Regulator positive and negative feedback

B. When the given value decreases and the controller output increases, the regulator is positive acting

Multiple choice C； Multiple choice: B, C. When discussing control principles (a key focus of university courses), feedback control (where the controlled element follows changes in the setpoint) is usually the main topic. The rule regarding the action of a controller is as follows: error = measurement – setpoint; when this value increases, the amplification factor of the control system is positive, indicating positive action. At this point, both B and C are correct. When teaching control engineering (a key focus in technical schools and training programs), only constant-value control (with a fixed setpoint) is usually covered. The rule regarding the forward and reverse action of a controller is as follows: when the measured value increases, the output of the regulator increases, which is considered forward action. At this point, we don’t talk about what’s given. So just choose C.

If it were me, I wouldn’t choose either; first, I would check what technical requirements the medium needs to meet. Choose whether the valve should operate on air opening or air closing principle. If air opening is selected, then for a falling liquid level a direct-acting regulator should be used; if the liquid level rises, an inverse-acting regulator should be chosen

Understand this question based on the way it was formulated. First, it is important to understand one concept: the controller and regulator in this question can be regarded as having the same meaning. Some also believe that the controller consists of regulators. But follow the meaning of the title. The input to the regulator is the deviation of the measured value at time t from the equilibrium state. That is, the regulated input = I_in(t) – I_in_balance; here, the balanced state refers to the input condition when the measured value equals the set value, that is, when the deviation is 0 and the output is at a stable value. The regulator output is the difference between the regulator output at a certain time t and the equilibrium state. That is, the regulated output = I_out(t) – I_out_balance. Once these concepts are understood, this problem becomes easier to grasp. First, looking at A: as the given value increases, the deviation decreases; however, since the output increases, the regulator must operate in a reactive manner. Therefore, A is incorrect ； Now looking at B: as the given value decreases, the deviation increases and the output increases; thus the regulator has a positive effect, so B is correct ； Finally, looking at C: as the measurement increases, the deviation increases and the output decreases; the regulator operates in a reactive manner, so C is correct. Therefore, the correct answer should be B and C