Table of Contents

LowpassFilter

Namespace
ZCore
Extends
Inherited Methods
FUNCTION_BLOCK LowpassFilter EXTENDS ZCore.FirFilter

In signal processing, a finite impulse response (FIR) filter is a filter whose impulse response (or response to any finite length input) is of finite duration, because it settles to zero in finite time. FirFilter implements main concepts of FIR Filters. Specializations implement the InitalizeCoefficents, which in turn controls the bahavior of the filter.

To use the FIR Filter

  • Call Update every cycle and submit a value
  • To use the filtered value use the Output method.

Frequencies below the cut-off frequency freq can pass through the filter. This filter is commonly used for smoothing a signal. The following code shows an example where a signal (sinus signal with amplitude 1 and frequency 1Hz) is noisy (50Hz and 100Hz noise). A lowpass filter with 8000 samples is applied to the signal filtering out the noise.

Lowpass Filter example
Lowpass Filter example
lowpassFilter : ZCore.LowpassFilter(cycletime:=0.001, samples:=8000, freq:=20);
signal, t, filtered : LREAL;

//--------------------------------------------------------

signal := SIN(1 * 2 * PI * t)  + 0.1*SIN(50 * 2 * PI * t) + 0.03*SIN(100 * 2 * PI * t);
t := t + 0.001;
lowpassFilter.Update(signal);
filtered := lowpassFilter.Output();

Constructor

FB_init 

METHOD FB_init (
 [input] bInitRetains : BOOL,
 [input] bInCopyCode : BOOL,
 [input] cycletime : LREAL,
 [input] samples : DINT,
 [input] freq : LREAL) : BOOL

Inputs

bInitRetains BOOL

if TRUE, the retain variables are initialized (warm start / cold start)

bInCopyCode BOOL

if TRUE, the instance afterwards gets moved into the copy code (online change)

cycletime LREAL

cycletime in seconds, this should be set to the cycle time of the PLC and the cycle that Update is called with, respectively.

samples DINT

number of samples, the actual sample count is capped by ParameterList.MovingAverageMaxBufferSize.

freq LREAL

Returns

BOOL

Methods

InitalizeCoefficents 

METHOD PROTECTED FINAL InitalizeCoefficents ()

Initializes the coefficent of the FIR filter for a lowpass