Loop method, open

Due to the simplicity with which the experimental data can be obtained, we preferred the open loop method. Moreover, application of this technique permits extensive elimination of the trial and error method. 

Perhaps the most well known and most frequently published method is that by Ziegler and Nichols (Reference 3). They developed an open loop method in addition to the more frequently published closed loop method. The criterion used by both methods is the quarter decay ratio. This is illustrated in Figure 3.21 following a SP change the PV response should be slightly oscillatory with the height of the second peak being one-quarter of that of the first. 

The semibatch approach, where policies are developed for selective reagent feeds to the reactor, has been extensively elaborated, especially for emulsion polymerization, and in the context of controlling composition during copolymerization reactions [68-72,109]. Discussions are provided in Chapters 4, 7, 12, 17-19, and 21. Sun et al. developed model-based semibatch monomer feeding policies for controlled radical polymerization (CRP) [73, 74]. Vicente et al. [75,76] controlled composition and molecular weight distribution in emulsion copolymerization in an open-loop method by maintaining the ratio of comonomers. Yanjarappa et al. [77] synthesized, via a sanibatch method, copolymers with constant composition for biofunctionalization. General semibatch policies are reviewed by Asua [78]. 

Atomic force microscopes may use sophisticated algorithms to modify the voltages applied to actuating elements (piezoelectric ceramics) that incorporate the effects of scan rate, scan size,non-linearities,andcross-talk.Thisapproach is referred to as an open loop method. Closed loop approaches are also available. These provide an independent method of dimensional measurement (strain gauges, capacitance sensors, or optical interferometers) in order to apply realtime correction to actuator motion. Closed loop is more readily applied in x and y dimensions recent advances include z motion. Closed loop calibration can introduce additional electric noise, and care is required to make sure that ultimate resolution is not compromised. 

The human body is a remarkably complex biochemical process, and it shares many attributes with more traditional process control problems that have been discussed in earlier chapters. In the event that a body fails to achieve the robust level of self-regulation that occurs naturally (cf. Chapter 24), there are opportunities for medical intervention, often involving the administration of a therapeutic agent (or drug) in a prescribed manner. The therapy can be optimized using open-loop methods, but it is often advantageous to automate the process, thus removing the human from the feedback loop (much as a chemical plant removes the operator from the loop in the transition from manual control to... 

As with the Ziegler-Nichols open-loop method recommendations, the Cohen-Coon values should be implemented and tested in closed loop and adjusted accordingly to achieve the QDR. 

There are t wo methods of treating the kinetics of a reactor. In the open-loop method, the inlet temperature is taken as constant. The justification for this procedure is that this condition generally prevails during rapid... 

A similar type of open-loop control response is combined with closed-loop control in a patented method of surge protection. In this control method an open-loop control response is added before surge occurs. 

Positioners should be used to aehieve aeeuraey and repeatability of the eontrol-loop response. This is partieularly important with the open-loop eontrol method diseussed above. 

The form of compensation is the 2-pole-2-zero method of compensation. This is to compensate for the effect of the double pole caused by the output filter inductor and capacitor. One starts by determining the control-to-output characteristic of the open-loop system. 

The schematic and Bode plot for the single-pole method of compensation are given in Figure B-16. At dc it exhibits the full open-loop gain of the op amp, and its gain drops at -20dB/decade from dc. It also has a constant -270 degree phase shift. Any phase shift contributed by the control-to-output characteristic... 

Note that the Proeess Reaetion Method eannot be used if the open-loop step response has an overshoot, or eontains a pure integrator(s). 

Note that equations (5.72) and (5.75) are identieal, and therefore give the same roots. The first root, —3.79 lies at a point where there are an even number of open-loop poles to the right, and therefore is not valid. The seeond root, —0.884 has odd open-loop poles to the right, and is valid. In general, method (a) requires less eomputation than method (b). 

Note that method (b) provides both the erossover value (i.e. the frequeney of oseillation at marginal stability) and the open-loop gain eonstant. 

The identification of plant models has traditionally been done in the open-loop mode. The desire to minimize the production of the off-spec product during an open-loop identification test and to avoid the unstable open-loop dynamics of certain systems has increased the need to develop methodologies suitable for the system identification. Open-loop identification techniques are not directly applicable to closed-loop data due to correlation between process input (i.e., controller output) and unmeasured disturbances. Based on Prediction Error Method (PEM), several closed-loop identification methods have been presented Direct, Indirect, Joint Input-Output, and Two-Step Methods. 

The PBL reactor considered in the present study is a typical batch process and the open-loop test is inadequate to identify the process. We employed a closed-loop subspace identification method. This method identifies the linear state-space model using high order ARX model. To apply the linear system identification method to the PBL reactor, we first divide a single batch into several sections according to the injection time of initiators, changes of the reactant temperature and changes of the setpoint profile, etc. Each section is assumed to be linear. The initial state values for each section should be computed in advance. The linear state models obtained for each section were evaluated through numerical simulations. 

Figure 6.2. Illustration of fitting Eq. (6-2, solid curve) to open-loop step test data representative of self-regulating and multi-capacity processes (dotted curve). The time constant estimation shown here is based on the initial slope and a visual estimation of dead time. The Ziegler-Nichols tuning relation (Table 6.1) also uses the slope through the inflection point of the data (not shown). Alternative estimation methods are provided on our Web Support.

As far as we are concerned, using the error integral criteria is just another empirical method. We are simply using the results of minimization obtained by other people, not to mention that the first order with dead time function is from an open-loop test. 

Several methods have been investigated to find correlations between physical properties of fuel gas mixtures and the excess air ratio to optimize the combustion procedure. In spite of the varying composition of natural gas it is said to be possible to control a heater system by measurements of the dynamic viscosity of the gas [7]. One explanation could be the correlation between Wobbe number and viscosity With increasing Wobbe numbers the viscosity decreases, and if the Wobbe number of a gas is known, the excess air ratio can be adjusted, resulting in an open loop control. 

Determine the open-loop response of the output of the measuring element in Problem 7.17 to a unit step change in input to the process. Hence determine the controller settings for the control loop by the Cohen-Coon and ITAE methods for P, PI and PID control actions. Compare the settings obtained with those in Problem 7.17. 

The USP Apparatus 4, also known as the flow-through cell, was introduced and extensively studied by Langenbucher (22). In the open loop configuration, this system offers the advantage of unlimited medium supply, which is of particular interest for the dissolution of poorly soluble drugs. The idea to develop a flow-through cell method dates back more than 45 years. As early as 1957, a flow-through cell method with a... 

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