Servomechanisms

Fig. 4.42 Hydraulic servomechanism with walking beam feedback linkage.

The tracking or servomechanism problem is defined in section 9.1.1(e), and is directed at applying a control u(t) to drive a plant so that the state vector t) follows a desired state trajectory r(t) in some optimal manner. 

James, H.M., Nichols, N.B. and Phillips, R.S. (1947) Theory of Servomechanisms, McGraw-Hill, New York. 

World-scale, single-train units, coupled with advances in automation, led to the use of servomechanisms for valve plug positioning. 

In Chapter 3 the steady-state hydrodynamic aspects of two-phase flow were discussed and reference was made to their potential for instabilities. The instability of a system may be either static or dynamic. A flow is subject to a static instability if, when the flow conditions change by a small step from the original steady-state ones, another steady state is not possible in the vicinity of the original state. The cause of the phenomenon lies in the steady-state laws hence, the threshold of the instability can be predicted only by using steady-state laws. A static instability can lead either to a different steady-state condition or to a periodic behavior (Boure et al., 1973). A flow is subject to a dynamic instability when the inertia and other feedback effects have an essential part in the process. The system behaves like a servomechanism, and knowledge of the steady-state laws is not sufficient even for the threshold prediction. The steady-state may be a solution of the equations of the system, but is not the only solution. The above-mentioned fluctuations in a steady flow may be sufficient to start the instability. Three conditions are required for a system to possess a potential for oscillating instabilities ... 

Fink G (1995) The self-priming effect of LHRH a unique servomechanism and possible cellular model of memory. Front Neuroendocrinol 16 18-190... 

Setpoint changes can also be made, particularly in batch processes or in changing from one operating condition to another in a continuous process. These setpoint changes also act as disturbances to the dosedloop system. The function of the feedback controller is to drive the controlled variable to match the new setpoint. The dosedloop response to a setpoint disturbance is called the servo response (from the early applications of feedback control in mechanical servomechanism tracking systems). 

A review on instrumental methods of analysis) (313 refs) 3) L.A. McColl, Fundamental Theory of Servomechanisms, Van Nostrand, NY (1945) 4) D.P. Eckman, Principles of Industrial Process Control, J Wiley, NY (1945)... 

Throttling of the suction of centrifugal and axial compressors wastes less power than throttling the discharge. Even less power is wasted by adjustment of built-in inlet guide vanes with a servomechanism which is a feedback control system in which the controlled variable is mechanical position. Speed control is a particularly effective control mode, applicable to large units that can utilize turbine or internal combustion drives control is by throttling of the supply of motive fluids, steam or fuel. 

Figure 3.23. Control of centrifugal compressors with turbine or motor drives, (a) Pressure control with turbine or motor drives, (b) Flow control with turbine or motor drives. SC is a servomechanism that adjusts the guide vanes in the suction of the compressor, (c) Surge and pressure control with either turbine or motor drive. The bypass valve opens only when the flow reaches the minimum calculated for surge protection.

Figure 3.24. Control of positive displacement compressors, rotary and reciprocating, (a) Flow control with variable speed drives, (b) Pressure control with bypass to the suction of the compressor, (c) Reciprocating compressor. SC is a servomechanism that opens some of suction valves during discharge, thus permitting stepwise internal bypass. The clearance unloader is controllable similarly. These built-in devices may be supplemented with external bypass to smooth out pressure fluctuations.

While most laboratory conductivity bridges are manually balanced, the Wheatstone bridge circuit also finds use in a variety of conductivity monitors, controllers, and recorders where it is mechanically rebalanced by a servomechanism operated by the detector. Generally in these devices, advantage is taken of the phase shift, which occurs in the detected signal as the bridge is driven through balance by the servo motor. 

Kochenburger, R. J. Trans. AIEE 69 (1950) 270. A frequency response method for analysing and synthesising contactor servomechanisms. 

The servomechanism incorporates a radioactive isotope feedback transducer system for detecting the position of the burning surface. 

Sarvomechanisms. Servomechanisms are part of a broad class of systems that operate on the principle of feedback. In a feedback control system, the output (response) signal is made to conform with the input (command) signal by feeding back to the input a signal that is a function of the output for the purpose of comparison. Should an error exist, a corrective action is automatically initiated to reduce the error toward zero. Thus, thru feedback, output and input signals are made to conform essentially with each other... 

At present, there is no standard definition of a servomechanism. Some engineers prefer to classify any system with a feedback loop as a servomechanism. According to this interpretation, an electronic amplifier with negative feedback is a servo. More frequently, however, the term servomechanism is reserved for a feedback control system containing a mechanical quantity. Thus, the IRE defines a servomechanism as "a feedback control system in which one or more of the system signals represents mechanical motion . Some would restrict the definition further by applying the term only to a special class of feedback control system in which the output is a mechanical position... 

Servomechanisms are an important part of nearly every pieee of modem mechanized Army equipment. They are used to automatically position gun mounts, missile launchers, and radar antennas. They aid in the control of the flight paths of jet-propelled rockets and ballistic... 

No single set of electrical and physical requirements can be stated for servomechanisms intended for these diverse military applications. The characteristics of each servomechanism are determined by the function it is to perform, by the characteristics of the other devices and equipments with which it is associated, and by the environment to which it is subjected. It will often be found that two or more servo-system configurations will meet a given set of performance specifications. Final choice of a system may then be determined by such factors as ability of the system to meet environmental specifications, availability of components, simplicity, reliability, ease of maintenance, ease of manufacture, and cost... 

Refs 1) Anon, Servomechanisms, Section 1, Theory , AMCP 706-136 (1965) 2) Anon,... 

Servomechanisms, Section 2, Measurement and Signal Converters , AMCP 706-137 (1965)... 

Anon, "Servomechanisms, Section 3, Amplification , AMCP 706-138 (1965) 4) Anon,... 

Unsteady flow also includes such topics as oscillations in connected reservoirs and in U tubes and such phenomena as tidal motion and flood waves. Likewise, the field of machinery regulation by servomechanisms is intimately connected with unsteady motion. However, all these topics are considered to be beyond the scope of the present text. 

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. 

Controllers can be designed on a regulation basis (d=K), x(0) = 0) or on a servomechanism basis (d 0, x(0)=0). When d+0, the controller must counteract the disturbance, although the addition of integral control can force the error of some outputs to zero, depending upon the nature of A. The closed loop transfer function for set point changes (servomechanism) is... 

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