Secondary controller

SPS = secondary control variable setpoint and PVS = secondary control variable measurement. The + and — indicate to multiply the signal by +1 or —1... 

Schemes to control the outlet temperature of a process furnace by adjusting the fuel gas flow are shown in Figure 13. In the scheme without cascade control (Fig. 13a), if a disturbance has occurred in the fuel gas supply pressure, a disturbance occurs in the fuel gas flow rate, hence, in the energy transferred to the process fluid and eventually to the process fluid furnace outlet temperature. At that point, the outlet temperature controller senses the deviation from setpoint and adjusts the valve in the fuel gas line. In the meantime, other disturbances may have occurred in the fuel gas pressure, etc. In the cascade control strategy (Fig. 13b), when the fuel gas pressure is disturbed, it causes the fuel gas flow rate to be disturbed. The secondary controller, ie, the fuel gas flow controller, immediately senses the deviation and adjusts the valve in the fuel gas line to maintain the set fuel gas rate. If the fuel gas flow controller is well tuned, the furnace outlet temperature experiences only a small disturbance owing to a fuel gas supply pressure disturbance.

Both control schemes react in a similar manner to disturbances in process fluid feed rate, feed temperature, feed composition, fuel gas heating value, etc. In fact, if the secondary controller is not properly tuned, the cascade control strategy can actually worsen control performance. Therefore, the key to an effective cascade control strategy is the proper selection of the secondary controlled variable considering the source and impact of particular disturbances and the associated process dynamics. 

The dynamics of the secondary control loop should be approximately two to four times as fast as the dynamics of the primary control loop in order to achieve stable control. The secondary controller is actually part of the primary controller s process system. Hence, changes in the secondary controller tuning constants change the process system of the primary controller. Therefore, cascade control loops should always be tuned by first tuning the secondary controller and then the primary controller. If the secondary controller tuning is changed for any reason, the primary controller may need to be retuned also. 

Many misconceptions exist about cascade control loops and their purpose. For example, many engineers specify a level-flow cascade for every level control situation. However, if the level controller is tightly tuned, the out-flow bounces around as does the level, regardless of whether the level controller output goes direcdy to a valve or to the setpoint of a flow controller. The secondary controller does not, in itself, smooth the outflow. In fact, the flow controller may actually cause control difficulties because it adds another time constant to the primary control loop, makes the proper functioning of the primary control loop dependent on two process variables rather than one, and requites two properly tuned controllers rather than one to function properly. However, as pointed out previously, the flow controller compensates for the effect of the upstream and downstream pressure variations and, in that respect, improves the performance of the primary control loop. Therefore, such a level-flow cascade may often be justified, but not for the smoothing of out-flow. 

Secondary Control of Wound-Rotor Motors Wound-rotor motors may be effectively reduced-voltage-started or have their speed controlled by using external secondaiy resistance. The addition of resistance into the secondaiy circuit of a wound-rotor motor reduces the starting current and affects the speed under load conditions. 

Provide check valve (s) in feed line (secondary control)... 

Fluoride Removal Efficiencies Selected Aluminum Industry Primary and Secondary Control Systems... 

On small and low-production furnaces, the enclosure can be used as both primary and secondary control, thereby reducing the need for other equipment. 

We can use a block diagram to describe Fig. 10.1. Cascade control adds an inner control loop with secondary controller function Gc2 (Fig. 10.2a). This implementation of cascade control requires two controllers and two measured variables (fuel... 

The slave loop will have a 10% offset with respect to desired set point changes in the secondary controller. 

With the choice of x = 0.5 s, but without the inner loop nor the secondary controller, the closed-loop equation is... 

Figure S.2b shows another common system where cascade control is used. The reactor temperature controller is the primary controller the jacket temperature controller is the secondary controller. The reactor temperature control is isolated by the cascade system from disturbances in cooling-water inlet temperature and supply pressure.

We will also talk in Chap. 11 about the two types of cascade control series cascade and parallel cascade. The two examples discussed above are both series cascade systems because the manipulated variable affects the secondary controlled variable, which then affects the primary variable. In a parallel cascade system the manipulated variable affects both the primary and the secondary controlled variables directly. Thus the two processes are basically different and result in different dynamic characteristics. We will quantify these ideas later. 

The VPC scheme is a different type of cascade control system. The primary control is the position of the valve. The secondary control is the column pressure. The pressure controller is PI and tuned fairly tightly so that it can prevent the sudden drops in pressure. Its setpoint is slowly changed by the VPC to drive the cooling water valve nearly wide open. A slow-acting, integral-only controller should be used in the VPC. 

Figure 11.1c shows a series cascade system. There are now two controllers, The secondary controller Bi adjusts M to control the secondary variable X. The setpoint signal Jfto the Bi controller comes from the primary controller, i.e., the output of the primary controller Bi is the setpoint for the controller. The Bi controller setpoint is Jfy -... 

So to design the secondary controller we use the dosedloop characteristic equation... 

Example 11.1. Consider the process with a series cascade control system sketched in Fig 11.le. The secondary controller B, and the primary controller Bj are both proportional only. 

It is useful to compare these values with those found for conventional control = 19.8 and oi. = 1.61. We can see that cascade control results in higher controller gain and smaller dosedloop time constant (the reciprocal of the frequency). Figure 11.26 gives a root locus plot for the primary controller with the secondary controller gain set at Two of the loci start at the complex poles s = rj which Come from the dosedloop secondary loop. The other curve... 

The reactor temperature controller (loop 2) is the primary controller, whereas the jacket temperature controller (loop 3) is the secondary controller. The advantage of the cascade control is that the reactor temperature control quickly reacts by the cascade system to disturbances in cooling fluid inlet conditions. The d3mamics of the transfer function G32 is faster than that of G 22-In the CSTR cascade control there are two control loops using two different measurements temperatures T and Tj, but only one manipulated variable Fj. The transfer function of the primary controller is the following ... 

The local control of the CSTR by using centralized and decentralized control has been also analyzed. The decentralized control is studied with a cascade control with two PI primary and secondary controllers. From the block diagram, the step response to change in the concentration and temperature references are deduced. A short reference to the decoupling problem is also discussed. 

An addition to the noted advantages is that the set point of the secondary controller can be limited. In addition, by speeding up the overall cascade loop response, the sensitivity of the primary process variable to process upsets is also reduced, whereas the secondary loop can reduce the effect of control valve sticking or actuator nonlinearity. The primary or outer control loop of a cascade system is usually a PI or PID controller. A properly selected secondary will reduce the proportional band of the primary controller. 

In reactor temperature control applications, a slave controlling the jacket outlet temperature is recommended so that the dynamics of the jacket is transferred from the primary to the secondary loop. In temperature-on-temperature cascade systems, such as shown in Figure 2.45, the secondary controller should have little or no integral. 

Whenever the secondary controller is in manual or on local set point, the status is communicated backward to the primary controller via the TCO-TCI communication link. At the same time, the set point of the secondary controller is communicated backward to the primary controller via the TRO-TRI communication link. The status notification puts the primary PID into an initialization mode. This forces the primary controller output to be the same as the secondary controller set point. (The secondary controller should be configured so that its set point tracks its measurement whenever it is in manual.)... 

In cascade control, we therefore have two control loops using two different measurements but sharing a common manipulated variable. The loop that measures the controlled variable (in the example, the reacting mixture temperature) is the dominant, or primary control loop (also referred to as the master loop) and uses a set point supplied by the operator, while the loop that measures the second variable (in the example, the cooling water temperature) is called the secondary (or slave) loop and uses the output from the primary controller as its set point. Cascade control is very common in chemical processes and the major benefit to be gained is that disturbances arising within the secondary loop are corrected by the secondary controller before they can affect the value of the primary controlled output. 

One of the obvious goals in the study of hemoproteins is an understanding of the mechanisms by which the physical and chemical properties of the heme group are modulated by the protein environment. The primary control of heme reactivity clearly is exercised in the selection of axial ligand(s) through molecular evolution. However, powerful secondary control mechanisms also exist. As the... 

An alternative or complementary solution to the introduction of storage is the utilisation of secondary controllable loads in the system that will be activated when the production of renewable generators exceeds the current demand. Such loads might be desalination units, water heating, house or district heating, etc. 

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