Reset, automatic

To improve the availability performance of the architecture when faced with an inevitable fleeting defect, the PIPC performs an automatic external reset. 

As soon as it is in the fallback position (field watchdog), the external power supply of the MCCS is cut for at least 5 seconds, the MCCS then restarts (by setting the outputs in restrictive mode, the watchdog is disabled). The power cut will remove potential residual erraticon on silicon junctions. 

Once the internal self-tests have been performed and the acquisition and alignment of functional contexts is made with the other MCCS (education phase and total transfer of the functional context of one MCCS to another), the watchdog is rearmed, thereby activating the electric outputs. 

The automatic reset capability is limited in number and time. Beyond that, we judge that the hypothesis of the fleeting character of the failure is not verified and the risk of a warning behavior is detrimental to the overall level of operational 

The reset principle constitutes a theoretical weakness in the demonstration of safety, but it is indispensable for the availability of the architecture. 

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Abnormally low pressure may not be an unsafe condition and the low-pressure switch may be automatic reset, closing again at a pressure corresponding to a temperature just below that of the load. 

Fig. A.3. Light meter used by the author (Model 8020, Pelagic Electronics). For total light measurements, the signals are integrated with capacitors. Milliammeter reading is automatically reset at full-scale position, and the number of resets is digitally indicated below the meter. The box at the right contains a photomultiplier and sample compartment.

The cool-down time should be as short as possible as this is unused analysis time. The controller as well as the oven should be capable of automatically resetting to the initial temperature. An indication that the column temperature has re-equilibrated after cool-down is very desirable. 

The integral and derivative modes are normally used in conjunction with the proportional mode. Integral action (or automatic reset) gives an output which is proportional to the time integral of the error. Proportional plus integral (PI) action may be represented thus ... 

The PTGC technique involves increasing the column temperature at a preset rate during the elution process. This rate may be constant throughout the run, or periods of isothermal operation may be automatically programmed at set times between temperature increases. Generally, the electronically controlled ovens are designed to increase temperature at rates from 0.5-30°C per minute. The initial temperature should be chosen to minimize the retention time for the least retained solute, while the final temperature must be sufficient to elute the least volatile compound in a reasonable time. The instrument then automatically resets the temperature to the initial value in preparation for the next sample. 

Indexing Windows 2000 implements an Index Service to catalog and improve the search capabilities of your drive. Once files are indexed you can search them more quickly by name, date, or other attributes. Setting the index option on a folder will cause a prompt to appear, asking whether you want the existing files in the folder to be indexed as well. If you choose to do this, Windows 2000 will automatically reset this attribute on subfolders and files. If not, only new files created in the directory will be indexed. 

These tertiary crushers employ smooth or toothed heavy-duty impact and abrasion-resistant steel-rimmed rolls. The rolls are mounted inline in a horizontal manner and turn toward each other at equal speeds to create a nip into which a friable feed material is introduced (Fig. 4). Heavy-duty compression springs with automatic reset are used to dampen crushing shock and to protect the crusher from tramp iron and oversize material. An adjustable screw that adjusts spring tension changes the crusher opening. A flywheel is used to even out pulses and economize on power consumption. These crushers have a theoretical maximum reduction ratio of 4 1 and will only crush materials to about 10 mesh. Roll crushers produce a controlled product size distribution without a lot of fines. The narrow particle size distribution is achieved by controlling a combination of variables including roll speed, gap measure, differential speed, feed rate, and roll surface. 

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. 

With the exception of derivative action any of these control modes may be used alone in certain applications. Integral and derivative actions are most usually combined with proportional control to give proportional plus integral control (proportional control with automatic reset) proportional plus derivative control or three-mode control, which is proportional plus integral plus derivative. 

AUTOMATIC RESET (INTEGRATION) - Is a type of control, in which the controller output, changes at a rate proportional to the deviation or error. The output will continue to change as long as any deviation or error exists. 

Fig. 1-8. Example of temperature variation with proportional control of an extruder barrel (no automatic reset).

Fig. 1-9. Variation of temperature in an extruder barrel with time, using proportional plus automatic reset control.

There is one major disadvantage to using an automatic reset barrel temperature controller the coefficients of the proportional, the reset, and the rate terms all have to be adjusted properly to obtain desired performance. It is not difficult to do this, but it can be time-consuming. One must follow the manufacturer s instructions. 

Many processes require speed controls. Performance and reliabilities of these controls are very similar to those of the temperature controls—you get what you purchase. Early speed controllers, like the temperature controllers, were mechanical. Speeds were held within 5 percent, resulting in poor plastic melt control. Where better speed control is desired, the solution is the same as in temperature control only the equipment names are changed. A device is added to the motor, and an integral characteristic is provided, corresponding to the automatic reset with heat. It brings the speed closer to the set point. A derivative characteristic, corresponding to the rate in heat, ensures a prompt response to any upsets. 

Automatic reset is done by using an electronic integrator to perform the reset function. The deviation or error is integrated with respect to time, and the result is added to the deviation signal to move the proportional band. The block diagram of proportional control with automatic reset is shown in Fig. 4.30. 

One drawback of the automatic reset is its relatively slow response. The response time can be reduced by addition of a second corrective term, which acts on the rate of change in temperature. A block diagram of a proportional plus integral plus derivative (PID) control system is shown in Fig. 4.32. 

These overload relays are highly resistant to vibration. They can be adjusted only by changing heaters, caimot be modified for automatic reset, and are inherently sensitive to ambient temperatures. 

A logical solution to the reset-windup problem is to add enough intelligence to the controller to make it aware of a shutdown condition. This is done by placing in the controller s reset circuit a switch energized by the output. Whenever the output exceeds 100 percent, the switch disables the reset circuit, leaving a proportional (or proportional-plus-deriva-tive) controller. In the absence of automatic reset action, a bias must take its place. Because this bias equals the output of the controller at zero deviation, it is ordinarily adjusted in relation to the expected process load. For this reason it is sometimes called the preload setting. 

Two-mode control combines the speed of response of proportional action with the elimination of offset brought about by automatic reset. The proportional mode is just as valuable in a sampled dead-time loop as it was in one without sampling. In fact, proportional action enables any loop whose dead time is less than the sampling interval to he critically damped. Figure 4.22 shows how this is done. 

A sampling integral controller is capable of critically damping a process dominated by dead time, while a continuous controller is not. This line of reasoning parallels that of complementary feedback, i.e., sampling is similar in nature to dead time whereas automatic reset is not. 

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