Thursday, April 2, 2009

Computer Control of processes

Not just specialized Computers but also General Purpose Computers (PC) is also used in automation. Here we look at how to Control processes solely using General Purpose computers (PC based control).

Software
Software used here is Adamview, a product of Advantech.
Hardware
The hardware needed is popularly referred to as Data Acquisition Modules. The ones we use here are the ADAM 4000 series modules, but usually any module can be used given that they confirm to the open Modbus standard.
The different modules have different uses,
1. Digital input - ADAM-4014
2. Digital output- ADAM-4060
3. Analog input-ADAM-4011/12/13
4. Analog output- ADAM-4021/4024
5. converters- ADAM-4522
6. GSM communication – ADAM-4581 // insert pictures from manual
The modules are supplied with voltage ~20v dc.

Communication
The modules are daisy chained using twisted pair cables using the RS 485 communication protocol. The Communication to computer occur using RS 232 serial (COM) port. A RS 232 to RS 485 converter module is used at the PC side to connect to the serial cable.
Any software can be used with modules of any manufacturer given that all conform to the open MODBUS standard.
Structure of PC based Control

Task
This is the place that the control logic is setup. There is GUI blocks to do mathematical logical operations PID control IO etc... The control logic is implemented using these blocks. If the given blocks are not sufficient the logic can be built by programming with inbuilt visual basic language.
This has DDE and OLE support, thus the software can be used as a data center and logic / GUI can be implemented in VB/C excel etc.
Display
This is the place where the GUI or HMI is setup. The HMI is needed when the set points need to be changed (when a process by process input is needed) or real time values of processes are needed to be displayed for monitoring.
The machine operator interacts solely with the HMI, which is usually a industrial PC with a touch panel screen.
Tag: A tag is an object which helps to connect user input to the control logic.
Note: The Inputs/outputs can be easily connected to the Display without hassle, but we need to make a tag to connect from display to Task.
Steps in designing a Control program
1. Install ADAMview provided with the CD
2. Configure the devices settings ( thermocouple type, IO, communication speed etc ) using ADAM-4000-5000 Utility
3. Start ADAMview Device manager and add required modules (for simulation add DEMO)
4. Start the ADAMview builder utility (This is the HMI/SCADA control software referred to as just ADAMview)
5. Let’s start with a simple application, Control of a Heater. The input is taken from a thermocouple (here it can be simulated (sine wave)) and the output is digital (to a heater).
6. Click on task pane. Add analog input by clicking on AI
7. Inside its properties select device as DEMO
8. Go to Display -> insert label ‘Temperature’->add GUI component -numeric display item-> right click on it ->select input from ->Task1->Al1. Now we have configured this display to get input from the Analog input we set up earlier.
9. Select run and see the analog input in display varying with time. Stop it.
10. Insert label ‘set point’->then insert numeric control display item beside it.
11. Go to Task and add a TAG. Configure it to point to DISPLAY1 -> NCTL1
12. Add digital output ‘DO’ to work area. select device (if not available select DEMO)
13. Now add (SOC1) single calculation block and connect Al1 and tag1 to it.
14. Click on soc1 and configure it as <= (tag1=operand1 , Al1=op2)
15. Connect output of SOC1 to DO1. We have completed the construction of the basic control program.
Now do this to see the status of the heater.
1. Go to Display -> add Indicator
2. Right click on it and choose select ->Task1-> DO1
Now run the program to see the simple control of heater using the PC.
A more complex Program will be included in the CD.

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