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DeltaV Tune is a flexible tool that is suitable for tuning most control loops. Because it is flexible and easy to use, DeltaV Tune requires little initial setup. However, to achieve the best results with DeltaV Tune, adhere to the guidelines specified in the Tuning Rules and Selecting a Controller Design topics.
Tuning Rules
To obtain proper tuning results, observe the following basic rules: Equipment must be in good operating condition.
Studies on thousands of loops showed that valve problems cause 30 to 35 percent of loop tuning problems. This means that one out of every three loops needs valve improvements to make proper loop control.
If the calculated values seem out of the ordinary, this might indicate valve problems. If the gain of the calculated values is too high or the reset is too small, check your valves. Many valves experience the stem's sticking or hysteresis in movement that cause improper model identification and loop tuning.
large. However, to achieve optimal results, make sure that PV is close to SP before you start tuning.
If the SP and PV values are not close, a warning is issued when Test is initiated. After you select Force, the identification continues. The Step size must be large enough for the tuner to oscillate.
The output (OUT) change (or Step size) must cause an appropriate change in the PV. If the change in the PV is not large enough, the tuning process might not start at all because there are no self-oscillations. Full oscillation might not be observed under the following conditions:
Load disturbances affect the PV. The system is too noisy.
A valve is stuck.
If no oscillations occur, stop DeltaV Tune by clicking the Abort button. Then, check for anything that might affect the oscillations. Your process must be able to make the step change required for tuning.
Make sure that the step change does not cause a relief valve to open or any interlocks to be triggered, either of which could affect the tuning results.
When tuning, neither the SP, the PV, nor the output (OUT) should be near its limits.
When you tune a loop, the SP and PV should not be within 10 percent of the loop's engineering unit limits and the output (OUT) should not be within 10 percent of the configured limits. If these values are outside of the specified range, the oscillations could be affected and the calculations of loop dynamics would be inaccurate.
Selecting a Controller Design
DeltaV Tune provides a wide choice of tuning rules and features that can be used to provide the best tuning for a loop. Here are some general guidelines:
After you change tuning values, observe the loop for a period of time to see its reaction to noise, disturbances, or small SP step changes.
modify the process type or click the Expert button to use an advanced tuning rule.
If the loop performance is still not satisfactory, you can try an alternative process type or the Expert tuning selection (without retuning the loop).
General guidelines for tuning rule selection:
The PI design is the most common choice for flow, liquid level, and gas pressure control.
PID control, along with feedforward and cascade strategies, is often used for temperature control, pH control, and composition control. (The composition control might require some limits on derivative action.)
If you have tuned a loop by trial and error and it works well, you might consider using DeltaV Tune to verify your settings and tune your loop more precisely. If the values calculated by DeltaV Tune are drastically different than your trial and error values, consider the new values closely before changing block tuning parameters.
The PID designs are limited for some applications. For example, if the ratio of the time delay of a loop to time constant exceeds 1, the PID performance cannot be guaranteed. The tuning results must be carefully verified. If the response is not acceptable, you might need to modify the tuning results. For such applications, using the Smith predictor for control might provide better results. (Refer to the PID_DEADTIME Module Template topic.)
For more information on rule selection and associated controller designs, refer to the Expert Feature topic.
Application Examples
This section provides some general application examples to help you use DeltaV Tune to tune your process loops. The following sections provide examples of a single feedback loop, a cascade control loop, and a feedforward control loop.
Tuning a Single Feedback Loop
For an example of tuning a single feedback loop, refer to the temperature loop in the following figure. In this example, steam is fed into the heat exchanger, and water is passed through a header in the steam heater. The water absorbs heat from the steam.
Steam Heater
In the Tuning Calculation panel, the process type should be Temperature. You would probably start tuning this loop with a Normal loop response setting, unless the loop has some unusual characteristics. If you select the Expert button, then you should start tuning with a tuning rule for PID.
After tuning the loop and placing it in operation, observe the response to SP changes and load disturbances caused by process upsets. If the loop demonstrates too much overshoot, first try changing the loop response to SLOW and update the tuning. If you still experience too much overshoot for the process, you can select a different advanced tuning rule with lower gain. You can experiment with different tuning rules until you get the results you want.
Tuning Cascade Control Loops
You can use DeltaV Tune to tune cascade control loops. However, because the loops for cascade control are interactive, you must tune them in a particular order. You need to tune the secondary (or slave) loop first and the primary (or master) loop last. For example, refer to the cascade loop for controlling level in the following figure. The primary loop controls the level. The secondary loop controls the inlet flow.
Cascade Level Control
1. Put the secondary controller into Auto or Man mode to allow the loop to settle. 2. Tune the secondary loop as if it were a single feedback loop using DeltaV Tune. 3. Switch the secondary controller to Cascade mode.
4. Adjust the SP of the primary controller to match the PV. 5. Tune the primary loop using DeltaV Tune.
Use these same steps for tuning most cascade loops.
Tuning Feedforward Control Loops
You can tune the feedback portion of a feedforward control loop using DeltaV Tune, but you must hold the feedforward value constant during tuning. Begin with the Normal dynamics setting.
The temperature control feedforward loop in the following figure is similar to the single-loop steam heater example, but the flow rate for the water flowing into the heat exchanger is fed forward to the temperature controller.
To use DeltaV Tune to tune the feedforward control loop in the following figure, perform these steps: 1. Disable the feedforward portion of the loop and hold the input constant.
Typically, you can disable the feedforward input by changing the FF_ENABLE parameter of the block from True to False. 2. Using DeltaV Tune, tune the control loop as if it were a single feedback loop.
Feedforward Control Diagram
Use the same procedure to tune most feedforward control loops.
The guidelines and examples contained in this section are suggestions for typical control loops. Select the approach that is best for your particular application.