FB_CMA_ZoomFFT

Efficient calculation of spectral values over a defined frequency band

The function block FB_CMA_ZoomFFT calculates a section (frequency band) of the Fourier transforms of the real-valued input signal x[n] applied to the function block. The result of the function block is identical for the frequency band under examination to the result from FB_CMA_RealFFT, but with correspondingly lower computing effort.

The frequency band to be calculated (the "zoom") is defined by a centering frequency fCenterf_c and a decimation factor nDecimationFactorD. The decimation factor must be a power of 2 (2, 4, 8, 16, ...). The frequency band is then defined as:

The frequency resolution Δf in the frequency band remains as one would expect with a normal FFT over the entire spectrum. The following applies:

Sampling theorem: The highest frequency of a component in the input signal should be at the most around half of the sampling rate of the input signal so that aliasing effects are avoided.

NaN occurrence

If the input vector contains one or more NaN (Not a Number) values, the entire output sector is filled with NaN. See separate section for further information on NaN values.

	Handling of NaN values If the situations described above, which lead to NaN values, cannot be ruled out or safely neglected, the application program must be able to handle these error values.

Behavior when processing multi-channel input data

When processing several channels (nChannels > 1), there is a possibility of each channel having different return values. In this case, return values can be queried separately on the function block. If the results from one or more channels are impermissible, but not all channels, the value on the function block corresponds to eCM_InfRTime_AmbiguousChannelResults. If the results of all channels are impermissible, then the value on the function block corresponds to eCM_ErrRTime_ErrornousChannelResults.

A list of return values of all channels can be queried using the method GetChannelErrors().

Sample implementation

A sample implementation is available under the following link: Zoom FFT

Inputs and outputs

The input and output buffers correspond to one of the following definitions (input / output shape). The variable parameters are part of the function block input stInitPars.

Versions	Input buffer (MultiArray input stream) element type, number of dimensions, dimension sizes	Output buffer (MultiArray output stream) element type, number of dimensions, dimension sizes
Standard variant (`nChannels = 1`)	`LREAL, 1,`nFFT_Length	`LCOMPLEX, 1,`nFFT_Length/(2*nDecimationFactor+1)
Standard variant (`nChannels = 1`)	`LCOMPLEX, 1,`nFFT_Length	`LCOMPLEX, 1,`nFFT_Length/(2*nDecimationFactor+1)
Multichannel variant (`nChannels > 1`)	`LREAL, 2,`nChannels `x nFFT_Length`	`LCOMPLEX, 2,`nChannels x nFFT_Length/(2*nDecimationFactor+1)
Multichannel variant (`nChannels > 1`)	`LCOMPLEX, 2,`nChannels `x nFFT_Length`	`LCOMPLEX, 2,`nChannels x nFFT_Length/(2*nDecimationFactor+1)

VAR_INPUT
    stInitPars       : ST_CM_ZoomFFT_InitPars;           // init parameter
    nOwnID           : UDINT;                            // ID for this FB instance
    aDestIDs         : ARRAY[1..cCMA_MaxDest] OF UDINT;  // IDs of destinations for output
    nResultBuffers   : UDINT := 4;                       // number of MultiArrays which should be initialized for results (0 for no initialization)
    tTransferTimeout : LTIME := LTIME#500US;             // timeout checking off during access to inter-task FIFOs
END_VAR

Input parameters

The input parameters of this function block represent initialization parameters and must already be assigned in the declaration of the FB instance! (Alternatively: Init() method). They may only be assigned once. A change at runtime is not possible.

stInitPars: Function block-specific structure with initialization parameters of the type ST_CM_ZoomFFT_InitPars. The parameters must correlate to the above definition of the input and output buffers.
nOwnID: Identifies the function block instance with a unique ID. This must always be greater than zero. A proven approach is to define an enumeration for this purpose.
aDestIDs: Defines the destinations to which the results are to be forwarded by specifying the IDs of the destinations. The definition of the output buffer (as described above) must correlate to the definition of the input buffer of each selected destination.
nResultBuffers: The function block initializes a Transfer Tray Stream with the specified number of MultiArray buffers. The default value is four.
tTransferTimeout: Setting of the synchronous timeout for internal MultiArray forwardings. See section Parallel processing.

Output parameters

VAR_OUTPUT
    bError         : BOOL;                           // TRUE if an error occurs. Reset by next method call.
    hrErrorCode    : HRESULT;                        // '< 0' = error; '> 0' = info; '0' = no error/info
    ipErrorMessage : I_TcMessage := fbErrorMessage;  // Shows detailed information about occurred errors, warnings and more.
    nCntResults    : ULINT;                          // Counts outgoing results (MultiArrays were calculated and sent to transfer tray).
END_VAR

bError: The output is TRUE if an error occurs.
hrErrorCode: If an error occurs, a corresponding error code of the type HRESULT is output. Possible values are described in the List of error codes.
ipErrorMessage: Contains more detailed information on the current return value. Refer here to the section Error description and information. This special interface pointer is internally secured so that it is always valid/assigned.

Methods

Call():

The method is called each cycle in order to apply the algorithm to the current input data. The function block waits for input data if the method indicates neither new results nor an error. This is a regular behavior in the process of the analysis chain.

Return value: If an error occurs, a corresponding error code of the type HRESULT is output. Possible values are described in the List of error codes.

METHOD Call : HRESULT
VAR_OUTPUT
    bNewResult   : BOOL;       // TRUE every time when outgoing MultiArray was calculated and sent to transfer tray.
    bError       : BOOL;       // TRUE if an error occurs.
    hrErrorCode  : HRESULT;    // '< 0' = error; '> 0' = info; '0' = no error/info
END_VAR

bError: The output is TRUE if an error occurs.
hrErrorCode: If an error occurs, a corresponding error code of the type HRESULT is output. Possible values are described in the List of error codes. This output is identical to the return value of the method.

	If a timeout occurs or no MultiArray buffer is available for the result, then neither the input data nor the result data are lost. They are forwarded on the next call.

Configure():

The parameter fCenter can be adjusted at runtime by calling this method. Here, the FFT index at the frequency f is calculated to k = f * fSampleRate / nFFT_Length. If f is not an integer multiple of the frequency resolution fSampleRate / nFFT_Length, the spectral components are divided into two consecutive spectral values.

The corresponding PLC array must be defined as follows. The Configure() method can also be used for a new configuration with a different set of arguments.

Return value: If an error occurs, a corresponding error code of the type HRESULT is output. Possible values are described in the List of error codes.

METHOD Configure : HRESULT
VAR_INPUT
    pArg     : PVOID;    // pointer to array of arguments
    nArgSize : UDINT;    // size of arguments buffer in bytes 
END_VAR

The input buffers correspond to one of the following definitions (input shape). The variable parameters are part of the function block input stInitPars.

Versions	Input buffer (MultiArray input stream) element type, number of dimensions, dimension sizes
Identical configuration of all channels	`LREAL, 1,`1
Channel-specific configuration (`nChannels > 1`)	`LREAL, 1,`nChannels

METHOD Init : HRESULT
VAR_INPUT
    stInitPars     : ST_CM_ZoomFFT_InitPars;           // init parameter
    nOwnID         : UDINT;                            // ID for this FB instance
    aDestIDs       : ARRAY[1..cCMA_MaxDest] OF UDINT;  // IDs of destinations for output
    nResultBuffers : UDINT := 4;                       // number of MultiArrays which should be initialized for results (0 for no initialization)
END_VAR

Init():

This method is not usually necessary in a Condition Monitoring application. It offers an alternative to the function block initialization. The Init() method may only be called during the initialization phase of the PLC. It cannot be used at runtime. You are referred to the use of an FB_init method or the attribute 'call_after_init' (see TwinCAT PLC reference). In addition, this facilitates the function block encapsulation.

The input parameters of the function block instance may not be assigned in the declaration if the initialization is to take place using the Init() method.

Return value: If an error occurs, a corresponding error code of the type HRESULT is output. Possible values are described in the List of error codes.

stInitPars: Function block-specific structure with initialization parameters of the type ST_CM_ZoomFFT_InitPars. The parameters must correlate to the above definition of the input and output buffers.
nOwnID: Identifies the function block instance with a unique ID. This must always be greater than zero. A proven approach is to define an enumeration for this purpose.
aDestIDs: Defines the destinations to which the results are to be forwarded by specifying the IDs of the destinations. The definition of the output buffer (as described above) must correlate to the definition of the input buffer of each selected destination.
nResultBuffers: The function block initializes a Transfer Tray Stream with the specified number of MultiArray buffers. The default value is four.

PassInputs():

As long as an FB_CMA_Source instance is called and signal data are thus transferred to a target block, all further function blocks of the analysis chain have to be called cyclically as explained in the API PLC Reference.
Sometimes it is useful not to execute an algorithm for a certain time. For example, some algorithms should be executed only after prior training or configuration. The function block must be called cyclically, but it is sufficient for the data arriving at the function block to be forwarded in the communication ring. This is done using the PassInputs() method in place of the Call() method. The algorithm itself is not called here, and accordingly no result is calculated and no output buffer generated.

Return value: If an error occurs, a corresponding error code of the type HRESULT is output. Possible values are described in the List of error codes.

METHOD PassInputs : HRESULT
VAR_INPUT
END_VAR

GetChannelErrors():

The method enables the querying of a list of the channel-specific return values when processing several channels (nChannels > 1). A call is useful in the case that the return value of the function block corresponds to one of the values eCM_InfRTime_AmbiguousChannelResults or eCM_ErrRTime_ErrornousChannelResults.

Return value: Information on the reading process of the list of error codes. The value is set to TRUE if the query was successful, otherwise to FALSE.

    METHOD GetChannelErrors : BOOL
VAR_IN_OUT
    aChannelErrors : ARRAY[*] OF HRESULT;
END_VAR

aChannelErrors: Error list of the type HRESULT of the length nChannels.

Similar function blocks

The function block FB_CMA_RealFFT calculates the Fast Fourier Transform for real-valued input signals.

The function block ST_CM_SparseSpectrum_InitPars calculates individual, configurable magnitude and spectral values.

Requirements

Development environment	Target platform	PLC libraries to include
TwinCAT v3.1.4022.25	PC or CX (x86, x64)	Tc3_CM, Tc3_CM_Base