A poor system design may cause to stop your assembly line or miss your products without recording their id to the database. This slide shows how may it happen.
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Deadlock situation and missed products
1. Sometimes, deadlocks in PC supported automation systems can be a real headache if it designed poorly. When a deadlock occurs, system stops by making it impossible to free up without intervention. Assume that we have a conveyor carrying boxes, and scanning each boxes barcode and store it in computer database. We should have an ordinary PC – PLC handshake system 1 2 3 Boxes move this way
2. Network delays or PC unstability may cause delays in this handshake. Following legend is intended to visualise PC – PLC handshake, only two signals are included. Barcode data is omitted. Initially, Box 1 is not on the station yet, and is ready for receiving a box data 1 2 3
3. Let's see a normal operation. When Box 1 comes to the station, PLC gets the product presence information from sensor and transfers it to the PC by setting 'Data Save Request' flag to 'True' 1 2 3
4. When PC gets boxes information, it updates a database table and informs PLC by setting 'Data Saved' flag to 'True' when completed. 1 2 3
5. Then PLC sends the Box 1, and clears its 'Data Save Request' 1 2 3
7. This is a complete cycle without any problem. In normal conditions, this cycle is repeated by many times between PC and PLC 2 3
8. However, PC is not stable as PLC, sometimes it can be late to give an answer to PLC. In this case, PLC will not send the box until it gets a response from PC. 2 3 4
9. Normally it is not a big problem, may be less than a second of waiting may occur. 2 3 4
10. But what may happen if PC have a temporary hang at this moment? PLC resets its 'Data Save Request' but PC could not find a chance to see it yet. 2 3 4
11. But what may happen if PC have a temporary hang at this moment? PLC resets its 'Data Save Request' but PC could not find a chance to see it yet. And unfortunately, Box 3 come to the station. 3 4 5
12. Before hanging, PC set the 'Data Saved' flag for Box 2 and did not realised that Box 3 is on the station. PC is assuming that Box 2 never leaved the station 3 4 5 2
13. Now there are two possibilities according to the poorness of design: Possibility 1: Deadlock. PC and PLC mutually waits each other to reset their flag. 3 4 5 2
14. Now there are two possibilities according to the poorness of design: Possibility 2: Miss the Box 3 without recording its data into database 3 4 5 Bye bye Box 2 !
15. 'State analysis' of a system is important. A poor design may cause unwanted results. In next post, we will do a state analysis for the single station with a single task. http://indanotes.blogspot.com
