Starting a Course on Process Control Instrumentation

From this post onwards we are starting a course on Process Control Instrumentation, so the students can now access info about analytical Instrumentation as well as PCI on my blog.


PROCESS CONTROL INSTRUMENTATION

1. Introduction

We often come across the term process indicating a set up or a plant that we want to control. Thus by a process we may mean a unit of chemical plant (say, a distillation column), or a manufacturing system (say, an assembly shop), or a food processing industry and so on. We may want to automate the process; we may also like to control certain parameters of the system output (say, level of a tank, pressure of steam etc.). Broadly speaking, there could be two types of control; we might want to carry out. The first one is called sequential control, where the control action is carried out in a sequence. A good example for this type of operation could be in an automated car manufacturing system, where the assembly of parts is carried out in a sequence (on a conveyor line). Here the control action is sequential in nature and works in a preprogrammed open loop fashion (implying that there is no feedback of the output signal to the controller). Programmable Logic Controller (PLC) is often used to carry out these operations.
But there are cases, where the control action needed is continuous in nature and precise control of the output variable is required. Take for example, the drum level control of a boiler. Here, the water level of the drum has to be maintained within a small band, in spite of variations of steam flow rate, steam pressure etc. This type of control is sometimes called modulating control, as the control variable is modulated to keep the process variable at a constant value. Feedback principle is used for these types of control. Now onwards, we would concentrate on the control of these types of processes. But in order to design a controller effectively, we must have a thorough knowledge about the dynamics of the process. A mathematical model of the process dynamics often helps us to understand the process behaviour under different operational conditions.
In this lesson, we would discuss the basic characteristics of this type of processes where continuous control is used for controlling certain variables at the outputs.

2. Characteristics of a Process

Different processes have different characteristics. But, broadly speaking, there are certain characteristics features those are more or less common to most of the processes. They are:
(i) The mathematical model of the process is nonlinear in nature.
(ii) The process model contains the disturbance input
(iii) The process model contains the time delay term.
In general a process may have several input variables and several output variables. But only one or two (at most few) of the input variables are used to control the process. These inputs, used for manipulating the process are called manipulating variables. The other inputs those are left uncontrolled are called disturbances. Few outputs are measured and fed back for comparison with the desired set values. The controller operates based on the error values and gives the command for controlling the manipulating variables. The block diagram of such a closed loop process can be drawn as shown in Fig. 1
In order to understand the behavour of a process, let us take up a simple open loop process as shown in Fig. 2. It is a tank containing certain liquid with an inflow line fitted with a valve V1 and an outflow line fitted with another valve V2. We want to maintain the level of the liquid in the tank; so the measured output variable is the liquid level h. It is evident from Fig.2 that there are two variables, which affect the measured output (henceforth we will call it only output) - the liquid level. These are the throttling of the valves V1 and V2. The valve V1 is in the inlet line, and it is used to vary the inflow rate, depending on the level of the tank. So we can call the inflow rate as the manipulating variable. The outflow rate (or the throttling of the valve V2 ) also affect the level of the tank, but that is decided by the demand, so not in our hand. We call it a disturbance (or sometimes as load).
The major feature of this process is that it has a single input (manipulating variable) and a single output (liquid level). So we call it a Single-Input-Single-Output (SISO) process. We would see afterwards that there are Multiple-Input-Multiple-Output (MIMO) processes also.