• Functions of a Process I/O Interface,
- Control the scanning and processing of all the inputs and outputs between the process and the computer.
- Provide conversions of signal levels, compensation for sensor range, and linearization of signals.
- Provide signal and sensor validity checks.
- Provide signal pre-processing and filtering.
- Provide measurements to the various other programming packages which have been converted to engineering units or interpreted in other ways.
- Provide alarm limit checks input-by-input.
• Some features found in computer control systems,
- timer initiated events - such as clock based sampling periods for a control loop
- 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.
- Control software - directs the operating points for the control system
- System and program initiated events - these are for system house keeping, and for communication to other computer systems in a network.
- Operator initiated events - requests for reports, changing of operating parameters, checking quality, etc.
• Each computer has the ability to process interrupts.
• These interrupts can stop the microprocessor, and make it execute other subroutines.
• If no interrupts are received, the computer will always run the same program
• When interrupts are received, they are processed as urgent, or non-urgent.
• When an interrupt is processed, a small subroutine is run, and then execution returns to the original point in the interrupted program.
• 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,
• Linear control theory can be converted into computer programs.
• Canonical form (Laplace/frequency domain) control rules can be converted into a form suitable for computers by using ‘z-transforms’
• State space control laws can be converted into computer code using matrix techniques, along with numerical integration.
• For example the PID control law has been used in many computer control programs to date.
• The figure below depicts a compute controlled process
- Multi-input/Multi-output control systems
• An example of Computer-process interface
