Glass fiber manufacture requires a high level of accuracy and reliability to achieve a consistent quality in the fiber produced.
- Increased efficiency
- Improved recovery time
- Increased availability and productivity
- Setpoint depression and ramping - allowing control of the temperature profile during a production cycle
- 'Cold bushing' protection - preventing thermal shock to the bushings in the event of operational or mechanical failure
- Better than 1°C resolution
- Operation with one or two thermocouples - efficient implementation in complex control systems requiring programmed start-up of a heat sensitive process from stand-by condition
The bushings control application offers better than 1°C resolution and can operate with one or two thermocouples, making its implementation in complex control systems requiring programmed start-up of a heat-sensitive process from stand-by condition highly efficient.
Setpoint depression and ramping
The main aim of the control module is to control the bushing temperature under PID action, and while doing this, ensure that the physical bushing is not subject to severe changes in temperature that may damage the bushing. Figure 1 shows a typical setpoint profile for the bushings control module.
During normal running mode, when actually winding glass fiber onto a drum, the target temperature of the bushing is entered by the operator. This value is never changed by the control strategy and is referred to as the nominal setpoint. The motor automatically ramps down its speed to compensate for the increase in diameter of the drum.
When the bushing is ready for production but the variable speed motor is not winding the glass fiber onto the storage reels, a setpoint depression, also entered by the operator, is used by the strategy to reduce the control setpoint. The bushing is in a ‘background’ mode with a limited amount of fiber being produced.
After changing the winding drum, the operator can initiate a setpoint depression recovery. The control setpoint is ramped at a configurable rate from the depressed setpoint to the local setpoint value. The bushing gets reheated to normal operating temperature ready to produce another drum of fiber.
On power-up or after a power failure to the bushing heating power supply (bushing ‘cool’), the controller enters a setpoint deviation recovery mode where the control setpoint is kept close to the current process variable, then ramped to its target value. This action prevents generation of large control errors.
Output limiting
When the bushing is ‘cool’, the control module limits the controller output to a value selected from a break point curve depicted in Figure 2. A high limit is generated based on the current value of the process variable. The operating region is represented by the shaded area in the graph.
