Let's get into the mood of things by reviewing some basic principles.
A system is a collection of objects related to each other by their interaction. A system may be either open or closed. An open system has inputs, outputs, or both. A system with just an input could be a light bulb in an electric environment A system with just one output could be a light bulb in a 3D image rendering system. A closed system has no inputs or outputs, is therefore neither measurable, nor controllable. We may look at, but not into a closed system until we find a way to open it up. Our objective is to design measure and control open systems.
A complex system may be broken down into smaller system or constituents. The act of dividing a system into parts is the inverse of that of composing a system from its parts. The system was built out of components by interconnecting their inputs and their outputs. An example of such a connection could be a forklift carrying materials from one part of the plant to another. We may choose to break a system down into pieces for a variety of reasons. To look at the accounting value of a company we would break it-up into units based on its fixed assets. To look at the cost of running it we would break it up using different criteria, such as the staff. Finally we could break the plant up into parts that are electro-mechanically connected.
The rule we use to decompose a system into its parts should be consistent, i.e. it needs to be applied in full in all directions amongst the objects of a system. A system may be indivisible with respect to the rule used to divide it. If the rule we used to divide a system was "all connections with nuts and bolts" then the parts become indivisible when they have no more nuts and bolts and the connections from this point on are made by welding. To divide the system beyond this point we would need to change the rule of division to say "all connections with welding".
For the purpose of the following discussion the words System, Object, Individual are synonyms. Generic objects are systems, just like the car object is a running system. Individuals, human or animal are too running systems.
Systems are built of subsystems using a composition rule and so forth. A complex system is a hierarchy of parts glued together at every level by a rule, like the plane in REF _Ref16852765 \h Figure 1.
Ribs&Beams Rib 1 Beam 2 Rib N bolted Sheets Sheet 1 Sheet N welded Sheet 2 Engines Engine L Engine R engineered Engine C Plane assembled
Figure SEQ Figure \* ARABIC 1: Plane as a hierarchy of systems
A process is a set of interactions between the objects of a system observed over a period of time. A simple example of a process would be hot cup of tea getting cold, a more complex one would be the set of transitions that an integrated circuit has undergone to perform a filtering function. More formally, a process is a function of between the input of a system and the output of that system.
We measure a process by transparently tapping the output interface of the target system and analyzing the output values over a period of time. Conversely, we control a process by transparently tapping the input interface of the target system and introducing values into the system over a period of time.
An Event is defined as the state of the output interface of a system at a particular point in time. A complete set of such events over a period of time defines the activity of the system during that period. This set of events is also the History of the process that ran the system. We can infer the process that ran the system by knowing the complete set of events at the input and output interfaces of the system.
System A System B O I Events Process Events History of process Task of process t0 t1 V(t) t
History of process
Task of process
Figure SEQ Figure \* ARABIC 2: Process and Events
Figure SEQ Figure \* ARABIC 3: Task and History of a Process
The output values of System A in Figure 2 are plotted versus the time axis in Figure 3. The process has performed between t0 and t1. From t0 looking forward in time we say that the graph is the “obligation” of the process, or its Task. In other words the process must produce the graph of values. From t1 looking back in time we say that the graph is the History of the process. So the Task and the History of a Process are essentially the same, both depending on the function of the process, the difference comes from the direction in which we choose to see the timeline.
The act of measuring a system may influence that system's characteristics. A measuring device should be invisible. The output of a measuring device is invariably used as input in other processes. The output of an Electrocardiograph is fed into the diagnostic process of a patient. Directly or indirectly a measuring device has a process control aspect to it and should therefore be accurate.
Created on 05/27/2009 06:25:10 AM, modified on 05/27/2009 06:25:10 AM
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