Types of Controllers in Process Control System

What is Controller?

In this universe, there are several types physical changes exists, among those changes, which are under our bounded observation is known as process and device by which we can obtain desired response from that process is commonly known as the controller.

Types of the controller

Types of Controllers in Process Control System
Types of Controllers in Process Control System

1. Discontinuous Controller

A. Two position mode controller

Working

Historically, two position controller was abundantly used among the controllers, since it has only two possible positions i.e. 0% or 100%. A simple example of this type of controller is a relay.

Whenever the measured value of the controlled variable is less than desired value i.e. potential difference between two terminal of the coil of relay then, normally open contact gets closed or normally closed contact gets opened.

Similarly, if the measured value is equal to desired value then, there is no change in state.

Applications

Room heater, if the temperature of the room goes below the desired temperature then, the heater turns ON and if the temperature is above the desired temperature then heater turns OFF.

Advantages

  1. Simple
  2. Easily adaptable to large scale systems which have slower process rates

Disadvantages

  1. Neutral zone exists, due to which though the difference between measured and desired value exists, there is no change in controller output.

B. Floating Position Mode Controller

a. Single Speed floating Position Mode Controller

Working

This controller’s output changes at fixed rate when the difference between desired value and the measured value exceeds neutral zone. Mathematically, it can be represented as,

Where,

  • dp/dt = rate change of controller output with respect to time
  • KF= rate constant
  • ∆ep = half of the neutral zone

By integrating equation (1) we get,

Where p(0) is initial controller’s output.

This indicates that current controller’s output keeps a history of previous control outputs. In many cases, such a kind of information is not available.

b. Multiple Speed floating mode controller

Unlike single speed controller, it’s ‘KF’ values increases or decreases as per deviation exceeds certain limits. It means that for large error (|desired value – measured value|) will have large ‘KF’ value and vice versa.

Application

In self-regulating processes such as liquid flow rate control in the pipe (as shown in below diagram) a single speed floating controller is used. The load is determined by the inlet and outlet pressures Pin and Pout, and the flow is determined in part by the pressure P, within the DP cell and control valve. This is an example of a system with self-regulation. We assume that small control valve opening has been found to maintain the desired flow rate inside the pipe. If larger than the neutral zone, the valve begins to open or close at a constant rate until an opening is found that supports the proper flow rate at the new load conditions. Clearly, the rate is very important, because especially fast process lags cause the valve to continue opening or closing beyond that optimum self-regulated position.

Advantages

  1. Effect of neutral zone can be minimized.
  2. Overshoots and undershoots are reduced compare to two position mode
  3. Gives better performances for self-regulating lower order small dead time systems.

Disadvantages

  1. When applied to large-scale systems such as room temperature control, this type of controller shows inevitable cycling i.e. measured value will fluctuate around desired value for a long time.

In upcoming articles, we will learn about Continuous controllers, till then if you like this article, share it with your friends, like our Facebook page and subscribe to our newsletter for future updates. Have a nice day!

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