NaN values

In some cases error handling by error codes is not the best choice, in particular if operations return undefined values on account of unusual, but in principle possible input data, or if values are to be excluded from the processing.

The IEEE 754 standard defines symbolic values of the category NaN (Not a Number) for these purposes. In the following situations these are generated or taken into account in the TwinCAT 3 Condition Monitoring library:

If insufficient valid values are available for a statistical evaluation, the result is output with NaN.
If certain values are to be excluded from a statistical evaluation, this is achieved by the function block ignoring NaN values at the input.
If the input vector of the frequency analysis of a time series contains one or more NaN values, the entire output vector is filled with NaN. This property can be used to mark results as undefined in case a gap in the input signal leads to jumps in the time series. Because it is not possible to calculate correct spectra on the basis of fragmented time series.

The ability of a function block to create NaN values is noted in its description.

The following points rank among the main features of NaN values:

All arithmetic operations that use NaN as input data return NaN as the result.
All relational operators =, !=, > < >= <= always return the value False if at least one of the operands is NaN.
The standard C functions isnan() or _isnan() or the PLC functions LrealIsNaN() and RealIsNaN (Tc2_Utilities library) return the value True if the argument has the value NaN.
The expression isnan(a) is equivalent to the expression !(a == a) or NOT(a = a).

The fact that NaN values reproduce themselves when used in further calculations is advantageous in that invalid values cannot be overlooked.

CAUTION

Software malfunctions

NaN values can lead to potentially dangerous malfunctions of the software concerned!

Only use expressly approved NaN values, specifically as control values in motion control and drive control functions.

Notice

Machine damage due to floating point exceptions (FP exceptions)

Comparisons with NaN values can lead to an exception, which result in the runtime stopping and potentially machine damage.

Switch off the FP exceptions if NaN values are to be used and processed in the application.

Further explanations on the option to switch the FP exceptions off and on can be found in chapter Task settings.