command at a frequency of 120 Hz or every half AC cycle, which is less than the traditional 60 Hz AC cycle, thereby reducing temperature errors and achieving more accurate and responsive temperature control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the duty cycle command is quantized with high resolution (e.g., 0.1%) using conventional methods, then the temperature control precision is improved, but the response time increases significantly (requiring 1000 AC cycles or 16.67 seconds)
Solution Approach 1:
The patent segments the duty cycle control into two independent components: a coarse quantized duty cycle command (QDC) that determines the base power level, and a pseudorandom duty cycle modulation (PDCM) that provides fine-resolution adjustments. This segmentation allows the system to achieve 0.1% resolution without requiring 1000 AC cycles, because the PDCM component adds precision through pseudorandom variations rather than through extensive cycle counting.
Solution Approach 2:
The patent employs periodic pseudorandom duty cycle modulation at a frequency of 120 Hz (every half AC cycle) to update the duty cycle command. This periodic action with pseudorandom patterns enables the system to maintain high resolution control while significantly reducing the response time from 16.67 seconds to 8.33 milliseconds, as the pseudorandom modulation provides continuous fine-adjustment capability within each periodic cycle.
2Measurement precision
If the conventional PWM method with long iteration intervals is used, then the duty cycle resolution requirement is met, but periodic thermal ripples and peak deviations from setpoint occur
Solution Approach 1:
The patent implements periodic pseudorandom duty cycle modulation that updates every half AC cycle (120 Hz), creating a periodic control action that continuously adjusts the heater power. This periodic modulation with pseudorandom patterns prevents the formation of regular periodic thermal ripples by introducing unpredictable variations, thereby improving temperature stability while maintaining 0.1% resolution and reducing peak deviations from the setpoint.
Solution Approach 2:
The system uses feedback from the temperature sensor to continuously monitor the load temperature and adjust the pseudorandom duty cycle modulation accordingly. This feedback mechanism ensures that the temperature remains stable around the setpoint by dynamically adjusting the PDCM component based on actual temperature deviations, thereby eliminating periodic thermal ripples and reducing peak deviations.
3Ease of operation
If the duty cycle command is updated every full AC cycle (60 Hz), then the system operation is simplified, but the response time and temperature control accuracy deteriorate
Solution Approach 1:
The patent segments the control update frequency into two levels: a base update rate every half AC cycle (120 Hz) for the coarse quantized duty cycle command, and a higher effective resolution through pseudorandom modulation within each update cycle. This segmentation allows the system to operate at a simplified 120 Hz update rate while achieving temperature control accuracy equivalent to or better than systems updating at higher frequencies, because the pseudorandom modulation provides continuous fine-adjustment capability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach provides a significantly shorter response time for updating duty cycle commands, reduces average temperature errors, and eliminates periodic thermal ripples, enabling more precise and dynamic temperature control suitable for various manufacturing processes.
Implementation Method 1
controlling an electric heater to drive a load temperature to approximate a setpoint
Data Source
AI summary
Systems and methods for controlling a heater to drive a load temperature to approximate a setpoint. At the beginning of each ½ AC cycle, a sigma delta modulation algorithm is computed to determine whether the AC cycle should be turned on or off. The running estimate of the past actual duty cycle is compared to the desired output duty cycle, and the difference between the two is used to adjust the next ½ cycle's output. This results in a pseudo-random pulse sequence output which does not contain significant periodic components that could cause undesirable small periodic thermal ‘ripples’ on the element being heated.