Browsing by Subject "Computer control systems"

Now showing 1 - 7 of 7

Open Access
Architecture framework for software safety
(Springer, 2014-09) Gürbüz, Havva Gülay; Pala Er, N.; Tekinerdoğan, Bedir
Currently, an increasing number of systems are controlled by soft- ware and rely on the correct operation of software. In this context, a safety- critical system is defined as a system in which malfunctioning software could result in death, injury or damage to environment. To mitigate these serious risks, the architecture of safety-critical systems needs to be carefully designed and analyzed. A common practice for modeling software architecture is the adoption of software architecture viewpoints to model the architecture for par- ticular stakeholders and concerns. Existing architecture viewpoints tend to be general purpose and do not explicitly focus on safety concerns in particular. To provide a complementary and dedicated support for designing safety critical systems, we propose an architecture framework for software safety. The archi- tecture framework is based on a metamodel that has been developed after a tho- rough domain analysis. The framework includes three coherent viewpoints, each of which addressing an important concern. The application of the view- points is illustrated for an industrial case of safety-critical avionics control computer system. © Springer International Publishing Switzerland 2014.
Open Access
An integrated process planning approach for CNC machine tools
(Springer-Verlag, 1996) Aktürk, M. S.; Avcı, S.
In view of the high investment and tooling cost of a CNC machining centre, the cutting and idle times should be optimised by considering the tool consumption and the non-machining time cost components. In this paper, we propose a detailed mathematical model for the operation of a CNC machine tool which includes the system characterisation, the cutting conditions and tool life relationship, and related constraints. This new module will be a part of an overall computer-aided process planning system to improve the system effectiveness and to provide consistent process plans. A hierarchical approach is presented for finding tool-operation assignments, machining conditions, appropriate tool magazine organisation and an operations sequence which results in the minimum production cost. © 1996 Springer-Verlag London Limited.
Open Access
Joint lot sizing and tool management in a CNC environment
(Elsevier, 1999) Aktuük, M. S.; Önen, S.
We propose a new algorithm to solve lot sizing, tool allocation and machining conditions optimization problems simultaneously to minimize total production cost in a CNC environment. Most of the existing lot sizing and tool management methods solve these problems independently using a two-level optimization approach. Thus, we not only improve the overall solution by exploiting the interactions among these decision making problems, but also prevent any infeasibility that might occur for the tool management problem due to decisions made at the lot sizing level. The computational experiments showed that in a set of randomly generated problems 22.5% of solutions found by the two-level approach were infeasible and the proposed joint approach improved the solution on the average by 6.79% for the remaining cases.
Open Access
Reconfigurable hardened latch and flip-flop for FPGAs
(IEEE, 2017-07) Ahangari, Hamzeh; Alouani, I.; Öztürk, Özcan; Niar, S.
In this paper, we propose Joint Latch (JLatch) and Joint Flip-Flop (JFF), two novel reconfigurable structures which bring the reconfigurability of reliability to user latches and flip-flops (FFs) in reconfigurable devices such as FPGAs. Specifically, we implement two reconfigurable storage elements that exploit a trade-off between reliability and amount of available resources. In fault prone conditions, JLatch (or JFF) is configured in such a way that four pre-selected normal static latches (or FFs) are combined together at circuit level to form one hardened storage cell. Solution focuses on transient faults such as soft errors, where we show that critical charge is increased by at least three orders of magnitude (1000X) to practically bring immunity against any Single Event Upset (SEU). If four latches inside an FPGA logic block are far enough, it can effectively cope with Multiple Bit Upsets (MBUs) as well. Additionally, provided that special transistor sizing is applied (only necessary for some latch structures), JLatch and JFF take advantage of a novel self-correcting technique to correct any single fault immediately. Our solution provides reconfigurability of reliability with negligible performance and area overhead with only one (two) extra transistor(s) per latch (FF). The delay of this technique is less than the delay of conventional TMR (Triple Modular Redundancy) technique with a majority voter at output. © 2017 IEEE.
Open Access
Scheduling with tool changes to minimize total completion time: a study of heuristics and their performance
(John Wiley & Sons, Inc., 2003) Aktürk, M. S.; Ghosh, J. B.; Güneş, E. D.
The machine scheduling literature does not consider the issue of tool change. The parallel literature on tool management addresses this issue but assumes that the change is due only to part mix. In practice, however, a tool change is caused most frequently by tool wear. That is why we consider here the problem of scheduling a set of jobs on a single CNC machine where the cutting tool is subject to wear; our objective is to minimize the total completion time. We first describe the problem and discuss its peculiarities. After briefly reviewing available theoretical results, we then go on to provide a mixed 0–1 linear programming model for the exact solution of the problem; this is useful in solving problem instances with up to 20 jobs and has been used in our computational study. As our main contribution, we next propose a number of heuristic algorithms based on simple dispatch rules and generic search. We then discuss the results of a computational study where the performance of the various heuristics is tested; we note that the well-known SPT rule remains good when the tool change time is small but deteriorates as this time increases and further that the proposed algorithms promise significant improvement over the SPT rule.
Open Access
Tool allocation and machining conditions optimization for CNC machines
(Elsevier, 1996) Aktürk, M. S.; Avcı, S.
In the literature, there exist many variations of machining economics problem in terms of modelling approaches and solution methodologies. However most of the existing studies focus on the single machining operation which is seldom in practice. On the other hand, tool management approaches at the system level fail to relate the tooling issues to the machining conditions, and ignore the tool availability and tool wear restrictions. A new solution methodology is developed to determine the optimum machining conditions and tool allocation simultaneously to minimize the production cost of a multiple operation case where there can be alternative tools for each operation. As a result, we can both improve the solution by exploiting the interaction between these two decisions, and also prevent any infeasibility that might occur for the tool allocation problem due to tool contention among the operations for a limited number of tool types by considering the tool availability and tool life limitations.
Open Access
Two-machine flowshop scheduling with flexible operations and controllable processing times
(2013) Uruk, Z.; Gultekin H.; Akturk, M. S.
We consider a two-machine flowshop scheduling problem with identical jobs. Each of these jobs has three operations, where the first operation must be performed on the first machine, the second operation must be performed on the second machine, and the third operation (named as flexible operation) can be performed on either machine but cannot be preempted. Highly flexible CNC machines are capable of performing different operations. Furthermore, the processing times on these machines can be changed easily in albeit of higher manufacturing cost by adjusting the machining parameters like the speed and/or feed rate of the machine. The overall problem is to determine the assignment of the flexible operations to the machines and processing times for each operation to minimize the total manufacturing cost and makespan simultaneously. For such a bicriteria problem, there is no unique optimum but a set of nondominated solutions. Using ε-constraint approach, the problem could be transformed to be minimizing total manufacturing cost for a given upper limit on the makespan. The resulting single criterion problem can be reformulated as a mixed integer nonlinear problem with a set of linear constraints. We use this formulation to optimally solve small instances of the problem while a heuristic procedure is constructed to solve larger instances in a reasonable time.