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PID stands for Port ID. In Spanning Tree Protocol (STP), when the Bridge ID (BID) and path cost of a received BPDU on a port are the same, the PID is compared to determine which port should be blocked. In digital TV multiplex systems, PID (Packet Identifier) acts like a file name, often referred to as a "flag code transmission package." In Engineering Control and Mathematical Physics, PID stands for Proportional-Integral-Derivative, an essential concept in classical control theory. The PID consists of an 8-bit port priority and a port number, with the default port number typically set at 128.
PID regulation is a fundamental method used in control systems within classical control theory. It combines proportional, integral, and derivative actions into a linear control law. The primary function of PID control is to minimize the deviation between the desired value (setpoint) and the actual measured value (process variable). By combining the proportional, integral, and derivative signals of the error, the controller generates a control output that adjusts the process accordingly.
The mathematical expression of this control quantity is typically represented as:
$$
u(t) = K_p e(t) + K_i \int_0^t e(\tau) d\tau + K_d \frac{de(t)}{dt}
$$
Where $ K_p $ is the proportional gain, $ K_i $ is the integral gain, and $ K_d $ is the derivative gain. Increasing $ K_p $ can reduce steady-state error, but if too large, it may lead to oscillations or instability. A larger $ K_i $ results in weaker integral action, slowing down the elimination of steady-state errors but improving stability. Increasing $ K_d $ enhances the system's responsiveness, reduces overshoot, and improves dynamic performance.
There are various analog and digital PID regulator products available on the market, and users can apply them based on their specific needs.
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**PID Parameter Tuning Methods and Setting Skills**
PID stands for Proportional-Integral-Derivative. While programming PID controllers isn’t the main challenge, tuning the parameters is often the most difficult part. Understanding the physical meaning of each parameter is key to effective tuning. The principle of PID control can be better understood by observing manual control strategies, such as adjusting furnace temperature. This article doesn't require advanced mathematics to grasp the basics.
**1. Proportional Control**
An experienced operator manually controls the temperature of an electric furnace to achieve excellent control quality. This approach shares similarities with PID control. In proportional control, the operator observes the current temperature, compares it to the setpoint, and adjusts the heating current proportionally to the error. If the temperature is below the setpoint, the potentiometer is turned clockwise to increase the current; if it’s above, it’s turned counterclockwise. This is known as proportional control, where the output is directly proportional to the error.
However, delays in the system can make adjustments less immediate. If the proportional gain is too low, the system responds slowly. If it's too high, it may cause overcorrection and instability.
**2. Integral Control**
Integral control corresponds to the area under the error curve over time. It accumulates past errors to eliminate steady-state error. For example, in manual control, the operator might adjust the potentiometer periodically based on the current error. The integral term ensures that the system continues to adjust until the error is zero. However, too much integral action can cause instability, while too little makes the system slow to respond.
**3. PI Control**
Combining proportional and integral control (PI) helps eliminate steady-state error while maintaining reasonable dynamic performance. The integral term has a delay effect, so it’s rarely used alone. PI and PID controllers are widely used because they overcome the limitations of pure proportional or integral control.
**4. Derivative Control**
Derivative control predicts future changes in the error based on its rate of change. It helps reduce overshoot and improve stability. For instance, if the temperature is rising too fast, the controller can anticipate this and reduce the heating current in advance, similar to aiming ahead when shooting a moving target.
**5. Sampling Period**
The PID control algorithm runs periodically, and the time between these runs is called the sampling period. A shorter sampling period provides more accurate data but increases computational load. Too short a period may not provide useful information, especially if the error changes slowly.
**6. PID Parameter Adjustment Method**
Tuning PID parameters involves experimenting with the effects of each parameter on the system’s performance. Experienced engineers can quickly adjust the settings based on observed behavior. Key steps include starting with conservative values, gradually increasing the proportional gain, adjusting the integral time to eliminate steady-state error, and adding derivative control if needed.
**7. Experimental Verification**
An experiment using the S7-300 PLC’s FB 41 PID block was conducted to test the control strategy. The controlled object consisted of two series inertia links with time constants of 2s and 5s, and a gain of 3.0. The results validated the effectiveness of the proposed PID tuning method.
The author plans to release a STEP 7 project for software-based PID tuning in the future. This article draws from the author’s previous work published in the “Automation Application†magazine, offering practical insights into PID parameter tuning.