Valve Positioner Cutoff Transition for Smooth Hard Stop Control
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Solution Overview
Problem
Conventional smart valve positioners experience undesirable valve responses due to rapid saturation at cutoff thresholds, leading to issues like valve hammering, calibration errors, and inaccurate diagnostics, especially in large valves with high pressure service.
Innovation Solution
Implementing a method with two cutoff thresholds, where the servo set-point is slowly ramped beyond the first threshold and immediately saturated at the second threshold, providing a smooth transition and eliminating sudden valve motion, thereby improving hard stop transients and diagnostics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a single cutoff threshold is used for valve control, then the valve response is fast and the I/P drive reaches full saturation quickly, but the valve plug or actuator piston hammers into the travel stop and process stability is disturbed
Solution Approach 1:
The single cutoff threshold is segmented into two distinct thresholds: a first cutoff threshold (e.g., 0.5%) and a second cutoff threshold (e.g., 99.5%). This segmentation allows the control system to differentiate between approaching the threshold and actually crossing it, enabling a staged response that prevents hammering while maintaining speed.
Solution Approach 2:
The control system takes preliminary action by detecting when the servo set-point approaches the first cutoff threshold before actually crossing it. This early detection triggers a preparatory response that modifies the I/P drive signal progression, preventing the abrupt saturation that causes hammering while still achieving full saturation eventually.
2Reliability
If the I/P drive is fully saturated to maintain full seat load, then valve seating integrity is maintained, but the valve plug or actuator piston hammers into the travel stop in high pressure service
Solution Approach 1:
The control system dynamically adjusts the I/P drive signal based on the servo set-point's position relative to the cutoff thresholds. Instead of a static fully saturated state, the system transitions from a ramping phase to a controlled saturation phase, maintaining adaptability to prevent component damage while ensuring seating integrity.
Solution Approach 2:
The control system applies a cushioning effect by controlling the rate of I/P drive saturation. Rather than immediately driving to full saturation, the system modulates the saturation process to reduce mechanical冲击, cushioning the transition to protect components while still achieving the necessary seat load.
3Object-affected harmful factors
If a mechanical air cushion is used to decelerate valve response, then valve hammering is reduced, but the valve response becomes slower and additional check valves are required
Solution Approach 1:
The control system replaces the mechanical air cushion system with an electronic control mechanism. By using the dual threshold detection and controlled I/P drive saturation, the system achieves deceleration of valve response electronically, eliminating the need for mechanical air cushions and associated check valves while maintaining or improving response speed.
4Object-affected harmful factors
If an electronic soft stop is used to decelerate valve response, then valve hammering is reduced, but chattering occurs near the soft stop threshold and travel calibration errors affect performance
Solution Approach 1:
The control system uses feedback from the servo set-point position relative to the dual thresholds to continuously adjust the I/P drive signal. This feedback mechanism ensures smooth transitions and prevents chattering by maintaining stable control near the cutoff thresholds, eliminating the chattering problem associated with electronic soft stops.
Data Source
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AI summary
The present invention includes a method, apparatus, and computer readable medium for controlling a valve. Example embodiments of the present invention described herein utilize a soft cutoff threshold and soft cutoff procedure at which point the controller monotonically ramps a servo set-point to slowly overdrive the servo beyond a calibrated set- point range, wherein the servo transitions into a hard cutoff when the servo set-point reaches a predefined overdrive threshold.