Using a PLC for Precision Temperature Control

Advantages of Using a PLC for Precision Temperature Control

Condensed article from the SECO/WARWICK Heat Treat Furnace Seminar

Temperature Control Panel for an Aluminum Log Homogenizing Furnace System

The purpose of temperature control is to facilitate heating a work load to a specified temperature, then maintain it at that temperature in a controlled manner. PLC control systems are the best way in which this type of precision control can be accomplished.

There are many types of instruments that can be used for temperature control or other process variables. In the past, dedicated instruments were used for each control loop. While this is still done, the advancement of Programmable Logic Controller, or PLC technology has made it possible, practical, and desirable to have the temperature control included in the same instrument that is also controlling the other aspects of the equipment.

An Operator Interface is a display device that an operator looks at to know what is happening on the equipment. The display will contain all important data, like the actual and set point temperatures, drive speeds, alarm messages, etc. This device usually has a keypad attached to it so that the operator can also enter information like new temperature set points. When an operator interface is used, it also eliminates many of the other instruments and displays required. In fact, excess temperature instruments are usually the only other instruments mounted on the control panels.

There are many benefits that can be obtained when equipment is controlled by a PLC. This is particularly true if all aspects of the equipment, including temperature control, are controlled by the PLC.

The number of different instruments in each system can be reduced. This is even more important than just the cost reduction of instruments. It also means that the operators need only to learn the operation of one device. When other pieces of equipment are added, they can use the same operator interface, greatly simplifying the operation learning curve. The display is also created by the equipment programmer, so it can be custom made to be as simple or complicated as desired.

Since all of the important data is monitored by the PLC, more alarms and status indicators can be added. Diagnostics that are not possible with other instrumentation can be easily added. For example, if in a multiple zone furnace a circuit breaker for one zone trips and the zones next to it try to carry the load, the PLC could check for this, and display a message to the operator to check the circuits. In addition to current alarms, an alarm history can also be viewed or printed. This would list all of the alarm messages that have occurred over a time period, as well as when the alarm was acknowledged and when the condition was fixed. This can be a very useful maintenance tool.

Troubleshooting is greatly simplified. Because all of the inputs and outputs go through the PLC, their status can be monitored and changed easily without rewiring. Alarm and status messages can be programmed, as well as self testing diagnostic routines to aid in trouble shooting. The PLC can also be connected to a modem so that the equipment can be monitored, and modified from remote locations. This can include service from SECO/WARWICK and, therefore, could eliminate the time it takes to get a field service person on site.

The power can be controlled such that not all zones are at 100% at the same time. While this does not have a large effect on the operation of the equipment, it would ensure that the power drawn at one time is kept to a minimum, thereby reducing the demand charge for the companies' utilities.

The utilities can be monitored for each piece of equipment. With this information, the actual costs per cycle or hour can be determined.

The program can be written to display a message to remind operators of scheduled maintenance, and then keep a record of when this maintenance was performed.

The equipment and/or the process can be modified usually without changing the equipment, only the control software. This saves labor and shut down time. Improvements can be made to control algorithms to customize the control to the application.

SUPERVISORY/CONTROL STATIONS

In many cases, it is desirable to monitor or control equipment from a remote location, such as a supervisors office or a production or maintenance office. This is possible through the use of personal computers (PC's) and man-machine interface (MMI) software. This type of system is capable of not only controlling and monitoring the equipment, it is also capable of data acquisition, storage, graphing, and printing.

Since the PC is operating in a Windows environment, it is able to multi-task. In other words, a portion of the processor is used for different applications, so more than one application can be operating at the very same time.

For this reason, the PC can also be used for other programs, such as word processing or spreadsheet programs at the same time that the equipment is in operation. During this time, the PC is still logging data and equipment status even though another application is actively being used.

Feature Article February 2005