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Control loops explained

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Quality in Control explains the importance of control loops in the manufacturing process.

By definition control means a continuous sequence of measuring, comparing to a set-point and correcting. This sequence is called a control loop and in most cases, it has one input and one output. Typically each manufacturing process involves numerous control loops that deal with a particular parameter of production. 

Control loops can be divided into three integrated levels for better understanding. The control loops are explained here using the example of a bakery.

Process/ Machine Control Loop - Fast Loop

The least complex of all control loops, the fast loop controls the operation of the production machinery. It has a fast response to a change of the actual value because the feedback is usually the aspect that is controlled. Typical examples are the speed of a conveyor, the temperature of an oven or ensuring a piston reaches its stroke before returning. The fast loop is the most important control loop with the purpose of providing consistent manufacturing conditions.

On-Line Product Control Loop - Medium Speed Loop

The medium speed loop controls various aspects of the end-product itself by providing the set-point for one or several process control loops. Usually a much slower loop because of a common time-delay caused by the product travelling from the point of control to the point of measurement, a typical medium speed loop would be the measurement of the dough temperature at the end of the oven and the fact that one cannot respond to the real effect of the last control action until another dough piece has passed right through the oven. 

This control loop is crucial where fastest possible compensation for raw material or environmental changes is important to maintaining manufacturing tolerances. Since this loop measures 100% of the production, it is a good QA tool and can also provide significant savings by avoiding scrap and reducing giveaway.

Off-Line Quality Control - Slow Loop

The slow loop involves measurement of all critical parameters of the end product and usually cannot be performed for 100% of the production. It is therefore done in intervals ranging from 30 minutes to eight hours depending on the stability of the process. In its most basic form, it will allow the operator to make corrections to the set-point of the two faster loops and ensure the product is to specifications.

Given the right tools, this loop can tell the operator when things start to go adrift before they start producing scrap, despite its relatively slow speed. Using statistical analysis allows the operator to pick a trend or identify machine problems before it's too late and any tolerances are exceeded. 

The primary objective of quality control is not to satisfy the customer but to produce the best quality product with the highest efficiency. This requires the quality control to be done as efficiently and effectively as possible, using the data it produces for the analysis of the processes.

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