Sampling strategies order tracking Flashcards
which are the aquisition strategies
- manual start/stop
- start/stop linked to the intensity of the phenomenon (trigger on signal level)
when does the trigger start the aquisition
when a reference signal (trigger signal) exceeds a fixed (TL = trigger level) value with a fixed slope
which is a parameter often used to set the threshold level
rms
why is it usefull to have a common start/stop in different time records
to compare/correlate them
in which types of signal the starting time is important and in which ones it is not
important in deterministic signals, not imp in random signals
def of internal and external trigger
in internal trigger the trigger signal is one of the signal to acquire in external trigger the trigger signal is an additional signal which is somehow related to the physical phenomenon
why is a trigger necessary in deterministic phenomena
to synchronise the different transducers
how can we eliminate random noise from our signal
we do the average of many records of a deterministic phenomenon
what happens if i make the average of some signals but they are out of phase? how can i obtain the signals in phase?
1) i eliminate the noise (good) but i also destroy the deterministic signal (bad)
2)we need a trigger
which are the 2 main sampling strategies
asynchronous and synchronous sampling
what is asynchronous sampling
samples are collected at constant time intervals delta t
what is synchronous sampling (order tracking)
samples acquired synchronously with a given phenomenon
when is synchronous sampling usefull
when we have a periodic physical phenomenon related to the signal to acquire ( for instance with a rotating shaft the sampling must be synchronous with the angular speed of the rotor)
what we need to avoid aliasing
at least 2 sampling points per cycle
which are the 2 order tracking techniques seen at lesson
encoder and 1*rev reference
describe the encoder
-acquisition with constant angle interval
-this allows to compensate possible changes of the rotational speed omega
1*rev reference
-one reference per revolution (acquisition always starts in the same angular position)
-sampling frequency constant on the single rotor revolution (no compensation for possible changes of rotational speed)
