MSA Measurement Systems Analysis

The purpose of the MSA Manual is to provide a guide to assess the quality of a measurement system. This tool, like the APQP, PPAP, AMEF and SPC is considered part of the Core Tools of the automotive sector and is a requirement of the IATF 16949 technical specification.

Measurement systems
The MSA Manual developed by the AIAG, deals with measurement systems, understood as the set of instruments or gages, patterns, operations, methods, devices, software, personnel, environment and assumptions used to quantify a unit of measurement or prepare the evaluation of a property or property to be measured. It is the complete process used to obtain measurements.

Quality of measurements
The basic concept of the MSA is the quality of the measurements, which are the statistical properties of multiple measurements obtained from a measurement system operating under stable conditions.

Bias and variance.
They are the statistical properties most commonly used to characterize the quality of the data. Bias refers to the location of the data in relation to the reference value (master). The variance refers to the dispersion of the data.

One of the most common reasons for the low quality of the data is the excessive variation of the measurement system.

A significant proportion of this variation may be due to the interaction of the measurement system and its environment. For example, a system used to measure the volume of liquid in a tank may be sensitive to changes in ambient temperature. Then, the changes in volume detected may be due to changes in the ambient temperature and volume changes.

If the variation due to environmental factors is very large, it can mask the variation in the process, and in that case the data from the measurement system is not useful. One of the most important parts of the study of measurement systems is aimed at monitoring and controlling their variation.

This means, among other things, that you must learn how the measurement system interacts with your environment so that only data of acceptable quality are generated. This is very similar to the approach applied to understand and control the variation of a manufacturing process.

Therefore, a measurement process can be seen as a manufacturing process that produces numbers (data) as results.

Seeing a measurement system in this way is useful because it allows us to bring all the concepts, philosophy and tools that have already been proven to be useful in the area of statistical process control.

During the measurement process the variation of the process is detected, in order to have knowledge of:

  • What the process should be doing
  • What may be wrong
  • What the process is doing

CONCEPTS

Measurement
Assigning values to material objects to represent the relationships between them with respect to a particular property.

Gage
Any device used to obtain measurements. It is often used to refer specifically to flooring devices. Includes devices pass / fail.

Standard

  • Base accepted for comparison
  • Acceptance requirements
  • Reference value
  • Known value accepted as true value, under established uncertainty limits.

Discrimination, readability, resolution
The smallest readable unit or limit of detection. It is the smallest scale of a measuring instrument.

Effective resolution
Sensitivity of a measurement system with respect to the variation of the process for a particular application.

Reference value
Accepted value of an artifact. It requires an operational definition. It is used as a substitute for true value.

True value
Real value of an artifact. It is unknown and can not be known.

Accuracy
“Proximity” to the true value or to an accepted reference value.

Bias
Difference between the observed average of measurements and the reference value. It is a component of the systematic error of the measurement system.

Stability
Change of bias over time. A stable measurement process is in statistical control with respect to location.

Linearity
Change in the bias over the normal operating range.

Precision
Closeness to each other of repeated readings.

Repeatability
Variation in the measurements obtained with a measuring instrument when it is used several times by the same evaluator, measuring the same characteristic in the same part. It is referred to as the equipment variation, capacity or potential of the instrument or system’s own variation.

Reproducibility
Variation in the average of measurements made by different evaluators using the same measurement equipment, in the same characteristic and in the same part. For qualification of products and processes, the error may come from the evaluator, the environment or the method. It is referred to as a variation of the evaluator.

Repeatability and Reproducibility Studies (Gage R & R – GRR).
Combined estimation of the repeatability and reproducibility of the measurement system. It is a measurement of the capacity of the system. Depending on the method used, it may or may not include the effects of time.

Three methods are accepted to develop the GRR:

  • Rank
  • Averages and rank
  • ANOVA

Measuring system capacity
Short-term estimation of the variation of the measurement system. For example, GRR, including graphics.

Performance of the measurement system
Long-term estimation of the variation of the measurement system. For example, method of control letter. Consider the total variation.

Sensitivity
The smallest input that results in a detectable output signal. Response of a measurement system to changes in the measured characteristic. It is determined by the design of the gage (discrimination), by the inherent quality of the equipment (manufacturer), maintenance and operating conditions.

Consistency
The degree of change in repeatability over time. A consistent measurement process is within statistical control with respect to width (variability).

Uniformity
It is the change in repeatability over the normal range of operation. It is the homogeneity of repeatability.

Measurement uncertainty
It is an estimate of the range of values in which the true value is believed to be contained. It is used to describe the quality of the measured value.

Standards and traceability
Most industrialized countries maintain an institution that represents the highest level of authority in metrology. They usually provide measurement services and maintain measurement standards to support the industry in having traceable measurements. These national institutions maintain relations between them and establish Mutual Recognition Arrangements (MRAs).

Traceability
Definition of ISO: It is the property of a measurement or the value of a standard by which it can be related to established references, usually national or international standards through an uninterrupted chain of comparisons, all of which have established uncertainties.

Traceability can be linked to reference values or “agreed standards” between the client and the supplier.

Not all organizations have metrology laboratories within their facilities, and depend on independent laboratories for traceability calibration services. In these cases, it must be ensured that the external laboratory is accredited. According to ISO / IEC 17025.

Calibration systems
It is a set of operations that establish, under specific conditions, the relationship between a measurement device and a traceable standard of known reference value and uncertainty. The calibration may also include steps to detect, correlate, report or eliminate by adjustments any discrepancy in the accuracy of the compared measuring device.

Each calibration event includes all the necessary elements, including: standards, measurement equipment to be verified, calibration methods and procedures, records and qualified personnel. The calibration system is part of the quality management system of an organization and must be included in the requirements of internal audits.

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