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Changhui Instruments discusses in this article the characteristics of 2-degree-of-freedom PID control, its implementation methods, and the application of 2-degree-of-freedom PID control in temperature controllers, which can be useful for instrument technicians to gain a deeper understanding of PID and to select appropriate temperature controllers. As measurement devices in the production process became more advanced, faster, more complex, flexible, and centralized, the most fundamental control algorithms evolved from the previously used PID control to digital regulators that utilized microprocessors. A variety of PID algorithms have since emerged, and 2-degree-of-freedom PID control is one of these control algorithms. Classic PID control still has shortcomings in practical applications: although actual PID control systems have two types of optimal control constants – those for suppressing external disturbances and those for tracking the setpoint – classic PID control is a 1-degree-of-freedom control method that allows only the setting of one type of control constant. Therefore, if the control constants are set to optimize the performance in tracking a given value, the ability to suppress external disturbances becomes poor; whereas if the control constants are set to optimize the ability to suppress external disturbances, the tracking performance for the given value becomes oscillatory. Therefore, it is necessary to make the settings during operation, resulting in poor control performance. In order to eliminate these inherent shortcomings of conventional PID control, Changhui Instruments has developed a 2-degree-of-freedom PID control method in which two types of control constants – those optimal for suppressing external disturbances and those optimal for tracking the set value – are adjusted independently, while the parameter that needs to be adjusted remains the same. This approach results in a simpler adjustment process and significantly improved control performance compared to traditional PID control; it has been applied in the YR-RJD series of general-purpose temperature controllers, achieving excellent control results in processes such as temperature, flow rate, and pressure control. The functions of 2-degree-of-freedom PID control must include all the basic functions of control systems discussed earlier, such as automatic regulation, adaptive control, and various types of lead (feedforward) control. If 2-degree-of-freedom PID control is used for process control, the various changes in the controlled object are eliminated due to the damping characteristics of the control loop, thereby allowing the operating characteristics of the device to be altered. Before introducing the 2-degree-of-freedom PID control method, Changhui Instruments discusses the lead differential control method first. The derivative lead-type lead-differential PID control, which is widely used in digital control, differs from conventional PID control based on control error in that it performs a PI operation on the control error in order to prevent sudden changes in the derivative when the setpoint changes; the derivative term only acts on the PV (process variable). The basic functions of this lead-differential PID control system are: ① Suppression of changes in external disturbances. ②Tracking performance for given value changes. However, for these two functions, the optimal values of the control constants (PID parameters) are not one but two; there is a control constant that is optimal for suppressing external disturbances, and another that is optimal for tracking a given value. As mentioned earlier, although there are two types of optimal control constants in PID control systems, the previous 1-degree-of-freedom PID control method could only set one type of optimal control constant. Figure 1 shows the variation in the tracking of the setpoint characteristic when the optimal control constant for suppressing external disturbances is set ； Figure 2 shows the variation in the characteristic of suppressing external disturbances when the optimal control constant for tracking a given value is set. Figure 1 shows that when PID control focuses on disturbance response, the response to the setpoint deteriorates. Figure 2 shows that when focus is given to the setpoint response, the disturbance response worsens. It is clear from Figures 1 and 2 that if the control constants are set to optimize the ability to suppress external disturbances, the tracking of the setpoint becomes oscillatory; conversely, if the control constants are set to optimize setpoint tracking, the ability to suppress external disturbances becomes less effective. In previous PID control, since it was not possible to set the control constants in such a way as to optimize both tracking of the setpoint and suppression of external disturbances, the control constants had to be set at two specific compromise points to achieve appropriate control characteristics, and this aspect needed improvement. Figure 3 shows the operational behavior of 2-degree-of-freedom PID control in meeting both the target value response and disturbance response requirements. Figure 3 is a schematic illustration of how 2-degree-of-freedom PID control takes into account both the target value response and disturbance response. Methods for implementing 2-degree-of-freedom PID control. There are many ways to implement 2-degree-of-freedom PID control, and Changhui Instruments introduces a method that uses a setpoint filter to achieve this control. The chosen fixed-value filter is used to implement 2-degree-of-freedom PID control for the following reasons: ① It is very similar to the measurement-value derivative lead-type PID control algorithm widely used in digital control systems, and its expression is straightforward. ②Since it only adds a given-value filter, it is easily applicable to already existing temperature controllers. ③The corresponding conversion to control algorithms such as deviation-square PID control and PID control with an additional margin is easy. In summary, the 2-degree-of-freedom PID control method using a given-value filter is universal and has great potential for further development. Advantages of the characteristics and applications of 2-degree-of-freedom control: Changhui Instruments summarizes the characteristics of 2-degree-of-freedom PID control as follows: ① It enables both the simplification of the PID control method and an improvement in control performance. ②The greater the number of stages (n) in cascade control, the better the effect (by a factor of nn). ③It has a simple structure, is easy to understand, and can be easily applied to existing systems. ④By setting the parameters α, β, and γ, various PID control configurations can be arbitrarily implemented. The recommended values of α, β, and γ for the filter H(S) when implementing 2-degree-of-freedom PID control ⑤ enable improvements in the operating characteristics of equipment through comprehensive application. ⑥It can be replaced by the previous PID control method. Source: Thermostat http://blog.sina.com.cn/u/6430376461