Open loop

Heating is usually accompHshed with a semiclosed loop which has a constant fresh gas makeup and a bleed to draw off the desorbed material. However, contaminant is at a higher level than in an open loop and product purity is harder to achieve. 

Flux is maximized when the upstream concentration is minimized. For any specific task, therefore, the most efficient (minimum membrane area) configuration is an open-loop system where retentate is returned to the feed tank (Fig. 8). When the objective is concentration (eg, enzyme), a batch system is employed. If the object is to produce a constant stream of uniform-quahty permeate, the system may be operated continuously (eg, electrocoating). 

Open-loop systems have inherently long residence times which may be detrimental if the retentate is susceptible to degradation by shear or microbiological contamination. A feed-bleed or closed-loop configuration is a one-stage continuous membrane system. At steady state, the upstream... 

Open-Loop versus Closed-Loop Dynamics. 8-4... 

An open-loop system positions the manipulated variable either manually or on a programmed basis, without using any process measurements. This operation is acceptable for well-defined processes without disturbances. An automanual transfer switch is provided to allow manual adjustment of the manipulated variable in case the process or the control system is not performing satisfac torily. 

Open-Loop versus Closed-Loop Dynamics It is common in industry to manipulate coolant in a jacketed reacdor in order to control conditions in the reacdor itself. A simplified schematic diagram of such a reactor control system is shown in Fig. 8-2. Assume that the reacdor temperature is adjusted by a controller that increases the coolant flow in proportion to the difference between the desired reactor temperature and the temperature that is measured. The proportionality constant is K. If a small change in the temperature of the inlet stream occurs, then depending on the value or K, one might observe the reactor temperature responses shown in Fig. 8-3. The top plot shows the case for no control (K = 0), which is called the open loop, or the normal dynamic response of the process by itself. As increases, several effects can be noted. First, the reactor temperature responds faster and faster. Second, for the initial increases in K, the maximum deviation in the reactor temperature becomes smaller. Both of these effects are desirable so that disturbances from normal operation have... 

In open-loop control applications wdiere best static acciiracv is needed. 

Open-loop behavior of multicomponent distillation may be studied by solving modifications of the multicomponent equations of Distefano [Am. Inst. Chem. Eng. J., 14, 190 (1968)] as presented in the subsection Batch Distillation. One frequent modification is to include an equation, such as the Francis weir formula, to relate liquid holdup on a tray to liquid flow rate leaving the tray. Applications to azeotropic-distillation towers are particularly interesting because, as discussed by and ihustrated in the Following example from Prokopalds and Seider... 

Tacho-generator for open loop or encoder for a closed loop confrol... 

These drives are normally open loop (sensor-less) without encoder. For higher regulation, it is better to adopt a two-phasor control, such as a field-oriented control (FOC) or a direct torque control (DTC) drive. 

Up to + 0.01% in open-loop and 0.001% in closed-loop control systems... 

The expander train supplier shall assist with the selection of the expander inlet and bypass valve configurations and closure rates by performing an open loop dynamic simulation study. For a main air blower train dais could be closed loop. 

The PI controller, even when optimally tuned, is also unable to prevent surge. Furthermore, it is unable to stop surge once it occurs. In the above situation, the operator would correctly identify the problem as instability of the closed-loop PI controller. The only viable action would be to open the closed control loop by placing the controller in manual, thereby freezing the valve open. In this scenario, open-loop control will stop surge. 

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. 

Figure 6-49. PI plus open-loop recycle trip strategy.

It has been observed that the value of the combined closed-loop PI control and the open-loop recycle trip control justifies its use on all turbocompressors. 

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. 

Breaker Trip Triggers Open-loop Control... 

The power applied by die expander to die shaft must be redueed in diis situation. Open-loop eontrol ean be applied, but an intelligent applieation of die open-loop response should be made. How mueh to reduee diis power is deseribed in the following paragraphs. The eharaeteristies deseribed in the seetion Expander eharaeteristies are needed to determine expander power output under any eonditions. 

To follow through with the open-loop approaeh, there must first be a signal that warns the speed eontrol funetion of load loss (or breaker opening) and impending speed inerease. This is provided by a digital eontaet that indieates motor/generator breaker status. Next, a load loss estimate is needed. This is provided by a kilowatt transdueer on the motor/generator. 

Many plants use off-line optimization. Off-line optimization is an open loop eontrol system. Instead of the elosed loop system, whieh eontrols the plant settings, data is provided to the operator so that he ean make the deeisions based on the findings of the operational data. Off-line systems are also used by engineers to design plants and by maintenanee personnel to plan plant maintenanee. Comparisons of the on-line systems to off-line systems ean be seen in Table 19-1. 

The iiitriiisie gain of the open loop power supply is... 

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. 

B.2 Defining the Open Loop Response of the Switching Power Supply—The Control-to-Output Characteristics... 

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... 

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