Objectives of control
- Maintain the process at desired operational state
- Safely transition the process between operational states
Measured parameter of interest that we want to control
Output
Parameters which may or may not be manipulated to affect this outcome
Inputs
Affect the outcome but cannot be manipulated. Inputs that cannot be controlled by an operator or control system
Disturbances
Produces a measured value for use in control system
Measurement device
Sends a signal to the final control element to maintain tank level by adjusting supply flowrate based on the error ε
Controller
Unpredictable changes in the process
Load changes
Value at which it is desired to
maintain the controlled variable
Set point
Error
Set point - Measured Value
Variable which is changed to control the controlled variable. Must be able to be changed by operator/control system and can be readily and directly manipulated,
Manipulated Variable
The condition of a system is monitored by a sensor, which provides
this information to the controller
2. The comparator in the controller determines the value of the error
signal (difference between actual and target value)
3. Based on the value of the error signal, the controller transmits a
signal to the final control element to change the value of the
manipulated variable
* The sensor, whose signal is fed into the control system to complete
the feedback loop, continually monitors the new value, of the
controlled variable
* In this way the control system acts continuously, to maintain the
controlled variable within acceptable limits of its set point
Closed Loop Control
Stop a process in an emergency
Safety Instrumented systems (SIS)
Economics: maintain optimum output
Product Specs: maintain product ‘on spec’
Environment : maintain effluent/atmospheric releases within allowable levels
Safety: Maintain equipment within operating specs, prevent hazardous situations
Motivations of control
Require continuous monitoring and control actions
Continuous processes
Cheaper and simpler than a controller - used for instructions like “start” and “stop”
Switch
Practical approach to problem solving that may not necessarily provide an optimum solution
Typically easier and more direct that other approaches
Heuristic Approach
1) Identify obvious control loops (e.g. level, flow, pressure, temperature)
2) Identify process variables that affect product quality and production spec.
3) Provide additional loops required for safe operation
4) Provide monitoring equipment
5) Provide sample points as necessary
6) Provide recorders if necessary
7) Provide interlocks and alarms
Heuristic Approach
Input variable that can affect the performance of a system
Load Variable
Uses secondary measurements to infer the value of primary variables that are difficult to measure directly e.g. composition
Inferential control
Two or more controllers are cascaded to control one or more process variable. Controllers are said to be “in cascade” when the output (OP.1) of the first or primary controller (master) is used to manipulate the set-point (SP.2) of another or secondary controller (slave)
Cascade control
More commonly uses switches (on/off) rather than controllers
Batch processes
Automatically stop the pump when the tank is full.
Level Switches
Required when basic process safety control system has failed to keep the process in a safe state.
Executes a specific safety-related task in the event of a specific dangerous condition.
Safety Instrumented Systems (SIS)
Safety instrumented systems process
- Sensor(s) to detect dangerous condition
- Controller (logic solver) to decide when to shut down the process
- Final control element(s) to perform shutdown function - isolation valve or override solenoid to force control valve to shutdown state
Use two in series for additional safety
Solenoid and pneumatic actutors to avoid common cause failure
