Before the early 1980s, most vacuum furnaces relied on relays, timers, manually operated push buttons and dedicated process-control instruments to control the furnace sequencing and operation. To execute a specific cycle, a furnace operator usually had to manually initiate a series of predetermined functions, such as starting the vacuum pumps, starting furnace heating, backfilling the furnace, ending the cycle and other operations. These vacuum furnaces could produce consistent batches of product with a high degree of reliability, but the level of operator skill and attention required often affected the final results. In addition, the reliability of mechanical relays, timers and push buttons exposed to dust, humidity and other factors dependent on a variety of environments was somewhat unpredictable. Thorough operator training and a well-planned preventative maintenance program were required to keep malfunctions and furnace downtime to a minimum.
Microprocessor-based control systems
The required level of operator intervention decreased and the reliability of overall furnace operation improved with the advent of microprocessor-based control instruments (the Honeywell DCP 7700 and Barber-Colman 570 temperature controllers, for example) together with microprocessor-based vacuum controllers. However, the introduction of microprocessor-based controllers and the continual stream of updated versions and new models demanded that personnel, such as a facilities or controls engineer, become familiar with a variety of instruments.
In addition, the operator had to become familiar with an assortment of display screens on several different process-control instruments, even though less attention was required during a cycle. The situation demanded one instrument/controller to manage individual control instruments and further reduce the required level of expertise and programming skills.
The development of the programmable logic controller (PLC) offered a solution to these problems. The PLC originally was designed to replace relays, timers and other hardwired logic-control systems and to simplify the management of several individual control instruments. While PLCs were introduced before the 1980s, their use on vacuum furnace-control systems was not widely accepted for several years.
Modern PLCs used as control devices on even the most complex vacuum furnace systems are reliable, powerful and practical. PLCs have become an important tool to help reduce furnace operator involvement, to produce consistent product quality, and reduce furnace downtime. Most modern vacuum furnaces equipped with a PLC require only that the operator load and unload the furnace, select the recipe to be run, and push the start button.
Today's PLC systems, including the PLC, operator interface panel, engineering programming, and installation labor generally cost much less than their predecessor relay /timer/push button systems. More importantly, most of the problems associated with the earlier systems, such as dust, mechanical wear, loose wires, etc. are virtually nonexistent.
Additional benefits are possible using PLC-controlled equipment, especially if the PLC controls all operating parameters of a vacuum furnace, including temperature, vacuum level, cooling pressure and time. The number of different control instruments used in each system can be reduced, which results in cost savings, and the operator only needs to learn the operation of one control device. When additional pieces of equipment are added, they can share the same operator interface, greatly simplifying the operator's learning curve.
Because all important data is monitored by the PLC, additional alarms and status indicators can be added. Diagnostics that are not possible using other instrumentation can be added easily. For example, in a multiple zone furnace, if the circuit breaker for one heating zone trips and adjacent zones try to carry the additional demand, the PLC can monitor the situation and display a message indicating the malfunction. This alerts the operator to check the circuits. Taking the example one step further, the current on each zone can be monitored, and if the indicated percent output is out of range, the problem can be displayed as an alarm.
The addition of a PC or operator interface panel allows viewing or printing an alarm history report. Information could consist of all alarm messages that occurred over a specific time interval, as well as the time and date of when the alarm was acknowledged and if/when the condition was remedied.
Such a control system greatly simplifies troubleshooting. Because all inputs and outputs go through the PLC, their status can be monitored continuously and changes can be executed easily without rewiring. Alarm and status messages and self-diagnostic test routines can be programmed to assist in troubleshooting.
The PLC also can be connected to a modem, so the equipment can be monitored, controlled and modified from a remote location. This can include service from the furnace manufacturer, which allows a quicker response time and less furnace downtime, and can eliminate the need for an onsite field service call. These benefits translate into cost savings.
Furnace power can be controlled so all heating zones are not at 100% at the same time. While this does not have a large effect on the operation of the equipment, it can ensure that the power drawn at any one time is kept to a minimum, thereby reducing the demand charge for electricity. Utilities can be monitored for each piece of equipment to determine actual costs per cycle or per hour.
Another advantage is the capability of writing a program to display a message reminding operators to perform scheduled maintenance and to log the maintenance performed.
Equipment and/or processes usually can be modified by changing the control software, not the equipment; this reduces labor costs and furnace downtime.
In many instances, it is desirable to monitor or control equipment from a remote location, such as a supervisor's or maintenance office. This is easily accomplished using a PC and/or human-machine interface (HMI) software. This type of system is capable of controlling and monitoring the equipment, as well as data storage and acquisition, constructing graphs, and printing.
The PC control station is capable of starting, stopping and interrupting an automatic or manual sequence, switching from automatic to manual and back again during a cycle. It also can display any alarm status and initiate (in some cases) the corrective action required. The entire operator interface is completed through graphic display screens, which are configured to appear as graphical representations of the furnace and related components. Various operator parameters are selected and entered using a mouse or keyboard.
All operator information, such as process variable values and sequence information, can be displayed on the color monitor. The operator simply clicks a mouse pointer on the area of interest to view more detailed information. For example, the main screen shows the furnace and related components together with critical information, such as temperature. More detailed information, such as percent of power output to the heating elements, is accessed by clicking on the furnace display. A second click returns to the main display.
Additional methods (not available on any other type of display) are available on graphic displays to indicate the status of equipment. For example, valves, fans, gas and water lines can change color and/or blink to indicate their status or problem, in addition to showing a digital indication of vacuum and pressure level. Or the furnace icon can change color in addition to showing a digital temperature reading, which presents fast, easy-to-understand information, thus minimizing the possibility of costly mistakes.
Data-logging instructions can be initiated from one of the operator interface screens, and data is collected only for a change in a process variable to save on the amount of memory used. A window can be configured to show real-time trends during the cycle and data can be exported into a spreadsheet program and manipulated to generate charts, graphs and tabular printouts.
The PC in this type of control system is configured to be an operator interface only; its primary function is to allow the operator to enter and retrieve data related to the process. Sequencing, safety and alarm functions are stored in the PLC. Should the PC "go down" for any reason, the PLC continues to control the automatic sequencing of the equipment through completion of the cycle.
The PC operates in a Windows environment, which allows multitasking; that is, it continues to log data and equipment status while being used for another application.
Optional capabilities and expansions
The PC, in addition to the above capabilities, can be loaded with development software, which is used for programming, editing and troubleshooting the PLC logic. This enables on-line changes, so troubleshooting is simplified and it is not necessary to put the PLC into the program mode.
The PC also can be equipped with a communications package, including a modem, so equipment can be supported by the furnace-manufacturer's engineering staff without the need for on-site, field service personnel. This option also allows the furnace manufacturer to monitor the equipment I/O and software, so both types of problems can be identified. Special communication software also can be included, which permits the remote user to run any software on the host computer.
Future control systems
What will future PLC/PC/ operator-interface-panel systems look like? The next logical progression in vacuum furnace-control systems probably will consist of a stand-alone PC, without a PLC. While many users of today's control systems are wary of committing to total dependence on a PC to control their equipment, several PC only systems already are in operation.
The main concern of PC only control is the possibility of the PC locking up or crashing. Either of these conditions on a PC-based control system requires shutting off the PC, rebooting, and in some cases, replacing the hard drive/computer. Control of the equipment and the process is lost during this period, but there are remedies available, which if not completely foolproof, will at least minimize the impact of these occurrences.
A remedy to reduce the possibility of PC lock up is the installation of Windows CE-a more reliable, simplified operating system than that used in most modern PCs. PCs usually lock up because something objectionable exists in the operating system, programming code, software or memory. The simpler the operating system, the less chance of lock up. While Windows CE does not offer as many multitasking operations as Windows NT, for example, it works well on furnace control system PCs, which benefit from a more dedicated, simplified operating system.
Installation of dual hard drives could provide a solution to a PC hard-drive crash. With the addition of a second hard drive, or mirror hard drive, failure of the primary hard drive initiates an automatic changeover to the secondary hard drive, thereby eliminating the interruption or loss of furnace control. With a dual hard-drive configuration, both hard drives are loaded with identical software and any modification made to the primary hard drive automatically is duplicated in the secondary drive.
Today, the preferred furnace control system still incorporates a PLC as the primary controller because of its proven reliability and simplified programming requirements. However, the PC-based system, with its inherent capability to provide additional functions, may evolve into the future furnace control system of choice.
For more information: Jim Terchila is applications engineer, Seco/Warwick Corp., 180 Mercer St., Meadville, Pa. 16335-3618; tel: 814-332-8509; fax: 814-724-1407; e-mail: email@example.com