Two position control

On/off control A simple two-position control system that is only capable of performing these two functions (on and off). 

Figure 31.2 UmWs of controlled variable with two-position control.

Figure 12 Two Position Controller Input/Output Relationship. 18...

EO 1.4 DESCRIBE the characteristics of the following types of automatic control systems a. Two position control system... 

A two position controller is a device that has two operating conditions completely on or completely off. 

Figure 12 shows the input to output, characteristic waveform for a two position controller that switches from its "OFF" state to its "ON" state when the measured variable increases above the setpoint. Conversely, it switches from its "ON" state to its "OFF" state when the measured variable decreases below the setpoint. This device provides an output determined by whether the error signal is above or below the setpoint. The magnitude of the error signal is above or below the setpoint. The magnitude of the error signal past that point is of no concern to the controller. 

A system using a two position controller is shown in Figure 13. 

For processes that can operate with continuous cycling, the relatively inexpensive two position controller is adequate. For processes that cannot tolerate continuous cycling, a proportional controller is often employed. For processes that can tolerate neither continuous cycling nor offset error, a proportional plus reset controller can be used. For processes that need improved stability and can tolerate an offset error, a proportional plus rate controller is employed. 

There are two frequently used algorithms to determine the control action in a feedback control system for bioprocess control, an on-off (two-positioned) control, and a PID (proportional-integral-derivative) control. 

On-Off (Two-Positioned) Control An on- off (two-positioned) controller is an algorithm that has two manipulating actions completely ON or completely OFF. Here, the controller gives the ON signal when the deviation exceeds a threshold value and the OFF signal when the deviation falls below another threshold value. Automatic on-off valves or switches are usually employed in actual processing plants in order to perform on-off (two-positioned) control. 

Two sources reported the results of a study of rats administered sodium selenate or sodium selenite in the diet for a lifetime (Harr et al. 1967 Tinsley et al. 1967). A vehicle control and two positive control groups (administered a known hepatocarcinogen, V-2-fluorenyl-acetamide [FAA]) were included. Mortality was high in the highest dose group (0.8 mg selenium/kg/day), and therefore, selenium administration was discontinued. Longevity was reduced in animals fed 0.4 mg selenium/kg/day, but not... 

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. 

The sensitivity of an immunoassay is typically defined by its LLOQ, which can only be determined when a reference standard is available. Alternatively, a detection limit of qualitative assays is used where a distinction is essentially made between positive and negative results only. Typically, the antibody data are reported as titer, the titer being the reciprocal of the highest dilution of a sample in which the instrument response is greater than the cutoff response [22]. At least two positive control analytes should be used. If a positive control antibody is available, a pseudocalibration curve can be generated by a series of dilutions. However, one should keep in mind that true quantitation is impossible because of the lack of a true reference compound. Determination of the dilutional linearity is not so important when the result is reported as a titer. However, if the determination of positive samples is based on the interpolation from a reference standard curve, it is essential to demonstrate that the QC samples fall within the (limited) linear range of the calibration curve and not on a plateau or a region that may include a hook effect. 

Two-Position Control. The simplest case is two-position (on-off) control. Here, any deviation of the measured value from a set point drives the final control-operator to either a full-on or full-off position. This forces the measured value back and forth across the set point, and the measurement signal cycles about this point. The amplitude and frequency of this cycle depend on the response characteristics of the process. As the process dead-time becomes small, the frequency of the cycle becomes high likewise, as the process capacitance becomes high, the amplitude of the cycle becomes small. This mode of control is used only for processes in which this cycling effect can be tolerated it is most successful with those having large capacitance. 

Control algorithms We have discussed that in closed-loop control systems a corrective action is taken by the controller in response to feedback from a transducer. The exact corrective action depends on the algorithm which has been developed. The simplest control approach is a two position control which turns the control element on and off based on the monitored value of the output. With an on/off strategy, the process value will typically oscillate above and below the set point. The most common controller is the PID (proportional, integral, and derivative) loop controller which is able to detect an early trend, adjust quickly, and prevent an over-correction. A PID controller can maintain temperatures within 1°F. The controller provides the means to define the control algorithm by assigning a constant for each of the three control modes. Typically, most of the adjustment is accomplished with the proportional control element, with the control action, u be-... 

A system of regulation in which the manipulated variable has only two possible values, high and low, maximum and minimum, or on and off. Also known as two-position control. 

Generally the two-position control mode is best applied to large-scale systems with slow process rates. This method leads to the oscillation of the variable about the setpoint, and therefore, in cases where overshooting cannot be allowed, the on-off method cannot be used. This method is used in the control of many home appliances, such as the oven control. 

In a two-position control mode the output has a value of either 100% or 0%, depending on the error being greater or less than the specified value. In a multi-position mode, the number of divisions of the controller output is increased. In a proportional control mode, a smooth linear relationship exists between the output and the error. Therefore, a one-to-one correspondence exists between each value of the eontroller output and the error. This mode can be represented as shown in Eq. 7.5. 

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