This preview shows page 1. Sign up to view the full content.
Unformatted text preview: ting error are not always comparable, however. For example, a, < 1~~1 not unusual
for a continuous-batch mixing process. Suppose that
the concentration of constituent A is at 10% and is
continuously reduced towards a target of 5%. Batches
might be produced with concentrations of lO%, 7%)
and 4%. In this case, perhaps 1~~1 l%, but a spec=
trograph may measure with a,,, = .l%. Another example is a thermostat, which often provides u,,, < l&sl,
especially if it has a “dead zone” in its logic.
Alternatively, one may find a, > 1~~1.
For example, physical laws may make it possible to accurately
set gas pressure in a sealed cavity by setting gas temperature, but there may be no precise way to directly
Through In the design of experiments classes he teaches at
Alcoa, J. S. Hunter gives a rule of thumb for experiments on existing processes. For each control
TECHNOMETRICS, 0, .015 inches Internal, *The x, y, and z axes are used to refer to the part and the CNC machine. Knowledge
Experimentation Predicted effects
responses) external (guess) Feed rate (% of
nominal) 4.3 Proposed settings,
predicted effects -1%
panel) 85-115% Fixture height C. MONTGOMERY FEBRUARY 1993, VOL. 35, NO. 1 The a axis refers only to the machine. variable, low/high settings should be selected to cause
a predicted effect (main effect) for the “key” response variable equal to one standard deviation of
its variation in ordinary use, C~ (if there is “ordinary
use”). This is a large enough change in response to
have practical consequence and also large enough to
likely be detected if measurement error is negligible
and the experiment has enough runs. If the rule of
thumb is followed, every control variable has “equal
opportunity” to affect the response variable.
Naturally, it is harder to suggest such a rule for
immature processes. Moreover, other issues and constraints must be taken into account when settings are
discreteness of settings, process
constraints, ease of changing a setting, and so forth.
These are s...
View Full Document
This document was uploaded on 02/12/2014.
- Fall '14
- The Land