Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

6. COMPUTER CONTROL OF PROCESSES

 

· Functions of a Process I/O Interface,

  1. - Control the scanning and processing of all the inputs and outputs between the process and the computer.
  2. - Provide conversions of signal levels, compensation for sensor range, and linearization of signals.
  3. - Provide signal and sensor validity checks.
  4. - Provide signal pre-processing and filtering.
  5. - Provide measurements to the various other programming packages which have been converted to engineering units or interpreted in other ways.
  6. - Provide alarm limit checks input-by-input.

· Some features found in computer control systems,

  1. - timer initiated events - such as clock based sampling periods for a control loop
  2. - Process initiated interrupts - Prioritized interrupt signals can be used to alert computers to standard notifications, or to initiate an emergency sequence when a critical failure has occurred.
  3. - Control software - directs the operating points for the control system
  4. - System and program initiated events - these are for system house keeping, and for communication to other computer systems in a network.
  5. - Operator initiated events - requests for reports, changing of operating parameters, checking quality, etc.

 

· Interrupts

  1. · Each computer has the ability to process interrupts.
  2. · These interrupts can stop the microprocessor, and make it execute other subroutines.
  3. · If no interrupts are received, the computer will always run the same program
  4. · When interrupts are received, they are processed as urgent, or non-urgent.
  5. · When an interrupt is processed, a small subroutine is run, and then execution returns to the original point in the interrupted program.
  6. · If interrupts are not used, then the computer must continuously scan all of its inputs to see if any are critical. But this is not practical for real time processing.

 

· Linear Computer Controllers,

  1. · Linear control theory can be converted into computer programs.
  2. · Canonical form (Laplace/frequency domain) control rules can be converted into a form suitable for computers by using `z-transforms'
  3. · State space control laws can be converted into computer code using matrix techniques, along with numerical integration.
  4. · For example the PID control law has been used in many computer control programs to date.
  5. · The figure below depicts a compute controlled process

 

· Advanced topics include,

  1. - Feed forward controllers
  2. - Adaptive controllers
  3. - Optimal controllers
  4. - Non-linear control systems
  5. - Multi-input/Multi-output control systems

 

· An example of Computer-process interface

 

 

6.1 TEMPERTURE CONTROL

6.2 BATCH PROCESSING

6.3 COMPUTER PROCESS MONITORING

6.4 IMPLEMENTING A COMPUTER CONTROL SYSTEM

6.5 PRACTICE PROBLEMS