MES 3.0

Statistical Process Control (SPC) is a technique of quality control which employs statistical methods to monitor and control a manufacturing process.

The SPC module has functionality to collect samples, analyze test data, check for rule violations to help to ensure that a process operates efficiently, producing more specification-conforming products with less waste.

What Is Quality

Quality is defined as a measure of excellence or a state of being free from defects, deficiencies and significant variations.

In manufacturing, quality is defined as a measure of excellence or a state of being free from defects, deficiencies and significant variations. It is brought about by strict and consistent commitment to certain standards that achieve uniformity of a product in order to satisfy specific customer or user requirements. Quality products help to maintain customer satisfaction and loyalty and reduce the risk and cost of replacing faulty goods.

Your customers expect you to deliver quality products. If you do not, they will quickly look for alternatives. Quality is critical to satisfying your customers and retaining their loyalty so they continue to buy from you in the future. Quality products make an important contribution to long-term revenue and profitability. The quality of a product is very important for a stable manufacturing industry. The defected or the faulted goods must be sorted out from the good ones. So it is extremely important to maintain the quality of any manufacturing product. This is why we have introduced the SPC module. This module ensures the quality of items and increase the productivity of efficient products.

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Quality Versus SPC

Quality is very broad and includes much more than just SPC. SPC is a tool in the Quality process.

Although Quality and SPC are sometimes used interchangeably, they are different. Quality is very broad and includes much more than just SPC, while SPC can be considered a tool in the Quality process.

A quick example may help to point out the difference. If product in the warehouse is going bad over time, then a process has to start to narrow in on the cause. It will involve brainstorming, perhaps creating fishbone diagrams to determine the possible causes. In the case of an off-color product, it could be rust building up in pipes, chemical formulation changes, or different raw materials being used. This part of the example refers to quality. Unlike SPC, quality requires more than installing software, collecting samples and analyzing the results. 

Once the most likely causes of the off-color product have been determined, SPC can be used to monitor the attributes and narrow down and isolate the cause. It may be determined that when the pH  of a sub-ingredient falls out of a certain range, the stability of the product color is degraded. With this knowledge, SPC can be used to monitor the pH so that if it falls outside of range, it can be corrected quickly. This prevents a bigger problem that may arise when the product stays in the warehouse for a period of time.

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SPC Variation

SPC variation includes gathering sample data in three main categories: Environment, Materials, Methods.

Some examples:

  • Environment
    • temperature
    • humidity
    • air-flow
  • Raw Materials

    • photo of raw material
    • pH
  • Methods
    • manual 
    • tag

When a manufacturing process involves complex machines to complete production, a temporary malfunction or a breakdown in an intricate piece of equipment can affect the manufacturing process. Identifying means of improving efficiency of all working parts of production promotes a continual and more efficient operation. Positioning of equipment and the personnel required to operate machines can also affect production.

Environment
The climatic conditions to which the commodity was exposed before receipt; what conditions are likely to occur whilst commodities are held in storage.

Raw materials
The availability of materials and the development of new, hi-technology materials will have an influence on the final design of a product. Quality of the finished product always depend on the quality of raw materials.

Methods
Quality also depends on the methods used to produce it and the chemicals added during production. To maintain high standards of quality, companies are investing in new machines and following new procedures and methods these days.

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SPC Samples

Sampling is the process of selecting units from a population of interest so that by studying the sample we can generalize our results back to the population from which they were chosen.

Every hour we may pull out some samples to represent the population. We shall define the sample size, which is basically equal to the number of measurements. The Sepasoft SPC module defines this with the following.

  • Interval Type: Manual, shift-based or time-based. When set to time-based interval and Auto Approve, sampling starts automatically. Using a shift- or time-based Interval Type and Auto Approve set to False is an example of semi-automatic interval. 
  • Measurement Count: How many measurements defines your sample size.

Attribute List

  • Datatype: Integer, Real, Boolean, Inspected Count, Nonconforming Count, Non-conformity Count.
  • Chart Type: value charts, attribute charts, analysis charts.
  • Order, Units, Weight, Default Value, Min/max values.


See:


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When the SPC module with the core Production module are installed, you access SPC settings in the Sample Definition Manager component:

  • Interval Type: when to sample. This is configured in the Sample Definition Manager.
Configure the Interval Type to specify when a sample it taken in Sample Definition Manager > Locations.



Location Settings

  • Interval Type
  • Tag
  • Coming Due
  • Interval
  • Duration
  • Enabled
  • Auto Approve

Location Settings exist for Locations that have been configured in the core Equipment Manager installed with the Production module.  You select location(s) in the Location Tree to view/edit settings.




  • Measurement Count is where you set the sample size and defines the number of measurements taken in a Sample.

This is configured in the Sample Definition Manager under, Attribute List > Measurement Count.

Editing the Measurement Count to set the Sample Size for a sample definition in the SPC component: Sample Definition Manager > Attribute List.


Example 1 Sample includes 3 measurements 

Measurement Count

SPC Values and Attributes

Attributes are added to Sample Definitions.

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SPC Standard Deviation

In statistics, the standard deviation (SD, also represented by the Greek letter sigma, σ for the population standard deviation or s for the sample standard deviation) is a measure that is used to quantify the amount of variation or dispersion of a set of data values. In other words it is the amount that we are deviating.

Standard deviation of sample means is calculated with the following equation:

where σx is the standard deviation of individual measurements and n is the sample size. The Upper (UCL) and Lower (LCL) control limits are calculated by the following equations:

where the z value is the number of standard deviations (sigmas) from the mean to put the control limits.

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