Oven and furnace controls have progressed rapidly in the last five years in an effort to provide tighter temperature uniformity, enhanced data-acquisition and storage capability, scalable architecture, improvements in the user interface and communication, and data security. In some instances, these improvements are driven by companies seeking to improve the quality of long-established products. However, oftentimes the need for these feature-rich thermal processing controls is driven by heating equipment manufacturers introducing new products that require technically complex, demanding process control technologies. These requirements are particularly evident in the manufacturing of composite parts and assemblies in the aerospace, wind turbine and automotive industries.
Programmable Logic Controllers for Industrial Ovens
The progression of oven controls has moved from processes managed by discrete temperature controllers with manual oven controls to those driven by programmable logic control (PLC) with temperature achieved by a discrete temperature controller or within the PLC. These approaches have worked reasonably well. They are cost effective for simple to moderately difficult oven applications where the numbers of zones and functions to be controlled are limited, and the decision loops for controlling temperature and other variables are moderate in scope.
However, new requirements thrust upon oven manufacturers are requiring the oven control, temperature measurement and data acquisition be more tightly integrated than possible with discrete controls or PLCs integrated with discrete temperature controllers. Temperature control suppliers have, in some instances, responded with programmable automation controllers that incorporate temperature and machine control as well as recipe management, temperature-sensing logic, data acquisition and communication with plant equipment, remote servers and compatibility with Internet-enabled devices such as tablets and smartphones. Programmable automation controllers offer these features in compact, modular control systems suited to thermal process applications. These controllers typically include the following capabilities:
- Temperature monitoring for one or more zones and monitoring of part temperature and other process variables such as pressure and vacuum levels, actuator positions, fan operation, door position and conveyor operation.
- Scalable architecture providing extensive numbers of I/O and PID loops.
- The ability to handle many types of analog and digital inputs, including thermocouple, RTD, voltage and current.
- Autotuning and overshoot prevention capability.
- Recipe management, allowing easy development and storage of programs.
- Full-featured alarms (both active and historical) for all process events with email alarm notification.
- Thermocouple status monitoring and automatic thermocouple burnout switching.
- On-board datalogging and archival with the ability to send data files to a plant server or other storage media.
These programmable automation controllers have been applied to many types of batch and continuous ovens for operation in single-zone, multi-zone and thermocouple-based part temperature control. For single-zone operation, single or multiple thermocouples can be utilized to provide area-wide temperature control. The programmable controller can control chamber temperature by monitoring the leading, lagging or average temperature of one or more thermocouples located in the oven chamber.
For multi-zone use, these programmable automation controllers can be utilized to control individual zones in a continuous oven by monitoring single or multiple thermocouples in each zone. Leading, lagging or average temperature can be monitored in each zone to provide enhanced temperature control and ensure temperature uniformity within a zone. Multiple-zone ovens typically are used where the oven chamber is too long to control effectively with one zone of control or for those where the application requires multiple zones to achieve the required process. Often, this is the case when the process requires a drying cycle before the product can be cured or calcined.
Thermocouple-based part temperature control frequently is used for critical temperature applications where the part temperature must be monitored to ensure the product reaches and is held at the desired setpoint for a prescribed period of time. During a soak cycle in a batch oven, for example, if one or more readings from thermocouples monitoring part temperature fall outside an acceptable range, the controller can pause the oven timer until all of the thermocouples reach the allowable temperature range. This feature ensures a guaranteed soak time, and it can be used to reduce the overall process time as compared to a process using only chamber temperature as the controlling variable. Thermocouple-based part temperature controls also can be used during a ramp/wait step to ensure the parts in the oven chamber are brought up to temperature at the same rate, irrespective of part thickness or part size. Thermocouple-based part temperature control can improve product quality and reduce process time in stress-relieving and aging applications.
In addition to monitoring chamber and part temperature, programmable automation controllers can monitor other process parameters and adjust them as required. These parameters can include conveyor speed in a continuous oven, exhaust damper position, chamber pressure, vacuum port selection and pressure negativity level when curing composite parts. Other variables that can be controlled include supply and return duct airflow rates and chamber moisture level in drying applications.
Data Acquisition for Process Control Based on Part Temperature
Sensor calibration and data acquisition have become particularly important in meeting the heat treatment standards of AMS-2750E (aerospace material specification), AMS-2770 (wrought aluminum heat treat standard) and CQI-9 (automotive heat treat standard). Programmable automation controllers and associated data-acquisition systems are particularly adept at meeting the requirements for Instrumentation Type and Temperature Uniformity Survey (TUS) and system accuracy tests for these standards. Often, a data-acquisition station can be ordered with calibration certificates that meet NIST and ISO 17025 standards. Calibration correction can be performed in the field on individual thermocouples over multiple temperature points to achieve consistently accurate and repeatable measurements.
Data-recording systems offer an array of data-transfer capabilities. For instance, data can be saved to removable memory such as compact flash, SD memory and USB media. Connectivity to popular PLC systems is supplied via serial Modbus RTU, Modbus TCP or Ethernet/IP. Ethernet connectivity will support automatic FTP file transfer and often provides email messaging and web browser viewing. Email messaging can include alarm notification reports, periodic process value updates and even scheduled report data. Additionally, process values and historical trends can be viewed on mobile platforms such as Android or Apple iOS.
The automation controllers and associated data-acquisition station can include multi-touch screens that allow viewing of the data over any time horizon. This allows users to “zoom out” to see trend lines over time or “zoom in” to see subtle changes in the oven chamber temperature or other process variables at particular points in time. Messages can be added to these trend lines by entering information with a keyboard and mouse (through a USB input), through a virtual pop-up keyboard, a stylus or even through finger touch on the screen.
In addition, data-recording systems are available with advanced security measures to ensure the data has not been altered after being recorded. The data is saved in a proprietary binary file format to non-volatile memory. Data traceability requires log-in names and passwords to record who logged in when accessing data and changing process parameters. This level of security ensures confidence in preventing changes to the process and the stored production data and is necessary in meeting AMS-2750E.