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process exam 3
Terms in this set (45)
proportional control (P)
the controller output is proportional to the error signal. almost always an offset. takes immediate corrective action as soon as an error is detected.
key concepts- controller gain can be adjusted to make the controller output changes as sensitive as desired to deviations between set point and controlled variable. the sign of Kc (controller gain) can be chosen to make the controller output increase or decrease as the error signal increases.
output depends on integral of the error signal over time. advantage of eliminating offset as long as the controller output or final control element doesn't saturate and become unable to bring the controlled variable back to the set point. very seldom used by itself bc little control action takes place until the error signal has persisted for some time. generally used with proportional control.
response to a step change- at time 0 the controller output changes instantaneously due to proportional action. integral action causes the ramp increase.
function is to anticipate the future behavior of the error signal by considering its rate of change. controller output can be made proportional to the rate of change of the error signal or the controlled variable by derivative control. by providing anticipatory control action, the derivative mode tends to stabilize the controlled process.
parallel form without derivative filter
issue with proportional only (P) control?
almost always an offset associated with using P only control. proportional controllers react to the deviation only with no distinction as to the time period over which the deviation develops.
issue with integral only (I) control?
tends to produce oscillatory responses of the controlled variable and reduces the stability of the feedback control system. limited amount of oscillation can be tolerated because it is often associated with a faster control. correct tuning can mitigate undesirable side effects. corrective action depends on the duration of the deviation.
what is resist windup
when a sustained error occurs and the integral term becomes large and the controller output eventually saturates- further build up of the integral term with the controller is saturated is known as reset windup. occurs when: 1. PI or PID controller encounters a sustained error (during a start up of a batch process or a large set point change) 2. consequence of a large sustained disturbance that is beyond the range of the manipulated variable.
issues with derivative control?
controller output is equal to the nominal value as long as error is constant so must be used in conjunction with P or PI control. if the process measurement is noisy (contains high-frequency, random fluctuations) then derivative action can amplify the noise unless the measurement is filtered.
what is derivative kick
occurs when a sudden set point change causes the derivative term in a PID and therefore a large spike in the final control element
simple, inexpensive feedback controllers that are commonly in noncritical industrial applications (as well as thermostats, refrigerators, etc) but are not as versatile or effective as PID controllers. on-off control can be considered a special case of proportional control with a very high controller gain.
how are changes in the controller settings expected to change the system (Kc)
kc- increasing tends to make response less sluggish, but increasing too much can cause the process to exhibit an undesirable degree of oscillation and perhaps become unstable. intermediate K values usually result in the best control. for P control, increasing Kc helps minimize offset.
how are changes in the controller settings expected to change the system (Ti)
increasing integral time usually makes PI and PID control more conservative (sluggish). theoretically offset will be eliminated for all positive values of Ti but for very large values of Ti the controlled variable will return to the set point very slowly after a disturbance or set point change occurs.
how are changes in the controller settings expected to change the system (Td)
more difficult to generalize. for small values, increasing Td tends to improve response by reducing maximum deviation, response time, and degree of oscillation. if Td is too large, measurement noise is amplified and the response can become oscillatory. intermediate Td is generally what you want.
composition sensor transmitter (analyzer)
negligible dynamics when Tau >> TauM. assume that the dynamic behavior can be approximated by a first order transfer function. useful approximation is set TauM=0 and let Gm=Km where Km depends on the input and output ranges of the composition sensor transmitter combination
if a reported controller gain is not dimensionless, it includes the gain of a least one other device (maybe an actuator) the controller gain is equal to Km
current-to-pressure (I/P) transducer
transducers are designed to have linear characteristics and negligible (fast) dynamics. therefore the transfer function is simply the steady stain gain of Kip
usually designed so that the flow rate through the valve is a nearly linear function of the signal to the valve actuator, so a first order transfer function usually provides an adequate model for operation of an installed valve in the vicinity of a nominal steady state.
set point changes
D (disturbance) = 0, want Y/Ysp
Ysp (set point) = 0 and need Y/D
general expression for feed back control systems
following the path between output to input variable π(forward transfer functions)/(1 + π(transfer functions around the loop). If have inner loops reduce them down first and assign a new variable. Use that in the overall transfer function
performance criteria for closed loop systems
must be stable, effects of disturbance are minimized (providing good disturbance rejection), rapid smooth responses to set point changes are obtained (good set point tracking), offset is eliminated, excessive control action is avoided, control system is robust (insensitive to changes in the process conditions and to inaccuracies in the process model)
high degree of performance if control system provides a rapid and smooth response to disturbance and set point changes with little to no oscillation
if a system is robust it provides satisfactory response for a wide range of process conditions and for a reasonable degree of model inaccuracy. can be achieved by choosing conservative controller settings (small values of Kc and large values of Ti) but conservative tend to cause poor performance. conservative controller settings to achieve robustness sacrifice performance.
tradeoff between set point tracking and disturbance rejection (for most single loop controllers in the process industry disturbance rejection is more important than set point tracking)
PID controllers that provide excellent disturbance rejection can produce large overshoots for set point changes. If controller settings are specified to provide excellent set point tracking, the disturbance response can be very sluggish.
determining PID controller settings
1. direct synthesis (DS) method
3. Ziegler- Nichols
4. Tyreus Leuben
Direct synthesis method (DS)
controller design is based on a process model and a desired closed loop transfer function. provides valuable insight in to the relationship between the process model and the resulting controller.
provides satisfactory set point responses but very slow disturbance responses bc TauI is very large
section of TauC
select TauC as the max (0.2tau, theta)
must have overall K and tau. penalizes errors that persist for long periods of time- results in the most conservative controller settings. optimal controllers settings are different for set point changes and step disturbances. controller settings are for set point changes are generally more conservative.
1/4 decay ration and is generally considered excessively oscillatory so its not the best approximation
online controller tuning
on-site adjustment of unsatisfactory controller settings. Involves plant testing on a trial and error basis. Good initial controller settings reduce the required time and effort during online controller tuning. `
Controller tuning inevitable involves a tradeoff between robustness and performance
The performance goals of excellent set point tracking and disturbance rejection should be balanced against the robustness goal of a stable operation over a wide range of conditions.
Controller settings do not have to be precisely determined
In general, a small change in a controller setting from its best value (+/- 10%) has little effect on closed-loop responses.
For most plants, it is not feasible to manually tune each controller.
Each control specialist is typically responsible fro 300 to 1000 control loops—only control loops that are perceived to be the most important or troublesome are manually tuned. All others typically operate using the preliminary settings from the control design system.
Continuous cycling method
based on the determination of Kcu and Pu the ultimate gain and the ultimate period (respectively).
the numerical value for the gain that produces continuous cycling for proportional only control.
the period of the corresponding sustained oscillation (to the value of Kcu)
step test method
after process reaches steady-state but controller in manual mode and introduce a small step change
guidelines for common control loops
flow rate, liquid level, gas pressure, temperature
flow rate guideline
flow control loops are generally characterized by fast responses and essentially no time delay. disturbances tend to be frequent but generally small. generally use PI control with intermediate values of the controller gain. (don't want to use PID because of the presence of high frequency noise that would be amplified by derivative control)
standard P or PI controllers are widely used for level control. level control problems are unusual because increasing the gain of a PI controller can increase stability while reducing the gain can increase oscillation and instability. (increasing Kc too much can still cause oscillatory behavior) If a small offset is tolerable (+/-5%) in the level then integral control can be omitted. Control objectives of surge tank to damp fluctuations in inlet streams can be achieved through averaging level control by allowing the liquid level to rise or fall in response to inlet flow disturbances.
1. The exit flow rate from the tank should change gradually rather than abruptly in order to avoid upsetting processes downstream.
2. The liquid level should be maintained within specified upper and lower limits.
3. The steady-state mass balance must be satisfied so that the inlet and outlet flows are equal.
high and low limits are more serious considerations from pressure control because of safety and operational issues. PI controllers are normally used with only a small amount of integral action (large tauI)
no general guidelines. PID controllers normally employed because produce a quicker response than PI, but temp control loops vary greatly from equipment to equipment and process to process. (heat transfer and different time scales.)
troubleshooting control loops
i. One-third of industrial control loops are in manual mode.
ii. Poor controller tuning means that another third of loops actually increase process variability.
iii. Control loops consist of many individual components ( sensor/transmitter, controller, final control element, instrument lines, etc...). Serious control problems can result from the malfunction of any one of the individual pieces, but even if every piece is operating properly that does not mean that the overall system will operate properly.
iv. Tuning is not a cure all for control loop problems!
v. Control loops can become unstable/sluggish due to a variety of reasons including:
1. Changing process conditions (usually changes in the through-put).
2. Sticking of a control valve stem.
3. Plugged line in a pressure or differential pressure transmitter.
4. Fouled heat exchangers (especially reboilers for distillation columns).
5. Cavitating pumps (usually caused by a suction pressure that is too low).
*only items 1 and 4 provide valid reasons for re-tuning the controller.
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