Vacuum heat treating and other heat treating processes as well require tighter process control to reduce operating costs and meet increasing customer demand for higher quality products. An advanced thermal process control and monitoring PC workstation (CompuVac 2000) not only offers an easy-to-use, reliable, and flexible system that provides critical furnace data in real time, but also provides the ability to archive acquired process data for later retrieval and evaluation. In addition, the system functions as a remote workstation, affording full protection against damage to a valuable furnace workload and potentially costly downtime. The workstation is available with new VFS vacuum furnaces, and also is available as a replacement for obsolete control systems of existing vacuum furnaces (figure 1).
ENGINEERING A SOLUTION
The integrated control system offers a wide range of features, including highly efficient data acquisition and control capabilities, advanced process diagnostic tools, alarm features, and batch-report capabilities. Paramount in the development of the furnace control system was ease of use for an operator through a user-friendly touch-screen approach.
A programmable logic controller (PLC) enables proportional integral derivative (PID) loop and logic control of all furnace functions. All input/output (I/O) is resident on the system's PLC. Today's advanced system includes fast-response computer hardware and a built-in color, flat-panel touch-screen monitor (figure 2).
Flexible design allows configuring the workstation to meet a variety of customer specific requirements including custom screens, customized report generation, and a printer or chart recorder for real-time cycle records.
The control system functions as a local supervisory station; that is, it not only provides a "window" for the operator to view the status of furnace system functions, but also offers a way for the operator to control all furnace functions. A remote supervisory workstation option (typically a desktop PC installed at a location remote from the furnace site accessible via modem) can be connected to the local workstation at a production site via the facility's LAN (local area network) or WAN (wide area network) communications link. This provides the user with the capability to closely monitor and control thermal processing conditions of one or more furnaces from a distant location (figure 3).
SIMPLE TO NAVIGATE
The workstation contains dynamic graphic displays that are easy to understand and simple to navigate through the entire furnace operation. Besides interactive functionality, the system offers powerful controls, data acquisition and report capability.
Graphic screens use Citect, an interactive WindowsR-based SCADA (supervisory control and data acquisition) industrial automation software package (Ci Technologies Inc., Charlotte, NC). Citect's open-architecture does not restrict the choice of system hardware; it provides nearly 130 different PLC drivers to accommodate virtually any PLC available.
For example, an overview screen helps the user to rapidly assess furnace operational status through an easy-to-follow graphic, which shows components such as chamber and hot zone, vacuum pumps and valve positions, and gas quench motor (figure 4).
The user can quickly and easily create, save, load, and modify process cycles on a recipe screen. Process variables in both real-time and as historical trends are displayed on two trend screens and an hour meter screen provides data such as total running time, maintenance time and total number of starts for critical components and systems. Other furnace and process information, such as workload temperature surveys, graphic representations of thermal profiles, PID loops, active alarms, and an historical alarm summary, are furnished on separate screens.
ACCURATE, EFFICIENT PROCESS CONTROL
Users can monitor and control thermal processing conditions with a high degree of confidence because the workstation allows greater process control and provides a broad spectrum of critical data in real time during any given process cycle.
For example, the system provides real-time temperature monitoring and control of multiple thermocouples placed in and around the workload. During the heat treating process the operator can monitor a "temps" screen, where bar graphs show the temperatures of the control thermocouple and all of the work thermocouples.
Uniform heating of a workload is essential to produce the required metallurgical properties of a particular material being heat treated, or to produce required brazing results. To achieve the proper processing conditions, the heat treater establishes the temperatures required at certain points in the process cycle. For instance, multiple work thermocouples might be placed in the furnace charge to monitor the temperature in and around the workload. The process cycle can be programmed to identify a specific number of the total thermocouples to ensure that the specified temperature is achieved and is uniform in the workload. The workload temperature must be uniform to within a defined range of the soak set-point temperature (I10F, or I6C, for example) by all the specified work thermocouples before proceeding to the next segment of the process cycle. As the process cycle progresses, the operator monitors all work thermocouple signals on the temps screen to ensure that the cycle progresses as planned. If a work thermocouple does not show the proper temperature, the operator can decide whether to proceed after evaluating the situation. The control system enables the operator to clearly and accurately observe the necessary process data and make a quick and informed decision.
An example of the benefit of remote control is illustrated in a situation experienced by Astro-Met Inc. (Cincinnati, OH), a manufacturer of titanium alloy and cobalt-chromium-molybdenum prosthetic implants and bone-joint replacements. The parts are treated in a vacuum furnace to produce a bright, oxide-free surface, which meets critical medical implant requirements.
Astro-Met operates four production vacuum furnaces, including a recently installed horizontal external quench (HEQ) vacuum furnace at its facility in Olive Branch, Miss. The furnace system includes a CompuVac 2000 PC workstation, which is linked via WAN to a remote supervisory workstation at the Cincinnati headquarters facility. Authorized personnel (in this instance, the Mississippi facilities manager) can access the local supervisory workstation directly through the remote supervisory workstation. Astro-Met's control system is designed to give operators at the Mississippi site ability to load and run, but not to create or modify, heat-treating process cycles. Only personnel having a higher level of security clearance can create and modify the process cycles.
During an unexpected power outage at the Mississippi facility, the Engineer/ Supervisor of Furnace Operations at the Cincinnati plant prevented the loss of a valuable workload at the Mississippi plant by using the remote supervisory workstation, which was accessed on-line through a PC at his home in Cincinnati.
The Mississippi plant experienced a power outage that completely shut down the vacuum furnace while in the middle of a heat treating cycle. Under such conditions, the internal battery in the CompuVac 2000 workstation ordinarily ensures that the process cycle and related data, such as the termination point in the heat-treat cycle, is saved. The control system automatically commands the furnace to resume operation when power is restored, beginning from the exact point where the thermal process was interrupted.
Because the duration of the power outage exceeded the time limit of the system's battery-powered back-up memory in this instance, the stored information was lost, and restarting the process cycle to save the workload needed to be handled remotely.
The furnace operations supervisor at Cincinnati began the data recovery and furnace restart process by logging onto Astro-Met's WAN from the PC in his home, accessing the CompuVac 2000, and determined from the trend screen that the process cycle was interrupted (terminated) while in a hold segment.
Coordination of furnace start-up activities with the on-site furnace operator is important, and it also is necessary to use caution in taking over control of the furnace operation because the position of production and maintenance personnel attending the furnace during the procedure are not observable from the off-site location. Restarting the furnace operation from the point of cycle disruption via the supervisory workstation enabled completing the heat treating process cycle, thereby preventing a costly loss of workload.