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01 February 2004

Heat is on

Feedforward improves temperature control.

By Ralph Staus

The feedforward feature of the proportional, integral, derivative (PID) control system provides improved control for known disturbances to a continuous process. The control system is then able to act with superior, undisrupted regulation of the process, rather than with a reaction to an upset.

Batch mode operations are common in manufacturing, and control systems experience repeated upsets due to the nature of the batch operation. Hybrid systems containing a continuous process with upsets as batch processing starts or stops production provide a challenge to ensure product quality. The control system must respond quickly to changes in the process to produce quality product starting with the first rather than providing out-of-specification product until the system stabilizes. The batch operation in manufacturing is often programmable logic controller (PLC) controlled with the PLC initiating the action that upsets the loop control system. This same PLC can provide the control action input to improve the PID response. Feedforward will improve the quality of the control action for these systems, acting quickly to correct for the upset instead of reacting to the process deviation.

The batch process

In one process, operators apply a protective coating to tin-plated steel as an intermediate step in making three-piece cans for the food industry. That coating provides a protective layer to separate the can from food products that will go inside. The acid level of the food determines the type and thickness of the coating. This process operates in a batch mode, processing several thousand pounds of steel in a batch. Each sheet of steel gets an individual coating, and then the makers place it in a curing oven. This mode of processing allows the coating equipment to efficiently process many different plate sizes for each setup of a specified lacquer. The typical batch operation lasts twenty minutes followed by two minutes of idle time as the manufacturer readies the next batch. Operators process batches sequentially in a campaign with the same lacquer applied to each batch. Between campaigns an extended downtime of several hours is required to clean the equipment prior to using a new or different lacquer.

The lacquer cures in an oven heated by the 1,400°F hot air supplied by the burner system. Each zone of the curing oven has a PID-loop-controlled damper for oven temperature that regulates the quantity of hot air mixed with recalculated oven air. The oven temperature set point varies depending on the oven zone and the lacquer applied. Each sheet of lacquer-coated steel is placed on a carrier and transported through the oven. In the first zone the lacquer heats up and drives off the volatile gases. Later zones continue the process of drying and baking the coating as each sheet continues on the conveying system through the oven.

The continuous system

The burner system is a continuous process providing 1,400°F hot air to the oven. This air not only heats the product, but it also heats the conveying system. The oven conveying system continues to operate with or without product processed by the coating system, providing a continuous load for the burner. Volatile gases driven off during the curing process supplement the natural gas used to heat the system. Burning the volatile gases not only reduces the quantity of the fuel required to heat the product but also reduces plant emissions. Previous ovens had separate equipment to provide heat to the oven and equipment to reduce the emission of volatile gases. Combining these requirements in one piece of equipment introduced the batch upset to the continuous process.

This system works well and economically processes the lacquer thicknesses. The fuel costs are significantly less when compared to a system that relies only on natural gas burners for heat. In addition, the emissions of volatile gases go down, eliminating the requirement for additional postprocessing of the fumes and exhaust.

The temperature swings as each batch process started or completed were significant but accepted as unavoidable during the first several years of operation. A change in products and an increase in lacquer thickness resulted in excessive temperature excursions when these products underwent curing. With the natural gas valve near maximum position to maintain the temperature of the oven during the idle time, the start of a batch would result in excessive temperatures, triggering a shutdown of the process. The process then had to stabilize before restarting.

Restarting production resulted in another temperature excursion, shutting the process down again. Only after repeated restarts would the natural gas valve be in a low fire position, allowing the batch to continue without a burner system temperature alarm shutting down the coating equipment. The manufacturer's specification for the burner and oven equipment prohibited raising the allowable temperature to ride through the excursion.

PID tuning

The first attempt to resolve the disruption in production with the thicker lacquer coatings was to retune the PID loop. Before discussing the tuning of a PID controller, it is important to note that safety circuitry is required with all control systems. You should verify proper operation of applicable safety circuits prior to making any changes to controller setup parameters. Controller action may be unpredictable during the tuning process, and you must ensure safe operation of the system.

The equation for the PID control in the SLC-504 is in the literature provided by the manufacturer. In this equation, the reset term (1/TI) provides the integral portion of the output. The rate term (TD) provides the derivative portion of the output.

Controller gain (Kc) provides the proportional portion of the output and also modifies both the integral and derivative portion of the output.

The manuals have for many years provided guidance on the tuning of the PID control for standard systems. The current instruction set manual has a brief cookbook approach indicating the proportional gain (Kc) should be set to one-half the value needed to cause the output to oscillate when the reset (T_I) and rate (T_D) terms are set to zero. Set the reset (T_I) to the natural period of the oscillation and the rate (T_D) to one-eighth of the reset.

The first step of the tuning is to set the integral and derivative gain to zero. Doing this allows the system to operate with only proportional gain. The proportional gain then increases until the system begins to oscillate. Both the process temperature and the output of the controller oscillate due to the linear relationship of the equation, but the output oscillation is more readily observable. These oscillations should be small but sustained.

Set the reset term (T_I) or integral gain equal to the period of oscillation. Allowable values are from 0 to 327.67 minutes with RG = 1 and from 0 to 3276.7 minutes with RG = 0 for the SLC-504 processor. Set the rate term (T_D) or derivative gain to one-eighth of the integral gain. Allowable values are 0 to 327.67 minutes. Set the controller gain (Kc) to one-half of the value where the system oscillates.

This temperature control system is a relatively slow system. The thermal inertia of the load requires time to react to additional fuel. The tuning of the control loop occurred without the need of a high-speed chart recorder with the system operating on natural gas. This is not the mode of operation used during production, but the tuning values originally determined during start-up worked well during production runs until the change in lacquer thickness.

The manufacturer states that the method of tuning in the manual provides acceptable results on most systems. Additional information for tuning is available from organizations such as ISA. You should review this information for critical applications or if you desire a more complete understanding of tuning.

The operator evaluated the rules provided in these publications and then tuned and operated the system with these revised parameters.

All tuning setups did not prevent the temperature excursion from exceeding allowable limits at the start of production when products with thicker lacquer coatings start to cure. The proportion and integral gain in a PID control system are too slow to react to process changes. The output from these gains changes only after the process has deviated from the desired set point as a result of the increase in temperature as the volatile off gases joined the burner fuel. The derivative gain portion of the controller output is fast acting and drives the output to close the natural gas control valve to provide the necessary corrective action. Even the derivative action was too slow when compared to the rapid temperature rise from the addition of significant quantities of volatile gases. The resultant temperature alarm halted production before the natural gas valve had time to close. The process now needed a control that would predict the temperature excursion and take corrective action to prevent the excursion rather than react to it.

Feedforward

The proposed solution of adding feedforward would allow the process to anticipate the arrival of additional fuel. This process is PLC controlled for the PID temperature control and the material handling logic. Using the signal indicating that production had started, a feedforward value could help reduce the natural gas control valve position before the volatile off gases arrived. The use of feedforward to the PID control loop required knowledge of the arrival time and quantity of the volatile gases.

The time from when the batch initiates until the temperature starts to rise rapidly as the volatile gases fuel the burner is the sum of three delay times. First is the time required for the lacquer-coated steel to convey to the oven entrance. The second delay is the time it takes for the lacquer to increase in temperature to the point it drives off the volatile gases. Third is the transport delay for the volatile gases to get through the ductwork to the burner system. The time for the first two varies with operating speed, product sheet size, and oven temperature. The overall delay for the full range of setups and products remained between thirty and thirty-five seconds.

The operator selected the time value of thirty seconds as a starting point to test the effect of adding feedforward to the PID control. This would be to start closing the natural gas control valve before the temperature rose due to the additional fuel from the lacquers. No one expected the temperature to fall significantly due to the lag of the control valve and system thermal inertia for those setups resulting in a thirty-five second delay.

The value needed to quantify the feedforward was not the amount of volatile gases but the amount the PID output action needed to compensate for the gases. The operator observed the PID block output during the periods of stable operation with and without production. The output of the PID block during idle and production times varied with several parameters including the oven operating temperature and lacquer thickness. A value for the proposed feedforward came by averaging the required change in output from idle to production for several of the lightweight lacquers. These product weights ran for several years without problems prior to this project. The temperature excursions for these products, while significant, were not large enough to trigger the alarm and stop production. Using this value for feedforward would eliminate the excursion for some products and reduce it for others.

For this system, the reduction of the PID output value during production was 40% of the normal output where no volatiles were present. With two ovens producing volatile gases, operators would apply half of this value when each of the ovens started production and remove it when the oven stopped production.

The output of the PID block has a range of 0 to 16383 and is the sum of the PID equation and the feedforward. The PID block on the SLC-500 uses 23 words for the configuration and data storage. The feedforward/bias value goes in word 6 of the configuration. The feedforward/bias value has a range of –16383 to 16383. With both the time and amount of feedforward obtained the relay ladder logic could implement this corrective action.

Relay ladder logic

Feedforward came from the use of the TON, MOVE, and ADD blocks in the relay ladder logic. Programming allowed timers for each oven to delay the application of the feedforward for thirty seconds after the start of production. After the delay, a MOVE instruction loaded the required predetermined value into an intermediate register. A second timer for each oven enabled a MOVE to replace the value with a zero into the intermediate register after production stopped in that oven. An ADD block summed the feedforward values for each oven and placed the sum in word 6 of the PID configuration file.

PID control applied to batch operations experiences repeated upsets. Tuning the control loop for stable operation in several of the normal operating batch process modes may not provide a smooth transition between modes. Feedforward is a valuable tool for corrective input for processes that experience excessive process variable swings as the result of batch operations.

Feedforward implementation is a software change for systems that have PLC control for logic and PID loops.

The batch mode operation would provide volatile gases to the burner system, which would result in a rapid rise in temperature. The temperature excursion for this burner system before the use of feedforward exceeded the allowable limit for thicker lacquer applications triggering a halt in production. After the application of feedforward in the PID loop, temperature excursions did not exceed the allowable limits for all products. CP

Behind the byline

Ralph Staus is an assistant professor of engineering at Penn State University, Berks-Lehigh Valley College. He has twenty years of industrial engineering experience, fourteen of which were in factory automation in engineering, design, installation, and maintenance.