Functional Elements of Instruments

 

Functional Elements of an Instrument

 

Any instrument or measuring system can be described in general with the help of a block diagram. While describing the general form of a measuring system, it is not necessary to go into the details of the physical aspects of a specific instrument. The block diagram indicates the necessary elements and their functions in a general measuring system. The entire operation of an instrument can be studied in terms of these functional elements.

Fig. Functional Block Diagram of Instrument

 

The various elements can be grouped as,

1.  Primary sensing element,

2.  Data conditioning elements,

3.  Data presentation element

Each element is made up of several distinct components which perform a

function in the measurement procedure. In the block diagram, the function of each element is more important than the construction of the element.

 

1.  Primary sensing element

An element of an instrument which makes first, contact with the quantity to be

measured is called the primary sensing element. This first detection of the measurement is done by the primary sensing element. In an ammeter, a coil carrying the current to be measured is a primary sensing element. In most cases, a transducer follows the primary sensing element which converts the measurement into a corresponding electrical signal.

A transducer converts a physical quantity from one form to another but in case of electrical and electronic measurements, it converts physical quantity to be measured into an analogous electrical signal.

2.  Data conditioning elements

Variable Conversion Element:

The output of the primary sensing element is in an electrical form such as voltage frequency or any other electrical parameter. Such an output may not be suitable for the actual measurement system. For example, if the measurement system is digital then the analogue signal obtained from the primary sensing element is not suitable for the digital system. Then analogue to a digital converter is required which is nothing but a variable conversion element.

The original information about the measurement must be retained as it is while doing such a conversion.

Variable Manipulation Element

The level of the output from the previous stage may not be enough to drive the next stage. Thus, the variable manipulation element manipulates the signal, preserving the original nature of the signal. The manipulation involves the change in the numerical value of the signal. For example, an amplifier just amplifies the magnitude of the input, at its output, retaining the original nature of the signal. It is not necessary that the variable manipulation element must be always after the variable conversion element. It may be before the variable conversion element. In some cases, the levels of the output of the previous stage are high and require to be lowered. In such cases, attenuators are used as variable manipulation elements.

Sometimes the output, of the transducer may get affected due to unwanted signals like noise. Thus, such signals are required to be processed with some processes like modulation, clipping, clamping etc., to obtain the signal in a pure and acceptable form from highly distorted form. Such a process is called signal conditioning. Thus in addition to variable conversion and variable manipulation, signal conditioning is also done in the second stage. Hence the second stage is called data conditioning or signal conditioning elements.

Data Transmission Element

When the elements of the system are physically separated, it is necessary to transmit the data from one stage to another. This is achieved by the data transmission element. The signal conditioning and data transmission together is called the intermediate stage of an instrument.

3.  Data presentation element

The transmitted data may be used by the system, finally for monitoring, controlling, or analyzing purposes, Thus the person handling the instrument must get the information in the proper form, according to the purpose for which it is intended. This function is done by the data presentation element. If the data is to be monitored, then Visual display devices are used as data presentation elements. If the signal is to be recorded for analyses purpose, then magnetic tapes, and recorders, high-speed cameras are used as data presentation elements. For control and analysis purposes microprocessors, computers and microcontrollers may be used as data presentation elements. The data presentation stage may be called the terminating stage of an instrument.