Many students are unfamiliar with what PID is, especially those who haven't studied automation. They often lack the necessary knowledge and procedures to understand it properly. This approach to learning is clearly inefficient. First and foremost, one must grasp the fundamental concept of PID before diving into its application.
So, why use a PID controller? Well, there are several reasons. One key reason is that the actual speed of a vehicle can be unstable due to external factors. Another is that we want the system to reach the target speed as quickly as possible. A speed control system typically operates in a closed-loop, ensuring stability throughout the process. Speed is an essential parameter in such systems.
Moreover, car speed is rarely linear. With so many external variables at play, it's nearly impossible to prove linearity. If the system were linear, a simple proportional control (P) would suffice. For instance, if 60% PWM results in 2 m/s, then 90% PWM might produce 3 m/s. However, perfect linearity is rare. Therefore, we need more advanced control methods—like PID—to achieve both speed and accuracy.
The challenge lies in adjusting the PWM to match the desired speed. It’s not just about being fast; it also needs to be precise and stable. The system must adjust using an algorithm, and PID is commonly used for this purpose. For example, if the current speed measured by an encoder is 2.0 m/s but the target is 2.3 m/s, you might increase the PWM slightly. But how much exactly? That's where the algorithm comes in.
PWM and speed are related, but the relationship isn’t known beforehand. You can’t just guess—trying small increments like 1%, 2%, or even 3% may not work. Instead, the PID algorithm calculates the change in PWM based on the error between the current and target speeds. It considers the error from previous adjustments and uses three parameters (P, I, D) to determine the optimal response.
In short, PID helps establish the relationship between PWM and speed, allowing the system to reach the target speed efficiently and accurately.
The input to the PID controller includes the current speed, the previous time step, and the previous speed. The output is the adjustment needed for the PWM—whether to increase or decrease it.
Imagine controlling a person to walk exactly 110 steps using PID. The controller would guide them to the correct position through proportional, integral, and derivative actions.
(1) **P (Proportional)**: The person walks toward 110 steps but overshoots or undershoots slightly. The controller adjusts based on the current error, but steady-state error remains.
(2) **PI (Proportional + Integral)**: The person oscillates around the target, gradually settling at 110 steps. The integral term eliminates steady-state error over time.
(3) **PD (Proportional + Derivative)**: The person approaches the target smoothly, avoiding overshoot. The derivative term predicts future errors, improving stability and reducing oscillation.
In summary, PID is a powerful tool for maintaining system stability and achieving precise control, making it essential in many engineering applications.
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