INSTRUMENTATION AND CONTROL SYSTEMS LAB

Transcription

1 INSTRUMENTATION AND CONTROL SYSTEMS LAB

2 INDEX S.No. Name of the Experiment Page No. 1 Linear Variable Differential Transformer (L.V.D.T) 2 Speed Measurement Module 3 Capacitive Pickup 4 Thermister Module 5 Strain Gauge 6 Resistance Temperature Detector (R.T.D) Module 7 Light Dependent Resistance

3 CALIBRATION OF LVDT FOR DISPLACEMENT MEASUREMENT

4 MEASUREMENT OF DISPLACEMENT USING L.V.D.T AIM: To measure the linear displacement using Linear Variable Differential Transformer (L.V.D.T.) APPARATUS: Digital instrumentation tutor, L.V.D.T. THEORY: LVDT converts linear motion into an electrical signal. It is used for measuring displacement. The LVDT consists of a primary winding and two secondary windings, which are wound on cylindrical former. The two secondary windings S 1 & S 2 have equal number of turns. The secondary windings are placed identically on either side of the primary winding. The primary winding is connected to an a.c. source. A movable core is placed inside the cylindrical former. PRINCIPLE: As the primary winding is connected to the a.c. source. It is excited and hence a magnetic field is produced. Due to this magnetic field, a voltage is induced in the secondary windings. The differential output is E 0 = E s1 -E s2 When the core is in the normal (null) position, the magnetic field linking with both the secondary windings S 1 & S 2 are equal. Hence the e.m.f. induced in them are also equal.

5 There fore null position E 0 = 0 (E s1 = E s2 ). When the core is moved to the right of the null position, more magnetic field links with the winding S 2 and less with winding S 1. Therefore E s2 > E s1. The output voltage E 0 = E s2 E s1 and is in the phase E s2. When the core is moved to the left of the null position more magnetic field links with the winding S 1 and less with the winding S 2. Therefore E s1 > E s2. The output voltage of the L.V.D.T. gives a measure of the physical position of the core and it s displacement. PROCEDURE: 1. Connect the terminals marked PRIMARY on the front panel of the instrument to the terminals marked PRIMARY on the transducer itself, with the help of flexible wires provided along with. Observe the colour code for the wires provided and the colour of the binding posts. 2. Similarly connect the terminals marked secondary. 3. Keep pot marked MAX in most anticlock- wise position. 4. The magnetic core may be displaced and the pointer may be brought to zero position. If the DPM is not indicating zero, use potentiometer marked MIN to get a zero on DPM at zero mechanical position. If the core is displaced in both directions, the meter must show indications with appropriate polarity. Now displace the core to 19 mm positions in one of the directions. Adjust the MAX pot to get an indication of on the DPM under this condition. Now the set up is ready for experimentation. You may again check for zero position also. 5. Now the core can be moved by a known amount in the range ± 19 mm and the DPM readings are taken in the table given below. It may noted that by inter changing the secondary terminals ro the primary, the polarity of the meter indication can be reversed for a given deirection of input displacement. 6. Plot the graph of input displacement and the output indication on the x and y axis respectively.

6 OBSERVATIONS: Sl.No. Input Displacement (mm) Output Indication (mm) Difference (mm) Percentage Error PRECAUTIONS: 1. Move the core with a gentle fashion. 2. While connecting lead wire from panel to transducer make proper connectionsfollowing colour code. Avoid 3. Avoid starting of the excitations source terminals. 4. Donot try to effect the core movement beyond 20 mm as per the given range. OBSERVATIONS : Study the linearity of input and output displacements. Note the effects of interchanging the secondary connections on the meter output polarity Note that when the core is mechanically at zero position, a small electrical output is obtained due to imperfections of the transducer.

7 GRAPH: Input Displacement Vs DPM Reading RESULT:

8 STUDY AND CALIBRATION OF PHOTO AND MAGNETIC PICKUP FOR SPEED MEASUREMENT

9 MEASUREMENT OF SPEED AIM: To measure the speed of a motor by Photo Pickup and Magnetic Pickup methods. APPARATUS:. Tachometer, DPM THEORY: There are two methods for the measurement of speed of a motor shaft with the help of non-contact pickups. i. Magnetic pickup: In this type of device, a small toothed wheel is to be attached to the shaft whose speed is to be determined. A parmanent magnet with a coil wound around around it is placed near the rotating toothed wheel. As the wheel rotates, the magnetic flux linking the magnet and coil changes. As a result voltage is induced in the coil. The frequency of the pulses depends upon the number of teeth on the wheel. Speed of the shaft = Pulses per Second Number of teeth on the wheel ii. Photo electric pickup: This is another method of speed measurement based on photo electric effect. An opaque disc with evenly spaced holes on its periphery is to be attatched to the shaft whose speed to be

10 measured. A light source is placed on one side and a light sensitive transducer on the other side of the disc. Both are in alignment with the holes in disc. As the disc rotates the intermittent light falling on the photocell produces voltage pulses whose frequency is a measure of the speed of the shaft. Remaining is same as inductive pickup. PROCEDURE: 1. Connect the calibration source to the input socket by the cable provided and connect the O/P terminals to the DPM observing polarity. 2. Adjust the pot marked max to get 1500 as the meter (DPM). Now the set up is calibrated from 1500rpm, Neglect decimal point. 3. Connect the photo electric and magnetic pickup in proper fashion. 4. Connect the motor terminal cable to the O/P terminals of D.C. supply ensure that dimmer knob is in zero position. Switch on the power supply and control slowly so on changing the speed of motor. 5. Motor speed will be indicated by the DPM for each pickup 6. Compare the DPM reading with digital tachometer reading.

11 OBSERVATIONS: S.No Tachomet er (RPM) Photo pickup (RPM) Percentage error Tachometer (RPM) Magnetic pickup (RPM) Percentage error PRECAUTIONS: 1. Always start the motor with zero speed and operate the knob slowly. 2. As soon as the experment is over disconnect the photo-electric and and magnetic pick-ups. RESULT:

12 ANGULAR DISPLACEMENT MEASUREMENT CAPACITIVE PICK UP

13 ANGULAR DISPLACEMENT MEASUREMENT CAPACITIVE PICK UP AIM: Study the measurement of angular displacement using capacitive pick up APPARATUS: Capacitive transducer, D.P.M THEORY: Capacitance is well known to be function of effective area of the conductors, separation between them, and the dielectric strength of the material in separation. Any one of these three can be the varying factor causing a change in capacitance. The capacitive transducer along with this equipment is based on the principle of variation of effective area of the conductors, other parameters i.e. separation, distance and dielectric strength being kept constant. A two ganged condenser normally used in transistor radio receivers, is used to demonstrate the measurement of angular displacement measurement.

14 OPERATION 1. Connect the capacitive pick up cable to the input socket on the main unit. 2. Keep the input angular displacement to zero position. 3. Check for zero indication on the DPM. Otherwise by operating potentiometer marked MIN (P2).obtain zero indication for zero angular position of the shaft. 4. Now turn the shaft of the capacitive pick up to fully clock wise position in a gentle manner corresponding to 180 degrees by operating knob marked MAX (P1). If required again check for zero indication. 5. Note down the reading corresponding to input angular displacement and indicated angular displacement on the DPM.

15 OBSERVATIONS S.No Input Angular Displacement DPM Indication Percentage error OBSEVATIONS : There is lot of non linearity of the indicated versus true reading. The reason is the non-linearity of the input angular displacement and the capacitance of the gang condenser. This method of angular displacement is free from mechanical friction (neglecting the friction in the ball bearings of the shaft).this a distinct advantage over potentiometric method. PRECAUTIONS : 1. Operate the sensor knob carefully 2. Do not stretch the wire coming from the capacitive sensor. RESULT:

16 TEMPERATURE MEASUREMENT USING THERMISTER

17 MEASUREMENT OF TEMPERATURE USING THERMISTOR AIM: To measure temperature using thermistor. APPARATUS: Thermistor, heater & container, DPM. THEORY: Thermistors are made of semi conductors of ceramic materials that are exceptionally sensitive to temperature. The main parts of the thermistor are: i. A metal tube which houses a thermistor sensing element. ii. An insulation sperates the thermistor sensing element from metal tube. Lead wire are drawn out from the thermistor sensing element. iii. The metal tube, sensing element and leads together became a thermistor used to measure temperature. iv. The material is made by sintering oxides of metals such as manganese, nickel, cobalt, copper, iron etc. Physical forms may be beads, discs, washer, rod type. The resistance of the thermistor decreases with increase in temperature. Hence they are called Negative temperature coefficient (N.T.C). They are highly sensitive to small temperature changes. v. A typical thermistor will exhibit a decrease in resistivity by a factor of 50 : 1 over temperature range of 0 to 100 O C.The highest temperature up to which thermistors can be used is limited to about 200 degree centigrade.

18 PROCEDURE: 1) Connect a shorting link across the input terminals and use pot P 1 on the front panel to get Zero indication on the DPM. The switch SW 2 may be kept in lower position corresponding to 2 kilo ohm range. Connect the thermistor across the input terminals remember the polarity is not important. 2) Connect the terminals marked O/P to DPM input terminal. 3) Now the thermistor may be immersed in boiling water and the resistance of the transducer should be noted down.

19 4) Switch off the heater supply and note the value of resistance for temperature reading from mercury in glass thermometer and note down the relationship between thermistor resistance and temperature up to ambient temperature. Enter the results in the following table. PRECAUTIONS: 1. Handle the eqipment carefully. 2. The thermistor wire should not touch the heater. OBSERVATIONS: S.No Thermometer temp reading ( o c) Resistance of thermistor in Ω Percentage error 1. The input/out put relationship (Temperature /Resistance) is highly non linear for this transducer. 2. In comparison with RTD, change in resistance for given change in temperature is very large I,e, it has higher sensitivity. Hence small i.e. differential change in temperature can be accurately monitored with the help of thermistors. GRAPH: Resistance versus temperature relationship can be plotted on a graph. RESULT:

20 STRAIN MEASUREMENT USING STRAIN GUAGE

21 STRAIN MEASUREMENT USING STRAIN GUAGE AIM: Measurement of strain using strain gauge. APPARATUS: Digital panel meter, cantilever beam, weights. THEORY: Strain gauges are devices used to measure the dimensional change of components under test. These are used in many applications like force measuring devices, measurement of vibration, measurement of pressure etc., In this experiment bonded strain gauges are used. These gauges are directly bonded (that is pasted) on the surface of the structure under study. In this fine wire strain gauges are used. A fine resistance wire of diameter of 0.025mm, which is bent again and again as shown in figure. This is due to increase the length of the wire so that it permits a uniform distribution of stress. This resistance wire is placed between the two carrier bases (paper, bakelite or Teflon), which are cemented to each other. The carrier base protects the gauge from damages. Loads are provided for electrically connecting the strain gauge to a measuring instrument (wheat stone bridge).

22 PROCEDURE: 1. Ensure that the instrument is switched off. 2. Connect the flexible wires provided with the strain gauge cantilever beam between the terminals 1-1,2-2,3-3 & Keep switch S1 to right position marked. 4. Turn ON the main supply by gently moving the balance. Put p1 and p2 obtain initial balance on the meter and wait for 5 minutes to allow the strain gauge temperature to stabilize. 5. Now apply a gentle pressure with hand on the end of the cantilever beam, the Digital Panel Meter (DPM) should indicate some change in reading. This indicates the strain gauge setup is ready for experiment dial. 6. Now keep p3 pot in minimum clock-wise position corresponding to position of gain =100. Check for null balance again. 7. Now apply weight of 1kg, 2kg etc., and note down the DPM reading, neglecting the decimal point.

23 CALCULATIONS: E 0 = E i *ΔR/R (E i =5V) Guage Factor = (ΔR/R)/( Δl/l) (G=2) E=Stress/Strain= 6Wl/bt 2 (E= 200Gpa) W= Applied Load, l= Length of Cantilever Beam, b= Breadth of Cantilever Beam t= Thickness of Cantilever Beam. OBSERVATION TABLE: Stress = WL/ ((1/6) BT²) W-Applied load L-length of cantilever beam B-breadth of cantilever beam T-thickness of cantilever beam Theoretical value of strain = stress\ E (E=2x 10) Strain = change in length / original length GAUGE FACTOR (Strain sensitivity factor) The fractional change in resistance due to a unit change in length (unit strain) is called as gauge factor. Gauge factor = (ΔR/R)/( ΔL/L) Where Robotics= Resistance L=Length SL.NO. LOAD DPM INDICATION STRAIN(N/mm 2 Percentage Strain error Theoretical Practical Theoretical Practical Percentage error Practical strain = (ΔL/L) = (Δ R/R)/(G) [G=2] ΔR/R= E out / E excitation [E excitation = 5V]

24 PRECAUTIONS: 1. Make the connection to the binding posts and terminals very care fully. 2. Provide a warm up time of about 10 to 15 minutes before taking readings. 3. Ensure that the cantilever beam arrangement is fixed to the table. GRAPHS: Load vs DPM reading Load vs practical strain Theritical strain vs practical strain RESULT:

25 TEMPERATURE MEASUREMENT USING RTD

26 MEASURING OF TEMPERATURE USING R.T.D AIM: To measure the temperature by using resistance temperature detector. APPARATUS: Resistance temperature detector (R.T.D), Digital panel meter, Heater and container, Thermometer THEORY: The principle of operation of R.T.D. is based as fact that the electrical resistance of many metals increase almost directly with temperature and is reproducable to a high degree of accuracy. The resistance of the R.T.D. increase as temperature increase. The resistance and temperature linearly relateed over a wide temperature range. The main parts of R.T.D. is i. A glass or metal tube which houses a ceramic mandrel on which a platinum resistance wire is wound. The lead wires of the sensing element projected out of the ceramic mandrel. This arrangement becomes the resistance thermometer. ii. The glass or metal bulb is evacuated or filled with inert gas to protect the resistance wire sensing element. BASIC PRINCIPLE: When an electric conductor is subjected to temperature change, the resistance of the conductor changes.thus, the change in resistance of the electric conductor becomes a measure of the change in temperature when calibrated.

27 PROCEDURE: 1. Keep switch SW 2 in position marked TEMP 2. Connect a precision resistance of 100 ohms across input terminals 3. Adjust pot marked MIN (P 2 ) to read 0.0 on DPM. This action simulates ice-bath temperature. Since at zero degree centigrade PT 100 exhibits 100 ohms. 4. Now connect a precision resistance of 139 ohms across the input terminals. Adjust MAX pot (P 1 ) to read on DPM. This action simulates boiling point temperature of water i.e.100 degree centigrade.

28 5. Connect the R.T.D. across the terminals marked input. Note that polarity is immaterial. 6. Connect terminals marked output to DPM input terminals. 7. Note that RTD may be immersed in the boiling water and the resistance of the transducer should be noted down. 8. Note down the readings of the thermometer and RTD. OBSERVATIONS: S.No Thermometer Temperature on Resistance on Percentage reading ( o c) DPM ( o c) DPM (ohms) Error Please note that RTD exhibits good linearity and good accuracy in comparison with thermocouple The time constant of the sensor is large Polarity of connections is immaterial as RTD is only a resistance sensor. PRECAUTIONS: Make the connections properly. Take the readings when they are steady. RTD should not touch the bottom of the vessal. Please handle RTD very carefully as it is very costly. Please ensure that is not dropped on the floor. GRAPH: Thermometer temperature vs RTD temperature Thermometer temperature vs RTD Resistance

29 RESULT:

30 DISPLACEMENT MEASUREMENT - LDR

31 DISPLACEMENT MEASUREMENT LIGHT DEPENDENT RESISTOR AIM: Measurement of displacement using light dependent resistor. APPARATUS: Light Dependent Resistor (LDR), DPM THEORY: If radiation falls on a semiconductor, its conductivity increases. This is called as photo conductive effect. This effect is the basis of operation of L.D.R. The conductivity of a material is proportional to the concentration of charge carriers present. Radiant energy supplied to the semi conductor causes covalent bonds to be broken & hole and electron pairs in excess of those generated thermally, are created.these increased current carriers decrease the resistance of the material and hence, such a device is called as photo resistor, photoconductor or L.D.R. The L.D.R, with widest application, is the cadmium sulphide cell. Cd S photoconductors have high dissipation capability, excellent sensitivity in the visible spectrum and low resistance when stimulated by light. In this set up the L.D.R is used as a displacement transducer.

32 OPERATION: 1. Establish a connection between the lamp and front panel socket for lamp supply 2. Connect the L.D.R to the input terminals on the front panel. Polarity is not important. 3. Connect the output terminals of transducer circuit to D.P.M observing polarity. 4. Adjust the channel on which L.D.R is mounted, so that full scale deflection is obtained on the meter. If required, use potentiometer marked MAX. on the panel. 5. Using the scale mounted on the bottom of C channel, measure the input displacement & the resultant DPM readings.

33 OBSERVATIONS Sr.No Scale reading on the set up Effective displacement D.P.M indication Percentage Error THE STUDENT MAY NOTE THAT The set up can be used to measure displacement over a wide range. How ever over a limited range the response is seen to be linear. The L.D.R is sensitive to temperature variation also. Hence at different temperatures, the out put-input responses are seen to be slightly changing. PRECAUTIONS: Do not expose the L.D.R to intense light. Do not connect the DPM before connecting lamp and L.D.R to the front panel.

34 GRAPH: Plot the graph between input Displacement vs DPM Reading RESULT: