Rockwell-automation 1771-IFE , D17716.5.90 USER MNL. ANALOG INPUT Modu Instrukcja Użytkownika Pobierz pdf (Strona 33)

Module Configuration

Chapter 4

4-4

The module has hardware-based high frequency filters on all channels to

reduce the effect of electrical noise on the input signal. Software digital

filtering is meant to reduce the effect of process noise on the input signal.

Digital filtering is selected using BTW word 3, bits 00-07.

The digital filter equation is a classic first order lag equation (Figure 4.1).

Using a step input change to illustrate the filter response (Figure 4.2), you

can see that when the digital filter constant time elapses, 63.2% of the total

response is reached. Each additional time constant achieves 63.2% of the

remaining response.

Figure 4.1

Digital

Filter Equation

= Y

n-1

t + TA

- Y

n-1

)

Where:

n = present output, filtered peak voltage (PV)

n -1 = previous output, filtered PV

t = module channel update time (seconds)

n = present input, unfiltered PV

= digital filter time constant (seconds)

Figure 4.2

Digital Filter Lag Equation Illustration

100%

63%

0 0.01 0.5 0.99 Time in Seconds

16723

TA = 0.99 sec

TA = 0.5 sec

TA = 0.01 sec

Unfiltered Input

Amplitude

Digital filter time constant values of 0.00 BCD to 0.99 BCD (0.00 BCD =

no filter; 0.99 BCD = maximum filter) are set in bits 00 through 07 of

word 3 of the block transfer write instruction. If an invalid digital filter

value is entered (i.e., 0.1F), bit 02, word 1 of the block transfer read

instruction will be set. If an invalid digital filter value is entered, the

module will not perform digital filtering. If you use the digital filtering

feature, the filter time constant value chosen will apply to all input signals.

Digital Filtering

1 2 ... 28 29 30 31 32 33 34 35 36 37 38 ... 62 63

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