Automatic control functions

Use of computer-based systems in systems important to safety Automatic control Functions of the protection system... 

It is automation that has allowed food and beverage industries to maintain the safety of operation in what has become very complex processing and routeing operations. Its success stems from the fact that control logic has been written specially for the industry, and it combines knowledge and an appreciation of the food process with the engineering and also the automatic control functions. 

Instrumentation A sensor that either simply or automatically controls HVAC equipment or that records some particular function of the plant operation, such as temperature, pressure, or flow. 

This form of implanned manual operation is unsatisfactory on a number of counts. The fact that the operator may normally be insulated from the process by the automatic control systems means that he or she will probably not be able to develop the knowledge of process dynamics ("process feel") necessary to control the system manually, particularly in extreme conditions. Also, the fact that manual control was not "designed into" the systems at the outset may mean that the display of process information and the facilities for direct control are inadequate. A number of techniques are available to assist designers in the allocation of function process. Some of these are described in Meister (1985). In a paper entitled "Ironies of Automation" Bainbridge (1987) notes four areas where the changed role of the human in relation to an automated system can lead to potential problems. These will be discussed below. 

In many applications, especially where process plant is concerned, the measurement information is required to be conveyed to a central point, which may be a control room manned by operators or a computer which carries out the control functions automatically. The connecting link between the various measurement points and the central control is usually a telemetry system, although in some cases the distances involved are so large that radio links need to be used. 

Whether the boiler is fired on oil, gas or solid fuel, it may be expected that it will operate automatically. When boiler plant is not run continuously initial start-up may be manual, time clock or through an energy-management system. Manual attendance may be limited to maintenance functions dictated by the type and size of plant. Automatic controls will cover three areas ... 

A starter or contactor with manual push-button or thermostatic operation to start and stop the fan normally controls simple systems. More complex systems that incorporate components that need control or monitoring are normally operated from purpose-built central control panels. The most common functions provided are fan motor stop, start and speed control, damper control, filter-condition indication and heater battery control. For optimum control, the system should be automatically controlled from thermostats or other sensors and a timeswitch. 

In any automatic control system, the four basic functions that occur are ... 

The three functional elements needed to perform the functions of an automatic control system are ... 

Figure 4 Relationships of Functions and Elements in an Automatic Control System...

Injection time-. Most modern instruments have a control function of the injection pressure that automatically corrects for hydrodynamic injection variability through the injected time. An injection time of at least 3 s is needed for this to function properly. Too short injection times decrease precision and too long injection times induce band broadening. Rather increase pressure if possible. 

The reference electrode is used as a potentiometric (always zero-current) probe to monitor A< >w relative to its own A< >r. This value is compared with Ea and if a difference (i.e., an error signal) exists, the potential impressed across the cell by the potentiometer is adjusted until balance (i.e., no error signal) is achieved. A device that accomplishes this control function automatically is called a potentiostat for obvious reasons. Such behavior can be mimicked by the experimenter. Although this is assuredly almost never done these days, it is useful to think about a manual potentiostat as a pedagogical device. 

The automatic apparatus consists of a viscosimeter and phototran-sistorized sensing devices mounted in a precision thermostat ( 0.005°C) connected to a cooled prethermostat ( 0.1°C). The base apparatus is commercially available (Schott Viscotimer, Jenaer Glaswerk, Schott Gen., Mainz), but the viscosimeter control functions and the time measurements are performed by using an electronic computer-controlled interface. This modification enables one to follow slow reactions and to reduce standard errors on the outflow times to 2 msec. The final results are evaluated numerically by an on-line computer-plotter system. 

Drugs that block beta-1 receptors on the myocardium are one of the mainstays in arrhythmia treatment. Beta blockers are effective because they decrease the excitatory effects of the sympathetic nervous system and related catecholamines (norepinephrine and epinephrine) on the heart.5,28 This effect typically decreases cardiac automaticity and prolongs the effective refractory period, thus slowing heart rate.5 Beta blockers also slow down conduction through the myocardium, and are especially useful in controlling function of the atrioventricular node.21 Hence, these drugs are most effective in treating atrial tachycardias such as atrial fibrillation.23 Some ventricular arrhythmias may also respond to treatment with beta blockers. 

The development of a tourniquet for battlefield application (Gooch et al. 2003) occurred in two stages. The first stage was the development of a one-handed ratcheting buckle strap tourniquet that would work reliably by itself and could be easily optionally augmented with an automatically controlled pneumatic pressure bladder. The second stage was the development of an automatically controlled pneumatic tourniquet that could occlude the blood flow within 15 s of application and would still function as a manually operated one-handed tourniquet in the event of a failure of the automatic system due to damage on the battlefield. 

General Second-Order Element Figure 8-3 illustrates the fact that closed-loop systems can exhibit oscillatory behavior. A general second-order transfer function that can exhibit oscillatory behavior is important for the study of automatic control systems. Such a transfer function is given in Fig. 8-15. For a unit step input, the transient responses shown in Fig. 8-16 result. As can be seen, when t, < 1, the response oscillates and when t, < 1, the response is S-shaped. Few open-loop chemical processes exhibit an oscillating response most exhibit an S-shaped step response. 

There are five basic types of synchros which are designated according to their function. The basic types are transmitters, differential transmitters, receivers, differential receivers, and control transformers. Figure 1 illustrates schematic diagrams used to show external connections and the relative positions of synchro windings. If the power required to operate a device is higher than the power available from a synchro, power amplification is required. Servomechanism is a term which refers to a variety of power-amplifiers. These devices are incorporated into synchro systems for automatic control rod positioning in some reactor facilities. 

Control functions are usually in the form of interlocks. Pump isolation valves are sometimes interlocked with the pump. In some applications, these interlocks act to prevent the pump from being started with the valves shut. The pump/valve interlocks can also be used to automatically turn off the pump if one of its isolation valves go shut or to open a discharge valve at some time interval after the pump starts. 

A novel approach to controlled release integrates biosensing and control functions in a single membrane device. As an example, a membrane responds to changes in glucose level in the body by automatically changing its permeability to insulin. The response mechanism is shown... 

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