HP 5528A

Mo del 552 8A
App e ndi x B: M eas ur eme nt Co ns id erations
YOll Cil n think of rep ea t. lbilit y in a similar way when making m eas ur ements with the l ase r.
Op era tor skill, m ac hine stabilit y, m ac hine geome tr y, and the environment affect repeatabilit y.
ACCURACY is h ow closely the m eas ur eme nt s agree to a given standard f or the m easu remenl.
Repeatability. by co
mp arison, is h ow d ose the measureme nt s agree to each othe r.
In our exa mple of the mar ks man, Ihe cl ose n ess of his shots to thc bull' s e ye is th e me as ur e of his
acc ur acy. It is p oss ible for all his shots to have been close together (rep ea table), but to have been
fa r from the b ull' s eye. Rep ea ta bilit y and acc ur acy are not thc same.
Accura
cy in lin ea r posit io ning is the diff erence between the a ve ra ge p os iti on of a gro up of
m easu rements and the locat io n of the target posit ion for th ose mea su rement s. Acc ur ate paris are
exac
tl y like the dr awing for th os e pa rt s. Repe at able parts ar e exactly lik e each othe r. A batch of
pa ri S, all rejected for the s am e flaw, are rep ea table but not accurat e.
R ESO LU TION is the small es t increment of sc al e. In the ta rget exampl e, resolut io n is the distance
between the r
in gs in the ta rgel.
FACTORS AFFECTING ACCURACY AND PRECISION
Th e accura cy and pr ecision of a m eas ur ement ar e influ enced by many fa ctor s, which can be
se parated into tw o grou ps -
• M ac hine-related, an d -
• Se t- up -related.
Ma chin e- related fa ctors in cl ude -
• M ac hine geom et r y,
• M achine sca le accura cy and pr ecision,
• M ac hine env ir o nm ent, and -
• M ac hine r es ponse to the changing environm ent.
One
se t- up -related error Is Abbe Error.
Machine-Related Factors
A m ac hine calibrat io n is more compl ex than checking p os itioning accur acy and pr e ci sion. Th e
accu racy and pre Cision of the work a m ac hine c an be ex pected to turn out depe nd s on -
• Th e machine's enviro nm ent ( in cl udin g te mp erature and vibrati on ), -
• The m ac hine's res pon se to chan gi ng operating and enviro nm ental co nd iti on s. ( Ma ch in e
stabilit y) -
• The accura cy an d pr ecision of Ih e ma c hin e's geome tr y, and -
• Th e acc ur acy and precision of the ma chine's sca le(s ).
E ac h of th ese f ac tors is equa ll y important. Me as ur ements of acc ur acy and pr ecision have
meaning only when the environment and geome
tr y ar e kn ow n.
B-3