Yarn Tension Control via Dynamic PI Coefficient Switching
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Solution Overview
Problem
Conventional yarn unwinding control systems are inadequate for applications requiring frequent changes in yarn tension at short time intervals, as they are designed to be indifferent to small, sudden variations and take several seconds to adjust, making them unsuitable for producing fancy patterns on less sophisticated knitting machines.
Innovation Solution
A tension-controlling method that uses a control unit with adjustable proportional-integral controller coefficients, switching between slow and fast response modes based on tension error thresholds, enabling quick adjustment of yarn tension and maintaining stability during occasional perturbations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the control loop is set to a very low frequency band to be indifferent to small sudden tension variations, then the system is stable against occasional tension peaks, but the response time to adjust yarn tension becomes several seconds which is too slow for frequent reference tension changes
Solution Approach 1:
The control system dynamically adjusts the proportional-integral controller coefficients based on the operating mode. In normal operation mode, the system uses coefficients optimized for stability and rejection of tension disturbances. When a reference tension change is detected, the system switches to a different set of coefficients that provide faster response. This dynamic adaptation of controller parameters resolves the contradiction between stability and response speed.
Solution Approach 2:
The invention changes the parameters of the proportional-integral controller (the Kp and Ki coefficients) depending on the operational requirements. By having multiple sets of coefficients and switching between them based on whether the system needs to maintain stability or respond quickly to reference changes, the system achieves both goals at different times without compromise.
2Reliability
If the control system is designed to compensate for slow changes in tension, then it maintains constant tension on a predetermined value, but it cannot quickly align yarn tension to new reference tension values when patterns need to be changed
Solution Approach 1:
The control system transitions from a static controller design to a dynamic one where the controller coefficients are selected based on the current operational phase. When the knitting machine operates in a steady state producing a particular pattern, the system uses coefficients optimized for tension stability. When a pattern change is initiated and the reference tension changes, the system automatically switches to coefficients that enable rapid tension adjustment, thus improving pattern change speed without sacrificing constant tension maintenance during production.
Solution Approach 2:
The single control system is designed to perform multiple functions by switching between different coefficient sets. It can maintain constant tension during normal production and quickly respond to reference tension changes during pattern transitions, making the system universally applicable to both steady-state operation and transient pattern change scenarios without requiring separate control systems.
3Reliability
If the control loop operates continuously, then it can respond to tension changes, but it operates improperly during particular operative conditions such as at the start of the weaving process or during the insertion step when the yarn is not running
Solution Approach 1:
The control system dynamically determines its operational state based on the yarn consumption speed and switches between different modes: disabled mode during insertion/start-up phases when yarn is stationary, and enabled mode during normal production when yarn is running. This dynamic state recognition and mode switching prevents improper operation during special conditions while ensuring accurate tension control during production.
Solution Approach 2:
The control system monitors the yarn consumption speed itself and uses this information to automatically determine whether it should be enabled or disabled, without requiring external intervention or complex external sensing systems. The system serves itself by using its own operational parameters to control its own activation state.
Data Source
AI summary
The yarn tension is modulated by a yarn-braking device controlled by a control unit which receives signals of measured tension from a tension sensor and compares them with a reference tension signal in a control loop; as long as the reference tension remains unchanged, a first set of coefficients is used in the control loop causing the control loop to have a relatively slow behavior in relation to the error-compensating action; when a change in the reference tension occurs, the system switches to a second set of coefficients causing the control loop to have a relatively fast behavior; the second set of coefficients is maintained as long as the difference between the second value of the reference tension and the measured tension remains greater or equal to a predetermined minimum value, then the system switches again to the first set of coefficients.