Fixed value is also called set value and set value. In this type of control system, there is a preset threshold. When the physical quantity in the system reaches or exceeds the threshold, the system will trigger a specific action. For example, the temperature control system will set a threshold. When the system temperature reaches or exceeds the threshold, the system will trigger the action of lowering the temperature, such as turning on the cooling equipment. A fixed value in mathematics. For example, suppose there is a function f(x) equal to 2x+3. For this function, when x equals 2, f(2) is a constant value, because when the given input value x equals 2, its output value is fixed, that is, f(2) equals 2x2+3 = 7. This 7 is a fixed value of f(2).