I have seen two control loops that did not go digital during the migration to a DCS. These electronic analog controllers stick out like a sore thumb in a modern day control room. The user would like to get rid of them along with the parts, maintenance, and operator interface issues. What keeps these relics from the 1970s hanging around?
The two analog control holdouts I have seen had 4 things in common: a variable speed drive, zero process dead time, a critical process impact, and an inability to run in manual.
If you don't have time to read on to get details on the particular loops, the most important "insights" are:
(1) Controller tuning tools and methods that rely on an open loop test cannot be used
(2) Digital adaptive controllers that identify tuning from set point changes are needed
(3) Must faster measurement update, communication, and controller scan and execution time intervals must be developed for valid holdouts to go digital
(4) If a loop has a control valve, it is rarely a valid holdout
The first application was polymer melt pressure controlled by the manipulation of melt pump speed. The melt pressure was important for throughput and relative viscosity control. An analog trend chart recorder showed what would appear to be a lot of noise. However, if the loop was taken out of auto, the amplitude of the fluctuations got so much worse you could not afford to stay in manual for more than a few seconds. The loop was reacting and compensating for incredibly fast disturbances. The process time constant can be estimated from the fluid inertia and viscosity and typically varies between 50 and 500 milliseconds. The process dead time can be estimated as the time it takes a pressure wave traveling at the speed of sound in the fluid to propagate from the final element to the first major resistance to change the pressure difference that is the driving force for the acceleration of a basically incompressible column of fluid. In other words, the process dead time was essentially zero. The dead time in the loop was all due to the automation system. The dead time of a variable speed drive (VSD) is nearly zero if the following conditions are met in the VSD application.
(1) The change in speed is larger than the resolution limits of the VSD A/D card
(2) The change in speed is larger than any dead band introduced by the user into the VSD configuration to suppress reaction to noise
(3) The rate of change of speed is smaller than any rate limiting introduced into the VSD configuration to reduce motor load and upsets to down stream equipment
(4) The rate of change of speed is smaller than any rate limiting from rotor inertia
These conditions are met for a well designed VSD for liquid pressure control, which leaves the measurement and controller as the sources of dead time. The process is self-regulating but it takes a high speed recorder to see any sort of time constant unless a signal filter is added. Note that I am not advocating replacing control valves with a VSD. There are practical problems when a control valve is omitted, such as the reversal of flow and the creation of incredibly fast flow upsets to other loops and unit operations.
There is an important exception to zero process dead time for liquid flow control. For highly viscous flows, a "ketchup bottle effect" has been observed where there is a huge dead time to initially start a flow through a small injection orifice as described in the first chapter of my book titled A Funny Thing happened on the Way the Control Room available at http://www.modelingandcontrol.com/FunnyThing/.
We all know about aliasing from digital communication, even more important here is introduction of a delay into a control loop that has essentially no process dead time.
Why am I obsessed with dead time? The ultimate performance (IAE) achievable for unmeasured disturbances with the fastest tuning is proportional to the dead time squared and the ultimate period for this dead time dominant loop is twice the dead time.
The second application was incinerator pressure controlled by the manipulation of an induced daft (ID) fan speed. The loop behaved like an integrator. If the controller was put in manual, the pressure could ramp and hit the interlock trip point in less than a second. Since open loop testing for exact quantification was not reasonable, a dynamic simulation was used to show it could occur in 0.25 seconds. While the residence time was on the order of 0.1 minute, the process gain was incredibly large because the measurement scale span was just a few inches of water column. The simulation also showed the decoupler between the forced draft (FD) fan and ID fan speed (air flow feedforward) was doing more harm than good because of the inverse response associated with the cold air flow. After elimination of the decoupler and retuning, the frequency of furnace trips was reduced but trips still occurred every couple of days. This process was controllable only because the process dead time associated with the furnace volume was essentially zero. For gas pressure systems the process dead time originates from gas volumes in series separated by flow resistances. The pressure sensor was seeing and the ID fan was acting on the same gas volume. The dead time in the loop was all due to the automation system.
Summarizing, a digital controller with a 0.1 second execution time was tried on startup but the furnace trips were excessive despite the best tuning and strategy. In 0.1 second, the pressure was almost half way to the trip point. When the digital controller was replaced with an analog electronic controller the pressure trips were eliminated.
This application and a phosphorous furnace application are discussed in chapter 3 titled "Pressure Control - Without Dead Time I would be out of a Job" in the aforementioned book A Funny Thing happened on the Way the Control Room.
Next week I will share my experiences with making control valves respond faster. With this under our belts, I will offer how fast digital devices and communication and data historians need to be once you get these valves to "turn on a dime or at least a quarter".
Note that the equations for computing process dead time and time constants for these systems is in Tuning and Control Loop Performance - 3rd edition, but is out of print.
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