Determination of positive and negative effects of regulators in cascade control and ratio control systems

To judge the positive and negative effects in the regulating system, generally check the drawing requirements of the design institute, or ask the craftsman to understand the control loop, which is generally in the form of negative feedback. The air opening and air closing of the regulating valve must be strictly in accordance with the design requirements, otherwise an accident may occur. The positive acting regulating valve means that as the air source pressure increases, the regulating valve stem gradually closes from top to bottom.

The positive and negative effects of the regulator mean that when the deviation value of the input regulator increases, the output of the regulator also increases. At this time, it is a positive effect, and if it decreases, it is a reaction. The positive effect of the valve refers to the downward movement of the valve stem when the signal input to the valve increases, and vice versa. There are two types of valve cores: positive and reverse. The valve has two positive and negative functions, so 2X2=4. The valve has 4 matching forms, corresponding to air opening and air closing. The selection of air opening and air closing of the valve mainly depends on the process requirements. Because the site must be in a fail-safe state, AFC and AFO are generally used to represent the air opening and air closing of the valve respectively.

The actuator has forward and reverse effects, and the control valve has forward and reverse effects. Air opening and air closing are related to the actuator and control valve above. The principle is to keep the valve in a certain state under accident conditions. Air opening is FC and air closing is FO. Executive agency plays a positive role: The gas signal increases and the push rod moves downward, which is a positive effect. The opposite is counterproductive. Formal installation of control valve: When the push rod moves downward and the valve opens wide, it is formal installation, and vice versa is reverse installation. If the positive action or formal installation is regarded as "+", and the reverse action or reverse installation is regarded as "-", then when the signs of the actuator and the control valve are the same, it is air closed, and when they are opposite, it is air open. The positive and negative effects of the regulator mean that as the input signal increases, the output signal also increases, and vice versa. The principle is that the entire control system is negative feedback. For example, a single-loop control system has a regulator, a transmitter, a control object, and an actuator. For example, if we choose the actuator to open the valve with air, then the sign is "+", the control object is positive action, the sign is "+", and the sign of measurement loosening is always "+", because the system is negative feedback, the sign of the result of the multiplication of the four signs is "-", so the sign of the regulator is "-", which is a reaction. If we choose the air-closed valve above, then the regulator sign is positive, which is positive action.

3# Thank you for the beating melody, but I still don’t understand the positive and negative functions of the valve. Please explain in detail!!!

Cascade control: The selection order of the forward and reverse actions of the main and auxiliary controllers should be first auxiliary and then main. (1) The forward and reverse effects of the auxiliary controller are determined according to the specific conditions of the auxiliary loop and have nothing to do with the main loop. The auxiliary loop can determine the forward and reverse effects of the auxiliary controller according to the method of determining the forward and reverse effects of a single-loop control system. (2) The forward and reverse effects of the main controller are determined according to each link included in the main loop. The amplification factor of the auxiliary loop can be regarded as "positive", because the transmitter is generally "positive", so the positive and negative characteristics of the main controller are the same as those of the main object.

Negative feedback is generally used in the regulating system, and positive feedback must not be used alone, otherwise accidents may occur. Principles for selecting air on and off.: 1. Under accident conditions, process equipment should be in a safe state as much as possible. For example, the washing line of the distillation tower should be open to ensure that it is fully closed in the event of an accident, so that the materials in the tower can be stabilized and avoid evacuation accidents. 2. Reduce raw material and power consumption under accident conditions and ensure production quality. 3. Consider the media characteristics. The liquid in the kettle of the distillation tower is easy to crystallize and solidify. The steam flow regulating valve of the boiler adopts an air-closed type to prevent crystallization or solidification in the kettle during an accident.

The air opening and air closing of the cascade controlled valve and the positive and negative effects of the regulator QQ593746553

It would be great if an experienced teacher could upload a picture to explain.