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Tools measurement and tolerances - Electronics

Created; 05/02/2015, Changed; 27/04/2018, 24/04/2018

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When you assemble things from components sometimes the completed assembly will not fit together and other times the components will go together so well that the assembled item is superior to the norm. Electronics components are now made with very good tolerance control so that in practice the extreme (worst and best) cases situation do not apply, defective parts almost never enter a legitimate supply line. It is usually easiest and cheapest way to put a bigger margin in the design so that assembly is consistently fault free due to the accumulation of tolerances. So instead of using a sum of squares use the sum of worst case errors or some factor between the two if that is not practical.

It you want a good camera now pick a make and model and the next box off the shelf will be fine. When my Granny bought her Rolliflex in the 1940's mum tells me she made a lot of fuss to get the one demonstrated to her rather than another off the shelf. The point must have been significant for granny to tell mum and for mum to tell me. But the story is generally very true and well understood until about 1980 then anything purchased had become consistent that is any one off the shelf is the same as any other off the shelf. The first Zero Defects programme was launched by the big semiconductor manufactures such as Motorola about 1980.
  • To qualify my point most designs use ICs that must be operated within all tolerances for each parameter; temperature, supply and other voltages and currents etc. Therefore a statistical analysis of a complex network is not applicable because it is significantly one part.
  • Complex resistor networks are not always necessary and have always been best avoided due to there poor long term stability for analogue conditioning. Much  close tolerance resistors of 1% are common now where 10% tolerance used to be are commonly used
  • A design that requires a close tolerance reference and a resistor or resistor network will have one of each for the function so the arithmetic comes down to calculating the extremes conditions and this is not complicated (up to 4 sums for two parts) and no statistical analysis is necessary.
  • To repeat the point in the text above and below things have changed. Much of the design work has been moved to the component manufacturer over the past 35 years.
  • There are exceptions such as a low power sensor IC may be operated above its maximum operating temperature but below its storage temperature but consequently at lower performance. Manufactures will explain how to first predict and qualify an IC this way. Take care, I was advised how to do this and warned that the IC was glued to the package and that that was the reason that there was an absolute storage temperature limit.
  • A circuit design become part of a system where trade-off between cost and risk is relevant. The PCB assembly and manufacture its testability, mounting rigidity, heat etc. do need to be either reckoned or calculated and analysed statistically. 

St. Marys, The Castle in the Rocks, Hastings, Easter Bank Holiday, April 2015. 
This is as good as it gets - the cars are there, they do not need to be seen as spoiling the picture - This is as it is. 
I held the camera above my head even so to reduce the impact of the foreground and in that way the photograph
is like as I see it (perceived). 
That is the cars are seen but you look past them therefore perceive the scene
 differently.
Camera is a Cannon IXUS 60.
The camera would have been the next one off the shelf and it would
perform as well any other of the same model. This camera is better than a very good film camera. This picture is
particularly sharp, I put down to experience and luck, given that I was holding the camera above my head.

Best tools for the job

If you are working on a higher performance instrument design there may be no accurate test instrument as good as the instrument you are designing. In any case the required test equipment for the particular job may not be available or may change the way an instrument under tests functions such as when considering using an oscilloscope with x10 attenuator probes often severely loads a high frequency circuits by it's capacitance. Either way spilt the system up into parts that you can test in different ways so that you can figure out logically the likely performance in the parameter you are interested in. In this case build up a pattern of cross checks that give you a subjective confidence. Continue to build up the system you are developing cross checking in the way I have described, and reviewing what you had done previously until the work is complete and you can say what you have achieved and how much confidence you have in its performance.

In conclusion you best tool is your reasoning, and to develop an ability to look at things in alternative ways. Nothing is absolute even the accuracy of a measurement, but by building up a pattern of measurement with tolerances you will come to a high degree of certainty.

Worst Case Design and understanding tolerances

This is about understanding shades gray. To start with, Sum the squares of the errors, to give you a figure of merit. You should, but it is not essential, square the sum of rooted numbers to give a meaningful value.

If you try calculating the values of resistor that you require in a network such as a Whetstone bridge using Supper-position and Maxwell's circulating currents the outcome is a set on non-preferred value resistors. So what they taught you at school was no use then, So what you do is create a model which you can repeatedly try preferred values of resistors and use the values that best give the lowest number using the least squares of the errors method. {squaring the errors and sum them - but don't bother with square rooting those sums that you would have been taught}

That was a lot of work but you can write a program such as written in BASIC or a spreadsheet to do that - the exercise will give you a better feel for tolerances and how to rationalise your design work down to the necessary parts. Eventually you should by inspection see what parts of the circuit need to be checked for tolerances, power rating etc. Do some sums with a calculator, pen and paper entering preferred values. All circuits with resistor networks can be split down to two or three resistor networks which come down to a voltage source with an impedance, which you then load with the next resistor network. In fact if you can avoid resistor networks that is even better, resistors age and have temperature coefficients.

My point is that the best lesson in tolerating and getting a feel circuit function and actual behaviour is to do the maths, until you feel it without having to go through everything to prove it. Once you are there use a simulator rather than spreadsheet if you want.

If you do start with a simulator, have a guess and do some arithmetic to support your guess first, Use the simulator to test your guess. If there is a disparity prototype it and see which is correct and analyse it to understand it.

I recommend that you download and use any spreadsheet or other models that an integrated circuit or other component the maker provides such as for a switch mode power supply coil maker and controller manufacturing provides they are well tested and good. But give yourself some margin the IC makers tend to specify to just the limit.

I find thermal modelling from data sheets difficult. An IC won't dissipate much without an area of copper. Hardly anything can be dissipated if the IC is faced down, face up is better, and with the board stood vertically is best. I have found exposed tinned or silvered copper better than with solder resist covering a copper area. Tinned or silvered copper does go mat white quickly which could mean the board tends to improve with age slightly. I have heard others contradict my observation on removing solder resist so try it for yourself.

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Cannon AE-1

This camera uses a very low power Microcontroller made by Texas Instrument and the low power is achieved using Ferranti IIL (I squared L) low voltage transistor Intellectual Property.

Cannon AE-1

My father bought his first AE-1 in 1976 with the 50mm lens when the camera was first launched. The camera was particularly popular with amateur photographers for at least a decade. The A-1 was launched later and was the professional version.

I find this camera has everything to hand and only features that you need and it does all that you expect of it well. The design was not compromised in order to persuade you to purchase the next model - this happens in capitalism occasionally during times of commercial pressure in this case to beat off any remaining German or Nikon camera supremacy. A classic surely?

This camera made photography consistent. To achieve the correct exposed pictures. The AE-1 cost about £250 and by comparison a teenage electronics technician would be earning £19 to £24 weeks before deductions. The camera has a low power Texas Instruments microprocessor in which one of the significant Intellectual Property parts was that it used Feranti I2L (I squared L) low voltage transistor integrated circuits. These I2L was developed in 1973 and probably continued being used in watch IC's for the next 20 years?

The right hand camera; Clockwise from the top.
  • Cannon 50mm, F1.8 lens - Normal perspective.
  • The cap covers the flash connector - I use the hot shoe.
  • Little silver button - Increases the exposure by 1.5 stops.
  • Red dot on the side of the lens - With the lens unlocked turn the lens so that the red dot lens is at 12-o-clock to fit or remove the lens.
  • The black button - Preview the exposure and is the same as pressing the shutter release down halfway.
  • The Slide switch on the body - is for stop down metering use with manual lenses. Dad told me not to use it, it will damage the camera but I have used it with a Signor 105mm, F2.8 lens. Reading the quaint Japanese English manual carefully first. You can adjust the aperture stopped down until the needle in the viewfinder lines up with the mark. (The manual lens has a ring to open up the aperture for focusing - remember to close it back down before taking the photo)
  • Silver button on the side of the camera - Lens lock - Press to release the lens. I read that this lens mounting is noted for its lack of wearing compared to later models such as EOS.
  • The camera base you can see the tripod/flash bar mount, The film wind back release button, The coupling for film motor accessory.
Left Camera;
  • 35-105mm F3.5 macro-zoom lens. These were made from about 1985 and use a significant Polaroid molded lens technology (IP of 1980) to make aspherical glass lens elements. A traditional spherical lens focuses in a radius not a flat plane of the film. I think the glass was ground and polished in the traditional way to make a spherical lens then the glass was heated and pressed into a mold to reshape the glass to aspherical shape.
  • The battery is 6V and is under the cover at the side of the lens.
  • The speed setting ring under the wind on leaver can easily be moved whilst looking at the meter in the view finder for aperture priority photography or stop down manual lens photography. But the camera is a shutter speed priority camera with an automatic lens.
  • You lift another ring and rotate it to set the film speed. That is a bit fiddly.
  • The wind on wind back and film loading is free of binding and as good as it gets for loading the 35mm film cassette reliably in any 35mm camera.
  • The black button is the shutter release you can screw a shutter release cable into it. Press the button halfway to down to preview and lock the exposure with the meter in the viewfinder.
  • The leaver is set to lock - unlock - self-timer which causes the led to flash for several seconds.
  • The button the other side of the flash hot shoe is battery check. Look in the viewfinder at the meter needle.
  • There is a circle with a line through it mark which indicates where the film is and where you would measure the focal distance from if you were to set the focus that way. It is important for close up large aperture photography.
  • Wind the film back handle when you have come to the end of the film - first press the button on the base of the camera to release the film.
  • Camera from the back has a clip to hold part of the film box with the film rating and type. The viewfinder has the meter, manual warning LED, exposure warning LED, Focus, clear screen, Fresnel ring and split image.

Significance difference between these two AE-1 cameras is that there is no difference between them. They were different to British, European, American products in the respect that any two of the same product would be the same by comparison. 

I took the photo with a digital Cannon IXUS 60 - that camera takes comparable quality photos. The camera's flash has done well I had zoomed to x3 and photo edited to crop and turn the picture black and white.

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Useful Links;

I have variously been recommended and received criticism of the following modelling tools (which are free or limited demo's); Personally I have not used any modelling type of tool and my view circuit board layout is the most significant factor. I can see the control loop stabilisation might be a good feature. I do use power supply component makers modelling tools, particularly for wound components they are very good. I use spreadsheet or perhaps basic program based tools provided by manufactures of inductive or integrated circuits such as for switch mode power supply design these are very good.

Conversely the discrete circuits on the previous pages could be modelled rather than prototyped and I can see a benefit with the high voltage circuit, if you wish to see how fast it can operate accurately. That was not a requirement though.
  • Tina http://www.tina.com/ - The not Texas Instruments version of the tool. The demo version does not allow outputs or any work to be save. 

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