【正文】 required, until the loop is acceptably quick to reach its reference after a load disturbance. However, too much I will cause excessive response and overshoot. A fast PID loop tuning usually overshoots slightly to reach the setpoint more quickly。s frequency response to design the PID loop values. In loops with response times of several minutes, mathematical loop tuning is remended, because trial and error can literally take days just to find a stable set of loop values. Optimal values are harder to find. Some digital loop controllers offer a selftuning feature in which very small setpoint changes are sent to the process, allowing the controller itself to calculate optimal tuning values. Other formulas are available to tune the loop according to different performance criteria. 4 Modifications to the PID algorithm The basic PID algorithm presents some challenges in control applications that have been addressed by minor modifications to the PID mon problem resulting from the ideal PID implementations is integral windup. This can be addressed by: Initializing the controller integral to a desired value Disabling the integral function until the PV has entered the controllable region Limiting the time period over which the integral error is calculated Preventing the integral term from accumulating above or below predetermined bounds 10 Many PID loops control a mechanical device (for example, a valve). Mechanical maintenance can be a major cost and wear leads to control degradation in the form of either stiction or a deadband in the mechanical response to an input signal. The rate of mechanical wear is mainly a function of how often a device is activated to make a change. Where wear is a significant concern, the PID loop may have an output deadband to reduce the frequency of activation of the output (valve). This is acplished by modifying the controller to hold its output steady if the change would be small (within the defined deadband range). The calculated output must leave the deadband before the actual output will proportional and derivative terms can produce excessive movement in the output when a system is subjected to an instantaneous step increase in the error, such as a large setpoint change. In the case of the derivative term, this is due to taking the derivative of the error, which is very large in the case of an instantaneous step change. 5. Limitations of PID control While PID controllers are applicable to many control problems, they can perform poorly in some controllers, when used alone, can give poor performance when the PID loop gains must be reduced so that the control system does not overshoot, oscillate or hunt about the control setpoint value. The control system performance can be improved by bining the feedback (or closedloop) control of a PID controller with feedforward (or openloop) control. Knowledge about the system (such as the desired acceleration and inertia) can be fed forward and bined with the PID output to improve the overall system performance. The feedforward value alone can often provide the major portion of the controller output. The PID controller can then be used primarily to respond to whatever difference or error remains between the setpoint (SP) and the actual value of the process variable (PV). Since the feedforward output is not affected by the process feedback, it can never cause the control system to oscillate, thus improving the system response and stability. For example, in most motion control systems, in order to accelerate a mechanical load under control, more force or torque is required from the prime mover, motor, or actuator. If a velocity loop PID controller is being used to control the speed of the load and mand the force or torque being applied by the prime mover, then it is beneficial to take the instantaneous acceleration desired for the load, scale that value appropriately and add it to the output of the PID velocity loop controller. This means that whenever the load is being accelerated or decelerated, a proportional 11 amount of force is manded from the prime mover regardless of the feedback value. The PID loop in this situation uses the feedback information to effect any increase or decrease of the bined output in order to reduce the remaining difference between the process setpoint and the feedback value. Working together, the bined openloop feedforward controller and closedloop PID controller can provide a more responsive, stable and reliable control system. Another problem faced with PID controllers is that they are linear. Thus, performance of PID controllers in nonlinear systems (such as HVAC systems) is variable. Often PID controllers are enhanced through methods such as PID gain scheduling or fuzzy logic. Further practical application issues can arise from instrumentation connected to the controller. A high enough sampling rate, measurement precision, and measurement accuracy are required to achieve adequate control performance. A problem with the Derivative term is that small amounts of measurement or process noise can cause large amounts of change in the output. It is often helpful to filter the measurements with a lowpass filter in order to remove higherfrequency noise ponents. However, lowpass filtering and derivative control can cancel each other out, so reducing noise by instrumentation means is a much better choice. Alternatively, the differential band can be turned off in many systems with little loss of control. This is equivalent to using the PID controller as a PI controller. 6. Cascade control One distinctive advantage of PID controllers is that two PID controllers can be used together to yield better dynamic performance. This is called cascaded PID control. In cascade control there are two PIDs arranged with one PID controlling the set point of anoth