Control Loops Explained

To understand measurement and control it is worthwhile to reflect on the subject of complete production control. This is especially important if one starts to look at data-logging and analysis of any measurement data.

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. Because there are so many control loops it helps our understanding if we split them into 3 integrated levels. We use the example of a bakery to help explain them.

(While analysing the control loops, it is important to remember that some of them involve humans. Fully automatic production control hardly exists !)



Process / Machine Control Loop - Fast Loop

This is the least complex and controls the operation of the production machinery. It has a fast response to a change of the actual value because the feed-back 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. It is the most important control loop with the purpose of providing CONSISTENT MANUFACTURING CONDITIONS.


On-Line Product Control Loop - Medium Speed Loop

Like Quality Control this loop controls aspects of the end-product itself by providing the set-point for one or several process control loops. It is usually a much slower loop because of a common time-delay caused by the product travelling between the point of control to the point of measurement. A typical example is the measurement of the dough temperature at the end of the oven and the fact that we cannot respond to the real effect of our 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. Because it measures 100 % of the production, it is a good QA tool and can also provide significant savings by avoiding scrap and reducing give-away.


Off-Line Quality Control - Slow Loop

Often considered a burden, it is the most important process for letting you know what happens out on the factory floor. It is a slow loop because it 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 from 30 minutes to 8 hours depending on the stability of the process.

In its most basic form it will allow you to make corrections to the set-point of the two faster loops above and thus ensure product is to specifications.

However, given the right tools, it can tell you when things start to go adrift before you start producing scrap, despite its relatively slow speed. This is because measuring all critical aspects allows you to see correlations between the movement of one parameter against another. Using statistical analysis allows you to pick a trend or identify machine problems before it's too late and any tolerances are exceeded.  

The purpose of Quality Control is not to satisfy your customer but to PRODUCE THE BEST QUALITY PRODUCT WITH THE HIGHEST EFFICENCY. Obviously this also means doing your QC as efficiently and effectively as possible and using the data it produces for the analysis of your processes.


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