Basic principles of design
By Vladimir Koren
What is called the design, is a logical process. The word "design" includes activities such as planning, reflection and production.
The designer must have sufficient cleverness, knowledge, imaginative thinking, and such a subjective quality, as endowment. Most of these properties can be gained by only one way: based on experience and ability to overcome all new and new, at first glances, stalemates.
A good designer, as a rule, is a specialist, which is constantly improved, enriching their knowledge. He subscribes to the major magazines for designers, newsletters, collects and stores the instructions for use, specifications and other special literature. Given, that even the usual catalogs and instructions for use, published by the manufacturer, provide an opportunity to expand an engineering education, it is natural, that such a person knows, which components are produced in the industry, and that is what helps him to be succeeded.
To ensure that your efforts were not in vain, you need to study the successful designs, methods of laboratory tests, as well as the maximum permissible operation conditions of the components.
Often, a mistake is noticed only a few milliseconds before the moment, when the light smoke will notify you that some, in its way, "unique element" has been "vaporized". However, this is not so important - this is just a "hardware".
Never forget that on how well your design is thought-out, depends not only its working capacity, but also, that is most importantly, safety and health of people who will operate and maintain it.
In this article, we will try to learn from some of my mistakes and of mistakes of others.
OK, let's begin.
The first step in any designing is the familiarization with the task. This postulate may seem too obvious, but believe me, it is dictated by common sense. In fact, how you can seriously think about the design of a device, without realizing, what it should do. Studying the problem consists of such action (but not limited to), as a search in the literature and internet, deliberation, consultation with customers and with people who have met with a similar task in the past (and especially with those, who have already solved it).
The next step is the formulation of an approach to the task solution. Here it is necessary to find a few methods or schemes, which could help achieve the required result. Very rarely it happens that there is only one single way to solve the problem, so you need to consider several options. As well, do not forget about the so-called contingency. That is, after the several alternative schemes will be tried out, you need to stop and ask yourself the question: "What it will be if..?" What if, at any functioning stage of the scheme or controller's program, some unplanned events will happen (the operator will press a wrong button, at the wrong time, or some of the components go out of action)? Believe me, after that your scheme will undergo the serious changes (or even the complete redesign).
The most difficult part in the design of automatic control systems is not the design of a control circuit as such, but the design of a control circuit that operates under a given algorithm in the light of unforeseen events and "human factor" (a broad field of action for a perverted fantasy).
"Always, first of all, the most undesirable option will be realized." - Finagle's Law.
During design of automatic control systems, I, for example, invited my colleague to "torture" the circuit, by sending the very unimaginable combination of different commands from the control panel. If during this test, the "improper state", i.e. disruption of the functioning occurred, I made alterations in the circuit or program of the controller, in order to protect the system from this kind of troubles. One of the ways that this can be achieved - the restriction of "freedom of choice" for the operator.
The less control elements used, the lower the probability of error. All that can be automated, should be automated.
However, do not get carried away, leave the possibility of switching to manual control in case of failure of the automation - you cannot stop the plant. There is a contradiction? Yes, of course. However, this is the sign of a good designer - to combine incompatible and find a middle ground. At least, this has to be sought. Try to determine all the possible malfunctions of the system, and divide them into groups. First - this is when the device must be unequivocally completely stopped, until the malfunction will be eliminated. Second - this is when you can bypass the faulty component either by automatic or by manual switching to backup.
And one more rule that can save your design time and your nerves:
Save all the intermediate variants of control circuits and controller's programs. During the design process, you will be needed to refer to them time after time.
Before beginning the manufacture of the device, you must prepare a complete diagram of the device and make drawings of all critical mechanical parts. It is not acceptable, to complete the project, no matter how simple it was, and not to leave the documentation, reflecting the purpose and principle of operation of the device. This observation may again seem obvious to you, but believe me, during my work in this area, I've met a such attitude to this important phase of work more than once.
Drawings and other design documentation play a critical role in the manufacture and testing of the newly designed devices.
During the test, you must carefully write down all main values of voltage, current and signals, as well as other important and necessary parameters. All the amendments, clarifications and changes that are made in the circuit, in accordance with the test results, should be introduced into the main drawing of a device.
The design of any new device cannot be considered as completed, until the device has not been adequately tested. Any idea should be regarded as hypothetical, until its validity is not proven.
In addition, you must make description of the methodology of adjustment and installation-specific settings, in order it could be used by the less qualified staff than you. For you, this technique can be self-evident, because it came up with you, but for others it may be a mystery. Furthermore, in six months or a year, you can be asked to repair or adjust your device, but you had already forgotten a lot. In such cases, well-designed documentation will be invaluable.
In the final stage, the circuit is assembled in the enclosure. Choice of the enclosure is carried out in accordance with safety requirements and depending on the conditions in which the device will be used. In any case, the quality of the housing for your circuit must be economically and technically valid. Furthermore, be sure always make a temperature balance calculation. Practically all the electric or electronic components are emitting the heat during the work. It would be a pity, if the circuit, well working on prototyping board, will not be workable due to overheating of components, after it was assembled in the enclosure.
During developing the drawings of arrangement of components, it needs to take into account the possibility of changing the circuit and installation of additional components.
The way from the idea to final product, in any case, is quite long, so do not complicate it with extra "zigzags" because of the abominable methods and poor planning. This, at least, will increase the device cost and can cause much distress.
So, let's list the main steps that are needed to design a viable device:
Examination of given task.
Formulation of several approaches to solve this task.
Preparation of the records, drawings and all necessary documents.
Testing the operation of the circuit and controller's program with the help of computer-simulation or at a breadboard prototype panel.
Testing the circuit in an environment as close as possible to real.
Assembling, and testing the product in its final form, using all your knowledge and skill.