Temperature control loops

The dominant pole of this temperature control system is also determined by the thermal time constant of the microhotplate, which is approximately 20 ms. The open-loop gain of the differential analog architecture (Aql daa) is given by Eq. (5.8)  

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 differential input voltage (vi = -17 ) is copied over the series impedance 

Transistors Mg and Mg are the main sources of noise of this ampHfier. Their area is hence optimized in order to meet the noise specifications. Table 5.3 summarizes the transistor dimensions, bias currents and resistance values. 

The experimental results of the fully differential low-noise amplifier are summarized in Tab. 5.4. 

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Figure 8-47 presents a P I diagram for a simple temperature control loop that adheres to the ISA symbology. The measurement... 

The lube oil must be maintained within a specific operating band to ensure optimum equipment performance. This is accomplished by controlling the flow rate of the cooling water with a temperature control loop. 

The temperature control loop consists of a temperature transmitter, a temperature controller, and a temperature control valve. The diagonally crossed lines indicate that the control signals are air (pneumatic). 

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 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. 

Systems are designed to function normally even when a single instrument or control function fails. This is achieved with redundant controls, including two or more measurements, processing paths, and actuators that ensure that the system operates safely and reliably. The degree of redundancy depends on the hazards of the process and on the potential for economic losses. An example of a redundant temperature measurement is an additional temperature probe. An example of a redundant temperature control loop is an additional temperature probe, controller, and actuator (for example, cooling water control valve). 

Temperature Control Loop and Geometric Mean Circuitry... 

The schematic of the temperature control loop and the geometric mean circuitry of the single-ended mixed-signal architecture is shown in Fig. 5.7. 

Fig. 5.17. Temperature control loop of the differential analog architecture...

A micrograph of the single-ended hotplate-based microsystem is shown in Fig. 6.2 and features a die size of 5.0 x 2.9 mm. This system is a minimal implementation of a temperature-controlled microhotplate system. Temperature modulation is facilitated by an direct access to the input voltage A modulation of the input voltage is translated into a modulation of the microhotplate temperature. Another interesting application of the system includes its use as a microcalorimeter or as a material research platform [145]. The schematic of the temperature-control loop is shown in Fig. 6.3. 

The converted temperature signal serves as input to the digital PID temperature controller so that the temperature control loop is closed (see Sect. 6.3.3 for a detailed description of the temperature control loop). 

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. 

The reactor temperature controller (loop 2) is the primary controller, whereas the jacket temperature controller (loop 3) is the secondary controller. The advantage of the cascade control is that the reactor temperature control quickly reacts by the cascade system to disturbances in cooling fluid inlet conditions. The d3mamics of the transfer function G32 is faster than that of G 22-In the CSTR cascade control there are two control loops using two different measurements temperatures T and Tj, but only one manipulated variable Fj. The transfer function of the primary controller is the following ... 

Although we do not necessarily know the relationships involved in the external world, they do exist and determine the values of the relevant load disturbances. Consequently, we can say that the external world removes as many degrees of freedom as the number of disturbances. In Fig. 7.9 we are introducing a flow control loop to keep F constant, and a temperature control loop with a preheater to maintain qF. The feed composition is fixed by a relationship that we do not know—but all the same must exist. The control objectives are achieved by fitting suitable control systems to the plant and we can say that these control systems remove as many degrees of freedom as the number of control objectives in the overall control strategy. 

From equation 7.10, nf= 8. The feed flow and feed temperature control loops remove two of these and the unknown relationship specifying xf (which may be dependent upon the configuration of a plant upstream) reduces nf again by one—giving nf = 5. Hence, we require five control objectives to specify the system uniquely, i.e. to control it adequately. These will be to maintain xD (as a market requirement) and to control S, Pc, and the levels in the base of the column and the reflux accumulator (for operational feasibility). 

Two 1-min temperature measurement lags are included in the temperature control loop. A 50°C temperature transmitter span is used. Controller gain is 10, and integral time is 10 min. 

Ziegler-Nichols (ZN) and Tyreus-Luyben (TL) PI tunings are evaluated. Ultimate gain and frequency are obtained by performing relay-feedback tests. Temperature control loops have three 20-s lags. The pressure control loop has two 30-s lags. There is a... 

Heater for start-up. Because positive feedback due to heat integration may lead to state multiplicity, the heater duty can be manipulated in a temperature control loop to ensure stable operation. 

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