Thus, cascade control works to guard against steam drum level instability resulting from changes in the feedwater flow caused by factors outside the boiler. We may better compensate for these loads by installing a second temperature transmitter at the inlet duct of the dryer, with its own controller to adjust steam flow at the command of the primary controller: Now, if any of the loads related to incoming air flow or temperature vary, the secondary controller (TC-1b) will immediately sense the change in dryer inlet temperature and compensate by adjusting steam flow through the heat exchanger. The control valve’s stem position is the process variable (PV) for the positioner, just as the command signal is the positioner’s setpoint (SP). Do you have any questions or suggestions for improvement? If any of these parameters were to suddenly change, the effect would be slow to register at the outlet temperature even though there would be immediate impact at the bottom of the dryer where the heated air enters. To re-emphasize an important point, one of the non-negotiable requirements for cascade control is that the secondary (slave) loop must be faster-responding than the primary (master) loop. Please contact us via this form! You’ll find one quickly with the handy installer search function. The subordinate takes care of all the little details that would otherwise burden the supervisor if the supervisor had no one to delegate to. By contrast, water level is a much slower-responding type of process. Since such systems are … The following illustration shows a triple-cascade control system994 for a motor-actuated elevator, precisely controlling the position of the elevator through cascaded velocity and motor current control: Hypothetically, the position of the elevator could be controlled with a single PID controller sensing platform position and directly sending power to the motor. If you perform a “thought experiment” where the feedwater valve is suddenly opened fully, it is easy to see that the feedwater flow rate will immediately reach its full (100%) value while the steam drum’s water level will merely begin to rise, taking time to reach its full (100%) value. Here with servomechanisms we see how cascading not only has the effect of “shielding” certain load variables from the master controller’s view, but it also simplifies the dynamic characteristics of the process from that same point of view. Cascade control cannot function if this speed relationship is reversed. Published under the terms and conditions of the, Basic Process Control Strategies and Control System Configurations, Techniques for Analyzing Control Strategies, Creative Commons Attribution 4.0 International Public License. This outlet temperature is fairly slow to react, as the solid material mass creates a large lag time: There are several parameters influencing the temperature of the outlet air other than the moisture content of the drying material. In the control valves chapter, we explore the phenomenon of installed valve characteristics (Section [installed_valve_characteristic] beginning on page ), specifically noting how changes in pressure drop across a control valve influences its throttling behavior. If a supervisor focuses on achieving goals that are shorter-term than the time required for subordinates to complete their assignments, the supervisor will inevitably call for “course changes” that are too quick for the subordinates to execute. http://www.econergy.ltd.uk/about/news/?artid=70&pageNum=0&blk=68. Larger turndown allows for higher system efficiency. The first controller (called the primary, or master) essentially “gives orders” to the second controller (called the secondary or slave) via a remote setpoint signal. As stated previously, the slave (flow) controller effectively shields the master (level) controller from loads in the feedwater supply system, so that master controller doesn’t have to deal with those loads. However, so long as the slave controller does a good job of stabilizing the air temperature entering the dryer, the master controller will never “see” the effects of those load changes. shorter lag and dead times) than the primary process variable. Valve positioners therefore act as “slave” controllers to “master” process controllers controlling pressure, temperature, flow, or some other process variable. These include air flow, ambient air temperature, and variations in steam temperature. Of course, these variables still act as loads to the slave controller, as it must continuously adjust the steam valve to compensate for changes in air flow, ambient air temperature, and steam temperature. Correspondingly, the control system would be slow to correct for any of these changing loads. This will lead to the subordinates “lagging” behind the supervisor’s orders, to the detriment of everyone’s satisfaction. Water flow is an inherently fast process, the flow rate responding immediately to changes in valve position. longer-term than the completion time of the tasks given to subordinates). A common implementation of cascade control is where a flow controller receives a setpoint from some other process controller (pressure, temperature, level, analytical, etc. Looking for an installer near you? Below are more advantages of using a cascaded system instead of a single larger boiler –, Above: three HTP Elite boiler units cascaded, (information from Econergy | Original article: http://www.econergy.ltd.uk/about/news/?artid=70&pageNum=0&blk=68 ). In this capacity, the motor is usually called a servo. However, the master controller most definitely depends on the slave controller being well-tuned in order to fulfill the master’s “expectations.” If the slave controller were placed in manual mode, the master controller would not be able to exert any control over its process variable whatsoever. Just like supervisory control systems where a process controller receives a “remote” setpoint signal from some other system, the secondary (“slave”) controller in a cascade system typically has three different operating modes: This means it is possible to defeat a cascade control system by placing the secondary controller in the wrong mode (automatic) just as it is possible to defeat any control system by placing the controller in manual mode. Responsibility for incoming air temperature has been delegated to the slave controller, and as a result the master controller is conveniently isolated from the loads impacting that loop. Robotic systems make extensive use of servo motors and cascaded control loops to modulate power to those motors. The result of these pressure changes is a non-linearization of valve response, such that the valve tends to be more responsive near its closed position and less responsive near its open position. Water-to-Air Geothermal Heat Pump Systems, Water-to-Water Geothermal Heat Pump Systems, Product Approvals, Certifications, Recalls & Regulatory Info. If it were not for cascading, a single PID controller would have to control position by manipulating the acceleration of the platform (i.e. Each one of these variables is a load on the process variable we are trying to control (outlet air temperature). This flow of information is collectively referred to as a feedback control loop. To cascade controllers means to connect the output signal of one controller to the setpoint of another controller, with each controller sensing a different aspect of the same process. However, much more precise control of the platform is achievable by sensing position, velocity, and motor current, and controlling each one of those variables with its own loop. (information from Econergy | Original article: Copyright © 2020 HTP Comfort Solutions LLC. Don't have an Control account? An essential requirement of cascaded control is that the secondary process variable be faster-responding (i.e. In motion control systems, each successive variable is the time-derivative of its precursor. It is worth noting that the inclusion of a flow control “slave” loop to this boiler water level control system also helps to overcome a potential problem of the control valve: nonlinear behavior.
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