statistical process control using control charts · pdf file · 2010-10-15tujuan...

Lecture 3

15 October 2010 Materi ke-3

Statistical Process ControlUsing Control Charts

ReviewReview

� Apakah yang anda ketahui tentangpeta kendali / Shewhart control chart?

� Jelaskan pentingnya diagram pareto


� Jelaskan pentingnya diagram paretodalam proses perbaikan kualitas?

OutlineOutline

� Pengantar� Penyebab Variasi� Statistik Dasar untuk Control Charts


� Statistik Dasar untuk Control Charts� Pemilihan Subkelompok rasional� Analisis Pola pada Control Chart� Pemeliharaan Control Chart

IntroductionIntroduction

� Proses kontrol statistik (Statistical process control) adalah kumpulan alatyang bila digunakan bersama-samadapat menghasilkan stabilitas proses


dapat menghasilkan stabilitas prosesdan pengurangan variabilitas

� Control Chart: alat grafisuntukmemonitor aktivitas dari proses yang sedang berlangsung

IntroductionIntroduction

� Manfaat menggunakan control chart� Ketika untuk mengambil tindakan korektif� Jenis tindakan perbaikan yang diperlukan

Ketika meninggalkan proses sendirian


� Ketika meninggalkan proses sendirian� Kapabilitas proses� Kemungkinan sarana peningkatan kualitas� Cara menetapkan spesifikasi produk

Cause of Variation

� Penyebab variasi� Chance Cause

� melekat proses Sesuatu (sebagai variasi alamidalam proses )


dalam proses )

� Assignable Cause� Sesuatu yang dapat diidentifikasi ditentukan� Contoh: alat yang salah, kesalahan operator

Cause of Variation

� Chance and Assignable Causes of Quality Variation� Sebuah proses yang hanya disebabkan chance causes ,

proses tersebut dikatakan dalam pengendalian statistik. Alam variabilitas atau kebisingan latar belakang. Fluktuasi


Alam variabilitas atau kebisingan latar belakang. Fluktuasi� Sebuah proses yang beroperasi di hadapan penyebab

dialihkan dikatakan di luar kendali. Misalnya Kesalahanoperator, bahan baku yang rusak, pengaturan yang tidaktepat.

� Tujuan akhirnya SPC adalah pengurangan ataupenghapusan variabilitas dalam proses identifikasipenyebab dialihkan.

� A process that is operating with only chance causes of variation present is said to be in statistical control. Natural

Statistical Basis for Control Charts

� Basic Principles� Assumed to have approximately

normal distribution


normal distribution� Control limits : 99.74 % ( 3σ limits )� A control chart : on line process

control� Making inference


� Selection of Control LimitsLet θ represent a quality characteristic of interest and represent an estimate of θ


( )( ) ( )( ) ( )

ˆ

ˆ ˆ

ˆ ˆ

CL E

UCL E k SD

LCL E k SD

θ

θ θ

θ θ

=

= +

= −

If k = 3 →0.0026 of a sample statistic falling outside


� Errors in making inference from control chart� Type I : process is out of control when it is actually in

control� Type II : process is in control when it is really out of control

Effect of control limits on errors in making inference


� Effect of control limits on errors in making inference� Warning limit � Usually 2 standard deviation� Effect of sample size on control limits� Influence in standard deviation


�� Basic PrinciplesBasic PrinciplesA typical control chart has control limits set at values such that if the process is in control, nearly all points will lie between the upper


nearly all points will lie between the upper control limit (UCL) and the lower control limit (LCL).


� A control chart contains� A center line� An upper control limit� A lower control limit

� A point that plots within the control limits indicates the process


� A point that plots within the control limits indicates the process is in control� No action is necessary

� A point that plots outside the control limits is evidence that the process is out of control� Investigation and corrective action are required to find and

eliminate assignable cause(s)� There is a close connection between control charts and

hypothesis testing

Photolithography Example

� Important quality characteristic in hard bake is resist flow width

� Process is monitored by average flow width� Sample of 5 wafers


� Sample of 5 wafers� Process mean is 1.5 microns� Process standard deviation is 0.15 microns

� Note that all plotted points fall inside the control limits� Process is considered to be in statistical control

Selection of rational Subgroups

� The premise : chosen is such manner that the variation within it is considered to due only to chance causes.

� Basis : Time order


� Two approaches ( Besterfield , 1990 )� Instance of time method� Period of time method

� Subgroup Size ( the number of items in each group )

� Frequency of sampling


� Subgroups or samples should be selected so that if assignable causes are present, the chance for differences between subgroups will be maximized, while the chance for


will be maximized, while the chance for differences due to these assignable causes within a subgroup will be minimized.


� Selection of Rational SubgroupsTwo general approaches to constructing rational subgroups.

� Select consecutive units of production. Each


� Select consecutive units of production. Each sample consists of units that were produced at the same time (or as closely together as possible)� Provides a “snapshot” of the process.� Effective at detecting process shifts.

� Select a random sample over the entire sampling interval. � Often used to make decisions about the

acceptance of all units of product that have been


acceptance of all units of product that have been produced since the last sample.

� Can be effective at detecting if the mean has wandered out-of-control and then back in-control.

Analysis of Patterns in Control Chart

Five Rules for identifying an out-of-control process1. A single point outside the control limits2. Two out of three consecutive points fall outside the

2σ warning limits on the same side


3. Four out of five consecutive points fall beyond the 1σwarning limits on the same side

4. Eight or more consecutive points fall to one side5. A run of eight or more consecutive points –up, down,

above or below the CL , or above or bellow the median

Analysis of Patterns in Control Chart

Nonrandom patterns can indicate out-of-control conditions� Patterns such as cycles, trends, are often of

considerable diagnostic value (more about this in Chapter 5)Look for “runs” - this is a sequence of


� Look for “runs” - this is a sequence of observations of the same type (all above the center line, or all below the center line)

� Runs of say 8 observations or more could indicate an out-of-control situation.� Run up: a series of observations are increasing� Run down: a series of observations are

decreasing


An x chart with a nonrandom, up-run, down-run patterns


An x chart with a cyclic pattern

Analysis of Patterns in Control Chart� Interpretation of Plots ( Non random pattern )� Determination of causes associated with out-of-control points

� Require a thorough knowledge of the process and the sensitivity of the output quality characteristic to the process parameters

� The pattern and associated causes Change in the level of the plotted pattern ( a jump ) ( change


� Change in the level of the plotted pattern ( a jump ) ( change quality raw material , change operator , failure component )

� Trend in the plotted pattern ( tool wear , change in pressure )� Cyclic behavior in the plotted pattern ( seasonal effects of

quality , operator fatigue )� Concentration of points near the control limits ( two or more

operator plotted on the same chart , different production method )

Maintenance of Control Chart

� Proper placement of the control charts on the shop floor is important → easy to access


� The control chart should draw the attention and curiosity of everyone involved

QuizQuiz1. What are benefits of using control chart ?2. Explain the different between chance

causes and assignable causes ? give example of each ?


example of each ?3. How are rational subgroups selected ?

explain the importance of this in the total quality systems approach?

4. 5-13 ?

Kata inspirasi pertemuan ini

Berfikir

Banyak orang yang berfikir. Tapi, sedikit


Banyak orang yang berfikir. Tapi, sedikit yang bertindak. Ingat, tak seorangpun

akan sukes hanya dengan berfikir, tanpa bertindak. Semua fikiran, harus diikuti

oleh tindakan.

statistical process control using control charts · pdf file · 2010-10-15tujuan...

Documents