Application for frequency measurement (EL1x1, 10 µs typ.)

Due to their fast input filters (typ. 10 µs, 10..50 µs), terminals from the EL1x1x series can be used for frequency measurement. The filter time refers to the time over which an external signal with rated voltage or zero signal level must be present at the terminal point until it reaches the evaluation unit via the input circuit.

The maximum limit frequency depends on the following factors:

• EtherCAT cycle time
• Filter time in the input circuit
• Asymmetry of the filter time for rising and falling edge
• Duty factor for input signal
• Edge steepness of the input signal

Due to the production-related fluctuation range of the filter time the maximum limit frequency may be lower than the upper limit defined by the cycle when the EL1x1x is used for frequency measurement.

Example:

• Task cycle: 100 µs
• Actual filter time for rising edge: 10 µs
• Actual filter time for falling edge: 50 µs
• Duty factor 1:1

Derivation:

• Limit frequency from cycle time: 5000 Hz; depending on the cycle a half-period must be detected
• Asymmetry of the filter time: 40 µs
• Therefore minimum detectable half-period: 100 + 40 = 140 µs
• Therefore maximum detectable limit frequency: 3500 Hz

If detection of higher frequencies is required, it is advisable to either use an adequately fast input circuit (e.g. EL1202), or the input signal must compensate the actual asymmetry in the terminal through a duty factor not equal to 1:1.