Questions regarding DCS high selection and low selection

1. High/low selection refers to the use of two PID controllers to control a valve; the outputs of these PID controllers are compared within the DCS system, and the larger output is sent to the valve actuator. Choosing the larger output corresponds to high selection, while choosing the smaller output corresponds to low selection. 2. Positive and negative action refer to the relationship between the change direction of the controlled variable and the output variable in a single PID loop; when they move in the same direction, it is positive action, while when they move in opposite directions, it is negative action ； The flow control loop is generally reactive; for example, if the flow rate increases, in order to maintain the controlled flow at the set value, it is necessary to close the valve, which is why it is reactive. 3. There is no inherent connection between forward/reverse action and high/low selection; in the complex circuits involving high/low selection, the forward/reverse action of a single circuit is determined first, and then high/low selection is decided based on the process control objectives.

High selection involves choosing the highest value among multiple inputs as the output； Low selection involves choosing the lowest value among multiple inputs as the output; it is a module used to implement selective control. Direct action and reverse action refer to the variation pattern of a controller’s output in response to an input; that is, when the input increases, the output also increases (and when the input decreases, the output decreases), which is considered direct action ； An increase in the controller input with a decrease in output (or a decrease in input with an increase in output) is feedback. The controller input is defined as the controller’s error input, and there are two definitions for the controller input error: system-defined and instrument-defined. The system is defined as: controller input error = controller setpoint signal – measured signal; that is, an increase in the measured signal reduces the error, while an increase in the setpoint signal increases the error ； The instrument is defined as follows: controller input error = controller measurement signal – setpoint signal; that is, an increase in the measurement signal leads to an increase in the error, while an increase in the setpoint signal results in a decrease in the error ； It is generally determined through instrument definitions whether a controller is positive or negative acting; that is, when the measurement signal of the controller increases, an increase in output indicates a positive-acting controller. The purpose of determining the controller’s reaction is to ensure that the control system forms negative feedback. Action-reaction has nothing to do with high selection or low selection.

High or low selection is set according to process requirements. . Action and reaction fall within the scope of consideration for instrumentation professionals

There are so many enthusiastic and patient people; if I heard such questions, I would get angry right away.

So the changes in the driving and reacting forces correspond to the changes in input deviation as defined by the instrument? Is this deviation an absolute value? For example, a tank has its liquid level controlled by a discharge valve at the bottom – referred to here as a level control valve. If the actual PV value of the level controller is increasing but remains below the set SV value, then in such a situation the OP value will increase, which in turn leads to an increased opening degree of the valve in order to maintain a stable liquid level. Based on this, it can be concluded that the level controller operates in a positive manner. So, when the input increases and the measurement signal increases, does it mean that the PV value goes up, or that the difference between PV and SV increases? But here, the difference between PV and SV is based on the absolute value, and it is decreasing; how to solve this? I’m a novice in process engineering

I don’t know much about instruments in terms of manufacturing processes; I just got one recently: L

In the complex control circuits provided by the design institute, the instrument controllers all have positive and negative actions; in some cases, there is also a selection between high and low levels among multiple controllers for one container. So, there’s no connection between the two, right? Won’t the positive or negative action of the instrument controller affect whether a high-or-low selection is used among multiple controllers?

Found online: In selective control systems, there is usually one normal controlled parameter and one emergency controlled parameter, with two controllers corresponding to them, namely the normal controller and the emergency controller. That is, in the case of emergency release, if the output of the emergency controller decreases, the low option is selected ； That is, in the case of emergency release, if the output of the emergency controller increases, the high selection is chosen. In the event of an emergency, the selector should select the output of the emergency controller to intervene urgently in the production process. As an example, suppose there is a container with a level controller (of the direct-acting type) and a controller for the bottom drain flow valve (of the reverse-acting type). If both controllers are designed to operate in a high-priority mode, and it is necessary to maintain a stable liquid level in the process, should the level controller be considered the primary control parameter while the flow controller serves as the emergency control parameter? In an emergency situation, would the output from the flow controller increase in order to control the opening degree of the flow valve?

This post was last edited by Ku Liang Zhi Le on 2019-6-19 22:13. Deviation is not an absolute value. The deviation can be positive or negative. When the PV value is less than the SV value, the controller deviation is negative. As the PV value increases, the deviation becomes less negative (its absolute value decreases); meanwhile, the controller’s deviation also increases. If PV keeps increasing until the deviation reaches zero, that value is greater than a negative deviation.

I have a case here: the level controller for the tank operates in a direct-acting manner, while the controller for the drainage flow valve at the bottom operates in a reverse-acting manner. The design specifies a high-priority selection; was it intended from the start that the level controller would be the main parameter to be controlled, with the flow controller serving as an emergency control parameter? If the level controller is indeed the main parameter to be controlled, then why does the high-priority selection result in the level controller being used under normal conditions? (From a process perspective, the liquid level should be kept stable.)