Predictive Material Delivery to A Batch System

Transcription

1 Presented at the World Batch Forum European Conference Mechelen, Belgium October 17 S. Southgate Drive Chandler, Arizona Fax Predictive Material Delivery to A Batch System David A. Chappell Section Manager Procter & Gamble 856 Union Centre Blvd. West Chester, Ohio 4569 Phone Fax chappell.da@pg.com KEY WORDS Adaptive, Predictive, Self-tuning, Inexpensive, Robust, Accurate, Reduced Maintenance, Improved Batch Cycle Time, Instrument Device, Controller, Final Control Element ABSTRACT Creating an inexpensive material delivery system to provide high accuracy and to do so at high speeds would seem to be an impossible task. Procter & Gamble s Adaptive Predictive Control (APC) makes this seemingly impossible task both possible and practical. Using this technology, P&G has demonstrated improvements in automated material delivery accuracies by a factor of more than 1! APC allows the control system to predict-the-future and then use this information to accurately add the formulated amount of material. APC allows for the use of simple and inexpensive ON-OFF flow control devices, reducing not only the installed cost of a system but also its long-term maintenance costs. Since APC controls at the full flow rate of a delivery system rather than reducing the flow at the end of a delivery, batch cycle time is improved. APC is self-tuning and will not only learn the characteristics of a delivery system, but also will track and adapt to the normal variations that occur over time. With APC overseeing the automated material delivery systems, the attention required by normal operations becomes almost nonexistent. P&G s APC technology has been licensed to an instrument vendor and has been incorporated into their factory floor devices. Several control companies are developing easy to use function blocks that will seamlessly integrate these Instrument Devices with APC into the controller architectures. Copyright World Batch Forum. All rights reserved. Page 1

2 Background: Manufacturing systems rely on exacting measurements provided by sensors and conditioning devices to make high quality products at effective rates by using a variety of techniques that integrate manual operations with state-of-the-art automation. Load cell and flow meter systems are integral to our automation efforts. When these systems are properly implemented, we gain unmatched competitive advantage in product quality and manufacturing rates. When improperly implemented these systems can become nightmares, which must eventually be replaced or become lower performing systems. This paper will describe how to avoid some of the process pitfalls and how to identify a system s capability, as well as techniques used to improve performance of these systems. Adaptive Predictive Control (APC): While the process industry has available many adaptive strategies for continuous systems there has been nothing of substance for batch material delivery systems, until now. P&G has developed a proprietary and patent pending method to improve the performance of material delivery systems. This allows us to install simple, inexpensive and robust process systems that have great accuracy (accuracy is defined as deviation from set-point of a delivered material) in the delivery of materials using Load-Cell Gain-In-Weight, Load-Cell Loss-In-Weight and Flow-Meter addition techniques. This method is referred to as Adaptive/Predictive Control or APC. Old % Accuracy APC % Accuracy % Improvement Duncan Hines Molasses Sunny Delight Water Sunny Delight Sugar Sunny Delight Flavor Table 1 Four APC examples Table 1 provides four examples of how APC has reduced a delivery system s variability at P&G. These examples demonstrate the potential of APC, the actual results will vary depending upon the exact application. APC technology allows P&G to use simple and inexpensive ON/OFF control equipment in the material delivery systems, as P&G s Traditional represented in figure 1, and Technology Adaptive achieve higher accuracy results Brute Force Technology than realized by using the traditional dribble/flow control elements used by most of the industry as represented in figure. This ON/OFF control also improves batch cycle times by not having to wait for the dribble control to reduce the flow, which takes more time to deliver the Figure 1 Adaptive same amount of material. Using Figure Traditional Dribble Copyright World Batch Forum. All rights reserved. Page

3 APC material deliver times have been reduced by % to 3%, which has translated in overall batch cycle improvements of 7% to 1%. The Predictive part of APC continuously monitors the process, determining a material s actual flow rate at all times during the material delivery. Based on the flow rate and APC s knowledge of the past performance of the delivery system, it continually predicts the optimum point at which the delivery system should stop (cutoff) so that the desired amount is transferred. The Adaptive part of the APC monitors the performance of each delivery and updates the factors used by the Predictive part. This allows APC to adapt to and compensate for normal process variations that can cause a material s delivery characteristics to change over time, (e.g. humidity which affects dry materials or viscosity which affects liquids). By using the flow rate of a material and monitoring batch-to-batch deviation from set point the APC can determine the amount of SPILL that it can expect. SPILL is the amount of material that will still be transported after the command to stop has been issued. The science behind the APC s advanced techniques are well understood by P&G with the graph shown in figure 3 demonstrating the four major components of SPILL and their relationships. The components are: instrument reading lag (W LAG ), material in suspension (W SUSP ), material which will flow through the restricting device after the closure command (W VLT ), and the kinetic energy stored in the material which is released as it impacts the vessel (F DEC(1) & F DEC() & F DEC ). For more details about these components see the definitions appendix. SPILL Total SPILL: Total SPILL (TS) = K 1 Q MAX + K Q MAX W LAG = Q MAX F W SUSP = Q MAX T W VLT = Q MAX K V Flow Q MAX NOTE: Total SPILL can be negative F DEC(1) = -Q MAX T = -W SUSP F DEC() = -Q MAX / (3. ñ A V ) F DEC = F DEC(1) + F DEC() = -Q MAX T -Q MAX / (3. ñ A V ) Figure 3 The Components of SPILL There are three types of algorithms supported by the APC for control. Spill Only Algorithm - This is used to control material movements that have very low flow rates, a widely varying flow rate during a single material movement or a very large component of SPILL. Generally a rate in units per seconds of less than.5% of Maximum Vessel Size or a flow to spill ratio greater than 1 to 5 will require a Spill Only algorithm. This is the least capable of the three algorithms and will have the largest variance, delivering capabilities in the range of.1% to.5 % full scale for load cell systems. When using this algorithm the time of Copyright World Batch Forum. All rights reserved. Page 3

4 material addition should be 1 seconds or greater. If the feed time is less than this special care will have to be taken in the configuration of the control system. K1 Algorithm - This can be used to control material movements that have a repeatable and reasonable flow rate, generally a rate in units per seconds between.5% and.5% of Maximum Vessel Size. It is also the most widely used and works well with flow-meters. This can deliver capabilities in the range of.5% to.5% full scale. When using this algorithm the time of material addition should be 3 seconds or greater. If the feed time is less than this special care will have to be taken in the configuration of the control system. K Algorithm - This is used to control material movements that have a high flow rate, generally a rate in units per seconds in the range of.1% to 5.% of Maximum Vessel Size. This is the most capable of the three algorithms and it can be used in most load-cell based Gain-In-Weight (GIW) or Loss-In-weight (LIW) applications. It is required for material movements that use gravity and head pressure of another vessel to deliver it to a receiving vessel. This special case generally can deliver capabilities in the range of.5% to.5% full scale. When using this algorithm the time of material addition should be 3 seconds or greater. If the feed time is less than this special care will have to be taken in the configuration of the control system. Tank Size Flow Rates in kg/sec Capability Range in kg Algorithm Type Live Load* (kg) Low Hi Low Hi Load Cell Systems: 1, Spill Only 1, K1 1, K Table : Example Algorithm Comparison *Live Load is the weight of the contents of the vessel. Resolution of the Instrument and Accuracy of the System: The load cell sensor provides a microvolt signal proportional to the load applied to a conditioner. The conditioner will amplify this signal and will convert it into a reading that can be used by humans or an automated system. The resolution of this reading is dependent upon many factors and is an important component in determining the minimum accuracy that a system can deliver. Today s instruments are capable of reliably delivering resolutions in excess of 1, counts of full scale of live load. As an example, a tank that can hold 1, kg of material theoretically could detect a change of.1kg. While very challenging it is possible to design and construct load cell systems that will deliver almost perfect linearity with very great resolution. System capability to deliver designed specifications can only be verified after testing the installed system with test weights and monitoring actually delivery performance. Issues such as supporting steel interaction, binding and load cell range selection must all be considered in design. Most will realize that the accuracy claims made for APC are far grater than accepted by Handbook 44, APC delivers this accuracy by using small segments of the full live range of the load-cell over a short period if time. Using this technique the Copyright World Batch Forum. All rights reserved. Page 4

5 repeatability and accuracy is far grater than that of the entire span over long periods of time and temperature. This makes it possible to deliver materials with far more accuracy than can be demonstrated by the accuracy of the full span of a load-cell system over a long period of time. Capability of the Delivery system: The resolution of a load cell system does not determine the capability of the material delivery systems. It determines how accurately the load cell system can report the change in the readings. Material delivery systems consist of a final control element (valves, slide-gates, etc) used to start and stop the flow of material and the equipment (pumps, screw-feeders, gravity, etc.) used to cause the material to move. Placement of the final control element is also very important, the closer to the receiving vessel than the better. In the end, the design and performance of the physical delivery system will determine how accurately you can deliver a given material on any given day. As described earlier, P&G s proprietary methods provide the potential to deliver an accuracy of 3 sigma that is.5% of the load cell full-scale range. This does not mean your systems will achieve this performance, only that it is possible. As a general rule.5% of the load cell full-scale range should almost always be obtainable. Less accurate feeds may indicate that something may be wrong with the delivery system. Tank Size Live Load as Percentage of Total Tank Size (kg) 1, Count Live Load* (kg) 1.%.5%.1%.5%.5% Resolution (kg) Table 3: Tank Conversion Table *Live Load is the weight of the contents of the vessel. Table 3 provides some example ranges for different sizes of vessels. You can identify your delivery system s current capability by applying industry standard Statistical Methods for Quality Improvement techniques and performing an X-Bar, R-Bar analysis on the error (deviation from setpoint). When performing your analysis, you should have at least 6 samples. It is necessary that every feed be used as a single sample rather than using data averaging. The histogram must be somewhat proportional with a Bell-Shaped curve as demonstrated in the two example Histograms (Figures 4 & 5). The average of the errors must approach zero. If any of these criteria are not met, Copyright World Batch Forum. All rights reserved. Page 5

6 your data may be inaccurate or the delivery system may be erratic and uncontrollable. Developing an Excel spread sheets that can be used for this analysis is a simple and straight forward task. Charting the flow rate and spill data can also provide useful information. The Spill & Rate Chart from Lima s Citric Acid (fig. 6) delivery system is an example of charting flow and spill. Notice the non-linearity of the spill in this example. Even though the flow rate is constant, other external forces act on the process to cause the delivery system to vary its spill. These variations are predicted and compensated for by the APC. This spill variation also occurs in flowmeter applications. Great care must be taken in acquiring accurate data. Make sure that sufficient time was allowed at the end of every material movement to guarantee all material is accounted for; this is a common problem where the final sample is taken too quickly. The Histogram Figure of a Dump and the Histogram Figure of a feed (Fig. 4 & Fig. 5) demonstrate two material movement systems that have dramatically different capabilities, yet they are both part of the same load cell system. The system used for these examples is a,- gram test stand located in P&G s Central Engineering Lab with a resolution of.1 gram. These results are consistent with other studies performed for several P&G Manufacturing sites making many different products. The example shown in Fig. 4 shows a 3-sigma capability of.15% full scale. The example shown in Fig. 5 shows a 3-sigma capability of.48% full scale. While it is desirable to predict a new system s capability, it is also a daunting challenge to do so and be right. The best way to predict the capability of a new system is to use the capability data of an existing system that is EXACTLY like the new one. If you can t find an exact example, your best option is to make your prediction and verify it during start-up by collecting and analyzing the data. Should your predictions be inaccurate your options then are to take the data and use it as the delivery systems capability or to modify the system to improve its performance. You do not need APC to develop Frequency Test of 1/8/97 for a dump, 414 points Min -5.91, Gram Lab System Max 3.38 Resolution of Scale.1Gram Avg STD.9683 New M New Driver Test of 1/8/97 for a feed, Min 1.75 Max, Gram Lab Avg Resolution of Scale STD New M New Driver Error Copyright World Batch Forum. All rights reserved. Page 6 Figure 5 Histogram Figure of a Error Figure 4: Histogram of a Dump Frequency Frequency

7 capability information on your current delivery system. You do need accurate information and data, this may prove very difficult to acquire. In many, if not most, of the systems we have applied APC the going in assumptions based on data from existing systems were wrong, very wrong. Often we are told the existing systems are perfect, just look at the data, only to find the data was improperly collected. When evaluating data from existing systems one must closely analyze the entire system before accepting the data at face value. Items like the resolution of the A-to-D converter used, the times the reported data samples were taken, and other factors should be considered. Number of s Agitation induced noise: Figure 6: Spill & Rate Chart When agitators are being used on a vessel, the reading of the load cell instrument will be affected. It is possible to program the load cell-conditioning instrument to filter this noise. This will improve the readings but will cause the response of the instrument to be reduced (W LAG ), which will impact capability. Another approach is to stop and start the agitator at the critical times during measurements. Some of these critical times are the start and stop of a feed, sampling for reporting purposes and cross checking other instruments. Flow meter systems: Flow meters share many characteristics of a Load Cell system, and their capability should be analyzed similarly. One thing you must do when sizing a flow meter for batch applications is determine the resolution of the reading such that the range of the totalizers will be sufficient. If you select an instrument range of 1 pulses per gram and need to deliver 1 KG your pulse count totalizer will have to be capable of at least 1 billion counts. Refer to your flow-meter vendor s literature to verify a proper selection. Flow meters can be used to allow parallel additions of materials and to deliver materials when the capability of the load cell system cannot meet accuracy requirements. Flow meters will indicate the amount of material that has passed through the flow tube; this only infers the amount that was actually added to the vessel. For flow meter technology to be successful in batch delivery systems great care in the installation of the device and system maintenance must be taken. A very common problem is that the delivery lines drain back to the supply tanks. When next used, the flow meter will indicate an amount delivered, but a portion of that amount could be line fill and is never received into the vessel. Installing re-circulation loops just before each final control element and maintaining a constant flow can help prevent this problem. To help identify if this is a problem instrument crosschecking can be very beneficial. Cross Checking of Measurement Systems K G Lima Citric Acid flow rate and spill Spill in KG Flow Rate KG/Second When there is more than one system used to measure process performance, some minor discrepancies are inevitable. These are expected and acceptable. It is beneficial if these systems are automatically used to cross check one another. This allows for an early warning when an instrument, Copyright World Batch Forum. All rights reserved. Page 7

8 Fixed Bias Adaptive Predictive Control Error Error Frequency Frequency Fixed Bias Error Adaptive Predictive Control Frequency or the process, is starting to malfunction and can prevent many days of down time as well as preventing miss-formulations and scrap batches. Adjusting targets (Set-point) to save raw material Merely improving the ability to deliver material more accurately may let us reduce process variation but it doesn t save material. Improved capability can let us greatly reduce the tolerance bands around the target or set-point. See Fig. 7. While this reduces process variations, raw material may not be saved. Over- and under-usage still balance each other; the same amount of material is used in both cases Error Frequency Old Tolerance Delivery Capability without APC Set-point Material Savings! New Set-point Delivery Capability with APC APC Tolerance Key X Axis = Deviation from Set-point, or Positive and negative Error Y Axis = Number of occurrences at that deviation from set-point Set-point = zero error Fig 7: Improved APC Tolerance Tolerance While true, it s not the end of the story. The target/set-point is usually selected to guarantee the amount of material actually delivered will meet the minimum tolerance. Fig 8: Adjusted Set-point This is often to meet product claims or to assure we have the minimum required active ingredients in the batch mixture required by our processes. With the improved capability realized by APC, we can take advantage of these reduced tolerances and modify the target/set-point. We can guarantee to meet or exceed minimum requirements and, we can reap major material savings. See Fig. 8. Some P&G businesses have financed installation of the automation that makes APC possible, on the material savings alone! Physical Material Delivery Systems Design Considerations There are many ways to deliver materials to a batching vessel. All have pluses and minuses. Used properly, they offer years of worry-free service. Used improperly, they become a long-term headache making it difficult or impossible to identify as the root cause of system problems. Many factors influence a systems capability as well as its robustness. Here are some to consider to improve system performance: Copyright World Batch Forum. All rights reserved. Page 8

9 Final control element placement Final control element reaction time and consistency of operation Multiple materials into a single delivery tube Material movement between vessels Gravity transfer Pump transfer Material delivery using gauge tanks Final Control Element Placement For both flow meter and load cell uses, locate the final control element (valve, slide-gate, etc.) as closely as possible to the receiving vessel. See Fig. 9. For best placement have the element flush with the vessel. If that s impractical, locate it a short distance away: Within one meter is good Within two meters is fair Within three meters is poor Beyond three meters means control is difficult Final Control Element Delivery System and Receiving Vessel Fig 9: Delivery System and Receiving Vessel Load Cell Generally, distances from vessel equates to greater spill and greater drain time. Batch cycle time lengthens due to the time needed to drain the delivery tube between the final control element and its receiving vessel. Installing the drain tube more vertically cuts this time. As the drain tube approaches horizontal, drain time grows. At some point, the tube never fully drains and may continuously dribble into a vessel. In Gain-In-Weight (GIW) uses, this will corrupt the readings, making data invalid. Or, you may face chemical reaction issues as the material dribbles in after the formulation expected it to already be in the vessel. Final Control Element Reaction Time and Consistency of Operation As a general rule, decreasing the closure time will improve control. Ranges of: Less than 1 second is very good 1 to 3 are good 3 to 7 are fair 7 to 9 are poor and Anything over 9 seconds is very difficult to control Elements must perform the same from operation to operation. If it takes 1.5 sec to close one time and.4 sec the next, control will be unacceptable. For air-to-open and air-to-close devices, use air regulators on the valves to provide more consistent operation. If air-to-open and spring-to-close valves are used, regulators are not needed. This spring-to-close design is also fail-safe ; it makes device close if the air pressure is lost. Copyright World Batch Forum. All rights reserved. Page 9

10 Multiple Materials into a Single Delivery Tube Sometimes, we must minimize the number of penetrations into a batch tank. Under these conditions, we use a Christmas Tree assembly. See Fig. 1, so called because it looks like an inverted evergreen tree. The same criteria apply to these control elements as mentioned in the Final Control Element Placement section. Also consider the chemical interactions that may occur in the delivery tube. Design the system so undesirable results can t occur when formulations change. Design the delivery tube to drain quickly. Multiple Materials into a Single Delivery Tube Fig 1: Christmas Tree Load Cell Material movement Between Vessels In many operations we have multiple vessels on load cells that make intermediate materials which are combined in a central vessel to make the final product. These vessels have names like Pre Mixes, Slurry Mixers, Preweighs, Meter Prover Tanks, etc. They improve batch cycle times by allowing parallel operations during the making process. They allow more accurate additions as they are often smaller than the final mixing vessel. They may perform a chemical or mechanical transformation on a material before it is combined with other materials in the final mixing vessel. These vessels can hold enough material to supply several batches in the final vessel or vessels. Once the materials are ready to transfer, they must be totally transferred to the receiving vessel. There are several ways to perform this transfer, some more effective than others. The biggest challenge is to guarantee that all the material that leaves an intermediate vessel arrives at the destination vessel. It can be a task tougher than it sounds. Gravity Transfer One of the best ways to move material from one vessel to another is via gravity. See Fig. 11. By mounting the intermediate vessel directly above the Gravity Transfer receiving vessel, the transfer tube will completely drain the intermediate vessel. Drain tubes issues Fig 11: Gravity Transfer Copyright World Batch Forum. All rights reserved. Page 1

11 exist for the gravity transfer as for all material deliveries, so the more vertical the tube the better. Consider, the material may be very viscous, requiring a hot water rinse to flush the vessel and drain tube. This is a common technique, and the water is accounted for in the formulation. Often some build-up (also called heel ) will occur in the intermediate vessel. This is common; plan for it. The vessel s zero weight will vary from batch to batch as heel occurs. Heel both grows and shrinks as batches are made. Pump Transfer Figure 1 shows that this transfer method is both more difficult to perform, and more difficult to assure that all material which leaves the intermediate vessel arrives in the receiving vessel. The problem: keeping the line between the intermediate and receiving vessels full, which means this method isn t as robust as it needs to be. The most successful pump transfer has involved using a water flush immediately after the transfer with enough water to guarantee that the line between the vessels was flushed at least 15%. This water was used in the recipe and the overall water volume had high enough tolerance that any discrepancy was acceptable. Pump Transfer Fig 1: Pump Transfer Material Delivery Using Gauge Tanks Gauge tanks are relatively simple devices that premeasure a raw material and then deliver it to a larger mixing vessel. They usually measure the material by volume using level indication instrumentation such as float switches or adjustable photo switches in sight glass on the exterior of the vessel. See Fig. 13. Note, it is important to include not only the target level but also an indication that a low level was reached to verify that the total amount of material transferred. This proves, within reason, that the entire amount of material in the gauge tank was transferred to the larger receiving vessel. Photo Sensor Sight Glass Photo Sensor Valve Volumetric Tank Drain Valve Material Transfer Control Considerations Having the best designed physical delivery systems are no guarantee that they will deliver the robustness and accuracies required by manufacturing. The Gauge Tank Fig 13: Gauge Tank Copyright World Batch Forum. All rights reserved. Page 11

12 proper operation of the physical equipment is also required, following is a description of the logical operations which when combined with the proper physical systems will deliver the robustness and accuracies required in high performance batch manufacturing systems. All material transfers have six basic phases (Fig. 14) that must occur to run a successful transfer. In designing a system, take care that an illogical condition does not occur, such Key SP = Set-point or Final Target Only SP - Spill = Final Target - the average SPILL value SP - K1 - K = Final Target updated based on APC prediction # = Operations within a Phase 1 16 Decision to start and complete a feed Basic Automation Active SP SP-Spill SP-K1-K Drain & Update Material Flow Pre Start Stop Finish Six Phases of a Material Transfer Post Fig 14: Material Transfer Diagram as a transfer that tries to stop feeding before a feed is detected. Conditions like a long Minimum Open Time and a very short feed can make this happen. There are many process conditions where you can create illogical control conditions. Be careful in tuning the system. These phases and their sub actions are: Phase 1 - Pre Phase - Start Phase 3 Copyright World Batch Forum. All rights reserved. Page 1

13 Phase 4 - Stop Phase 5 - Finish Phase 6 - Post Phase 1 - Pre The decision to start a material transfer occurs and everything required to perform it is verified. If anything is detected which would prevent the feed from successfully completing, the operation is aborted. The operator is informed of the failure and the reasons. During this phase: 1. Set point and tolerances are generated. The automation is requested to activate.. Automation evaluates the request and verifies reasonability ranges. If all is OK, the request continues to be processed. 3. Automation acquires a stable instrument reading, generates a target to use as the final delivery value, and verifies the final value will not over-range the instrument. 4. Automation requests the load-cell/flow-meter high-speed controller to activate, develops an Estimated Time To Complete (ETC) the material transfer and starts the Slow Step Timer (SST). 5. Automation verifies that the high-speed controller activated and moves into Phase the Start. Phase - Start Automation is waiting to verify that material transfer has started, by monitoring flow rate. If this fails to happen after a reasonable time, the automation aborts the feed operation and informs the operator of the failure. During this phase: 6. To guarantee that a control element will activate, there is a minimum open time during which the actual target is used as the control target. This will prevent a situation where the spill, when applied to the target to create a control target, will cause the high speed controller to believe it has already reached the activation target and never cause the actuator to activate. This situation can cause a great deal of confusion for plant maintenance and operations if it ever occurs. 7. After the Minimum Open Time has expired, the current spill is applied to the target to create the control target, which is used by the high speed controller to determine when to instruct the final control element to close. Phase 3 The material has established an acceptable flow-rate for APC control to be applied, and APC now manages the control target. This phase stays active until very close to feed completion. 8. When the material flow rate exceeds the minimum acceptable for APC control, APC starts to update the target using the K1 or K algorithm. Phase 4 Stop The material transfer is approaching completion and is close to where the high-speed instrument will interrupt the transfer when it detects the control target is equal to the high-speed controller instrument reading. Copyright World Batch Forum. All rights reserved. Page 13

14 9. The material transfer has reached a predetermined time before the feed should be completed, usually 5 to 1 sec. If the flow rate falls below the minimum acceptable for APC control anytime after this point, data from this feed will be considered unfit for use during the APC update process. 1. When the material transfer has reached this second predetermined time before the feed completes, usually to 3 sec, the APC function stops making a prediction and uses the last prediction made before this time for the remainder of the feed. 11. When the control target is equal to or less than the high speed controller instrument reading, the final control element is de-energized. The material flow is terminated. Phase 5 - Finish The logic controller identifies that the final control element has closed, the APC acquires accurate information about the transfer, determines if completion of the operation was within tolerances and sets the success/fail status and updates all data about the transfer. 1. The logic control system identifies the closure of the final control element. 13. After a predetermined time (adjustable for each different material) final instrument readings are taken. This allows for material drainage and valve reaction time to be taken into account. 14. If the data taken is acceptable, it is used to update the APC constants or else it is discarded. Phase 6 - Post The operator and/or equipment phase are told of the transfer completion and take actions indicated by the resulting information, such as tolerance errors. At the end of this phase the material transfer is complete and the control system can continue. 15. Evaluate results of the transfer in relation to the overall batch. If actions are required inform operations and wait for instructions. 16. The material transfer is determined to be satisfactory or operations determined it is necessary to continue, accepting any errors. Material Transfer Control Residency As can be seen by the 16 logical operations described above designing an automated material delivery operation is a significant amount of work and can consume a large part of any process controller. By moving the Start, ing, Stop and Finish Phases out of the controller and into the instrument device we have removed a significant amount of custom automation that a control engineer is responsible for. Our analysis indicates we have moved at least 8% of the custom programming normally required in a material delivery system. This not only relieves the control engineer of having to manage this custom code, it moves it to a location that is much more responsive and closer to the process. In the architecture shown in the Control Residency Diagram in Fig. 15 the controller selects a path through a steering matrix using isolated I/O through which the instrument device will provide the power signal used to directly open and close the final control element. Using this method any reasonable number of material feeds (at least 1) can be controlled by an instrument device cluster of up to flow meter/load cell delivery systems. The reaction time of cutoff detection to removal Copyright World Batch Forum. All rights reserved. Page 14

15 of power to the final control element should be less than milli-seconds, much faster than can be accomplished by a controller which must first acquire the instrument reading and THEN make a decision as to what to do with it and then do it. This process in many cases can take several seconds, much longer then the milli-seconds taken by the instrument device who s primary mission is to monitor the process and react as fast a possible. Architecture E-Net Instrument Device Start Finish ing Stop Pulse Signal Power to Output Card Steering Matrix X X X Output Card Output Card Output Card Controller X X X Power to Valve Power to Valve Power to Valve Control Net-Work Instrument Device E-Net Flow Meter Pre Pre Pre Start ing Load Cell Post Function Block Post Post Finish Stop Power to Output Card Controller Pre Start Instrument Device Stop Finish Material Transfer Phase Control Residency Controller Post Fig.15 Control Residency Diagram Conclusion In this discussion on material delivery you have seen that there are many factors that can influence a system s capability and that an instrument s capabilities are not the same as a delivery system s. Using APC s potential to reduce variability of material delivery can have a significant impact on process performance. We can develop real material savings by using Adaptive Predictive Control to reduce variation and then lowering the formulated targets of materials. The techniques to determine a delivery system s capability are not difficult and when properly used they provide detailed understanding of our processes. Being able to predict the future and be right is a real benefit, even if it s only a few seconds into the future. Using APC can reduce a systems physical cost and as well improving it s capability, a win in all aspects. By moving the APC function out of the controller and into the instruments greatly simplify the custom code in a controller and provides significant reaction time improvement that are beneficial to all concerned. Copyright World Batch Forum. All rights reserved. Page 15

16 Definitions Appendix: Material In Suspension V V 1 D W SUSP = Q MAX T Q MAX = Flow with wide open valve T = Time in suspension (varies with distance D and initial velocity V o ) Scale/Filter Lag W LAG = Q MAX F With constant flow rate Q MAX Weight Actual Weight Fed W(t) = Q MAX t W LAG(t) R(t) = Scale Starting Reading Weight Time F = Filter time lag Start F Valve Let-Through W VLT = Q MAX K V Q MAX = Flow rate with wide open valve K V = Constant depending on valve characteristics but Independent of flow rate Q MAX Deceleration Force F DEC = - Q MAX T - Q MAX V / 3. V V 1 D F DEC(1) F DEC() Q MAX = Flow with wide open valve T = Time in suspension V = Initial downward velocity NOTE: FDEC(1) = -WSUSP Material in suspension is offset by (part of) deceleration force SPILL = Q MAX [ F + K V -V o / 3.] V O = initial downward velocity Case 1: V O = or is independent of flow rate SPILL = K 1 Q MAX K 1 = F + K V -V o / 3. K 1 is independent of flow rate Q MAX SPILL = Q MAX [ F + K V -V o / 3.] V O = initial downward velocity Case : Vertical feed through valve V O is proportional to flow rate Q MAX Q i.e. V O = MAX ρ = material density ρ Α v Α v = valve area 1 SPILL = Q MAX [ F + K V ] + Q MAX [- ] ρ Αv 3. K 1 K SPILL = K 1 Q MAX + K Q MAX Copyright World Batch Forum. All rights reserved. Page 16