Many students don't know what PID is, because many students are not automated. They need information and procedures. This is obviously the wrong way to learn, at least, first of all, you have to understand what PID is.
First, why do you want to do a PID?
Due to external reasons, the actual speed of the car is sometimes unstable. This is one of the two. It is the second time that the car can reach the set target speed in the fastest time. The speed control system is a closed loop, which can meet the stability requirements of the whole system. The speed must be one of the system parameters. This is the third. The car speed is certainly not linear. There are so many external factors, no one can prove to be linear. If it is linear, use P directly. For example, when PWM=60%, the speed is 2M/S, then if you want it 3M/S, increase the PWM to 90%. Because 90/60=3/2, this is perfect. Perfection is impossible. Then it is not linear. How do you control the PWM to achieve the speed? That is to be fast, but also to be accurate, but also to be jealous. (that is, fast-tracking) The speed adjustment process of the system must be adjusted by an algorithm. The general PID is the algorithm used. Maybe you will think that if the current speed measured by the encoder is 2.0m/s and the speed is 2.3m/s, then can I increase the pwm by a little bit? Yes, how much is the increase in pwm? It is necessary to pass the algorithm, because PWM and speed are related to each other, and no one knows the whole system. To try it a little bit, add 1%, not enough, plus 1% is not enough, then you will add 1% for the third time? It is very likely that 2% will be added. An expression is obtained by three parameters of PID: △PWM=a *ΔV1+b *ΔV2+c *△V3, abc is developed by the long formula of PID, and then the reduced number, ΔV1, △V2, △V3 The speed difference after the first adjustment, the speed difference after the second adjustment, the third time. . . . . In a word, PID needs to establish the relationship between pwm and speed to make the current speed reach the target speed.
What input and output is the previous speed, the previous time and the previous speed. The output is how much your PWM should be increased or decreased.
Control model: You control a person to let him walk 110 steps in a PID controlled manner and then stop.
(1) P proportional control, that is, let him take 110 steps, he went to a hundred steps (such as 108 steps) or more than 100 steps (such as 112 steps) at a certain pace to stop.
Explain that P proportional control is one of the simplest control methods. The output of its controller is proportional to the input error signal. The system output has a Steady-state error when there is only proportional control.
(2) PI integral control, that is, he walked to step 112 at a certain pace and then went back and forth, went to the 108 step position, and then turned back to the 110 step position. Shake back and forth a few times at the 110 step position and finally stop at the 110 step position.
Note In the integral I control, the output of the controller is proportional to the integral of the input error signal. For an automatic control system, if there is a steady state error after entering the steady state, the control system is said to have a steady state error or a system with Steady-state Error. In order to eliminate the steady-state error, an "integral term" must be introduced in the controller. The integral term versus the error is integrated over time, and as time increases, the integral term increases. Thus, even if the error is small, the integral term increases with time, which pushes the controller's output up so that the steady-state error is further reduced until it equals zero. Therefore, the proportional + integral (PI) controller can make the system have no steady-state error after entering the steady state.
(3) PD differential control, that is, he walks to a certain step at a certain step, and then slowly approaches the position of 110 steps. If the position can be accurately stopped at 110 steps, there is no static difference control; If you stop near step 110 (such as 109 or 111), there is static control.
It is explained that in the differential control D, the output of the controller is proportional to the differential of the input error signal (ie, the rate of change of the error). The automatic control system may experience oscillation or even instability in the process of overcoming the error. The reason is that there is a large inertia component (link) or a delay component, which has the effect of suppressing the error, and the change is always behind. The change in error. The solution is to make the change of the suppression error "advance", that is, when the error is close to zero, the effect of suppressing the error should be zero. That is to say, it is often not enough to introduce only the “proportional P†term in the controller. The proportional term is only the amplitude of the amplification error, and the current need to increase is the “differential termâ€, which can predict the trend of error variation. . In this way, the controller with proportional + differential can make the control effect of the suppression error equal to zero or even a negative value in advance, thereby avoiding the serious overshoot of the controlled amount. Therefore, for a controlled object with large inertia or hysteresis, the proportional P+ differential D (PD) controller can improve the dynamic characteristics of the system during the adjustment process.
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