Term
| Describe Open Loop Control |
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Definition
| Not self correcting. A single setting controls the process at one point. No feedback. |
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Term
| Describe Closed Loop Control |
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Definition
| Self correcting. Has feedback. A variable is measured and compared to make adjustments to desired setpoint. |
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Term
| What are the 5 modes of control? |
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Definition
1. On-Off Control
2. Proportional Control(P)
3. Proportional plus Integral Control(Reset)(P+I)
4. Proportional plus Derivative Control(Rate)(P+D)
5. Proportional plus Integral plus Derivative Control(P+I+D) |
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Term
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Definition
| Two position control action. On or off. Controller outputs a signal that will position the final control element to one of its two extremes. |
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Term
| Describe Proportional Band |
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Definition
| Percent of span measured variable mus change in order to cause controller output(Final Control Element) to change 100%. |
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Term
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Definition
| Assume 25 degrees F change in input causes full output change. If the set point can be adjusted between 60 and 300 F, then the span = 240 F. The proportional band expressed as percent is calculated by 25/240x100 = 10.4%. |
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Term
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Definition
| Residual eror between set point and measured variable. |
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Term
| Convert proportional band and gain. |
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Definition
| Both are inversely related. Gain = Output/Input = KC(controller gain). Ex: Gain = 1/PB, PB = 1/Gain. |
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Term
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Definition
| A phenomenon when a controller drives its output back and forth between fully on and fully off. This is caused when the integral action is too aggressive. |
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Term
| Describe Proportional Integral control. |
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Definition
| Controller that will continue to generate an ever-increasing output until the error has been eliminated. Eliminates offset. |
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Term
| In a P+I controller. Output variations are due to two components: |
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Definition
1. The magnitutde of the error signal(proportional component).
2. The integral action due to the error signal and PB. |
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Term
| What is reset wind-up and its effect on a control loop? |
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Definition
| Reset wind-up happens if the process deviates from set point over an extended period of time(ex: system not in service) and the controller saturates. When the process is returned to service, large process overshoot could occur. |
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Term
| What is a common problem with reset wind-up? |
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Definition
| When the condition causing the windup is eliminated, the output must "wind down" for a period of time before the decreasing output has any effect on the process. |
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Term
| In PID control, corrective action(Positioning of the FCE) is determined by which 3 factors? |
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Definition
| Magnitude of the error, length of time the error has exited and rate of change of the error. |
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Term
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Definition
| Derivative is active only when the error is changing. Derivative provides deceleration and reduces the likelihood of overshoot or hunting. It can speed the process up. |
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Term
| How is rate achieved in a PID controller? |
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Definition
| By adding an adjustable restriction in the repsonse line to the Proportional bellows. It slows down the repositioning of the nozzle. |
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Term
| What is the main purpose of Rate? |
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Definition
| To prevent overshoot in processes with extensive lag. |
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Term
| Derivative action is _______ to the rate of change in the input. |
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Definition
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Term
| What are control system configurations that provide for sae plant operation during maintenance? |
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Definition
| The complete isolation, depressurization, cooling, and draining of a system is the safest configuration. Ex: An ECO |
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Term
| If isolating a controller from a system is not possible, then: |
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Definition
| Verify that the work scope does not breach the pressure boundaries. Transfer process control to an alternate controller. |
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Term
| When the High Level Controller is used: |
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Definition
| It reduces plant efficiency and FW heaters could BYPASS. |
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Term
| Taking the controller to Manual: |
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Definition
| Requires constant monitoring by operator. Manual feature not available on all controlers. |
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Term
| How do you calibrate/tune a pneumatic controller? |
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Definition
- Nozzle may need adjustment when Controller is placed in serivce due to specific gravity changing with temperature.
- Conduct a facilitated discussion of PMI-IC-ZI-0017, Pneumatic Controller Calibration, a generic instruction for pneumatic controller calibration.
- Certian Pneumatic Controllers have process indication. |
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Term
| What do you do before opening a controller's cover? |
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Definition
| Notify Operations and ensure the system is in a safe condition before you work on a controller i.e. system is bypassed, ECOed or out of service. |
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Term
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Definition
| There are several methods. The method depends on whether or not the controller can be taken "offline" for tuning, and the reponse speed of the system. |
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Term
| When tuning a controller, if the system can be taken offline, the best tuning method: |
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Definition
| Often involves subjecting the system to a step change input, measring the output as a function of time, and using this reponse to determine the control parameters. |
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Term
| When tuning a controller, if the controller has to remain online, one tuining method is to: |
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Definition
| First set the I and D values to zero. Increase the P until the output of the loop oscillates. Then increase I until iscillation stops. Finally, increase D until the loop is acceptably quick to reach its reference, A fast PID loop tuning usually overshoots slightly to reach the setpoint more quickly; however, some systems cannot accept overshoot. |
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Term
| What loops is Derivative usually reserved for? |
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Definition
| Slow acting loops such as temperature to anticipate the response needed. |
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Term
| How do you perform a bumpless transfer? |
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Definition
| By matching pressures and tranferring from manual to automatic with minimum disturbance. |
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Term
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Definition
| The smallest range of values the measured variable must pass through to cause the correcting device to go from one position to the other. It only applies to On-Off Control. |
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Term
| Advantage to proportional control |
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Definition
| There is less wear on final control element with more precise control. |
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Term
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Definition
| Approach to the control is fast, but the process temperature oscillates excessively before stabilizing. |
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Term
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Definition
| Slower approach to control point with almost no oscillation. |
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Term
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Definition
| The time for process temperature to reach control point is excessive, but there are no oscillations. |
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