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Feedback Controllers Chapter 7
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Chapter 7 On-off Controllers Examples Simple Cheap
Used In residential heating and domestic refrigerators Limited use in process control due to continuous cycling of controlled variable excessive wear on control valve. Examples Batch process control (PLC = programmable logic controller) Solenoid in home heating unit Sprinkler systems Cruise control? Chapter 7
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On-Off Controllers Chapter 7 Synonyms:
“two-position” or “bang-bang” controllers. e = error = set point – measured variable Chapter 7 Controller output has two possible values.
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Chapter 7 Practical case (dead band) δ = tolerance
system never reaches steady-state
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Chapter 7
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Chapter 7 Three Mode (PID) Controller Proportional Integral Derivative
Proportional Control Define an error signal, e, by e = Ysp – Ym where Ysp = set point Ym = measured value of the controlled variable (or equivalent signal from transmitter) Chapter 7
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Chapter 7 Since signals are time varying, e(t) = Ysp(t) - Ym (t)
n.b. Watch units!! For proportional control: where, p(t) = controller output = bias value (adjustable) Kc = controller gain (dimensionless, adjustable) Chapter 7
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Chapter 7 Standards (ISO/ISA) 3 – 15 psi ma 0 – 10 VDC
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Chapter 7 Proportional Band, PB Reverse or Direct Acting Controller
Kc can be made positive or negative Recall for proportional FB control: or Direct-Acting (Kc < 0) “output increases as input increases" p(t) Ym(t) Reverse-Acting (Kc > 0) “output increases as input decreases" Chapter 7
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Chapter 7 Example 2: Flow Control Loop
Assume FT is direct-acting. Select sign of Kc so that KcKv > 0 1.) Air-to-open (fail close) valve ==> ? 2.) Air-to-close (fail open) valve ==> ? Consequences of wrong controller action??
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Chapter 7 Transfer Function for Proportional Control: Let
Then controller input/output relation can written as Take Laplace transform of each side, or INTEGRAL CONTROL ACTION Synonyms: "reset", "floating control" I reset time (or integral time) - adjustable Chapter 7
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Chapter 7 Proportional-Integral (PI) Control
integral provides memory of e most popular controller Response to unit step change in e: Chapter 7
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Chapter 7 Integral action eliminates steady-state error
(i.e., offset) Why??? e 0 p is changing with time until e = 0, where p reaches steady state. Transfer function for PI control Chapter 7
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Chapter 7 ("repeats per minute") instead of I . For PI controllers,
Some controllers are calibrated in 1/I ("repeats per minute") instead of I . For PI controllers, is not adjustable. Derivative Control Action Ideal derivative action Used to improve dynamic response of the controlled variable Derivative kick (use -dym/dt ) Use alone? Chapter 7
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Chapter 7
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Chapter 7 Proportional-Integral-Derivative (PID) Control
Now we consider the combination of the proportional, integral, and derivative control modes as a PID controller. Many variations of PID control are used in practice. Next, we consider the three most common forms. Chapter 7 Parallel Form of PID Control The parallel form of the PID control algorithm (without a derivative filter) is given by
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Chapter 7 The corresponding transfer function is:
Series Form of PID Control Historically, it was convenient to construct early analog controllers (both electronic and pneumatic) so that a PI element and a PD element operated in series. Commercial versions of the series-form controller have a derivative filter that is applied to either the derivative term, as in Eq. 8-12, or to the PD term, as in Eq. 8-15: Chapter 7
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Features of PID Controllers
Expanded Form of PID Control In addition to the well-known series and parallel forms, the expanded form of PID control in Eq is sometimes used: Features of PID Controllers Chapter 7 Elimination of Derivative and Proportional Kick One disadvantage of the previous PID controllers is that a sudden change in set point (and hence the error, e) will cause the derivative term momentarily to become very large and thus provide a derivative kick to the final control element.
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Chapter 7
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Chapter 7 Automatic and Manual Control Modes Automatic Mode
Controller output, p(t), depends on e(t), controller constants, and type of controller used. ( PI vs. PID etc.) Manual Mode Controller output, p(t), is adjusted manually. Manual Mode is very useful when unusual conditions exist: plant start-up plant shut-down emergencies Percentage of controllers "on manual” ?? (30% in 2001, Honeywell survey) Chapter 7
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Chapter 7 Digital PID Controller finite difference approximation
where, = the sampling period (the time between successive samples of the controlled variable) = controller output at the nth sampling instant, n=1,2,… = error at the nth sampling unit velocity form - see Equation (8-19) (pn)- incremental change Chapter 7
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Chapter 7
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Chapter 7 Typical Response of Feedback Control Systems
Consider response of a controlled system after a sustained disturbance occurs (e.g., step change in disturbance variable); y > 0 is off-spec. Chapter 7
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Chapter 7
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Chapter 7 integral action ~
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Summary of the Characteristics of the Most Commonly Used Controller Modes
1. Two Position: Inexpensive. Extremely simple. 2. Proportional: Simple. Inherently stable when properly tuned. Easy to tune. Experiences offset at steady state. (OK for level control) 3. Proportional plus integral: No offset. Better dynamic response than reset alone. Possibilities exist for instability due to lag introduced. Chapter 7
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Chapter 7 4. Proportional plus derivative: Stable.
Less offset than proportional alone (use of higher gain possible). Reduces lags, i.e., more rapid response. 5. Proportional plus integral plus derivative: Most complex Rapid response No offset. Best control if properly tuned. Chapter 7
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Chapter 7 Example 3: Liquid Level Control
Control valves are air-to-open Level transmitters are direct acting Chapter 7
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Chapter 7 Question: 1. Type of controller action? Select Kc so that
air-to-open valve: sign of Kv? sign of process gain? Chapter 7
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Chapter 7 Previous chapter Next chapter
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