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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 5th Apr 2019, 6:40 pm   #1
astral highway
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Default Shock detecting mechanisms?

A good friend of mine has a very expensive, bespoke piece of equipment which was manufactured in a small factory in Austria to order.

It doesn't classify as consumer electronics. Just for interest (it doesn't help the diagnosis) it generates <200Hz pulses from a 20MHz clock on board with various divide-by chips and various forms of modulation.

I am very familiar with its construction.

He has asked me to investigate a shock detecting mechanism which disables the device. It is particularly active when the device is mounted horizonatally, but is false-triggering at the moment, in the absence of any shock.

I know from previous examination that it is not an obvious, clumsy mechanical mechanism.

Please note
OT for this thread is any discussion of mechanisms that are used in ordnance etc.

Thank you!
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Old 5th Apr 2019, 8:19 pm   #2
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Default Re: Shock detecting mechanisms?

This seems to be a contender in the absence of a mechanical sensor: https://en.wikipedia.org/wiki/Microe...anical_systems
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Old 5th Apr 2019, 8:30 pm   #3
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Default Re: Shock detecting mechanisms?

Hi Bill,

MEMs... that seems to be relevant and is very useful. Thanks for finding that.

I’d be interested if anyone has experience of these?

Cheers
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Old 5th Apr 2019, 9:43 pm   #4
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Default Re: Shock detecting mechanisms?

Mems accelerometers are all over the place.

Cellphones to tell which way you're holding them or moving them
Cars to trigger airbags and for stability control systems
Laptops with spinning disc drives to sense being dropped to quickly park the heads.
Wii type game controllers.

David
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Old 5th Apr 2019, 10:18 pm   #5
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Default Re: Shock detecting mechanisms?

I used MEMS accelerometers quite a long time ago, Analog Devices ADXL50. A bit power-hungry at the time, but they worked well and would have done the job. This was a first-generation device. I'd expect things to have massively improved by now.
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Old 6th Apr 2019, 2:23 pm   #6
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Default Re: Shock detecting mechanisms?

Al,

Geophone sensors are very good. These use a magnet on a spring moving inside a coil. It generates voltage proportional to the velocity of the magnet, different from a MEMS sensor that produces a signal proportional to acceleration, and the Geophone responds over a range from a few Hz to 1000Hz. There is a small peak in the response at 10Hz but you can flatten that with a parallel resistor.

They both can be used to make a Seismograph. For example Silicon Chip published a MEMS sensor seismograph with an Arduino and they recently upgraded it by adding the Geophone sensor, because it is much more sensitive than the MEMS sensor.

Here is a typical one from one of the many ebay sellers, this link is merely to show you the picture of one:

https://www.ebay.com/itm/Geophone-Se...gAAOSw05tacOhs
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Old 6th Apr 2019, 2:30 pm   #7
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Default Re: Shock detecting mechanisms?

How about these?
Although called vibration sensors they are sensitive to shock.
Perhaps use one to trigger a small thyristor or SCS if you want a latched output.

Jim
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Old 8th Apr 2019, 1:16 pm   #8
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Default Re: Shock detecting mechanisms?

In the past there were sensors (piezo?) designed to be glued to shop windows, the staff-protection screens in banks/post-offices etc. and which gave a suitable alarm-triggering signal if the window was smashed (or smashing was attempted).

Perhaps you could use something like RS Stock No. 524-5040 ??
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Old 8th Apr 2019, 1:32 pm   #9
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Default Re: Shock detecting mechanisms?

Quote:
Originally Posted by Argus25 View Post
Geophone sensors are very good. These use a magnet on a spring moving inside a coil. It generates voltage proportional to the velocity of the magnet, different from a MEMS sensor that produces a signal proportional to acceleration,
Agree that the magnet-on-a-spring will produce a signal proportional to magnet velocity, but how does that magnet velocity arise?

If the sensor is moved at constant velocity, steady state there will be no force needed to be applied to the magnet. So it will stay in its rest position with regard to its supports, the coil, etc. Zero output.

If the sensor is moved with constant acceleration, then the magnet will need a constant force applied to it to accelerate it. So it will be displaced from its rest position such that the spring displacement applies this constant force (Hooke's law, force is proportional to displacement). Since the magnet displacement from its rest position is constant, there is again no movement between it and its supports, the coil, etc.

Only if there is a rate of change of acceleration will there be a change of spring displacement, hence a non-zero relative velocity between magnet and coil. So this sensor gives an output proportional to rate of change of acceleration, whereas the MEMS is proportional to acceleration itself.

This might of course make it ideal as a shock sensor of course, but on reading your post it came across that the magnet/spring is a velocity sensor as against the MEMS being acceleration. It isn't - to get an output from the magnet/spring you have to differentiate velocity twice more!
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Old 8th Apr 2019, 1:50 pm   #10
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Default Re: Shock detecting mechanisms?

Quote:
Originally Posted by kalee20 View Post
This might of course make it ideal as a shock sensor of course, but on reading your post it came across that the magnet/spring is a velocity sensor as against the MEMS being acceleration. It isn't - to get an output from the magnet/spring you have to differentiate velocity twice more!
The "velocity" was merely the way they described it in Silicon Chip because they were referring to the magnet velocity inside it with respect to the coil and they were trying to explain it to the readers. Obviously if there is no rate of change of magnetic flux in the coil or no relative motion of the magnet in the coil, there would be no signal generated.

So as you point out there has to be a change in acceleration of the sensor body to get signal.So if its in a resting state and gets bumped, there is a change in acceleration, so it makes a good shock sensor.
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Old 8th Apr 2019, 5:19 pm   #11
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Default Re: Shock detecting mechanisms?

Hi everyone, thank you to all who have posted.

I won't reply individually this time, but just to note, it's great for me that I now know how these things work.

The issue, though, is to find out why a false trigger is happening with the existing arrangement. As the equipment is now quite 'elderly' (ie, 11 years old), it may be that a chip has had it or there may be power supply issues.

The manufacturers were really careful to design against cloning or self-maintenance and scrubbed out all the numbering/ lettering on every IC onboard! I've never seen anything like it!
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Old 8th Apr 2019, 11:38 pm   #12
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Quote:
Originally Posted by astral highway View Post

The manufacturers were really careful to design against cloning or self-maintenance and scrubbed out all the numbering/ lettering on every IC onboard! I've never seen anything like it!
I have seen this sort of thing quite a lot. It was very popular in auto electronics. Mostly, if you draw out or look at the schematic its pretty easy to deduce the type of IC, usually logic gates , op amps etc, check first the power supply pins. Usually I have been able to figure out the IC from its application and the components around it, it actually makes for a fun challenge if you like that kind of thing.

The other trick was potting things in rock had resin, but over the years I have become good at slowly removing that. This is how I discovered the secret 4 legged device created by Lucas. But after their factory went defunct, some photos of un-potted pcb's with the exposed device on them were found in their filing system and documented by an Archivist.

Generally for devices with micro-controllers its rarer that type numbers are rubbed off because the manufacturers read protect the firmware and rely on that, although that can be hacked in some cases in the far east I have been told.
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Old 9th Apr 2019, 9:08 am   #13
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Default Re: Shock detecting mechanisms?

Functional chips can be de-encapsulated and exercised while the patterns of voltage on their upper surface are watched with an electron microscope. Patterns of fusible links can be photographed.

On the whole, such analysis might be interesting, but it's often easier to design your own, write your own software and probably make a better job of it.

A lot of these firms who erase device markings are really trying to prevent repair (Boo! Hiss!) Their competitors won't be slowed down much. So when you see that the people they are trying to hurt are their customers, going definct seems like justice.

David
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Old 9th Apr 2019, 12:01 pm   #14
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Default Re: Shock detecting mechanisms?

Some types of one time programable chips can be put under a dental X-ray machine to see what links have been blown.
I was told that the device was indeed successfully reverse engineered.
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Old 9th Apr 2019, 3:46 pm   #15
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Default Re: Shock detecting mechanisms?

Was there not some device that could be packed in a parcel to see how roughly it was handled in delivery?
Is it just a daydream?
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Old 9th Apr 2019, 7:17 pm   #16
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Default Re: Shock detecting mechanisms?

Not electronic, but do a google search for 'Shockwatch'. They make labels that turn red if a package has been dropped or whatever.

I first came across them on the disk packs used in some of my minicomputer drives. If the pack has been mishandled it can cause a headcrash when it is loaded, so it is useful to know...
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