CCP class
- 1. COMPUTER CONTROL OF PROCESS UNIT – I (SAMPLED DATA CONTROL SYSTEMS)
- 2. Supervisory Control Supervisory control is a system which is controlling whole plant or more than one field devices through a master system. There will many types of field devices in a plant, each of the devices will have controllers to control the control action. We can’t install a signal controller to control the whole plant, it’s logic less. But can connect all the controller to a single master device. So a human can verify the process variable and set the set point to each controller distinctly.
- 3. A natural extension of a computer data logging system involves computer feedback on the process through automatic adjustment of loop set points. As various loads in a process change, it is often advantageous to alter set points in certain loops to increase efficiency as to maintain the operation within certain precalculated limits. The choice of set point is a function of many other parameters in the process. In fact, a decision to alter one set point may necessitate the alteration of many other loop set points as interactive effects are taken into account. Given the number of loops, interactions, and calculations required in such decisions, it is more natural and expedient to let a computer perform these operations under program control.
- 5. Here, a computer “supervises” the furnace’s temperature by communicating setpoint values to the temperature indicating controller (TIC) over a digital network interface such as Ethernet. From the temperature controller’s perspective, this is a remote setpoint signal, as opposed to a local setpoint value which would be set by a human operator at the controller faceplate. Since the heat-treatment of metals requires particular temperature ranges and rates of change over time, this control system relieves the human operator of having to manually adjust setpoint values again and again during heat- treatment cycles. Instead, the computer sets different setpoint at different stages according to the needs.
- 6. Process controllers configured for supervisory setpoint control typically have three operating modes: Manual mode Automatic mode with local SP Automatic mode with remote SP
- 7. Supervisory Control Structure Now we consider the control structure of supervisory control as
- 8. The master computer or supervisor makes the setpoint and gives to the controllers. Controllers adjust the PV with respect to the setpoint using PID controller. This forms a two-layer process control system: the “base” or “regulatory” layer of control (PID loop controllers) and the “high” or “supervisory” level of control (the powerful computer with the mathematical process models). A single cable can’t do the setpoint changes at once or much frequently, so a series of digital data lines are implemented to transfer setpoint from supervisor to controller. But it may also carry process variable information from those controllers back to the supervisory computer so it has data for its optimization algorithms to operate on.
- 9. Basic functions of Supervisory control The basic functions performed by supervisory controller in a plant are: Channel Scanning Conversion into Engineering Units Data Processing
- 10. (i) Channel Scanning: There are many ways in which microprocessor can address the various channels and need the data. Polling: In polling technique, each interrupting channel will have an interrupt or status flag which is checked by the processor one by one. The channel selection may be sequential or in any particular order decided by the designer. It is also possible to assign priority to some channels over others, i.e., some channels can be scanned more frequently than others.
- 11. Channel scanning and reading of data requires, the following actions to be taken: Sending channel address to multiplexer Sending start convert pulse to ADC. Reading the digital data. For reading the digital data at ADC output, the end of conversion signal of ADC chip can be read by processor and when it is ‘ON’ the digital data can be read.
- 12. Channel Scan Array: The scan array contains the addresses of the channels in the order in which they should be addressed.
- 13. Interrupt Scanning: Another method of scanning is interrupt scanning. It sends interrupt request signal to processor when the analog signal from transducer is not within High and Low limits boundary set by Analog High and Analog Low Signals. This is also called scanning by Exception. When any parameter exceeds the limits than the limits checking circuit would send interrupt request to microprocessor which in turn would monitor all parameters till the parameter values come back within pre - specified limits.
- 14. (ii) Conversion to Engineering Units: The data read from the output of ADC should be converted to the equivalent engineering units before any analysis is done as the data is sent for display or printing. For an 8-bit ADC working in unipolar mode the output ranges between 0 and 255. Conversion is done by the following parameters. The basic voltage and current range of transmitter is 0-5 V (or) 4-20 mA range
- 15. (iii) Data Processing: The data read from the ADC output for various channels is processed by the microprocessor to carry out limit checking and performance analysis. For limit checking the highest and lowest limits for each channel are stored in array. The limit array unit is shown
- 16. The limit array simplifies the limit checking routine. Through this, the facility to dynamically change the limits for any channel may also be provided, on the lines similar to scan array. In addition to limit checking, the system performance may also be analysed and report could be generated for the manager level. This report will enable the manager to visualise the problems in the system and to take decisions regarding system modification or alternate operational strategy to increase the system performance. The analysis may include histogram generation, standard deviation calculation, plotting one parameter with respect to another and so on.
- 17. Advantages of Supervisory control More productive, remote access Sustainability Interoperatability Increases efficiency of process control Easy maintenance
- 18. Disadvantages External interference Need trained personnel High investment needed