Digital filters

NAVIGATION: Closed-loop control > Speed control > Digital filters

Digital filters

Setting of filter combinations

To filter any noise on the actual speed value, or to damp resonance frequencies, various filter combinations can be used. A range of filter variants are available. The coefficients of the transfer function are automatically determined as soon as the values for the middle and limit frequency and the width have been entered.

Bild: Selection of various digital filters

Bode diagrams PT1 to PT4	Phase response

Bild: Bode diagrams PT1 to PT4

Parameters

P. no.	Parameter name	Function
P 0325	CON_SCON_FilterFreq	Limit frequencies
(0)	1 - 8000 Hz	Middle, limit frequency
(1)	1 - 8000 Hz	Width
(2)	1 - 8000 Hz	Middle, limit frequency
(3)	1 - 8000 Hz	Width
P 0326	CON_SCON_FilterAssi	Filter selector
(0)	Off	No filter active
(1)	USER	Manual writing of filter coefficients
(2)	Notch	Selection of a notch filter with the limit frequency from P 0325(0) CON_SCON_FilterFreq and the bandwidth from P 0325(1).
(3)	Notch_Notch	Selection of a notch filter with the limit frequency from P 0325(0) and bandwidth from P 0325(1) in series with a notch filter with the limit frequency fromP 0325(2) and bandwidth fromP 0325(3)
(4)	Notch_PT1	NOTCH_PT1(4) and NOTCH_PT2(5): A notch filter with the blocking frequency in P 0325(0) and bandwidth in P 0325(1) in series with a low-pass filter with limit frequency in P 0325(2).
(5)	Notch_PT2
(6)	PT1	PT1(6), PT2(7), PT3(8), PT4(9): A low-pass filter with the limit frequency in P 0325(2)At lower frequencies higher-order filters (PT3, PT4) should not be used.
(7)	PT2
(8)	PT3
(9)	PT4
P 0327	CON_SCON_FilterPara	Coefficients of the digital filter
(0)	b0	Filter coefficients
(1)	b1
(2)	b2
(3)	b3
(4)	b4
(5)	a1
(6)	a2
(7)	a3
(8)	a4

Tabelle: Parameters to set the filter constants

Vorgehensweise: FFT signal analysis

Scope setting:
isq (unfiltered, torque-forming current) Set shortest sampling time Create scope plot without notch filtering
On the oscilloscope click the "Mathematical functions" > FFT (Fourier analysis) icon. From the following pop-up menu choose isq. Disturbance frequency is displayed.
"Select filter": Select filter
Enter middle/limit frequency
Width: Enter the bandwidth of the limit frequency; the width has no effect when using PTx filters
Create scope plot with notch filtering

FFT without filtering	FFT with filtering

Tabelle: FFT transformation

NOTE:

Note that the filters not only have an effect on the amount but also on the phase of the frequency response. At lower frequencies higher-order filters (PT3, PT4) should not be used, as the phase within the control bandwidth is negatively influenced.

The coefficients can also be specified directly via parameter P 0327 CON_SCON_FilterPara. They take effect directly, so changing them is only recommended when the control is switched off.

NOTE:

A large bandwidth results in less attenuation of the limit frequency.