Using HC900 HC900 Acc utu ne III Introduction
This document provides information information on using the Accutune III features that are available in a PID block in the HC900 controller. All of the following features are available on controllers with firmware version 4.0 and later. Earlier versions had Accutune II, which provided a subset of the features listed below. Accutune III is an automatic tuning algorithm which will accurately ac curately identify and tune any process. This speeds up and simplifies start-up and also allows retuning at any setpoint. It may be used for self-regulating and single integrating processes. It can be initiated on-demand, whenever loop tuning is required. Prior knowledge of the process dynamics or initial process line-out to setpoint is not necessary. The setpoint value is not required to change in order to initiate the tuning process, but the controller must be in automatic mode to start tuning. The process need not be in a static (lined out) state and may be dynamic (changing with a steady output). At the end of the tuning process, the controller c ontroller immediately immediately calculates the new tuning constants and stores them in the PID block. Control resumes with the new tuning parameters. Remember Remember to upload your configuration and save it once tuning is complete. Tuning Criteria
Tuning criteria can be set for either Normal or Fast. For the Normal selection, the controller calculates conservative tuning constants with the objective of minimizing overshoot. If the controller determines that the process has appreciable a ppreciable deadtime, it will automatically default to use Dahlin Tuning, which produces very conservative tuning constants. The Normal selection may be useful for Three Position Step Control (TPSC) and Position Proportional applications, as it reduces “hunt” problems for the motor. This selection is also recommended for applications that have significant deadtime. For the Fast selection, the controller calculates aggressive tuning constants with the objective of producing quarter-damped response. response. Depending upon the process, this selection will usually result in some overshoot. Fuzzy Fuzzy Overshoo Overshoo t Suppressio n
Fuzzy Overshoot Suppression minimizes process variable (PV) overshoot following a setpoint (SP) change or a process disturbance. This is especially useful in processes that experience load changes or where even a small overshoot beyond the setpoint may result in damage or lost product. The fuzzy logic in the controller observes the speed and direction of the PV signal as it approaches the setpoint and temporarily modifies the internal controller response action as necessary to avoid an overshoot. There is no change to the PID algorithm, and the fuzzy logic does not alter the PID tuning parameters. This feature can be independently enabled or disabled as required by the application to work with the Accutune algorithm. Fuzzy Overshoot Suppression Suppression should not be enabled for processes that have an appreciable amount of deadtime. Fuzzy Overshoot Suppression should should be disabled during Accutune and can be enabled once tuning is complete.
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Using HC900 Acc utu ne III Cycle Tuning
With cycle tuning, tuning parameter values are derived from the process response to the action of causing the PV to oscillate about the SP value. This tuning method uses the measured ultimate gain and period to produce tuning parameter values. Cycle tuning does not distinguish between process lags and always results in Gain based on PV amplitude. It calculates values of Reset and Rate based on the time of the SP crossings using a fixed ratio of 4 to 1. (The Reset value is always 4 times the Rate value.) This method does not require a stable process initially and the process may be moving. Cycle tuning is applicable to TPSC and can be used for integrating processes such as Level control. This tuning method will cycle the controller’s output three full cycles between the low and high output limits while allowing only a very small Process Variable (PV) change above and below the Setpoint (SP). During cycle tuning, the control output will automatically switch between output limits when the PV deviates from SP by +/- 0.22% of the full PV range. For example, using cycle tuning on a loop with a PV range from 0 to 500 degrees F, the Accutune III algorithm will trigger a switch of the output when the PV deviates 1.1 deg F from SP. This results in a very minor disturbance to the running control loop and allows the user to invoke cycle tuning as required. Configure the PID block as follows: 1. On the PID block Accutune III tab, set the PID block Accutune III type to Cycle Tuning. 2. Disable PV Adaptive Tuning and Fuzzy Overshoot Suppression. 3. Set the Tuning Criteria to Normal or Fast depending on the required process response. You can make the following changes using Hybrid Control Designer software (HCD) in monitor mode or from the Operator Interface. 4. Put the loop in Automatic and check that Tune Status shows “Ready.” Start Accutune, Accutune in Progress will turn on. Tune Status will change to “Tune Running.” The PID block’s output will cycle three full cycles between the low and high output limits. 5. Once tuning is complete, Accutune In Progress will turn off, and Tune Status will change to “Ready.” Upload your configuration and save it once tuning is complete. Setpoin t Tuning
Setpoint tuning is based on the process response to a SP change. This tuning method identifies the process characteristics such as lags (one or two), measures process deadtime and offsets, and from this data calculates new tuning parameter values. Configure the PID block as follows: 1. On the PID block Accutune III tab, set the PID block Accutune III type to SP Tuning. 2. Disable PV Adaptive Tuning and Fuzzy Overshoot Suppression. 3. Set the Tuning Criteria to Normal and SP Tuning Direction to Up. 4. Set the Process Gain to 1, the autotune algorithm will recalculate it for you. The SP Tune Change can be between 5 and 15 percent. Note that this is actually the percent change in the PID output during tuning. Start with the Gain set to 1.0 (or Proportional Band=100%) and the Reset set to .05 minutes (or 20 repeats per minute) . Page 2 of 3
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Using HC900 Acc utu ne III Setpoin t Tuning (conti nued)
Please note, although the tuning algorithm optimizes PID response resulting from setpoint changes, the percent change value used to initiate an upset for identifying process dynamics represents an output value change of the control algorithm. During the tuning procedure there is no visible change to the controller’s working setpoint. The recommendation is to use the largest output change your process can tolerate for the identification process. If the process upset is too small, the algorithm may fail to converge on suitable tuning parameters. You can make the following changes using HCD in monitor mode or from the Operator Interface. 5. Put the loop in Manual mode and adjust the output until the PV equals the desired starting SP. Let the loop stabilize. The process must be stable for the tuning constants to be calculated correctly. 6. Put the loop in Automatic. Wait for the Tune Status to change to “Ready.” Start Accutune, and Accutune in Progress will turn on. Tune Status will change to “Tune Running.” 7. The SP tuning algorithm will begin to tune using the percent output change set above. The PID block output will change on its own with no visible setpoint change. 8. Once the tuning is complete, Accutune in Progress will turn off. The Tune Status will change to “Ready.” Upload your configuration and save it once tuning is complete. Duplex Tuning
This method is used in heat/cool applications. It can be set for Manual, Automatic or Disable. With manual, tuning must be initiated manually for each side. The current LSP or RSP value is used as the target SP for the desired heat or cool side tuning. For the heat side, the output cycles between 50 percent and the high output limit and for the cool side the output cycles between 50 percent and the low output limit. Tuning values are calculated and stored only for the side tuned. With Automatic tuning, heat and cool tuning are performed sequentially. When set for Disable, duplex type tuning is disabled and simplex type tuning is used instead. PV Adaptive Tunin g
This method continuously adapts the process tuning to changing system characteristics over time. When the PV deviates from the SP by a certain amount for any reason, the adaptive tuning algorithm becomes active and begins to monitor the resulting PV action. If the process becomes unstable and oscillates, it will eventually bring the process into control by retuning gain, rate and reset as needed using a systematic approach defined by an expert based method of tuning rules. Should the process not oscillate but instead be either too fast or sluggish, a different set of expert rules is applied to retune the controller for proper response. Need More Help?
Refer to the PID block section in the HC900 Function Block Reference Guide (51-52-25-109) for examples of different types of PID control. You can also contact our Global Technical Support at (800) 423-9883.
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