what's the difference between systems and engineering
Utilitarian systems, as previously discussed, are the means we use to transform resource inputs into useful goods and services. Any system can be divided into a set of input-transformation-output blocks. These are usually represented as in Figure 31. This way of looking at systems can be used as an analytical and design tool.
Take a plain blank sheet of paper. In the middle draw a rectangle like this:
Label your rectangle ‘operations transformation system’. From your knowledge of operations, make a list down the right-hand side of the sheet of all the outputs of an operations system that you can think of. Join these to your process rectangle with arrows coming from the rectangle.
Transformation processes should always have a verb attributed to them: forming, painting, assembling, packaging are all examples of transformation processes. Transporting is another example, since a component or product is ‘transformed’ from being in one place to being in another.
Outputs are what the system is all about - or at least some of them are. Unless a utilitarian system produces goods, services and information, and produces them in the right quantity, at the right time, of the appropriate quality and at planned cost, its purchasers or creators might as well save their time, money and effort.
(a) From your knowledge of operations, make a list down the left-hand side of the diagram that you drew in answer to the previous activity of all the important inputs to the system. Connect them to the operations system rectangle with arrows.
(b) The inputs to the transformation system are of three sorts:
• those that it uses;
• those that it transforms;
• those that it uses for planning and control.
Decide which of the inputs in your answer to (a) are used by the system, which are transformed, and which are used for planning and control. Then compare your diagram and responses to the ones given in the answers to the previous activity and this one.
(b) The inputs that the system uses are:
The inputs that the system transforms are:
The input that the system uses for planning and control is:
There are three reasons for getting you to carry out the input-transformation process-output exercise in the previous two activities;
• To make you think about systems engineering in terms of inputs, processes and outputs;
• To introduce input-output diagramming, which is a common technique used in the analysis and development of systems;
• To illustrate how using tools of this kind can help you to think in a holistic way about a system and can promote further analysis.
Look again at the outputs shown in the diagram associated with the answers to the previous two activities. Some of the outputs are planned, some are accepted and some are undesirable. It would be possible, for example, to carry out a further, more detailed, analysis on the scrap and waste producing sub-system.
Input-output diagrams are good for identifying and analyzing sub-systems, highlighting relevant systems in the environment, and for examining interactions between the various elements that have been identified.
A system does not usually behave in a random manner - its actions are governed in some way. This can be achieved by using the control models, either singly or in combination, shown in Figure 32(a) and (b).
The feedback (or closed loop) control model in Figure 32(a) works as follows:
• A feature of the output from the transformation process is monitored;
• Information regarding this is ‘fed back’ to a comparator process that compares the value of the feature with a predetermined ‘objective’ value;
• If the value falls within the predetermined limits, no further action is taken;
• If the value is outside the predetermined limits, a signal is sent to alter an input to the process in order to correct its operation.
For example, suppose that the transformation process is drying in an oven. The objective is to maintain the temperature of the oven to within ±8 °C of a particular value. A sensor monitors the temperature of the oven. This is then compared with the preset limits, and corrective action is taken through an actuator if necessary. The important features of the model are as follows:
• It repeats the basic structural pattern of input-process-output, with the addition of a feedback loop.
• It shows two different levels of control: the first deals with feedback directly from the transformation process and the second with feedback from the lower level. This pattern of hierarchy of control is a feature of nearly all operations systems. The function of the higher level system is to provide standards for those at a lower level and to take corrective actions that are not within the competence of the lower level. The hierarchy is not, of course, restricted to two levels.
• All or part of the output of the transformation process is monitored, either continuously or at intervals.
• Information from the monitoring sensor is fed back to a comparator, which, as its name suggests, compares the value with a predetermined standard. If the value falls within the permitted range, no further action is taken. If the value is outside the limits, two courses of action are open. If the deviation can be corrected at that level of the system, an actuating signal is sent to alter one of the inputs to the process. If the deviation is outside the competence of this control level, the occurrence is reported to the higher level of control. The comparator may be a physical device (a microcomputer, for example), a person or a committee - though that would be rare at this level.
• Actuation involves altering one of the inputs to the system to create the desired change in output.
• The feedforward (or open loop) control model in Figure 32(b) is a control strategy used to correct or compensate for the variance of an input, to achieve a desired output variable.
• Feedforward can be used when the relationships between input, process and output are so well understood that it is possible to predict the effect of a particular change to an input.
• Feedforward control involves monitoring the condition of a critical input variable and predicting, using a model of the process, the effect that a variation will have on output.
This approach means that changes to the measured input or some other input variable can be made ‘before the event’. For example, suppose a strip is to be rolled to a thickness of 2 millimeters. The thickness of the incoming material could be monitored and roller pressure increased or decreased as necessary.
Particular attention has to be paid to the nature of the control parameters that are used as objectives in a control loop. For example, measuring machine hours and utilization may, in the first instance, seem like a good idea. But if this results in the overproduction of components for stock it may be detrimental to the effectiveness of the system as a whole. This is another example of the importance of taking a holistic view.
The final concept associated with control is lag. This can be defined as the length of time that the control loop takes to correct an out-of-tolerance condition and can be expressed as:
Mean control lag =
Mean monitoring interval + Monitoring
time + Feedback (2) + Comparison
time + Feedback (1) + Actuation time
The length of the lag in the control loop can have a significant influence on system performance. In a fully automated control system, total reaction time can be very small, but where clerical procedures are involved it may be lengthier.
Figure 33 shows an operations system consisting of five linked processes involved in the manufacture of printed circuit boards (PCBs) and the time (in seconds) that each operation takes.
Each operation in Figure 33 involves:
A: Coding. The empty board is coded with part number and serial number.
B: Automatic insertion or mounting. Components are manually loaded and unloaded into an automatic mounting machine.
C: Manual assembly. Not all components can be mounted automatically. For example, block capacitors and switches have to be inserted into the PCB. The operator takes two boards and loads them into a carrier. The components are mounted. The fully populated board is mounted onto an in-feed conveyor that takes it through the soldering machine.
The policy is to test one board fully every 10 minutes. In practice, depending on how busy the test technicians are with other work, the interval varies between 8 and 14 minutes, with the average at about 10 minutes. If a fault is detected, further tests are carried out to discover whether the problem is isolated or persistent.
The corrective action taken depends on the type of fault and where it occurred. When problems occur with the settings on the automatic mounting machine they take about 10 minutes to correct from first detection during testing.
If a fault occurs due to the settings on the automatic mounting machine, how long is the total mean control lag?
The mean control lag in seconds is calculated as follows: