Aliasing with data sampling

Aliasing with data sampling

This is very familiar to control engineers. Put in very simple language we have a particular print interval and this means we are sampling the results of the system at a particular frequency. If there is a phenomenon occurring in the results with another frequency, in order to see this phenomenon we must have a sampling frequency which is significantly higher.

In our example, the results are sampled every 0.1 s or at 10 Hz.

Procedure

1. Plot the rotary speed of system B.

Figure 104: Rotary speed of system B

You see that the shaft is turning at 600 rev/min at the end of the simulation. This corresponds to 600/60 = 10 Hz. The data has been rather artificially created so that the print interval frequency coincides with the frequency of rotation.

2. Try altering the constant torque to 610 Nm and to 590 Nm.

We still cannot properly see the phenomenon we are looking for: the output of the modulo block. To get a meaningful graph, we should sample at a significantly higher frequency than is contained in the data you are plotting.

3. Try changing the print interval to 0.01 s (corresponding to 100 Hz) and to 0.001 s (1000 Hz).

An alternative is to select Discontinuity printout in the Run parameters dialog box.