I came across negative feedback and positive feedback while reading, and I’m not sure what they mean; I’d like to ask the experts here for help

Simply put, the output signal is fed back to the input terminal and compared with the given signal before being output; feedback that results in a decreasing deviation is negative feedback, while feedback that causes the deviation to increase is positive feedback.

Could you be more specific? This is how I understand it: for example, in a pressure control loop, I set it to automatic adjustment; then a signal is sent to open the valve, and the pressure transmitter on the other side sends back a pressure signal. As the pressure keeps increasing, the degree of opening of the valve per unit of time gradually decreases – is this what negative feedback means?

It is not possible to determine whether there is negative feedback based on the fact that the deviation keeps increasing, because even in the case of negative feedback, if the system’s sensitivity is too high, the system will become unstable and the deviations will continue to increase.

Feedback can be divided into negative feedback and positive feedback. The former causes the output to have the opposite effect of the input, reducing the error between the system’s output and the system’s target, thereby bringing the system toward stability ； The latter causes the output to have a similar effect to the input, leading to an increasing system deviation and system oscillation, which can amplify the control effect.

I’ve seen these concepts in books before, but I just don’t understand them; so could you give an example to explain what negative feedback means?

There is a certain liquid level in the tank, and there is a valve at the outlet. If the actual liquid level is higher than the set level, then water needs to be released to lower the level. By opening the valve to allow water to flow out, the actual liquid level decreases, and the difference between it and the set value becomes smaller; this difference is negative

For example, in a pressure control loop with a set value of 5 kg, when the pressure exceeds 5 kg, the feedback is sent to the input side and compared with the set value; this generates a signal that causes the control valve to close, thereby reducing the pressure. This is negative feedback. Conversely, as the valve opens wider, the pressure increases; this is positive feedback.

For example, when you want to fill a basin with hot water, you naturally have an expected temperature in mind; this is the given or set value； Feel the temperature with your hand (to check the current temperature); if it’s a bit too hot, add some cold water (to lower the temperature of the water in the basin), and if it’s a bit too cold, add some hot water (to raise the temperature of the water in the basin) ； After several mixings, the desired temperature can be achieved. Note that the water added (hot or cold) causes the temperature in the basin to change in the opposite direction of its current temperature (it decreases if it’s high, and increases if it’s low); this is negative feedback. If the added water (hot or cold) causes the temperature of the water in the basin to change in the same direction as its current temperature – increasing it further if it’s already high, and decreasing it further if it’s already low – then it is positive feedback. (That guy’s head is full of water, ha! It should be noted that even with negative feedback, it may not be possible to achieve the desired temperature, for example due to being too sensitive to temperature deviations. When the temperature drops a little, a lot of hot water is added, which raises the temperature again; then a lot of cold water is added, causing the temperature to drop once more. In the end, it’s not a basin of water but a pool of water, and it’s not yet at the desired temperature.

In the case of negative feedback, when the system is unstable and experiences large fluctuations, it still oscillates around the set value; it does not move further and further away from this value, and the deviation does not increase. Otherwise, it is positive feedback.