***interstates
Author |benjamin langton
It is important to understand the principles of proportional-integral-derivative (PID) control and the parameterization involved in closed-loop, open-loop, and cascading loops.
Tuning a control loop is a complex activity driven by relatively simple control laws. The goal is to coordinate one or more parameters to achieve a process that is stable within the boundaries of the property. This article is an introduction to the scaling-integral-derivative (PID) control loop process.
PID control is based on feedback. Measure the output of a device or process and compare it to a target or set value. If a discrepancy is detected, a correction is calculated and implemented. Measure the output again and recalculate any necessary corrections.
Not every controller uses all three math functions in the PID. Many processes can be processed to an acceptable level with proportional-integral terms. However, fine control, especially to avoid overshoot, requires the addition of differential control.
In proportional control, the correction factor is determined by the size of the difference between the set value and the measured value. The problem is that when the difference is close to zero, the correction is also close to zero, with the result that the difference never becomes zero.
The integral function solves this problem by considering the cumulative bias. The longer the difference between the set value and the actual value lasts, the greater the calculated correction factor. However, when there is a delay in the response to the correction, this can lead to an overshoot and may oscillate around the set point. Avoiding such an outcome requires a differential function, which looks at the rate of change achieved, progressively modifying the correction factor to reduce its effect as it approaches the set value.
Even if the equipment is essentially the same, each process has its own unique properties. For example, the airflow around the oven changes, the ambient temperature changes the density and viscosity of the fluid, and the atmospheric pressure changes over time. The PID settings (mainly the gain applied to the correction factor and the time used in the integral and derivative calculations, known as "reset" and "rate") must be selected to accommodate these local differences.
It may be beneficial to divide the process into the following four categories:
fast circuits, such as flow and pressure;
Slow loops, such as temperature;
integration processes such as liquid level and insulation temperature;
Noisy loops with constantly changing measured values.
Closed-loop tuning process
The first step in adjusting a closed-loop loop is to understand the process. Identify the loops that need to be adjusted and determine the speed of the loops. If the response time of the loop goes from less than 1 second to about 10 seconds, it is a fast loop, and the use of a PI controller is sufficient. If the response time of the loop is a few seconds to 30 seconds, a PI or PID controller can be selected. For slow loops with a response time of more than 30 seconds, a PID controller is recommended.
The second step is to understand the controller. Proportional items can be proportional gains or proportional bands. The integral term can be a time constant, a reset rate, or an integral gain (reset rate multiplied by proportional gain). The differential term can be either a time constant or a differential gain (the differential time constant multiplied by the proportional gain). In this article, proportional gain, integral replacement rate, and differential gain are assumed.
The final step is to observe the response. First, change the setpoint (less than 5%) slightly, or wait for the process to interfere. The process variables and control output responses are then observed.
If there is no transient change in the control output or no significant overshoot, increase the proportional gain by 50%.
If the process variable is unstable or oscillates continuously, and the overshoot is greater than 25%, the proportional gain is reduced by 50% and the integral replacement rate is reduced by 50%.
If there is a tolerable overshoot for the process variable oscillation, reduce the proportional gain by 20% and the integral replacement rate by 50%.
If there are three or more consecutive peaks when the setpoint changes, reduce the integral replacement rate by 30% and increase the differential gain by 50%.
If the process variable remains relatively stable around the setpoint for a long period of time after changing the setpoint or after the disturbance has begun, the integral replacement rate is increased by 100%.
Repeat the above steps until the closed-loop response is satisfactory.
Figure: Non-integral process model. **controlsoft
Open-loop tuning process
Similar to a closed-loop process, it starts with an understanding of the process and the controller. For non-integral loops, use the following procedure:
Put the loop into manual control mode, keep the control output constant, and wait for the process to be stable.
Make a small step change (less than 10%) on the control output and observe the response.
Evaluate the process model, where:
The model gain is equal to the process variable divided by the control output variation.
The dead time is equal to the time interval between the change in control output and the observable change in the process variable.
The time constant is equal to the time it takes for a process variable to reach 63% of the total variation.
Select an initial PID value, for example:
p is equal to 2 divided by the model gain.
i is equal to the dead time plus the time constant.
d is equal to dead time divided by 3 or time constant divided by 6.
These initial PID values should provide a reasonable closed-loop response. The controller is fine-tuned using a closed-loop tuning method.
For cascade control applications, such as tanks heated by steam valves, the adjustment procedure starts with the inner circuit, followed by the outer circuit. Due to the dynamic nature of the inner loop and the interaction of the outer loop, only one loop is adjusted at a time.
Put the outer loop in manual mode.
A closed-loop tuning procedure is performed on the inner circuit, and then the inner loop is placed in an automatic state.
Wait for the outer loop to stabilize.
Closed-loop tuning procedures are performed on the outer circuit.
PID controllers are commonly used in industrial automation for process control to tune flow, temperature, pressure, level, and other process variables. Proportional and integral controllers are essential for most control loops; Differential mode is often used for motion control. Temperature control typically uses all three control modes.
Key Concepts:
PID loop tuning comes in many forms, and engineers must understand what loop tuning is required for the application, as well as ensure that the parameters required for accurate measurements are provided.
Learn the difference between closed-loop and open-loop tuning processes, and where they are best suited.
Think about it:
What role does PID tuning play in your facility?
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In the October 2023 issue of Control Engineering Chinese China in the "Application Cases" section: Sustainable machine design benefits intralogistics.
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