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Quality Control

Quality Control. IENG102 Prof. Dr. Alagar Rangan. Quality Control. Quality is conformance to specifications Quality characteristic Variable Attribute Variable: Quality characteristic that can be measured on a numerical scale length, diameter, density, etc.

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Quality Control

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  1. Quality Control IENG102 Prof. Dr. Alagar Rangan

  2. Quality Control • Quality is conformance to specifications • Quality characteristic • Variable • Attribute • Variable: Quality characteristic that can be measured on a numerical scale length, diameter, density, etc. • Attribute: That cannot be measured on a numerical scale, Yes, No; Good, Ok, Bad; etc. • Quality Control: A system that is used to maintain designed Quality in a product or service. • Father of Quality Control: Edward Deming.

  3. Quality Control • comprehensive approach to improving Quality is TQM (Total Quality Management). • Graphical Methods: Histograms, Cause and effect Diagrams, etc.

  4. Quality Control • Control charts: CL, UCL, and LCL. • Average and Variations. • Average – Mean. • Variation – Range.

  5. Quality Control • Control Charts based on Normal distribution. • Normal distribution is symmetric. • In a production process, measurements of a Quality are normally distributed.

  6. Quality Control • Reliability T: Failure time, a random variable Reliability R(t) is the probability that a system is working at time t. R(t) = P(T>t)

  7. Quality Control • Series system A series system works if all the components work. • Parallel system A parallel system works if at least one of the components works.

  8. EXAMPLESExample 1. Quality Control Lathe Machining Car Parts: • Quality Characteristic: Thickness • Variable Characteristic • Process Mean Thickness • Process Standard Deviation of Thickness • Known or Estimated • Company wishes to control the thickness of parts

  9. Take samples of size 5 parts every hour • Calculate sample mean (x1+x2+...+x5)/5 • These means are normally distributed • Using this idea, draw control lines • Upper and lower control line • If every value of sample mean falls within UCL & LCL then process is under control • If not, stop production

  10. Check for assignable causes • Then restart production • Improves quality, saves cost, reduces lead time

  11. Example 2. Reliability • Failure Time of a Solid State Power Unit • Failure times are random variables • Reliability : The power unit operates continuously for a fixed period without failure • Expected time to failure • Design Life: How to design power units to have a required reliability • Rate of failure of systems

  12. Thank you

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