Vacuum Furnace Control Systems:  Past, Present, and Future

Jim Terchila, Sales Applications Engineer

In the last several years, we have seen some exciting changes and developments in vacuum furnace control systems. This article presents an overview of the latest developments in PLC/PC control systems.   For more information on Vacuum Furnace Technology, visit our website at www.secowarwick.com/vacuum.html. 

Until the early 1980’s, most vacuum furnaces relied on relays, timers, manually operated push buttons and dedicated process control instruments to control the sequencing and operation of the furnace equipment. To execute a specific cycle the operator was usually required to perform a series of predetermined functions such as starting the vacuum pumps, turning the heat on, backfilling the furnace, ending the cycle, and other manually initiated operations. While these vacuum furnaces produced consistent loads of product with a high degree of reliability, the level of operator attention and skill 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 was required to keep malfunctions and furnace downtime to a minimum. 

With the advent of microprocessor based control instruments such as the Honeywell DCP 7700 and Barber-Coleman 570 temperature controllers along with microprocessor based vacuum controllers, the required level of operator intervention decreased, and the reliability of overall furnace operation improved. However, the introduction of microprocessor based controllers and the continual stream of updated versions and new models offered, demanded that someone, usually a facilities engineer or controls engineer, become familiar with a variety of microprocessor based instruments.  While less operator attention was required during a cycle, the operator was required to become familiar with an assortment of display screens on several different process control instruments.  The situation demanded one instrument/controller to manage these individual control instruments and further reduce the level of expertise, and programming skills required. 

The development of the programmable logic controller (PLC) offered a solution to these problems. The PLC was originally designed to replace relays, timers, and other hardwired logic control systems, and basically to simplify the management of several individual control instruments. While PLCs were actually introduced well before 1980, their use on vacuum furnace control systems was not widely accepted for several years afterward. However, the modern PLC is reliable, extremely powerful, and very practical for use as control devices on even the most complex vacuum furnace systems. PLC technology is an important tool in helping plant managers achieve their goals of reduced furnace operator involvement, consistent product quality, and less furnace down time. Most modern vacuum furnaces equipped with a PLC require only that the operator load/unload the furnace, select the recipe to be run, and push the start button. The cost of  PLC systems, including the PLC, operator interface panel, engineering programming, and installation labor is generally well below the costs of hard wired relay/timer/push button systems, and more importantly, most of the problems associated with the older systems, such as dust, mechanical wear, loose wires, etc. are virtually gone.

Several additional benefits can be realized when equipment is controlled by a PLC. This is especially true if all operating parameters of a vacuum furnace, including temperature, vacuum level, cooling pressure and time are controlled by the PLC. 

• The number of different control instruments used in each system can be reduced. Not only does this mean a cost savings on the number of instruments used, but it also means that the operator need only 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.
• Since all of the important data is monitored by the PLC, additional alarms and status indicators can be added. Diagnostics that are not possible with other instrumentation can be added easily. For example, in a multiple zone furnace if the circuit breaker for one heating zone trips, and the adjacent zones try to carry the additional demand, the PLC can monitor this and display a message indicating the malfunction. The operator would then know to check the circuits. Taking this 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.
• With the addition of a PC or Operator Interface Panel, an alarm history can also be viewed or printed. This data would list all of the alarm messages that have occurred over a specific time interval, as well as the time and date of when the alarm was acknowledged and the condition remedied. 
• Troubleshooting can be greatly simplified. Since all inputs and outputs go through the PLC, their status can be monitored at all times and changes executed easily without rewiring. Alarm and status messages can be programmed, as well as self diagnostic test routines to aid in troubleshooting. The PLC can also be connected to a modem so that the equipment can be monitored, controlled, or modified from a remote location. This can include service from the furnace manufacturer which means a quicker response time, less furnace down time, and can eliminate the need for an onsite field service call. All of which translates into costs savings.
• The power can be controlled such that 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. 
• The utilities can be monitored for each piece of equipment and the actual costs per cycle or hour can be determined.
• The program can be written to display a message reminding operators to perform scheduled maintenance and a record of the maintenance performed can be logged.
• The equipment and/or process can usually be modified without changing the equipment, only the control software. This reduces labor costs and furnace down time.

Supervisory/Control Stations

In many cases, it is desirable to monitor or control the equipment from a remote location, such as a supervisory office or maintenance office. This can easily be accomplished through the use of a personal computer (PC) and/or human-machine interface (HMI) software. This type of system is capable of not only controlling and monitoring the equipment, it is also capable of data acquisition, data storage, graphing, and printing. The following describes this type of system in more detail:

Typical schematic of control system with remote computer station

The graphic above shows a typical schematic of a control system with remote computer station. The PC control station is capable of starting, stopping, and interrupting an automatic or manual sequence, as well as being able to switch from automatic to manual and back again, during a cycle. It is also capable of displaying any alarm status and and in some cases, initiating the corrective action required The entire operator interfacing is completed through graphic display screens, which are configured to appear as graphical representations of the furnace and related components. The various operator parameters are selected and entered through the use of a mouse, or through the keyboard.

Main Screen

All of the 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, from the main screen, the furnace and related components are shown, along with critical information, such as temperature, however, if the operator would want more detailed information, such as percent of power output to the heating elements, the operator would click the mouse on the furnace display and the detailed furnace information would appear. A second click would take him back to the main display.

Control Screen

On the graphic displays, there are additional methods available to indicate the status of equipment not available on any other type of display. For example, in addition to showing a digital indication of vacuum and pressure level, valves, fans, gas and water lines can change color and /or blink to indicate their status or problem. In addition to digital temperature indication, the furnace icon can change in color which presents fast and easy to understand information for the operator, thus minimizing the possibility of costly mistakes.

The data logging instructions may be initiated from one of the operator interface screens, and data is collected based on change of process variable to save on the actual amount of memory used. During the cycle, a window can be configured to show real time trends, and all of this data can be called into a spreadsheet program and manipulated in any number of ways, i.e. charts, graphs, or digital printouts.

Trend Screen

The PC, in this type of control system, is configured to be an operator interface only. It’s primary function is to allow the operator to enter and retrieve data related to the process. All of the sequencing, safety, and alarm functions are stored in the PLC. Should the PC “go down “ for any reason, the PLC would continue to control the automatic sequencing of the equipment through completion of the cycle.

Since the PC is operating in a Windows® environment, it is able to "multi-task" so even though another application, such as word processing, is actively being used, the PC is still logging data and equipment status.

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 would enable on-line changes, so that troubleshooting would be simplified and not require putting the PLC into the program mode.

The PC can also be equipped with a communications package, including a modem, so equipment can be supported from the furnace manufacturer’s engineering staff without the need for travel or on-site field service personnel. This option also allows the furnace manufacturer to monitor the equipment I/O, as well as the software, so both types of problems can be identified. Special communication software can also be included, which would permit the remote user to run any software on the host computer.

Advantages

• Simple to operate. The operator interface screens can be configured to graphically show the equipment and it’s status. This is much simpler and quicker than LED digital indicators.
• Data acquisition. The process data and control data can be recorded onto thePC hard drive. This data can be configured, manipulated, duplicated easily, and then displayed in a variety of ways. The information is much easier to decipher than a trend recorder.
• Additional PC’s can be networked so that control or monitoring can be done from one or more remote locations.
• Additional equipment can be controlled from the same PC at the same time, if the equipment includes a compatible PLC.
• The PC is able to support other programs, such as word processing programs, while operating the furnace equipment.

Future Control Systems

So what will future PLC/PC/Operator Interface Panel systems look like? The next logical progression in vacuum furnace control systems will probably consist of a stand alone PC, without the PLC. While many users of today’s control systems are leery of committing to total dependence on a PC to control their equipment, several PC only systems are already in operation.

Why are plant managers and supervisors dubious of PC only control? Well, as most of us have probably experienced, PC’s can and will lock up, or crash. When either of these conditions occur on a PC based control system, the PC must be shut off, rebooted, or in some cases the hard drive/computer will require replacement. Control of the equipment and the process is lost during this period and that’s enough to scare anyone. However, there are remedies available, that if not completely foolproof, will at least minimize the impact of these occurrences.

One remedy, that will at least reduce the possibility of PC lock up, is the installation of Windows CE – a more reliable and simplified operating system than used in most modern day PCs. PCs usually lock up because something objectionable exists in the operating system, programming code, software, or memory capacity. The simpler the operating system, the less chance of lock up. While Windows CE does not offer as many multi-tasking operations as say NT, it does work well on furnace control system PC’s that benefit from a more dedicated and simplified operating system.

A solution to a crash of the PC  hard drive, could be the installation of dual hard drives in the PC. With the addition of a second hard drive, or mirror hard drive, failure of the primary hard drive would initiate an automatic change over to the secondary hard drive, thereby eliminating the interruption or loss of furnace control. With the dual hard drive configuration, both hard drives would be loaded with identical software and any modification made to the primary hard drive would automatically be duplicated in the secondary drive.

While today, the preferred furnace control system still incorporates a PLC as the primary controller because of the proven reliability and simplified programming requirements, the PC based system, with the capacity to provide additional functions, may evolve into the future furnace control system of choice.