IFM Electronic ecomot mobile R 360 Serie Starterset-Handbuch Seite 144

Features of a PLC
An unfavourable signal shape does indeed occur in practical use. If, for
example, speed is to be monitored by evaluating the pulses of a puck, the
TRUE signal is short if the diameter of the puck is small.
An example is shown on the left side of Figure 3. Often metal screws are used
in a plastic disc. If the signals are not detected reliably, a metal strip can be
used. Its length should be 1/3 to 1/2 of the circumference. The optimum mark-
to-space ratio of 1:1 can thus be easily obtained.
Figure 3: Target Pucks for Rotational Speed Monitoring
Frequency Usually there are only higher frequencies if encoders with a high resolution are
inputs used. To be able to process higher frequencies the controllers have so-called
frequency inputs. This ensures that signals up to 50 kHz are reliably detected.
This is an exception from the usual cyclical program cycle.
Filtering The maximum frequency for standard inputs is limited since they have a low-
pass filter. Thus increased protection in the event of noise against interference
peaks is obtained. On the other hand the frequency inputs are more
susceptible.
Optimisation The experienced PLC programmer knows ways to optimise the cycle time for
time-critical processes. Since this manual is an introduction we will not discuss
this any further here.
Bus systems Using a bus system provides many advantages. In applications for mobile
machines CAN is used as a rule (
working with CAN). The time response, however, can be negatively affected. If,
for example, the input and the output are on separate CAN modules, the
response time increases by the time needed for the communication.
Time-critical pulses should therefore be put directly on inputs of the controller.
1-8
Training manual 2: Introduction into