Transcript (Ratio) Controller

AUTOMATION & ROBOTICS
LECTURE#09
PROCESS CONTROL STRATEGIES
By: Engr. Irfan Ahmed Halepoto
Assistant Professor
• Two position control (On-Off Controller)
• Ratio control
• Multiple position control
– Cascade Control
– Feed Forward Plus Feedback Control (Hybrid)
• On/off control activates an output until the measured value
reaches the reference value.
– A common example is the household thermostat.
• No control action takes place until the measured value
deviates from the setpoint by a minimum amount (dead
band).
• The output then goes from full off to full on, turning off again
when the setpoint is reached.
Typical ON/OFF Response
• The input to output characteristic
waveform for a two position controller
is a step function.
• The controller switches from its
"OFF" state to its "ON" state when
the error signal (set point - measured
variable) becomes positive.
ON/OFF Control Action: Heater Control
• If the process value is lower than the set
point, output will be turned ON and
power will be supplied to the heater.
• If the process value is higher than the set
point, output will be turned OFF and
power to the heater will be shut off.
• This control method, in which the output
is turned ON and OFF based on the set
point in order to keep the temperature
constant, known as ON/OFF control
action.
• With this action, the temperature is
controlled using two values (i.e., 0% and
100% of the set point), therefore, the
operation is also called two-position
control action.
Heater ON/OFF Control Action Mechanism
On-Off Control: Tank Level control Example
• Level control in a water tank can be as simple as on-off.
• Water level is the measured and regard as controlled
variable.
• Inlet water flow rate is the manipulated variable.
On/off temperature control of water in a tank
thermostat On/off switching action
On/off temperature control of water in a tank
Tank temperature versus time
Ratio Control Systems
• Ratio control systems are installed to maintain the
relationship b/w two variables to control a third variable.
• Ratio control systems are the elementary form of
feedforward control.
• Ratio control is applied almost exclusively to flows, known
as wild flow.
• Wild flow can be uncontrolled, controlled independently, or
controlled by another controller that responds to variables of
pressure, level, etc..
• Objective of a ratio control scheme is to keep the ratio of
two variables at a specified value, so ratio (R) of two
variables (A & B), R=A/B is controlled rather than controlling
the individual variables.
– Note: A (disturbance) and B (manipulated) are physical
variables, not deviation variables.
Ratio Control Architecture
• Ratio control reduce the effects of
variations in the feed flow rates.
• Flow rate for stream A is a
disturbance, referred to as the
“wild” stream.
• The controller takes measurements
of the disturbance, stream A, and
then applies ratio control to
immediately bring about the
appropriate change in the flow rate
of stream B, the manipulated
stream.
• The output and the manipulated
variable of the controller is,
therefore, a ratio, as opposed to a
single variable.
• The controller takes a measurement
of the flow rate of “A”, enacts ratio
control, and immediately sets a new
flow rate for “B.
Block diagram of a ratio control loop
Ratio Control: Method I
• The flow rate of the two
streams is measured and
their ratio calculated using a
'divider' (just a piece of extra
electronics).
• The output of the divider is
sent to the ratio controller
(which is actually a standard
PI controller).
• The controller compares the
actual ratio with that of the
desired ratio and computes
any necessary change in the
manipulated variable
Ratio Control: Method II
• one
stream
is
under
standard feedback control.
• The flow of the second
stream is measured and
sent to a 'multiplier' (again
just a piece of extra
electronics) which multiplies
the signal by the desired
ratio yielding the setpoint for
the feedback control law.
Ratio Control Conceptual Diagram
• Conceptual diagram shows that the
flow rate of one of the streams feeding
the mixed flow, designated as the wild
feed, can change freely based on
maintenance
options,
product
demand,energy availability, the actions
of another controller in the plant.
• The other stream shown feeding the
mixed flow is designated as
the controlled feed.
• A final control element (FCE) in the
controlled feed stream receives and
reacts to the controller output signal,
COc, from the ratio control
architecture.
• Note: other flow manipulation devices
such as variable speed pumps or
compressors may also be used in ratio
control implementations.
Ratio Control Conceptual Diagram
Relays in the Ratio Architecture
• As the conceptual diagram illustrates, we measure the flow
rate of the wild feed and pass the signal to a relay, designated
as RY in the diagram.
• The relay is typically one of two types:
– Ratio relay: where the mix ratio is entered once during
configuration and is not accessible for change during
normal operation.
– Multiplying relay: where the mix ratio is presented as an
adjustable parameter on the operations display and is thus
readily accessible for change.
• In either case, the relay multiplies the measured flow rate of
the wild feed stream (PVw), by the entered mix ratio to arrive at
a desired or set point value (SPc),for the controlled feed
stream.
• A flow controller then regulates the controlled feed flow rate to
this set point value (SPc), resulting in a mixed flow stream of
specified proportions between the controlled and wild streams.
Ratio controllers: Linear Flow Signals Required
• A ratio controller architecture requires that the signal
from each flow sensor/transmitter change linearly with
flow rate.
• Thus, the signals from the wild stream process variable,
(PVw), and the controlled stream process variable (PVc),
should increase and decrease in a straight-line fashion as
the individual flow rates increase and decrease.
• In case of any abnormality, additional computations
(function blocks) must then be included between the
sensor and the ratio relay to transform the nonlinear
signal into the required linear flow-to-signal relationship.
• Turbine flow meters and certain other sensors can
provide a signal that changes linearly with flow rate.
Flow Fraction (Ratio) Controller
• Instead of using a relay, an alternative ratio control architecture
based on a flow fraction controller (FFC) can also be used.
1. The FFC is essentially a "pure" ratio controller in that it receives
the wild feed and controlled feed signals directly as inputs.
2. Ratio set point value is entered into the FCC, along with tuning
parameters and other values required for any controller
implementation.
• Two popular control strategies for improved disturbance
rejection performance are cascade control and feed forward
with feedback trim.
• Improved performance comes at a price.
– Both strategies require that additional instrumentation be
purchased, installed and maintained.
– Both also require additional engineering time for strategy
design, tuning and implementation.
• The cascade architecture offers attractive additional benefits
such as the ability to address multiple disturbances to our
process and to improve set point response performance.
• In contrast, feed forward with feedback trim architecture is
designed to address a single measured disturbance and
does not impact set point response performance in any
fashion.
Cascade Control
• Cascade control is widely used within the process industries.
• Cascade control is used to improve the response of a single
feedback strategy.
• The idea is similar to that of feedforward control: to take
corrective action in response to disturbance variable (DV) before
the CV deviates from setpoint.
• Cascade control schemes have two distinct features:
1. There are two nested feedback control loops.
• There is a secondary control loop located inside a primary
control loop.
2. The primary loop controller is used to calculate the setpoint
for the inner (secondary) control loop.
• The secondary control loop is located so that it recognises the
upset condition sooner than the primary loop.
Cascade control: block diagram
Design of Cascade control system
The Inner Secondary Loop
• The dashed line in the block diagram, circles a feedback control loop.
• Here "inner secondary" have been added to the block descriptions.
• Variable labels also have a "2" (secondary) after them.
Nested Cascade Architecture
• To construct a cascade architecture, we nest the secondary control loop
inside a primary loop as shown in the block diagram.
• Note that outer primary PV1 is our process variable of interest in this
implementation.
• PV1 is the variable we would be measuring and controlling if we had
chosen a traditional single loop architecture instead of a cascade.
Cascade control: Early Warning System
• Measurement and Control of an "early warning" process variable
is essential element for success in a cascade design.
Cascade control: Early Warning System
• In the cascade architecture, inner secondary PV2 serves as a early
warning process variable.
• Essential design characteristics for selecting PV2 include that:
– it be measurable with a sensor,
– same FCE (valve) used to manipulate PV1 also manipulates PV2,
– the same disturbances that are of concern for PV1 also disrupt PV2,
– PV2 responds before PV1 to disturbances of concern and to FCE
manipulations.
• Since PV2 sees the disruption first, it provides our "early warning" that a
disturbance has occurred and is heading toward PV1.
• The inner secondary controller can begin corrective action immediately.
• Since PV2 responds first to final control element (e.g., valve)
manipulations, disturbance rejection can be well underway even before
primary variable PV1 has been substantially impacted by the
disturbance.
• With such a cascade architecture, the control of the outer primary
process variable PV1 benefits from the corrective actions applied to the
upstream early warning measurement PV2.
Outer Disturbance must impact Early Warning Variable PV2
• With a cascade structure, there will likely be disturbances that impact
PV1 but do not impact early warning variable PV2.
• The inner secondary controller offers no "early action" benefit for these
outer disturbances.
• They are ultimately addressed by the outer primary controller as the
disturbance moves PV1 from set point.
• So, a proper cascade can improve rejection performance for any of a
host of disturbances that directly impact PV2 before disrupting PV1.
Level-to-Flow Cascade Block Diagram
Level-to-Flow Cascade Block Diagram
• A level-to-flow cascade structure includes:
– Two controllers: the outer primary level controller (LC) and inner
secondary feed flow controller (FC)
– Two measured process variable sensors: the outer primary liquid
level (PV1) and inner secondary feed flow rate (PV2)
– One final control element (FCE): the valve in the liquid feed stream.
• As required for a successful design, the inner secondary flow control loop
is nested inside the primary outer level control loop. That is:
– The feed flow rate (PV2) responds before the tank level (PV1) when
header pressure disturbs the process or when the feed valve moves.
– The output of the primary controller, CO1, is wired such that it
becomes the set point of the secondary controller, SP2.
– Ultimately, level measurement, PV1, is our process variable of
primary concern.
– Protecting PV1 from header pressure disturbances is the goal of the
cascade.
• The feed forward with feedback architecture is
constructed by coupling a feed-forward-only controller
to a traditional feedback controller.
• The feed forward controller seeks to reject the impact
of one specific disturbance (D), that is measured before
it reaches our primary process variable, PV, and starts
its disruption to stable operation.
• Typically, this disturbance is one that has been
identified as causing repeated and costly upsets, thus
justifying the expense of both installing a sensor to
measure it, and developing and implementing the feed
forward computation element to counteract it.
Feed Forward with Feedback Trim Architecture
Combined Feed forward & Feedback Controllers
Combinations of feedback and feedforward control give us :
• Benefits of feedback control: controlling unknown disturbances
and not having to know exactly how a system will respond
• Benefits of feedforward control: responding to disturbances
before they can affect the system
Comparison of Feedback & Feedforward Control
Feedback (FB) Control
Advantages
• Corrective action occurs regardless of the source and type of
disturbances.
• Requires little knowledge about the process (process model is not
necessary).
• Versatile and robust (Conditions change? May have to re-tune
controller).
Disadvantages
• FB control takes no corrective action until a deviation in the
controlled variable occurs.
• FB control is incapable of correcting a deviation from set point at
the time of its detection.
• Theoretically not capable of achieving “perfect control.”
• For frequent and severe disturbances, process may not settle out.
Comparison of Feedback & Feedforward Control
Feedforward (FF) Control
Advantages:
• Takes corrective action before the process is upset.
• Theoretically capable of "perfect control“
• Does not affect system stability
Disadvantages:
• Disturbance must be measured (capital, operating costs)
• Requires more knowledge of the process to be controlled
(process model)
• Ideal controllers that result in "perfect control”: may be
physically unrealizable. Use practical controllers such as lead-lag
units