DSCUSB Toolkit Manual Strain Gauge or Load Cell Digitiser Module

Chapter 3 Explanation of Filter

Dynamic Filtering, Filter Level & Filter Steps

The Dynamic filter is basically a recursive filter and therefore behaves like an ‘RC’ circuit. It has two user settings, a level set in mV/V and the maximum number of steps (up to 255).

Instead of outputting every new value, a fraction of the difference between the new input value and the current filtered value is added to the current filtered value to produce the filtering action.

If this difference is less than the value set in the Filter Level then the fractional amount added each time is decremented until it reaches the minimum level set by FFST i.e. FFST is the limit of the divisor. e.g. if Filter Steps = 10 the fractional part of the difference between the new value and the current filtered value will be decremented as follows: 1/1, 1/2, 1/3, 1/4, 1/5, 1/6 . . . 1/10, 1/10, 1/10 before being added to the current filtered value.

If a rapidly changing or step input occurs and the difference between the new input value and the current filtered value is greater than the value set in

Filter Level then the output of the filter will be made equal to the new input reading i.e. the fractional amount of the new reading added to the current reading is reset to 1

This allows the Filter to respond rapidly to fast moving input signals.

When a step change occurs which does not exceed Filter Level, the new filtered value is calculated as follows:

New Filter Output value = Current Filter Output Value + ((Input Value - Current Filter Output Value) / Filter

Steps)

The time taken to reach 63% of a step change input (which is less than Filter Level) is the frequency at which values are passed to the dynamic filter, set in

Measurement Rate, multiplied by Filter Steps.

The table below gives an indication of the response to a step input which is less than Filter Level.

% Of Final Value

63%

Time To settle

Measurement Rate * Filter Steps

99%

99.9%

Measurement Rate * Filter Steps * 5

Measurement Rate * Filter Steps * 7

For example, If Measurement Rate is set to 100Hz = 0.01s and Filter Steps is set to 30 then the time taken to reach a % of step change value is as follows.

% Of Final Value Time To settle

63% 0.01 x 30 = 0.3 seconds

99%

99.9%

0.01 x 30 x 5 = 1.5 seconds

0.01 x 30 x 7 = 2.1 seconds

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The following table shows the number of updates ‘x Filter StepsT’ and the ‘% Error’ that the Filtered Output value will differ from the constant Input Value.

11

12

13

14

7

8

9

10

x FFST

1

2

3

4

5

6

% Error

36.78794412

13.53352832

4.97870684

1.83156389

0.67379470

0.24787522

0.09118820

0.03354626

0.01234098

0.00453999

0.00167017

0.00061442

0.00022603

0.00008315

15

16

17

0.00003059

0.00001125

0.00000414

18

19

0.00000152

0.00000056

20 0.00000021

Remember: if the step change in mV/V is greater than the value set in Filter Level then:

New Filter Output value = New Input Value i.e. the output jumps to the new input value and the internal working value of Filter Steps is reset to 1. This is then incremented each update (set by Measurement Rate) until it reaches the user set value of Filter Steps.

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