Sensors, control loops

G. D. Anderson s article recommends initial controller settings for those control loops set on automatic rather than manual for a plant startup. For liquid level, the settings depend upon whether the sensor is a displacer type or differential pressure type, or a surge tank (or other surge) is installed in the process ... 

FIGURE 9.54 The controlling loop with sensors, actuators, and controller. 

Digital control A control loop in which a microprocessor-based controller directly controls equipment by means of sensors. Its operation depends on a series of on-off pulses arranged to convey information. 

The control system will consist of a loop, with detector (sensor), controller and controlled device. The communication between these parts of the loop will be electric, pneumatic or mechanical (see Figure 31.1) [62]. 

LGS/AO System Integration. A number of modifications have been implemented in the Keck II AO system to accommodate LGS/AO (Contos et al., 2002) a tip-tilt sensor to provide NGS- based tip-tilt, a low bandwidth WFS (LBWFS) to measure long term focus using a NGS, focus tracking for the high bandwidth WFS (HBWFS) looking at the LGS, and a tip-tilt control loop for the laser pointing. 

The sensor element constitutes a palladium-nickel alloy resistor with a temperature sensor and a proprietary coating. The sensor has a broad operating temperature range and a sophisticated temperature control loop that includes a heater and a temperature sensor, which controls the die temperature within 0.1°C. 

A wide range of conttol systems is used in household appliances. A standard conttol loop consists of sensors, control units and actuators. The appliances become more powerful and efficient, as the technology is developed and integrated into microsystems. Matchbox-sized sensors can be equipped with wireless radio transceivers and their own miniature operating system to tiansmit continuous data to the facility manager. 

Figure 6. Left Conventional control loops provide a localised interface between the real and the digital world. Sensors targeted at important control parameters feed information into digital control routines that can respond via actuators e.g. to maintain parameters within specified limits. Right A vital step on the route to the realisation of the concept of internet scale sensing is to adopt the principle that all analytical measurements should be capable of being internet-linked. The localised control of important parameters is maintained, but the information is shared via the internet with external users [4]. (Reprinted with permission from Anal. Chem., August 1, 2004, 75 (15), 278A-286A. Copyright 2004 American Chemical Society.)...

Area 300 is controlled using a distributed control system (DCS). The DCS monitors and controls all aspects of the SCWO process, including the ignition system, the reactor pressure, the pressure drop across the transpiring wall, the reactor axial temperature profile, the effluent system, and the evaporation/crystallization system. Each of these control functions is accomplished using a network of pressure, flow, temperature, and analytical sensors linked to control valves through DCS control loops. The measurements of reactor pressure and the pressure differential across the reactor liner are especially important since they determine when shutdowns are needed. Reactor pressure and temperature measurements are important because they can indicate unstable operation that causes incomplete reaction. 

Temperature Control Loop and Sensor Resistance Readout... 

We will consider all the components of this temperature control loop in more detail later in this book. For now we need only appreciate the fact that the automatic control of some variable in a process requires the installation of a sensor, a transmitter, a controller, and a final control element (usually a control valve). Most of this book is aimed at learning how to decide what type of controller should be used and how it should be tuned, i.e., how should the adjustable tuning parameters in the controller be set so that we do a good job of controlling temperature. 

Let s start from the beginning of the control loop, at the sensor. Instruments for on-line measurement of many properties have been developed. The most... 

Figure 3 shows a TWC system and a typical performance of the TWC. The three components are highly purified over the catalyst around the stoichiometric point. The oxidizing and reducing components have almost the same chemical equivalent in the narrow shadowed region, and CO, HC and NOx are converted into H20, C02 and N2 (Fig. 3b). The atmosphere of the TWC is automatically controlled around the stoichiometric point by the TWC system. The flow rate of air is monitored and the fuel injection is controlled by a computerized system to obtain a suitable A/F ratio (Fig. 3c). The signal from oxygen sensor is used as a feedback for the fuel and air injection control loop. Therefore, the exhaust gases are fluctuating streams between oxidizing and reducing periodically and alternatively.

Does not have a Sensor-Feedback Control Loop. Digital Setpoint Only. 

This article presents the design and implementation of a software sensor for the continuous determination of substrate concentration based on a simple model of a fed-batch fermentation process and the available signals of two other sensors—one for on-line biomass determination (7) and the other for on-line ethanol determination (8)—developed in previous works. The software sensor proposed provides a continuous signal that can be used in a control loop to manipulate the substrate feed flow in order to maintain almost constant substrate concentration and obtain an excellent level of productivity and yield during all of the process, as shown in experimental control strategy studies in previous works (9). 

As can be observed in Figure 10.1, information from the process (value of the temperature measured by the sensor) arrives at the control loop, which defines it as a closed loop. This is the case for most controllers installed in bioreactors. If the decisions of a controller do not take into account any of the monitored information of the process, it is called an open loop. An example of an open loop is a fed-batch process, where it... 

Flow chart of a typical control loop showing temperature control elements a desired temperature value (set-point) is compared to the measured value by the thermometer (sensor) and, based on the error measurement, a signal to the electric resistance (actuator) is generated by the controller, that will heat up the bioreactor (process). 

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