Download
1 / 32
320 likes | 325 Vues
Instrumentation and Product Testing. Third Lecture. Measurement System and Its Static Characteristics. Introduction to Instrumentation and Product Testing Why Need Instrumentation and Product Testing in Manufacturing?
E N D
Instrumentation and Product Testing Third Lecture Measurement System and Its Static Characteristics
Introduction to Instrumentation and Product Testing • Why Need Instrumentation and Product Testing in Manufacturing? • The goal of Manufacturing or Production of parts/products is to produce them economically, in compliance with design specifications, which assure the proper function and service of the manufactured product in its expected life. The roles of Instrumentation and Product Testing are: • Quality, Safety and Reliability Control • Process Control and Automation • Design and Production of Intelligent Products
Some application examples: (a) Manual Process Control
(c) Quality Inspection (d) Indexing for Process Control Therefore, the basic purpose of Instrumentation (or Engineering Measurement) in Manufacturing is to ensure and verify the agreement of the product, in various stage of manufacturing, with compliance to the specifications of the design, aiming to achieve and Economical and Efficient production of High Quality and Reliable products.
(e) Intelligent Products • Toys • Automobiles • Audio/Video Instruments • Watches • Electric Appliance (Rice cooker, Washing machine, etc.)
Unit of Measurement It is a well-known saying that the knowledge about anything is complete only when if it can be quantified (expressed in numbers) correctly. Therefore, in conjunction with the above need, for every kind of quantity measured, there must be a unit to measure it and express it in numbers of that unit. Furthermore, in order that this unit is followed by all and not one who is taking the measurements, there must be a universal standard and the various units for various parameters of importance must be standardised.
What does a ‘Standard’ mean? • Two meanings: • Documents of regulations or guidelines for standardisation, and • ...
Standard • Unit has to have some relation to physical world, Therefore, physicalrecords, called standards, are used to permanently record the size of units. • Definition of standard: • A standard is a permanent or readily reproducible physical record of the size of a unit of measurement. • A universal standard must be one which is reproducible with such a degree of accuracy that for all industrial and scientific purposes it may be considered as absolute.
Primary Standard and Secondary Standard • A primary standard is used as a fundamental definition of the size of a physical unit. Primary standards are specified by the provisions of an international treaty. • A secondary standard is a copy of a primary standard that can be used routinely for making measurements. • Few of us will ever see/use a primary standard. Rather, we will generally deal with a secondary standard (say, laboratory standard) that has been copies from another secondary standard that itself may be many steps removed from the primary standard.
Error and uncertainty • The difference between a measured value and its true value is the ‘error’ in measurement. The essence of measurement is not only to know the magnitude of an unknown quantity, but also some knowledge of the degree of accuracy of the measurement. • Types of errors • Systematic errors are those caused by the measuring procedure, e.g. errors due to excessive measuring pressure, sine errors etc. Constant errors are those which affect all of a series of measurements by the same amount. e.g. errors in calibration, zero errors, wear of measuring contacts etc. • Random errors (uncertainties) are caused by incapability of technology and skill.
Factors affecting accuracy in measurements In measurement work, it is important to minimize errors and thus lead to increased accuracy. The choice/design of appropriate method is crucial. For example, measurement of an angle: The error becomes very large when measuring an angle close to 90.
Moreover, it is necessary to identify the various factors affecting accuracy. These can be due to one or more of the following: • Standard S • Workpiece W • Instrument I • Person P • Environment E
Measurement systems With a measurement system, physical quantities are measured so as to obtain data which can be transmitted to recording and display devices. The measurand is the physical quantity to be measured, i.e. temperature, pressure, flow rate, strain, displacement, etc.
A complete measurement system All measuring systems include three basic elements: Recorder or Display Transducer Signal conditioner
A complete measurement system All measuring systems include three basic elements: Recorder or Display Transducer Signal conditioner detect and convert the physical quantity into a more usable form: a mechanical, pneumatic, hydraulic or electrical signal.
A complete measurement system All measuring systems include three basic elements: Recorder or Display Transducer Signal conditioner modify the signal to a desirable output: amplifier, filter, A/D converter, etc.
Transmission Path: noise and attenuation A complete measurement system All measuring systems include three basic elements: Recorder or Display Transducer Signal conditioner show and record the measurand: analog or digital
Example: A Pressure gauge 60 50 70 40 80 30 90 20 100 10 kPa 110 0 Pressurized Gas Pipe
Bourdon-tube (oval cross-section) detector transducer stage Increased pressure causes movement of tube in this direction Sector and pinion are modifying stage Sector Pointer and dial are indicator stage Pressure source
Pedometer Transducer? Signal Conditioner? Display? Mass Counter LCD
4. Characteristics of measurement systems • To choose the one most suited to a particular measurement application, we have to know the system characteristics. • The performance characteristics may be broadly divided into two groups, namely ‘static’ and ‘dynamic’ characteristics.
Static characteristics • the performance criteria for the measurement of quantities that remain constant, or vary only quite slowly. • Dynamic characteristics • the relationship between the system input and output when the measured quantity (measurand) is varying rapidly. • In practice, the characteristics of the one group may well influence the characteristics of the other. In order to access overall instrument performance, however, the two groups of characteristics are normally studied separately and then a semi-quantitative superposition is carried out.
Static characteristics (Ref: Principles of Measurement Systems, J.P. Bentley, Longman; BS 5233, BS 5532, and ISO 3534) Determination of static characteristics is by calibration: Environmental Inputs, Ei Standard instrument(s) Element or System to be calibrated Output, O Input, I Standard instrument Standard instrument
Accuracy This is the closeness with which the measuring instrument can measure the ‘true value’ of the measurand under stated conditions of use, i.e. its ability to ‘tell the truth’. The accuracy of an instrument is quantified by the difference of its readings and the one given by the ultimate or primary standard.
Bias Bias (constant error) describes a constant error which exists over the full range of measurement of an instrument. This error is normally removable by calibration.
Precision Precision describes an instrument’s degree of random variations in its output when measuring a constant quantity. Precision is often confused with accuracy. High precision does not imply anything about measurement accuracy.
Measuring a fixed target position from a satellite Satellite Apollo 13: Low precision, low accuracy Satellite PolyU 18: High precision, low accuracy Satellite CYT 118: High precision, High accuracy
