Elements a Generalized Measurement System

Elements a Generalized Measurement System

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Any instrument or a 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 detail of the physical aspects of a specific instrument. The block diagram indicates the necessary elements and their function in a general measuring system. The entire operation of an instrument can be studied interms of these functional elements. The Fig 1.3.1 shows the block diagram showing the functional elements of an instrument.  

Fig. 1.3.1 Functional elements of an 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 number of distinct components which perform a particular function in the measurement procedure. In the block diagram, the function of each element is important father than the construction of the element.

1.Primary Sensing Element

An element of an instrument which makes first, the contact with the quantity to be measured is called primary sensing element. Thus first detection of the measurand is done by the primary sensing element. In ammeter, coil carrying current to be measured is a primary sensing element. In most of the cases, a transducer follows primary sensing element which converts the measurand into a corresponding electrical signal.

Key Point In general, a transducer converts a physical quantity from one form to other but in case of electrical and electronic measurements, it converts physical quantity to be measured into an analogous electrical signal.

2.Variable Conversion Element

The output of the primary sensing element is in 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 analog signal obtained from the primary sensing element is not suitable for the digital system. Thus analog to digital converter is required which is nothing but variable conversion element.

Key Point Some instruments do not need variable conversion element while some need more than one.

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

3. Variable Manipulation Element

The level of the output from the previous stage may not be enough to drive the next stage. Thus variable manipulation element manipulates the signal, preserving the original nature of the signal.

Key Point The manipulation involves the change in numerical value of the signal.

For example an amplifier which just amplifies the magnitude of the input, at its output, retaining the original nature of the signal. It is not necessary that variable manipulation element must be always after variable conversion element. It may be before variable conversion element. In some cases, the levels of the output of the previous stage are high and required to be lowered. In such case attenuators are used as the 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 pure and acceptable form from highly distorted form. Such a process is called signal conditioning. Thus in addition to variable conversion and variable manipulation, the signal conditioning is also done in the second stage. Hence second stage is called data conditioning or signal conditioning elements.

4. Data Transmission Element

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

5. Data Presentation Element

The transmitted data may be used by the system, finally for monitoring, controlling or analysing 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 element. If the signal is to be recorded for analysis purpose then magnetic tapes, recorders, high speed cameras are used as data presentation elements. For control and analysis purpose, the microprocessors, computers and microcontrollers may be used as data presentation elements. The data presentation stage may be called terminating stage of an instrument.

For example consider a simple analog meter used to measure current or voltage as shown in the Fig. 1.3.2 The moving coil is primary sensing element. The magnets and coil together act as data conditioning stage to convert current in a coil to a force. This force is transmitted to the pointer through mechanical linkages which act as data transmission element. The pointer and scale act as data presentation element.