27-11-2010, 10:20 AM
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DEFINITIONS
WHAT IS MEASUREMENT
MEASUREMENT IS ASSIGNMENT OF NUMBERS (OR VALUES) TO MATERIAL THINGS TO REPRESENT THE RELTIONS AMONG THEM WITH RESPECT TO PARTICULAR PROPERTIES.
BY EISENHART (1963)
WHAT IS MEASUREMENT SYSTEM
MEASUREMENT SYSTEM IS THE COLLECTION OF INSTRUMENTS,STANDARDS,OPERATIONS,METHODS,FIXTURES,SOFTWARE,PERSONNEL,ENVIRONMENT & ASSUMPTIONS
Measurement system analysis
Study of combined effect of all measurement contributors.
Assessing their suitability against measurement objective.
Objectives of Measurement Process
Product control-
Is the part in a specific category i.e. meets specification requirements ?
process control-
Is the process variation stable & acceptable
STATISTICAL PROPERTIES TO DEFINE A “GOOD” MEASUREMENT SYSTEM (MS)
1. ADEQUATE DISCRIMINATION & SENSITIVITY:
Should be small relative to PROCESS VARIATION or SPECIFICATION LIMIT.
Rule of 10’s i.e. 1/10th .
i.e. least count/ resolution of equipment should be 1/10th of process Variation (10 data categories).
2. MS should be in statistical control
- Common cause variations only
- No special cause variation
3. Variability of MS < mfg. Variability
-For process control
4. Variability < specification limits (sl)
- for product control
Bias
Difference between the observed average of measurements and the true value (reference value) on the same characteristics on the same part.
A measure of the systematic error of the measurement system.
It is the contribution of the total error comprised of the combined effects of all sources of variation, known or unknown.
Stability (Drift)
The total variation in the measurements obtained with a measurement system-
on the same master or parts,
when measuring a single characteristic,
over an extended time period.
i.e. Stability is the change of bias over time
Stability (Drift)
CAUSES OF INSTABILITY-
Mechanical wear,
Electrical instability,
Changes in external environment,
Degradation in components with time
Quite Common with electrical based gauges, optical gauges(dependent on light sources, spectrophotometer etc.
Reproducibility (Between system variation)
The variation in the average of the measurements
made by different appraisers
using the same measuring instrument
when measuring the identical characteristic
on the same part.
MSA
MEASUREMENT SYSTEM ANALYSIS
(THIRD EDITION)
MEASUREMENT SYSTEM ANALYSIS
(THIRD EDITION)
DEFINITIONS
WHAT IS MEASUREMENT
MEASUREMENT IS ASSIGNMENT OF NUMBERS (OR VALUES) TO MATERIAL THINGS TO REPRESENT THE RELTIONS AMONG THEM WITH RESPECT TO PARTICULAR PROPERTIES.
BY EISENHART (1963)
WHAT IS MEASUREMENT SYSTEM
MEASUREMENT SYSTEM IS THE COLLECTION OF INSTRUMENTS,STANDARDS,OPERATIONS,METHODS,FIXTURES,SOFTWARE,PERSONNEL,ENVIRONMENT & ASSUMPTIONS
Measurement system analysis
Study of combined effect of all measurement contributors.
Assessing their suitability against measurement objective.
Objectives of Measurement Process
Product control-
Is the part in a specific category i.e. meets specification requirements ?
process control-
Is the process variation stable & acceptable
STATISTICAL PROPERTIES TO DEFINE A “GOOD” MEASUREMENT SYSTEM (MS)
1. ADEQUATE DISCRIMINATION & SENSITIVITY:
Should be small relative to PROCESS VARIATION or SPECIFICATION LIMIT.
Rule of 10’s i.e. 1/10th .
i.e. least count/ resolution of equipment should be 1/10th of process Variation (10 data categories).
2. MS should be in statistical control
- Common cause variations only
- No special cause variation
3. Variability of MS < mfg. Variability
-For process control
4. Variability < specification limits (sl)
- for product control
Bias
Difference between the observed average of measurements and the true value (reference value) on the same characteristics on the same part.
A measure of the systematic error of the measurement system.
It is the contribution of the total error comprised of the combined effects of all sources of variation, known or unknown.
Stability (Drift)
The total variation in the measurements obtained with a measurement system-
on the same master or parts,
when measuring a single characteristic,
over an extended time period.
i.e. Stability is the change of bias over time
Stability (Drift)
CAUSES OF INSTABILITY-
Mechanical wear,
Electrical instability,
Changes in external environment,
Degradation in components with time
Quite Common with electrical based gauges, optical gauges(dependent on light sources, spectrophotometer etc.
Reproducibility (Between system variation)
The variation in the average of the measurements
made by different appraisers
using the same measuring instrument
when measuring the identical characteristic
on the same part.
