Monitors and Controls
Process Variability and
Every process varies. If
you try to bake ten cookies using the same mold, mix, and procedure,
none of them will come out of the oven identical. To a certain extent
they will all look similar, but no two of them will be exactly the same.
This inherent tendency of your cookies not to look identical is known as
process variability, while the ability of your baking
prowess to make your cookies stay within predictable limits of
similarity is your
Using the same process to
bake cookies over and over again will result in many batches of cookies
that look similar, but once in a while you'll get a batch that's quite
different. One time you may get a batch of darker cookies, and another
time you may get a batch of sweeter cookies. When this happens, the
cookies are outside the normal variability of the rest of your cookies,
so they are called
Something special outside of
your defined process happened that resulted in such outliers, e.g., you
probably set your oven too high or you inadvertently added more sugar.
Your process is said to have gone
are unpredictable and should be avoided. In processes that are more
complex than baking, the occurrence of out-of-control incidents can be
manufacturing for instance, a single misprocessing incident due to an
assignable cause can mean losses in the hundreds of thousands of
dollars. This is why in this industry, it is everybody's responsibility
to keep the process in control.
This can only be done if
there's a means of observing the process for any abnormalities and to
correct the situation before the process goes out of control. Observing
how the process takes place and behaves is known as
monitoring, while responding to changes to keep the process
within its normal variability is known as
Thanks to the
invention of statistics, nobody has to stare at a process all the time
to know how it is behaving. It is just necessary to look at how a
process looks like at specific
and a fair assessment of how the process generally behaves may be
achieved. In fact, it is not even necessary to scrutinize all aspects of
the process at these intervals. One simply needs to check the
process aspects that matter most.
monitoring consists of observing and
the critical parameters affecting a process at pre-defined, regular
intervals and recording the observations and results. The critical
parameters monitored are chosen in such a way that they collectively
represent the state of the entire process. Since monitoring costs
money, it is necessary to limit the number of parameters monitored to
the minimum required to ensure that the process is meeting the company's
way of reducing the parameters that need to be monitored is to look for
correlations between parameters. If the behavior of one parameter
can be reliably predicted from the behavior of another parameter, then
only one parameter has to be monitored. The process owner should
eliminate all redundancies in process monitoring.
results may just be jotted down on record books, but the simplicity of
this method has a big drawback. Numbers written on paper are difficult to analyze visually and
will not catch the attention of the process owner when an anomaly
arises. This is why many companies employ control charts
instead to record measurements from their monitors.
chart plots the measurement data for the parameter being monitored,
usually with time on the x-axis and the measurements on the y-axis.
These charts are special in the sense that they show the
behavior of the parameter in terms of the mean and variance of past
data. The variance is expressed in terms of upper and lower control
limits, which are three (3) standard deviations away from the mean of
the data distribution.
process monitoring, the process owners looks out for
in the control charts. For instance, any measurement outside the
control limits is an automatic cause for alarm, because this is an
outlier. Four (4) or more consecutively increasing or decreasing
points form a trend that is not normal, and therefore deserves
attention. Six (6) consecutive points on one side of the mean also
deserve investigation. When such abnormalities are observed, the process
owner must take an action to bring the process back to its normal
which is the means by which a process is kept
behavior, comes in many forms. It can be as simple as assigning a
person to monitor the progress of the process (e.g., watching the
cookies as they bake inside the oven) and responding in accordance with
the state of the process observed (turning off the oven when the cookies
are brown enough). Or it can be as complicated as computer-based
real-time monitoring of many parameters at the same time and automated
control of equipment based on what the parameter readings are.
cost and reliability play an important part in determining what process
controls to implement on your manufacturing line. Remember,
sophisticated computer systems need expensive maintenance, and a single
downtime can result in total breakdown of your process control system.
All things considered, many companies today (including the ones
from the semiconductor industry) prefer to use
Statistical Process Control
to keep their processes in check.
The ISO9000 Standard;
Metrology and Calibration
Books recommended for you:
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