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
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.