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Heavy Filtering?

Should we configure a filter for every measurement?

What is the impact to adding a filter?

What are the advantages and disadvantages to adding a filter?

What value should be used?

A low-pass filter is a filter that passes low-frequency signals but reduces the amplitude of signals with frequencies higher than the cutoff frequency. In control systems, usually the filter is a first order but second order and moving average are also used.

Adding a filter will reduce noise but could impact the performance.

Frequently we observe that filters used are hiding the reality. Indeed, when the filter time constant is too large, the measurement looks nice on the screen but in fact, the control deteriorates.

When filter time constant is too long, apparent dead time will increase and stability will be reduced and settling time will be longer.

What is filtering?

A filter will attenuate noise coming from the process. Some examples of process noise:

·waves in a large reservoir where level is measured,

·fast fluctuations in a pipe where flow is measured,

·impeller pulses in a pressure loop.

In each case, the variations are too fast compare to the settling time. Adding a filter will reduce noise amplitude and improve control element life since noise coming to the controller will also be present in the controller output.

What should be the filter time constant?

The cutoff frequency (w_c[rad/s] ) is the frequency dividing the attenuated zone from the non attenuated zone. The time constant (t_c[sec] ) is the inverse of cutoff frequency:

Tau_c =t_c= 1/w_c

The filter time constant is selected to determine filtration expected. This filter time constant should be less than the dead time (t_d) and a lot smaller than settling time (t_s).

Optimization software will have tools to select the filter time constant accordingly to the process model and control objectives.

Rules of thumb can also be used :

For example, Top Control suggests these values :

 

Where should the filter be?

The filter function can be in the transmitter, the input card, in a function block or in the PID function. We recommend to use the filter in the control system where it can be easily adjusted. Do not add more than on filter…

Optimization is a very good investment! Usually, within few weeks, money invested is recovered.

See in November and December issues of Control Magazine, an interview of Michel Ruel by Greg McMillan. See Control Talk :
«Show Me the Money», Control Magazine, November 2009, P 55.

At Paptac, MontréalQc, February 2-3 2010, two conferences by Top Control Inc.

Closed Loop Tuning vs Open Loop Tuning: Tuning all your Loops while the Process is Running is now Possible, M. RUEL, Top Control Inc.,

Recycling Session, Wednesday, February 3, 2010

Tracking Performance after Optimizing a Deinked Plant, M. RUEL, Top Control Inc.,