1.1       Introduction to measurement and instrumentation

1.1.1        Measurement

Measurement refers to the process of comparing unknown quantity with the standard one. It provides us with the means for describing chemical and physical parameters of materials quantitatively. Therefore, when one mentions the word measurement, what comes into the mind is the weight, length, distance, time, height, temperature, color of a material. The standard used for measurement should be accurate and internationally accepted. It also important that the experimental procedure adopted while performing the measurement should be scientifically provable.

The International System of Units (SI) defined seven base units of measurement known as the SI base units. The SI base units are;

Table 1. 1 The seven Base Units

The base units are the building blocks of the system. All the other units are derived from the base units and are known as SI derived units and the quantity as the SI derived quantity. There are unlimited number of SI derived quantities and units.  An example of the SI derived quantity is the velocity (with the derived unit as m/s) which is derived from dividing length by time.

1.1.2        Instrumentation

Instrumentation refers to the technology of using instruments to measure and control physical and chemical properties of a system. It is important for monitoring and maintaining the operation of a system within the design expectations so as to achieve good quality, safety and efficiency of the process. It helps achieve automatic control of process there by reducing the dependency on human labor.

Measurement and instrumentation are very essential to system control. Be it a spaceship, air conditioner, aircraft, etc. they all rely on measurement and instrumentation in order to make a decision. We are going to construct a number of instruments for measurement and use them to achieve automatic control of systems.

1.2       Functional Elements of a Measurement System (Instrument)

The functional elements of a measurement system are;

1.2.1        Sensing element

This is the primary sensing object on the instrument. An element that is sensitive to the variable being measured. It is the first part of the instrument to sense or detect the measurement and then produces an output that is proportional to the measurement.

1.2.2        Transducer (Variable Conversion) element

The output signal of the sensing element can be of any form. Sometimes, this output is not suited to the measurement system. The transducer element converts the signal from one physical form to another while maintaining the information content of the original signal.

1.2.3        Variable manipulation element

Variable manipulation element modifies the signal presented to it while preserving the original nature of the signal. It does this by amplification, attenuation and filtration so that the desired output is produced.

1.2.4        Signal conditioning element

Signal conditioning involves the removal of the unwanted noise and distortion from the signal through filtration.

1.2.5        Data transmission element

In a situation whereby the elements of an instrument are physically separated, it becomes necessary to transmit data from an element to another. The data transmission element transmits data from one location to another while preserving the information content of the data.

1.2.6        Data presentation element

This is the element that provides record or indication of the output on a measurement instrument. If the data is to be monitored, visual display devices are necessary. In case the data is to be recorded, recorders like SD cards, magnetic tapes, and high speed camera are necessary.

1.3       Input-Output configuration of a measurement system

A measurement instrument performs an operation on measured input (i) to provide an output (o) called measurement. The performance of the instrument can be expressed in terms of the operational transfer function (G). The relationship between the input and output is characterized by the transfer function such that;

There are three categories of inputs in a measurement system;

1.3.1        Desired input

This is the quantity that the instrument is intended to measure during the measurement.

1.3.2        Interfering input

This is the quantity that the instrument is not intended to measure but the instrument is unintentionally sensitive to the quantity.

1.3.3        Modifying input

This quantity takes into account the desired and interfering inputs to modify the input-output relation.

Generalization of the Measurement System

The measurement system can be generalized into input stage, intermediate stage and output stage according to the figure below;

Fig. 1. 1 Generalization of the measurement system

1.4       Performance characteristics of a measurement system

The following terms are used to describe the performance characteristics of an instrument;

1.4.1        Accuracy

Taking into account that no instrument gives the exact value of what is being measured, there is always uncertainties in the measured values. Accuracy refers to how close the measured value is to the true value.

1.4.2        Calibration

These are the procedures undertaken to for checking and adjusting instrument’s scale so that the readings conform to an accepted standard.

1.4.3        Repeatability

This describes how close the outputs are when the same input under the same conditions are applied repeatedly.

1.4.4        Range and Span

Range is the region between the limits within which a measuring instrument is designed to operate, while the span represents the algebraic difference between those upper and lower limits.

1.4.5        Precision

This is the ability of the measuring instrument to give a certain group of readings with a certain accuracy.

1.4.6        Sensitivity

This is expressed as a ratio of the output signal to the input signal.

1.4.7        Speed of response

This is the rapidity within which an instrument responds to the quantity being measured.

1.5       Introduction to Control Systems

Before we consider a control system lets first consider the following two terms;

1.5.1        System

A system is an assemblage of devices connected to form an organized structure in order to perform a specific task.

1.5.2        Control

The ability to influence, direct, command or regulate the behavior of a system or course of events. It’s a specific action to influence the behavior of a system in order to reach the desirables.

Therefore, a Control System may be defined as the;

assemblage of components that manages, influences, directs, commands or regulates the activities of a system to achieve the desired results. Measurement instruments are needed to obtain the different physical quantities involved in the control system.

There are some basic terms used along with the control systems

1.5.3        Process/Plant

A body or a system of which a particular quantity or condition is to be controlled. This is the control system.

1.5.4        Set point

Desired value which acts as a standard reference for the feedback control system. It the set point value that an automatic control system aims to reach.

1.5.5        Regulator

This is a device which performs the function of maintaining a given quantity at the desired level.

1.5.6        Controlled Variable

This is the quantity or variable kept constant in the control system.

1.5.7        Disturbance

The unwanted signal which tends to adversely affect the value of the variable being controlled.

1.5.8        Manipulated Variable

This the independent variable that one controls. It is varied as a function of the actuating signal so as to change the value of the actuating signal.

1.5.9        Actuator

This is a mechanical or electro-mechanical device that converts the control signal into mechanical motion. It allows for controlled movement or positioning.

1.5.10    Actuating Signal (Error signal)

This is the algebraic sum of the reference input signal and feedback signal. It represents the control action of the control loop.

1.5.11    Control Action

The manner in which the automatic controller produces the control signal.

1.6       Manual Vs Automatic Control

Control action can either be achieved manually or automatically. In manual control, it’s the human operator who observes the output quantity then makes the decision and modifies the input variables. This means that human must review and approve a given action on such a system.

In automatic control, it’s the automatic controller that executes the decision and modifies the input variables based on the value of the measured output quantity. This means that the system relies on some computerized actions. Thus based on the data obtained by the measurement system, an automatic control system directly and independently modifies the input variable which in turn would indirectly influence the output variable and as a result achieve the control action.