Feedback Signals

The Smith dead-time compensator is designed to aUow the controUer to be tuned as tightly as it would be if there were no dead time, without the concern for cycling and stabUity. Therefore, the controUer can exert more reactive control. The dead-time compensator utilizes a two-part model of the process, ie, Gp, which models the portion of the process without dead time, and exp — sTp,pj ), which models the dead time. As seen from Figure 18b, the feedback signal is composed of the sum of the model (without dead time) and the error in the overaU model Gpj exp — sTppj )), ie, C —. Using... 

Fig. 4.1 Block diagram of a closed-loop control system. R s) = Laplace transform of reference input r(t) C(s) = Laplace transform of controlled output c(t) B s) = Primary feedback signal, of value H(s)C(s) E s) = Actuating or error signal, of value R s) - B s), G s) = Product of all transfer functions along the forward path H s) = Product of all transfer functions along the feedback path G s)H s) = Open-loop transfer function = summing point symbol, used to denote algebraic summation = Signal take-off point Direction of information flow.

Electromechanical Controls. Electro-mechanical control devices are typically used for load control (lighting, ventilation, and heating) in buildings with no feedback signal. The most common device is the electromechanical timer, in which a small motor coupled to a gearbox is able to switch electrical contacts according to a predefined time schedule. They are still in use today, applied to loads with simple scheduling requirements. 

The pole placement design predicates on the feedback of all the state variables x (Fig. 9.1). Under many circumstances, this may not be true. We have to estimate unmeasureable state variables or signals that are too noisy to be measured accurately. One approach to work around this problem is to estimate the state vector with a model. The algorithm that performs this estimation is called the state observer or the state estimator. The estimated state X is then used as the feedback signal in a control system (Fig. 9.3). A full-order state observer estimates all the states even when some of them are measured. A reduced-order observer does the smart thing and skip these measurable states. 

The feedback elements are components needed to identify the functional relationship between the feedback signal and the controlled output. 

The feedback signal is a function of the output signal. It is sent to the summing point and algebraically added to the reference input signal to obtain the actuating signal. 

The actuating signal represents the control action of the control loop and is equal to the algebraic sum of the reference input signal and feedback signal. This is also called the "error signal."... 

B) in Figure 9 represents the lube oil temperature control loop in block diagram form. The lube oil cooler is the plant in this example, and its controlled output is the lube oil temperature. The temperature transmitter is the feedback element. It senses the controlled output and lube oil temperature and produces the feedback signal. 

The feedback signal. This usually comes from the opto-coupler. We do try to keep it away from noisy traces and components, but in reality, it is not that prone to noise pickup as many tend to instinctively believe. 

The voltage drop across the platinum temperature sensor is small since the platinum resistor has a nominal resistance of only 75 Q. The fully-differential LNA amplifies the minute voltage drop in order to provide an useful feedback signal to the differential-analog proportional controller. A simplified schematic of the fully-differential low-noise amplifier is shown in Fig. 5.18. 

The temperature sensor is located in the microhotplate center (J2x, 10 kO nominal). This polysilicon resistor is biased with a temperature-independent current source (/bias)- The voltage-drop across the polysilicon temperature sensor provides the feedback signal for the temperature controller. 

Figure 11.5h shows a combined feedforward-feedback system where the feedback signal is added to the feedforward signal in a summing device. Figure 11.Sc shows another combined system where the feedback signal is used to change the feedforward controller gain in the ratio device. Figure 11.6 shows a combined feedforward-feedback control system for a distiltetion column where feed-rate disturbances are detected and both steam flow and reflux flow arc changed to hold both overhead and bottoms compositions constant. Two feedforward controllers are required.

Fig. 1.36. The topografiner. An instrument developed by Young, Ward, and Scire in the late 1960s, which is the closest ancestor of the STM. (a) The tip is driven by the x and y piezos, and the sample is mounted on the z piezo. By applying a high voltage between the tip and the sample, a field-emission current is induced. Using the field-emission current as the feedback signal, topography of the sample surface is obtained, (b) Close-up of the tip and the sample. The end of the tip has a small radius, typically a few hundred A. The typical tip-sample distance is a few thousand A. (After Young, 1971.)...

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