Control System Improves Precision of Heat-Treating Processes
The heat-treating industry is continually searching for new processes to achieve more predictable, precise results in controlling the properties of steel. Meeting the goal of achieving ever tighter process specifications to ensure successful results involves gathering more information regarding heat-treating process control and, in many cases, giving customers additional data relative to actual heat-treating production processes involving their parts. There is a continual need for timelier, more accurate data with which to ensure customer confidence and satisfaction.
Precise process control system
To address the need for precise process control in the heat-treating laboratory, Applied Test Systems Inc. (ATS) developed a computer-controlled heat treat system derived from its mature computer-control system in use for several years in the automation of precision material creep and stress rupture testing in the company's line of creep testing frames.
Many of the requirements of creep testing, such as precision temperature control, data acquisition and retrieval of archival calibration records, are consistent with the requirements of a heat-treating laboratory. Thus, the salient features of the existing creep system have been incorporated in this new heat treat system.
Temperature control is just one of many important requirements of a furnace-control system. Much more is required of any system where operational and equipment costs must be justified. The ATS control system can be thought of as a heat-treating facility automation system having the capability to:
- Document, track the progress and archive the results of a heat-treating process for any job or lot of material, automatically attaching an event log to the job record, and thus, eliminating the need to manually maintain a traveler or other progress record
- Write and store temperature profiles and specifications to be used
- Input and store system-calibration results and automatically attach the files to the job record, which eliminates the need to separately maintain calibration records
- Display on a computer screen the status of the entire facility (furnaces and jobs to be processed), which improves facility efficiency through better use of resources
These features reduce the amount of "process uncertainty," which has an inverse relationship with process predictability (also called quality assurance). The positive implications for plants using the system can be immense. In addition to the normal concerns relating to quality assurance as it relates to the customer, reducing process uncertainty can also serve to reduce perceived risk with respect to product liability.
Current computer-control systems range from simple computer control of discrete set-point controllers to centralized systems, which require thermocouple input from each furnace wired back to the central computer. Centralized systems have high installation costs, and the possibility of computer failure could disable the entire lab.
ATS' system strikes a balance, minimizing the amount of thermocouple wiring and reducing the possibility of hardware failure to a minimum number of furnaces. Each controller is in charge of only four furnaces and is independent of other controllers within the facility system. Thus, a catastrophic controller loss can only adversely affect the operation of four furnaces.
Figure 1 shows the arrangement of master and slave controllers. All thermocouple wires from the three slave furnaces are wired to the associated master controller, which contains the precision cold junction compensation circuit for a maximum of eight thermocouples. Any combination of NIST-listed thermocouples can be used, and the type can be changed with a simple notation in the menu-driven software.
A maximum of two thermocouple inputs from each furnace are accommodated. A control thermocouple is located in the furnace wall, and the other normally is placed near, or is attached to, the material being processed. Temperature signals from the furnaces scanned and digitized using an 8-line scanner and a 24-bit A/D converter, respectively.
An addressable RS485 communications cable is used as the communication link between computer and master controllers, as well as between a master controller and its associated three slave controllers. A maximum of 32 master controllers can be accommodated by one computer port.
Each master controller houses a MC68331 32-bit processor that provides processing power for the master and three slave controllers. Additionally, each furnace has its own dedicated 8-bit processor. Sufficient memory is located in each controller such that once downloaded from the computer, the heat profile can continue to run for several days without additional computer intervention. Even in the event of a computer failure, furnace control and data logging continues, and after computer communication is restored, the archived data is downloaded and data display is restored.
Inputs are available to detect open doors and other situations, and outputs are available to control items such as gas flow and control valves. The provision for operator annunciators is incorporated in the software, and outputs are provided. All input and output lines are optically isolated to eliminate the unwanted effects of ground loop potential.
Each controller, whether master or slave, controls one furnace. Possible control methods depending on the type of heating element involved are solid state relay, switching at zero-voltage crossing points and an analog signal to drive the silicon-controlled rectifier (SCR) systems required for silicon carbide and other high-temperature elements, which require current-limiting controllers.
A significant advantage of the system is the elimination of phase angle-fired SCRs when used in furnaces with a temperature rating below 1200 C (2190 F), as they can use the zero-crossing solid-state relay-control method. The elimination of the electrical noise generated by phase angle-fired SCRs is welcome in any installation. Another benefit of using of zero-crossing solid state switching is the reduction of power factor corruption, sometimes resulting in significant savings in electric power costs.
A unique feature of the algorithm used in this system is the preemptive correction of the effects of line-voltage variation. By comparison, in many other temperature-control systems, the result of a line voltage change is a corresponding change in furnace temperature, and the unavoidable lag in correction by the controller.
This system monitors the power-line voltage, and upon sensing a change, immediately applies an appropriate correction in power to the furnace, canceling the effect of the voltage variation, resulting in no change to furnace temperature. For example, in the event of a line voltage reduction, the controller automatically increases the amperage in such a way as to maintain constant wattage being delivered to the furnace heating elements. At the termination of the voltage reduction, the system reverts back to the original amperage draw, again ensuring a constant wattage delivery to the elements. Due to the wide variety of furnaces used in the heat-treating industry, a provision for easy access to the PID tuning constants simplifies the tuning process.
Several levels of user-programmable over- and under-temperature alarm levels ensure furnace shut down when the appropriate temperature deviation is experienced. An independent watchdog feature ensures furnace shut down if processor function is interrupted. A totally redundant "overtemp" controller is available to meet required safety codes.
Even though thermocouples attempt to match the NIST characterization by type, no batch of wire is perfect, and few thermocouples ever match their predicted performance exactly. Thus, the system allows for sample thermocouples to be calibrated, and a correction factor established for temperatures over the expected operating range of the thermocouple. Calibration data can be input into the system and used to correct the temperature information recorded and displayed by the system. Thus, no manual correction is required, and all reference to temperature is always the corrected value.
An unlimited number of calibration records can be stored in the system, and the appropriate record is linked to the specific thermocouple in use during the set up process, and can be changed as necessary as thermocouples are replaced.
Dirats Laboratories, a materials-testing laboratory located in Westfield, Mass., routinely machines and heat treats customer material prior to testing as part of its services, and maintains a heat-treating facility consisting of 12 furnaces having an operating temperature capability to 1540 C (2805 F).
An ATS computer-controlled heat treat system was installed at the Dirats facility on a beta-site basis. According to Dirats president Eric Dirats, the laboratory is experiencing an order-of-magnitude better temperature control than previously found in the heat-treating environment. The lab is able to change temperature without overshooting, and maintain temperature to tighter tolerances. This results in less dead time in the furnace and an enhancement of production rates.
"The ability of the system to automatically correct temperature using the thermocouple wire-calibration data, and to display and record the actual corrected temperatures along with the tolerances allowed within the test specification is unique and very useful. We never had anything like that with our strip chart recorder system," says Dirats.
"The program architecture makes it easy to replicate the profile for subsequent jobs, thereby eliminating the chance for human error. The fact that the computer is keeping track of jobs even before they go to the furnace allows the department to run more efficiently. It's not just a temperature-control system. It is a work-management system.
The autonomy of the individual controller eliminates concern for computer failure. The architecture of the hardware is much more flexible than PLC based systems and allows the lab to be set up for the convenience and safety of the operating personnel," he added.