Deadband block

Even if the manipulated variable seems to follow the controller ontput, there could be a problem with the actuator. Estimates of the actuator deadband and dynamic response are required to determine if the actuator system is performing properly (both of which can be determined by a block sine wave test). This test is shown in Figure 15.21. A block sine wave is a series of eqnally sized step changes that approximate a sine wave. For the test shown in the figure, the amplitude... 

FIGURE 15.21 Results of a series of block sine wave tests to determine the deadband of the actuator. Note that the signal to the actuator and the measured value of the flow rate are plotted on different scales. 

The performance of a closed-loop system can be assessed by the settling time, closed-loop deadband, and the variability of the controlled variable evaluated over an extended period of time. The settling time and the closed-loop deadband can be determined using a closed-loop block sine wave test. For a closed-loop block sine wave test, the setpoint for the control loop is applied in the form of a block sine wave, and the amplitude of the block sine wave is varied until the deadband is determined. During these tests, the settling time of the controller can also be estimated. An accurate determination of the variability of a controlled variable generally requires an extended period of operation. An evaluation of the variability based on a short period of time may not be representative of true system performance. 

Step 1. Determine the deadband of the final control element using a series of block sine wave tests. Result the deadband of the final control element was less than 0.4%, and the dynamic response time of the final control element was 2 s therefore, the final control element was found to be functioning properly. 

Final elements must provide the desired capacity with the required precision of flow throttling over the desired range, usually 10% to 95% of maximum flow. The valve characteristic should provide a linear closed-loop gain, except choose linear or quick-opening characteristics for valves that are normally closed but must open quickly. Select the valve failure position for safety. The valve body should satisfy such requirements as required flow at 0% stem position, plugging, pressure drop, or flashing. The nonideal final element behavior, such as friction and deadband, should be small, as required by each application. Control valves should have manual bypass and block valves to allow temporary valve maintenance when short process interruptions are not acceptable. However, the bypass should never compromise safety interlock systems. 

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