While approaching an optimum something can sneak up that catches the loop off guard. Because of the deadly foe dead time, by the time the loop sees and reacts, it may be too late, particularly if it was blind sided.
The classic example is compressor anti-surge control. When moving to a lower discharge pressure or recycle flow (lower energy use), an inaccurate surge curve or untimely dip in feed can cause a precipitous drop to zero or negative flow in 0.03 seconds followed by huge reversals in flow from surge. Just a few of these surge cycles can damage the seals enough to reduce the efficiency of an axial compressor.
Another impressive case can occur for exothermic reactor control. During the approach to a higher reaction temperature and higher reaction rate (lower batch time) a higher than expected raw material concentration or catalyst activity can initiate a runaway acceleration of temperature and reaction rate.
Not quite as dramatic but still important in terms of environmental scrutiny occurs for an approach to a lower pH set point in a static mixer (lower base reagent use). A strong acid upset from a batch operation or level switch controlled sump can cause a low RCRA pH violation within seconds. Even if it lasts a few seconds and therefore has no measurable affect on any decent downstream volume, it can be a recordable environmental violation. In one particularly large application, an interlock diverted the feed from the plant waste treatment system if the control system could not do its job and a violation was eminent.The open loop backup successfully eliminated nearly all of these diversions.
A much slower but still important situation can occur for a bioreactor. During the approach to a lower substrate (glucose) concentration with less substrate inhibition (greater yield), non-ideal mixing and a drop in substrate feed can trigger starved biomass to eat their own product (ugh).
In each of these cases, there is a significant undesirable event that requires a slow approach to an optimum and a fast recovery from an inadvertent excursion into an extremely undesirable operating region. This is particularly true for the first three cases, which involve environmental and property protection. The last thing you want is to test the adequacy of your interlock system or have a recordable incident.
(1) Compressor Anti-Surge Control
(2) Exothermic Reactor Temperature Control
(3) Static Mixer RCRA pH Control
(4) Bioreactor Substrate Concentration Control
An open loop back up has been applied in the above applications to assist but not interfere with the PID controller trying to do its job. The calculation simply consists of incrementing the controller output from its last value via the ROUT mode every module execution when the process variable has exceeded a limit. The increment is stopped when the process variable has recovered beyond the trigger point plus some differential (e.g. noise band). It is normally only activated only when the controller is not in manual. There is a bumpless transition to PID action when the open loop backup is cleared.
For surge control, the clearing of the open loop back up has a time delay to insure the compressor is out of surge and the control system is not fooled by a flow reversal.
In each case, the need to get out of trouble as quick as possible overshadows any temporary loss in efficiency.
Another strategy is to use a fast opening but slow closing of the control valves for compressor vent or recycle flow, reactor coolant flow, pH reagent flow, and bioreactor substrate feed. This can be implemented by putting a rate (velocity) limit on a decreasing signal to the control valve. This can be implemented in the analog output block via the SP_RATE_DN parameter, which in this block is active on the set point even when the block is in the CAS mode. To insure the reset action in the PID block is not faster than the rate limiting in the AO block, the “Dynamic Reset Limit” option must be enabled in the PID and the “Use PV for BKCAL_OUT” option enabled in the AO block to use the working set point for the BKCAL_OUT. Any rate limit will affect tuning and must be implemented before running any tests to identify dynamics or tuning settings. The strategy also works on variable speed drives for reagent and substrate feeds to allow a fast increase but insure a slow decrease in speed.
The attached screen prints show a simple example of an open loop calculation and enabling of the above options. As with any new technique, the configuration should be thoroughly tested by a realistic simulation before used in an actual application.
Another option is to schedule the controller reset action to be much faster (reset time much smaller) when the process variable approaches a risky region to promote a fast recovery. There may be some overshoot of the set point but a slow approach back should prevent a second crossing to the more eventful side of the set point. Scheduling a drastically higher controller gain may not be a good idea because it can cause a bounce back toward the undesirable region from proportional action before the process variable even gets near the set point. Some new DCS software, such as DeltaV Insight, can automatically identify process dynamics and schedule the corresponding tuning settings.
Sometimes the open loop back up is called a kicker. The following is an excerpt from the January 2005 Control Talk column in Control magazine that describes a kicker used by Terry Chmelyk to reduce the number of feed diversions required to prevent the violation of an environmental constraint. It is similar conceptually to the previously described RCRA limit application, but here the measurement was conductivity instead of pH.
Terry: In a multi-effect evaporator system, we used built-in and integrated model predictive control (MPC) and optimization to reduce variability in the product density from 2.8% to 0.3% and increase throughput by 6 to 8%. We also used innovative environmental constraint handling to increase the interval between diversions by an order of magnitude.
Greg: Environmental limits can come on suddenly and unexpectedly. My experience is that these involve unmeasured disturbances and scenarios you can not initiate to develop a model. There is nothing sadder than an advanced control engineer without a model. What did you do?
Terry: We added an external “kicker” algorithm around the MPC because of the highly non-linear characteristics of the constraint variables (in this case it was condensate conductivities). The environmental impact required us to take immediate and “substantial” action to eliminate the contamination in the condensates. In essence, we built a basic fuzzy algorithm that “kicked” the weak black liquor (WBL) feed to the evaporators during a significant upset.
The first slide in the attached file summarizes the achievements of the MPC/kicker application. The second slide shows how the “kicker” backed out the WBL flow on high condensate conductivity to prevent a diversion yet allowed the MPC to recover quite well from the disturbance.