Instrument Classification

Instrument Classification

There are many ways by which the instruments can be classified. Broadly the instruments are classified as,

1. Active/passive instruments

2. Null/deflection type instruments

3. Monitoring/control instruments

4. Analog/digital instruments

5. Absolute/secondary instruments

 

1.Active/Passive Instruments

The instruments in which the output is produced entirely by the quantity being measured, are called passive instruments. The example of such an instrument is pressure gauge shown in the Fig. 1.2.1.

As the liquid pressure changes, the piston moves to which pointer is connected. Thus the liquid pressure, due to which piston and hence pointer moves, is solely responsible for the measurement. No other input energy source other than liquid pressure is used in this instrument.

The instruments in which the quantity to be measured just activates the magnitude of some external power input source which intum produces the measurement (instrument output) are called active instruments. Apart from quantity to be measured, another external energy input source is present in such instruments. The example of such an active instrument is the liquid level indicator as shown in the Fig. 1.2.2.

The potentiometer and the external power input is used to sense the position of float which is proportional to the level of the liquid in a tank. When the level changes, the float moves and hence slider of the potentiometer also moves. This derives the voltage, which is the part of the external power input, which produces the output. Thus the variable to be measured just modulates the magnitude of external power source.

In passive instrument, resolution is less and cannot be increased very easily. In active instrument, by adjusting the magnitude of the external energy input, control over resolution can be obtained. The passive instruments are simple to design and hence cheaper. The active instruments are complicated to design and hence costly. Depending on the required resolution, the passive or active instrument can be selected for the measurement purpose.

The differences between passive and active instruments can be summarized as,

Passive Instruments     

1. The output is produced entirely by the quantity being measured.

2. Additional energy input source not required.  

3. The resolution is less.         

4. The resolution can not be easily adjusted.       

5. Simple to design.

6. Cheaper hence economical.

7. Examples are pressure gauge, voltmeter, ammeter.  

Active Instruments

1. The quantity to be measured activates some external power input source, which intum produces the output.

2. Additional external energy input source is required.

3. The resolution is high.

4. The resolution can be adjusted by adjusting the magnitude of the external energy input.

5. Complicated to design.

6. Due to complex design and higher number of elements, it is costlier.

7. Examples are liquid level indicator, flow indicator.

 

2. Null/Deflection Type Instruments

The instruments in which a zero or null indication leads to the determination of the magnitude of the measured quantity are called null type instruments. The null condition depends on some other known conditions. In null type instrument, an attempt is made to maintain the deflection at zero by suitable application of an effect which is opposite to that produced by the quantity to be measured. A null type instrument requires,

1. The effect produced by measured quantity.

2. The opposite effect the value of which is known.

3. The null detector.

The example of such null type instrument is d.c. potentiometer as shown in Fig. 1.2.3.

In this instrument, the slide wire is calibrated interms of e.m.f. with the help of standard e.m.f. source. The null detector is generally a galvanometer. The deflection of galvanometer is proportional to the difference between the e.m.f.s Eab and unknown e.m.f. E_x. When these two e.m.f.s are equal the galvanometer shows null deflection. In such case E_x is equal to E_ab, where E_ab is known which is directly indicated on slide wire. Thus unknown e.m.f.s E_x can be determined.

The instruments in which the quantity to be measured produces some effect due to which pointer deflects, are called deflection type instruments. The pointer deflection is proportional to the quantity to be measured. The controlling torque which acts opposite to the pointer deflecting torque is provided in such instruments. When the opposing torque is equal to deflecting torque, the pointer is in balanced condition showing the reading on the calibrated scale which is the value of the quantity to be measured. The example of deflection type instrument is permanent magnet moving coil ammeter. This is shown in the Fig. 1.2.4.

The moving coil carries a current to be measured. It produces its own flux. It is placed under the magnetic field of permanent magnet. Due to interaction the two fluxes, coil moves. Hence the pointer deflects under the influence of the deflecting torque T_d. The controlling torque Tc is provided by spring. When T_d = T_C, the pointer attains the steady position. The deflection θ is proportional to the current through the coil which is to be measured.

The accuracy of the null type instruments is more than the deflection type. The deflection type instruments are more convenient from the usage point of view. The null type instruments can be highly sensitive as the null detector has to cover a small range around the null point. This null detector need not be calibrated. The deflection type of instruments are more preferred for the dynamic measurements as the pointer deflection can follow the variations of the quantity to be measured, more rapidly. So null type instruments are highly accurate and sensitive than the deflection type.

Null Type Instrument  

1. It uses null detector, the effect produced by measured quantity and opposite effect to obtain null condition.

2. The accuracy is high.

3. Highly sensitive as null detector has to cover a small range, around the nullpoint.

4. Not suitable for the dynamic and rapid measurements.     

5. The example is d.c. potentiometer.

Deflection Type Instrument

1. In this instrument the quantity to be measured produces some effect which deflects  the pointer against controlling  torque.

2. The accuracy is less.

3. Less sensitive.                                                                    

4. Preferred for the dynamic measurements.

5. The example is moving coil ammeter.

 

3. Monitoring/Control Instruments

The instruments which are used to monitor the process, indicating the value or condition of parameter under study are called the monitoring instruments. Such instruments give as audio or visual indication of the magnitude of the quantity to be measured. All the deflection type instruments like voltmeter, ammeter etc. are all the null type instruments, thermometers and passive transducers are the examples of the monitoring instruments.

The instruments which are used in automatic control systems are called control instruments. Generally such instruments have an electrical output. Such instruments are used in the feedback path to measure the output and feedback the information to the controller. Thus the output of such instrument must be in a suitable form for direct input to the controller.

The Fig. 1.2.5 shows the use of control instrument in a simple control system. The control instrument measures the output and sends it to the input side in the proper form. There it gets compared with the reference input to produce an error signal. The error signal is amplified and given to the controller. The controller decides the proper controlling action and controls the process. This produces the required output which is a controlled variable.

 

4. Analog/Digital Instruments

The instrument which gives the output which varies in continuous fashion as the quantity being measured changes, taking infinite number of values in any given range is called analog instrument. The representation of an analog signal is shown in the Fig. 1.2.6. The voltmeter, ammeter which are deflection type instruments are the good examples of the analog instruments. As the input value changes, the pointer immediately moves with a smooth continuous motion. Thus the pointer can be in an infinite number of positions within its range of movement.

The instrument which gives the output which varies in discrete steps and thus take only finite different values in the given range is called digital instrument. For example if the value shown by analog instrument having a range of 0−10 is 3.5 unit, then the digital instrument, with 10 equal parts show the same reading as 3. The analog reading of 0.5 means a reading zero of a digital instrument. The digital signal is shown in the Fig. 1.2.7.

By subdividing the various steps, the accuracy of digital instruments can be improved. In digital instruments, the magnitude is measured only at the instant the reading is taken. This reading persists till the another reading is taken.

In present days, the digital instruments are well suited for the digital computer control systems. As computer input and output both are in digital form, the use of digital instruments is very easy in computer control systems. To interface analog instrument to computer, it is necessary to use analog to digital converter to convert analog signal to digital, which is complicated. This reduces the speed of operation, affecting the accuracy. This is because finite time is required to convert analog signal to digital and this time may be very critical in the fast processes. 

 

Comparision of Analog and Digital Instrument

 

5. Absolute/Secondary Instruments

The instrument which gives the magnitude of the quantity to be measured interms of the physical constants of the instrument, is called an absolute instrument. The tangent galvanometer is the example of an absolute instrument.

The instrument in which the reading shown by the instrument gives directly the measurement of the quantity to be measured is called a secondary instrument. These are calibrated by the comparison with the absolute instruments. The ammeters, voltmeters, thermometers are the examples of the secondary instruments.

Obtaining reading by an absolute instrument is tedious and time consuming as the reading is required to be calculated. While the secondary instruments give direct readings without any calculations. Hence absolute instruments are rarely used while the secondary instruments are very commonly used.

Review Questions

1. Compare active and passive instruments.

2. Compare null and deflection type instruments.

3. How the instruments are classified ?

4. Compare analog and digital instruments.