Instrumentation and process controls used on vacuum furnaces in the heat treat industry are extremely diverse since vacuum furnaces can range from 20 to 50 years old. Vacuum furnace process control systems are somewhat like atmosphere style batch control systems; however, tend to be somewhat more sophisticated especially from a temperature uniformity control standpoint.
Most atmosphere furnaces tend to change set points only a couple times per cycle and usually heat as fast as possible, vacuum furnace cycles usually incorporate a heating ramp and quite often a cooling ramp. A traditional single loop PID controller would not typically be used for temperature control alone. Based on this need for more complicated temperature cycles, a control system with setpoint ramping, multiple setpoints and multiple tuning sets are required. It is not uncommon for vacuum furnace recipes to have 10 or more segments in a recipe. The controller could also have “auto tune” capability, along with anti-overshoot algorithms called “Fuzzy Logic”.
Hybrid Process Control Systems
A vacuum heat-treating environment can be difficult because of the changing heat transfer characteristics of the furnace as it moves from convection to radiation and conduction. The rapid heating rate of a vacuum furnace demands precise control, including setpoint program control with soak guarantee inputs. Overshoot of temperature set points is usually not tolerated for metal treating applications. Setpoint program control is often applied to the temperature, vacuum level and gas pressure with extensive interaction between the logic control. An alternate process control system often used is a “hybrid process control system”. The hybrid controller is a combination PID process controller, setpoint programmer and logic control.
Controledge HC900 Hybrid Process Control
Load Base Control Algorithm
- The HC900 Hybrid Controller combined with Experion Vista interface meets all the requirements for safe and productive process operation with maximum operator convenience including: Program control of sequencing and variables versus time
- Proportional (PID) modulating loop control
- Logic functions for equipment and process status
- Alarm detection, annunciation, and logging
- Data acquisition and data logging
- Recipe configuration, local storage and download capability
- Easily programmable by operators in engineering units.
- Sixteen (16) programmable events for integration with sequence control functions.
- Alarms and events may be programmed to send an e-mail message.
- Modbus/TCP protocol allows interfacing to HMI, data acquisition and OPC server software.
- Ethernet port supports direct PC connection or external Modem connection for configuration upload, download and maintenance.
- Isolated, universal analog inputs allow mix of analog input types on same card, saving I/O cost
- Auto tuning and fuzzy overshoot protection for quick start-up and proper control operation
- Storage of up to 1000 recipes for fast, error-free product selection
- Storage of up to 1000 time/temperature profiles. Each profile may be part of a recipe.
- Any HC900 can support up to 8 peer controllers for exchange of analog or digital data over Ethernet.
For years customers have been struggling with many issues surrounding their furnace controls including:
- Inability to achieve furnace temperature uniformity causing out of spec parts and in some cases lost product.
- Inaccurate control of setpoint program forcing operator to put program into HOLD allowing load temperatures to stabilize before entering SOAK. This manual operator intervention results in loss of production time and possible out of spec product.
- Loss of furnace classification per AMS2750 due to lack of control of load temperatures
- Unable to track Thermocouple usage
- Ineffective central alarm notification which inhibits operators for performing other tasks outside of monitoring heat treatment cycle
- Lack of predictive maintenance notifications
Loy Instrument, Inc has over 29 years of experience in Vacuum Furnace and Batch Furnace Control Solutions helping customers tackle these issues. Loy Instrument, Inc has developed a Vacuum Furnace Control and Load based Control Solution using Honeywell’s Hybrid Controledge HC900. This control algorithm has been implemented on many Vacuum and Batch Style Heat Treat furnaces solving the problem areas identified in the above list.
The Load Base Control Solution monitors the control, load, and zone thermocouples. The Controledge HC900 centers these temperatures on the control Setpoint. This control solution ensures that your temperature uniformity tolerance is always met while allowing the operator to accomplish other tasks rather than monitoring the vacuum furnace control. This control solution also greatly reduces out of spec overshoot and undershoot while entering a soak segment.
The Controledge HC900 is programmed to monitor the Thermocouple usage based on temperature (per AMS2750) and notifies the operator when these need to be changed. Another predictive maintenance notification programmed into the Controledge HC900 is the Thermocouple health monitor which notifies the operator that a thermocouple is approaching failure. This allows the operator to change the Thermocouple before it fails in the middle of a run resulting in lost production.
Control of temperature is executed with a powerful algorithm set that satisfies the most stringent application requirements. Multiple tuning constants may be used to tailor the control response to the dynamic characteristics of the furnace.
Approach limits allow maximum heating rates without overshoot, reducing cycle time and optimizing efficiency. The HC900 integrates the setpoint programmer, loop and logic functions within a single device. The Setpoint Program capability of the HC900 is used to control the temperature profiles with up to 1000 different profiles appropriate for a wide range of products, can be created and stored for use when these products are processed.
A typical heat treat cycle profile (Fig. 18) uses load guarantee soak function (as event 7) to control critical soak temperature. The cycle profile also contains other events used to control diverse functions required by a heat treatment cycle (high and low vacuum level, partial pressure, quench, etc.).
A single configurable database integrates both the loop (proportional, modulating) functions and the logic (discrete, Boolean) functions required by the process. User-friendly operator displays provide the operator with dynamic information about the status of each run as it progresses. Alarms are announced in color on dedicated displays and can be acknowledged directly from the operator interface.