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7 - Electronic devices and appliances

7.2. Basics of appliance design

Main development phases of appliances
Proportion of importance of sucess of the product:
  1. Setting up the specification (50%): Matching opinions, marketing, bench-marking, existing and expected rules, official regulations, etc.
  2. Development of a prototype (30%): Specifications, tests, manufacturability, price, etc.
  3. Development of the manufacturing technology (10%): manufacturing costs, capacity, quality, reliability, etc.
  4. Trial manufacturing (10%): tests.
  5. Manufacturing (0%): quality inspection, SPC.
Route to the specification:
  1. What has to be realized?
    In the engineering practice we deal with appliances, on which there is an existing demand. Demand can be real (unique e.g. nuclear plant), market (e.g. mobile phone), latent (i.e. SMS), objects existing not yet (e.g. Rubik’s cube).
  2. Who will be the customer/user?
    child, adult (man or woman), elderly/ill person, ordinary user, expert, technician, specialist, etc
  3. What kind of operating circumstances do we expect? (present and future)
    Indoor/outdoor, cold/hot (kitchen, bathroom); on the beach, underwater, in 20 000 m height; in an oven, in the gear (in hot oil), in the exhaust pipe; on a satellite, etc.
  4. What is the deadline?
    The moment of market launch has an optimum. Long development time: the appliance can have better properties than the product of the competitors; short development time: the appliance is a novelty leaving other aspects out of consideraton, decreasing the time until market launch will increase the development cost. Deadline: in most cases a short delay can be tolerated but there are special cases, where it has a key of importance (i.e. Spirit Rover)
  5. What does the appliance cost?
    More exactly, is the development, production, manufacturing of the appliance profitable ? How much will it take until the launch? The preliminary cost-estimation can revise the plan before its birth. An idea, which can be realizable and marketable, is useless, if the manufacturing is uneconomical. Main components of the cost:s development, design of manufacturing, assembly line, manufacturing, others (product assistance, maintenance, warranty, recycling)
  6. Further questions:
    In most cases, exact answers have to be given at this phase as well. Designed and realized mass and volume demand of the appliance, power demand, designed life time, meeting standards and directives. Could we have omitted something from the strategic questions? In case of complex projects, a feasibility study has to be made.
Main parts of the specification: Picture
Electronic design – circuit design
  1. Schematic design.
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  3. Dividing the circuit according to functions (subassemblies), connector distribuitions.
  4. Design of printed circuit boards: CADs (ORCAD, Pads..), component placement, track router.
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  6. Wiring of the appliance.
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Mechanical design
Mechanical design of the frame(s) of the appliance, housing design, arrangement of subassemblies (motherboard, cards, racks), ergonomics.
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Thermal design
Very important in case of high device density (e.g.notebook) and in case of high power (e.g. power supply). Software: thermal simulation. Hardware: thermal interface, heat sink, ventillators, fans and heat pipe.
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Ergonomic design
Optimal design of appliances from viewpoint of handling. Example: electronic instruments: unambiguous, aesthetic labelling, arrangement of displays, buttons and switches according their function, arranging elements belonging together into groups, marked with colors, LED indicators close to important switches, high power buttons and switches – larger size, more important function/power – at an edge of the appliance, place the most important indicator in the left upper corner. Optimal design of working circumstances, workstations. Example: assembling workstation.
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Functional safety
Functional safety: safety of life, accident and valuables. A functional safe appliance must not cause damage or danger even in case of failure. The designer and manufacturer is responsible for the caused damage as „safety engineer”.
Range of subjects in functional safety and environment endurance: Protection against environmental stresses (climatic, chemical, biological, mechanical, e.g. vibrations in automotive industry), protection against overcurrent, overheating, harmful radiations and explosion..
Design for shock protection
Electric appliances must not cause electric shock even in case of failure. Shock protection classes:
Shock protection class I.: These appliances must have their chassis connected to the electrical earth by an earth conductor. A fault in the appliance which causes a live conductor to contact the casing will cause the current to flow in the earth conductor. Protective separation + the touchable metal parts connected together (e.g. appliance door +chassis) and to the protective earthing or earth grounding connector (grounded power plug, colour code: green-yellow).
Shock protection class II.: These appliances has been designed in such a way that it does not require a safety connection to electrical earth. The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing. The appliance has insulator chassis, which provides a complete covering (e.g. hair-dryer). The chassis combines housing and protective separation in one.
Shock protection class III.: A Class III appliance is designed to be supplied from a SELV (Separated or Safety Extra-Low Voltage) power source. The voltage from a SELV supply is low enough that under normal conditions, a person can safely come into contact with it without risk of electrical shock. Contact voltage: 24-50 Veff AC. There are no circuit parts in the whole electrical system/appliance, which operates at voltages higher than this value.
Design for manufacturability (DFM)
Steps: Quality control, 6 sigma. Design, which takes manufacturing requirements into consideration. A design step where the product is designed by teamwork. System consisting of several tools and technics.
Advantages: development cost can be reduced, time to production can be reduced, manufacturing and testing costs can be reduced, better appliance quality.
Product development: conceptual design, optimizing the product, simplifying of manufacturing, starting the manufacturing, transporting, launching the product to the market.
Product development team: product requirements, ollaborating teams, applying DFM tools and technics.
Guiding principles:minimize the number of elements, apply standardized and identical elements, minimize the copper layers in the PCB, use standardized drills, tools, avoid small diameter vias, use common size for tool fastening, minimize assembling directions, maximize accessibility (for manual assembling steps), minimize manual operations, avoid post-adjustments, use well-known processes, design for testability, avoid hidden details, use symmetries, avoid possibility of mixing up, use self-aligning elements.
Design for reliability
Attributes of serial system: the system consists of finite number of identical elements, failure of one element causes the failure of the system, failures are independent, commercial appliances are serial systems.
Attributes of warm redundant (paralell) system: the system consists of finite number of identical elements, failure of all elements causes the failure of the system, error recognizing, switching components are needed, state of the reserve is known, the reserve also consumes energy, and wears out.
Attributes of cold redundant (parelell) system: the system consists of finite number of identical elements, failure of all elements causes the failure of the system, the redundacy is not on all the time, does not use energy, the reserve can not break down, the system needs an error recognising switching element, the switching takes time.
Design for reliability in the practice: Several reliability softwares are available on the market. Reliability analysis of components can be selected according to standards. Analysis of system: according to block diagram. Maintenance analysis: simualtion of failures and their repairing. Avoiding of „weak points”.
Realization based on standards
Advantages: intuitive design is not necessary, the parameters (dimensions, dissipations per volume, etc.) can be selected from standards, chance of appearance of hidden mistakes is less.
Disadvantages: hands of designers are tied, realization of unique ideas is not possible, the appliance is „overdesigned” in most cases manufacturing can be uneconomical in case of larger batch.
Standardized chassis and frame system (IEC TC 48) constitutes the bases of design, which is complemented witch thermal, EMC, shock protection, etc. standards and directives.
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Realization based partly on standards: very widespread; it applies the compulsory (EMC, shock protection, etc.) standards and directives only; possibility of optimizing price/batch size/manufacturing capacity; all of designing phases have to be applied; possibility of continuous product development in all parameters; example: construction of a commercial notebook.

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