Automation for Standardisation and Blending Systems
Christian Isaksson · 2006
The purpose of a standardisation and blending system is to produce milk products after a certain recipe with a uniform composition, e.g. with a uniform fat and/or protein content. To get high accuracy in the outgoing product, high accuracy is needed in all parts of the process; i.e. sensors, actuators and calculations. During recent years more complex compositions with more ingredients are wanted which puts further requirements on the control system. The combination of high performance and low price of SoftPLCs make them interesting. There is however some questions about the reliability of industrial PCs and with that SoftPLCs. The standardisation and blending process requires high performance but also high reliability. The SoftPLCs undoubtedly have higher performance but it is the combination of sufficient performance and sufficient reliability that is the optimal solution. To achieve higher reliability with SoftPLC the industrial PC should not have any moving parts, e.g. hard drives or fans. An UPS should also be used to maintain service even during short power failures. Both PLCs and SoftPLCs were investigated to see if they could be used for the standardisation and blending process. The investigation analysed each company’s control system according to some specified criterions, e.g. performance, memory capacity and I/O-possibilites. Larger companies who’s control systems already were used within Tetra Pak were the main candidates; i.e. Rockwell, Siemens and ABB (which was used for the current version of the standardisation machine). The smaller companies B&R and Beckhoff was also investigated (though only their SoftPLC alternatives). Here ABB was considered to have the best PLC while Siemens considered to have the best SoftPLC. Since the current machine has a limitation of the number of ingredients that can be mixed in the program code, one task was to develop and implement a program structure without this limitation. This was done with a decentralised calculation principle where the calculations that involve a certain ingredient is moved to the program controlling that ingredient instead of being centrally located in the program. The program structure was also implemented together with a process simulation in an ABB system. Also, more complex controller structures was investigated. This was because many ingredients may cause that the system becomes less robust. To increase robustness an alternative controller structure was proposed, based on local compensation in each ingredient. The user interface of the current machine was analysed according to some usability theories. Some suggestions for improvements were given.