Intro to Process Control--part 1

Learning Goals:

• Describe the three different types of processes

• Indicate the meaning of a time constant

• Describe the meaning of process variable, setpoint and output.

• Outline the meaning of first and second order systems

• List the different modes of operation of a control system.

Introduction

To succeed in process control the designer must first establish a good understanding of the process to be controlled. Since we don’t wish to become too deeply involved in chemical or process engineering, we need to find a way of simplifying the representation of the process we wish to control. This is done by adopting a technique of block diagram modeling of the process.

All processes have some basic characteristics in common, and if we can identify these, the job of designing a suitable controller can be made to follow a well-proven and consistent path. The trick is to learn how to make a reasonably accurate mathematical model of the process and use this model to find out what typical control actions we can use to make the process operate at the desired conditions.

Let us then start by examining the component parts of the more important dynamics that are common to many processes. This will be the topic covered in the next few sections of this Section, and upon completion we should be able to draw a block diagram model for a simple process; For example, one that says: 'It’s a system with high gain and a first order dynamic lag and, as such, we can expect it to perform in the following way', regardless of what the process is manufacturing or its final product.

From this analytical result, an accurate selection of the type of measuring transducer can be selected, this being covered in Section 2. Likewise, the selection of the final control element can be correctly selected.

From there on, Sections 4 through 14 deal with all the other aspects of Process Control, namely the controller(s), functions, actions and reactions, function combinations and various modes of operation. By way of introduction to the controller itself, the last sections of this Section are introductions to the basic definitions of controller terms and types of control modes that are available.

Basic definitions and terms used in process control

Most basic process control systems consist of a control loop, having four main components:

1. A measurement of the state or condition of a process

2. A controller calculating an action based on this measured value against a pre set or desired value (setpoint)

3. An output signal resulting from the controller calculation, which is used to manipulate the process action through some form of actuator

4. The process itself reacting to this signal, and changing its state or condition.

Control input; Process, Output, Disturbance inputs; Measurement, Setpoint, Process variable; Controller, -, + Control action, Actuator

+++++ Block diagram showing the elements of a process control loop

As we will see in the next sections, two of the most important signals used in process control are called …

1. Process variable or PV

2. Manipulated variable or MV

. In industrial process control, the PV is measured by an instrument in the field, and acts as an input to an automatic controller which takes action based on the value of it.

Alternatively, the PV can be an input to a data display so that the operator can use the reading to adjust the process through manual control and supervision. The variable to be manipulated, in order to have control over the PV, is called the MV. For instance, if we control a particular flow, we manipulate a valve to control the flow.

Here, the valve position is called the MV and the measured flow becomes the PV. In the case of a simple automatic controller, the Controller Output Signal (OP) drives the MV. In more complex automatic control systems, a controller output signal may drive the target values or reference values for other controllers.

The ideal value of the PV is often called the target value, and in the case of an automatic control, the term setpoint (SP) value is preferred

Process modeling

To perform an effective job of controlling a process, we need to know how the control input we are proposing to use will affect the output of the process. If we change the input conditions we shall need to know the following:

• Will the output rise or fall?
• How much response will we get?
• How long will it take for the output to change?
• What will be the response curve or trajectory of the response?

The answers to these questions are best obtained by creating a mathematical model of the relationship between the chosen input and the output of the process in question.

Process control designers use a very useful technique of block diagram modeling to assist in the representation of the process and its control system. The principles that we should be able to apply to most practical control loop situations are given below. The process plant is represented by an input/output block as shown.

Control input, Process, Output, Disturbance inputs

Control inputs are also known as manipulated variables.' The output is the process variable to be controlled.

+++++2 Basic block diagram for the process being controlled

In +++++2 we see a controller signal that will operate on an input to the process, known as the MV. We try to drive the output of the process to a particular value or SP by changing the input. The output may also be affected by other conditions in the process or by external actions such as changes in supply pressures or in the quality of materials being used in the process. These are all regarded as disturbance inputs and our control action will need to overcome their influences as best as possible.

The challenge for the process control designer is to maintain the controlled process variable at the target value or change it to meet production needs, whilst compensating for the disturbances that may arise from other inputs. So, for example, if you want to keep the level of water in a tank at a constant height whilst others are drawing off from it, you will manipulate the input flow to keep the level steady.

The value of a process model is that it provides a means of showing the way the output will respond to the actions of the input. This is done by having a mathematical model based on the physical and chemical laws affecting the process. For example, an open tank with cross-sectional area A is supplied with an inflow of water Q1 that can be controlled or manipulated. The outflow from the tank passes through a valve with a resistance R to the output flow Q2. The level of water or pressure head in the tank is denoted as H. We know that Q2 will increase as H increases, and when Q2 equals Q1 the level will become steady. The block diagram version of this process is provided below.

Note that the diagram simply shows the flow of variables into function blocks and summing points, so that we can identify the input and output variables of each block. We want this model to tell us how H will change if we adjust the inflow Q1 whilst we keep the outflow valve at a constant setting. The model equations can be written as follows:

== The first equation says the rate of change of level is proportional to the difference between inflow and outflow divided by the cross-sectional area of the tank. The second equation says the outflow will increase in proportion to the pressure head divided by the flow resistance R. ===

Control input is the valve position: Controlled variable (output) is the level in the tank; Disturbance is the variation in draw-off rate according to user needs H Q1 Q2 Cross-section area= A

+++++3 Example of a water tank with controlled inflow Control input = Y (valve opening) Inlet valve Flow to tank =Q1 (manipulated variable) Tank Output=H Tank level Outlet valve Flow from tank =Q2, +, - Supply pressure

+++++Elementary block diagram of tank process

===

Cautionary note: For turbulent flow conditions in the exit pipe and the valve, the effective resistance to flow R will actually change in proportion to the square root of the pressure drop so we should also note that R = a constant x × H. This creates a non-linear element in the model which makes things more complicated. However, in control modeling it’s common practice to simplify the nonlinear elements when we are studying dynamic performance around a limited area of disturbance. So, for a narrow range of level we can treat R as a constant. It’s important that this approximation is kept in mind because in many applications it often leads to problems when loop tuning is being set up on the plant at conditions away from the original working point.

The process input/output relationship is therefore defined by substituting for Q2 in the linear differential equation ... which is rearranged to a standard form as ...

Using this equation we can show that if a step change in flow delta_Q1 is applied to the system, the level will rise by the amount delta_Q1R, by following an exponential rise vs time.

This is the characteristic of a first order dynamic process and is very commonly seen in many physical processes. These are sometimes called capacitive and resistive processes, and include examples such as charging a capacitor through a resistance circuit and heating of a well-mixed hot water supply tank.

+++++Resistance and capacitor circuit with first order response

Cool water, Steam, Tank, Steam coil, Hot water, Drain

+++++ Resistance and capacitance effects in a water heater

NEXT: Intro to Process Control--part 2

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