In industrial automation control systems, regulating valves, as actuators, undertake the important task of regulating parameters such as flow, pressure, and temperature. However, due to the nonlinear characteristics of regulating valves (such as dead zone, friction, hysteresis, etc.), they are prone to cause system instability or slow response in the actual control process. To solve this problem, feedforward compensation technology is widely used to improve control accuracy and response speed.
The basic idea of feedforward compensation is: before the system is disturbed, the corresponding control action is applied in advance according to the magnitude and direction of the disturbance, so as to offset the influence of the disturbance on the system output. For regulating valves, feedforward compensation is usually used to overcome their nonlinear input-output relationship and improve control accuracy.
I. Principle of feedforward compensation
Feedforward compensation control does not depend on feedback error, but is based on direct measurement or estimation of disturbances or input signals. By designing a feedforward controller, the regulating valve is adjusted in advance. The combination of feedforward control and feedback control can significantly improve the dynamic performance and disturbance resistance of the system.
In regulating valve systems, common feedforward structures include:
1. Open-loop feedforward control: compensation is made only based on input or disturbance signals, without relying on feedback.
2. Composite feedforward-feedback control: feedforward is used for rapid response to disturbances, feedbac
is
used to correct residual error, ensuring system stability.
Two, Design and calculation of feedforward compensator
To design the feedforward compensator of the control valve, it is first necessary to obtain the static and dynamic characteristic curves of the control valve, usually including:
- The relationship between the input signal (such as 4~20mA current signal) and the valve opening;
- Valve flow characteristics (linear, proportional, quick opening, etc.);
- Dead zone and hysteresis of the valve, etc. nonlinear factors.
# 1. Establishment of static feedforward compensation function
Assuming the input signal of the valve is u and the output is the actual opening of the valve x, in an ideal case, we hope that x is linearly related to u:
x = K \cdot u
But in reality, the
valve may have nonlinearity, that is:
x = f(u) + d(u)
Among them, f(u) is the ideal input-output relationship, and d(u) is the nonlinear deviation. The goal of feedforward compensation is to design a compensation function g(u) such that:
x_{\text{comp}} = f^{-1}(x_{\text{desired}})
That is, the control signal output by the feedforward controller is the inverse function of the desired opening, used to offset the nonlinear effect.
# 2. Design of dynamic feedforward compensation
For dynamic systems, the response time and inertia of the control valve also need to be considered. Dynamic feedforward compensation usually uses transfer function models or state space models to describe the dynamic behavior of the system. For example, if the dynamic model of the control valve is:
G(s) = \frac{X(s)}{U(s)}
Then the transfer function of its feedforward compensator should be:
G_{\text{ff}}(s) = G^{-1}(s)
In this way, the system can achieve 'perfect' tracking of the input signal.
Three, Matters to be noted in practical applications
1. Difficulty in precise modeling: Due to the influence of factors such as environment, medium, and wear on the control valve, precise modeling is relatively difficult, and often requires identification through experimental data.
2. Robustness requirements: To cope with model errors, the feedforward controller should have a certain degree of robustness, which is usually combined with PID feedback control.
3. Online adaptation: Some advanced control systems use online identification technology to dynamically adjust feedforward compensation parameters to adapt to the working condition changes of the control valve.
4. Safety considerations: The feedforward control should have upper and lower limit protection and fault safety mechanisms to prevent control failure due to model errors.
Four, Conclusion
The feedforward compensation technology for control valves plays an important role in improving system response speed and control accuracy by predicting and compensating for the nonlinear characteristics of the controlled object in advance. Although its design and implementation are relatively complex, combined with modern control theory and computer technology, it has been widely used in industrial process control. In the future, with the development of artificial intelligence and big data technology, intelligent feedforward compensation will become more accurate and adaptive, further enhancing the performance level of the control valve control system.
