Analog & digital measuring instruments
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The document discusses the characteristics, construction, and principles of operation of Permanent Magnet Moving Coil (PMMC) instruments, used for measuring direct current. It highlights their advantages such as high accuracy and low power consumption, alongside disadvantages including high cost and age-related errors. Additionally, the document contrasts analog and digital measuring instruments, detailing their differences in accuracy, power consumption, sensitivity, and portability.
Analog & digital measuring instruments
- 1. Analog & Digital Measuring Instruments
- 2. 6/14/2020 RAJA SURESH CHARACTERISTICS OF IDEAL VOLTMETER AND IDEALAMMETER 1. Accuracy: The degree of exactness (closeness) of a measurement compared to the expected (desired) value. 2. Resolution: The smallest change in a measured variable to which an instrument will respond 3. Precision: A measure of the consistency or repeatability of measurements i.e. successive reading do not differ. 4. Error: The deviation of the true value from the desired value. 5. Sensitivity: The ratio of the change in output (response) of the instrument to a change of input or measured variable.
- 3. 6/14/2020 RAJA SURESH PMMC ( PERMANENT MAGNET MOVING COIL ) INSTRUMENT • The permanent magnet moving coil instruments are most accurate type for direct current measurements. • The action of these instruments is based on the motoring principle. • When a current carrying coil is placed in the magnetic field produced by permanent magnet, the coil experiences a force and moves. • As the coil is moving and the magnet is permanent, the instrument is called permanent magnet moving coil instrument. • This basic principle is called D’Arsonval principle. • The amount of force experienced by the coil is proportional to the current passing through the coil. WHAT IS APMMC?
- 4. 6/14/2020 RAJA SURESH WORKING PRINCIPLE OF PMMC INSTRUMENT • D’Arsonval Movement Principle : • An action caused by electromagnetic deflection, using a coil of wire and a magnetized field. When current passes through the coil, a needle is deflected. • Whenever electrons flow through a conductor, a magnetic field proportional to the current is created. This effect is useful for measuring current and is employed in many practicalmeters. • This type of meter movement is a current measuring device which is used in the ammeter, voltmeter, and ohmmeter. Basically, both the ammeter and the voltmeter are current measuring instruments, the principal difference being the method in which they are connected in a circuit..
- 5. 6/14/2020 RAJA SURESH CONSTRUCTION OF PMMC INSTRUMENT The PMMC instrument consists of a permanent powerful magnet of a horse-shoe form with soft-iron pole pieces attached to it. Between the magnetic pole pieces a soft iron cylinder is fixed, which serves to provide a uniform magnetic field in the air-gap between the pole pieces and the cylinder. The coil is wound on a light metal frame and is mounted so that it can rotate freely in the air-gap. The pointer attached to the coil, moves over a graduated scale and indicates the angular deflection of the coil and therefore the current through the coil.
- 6. 6/14/2020 RAJA SURESH Two Phosphor-Bronze conductive springs normally equal in strength, provide the calibrated force opposing the moving coil torque. The entire moving system is statistically balanced all deflection positions by three balance weights. The pointer, springs and pivots are assembled to the coil structure by means of pivot base, and the entire movable coil element is supported by jewel bearings. Parts: Permanent Magnet, Controlled Spring, Former, Coil, Pivot, Pointer,Scale,
- 7. 6/14/2020 RAJA SURESH WORKING When current is passed through the coil, force is setup on its both sides which produces a deflection torque and the direction can be determined by applying Fleming’s Left Hand Rule. Torque Equation The deflection torque Td is given by, Td = Force * Perpendicular distance Td = BNAI N - mt
- 8. 6/14/2020 RAJA SURESH Td = BNAI N - mt Where, Td = deflecting torque in N-m B = flux density in air gap, Wb/m2 N = Number of turns of the coils A = effective area of coil m2 I = current in the moving coil, amperes Therefore, Td = GI Where, G = NBA = constant The controlling torque is provided by the springs and is proportional to the angular deflection of the pointer.
- 9. 6/14/2020 RAJA SURESH Tc = KØ Where, Tc = Controlling Torque K = Spring Constant Nm/rad or Nm/deg Ø = angular deflection For the final steady state position, Td = Tc Therefore GI = KØ So, Ø = (G/K)I or I = (K/G) Ø Thus the deflection is directly proportional to the current passing through the coil. The pointer deflection can therefore be used to measure current.
- 10. 6/14/2020 RAJA SURESH ADVANTAGES It has a very high torque to weight ratio. The PMMC has consumes low power. The PMMC has a very high accuracy. The PMMC is free from hysteresis error. Extension of instrument range is possible with the help of shunt and series resistances. The PMMC has efficient damping characteristics and is not affected by stray magnetic field.
- 11. 6/14/2020 RAJA SURESH DISADVANTAGES It has comparatively high cost. The PMMC has only suitable for d.c. measurements. Aging of permanent magnets and control springs introduces errors.
- 12. 6/14/2020 RAJA SURESH DC AMMETER & VOLTMETER
- 23. 6/14/2020 RAJA SURESH FET INPUT VOLTMETER
- 24. 6/14/2020 RAJA SURESH CIRCUIT DIAGRAM OF FET INPUT VOLTMETER
- 25. 6/14/2020 RAJA SURESH RAMP TYPE DVM
- 27. 6/14/2020 RAJA SURESH PRINCIPLE Input voltage is converted into digital equivalent by counting the time taken for the ramp wave to decrease from the magnitude of input voltage to 0V. CONSTRUCTION The block diagram of the Ramp-type ADC can be divided into two sections as follows: 1. Voltage to time conversion section 2. Time measurement section
- 28. 6/14/2020 RAJA SURESH VOLTAGE TO TIME CONVERSION SECTION In the voltage to time conversion section, the analog input voltage is fed to the attenuation circuit. The attenuated signal is compared with the ramp signal generated by the ramp generator given in the block diagram by the input comparator 'C1'. Similarly, The ramp signal generated is compared with 0V via a zero-crossing detector 'C2'. A sample rate multivibrator is connected to the ramp generator whose purpose is to provide an initiating pulse for the ramp generator to start the next ramp voltage for the next measurement. It is also used to reset the counter before generating the next ramp voltage.
- 29. 6/14/2020 RAJA SURESH The counter is reset after each successful completion of time measurement by a control signal from the sample rate multivibrator. The count produced is displayed by connecting suitable display device. TIME MEASUREMENT SECTION In the time measurement section, there is counter which is triggered by a gating pulse. The inputs of the gating pulse are (i) Output of 'C1' (ii) Output of 'C2' (iii) Clock pulse from the oscillator.
- 30. 6/14/2020 RAJA SURESH OPERATION Initially, the attenuated signal is compared with a negative going ramp signal generated by the ramp generator. When the ramp voltage coincides with the input signal, the output of 'C1' becomes low. This point is called coincidence point. This initiates the counting process ( start of count ). The counter continues to count until the ramp voltage reduces and crosses zero (0V). This is detected by zero crossing detector 'C2'. The output of 'C2' becomes high which ends the counting process (end of count).
- 31. 6/14/2020 RAJA SURESH The count displayed is the count of number of clock pulses produced by the oscillator during the time in which the ramp signal is less than the input signal and greater than 0V (ie) |input signal| > ramp > 0V. This count gives the digital equivalent of input analog voltage.
- 32. 6/14/2020 RAJA SURESH RAMP TYPE DVM
- 34. 6/14/2020 RAJA SURESH The basic digital frequency meter needs the following components:- 1. Amplifier 2. Schmitt trigger 3. AND gate 4. A counter 5. Crystal oscillator 6. Time base selector 7. A flip-flop
- 35. 6/14/2020 RAJA SURESH OPERATION When we apply an input signal to the meter, the amplifier present in it starts amplifying the signal. The amplification is done in order to strengthen the weak magnitude of input signal. The amplified input is now given to Schmitt trigger which then converts the input signal into square wave. The square waves are then separated and clipped to obtain a train of pulses. Similarly, the oscillator produces a sine wave which is then converted into square wave which produces a square wave and is applied to the flip-flop. Now, the first pulse activates the gate control FF connected after the Schmitt trigger which enables the AND gate.
- 36. 6/14/2020 RAJA SURESH Now, the train of pulses is then applied to AND gate due to which AND gate produces an Output. This Output is then given to the decimal counter which counts the first decimal number. Now when the second pulse is given to the Gate Flip-flop which removes the AND gate to “turn ON”. Thus, when the third pulse arrives at the flip-flop the AND gate is then enabled until then the counter shows the previous counted decimal. The decimal counter and display unit shows the output corresponding to the train of pulses received for a precise time interval. Thus, the counter output shows only the frequency of signal.
- 37. 6/14/2020 RAJA SURESH APPLICATIONS OF DIGITAL FREQUENCY METER It is used for testing radio equipment. It is used for measuring the temperature, pressure, and other physical values. It is used for measuring vibration, strain
- 38. 6/14/2020 RAJA SURESH DIFFERENCE BETWEEN ANALOG INSTRUMENTS AND DIGITAL INSTRUMENTS ANALOG INSTRUMENT DIGITAL INSTRUMENT The instrument which gives output that varies continuously as quantity to be measured is known as analog instrument. The instrument which gives output that varies in discrete steps and only has finite number of values is known as digital instrument. The accuracy of analog instrument is less. The accuracy of digital instrument is more. The analog instruments required more power. The digital instruments required less power.
- 39. 6/14/2020 RAJA SURESH Sensitivity of analog instrument is more. Sensitivity of digital instrument is less. The analog instruments are cheap. The digital instruments are expensive. The analog instruments are extremely portable. The digital instruments are not easily portable. The resolution of analog instruments is less. The resolution of digital instruments is more.
- 40. 6/14/2020 RAJA SURESH THANK YOU