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Induction hardening

In industrial furnaces, several physical and chemical quantities must remain essentially constant or be operated according to a predetermined procedure if a high-quality product is to be produced. These quantities include temperature, pressure, gas composition, and indirectly flow rate. The principles of automatic control are the same for any of these quantities.

It is therefore best to briefly discuss these principles before discussing the control of each quantity. The quantity to be controlled (temperature, pressure, etc.) is called the controlled variable. The quantity that changes and can change the controlled variable is called the independent variable. In some processes, there are several independent variables. Automatic control is achieved by the following method: A force is derived (directly or indirectly) from the controlled variable. This force is balanced by gravity or mechanical spring force, or by a corresponding force generated by gas or electromagnetism.

The element that produces the balancing force has many different forms. If the value of the independent variable changes, then the value of the controlled variable also changes, and the balance between the derived force and the opposing force is destroyed. The slight difference in the two forces sets in motion a mechanism that attempts to return the controllable variable to its original value, which is called the “set value” or “control point”. This chapter presents a very simple control system.

The water level in the water (or other liquid) tank is the controllable variable. At the set value, the upward force of the water on the float is exactly balanced by the weight of the float. At the set value, the inflow and outflow of water are equal. If the drain valve (independent variable) is opened wider and the water level drops below the set value, the upward force of the water on the float becomes smaller. The float drops, thereby increasing the inflow of water, until the water level rises to a position close to the set value. The two drain valves constitute two independent variables. The set value is usually adjustable, so that the controllable variable can be changed as needed. In this article, the method of adjusting the water level is to bend the float rod or change the length of the connecting rod between the float rod and the valve rod.