Fluid Level Dynamics and Control - Single Tank Control Virtual Laboratory

Summary

Storage Tanks of various types are frequently used in industry to store fluids. Also, tanks can be used as a buffer between different unit operations so that a variable “incoming flow” can be averaged out so as to present a more uniform “outgoing flow” to the next unit operation.

This laboratory concentrates on the control of the level of fluid in a single tank.  The apparatus that the laboratory is based on contains two tanks but this laboratory will only be concerned with the level in the first tank.

This virtual laboratory has been designed to closely emulate a physical laboratory.

Figure 3.1: Screenshot of Program
Figure 3.1: Screenshot of Program

The Physical Apparatus

Storage Tanks of various types are frequently used in industry to store fluids. Also, tanks can be used as a buffer between different unit operations so that a variable “incoming flow” can be averaged so as to present a more uniform “outgoing flow” to the next unit operation. Thus the control of tanks and pumps is a very common problem met in industry.

In this virtual laboratory we will look at the single tank case as an introduction to the control process. Many University Laboratories use a similar system of tanks to illustrate basic control ideas.

This virtual laboratory has been designed to closely emulate a physical laboratory located at the University of Newcastle, Australia. Photos of the physical system are shown in Figures 3.2 and 3.3. The basic system comprises two identical tanks. Each tank can be filled and drained independent of the other.

The tanks can also be coupled together by the opening of a valve but we will leave this valve closed during this laboratory. Flow into the tanks is controlled by regulating the voltage applied to the armature of a DC motor (which in turn drives a pump). The level of the liquid in the tank is measured by a piezoelectric device.

Figure 3.2: The Coupled Tanks Apparatus
Figure 3.2: The Coupled Tanks Apparatus

Figure 3.3: Rear View Apparatus
Figure 3.3: Rear View Apparatus

Actually the physical coupled tanks apparatus is tedious to use in practice because of the long time constraints. Thus, in the virtual laboratory, we have elected to speed up the response by approximately a factor of 5:1.

A schematic diagram of the physical system is shown in Figure 3.4.

Figure 3.4: Coupled Tanks Schematic
Figure 3.4: Coupled Tanks Schematic

Pump P1 can be controlled to regulate the level of liquid in either tank T1 or tank T2 depending on the position of valve V3. When valve V3 is closed the system consists of two single tanks. Pump P2 can be used to provide an uncontrolled (or disturbance) flow into either T1 or T2.

The block diagram, shown in Figure 3.5, describes the system in terms of its electrical input and output.

Figure 3.5: Block Diagram of Coupled Tanks System
Figure 3.5: Block Diagram of Coupled Tanks System

For the purpose of the laboratory we will consider the gain associated with the pump (which includes the DC motor, pump and associated drive circuit) an integral component of the coupled tank apparatus.

In this laboratory you will design closed loop control systems of the form shown in Figure 3.6.

Figure 3.6: Control System Block Diagram
Figure 3.6: Control System Block Diagram

Prerequisites

This laboratory is typical of the type used in a first course in control. The assumed knowledge includes:

  • linearisation of a nonlinear system at an operating point
  • step responses
  • transfer functions
  • PID control

Learning Objectives

The objectives of this laboratory are to familiarize you with:

  • The experimental determination of models for dynamic systems.
  • The design of simple feedback control systems.