Writer: JUHA KARISOLA, GLASSROBOTS OY

The new glass types and ever rising quality requirements set the corporate research centres, development units and universities to face up to a challenge to create new, better ways to improve the reliability, quality and repeatability of industrial processes. Fuzzy control is a completely new, exciting form of computer logic to improve process performance in a number of different applications.

Fuzzy logic provides a systematic way to handle qualitative information. It enables an automated control system to reason and make judgements in similar way to human beings. Simply put, Fuzzy logic converts vague concepts or values into mathematical format, which is then used by the computer to control the process precisely.

Our company has developed a system that uses this technique to control the flat tempering process. The main benefit is the ability to repeat a process consistently and accurately, but the new system also makes it possible to mix different glass types and sizes in daily production without compromising the output or repeatability of the production. This article gives some general information about fuzzy controls and explains how and why it has been applied to the convectional tempering line.

Fuzzy Logic Controls

Fuzzy logic is based on the theory of fuzzy sets developed by Prof. L. Zadeh in the 1960’s and 70’s.  The theory of fuzzy systems is an area of study where primarily inexactness (vagueness, imprecision) or uncertainty is being studied by the means of the logic and set theory particularly formulated for this purpose.  Normal binary logic states that something either belongs to a group or it does not, but fuzzy logic accepts that something can belong partially to a group. For instance, binary logic says that when it is 30 ºC, water is warm and also when it is 20 ºC, but, abruptly, when it is 19 ºC it is not. Warm water is an unclear, ‘fuzzy’ concept, something that binary logic cannot understand.

By using membership functions (relationships of degree), Fuzzy Logic is able to assign quantitative values to something that partially belongs to a group.  Water at 20 ºC belongs entirely to the group ”warm”, while water at 19 ºC only belongs to this group to a certain degree.

The membership functions in each group may be bell shaped, triangular or trapezoidal and usually there are 3,5 or 7 groups.

Fuzzy Control consists of the following steps (see figure 1):

• fuzzification converting the measured process values into membership functions
• processing using set of rules
• defuzzification, to produce precise control values

To explain how a fuzzy logic control procedure works, let us look at a simplified example of target temperature and actual temperature. The aim is to define controls that keep the actual temperature steadily on the target curve by adjusting the heating power. The first step is to define the membership functions in the fuzzy sets for the input and output values (see figure 2).

Figure 2 shows the fuzzy set for the temperature difference between the actual and the target temperatures (T). Corresponding sets also have to be defined for other input values (in this example for the derivate of T, which indicates whether the difference is increasing or decreasing.

The fuzzy input values are then processed through the set of rules. The rules in fuzzy control consist of a condition, IF, followed by a control action, THEN. In normal language a rule would state something like: if the actual temperature is slightly lower than the target temperature and the difference is slightly increasing, then increase the heating power considerably. As a logic rule it states: if T is NS (negative small) and T/dt is NS (negative small), then P is PL (positive large). A simple matrix of the processing rules is shown in figure 3).

Each rule processes the information using different parameters; the output of each rule is different. The correct heating power required is obtained by combining the results from all the rules and finding its “center of mass” (see figure 4).

Fuzzy Control Benefits

Fuzzy control is simple for the user. It provides precise control values from non-precise input data, a situation in which a conventional controller fails. Since fuzzy control systems process rules in parallel they are very fast. Since each rule operates in parallel, the effect of individual errors is minimal. The whole system is resistant to errors and more reliable than conventional controls that process data in series. Fuzzy control constantly adapts automatically to changing conditions, something that normal PID controls, for example, are unable to do.

Full convection set the requirement for a new control

RoboTemp FTM is a horizontal roller hearth furnace with forced convection based furnace followed by high efficiency quench. The convection was chosen as it is the most efficient way to heat up the glass and especially suitable for the newest coated glass types with very low emissivity (e~0,02). Fast heating and low furnace temperature guarantees the best achievable optical quality and flatness. The distortions, roller waves and coating defects inherent to a radiant furnace are mainly due on extensive heating times in an overheated furnace and convection based system provides the easiest way to avoid such problems.

The glass is heated by means of hot air, which is blown to the upper and lower glass surface through nozzles and then circulated in the furnace. The amount of convection can be adjusted over the whole heating period, which eases the control the heating of the coated glasses. This unique construction allows also precise crosswise heating profiling, as the heating elements are encapsulated inside the nozzles. This is not provided by any other full convection furnace. Direct radiation has been completely eliminated. As the emissivity does not have much effect on heat transfer through convection, the furnace is versatile and suitable for production of different glass thicknesses, types and sizes in mixed production.

The process parameters (heating time, convection curve and eventually furnace temperature, as well as all the quench related adjustments) are stored under tempering programs in the computer.

FuzzyTemp™ How does it work?

The biggest challenge in the system was to find a control, which is fast and precise enough to control the heating profile. The traditional systems allow one profile for each loading. The ever rising requirements for thinner and larger glasses make such systems obsolete. Take a piece of coated high performance glass and even smaller pieces would not match the new quality requirements. Convection was the key to a solution, but it set tight requirement on the control system.

The FuzzyTemp™ is based on automatic, on-line temperature control; the temperature is measured over the whole width of the furnace by thermocouples, which are installed under upper nozzles. Based on this measured temperature profile of the glass, the FuzzyTemp™ automatically adjusts the required heating profile for each load to compensate the temperature differences. The measurement and following adjustment is continuously done, once in a second over the whole heating time. Simply put, towards the end of an ideal heating cycle the temperature over the glass surface gets completely even and the profile can be smoothly levelled.

As with any process the tempering process is subject to a number of external factors which further reduce the repeatability. Take an example of production mix: Whenever the furnace is loaded for small or medium runs, the process involves varying temperatures inside the furnace and roller bed. Another is the heat balance of the furnace: Unless the furnace is being operated on a full 24-hour, 3-shift basis, then the operators face the same problem every day. Even if a furnace can be taken into production after a couple of hour’s preheating, it still continues to heat up until reaching the heat balance. Thus a piece of glass produced in the morning may need compensation in the program when tempered in the afternoon. The same applies the external temperature around the furnace, which may vary considerably between morning and afternoon, summer and winter. Some operators compensate these differences by using different programs for different conditions while others may use adjustment in the heating time, profile or temperature to compensate for changes, but unless done on-line and in precise way as FuzzyTemp™ does, this may cause deviations in the flatness.

As explained, FuzzyTemp™ does not only provide easy, automatic way to create the right heating profile for each loading; it also compensate automatically the negative effects of varying production and external conditions. After extensive research and development, fuzzy logic was found the only available technique to realize the control satisfactorily under the given conditions.

Operator Experiences

Thanks to its versatility, the furnace has been adopted rapidly by the glass industry – not only by large float producers but also by small and medium size family businesses. The FuzzyTemp™ has already been installed in a number of furnaces around the world. Most of the furnaces are being used for production of large coated fasade glasses. Other applications where flatness is of major importance, benefit of the new control system as well. Many people may consider fuzzy controls odd and too complicated. Even though fuzzy control in itself may be a complex technique, its user interface is very simple. For the furnace operator, it could not be easier. Just select the required process curve and switch on FuzzyTemp™. All the controls then work automatically in the background.