Does anybody know what the damping function available in modern smart transmitters does? Is it a kind of moving average filter or something else?

Thanks in advance.

Sumeet Chimalkar

 

It is most probably a standard first order low pass filter. ie the thing you would get if you fed the signal thru a simple RC filter without any load affecting the signal.

in a microcontroller it would probably be implemented something like this:

value=value*(1-timeStep/dampTime)+thisRead*timeStep/dampTime;
or
value=value+(thisRead-value)*timeStep/dampTime;

essentially giving the same caracteristics.

where
timeStep is the (current) scan time
dampTime is the damping time set
thisRead is the value read by the current scan
value is the signal you send out

timeStep abd dampTime might be in any time unit as long as it is the same.


I don't say it always is stis formula, just that it is most probably this one used if nothing else is said about it.

(a side note: this formula will seriously damage the signal if ever timeStep>dampTime/2)


/Johan Bengtsson

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Sumeet,

The HART Protocol requires all compliant devices to return the damping value for the field device. This value is specified to be equivalent to the time constant of an RC filter. Since HART is an enhancement to 4-20mA instruments and RC best simulates how damping is performed in analog only instruments.

This value is returned in a universal command (all HART Devices are required to support all universal commands) and can be set in a common
practice command. It is an IEEE Floating point number and the unit code is seconds. Common practice commands are optional so, some devices may not allow you to set the damping value. In addition some devices only support discrete damping values. In other words, you may not be able to set the damping (in seconds) to any random number you want.

If I can help any further, let me know.

Best Regards
Wally Pratt
Chief Engineer
HART Communication Foundation
http://www.hartcomm.org

 

Besides the tutorial above, filtering in data acquisition is the application of numerical smoothing (we talk numerical somewhere, even if it is transparent to you).

Data smoothing and moving averages are differents animals, at least from the formulation.

In data smoothing (filtering), so many data are acumulated (correspondig to the degre of smoothing), then depending upon the degree of smoothing, each piece of accumulated data is factored correspondingly to the degree of smoothing and the formula that applies.
This method is called 'point by point modification of data'.
It is used in place of approximating by a single least-square polynomial of degree 'n'over the entire range of an extensive tabulation.

See also 'Gram polynomial'.

[email protected]

 

The damping circuit slows the response time of the final output of a transmitter. If you observed a constantly changing output as it increased and decreased rapidly, increasing the damping will cause the output to change at a slower smoother rate