Mantracourt Electronics Limited
DCell & DSC MantraCAN User Manual
29
filter will be equal to the new input reading. If the difference is less than
FFLV
then the fractional amount added is
incremented until it reaches the maximum level set by
FFST
.IE if
FFST
= 10 then after a step change the fractional
part of a new reading is incremented as follows
1/1, 1/2, 1/3, 1/4, 1/5, 1/6…. 1/10, 1/10, 1/10
This allows the Filter to respond rapidly to a fast moving input signal.
With a step change, which does not exceed
FFLV
, the calculated new filtered value can be calculated as follows
New Filter Output value = Current Filter Output Value + ((Input Value - Current Filter Output Value) / FFST)
The time taken to reach 63% of a step change input (which is less than
FFLV
) is the frequency at which values are
passed to the dynamic filter, set in
RATE
, multiplied by
FFST
.
The table below gives an indication of the response to a step input less than
FFLV
.
Update Rate is 1/table value of
RATE
see Chapter 3 Output Rate Control.
% Of Final Value
Time To settle
63%
Update Rate *
FFST
1%
Update Rate *
FFST
* 5
0.1%
Update Rate *
FFST
* 7
For example, If
RATE
is set to 7 = 100Hz = 0.01s and
FFST
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
1%
0.01 x 30 x 5 = 1.5 seconds
0.1%
0.01 x 30 x 7 = 2.1 seconds
The following table shows the number of updates x FFST and the error %
New Filter Output value
will differ from a
constant
Input Value
.
x FFST
% Error
1
36.78794412
2
13.53352832
3
4.97870684
4
1.83156389
5
0.67379470
6
0.24787522
7
0.09118820
8
0.03354626
9
0.01234098
10
0.00453999
11
0.00167017
12
0.00061442
13
0.00022603
14
0.00008315
15
0.00003059
16
0.00001125
17
0.00000414
18
0.00000152
19
0.00000056
20
0.00000021
Remember that if the step change in mV/V is greater than the value set in
FFLV
then
