Hempenius, Makarovié, Van der Weele, Tests of Restitution Instruments 
III. 5. Evaluation. 
When the dynamic system is applied to a sinusoidal* input, there are two distinctive 
distortions in the output. 
a. the periodical distortions R (systematic), and 
b. the stochastical distortions M (noise). 
The first group contains the disturbances classified under r, t and T, while to the 
second includes those denoted as k, l and L. 
Y 
Fig. 9. The combined periodical distortion 
Periodical distortions are consequences of the backlash, elastic deformations, time 
lags or their electric equivalents. They have characteristic effects on the crests and 
troughs of the sine* wave; therefore the amplitudes are effected systematically [6]. 
Stochastical fluctuations or the noise, are superimposed on the periodical distortions. 
Both characteristic distortions R and M can be evaluated by using the sinusoidal* 
targets as measuring scales. These can be brought into register with the plotted patterns 
due to the linear modulation. The periodical distortions R are the average discrepancies of 
the coinciding crests and troughs, while the noise M is represented by the random fluctu 
ations. 
The prevailing value R or M can be read directly on the reproduced graduation of 
amplitudes (or the frequency) at the point where the regular undulations disappear. 
The periodical distortions should be investigated separately for the two perpendicular 
directions (usually X and Y). The effect in an arbitrary direction is according to fig. 9: 
r a = r x sin a + r y cos a, 
n 
and the average value: 
0 
(9a)