Comparators

Mechanical Measurements and Metrology

1. Department of Mechanical Engineering JSS Academy of Technical Education, Bangalore-560060 MECHANICAL MEASUREMENTS AND METROLOGY (Course Code:18ME36B )

2. TEXT BOOKS Mechanical Measurements , Beckwith Marangoni and Lienhard , Pearson Education, 6th Ed., 2006 . Instrumentation, Measurement and Analysis , B C Nakra , K K Chaudhry, 4th Edition, McGraw Hill. Engineering Metrology , R.K. Jain, Khanna Publishers, Delhi, 2009 REFERENCE BOOKS: Engineering Metrology and Measurements , N.V.Raghavendra and L.Krishnamurthy , Oxford University Press.. Further Reference: National Programme on Technology Enhanced Learning ( NPTEL ) http://nptel.ac.in/courses/112104121/1

3. MECHANICAL MEASUREMENTS AND METROLOGY CHAPTER 3: Comparators

4. Learning Objectives Explain the basic principle of construction and operation of different types of comparator.

5. Explain the basic measurement principles of comparators Outcomes

6. Module 2 Comparators: Functional requirements & Classification Mechanical comparators - Johnson Mikrokator , Sigma comparators, Dial indicator. Electrical comparators - LVDT Pneumatic comparators - Back pressure, Solex Optical comparators - Zeiss ultra optimeter.

7. Direct measurement vs. Comparison Introduction Accuracy of direct measurement depends on four factors: Accuracy of the standard Accuracy of scale Least count of the scale Accuracy of reading the scale. Human element

8. Direct measurement vs. Comparison Introduction Accuracy of comparison measurement primarily depends Accuracy of the standard used for setting the comparator Least count of the standard Sensitivity of the comparator Accuracy of reading the scale

9. FUNCTIONAL REQUIREMENTS 1. A comparator should have a high degree of accuracy and precision. 2. Scale should be linear and have a wide range . 3. Should have high amplification . 4. Should have good resolution (least possible unit of measurement). 5. Provision for incorporating to compensate for temperature effects. 6. Should be versatile (provisions for different ranges, attachments).

10. CLASSIFICATION OF COMPARATORS Based on the principle used for amplifying and recording measurements Mechanical comparators Mechanical–optical comparators Electrical and electronic comparators Pneumatic comparators Projection comparators and multi-check comparators

11. CLASSIFICATION OF COMPARATORS Mechanical comparators Johnson Mikrokator Sigma comparators Dial indicator Mechanical Comparators are made up of mechanical means .

12. MECHANICAL COMPARATORS Dial Indicator / Dial Gauge

13. MECHANICAL COMPARATORS Dial Indicator / Dial Gauge The movement of the Plunger (spindle) will be multiplied thru the series of gears and pinions and indicated on the main scale on the dial by the indicator (Needle). Principle (Gear & Pinion type)

14. MECHANICAL COMPARATORS Dial Indicator / Dial Gauge Principle (Gear & Pinion type)

15. MECHANICAL COMPARATORS Dial Indicator / Dial Gauge Principle (Gear & Pinion type) The  plunger moves linearly wrt. change on the workpiece while taking measurements. The plunger consist of a rack and it meshes with a pinion(P1) on the gear(G1) This Gear (G1) mesh with the series pinions and gears to multiply the movement to increase the accuracy of measurement. Final Pinion (P3) is connected to the Indicator(Needle) this indicator will show the deflection on the main scale.

16. MECHANICAL COMPARATORS Johnson Mikrokator / Abramson's movement Designed by Hugo Abramson in 1938 Developed by C.F. Johansson . Used to obtain mechanical magnification of the difference in length as compared to a standard .

17. MECHANICAL COMPARATORS Johnson Mikrokator / Abramson's movement It works on the principle of a button spinning on a loop of string. A twisted thin metal strip holds a pointer, which shows the reading on a suitable scale. Since there is no friction involved in the transfer of movement from the strip to the pointer , it is free from backlash .

18. MECHANICAL COMPARATORS Johnson Mikrokator / Abramson's movement

19. Johnson Mikrokator / Abramson's movement A metallic strip is twisted and fixed between two ends. Longitudinal movement will cause the central portion of the strip to rotate . One end of the strip is fixed to an adjustable cantilever and the other end is fixed to the spring elbow. The spring elbow is connected to a plunger , which moves upwards or downwards . The spring elbow consists of flexible strips and stiff diagonal acts as bell crank lever , causing the twisted strip to change length when there is a movement in the plunger ; This change in length results in a proportional amount of twist of the metallic strip . The magnification can be varied by changing the length of the spring elbow

20. Sigma Comparator

21. Sigma Comparator Linear displacement of a plunger is translated into the movement of a pointer over a calibrated scale. Plunger is the sensing element that is in contact with the work part. It moves on a slit washer , providing frictionless linear movement , also arrests rotation of the plunger about its axis . A knife edge is attached onto the plunger, bears upon the face of the moving member of a cross-strip hinge . This unit comprises a fixed member and moving block , connected by thin flexible strips at right angles to each other.

22. Sigma Comparator When the plunger moves up or down , the knife edge drives the moving member of the cross-strip hinge assembly, deflecting an arm , which divides into ‘Y’ form. Extreme ends of Y-arm is connected to a driving drum by means of phosphor-bronze strips. The movement of the Y-arm rotates the driving drum, in turn, the pointer spindle . Working

23. CLASSIFICATION OF COMPARATORS Electrical comparators LVDT Electrical comparator Electrical comparators convert the linear movement of the plunger into electrical signals and these signals. They rely on a Wheatstone bridge circuit for measurement.

24. LVDT (Linear Variable Differential Transformer) Electromechanical transducer, convert the rectilinear motion of an object to into a electrical signal.

25. LVDT (Linear Variable Differential Transformer) The main components of an LVDT are transformer and a core . The transformer consists of three coils - a primary and two secondaries wound on a hollow cylindrical tube. The primary coil is located between the two secondary coil s. The ferro magnetic core moves freely inside the cylindrical tube. Non-ferromagnetic shaft / push ro d, is coupled to the core and connects to the object being measured.

26. Electrical comparator

27. Electrical comparator The plunger is the sensing element , the movement of which displaces an armature inside a pair of coils. Movement of the armature causes change in inductance in the two coils , resulting in a net change in inductance . This change causes an imbalance in the bridge circuit , resulting in an output. The output display device (analog or digital) is calibrated to show the readings in units of length, i.e. linear displacement.

28. CLASSIFICATION OF COMPARATORS Pneumatic comparators Operated by the gas or air under pressure. Pneumatic comparators enables two approaches for measuring the Deflection in measurement; Measuring the Air pressure Measuring the Velocity of Airflow

29. Back Pressure comparator It works on the principle of pressure difference generated by the air flow . Air is supplied at constant pressure through the orifice and the air escapes in the form of jets through a restricted space which exerts a back pressure . The variation in the back pressure is used to find the dimensions of a component. Principle

30. Back Pressure comparator Principle

31. Back Pressure comparator Principle The system uses a two-orifice, Orifice O1 (control orifice) & Orifice O2 (measuring orifice). The measuring head gets compressed air supply at a constant pressure P (source pressure). It passes through the control orifice into an intermediate chamber . Air exits the through the measuring orifice . Size of the control orifice remains constant. Effective size of the measuring orifice varies, as the gap d between the measuring orifice and the work surface . Depending on the gap d, the back pressure P b changes, providing a means for measuring dimension d.

32. Solex comparator Principle

33. Solex comparator The instrument is produced commercially by Solex air gauges Ltd. USA Designed for internal measurement Consists of a tank, water is filled up to a certain level (H) Dip tube is immersed . Air is passed at high pressure , some air will leak out from the dip tube in the form of air bubbles. Principle

34. Solex comparator This ensures the air moving towards control orifice at a constant pressure . The air at a reduced pressure then passes through the control orifice . Then air escapes from the measuring orifice in the measuring head . Based on the clearance between work part and measuring orifice, back pressure is created, resulting in the head of water being displaced in the manometer tube. Principle

35. Optical comparators Device that applies the principles of optics to the inspection of parts. In optical comparator , a light source and a reflecting surface(Mirror) are used as the optical means. An incident ray will hit the mirror & gets reflected, this ray is projected on to the scale. Principle Incident angle = Reflecting angle

36. Zeiss Ultra-optimeter Light Source Green Filter Condenser Two Reflecting Mirrors (M1, M2) Plunger Objective Lens Graticule Screen Principle

37. Principle The Light from the source passes through the green filter and condenser. The function of the Green filter is to allow only green light The filtered beam light passes through the Condenser. The function of the condenser is to focus the light to the movable mirror (M1). There are two mirrors, Mirror 1 (M1) and Mirror 2 (M2). Mirror 1 is a movable mirror , operated by the Plunger & Mirror 2 is fixed. The plunger is the measuring tip (Contacts to the workpiece to take the deviation. Once the focused light from the condenser falls on the mirror M and get reflected on to the Mirror 2 and reflected back to the Mirror 1. The objective lens will collect this image and projects on to the graticule. This image on the Graticule is observed by the Eye Piece.

38. End of Module