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. 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. 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. 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. The controller’s task is to inject the proper amount of negative feedback such that the process variable stabilizes over time. If a supervisor delegates some task to a subordinate, and that subordinate performs the task without further need of guidance or assistance from the supervisor, the supervisor’s job is made easier. 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. (information from Econergy | Original article: Copyright © 2020 HTP Comfort Solutions LLC. This analogy also makes it clear why the secondary process variable must be faster-responding than the primary process variable: the supervisor-subordinate management structure fails to work if the supervisor does not maintain focus on long-term goals (i.e. Water-to-Air Geothermal Heat Pump Systems, Water-to-Water Geothermal Heat Pump Systems, Product Approvals, Certifications, Recalls & Regulatory Info. Just a moment’s thought is all that is needed to understand why this precedence in tuning must be: it is a simple matter of dependence. 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. By contrast, water level is a much slower-responding type of process. 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. Larger turndown allows for higher system efficiency. These include air flow, ambient air temperature, and variations in steam temperature. Large space heating or domestic jobs can include multiple units in a “cascade system,” allowing for higher turndown ratio. 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. An analogy for understanding cascade control is that of delegation in a work environment. 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. 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. This will lead to the subordinates “lagging” behind the supervisor’s orders, to the detriment of everyone’s satisfaction. shorter lag and dead times) than the primary process variable. The result is a level control system with more predictable response. Optional Controls for Greenstar Boilers. Temperature controller TC-1b is able to be a slave to controller TC-1a because the natural response time of the temperature at the dryer’s bottom is much shorter than at the dryer’s top with respect to any changes in steam valve position. http://www.econergy.ltd.uk/about/news/?artid=70&pageNum=0&blk=68. A classic example of cascade control strategy is found in motion control applications, where an electric motor is used as the final control element to precisely position a piece of machinery. Larger turndown allows for higher system efficiency. If the master controller were placed in manual (effectively turning off its automatic response), the slave controller would simply control to a constant setpoint. ), fluid flow being one of the fastest-responding process types in existence. One of the benefits of cascaded flow control is that this problem becomes confined to the secondary (flow control) loop, and is effectively removed from the primary control loop. 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 ). An essential requirement of cascaded control is that the secondary process variable be faster-responding (i.e. High turndown capability when only one boiler is required, Flexibility with footprint allowing installation in irregular spaces, Increased reliability with heat provided by several boilers, Smaller boilers can be maintained by a single engineer on site, Boiler control by priority, delivering excellent efficiency and even usage. ), fluid flow being one of the fastest-responding process types in existence. Don't have an Control account? You’ll find one quickly with the handy installer search function. Today, HTP is enjoying its second generation of family leadership and is a full-fledged manufacturer on the cutting edge of energy-efficient space and water heating technology. Large space heating or domestic jobs can include multiple units in a “cascade system,” allowing for higher turndown ratio. Our vision is to provide comfort and peace of mind to our customers with compelling solutions created by passionate people. Since such systems are … 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. 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. Established in 1974 as a designer and fabricator of heat exchangers for the boiler and solar industries. Cascade control cannot function if this speed relationship is reversed. Each one of these variables is a load on the process variable we are trying to control (outlet air 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. ICM Cascade Module for Greenstar Boilers designed to control cascade system–where several smaller boilers are connected in parallel to achieve a higher output. In motion control systems, each successive variable is the time-derivative of its precursor. Correspondingly, the control system would be slow to correct for any of these changing loads. 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. Please contact us via this form! If feedwater pressure happens to increase or decrease, any resulting changes in flow will be quickly countered by the flow controller without the level controller having to react to a consequent upset in steam drum water level. Thus, cascade control works to guard against steam drum level instability resulting from changes in the feedwater flow caused by factors outside the boiler. The slave controller does not depend on good tuning in the master controller in order to control the slave loop. Cascade Control is an advanced application of the PID that can improve control of systems that are subject to significant lag. 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. 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. This level/flow cascade control system also embodies the principle of the secondary (slave) loop being faster-responding than the primary (master) loop. Below are more advantages of using a cascaded system instead of a single larger boiler – High turndown capability when only one boiler is required 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. Water flow is an inherently fast process, the flow rate responding immediately to changes in valve position. Clearly then, the slave controller’s response is essential to the master controller being able to control its process variable, therefore the slave controller should be tuned first when initially commissioning or optimizing a cascade control system. transmitter) goes to the controller, then to the final control device (e.g. Thus, the “slave” control loop (1b) helps stabilize the “master” control loop (1a) by reacting to load changes long before any effect might manifest at the dryer outlet. A necessary step in implementing cascade control is to ensure the secondary (“slave”) controller is well-tuned before any attempt is made to tune the primary (“master”) controller. A common implementation of cascade control is where a flow controller receives a setpoint from some other process controller (pressure, temperature, level, analytical, etc. This flow of information is collectively referred to as a feedback control loop. A simple control system drawn in block diagram form looks like this: Information from the measuring device (e.g. This would make the process characteristic “runaway” in nature, as any fixed amount of current will cause the platform to accelerate995. To phrase it simply, distorted valve response becomes “the flow controller’s problem” rather than something the level controller must manage. control valve), influencing the process which is sensed again by the measuring device. 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. Create one now. Velocity, for instance, is the time-derivative of position (\(v = {dx \over dt}\)). If a controller is “slaved” to another controller, it must be left in cascade mode in order for the control strategy to function as designed.

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