Close Process Control Translates To Quality Heat Treated Parts
After purchasing a nearly closed heat treating facility (Anderson Heat Treating in Rockford, Ill.) in 2000, the new owner, Dick Francis set out to build the finest commercial heat-treat shop in northern Illinois focused on heat treating small, high-quality precision work. This presented a considerable challenge due to the condition of the furnaces and controls. The goal was to bring the shop to top-shelf condition by hiring the best personnel and focus on processing small runs of high quality parts not easily handled in large commercial heat treat shops. The philosophy was that exceptional heat-treating quality starts with exceptional temperature and atmosphere control. In this case, the furnaces were equipped with outdated control systems, so the plan was to bring the plant equipment up to date by rebricking and meticulously maintaining all furnaces and upgrading all furnace controls.
The first upgrade was to install a Yokogawa UT350 controller on one of the draw furnaces; a furnace in which it was difficult to maintain tight temperature control. The UT350 was set up using Yokogawa's Zone PID function to provide better control over the 350 to 1350 F (175 to 730 C) operating range. The Super Control "Fuzzy Logic" overshoot suppression system was enabled, and the system was auto-tuned. The unit provided very good control, holding a +/-2 F tolerance on the old furnace-considerably better than the owner had previously seen.
One of the four main carburizing furnaces was the next piece of equipment to be upgraded, which involved the installation of a UP750 Carbon Master controller, a dual-loop programmable controller that controls both carbon atmosphere and temperature with up to 300 program patterns. The Carbon Master controller delivered superior control and saved up to 2 hours furnace time on some loads according to the company.
The reliability of the Carbon Master was verified on a regular basis by hardness tests. The technology behind this type of control is Yokogawa's proprietary SUPER2 hunting suppression system. As carbon is absorbed into the steel parts, the demand for carbon in the atmosphere is reduced and the dynamics of the process is changed. The PID variables for carbon potential change from the beginning of the cycle to the end of the cycle. In many cases, a furnace will experience hunting at some point during the process when the PID variables are no longer optimized. SUPER2 observes the phase between the process variable (PV) and the output (MV). If the phase between PV and MV deviates from normal operating conditions, SUPER2 shifts the phase of the PV to suppress the hunting.The process model block is simulating the process in Fig. 1. There are two process models for the block. The (SC = 2) process model is optimized for processes that run at a constant setpoint for long periods of time, and the dynamics of the process changes during the run period. The (SC = 3) process model is optimized for processes that operate over a wide range of temperatures, and setpoints are changed frequently. Each process model is approximately estimated by the current PID parameters. The compensator block observes the PV and signals from process model block. As the phase deviation becomes larger, the compensator computes a new PV value and feeds that value to the PID block to suppress the hunting. The PID block executes normal PID calculations. The new calculated PV is invisible to the operator. A furnace temperature of 1000 F (540 C) on the controller is the actual furnace temperature, but the PID block may be responding as though it is 974 F (523 C).
This allows achieving stable, accurate control in applications where process conditions change over time during the treatment, such as carbon potential, in process drying of ceramics and where setpoints change over a wide range of temperatures during the process. The hunting suppression system can only be found in Yokogawa controllers.
Figure 2 shows test results from an actual furnace. The furnace was auto tuned at 1470 F (800 C) and it calculated PID values of P = 2%, I = 128s and D = 32s. Figure 2a shows how the furnace responds when the temperature is set for 200, 400, 600 and then 800 C, respectively. Figure 2b shows the same furnace running under the same conditions with the exception that the Super 2 hunting suppression system is operating.
Over the past two years, nearly all the furnaces at Anderson have been upgraded with Yokogawa single-loop controllers, Carbon Masters and actuators, which has allowed the company to provide high quality heat-treated parts. The next step for the company is to obtain AS9100 certification and to acquire NADCAP accreditation, which involve meeting a number of requirements including:
- Minimizing areas where human error mistakes are possible
- Capability for complete process data acquisition, retention and report generation
- Setting furnace cycles from bar-code scans
- Tracking customer work POs and invoice numbers
Another area requiring tighter control is controlling retained austenite to prevent the need to reheat treat or scrap parts. The method used to control retained austenite at Anderson is a plunge-diffuse technique, which involves, in part, "blowing down" the furnace to quickly to lower the process temperature while maintaining a new lower carbon potential. The process works well to keep retained austenite to a minimum and distortion is virtually eliminated. A problem with this process was a two-hour period between the end of the soak cycle and the point of plunge-diffuse, when the furnace had to be blown down manually. A Yokogawa CX1000 control station was installed to eliminate the two-hour period. Its logic math function saved two hours on the furnace by converting the furnace from a fully manual operation to semiautomatic.
The CX1000 control station also enables Anderson to meet customer's increasing demands for quality work and an audit trail. With the CX, the company has excellent control, complete data acquisition, heat treat recipe management and complete furnace automation for one product (Fig. 3). Anderson hopes to install CX control stations on every carburizing furnace in the shop.
The CX1000 and its larger counterpart (the CX2000) combine control, data acquisition and networking into one package. The combination of menu-driven screens and function keys enables the control and measurement parameters to be easily entered into the CX. Data from externally wired Modbus-compatible controllers and devices can also be easily monitored and logged through the use of prebuilt screen combined with the built-in serial interface. The embedded control loops in the CX enable the user to perform a variety of control algorithms such as single, cascaded and PV switching with independent program profiles. Combining these features minimizes panel space, wiring and cost.
CX1000 features a high resolution, color TFT display screen with up to six universal inputs, two embedded control loops and built-in serial interface to four external controllers (Fig. 4). The CX2000 has a larger display screen, up to 20 universal inputs, six embedded control loops and built-in serial interface to 16 external controllers. Each embedded control module is provided with universal control outputs and DIO terminals for two control loops.
Additional DIO extension modules are also available. A variety of standard on-board data storage media include floppy disk, ZIP drive or 160-MB ATA PCMCIA memory card (Fig. 5). Advanced networking features include FTP client mode that allows automatic batch record archiving to a designated network FTP Server. The web server function allows a standard PC web browser to display real time DAQSTATION screen images on any PC having Internet access. With a web browser, users can view all data display screens, as well as alarm summary, instantaneous channel overview and system status log display information. Because this is a read-only operation, it is not possible to remotely access and alter any CX1000/CX2000 settings or records. This requires the use of either the free DAQ Standard software or the simple control panel software. Also, the CX1000 and CX2000 can send Internet SMTP e-mail messages and reports to designated recipients, triggered by alarms, power failures and other system events (Fig. 6). New versions of Yokogawa's DAQWORX'S software add to the CX ease of setup and the capability to solve demanding control and data acquisition applications.