Introduction to Mechanical Measurement

CHAPTER 1 Mechanical Measurement

Presentation Prepared by Prof. Naman M. Dave Assistant Prof. (Mechanical Dept.) Gandhinagar Institute of Technology

Please refer this file just as reference material. More concentration should on class room work and text book methodology.

Measurement and Metrology Science of measurement

Depending on field of application

Today (in broader sense) with some

practical terms

Metrology Practical terms related with

measurement: o Errors in measurements o Methods of measurements o Measuring Instruments o Units of measurement and their standards o Industrial inspection and its different

techniques o Measuring instruments and accuracy

Needs of Measurement and Metrology To ensure that the part to be measured conforms

to the established standard. To meet the interchangeability of manufacture. To provide customer satisfaction by ensuring that

no faulty product reaches the customers. To coordinate the functions of quality control,

production, procurement & other departments of the organization.

To judge the possibility of making some of the defective parts acceptable after minor repairs.

Objectives of Measurement and Metrology Although the basic objective of a measurement is to

provide the required accuracy at a minimum cost, metrology has further objectives in a modem engineering plant with different shapes which are:

To minimize the cost of inspection by efficient and effective use of available facilities,

To minimize the cost of rejection and re-work through application of statistical quality control techniques.

To maintain the accuracies of measurement. To determine the process capabilities and ensure that

these are better than relevant component tolerances. To do complete evaluation of newly developed products.

Modes of Measurement Based upon the number of conversions, three

basic categories of measurements have been developed.

They are; 1. Primary measurement 2. Secondary measurement 3. Tertiary measurement

Modes of Measurement 1. Primary measurement Direct observation and comparison Not involvement of any conversion

Ex. Length, Height, Depth or Width etc. measurement.

Modes of Measurement 2. Secondary measurement >Indirect method >Involvement of one conversion Ex. Pressure or Temperature measurement

3. Tertiary measurement >Indirect method >Involvement of 2 conversion Ex. Measurement of rotating shaft

Methods of Measurement

Methods of Measurement Type of Method Technique to measure With contact Instrument is placed in contact

with the object. For ex. vernier calliper

Without contact Instrument not placed in contact with the object. (use of sensor)

Absolute or Fundamental

Based on the measurements of base quantities entering into the definition of the quantity.

Methods of Measurement Type of Method Technique to measure Comparative Based on the comparison of the

value of a quantity to be measured with a known value of the same quantity.

Null measurement Here, difference between measurand value and known value of same quantity with which it is compared is brought to zero.

Presenter

Presentation Notes

The way to measure will define the method

Methods of Measurement Type of Method Technique to measure Substitutional Method Quantity to be measured is

replaced by a known value of the same quantity, so selected that the effects produced in the indicating device by these two values are the same (a type of direct comparison).

Complementary Method The value of quantity to be measured is combined with known value of the same quantity. Ex: Volume determination by liquid displacement.

Methods of Measurement Type of Method Technique to measure Transposition Quantity to be measured is first balanced by a known value and then balanced by an other new known value.

Value of quantity measured is first balanced by an initial known value A of same quantity, then measured by quantity is put in place of this known value. Then, it is balanced again by another value B. If the position of element in equilibrium.

Methods of Measurement Type of Method Technique to measure Coincidence Measurements coincide with

certain lines and signals. Ex. Callipers

Deflection The value of the quantity to be measured is directly indicated by the deflection of a pointer on a calibrated scale

Generalized Measurement System


Elements of measuring system

1. Primary sensing element Quantity under measurement makes its first

contact with primary sensing element. Sense the condition, state or value of the process

variable by extracting a small part of energy from the measurand,

and then produce an output which reflects this condition, state or value of measurand.


Elements of measuring system 2. Variable conversion (transducer) element Convert one physical form into another form without

changing the information content of the signal.

Presenter

Presentation Notes

Instrument maker division


Elements of measuring system 3. Variable manipulation element Modifies the signal by amplification, filtration or other

means so that desired output produced according to some mathematical rule for Ex. i/p x constant = o/p


Elements of measuring system 4. Data transmission element Transmits the signal from one location to another

without changing its information contents.


Elements of measuring system 5. Data processing element modifies the data before it displayed or finally

recorded • perform mathematical operation such as addition,

subtraction, multiplication, division, etc. • to calculate average, statistical and logarithmic values • to convert data into desired form. • to separate undesired signal from output signal. • to provide correction on the output signal.


Elements of measuring system 6. Data presentation elements Provides a record or indication of the output

(i) Transmitting information (measured quantity) to another location or devices. (ii) Signaling : To give a signal that the pre-defined value has been reached. (iii) Recording : To produce a continuous record of measured quantity in written form. (iv) Indicating : To indicate the specific value on calibrated scale.

Performance characteristics Important to select most suitable instrument for specific

measurement.

Static Performance characteristics: • Desired input to the instrument not change w.r.t time.

Dynamic Performance characteristics:

Static Performance characteristics Readability: This term indicates the

closeness with which the scale of the instrument may be read.

Susceptibility of device to have its indications converted into meaningful number.

Least count: It is the smallest difference between two indications that can be detected on the instrument scale.

Static Performance characteristics Range: It represents the highest possible value that can be

measured by an instrument or limits within which instrument is designed to operate.

Linearity: A measuring system is said to be linear if the output is linearly proportional to the input.

Static Performance characteristics Repeatability: It is defined as the ability of a measuring

system to repeat output readings when the same input is applied to it consecutively, under the same conditions, and in the same direction.

Reproducibility: It is defined as the degree of closeness with which the same value of a variable may be measured at different times.

System response: Response of a system may be defined as the ability of the system to transmit & present all the relevant information contained in the input signal.

Static Performance characteristics Threshold: Min. value of i/p required to cause a

detectable change from ‘0(zero)’ o/p. If i/p increased gradually from ‘0(zero)’, there will

be some min. value below which no o/p change can be detected.

Static Performance characteristics Hysteresis: Hysteresis is the maximum

differences in two output (indicated values) at same input (measurand) value within the specified range when input is continuously increased from zero and when input is continuously decrcased for maximum value.

Maximum diff. between increasing input value and the decreasing input value at the same output.

Presenter

Presentation Notes

http://ecoursesonline.iasri.res.in/mod/resource/view.php?id=147084 Hysteresis is a phenomenon under which the measuring instrument shows different output effects during loading and unloading. Hysteresis results from the inelastic quality of an element or device. It is the maximum difference between corresponding upscale and downscale outputs for any single test cycle, that is, the maximum difference in output at any given value of the measured variable within the specified range when approaching that point first with increasing the input and then decreasing the input. Hysteresis results from the presence of irreversible phenomenon such as (i) mechanical friction (ii) free-play or backlash in mechanism (iii) slack motion in bearings, (iv) magnetic and thermal effects.

Static Performance characteristics Calibration: Procedure of

making, adjusting or checking a scale so that readings of an instrument conforms to an accepted standard.

Sensitivity: Ratio of o/p response to a specific range in i/p.

Dead zone: Largest change of i/p quantity for which instrument does not indicate output.

Static Performance characteristics Drift: gradual variation or undesired change in o/p during constant i/p.

Zonal Drift: Combination of both

Zero Drift: whole calibration gradually shifts due to slippage

• Span: proportional change in the indication or change along the upward scale

Static Performance characteristics

Loading effect: Any instrument, invariably extracts energy from system, the original signal should remain undistorted. This is incapability of system to faithfully measure signal in undistorted form.

Presenter

Presentation Notes

http://ecoursesonline.iasri.res.in/mod/resource/view.php?id=147084 The loading effects may occur due to both electrical and mechanical elements. The loading effects of an Instrument are the alternations that are caused in the circuit conditions such as voltage, current etc. when the instrument is introduced in the circuit for the purpose of measurement. In simple terms, loading effects of an instrument ends up distorting the signal they are supposed to measure in the form of attenuation or phase shift. The instrument therefore reads the altered value of the quantity and thus an erroneous measurement is resulted. These loading effects can be better explained by the following examples. Let a voltage has to be measured across a resistance R in the circuit. For this purpose, a voltmeter V is connected across the resistor R in parallel. We know that a voltmeter has a very high resistance value. But since this value is finite, a fraction of the total current passing through R will pass through the Voltmeter V. This will lead to power dissipation in the Voltmeter. The voltmeter extracts this power out of the circuit and thus end up varying the values of the circuit parameters on being introduced. Another example of loading effects is found in the measurement of current by an Ammeter. An ammeter is a very low resistance device that is connected in series in a circuit for the measurement of current. When current passes through it, because of the low finite resistance of the ammeter, there is a small voltage drop across the ammeter which results in power dissipation. This power is again borrowed from the circuit and therefore affects the circuit parameters. So to minimize this �loading effect�, the best you can do is to use a measuring instrument that has very high impedance so that the current it draws is minimal. But practically an infinite or zero impedance in an instrument is not possible. So a condition of minimum loading effects is tried to be reached by making the values of the impedances of instruments very high or very low according to their use. That is why Voltmeters always have very high while Ammeters always have very low impedance values.

Static Performance characteristics

Accuracy: It is degree to which the measured value agrees with true value. Max. amount by which result differs from the true value.

Precision: It is repeatability or reproducibility of the measurement. If instrument is not precise, great difference in dimension measured again and again.

Measurement Errors What is Error ?

It is difference between indicated or

measured value and true value.

It is impossible to made measurement with perfect accuracy

Measurement Errors Types (Classification of Errors)

Measurement Errors Gross errors • Human mistakes • Careless readings, mistake in recordings, • improper application of instrument • Can not treated mathematically • Can be avoided only by taking care in reading

and recording

Measurement Errors Systematic error • Have definite magnitude and direction. • Can be repeated consistently with repetition of

experiments. • To locate these errors: repeated measurements

under different conditions or with different equipment or possible by an entirely different methods.

Measurement Errors Instrumental error Due to design or construction /assembly of

instruments Limiting accuracy Improper selection of instrument Poor maintenance For Ex. Errors due to friction, wear, slips,

vibration Errors due to incorrect fitting of scale at zero,

non-uniform division of scale, bent pointer.

Measurement Errors Operational error Misuse of instrument Poor operational techniques For Ex. Errors in flow measurement if flow-

meter is placed immediately after a valve or a bend.

Measurement Errors Environmental errors due to conditions external to the measuring

instrument, including conditions in the area surrounding the instrument,

such as effects of change in temperature, humidity, barometric pressure, or magnetic or electrostatic fields.

For ex. Buoyant effect of the wind causes errors on precise measurement of weights by pan balance.

Measurement Errors

These errors may be avoided by (i) Use instrument under conditions for

which it was design and calibrated. This atmospheric condition can be maintain by air conditioning.

(ii) Provide sealing certain components in the instrument.

(iiii) Make calibration of instrument under the local atmospheric conditions

Environmental errors

Measurement Errors System interaction errors Interaction between system (to be measured)

and instrument body. So it change the condition of the system.

For Ex. A ruler pressed against a body (system) resulting the deformation of the body.

Measurement Errors Observation errors : Due to poor capabilities and carelessness of

operators. i. Parallax : These errors may arise when the

pointer and scale not in same plane or line of vision of observer is not normal to the scale.

Measurement Errors Observation errors : Due to poor capabilities and carelessness of

operators. ii. Personal bias: Observer tendency to read high

or low, anticipate a signal and read too iii. Wrong reading,

wrong calculations, wrong recording data, etc.

Measurement Errors Random Error Accidental in their incidence Variable in magnitude and usually follow a

certain statistical (probability) law. Friction and stickiness in instrument Vibration in instrument frame or supports Elastic deformation Large dimensional tolerances between the

mating parts. Supply power fluctuations Backlash in the movement.

CHAPTER 1

Mechanical Measurement

Introduction to Mechanical Measurement

Engineering

Transcript of Introduction to Mechanical Measurement