What is damping in process transmitters and how is it useful?

What is damping?

Process transmitters differ from pressure sensors in terms of functionality: they have integrated displays, high measurement accuracies, and freely scalable measuring ranges, communicate via digital bus signals and can be delivered with a variety of case variants.

The majority of advanced process transmitters, both analog and digital, are comes with a feature called damping. The damping feature is very much needed in the low-pass filter function to reduce the noise produced by the transmitter.

Suppose a pressure transmitter is sensing the water pressure of a large pump. The water flow from the pump is extremely turbulent, and any pressure-sensing device connected to the pump’s immediate discharge port will interpret this turbulence as pressure fluctuations. This means that the pressure signal output by the transmitter will fluctuate as well, causing any indicator or control system connected to that transmitter to register “noisy” water pressure, i.e., a continuously fluctuating pressure signal reading of the transmitter.

Because this noise has a much higher frequency than the normal pressure cycles in a process system, it is relatively simple to reduce the amount of noise in the transmitter signal by filtering it with a low-pass filter circuit.

Because the reactance of the capacitor is quite large at low frequencies, low-frequency voltage signals applied to this circuit emerge relatively unattenuated at the output terminal. High-frequency signals applied to the same circuit are attenuated by the capacitor, which has a low reactance to high frequencies and tends to “short” those signals to the ground. The performance of such a filter circuit is primarily defined by its cut-off frequency, which is defined mathematically as f = 1/2RC. The cut-off frequency is the frequency at which only 70.7 percent of the input signal appears at the output (a 3 dB voltage attenuation).

The effect of intentional low-pass filtering of process measurement signals is commonly referred to as damping in the world of process control because it “damps” (turns down) the effects of process noise:

Damping is a useful tool for technicians to reduce noise while measuring. The cut-off frequency (degree of damping) can be changed by adjusting the value of R and C in the RC filter circuit.

Damping can be adjusted in digital transmitters where damping is performed by a digital algorithm by entering a numerical value into the transmitter’s configuration parameters. Damping in pneumatic transmitters could be accomplished by incorporating viscous elements into the mechanism, or more simply by adding volume to the signal line (For example, longer tubing, larger tubing diameter, or even “capacity tanks” connected to the tube to increase volume).

If the operator uses insufficient damping will lead to high noise reaching the control system and the excessive damping leads the transmitter to fail to understand the use of process changes. So, the technician should be aware of how much damping should the operator use.

The excessive damping of the transmitter leads the trend graph to be smooth. The entire purpose of a control system is to keep the process variable close to the setpoint, so the appearance of a “flatline” process variable trend is very appealing. The issue with excessive damping is that the transmitter responds slowly to any sudden changes in the real process variable. This principle is illustrated by a dual-trend graph of a pressure transmitter experiencing a sudden increase in process pressure, with the undamped transmitter signal in the upper portion and the over-damped signal in the lower portion (please note that the vertical offset between these two trends is shown only for convenience in comparing the two trend shapes):

Because of the excessive damping, the transmitter “lies” to the control system by reporting a process variable that changes much more slowly than it actually does. The extent to which this “lie” has a negative impact on the control system (and/or the human operator’s judgment in manually responding to the change in pressure) is highly dependent on the nature of the control system and its importance to the overall plant operation.

Sometimes one-way damping leads causes control problems in a system where the loop controller is aggressively tuned. In systems where the loop controller is aggressively tuned, damping can cause control problems. Because the controller “thinks” the process variable is responding too slowly and takes action to speed it up, even small amounts of damping can cause the actual process variable to overshoot the setpoint in such systems. Because the controller is trying to get a “sluggish” process variable to respond faster than the transmitter filtering will allow the signal to change, a technician may introduce damping to the transmitter with good intentions, but this causes the control system to wildly overshoot the setpoint (or even oscillate) because the controller is trying to get a “sluggish” process variable to respond faster than the transmitter filtering will allow the signal to change. The process variable (fluid flow rate) is not sluggish in reality; it only appears so because the transmitter is damped. Worse, because the control system never sees the real process variable, only the “lie” reported by the over-damped transmitter, this instability will not appear on a trend of the process variable.

When calibrating a transmitter in a shop, the damping adjustment should be set to its absolute minimum so that the technician can immediately see the results of applying stimuli to the transmitter. Any damping in a calibrated transmitter serves no purpose other than to slow down the calibration procedure.