The move to a greater degree of furnace compliance for thermal treatments is building confidence between thermal processing equipment manufacturers and customers. This is especially true when the furnace design is coupled to the application of expert process control systems.

Fig. 1. Graph of typical thermal cycle.

Furnace builders are employing new design features to improve the overall temperature distribution of their equipment. Careful design of energy and power densities, along with improved atmosphere flow patterns, has led to good useful working volumes.

Process conformity is a major issue for aerospace (typically NADCAP AMS2750) and automotive (TS16949) customers. The move to a greater degree of furnace compliance for thermal treatments is building confidence between thermal processing equipment manufacturers and their customers. This is especially true when the furnace design is coupled to the design of advanced process control systems. Along with our company's experience with aerospace and automotive thermal processing specifications, our products and systems help ensure the controlled parameters are maintained inside accredited limits at all times.

Three issues drive consideration for modern control system design:

• The gain and thermal lag in most modern furnaces with low thermal mass insulation is principally becoming a factor of the load size and set point value, rather than the furnace structure or its inherent components.
• Customers are seeking ways to take better account of actual workload temperatures in their control algorithms, so a greater number of processes are utilizing some method of workload temperature optimization.
• Accredited processes often define furnace approvals by specific classification. To achieve maximum efficiency in production planning with the highest workload capability, it is critical that thermal processing equipment performs consistently to its most appropriate class.

Fig. 2. Flow chart of multi-loop temperature control strategy.

A recurring problem for customers is excessive workload temperature excursions that can occur on start-up or on profiled ramps, even when the controlled zones stay close to the desired thermal profile. Since many work piece components have variable cross sections, undesirable temperature excursions can have a detrimental effect, particularly on thin-section parts. These excursions can cause furnaces to fail audit. Because of this, many users resort to crude methods of overshoot elimination and gain optimization, which invariably add to process cycle time.

A typical thermal cycle is shown in Figure 1. No account has been made of the need to optimize the furnace control based on the load requirements. The graph shows a very cautious approach to control. While this ensures there are no excessive temperature excursions, it exposes parts of the work piece to long process delays and extends work-in-progress time.

Eurotherm offers a variety of solutions within its multi-loop, soft-wired products to build a range of control strategies. A typical example is shown in Figure 2.

Some of the benefits of such an arrangement include:

• The ability to select from a number of work thermocouples based on mathematical functions.
• A look ahead on program ramp/dwell transitions to avoid overshoot on profiled ramps.
• The cutback feature on both master and slave loops provides the ability to achieve fastest work temperature settling time with no compromise to tuning for steady state control.
• Shared integral action intelligence between master and slave control loops.
• Holdback on programming segments gives guaranteed dwell times.
• The ability to provide cascade-feed-forward based on set point or process value with defined limits to ensure that thin/thick section material temperature differentials are tightly controlled.
• PID control on both the master and slave loops ensure correct control optimization for the fast furnace loop and slower work piece loop.
• Variable dynamic tuning to improve black-heat/radiant-heat boundary transition.
• Output valve gear characterization for non-linear burner systems.
• Excess air profiling.

Fig. 3. Graph showing changes in thermal cycle with the application of precise thermal control system.

Thermal cycle changes are shown in Figure 3, which indicates that when the solution is able to precisely control the thermal head (the stored heat in a furnace), it is possible to achieve fast work piece temperature settling times within approved accuracy limits.

As demand for equipment, improved processing costs, and compliance to higher accredited standards grows, users of thermal processing equipment should recognize that attention to their furnaces' control performance can bring benefits in processing time, energy usage, and audited compliance. IH