Types of Controller Action

In the basic conventional feedback control strategy the value of the measured variable is compared with that for the desired value of that variable and if a difference exists, a controller output is generated to eliminate the error. 

The simplest and, despite its several drawbacks, the most widely used type of control is the on/off control system. An example is a contact thermometer, which closes or opens a heater circuit. The designation on/off means that the controller output, or the manipulated variable (electric current) is either fully on or completely off. To avoid oscillations around the setpoint, the real on/off controller has built into it, a small interval on either side of the setpoint, within which the controller does not respond, and which is called the differential gap or deadzone. When the controlled variable moves outside the deadzone, the manipulated variable is set either on or off. This is illustrated in Fig. 2.30. Such shifts from the set point are known as offset. 

The oscillatory nature of the action and the offset make the resulting control rather imperfect, but the use of on/off control can be justified by its simplicity and low price, and the reasonable control obtained, especially for systems which respond slowly. 

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The disadvantages of PI control are that it gives rise to a higher maximum deviation, a longer response time and a longer period of oscillation than with proportional control alone (see Fig. 7.S). Hence, this type of control action is used... 

There are different types of controller actions that will make adjustments in the input, Z, based on the error between the output and set point. We shall consider four types proportional, integral, derivative, and proportional-integral actions. 

A frequent application of dynamic simulation is to investigate a proposed control strategy for a chemical plant. It is usually assumed that the plant is operating at steady-state initially. The objective is to investigate the response of the system including controllers to process disturbances and evaluate the performance obtained for different types of control action. What... 

In process control only a few types of control action (control modes) are important, namely (1) on-off or two-position control (2) proportional control (3) integral control or automatic reset (4) derivative or rate action. 

Figure 9. Effect of different types of control action (reproduced from Thomas J. Rhodes, Industrial Instruments for Measurement and Control, [New York McGraw-Hill, 1941], p. 422).

To assist in preventing sequencing errors, controls should be placed in the sequence in which they are to be used. At the same time, similarity, proximity, interference, or awkward location of critical controls should be avoided. Where operators have to perform different classes or types of control actions, sequences should be made as dissimilar as possible. 

Types of control action required whether a smooth control action of the system is required or an ON/OFF action is ok The latter is more drastic in nature, and the control action starts and stops only after the parameter values reach the upper and lower set points. The implications of this should be discussed with shop floor engineers before ordering the unit. 

Main Types of Control Actions In Figure 13.49, a bloek scheme of a control system is presented. In the case when the set point value xq is constant the control is called value keeping. If the set point is a function of time, that is, a signal is needed for operating the set point deviee. Various principles can be used to form commands for interventions by the controller. The basis of the methods is the error e, whieh is the difference between the controlled parameter (control signal x ) and the set point... 

The shape of the load response curve depends to a considerable degree on the type of control action used and the settings of the parameters involved. Furthermore, the penalty ascribed to a typical response curve is determined by the specifications of the process. Several means of weighing the response curve suggest themselves ... 

All nonconformities are caused, all causes within your control can be avoided - all that is needed is concerted action to prevent recurrence. There are three types of corrective action product-related, process-related, and system-related. Product-related nonconformities can be either internal or external, as addressed previously, and you will have nonconformity reports to analyze. Process-related nonconformities may arise out of a product nonconformity but if you expect something less than 100% yield from the process, the reject items may not be considered nonconformities. They may be regarded as scrap. By analyzing the process you can find the cause of low yield and improve... 

The performance of reciprocating pumps provides for ease of operation and control. Depending upon the type of piston action, the fluid may be subject to pulsations unless accumulator or surge drums are provided. 

When close control is desired, usually the variable that is to be closely controlled is monitored and no changes are made until the measurement differs from what is desired. This is feedback control. It obviously is not an ideal system, since the controller can only react to changes. A better system would be one that anticipates a change and takes corrective action that ensures an unvarying output. This is a feedforward control system. This type of control is very advantageous when the input variables have a wide range of variation. 

It is proposed to use one of two types of controller in this control loop, either (a) a PD controller whose action approximates to ... 

Besides, the pharmacological actions of many compounds are invariably dependent on the shape of molecules and hence, usually play a very significant role. Therefore, if both cis- and tram-isomers are produced in the course of a particular synthesis it may be absolutely necessary to incorporate in the product profile a specific test for the relative proportions of one to the other. This type of control measure strictly conforms the uniformity of composition in the bulk-drug industry and ensures a check on the batch-to-batch variation. 

The common types of control loops are level, flow, temperature, and pressure. The type of controller and the settings used for any one type are sometimes pretty much the same from one application to another. For example, most flow control loops use PI controllers with wide proportional band and fast integral action. 

In allosteric enzymes, the activity of the enzyme is modulated by a non-covalently bound metabolite at a site on a protein other than the catalytic site. Normally, this results in a conformational change, which makes the catalytic site inactive or less active. Covalent modulated enzymes are interconverted between active and inactive forms by the action of other enzymes, some of which are modulated by allosteric-type control. Both of these control mechanisms are responsive to changes in cell conditions and typically the response time in allosteric control is a matter of seconds as compared with minutes in covalent modulation. A third type of control, the control of enzyme synthesis at the transcription stage of protein synthesis (see Appendix 5.6), can take several hours to take effect. 

The simplest type of control which is commonly experienced is that of having an on-off or two-position action (often called bang-bang control). A typical example is the thermostatically controlled domestic immersion heater. Depending on the temperature of the water in the tank, the power supply is either connected to, or disconnected from, the heater. The relationship between controller input and output might appear as in Fig. 7.4. Such a system is simple and inexpensive. However, the oscillatory nature of the control makes it suitable only for those purposes where close control is not essential and/or where its non-linear action can be taken into account. This is considered in more detail in Section 7.16.2. 

There are three principal types or modes of control action which are more generally employed, viz. proportional (P), integral (I), and derivative (D). In the first the controller produces an output signal J which is proportional to the error, i.e. ... 

The forced swim test and other assays that are used to identify antidepressant compounds detect compounds with different types of drug action—i.e., serotonin reuptake blockers, norepinephrine reuptake blockers, and atypical antidepressants. A major concern with these types of assays is the identification of false positive compounds. Traditionally, test compounds are evaluated in the forced swim test and in locomotor activity assays to test for stimulant activity. Stimulant compounds are considered false positives in the forced swim test since swimming is considered by some a form of locomotion. However, there are some compounds that increase dopaminergic signaling, such as nomifensine and bupropion, that had antidepressant-like effects in clinical and preclinical tests and demonstrated stimulant activity in some studies [37,38]. Interestingly, stimulant drugs are normally considered false positives in the forced swim test because they are not prescribed for depressed patients however, no controlled studies have been conducted to test this assumption. 

One of the basic types of controls exercised over the nation s industry during the Korean action was the Controlled Materials Plan (CMP). Under CMP, steel, copper, and aluminum were allocated to industry. The Chemical Division was responsible for allocating these basic materials to manufacturers of 16 groups of chemical products such as paint, catalysts, and plastics. On July 1, 1953, CMP was replaced by the Defense Materials System (DMS). 

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