Frequency measurement

Operating mode up/down counter, if only track A is connected If only track A is connected, proper execution of the frequency measurement can only be ensured in the up/down counter mode. To enable the up/down counter set the bit in index 0x80n0:03 “Enable up/down counter”.

• The timeframe for the frequency calculation as well as the resolution can be parameterized in the CoE objects Frequency window (index: 0x80n0:11), Frequency scaling (index: 0x80n0:13), Frequency resolution (index: 0x80n0:15) and Frequency wait time (index: 0x80n0:17).
• The positive edges of track A are counted in the specified timeframe (see Frequency modes) and the next subsequent edge including the time until it arrives is counted. The waiting time for the edge can be set in the CoE object Frequency Wait Time (index: 0x80n0:17) (unit: ms) and is set as standard to 1.6 seconds. This is also the maximum value.
• The frequency is always specified as a positive number, irrespective of the sense of rotation.
• The size of the timeframe is 10 ms (default), but at the least the basic unit Frequency window base (index: 0x80n0:0F).
• This calculation is carried out in the terminal in free-running mode without reference to the distributed clocks system. It is therefore independent of the DC mode.
• EL5151: No frequency measurement is possible if the counter is blocked by the gate. In this case the period can be measured regardless.
• EL5151: A C or external reset restarts the frequency measurement. The last frequency value remains unchanged until a new frequency value is determined.
• The object Frequency window base (index: 0x80n0:0F) is used for switching the basic unit for the Frequency window between 1 µs and 1 ms, in order to adjust the time window for the measurement. The following maximum measuring windows are therefore possible:

• on expiry of the measuring window Frequency window (index: 0x80n0:11), the subsequent positive edge on track A is awaited, but at the longest for 1.6 s or the time from Frequency wait time (index: 0x80n0:17).
• The frequency is measured with different accuracies depending on the selected basic unit Frequency window base (index 0x80n0:0F) and the window size.

Frequency mode A

The measurement is automatically performed in frequency mode A if the window size is smaller than or equal to 600 ms.

• Basic unit 1 µs: all window sizes
• Basic unit 1 ms: up to 600 ms window size

Measurement sequence

• The measurement starts with a positive edge at track A. The current counter value and time (resolution: 100 ns) are stored.
• On expiry of the measuring window Frequency window (index: 0x80n0:11), the subsequent rising edge on track A is awaited, but at the longest for 1.6 s or the time from Frequency wait time (index: 0x80n0:17).
• The frequency is calculated from the edge difference and the actual elapsed time.

Frequency mode B

The measurement is automatically performed in frequency mode B if the window size is greater than 600 ms.

• Basic unit 1 ms: from 601 ms window size

Measurement sequence

• At the start of the measurement the time and the current position are stored with a resolution of 100 ns, irrespective of the current signal position.
• After the measurement the current position is stored irrespective of the current signal position.
• The frequency is calculated from the number of increments and the actual elapsed time.
• The frequency measurement therefore takes place with reduced accuracy.
• The larger the measuring window in relation to the basic unit, the more precise the frequency calculation.