[chbot] Quad chock fish challenge
Mark Atherton
markaren1 at xtra.co.nz
Sat Jul 24 08:27:09 BST 2021
thank guys.
another pairs of eyes always appreciated
-mark
On 24/07/2021 6:58 PM, Andrew Dachs wrote:
> Yup, agree with Trevor. that’s what my scribblings work out to as well.
> I had to assume all legs in the bridge are equal otherwise there are 3
> equations and four unknowns.
> Been fun :-)
> Andy
>
> Sent from my iPhone
>
>> On 24/07/2021, at 5:48 PM, Trevor Wignall <zl3adz at gmail.com> wrote:
>>
>>
>> Red-white measures more than black-yellow so that suggests more
>> resistance in series with red and white. If there were 33 Ohms in
>> series with red and white but not black and yellow and all four bridge
>> legs were equal, then to get 700 ohms black-yellow and 766 ohms (to
>> make the maths easier) red-white, the four bridge legs would need to
>> be 700 Ohms. However that would give 525+33 = 558 Ohms for black-red,
>> black-white, yellow-red and yellow-white.
>> A bit of simultaneous equation solving came up with the four legs
>> being 530 Ohms, and 85 Ohms in series with black and yellow and 117.5
>> Ohms in series with red and white. I am not saying that this is what
>> actually is there, but it does fit all the evidence supplied. Or I
>> have made an error in my maths.
>> Trevor
>>
>> On Sat, Jul 24, 2021 at 9:49 AM Mark Atherton <markaren1 at xtra.co.nz
>> <mailto:markaren1 at xtra.co.nz>> wrote:
>>
>> Hi Martin,
>>
>> Interesting. I have seen the 33 ohm pad value mentioned in some
>> load-cell manufacturer data (found this morning).
>>
>> Wallace,
>>
>> Evil intent? Most interested to know what that might be !!
>>
>> I will keep poking when time permits; the network is more complex
>> than anticipated, and I have found some interesting reading on the
>> subject.
>>
>> At the end of the day, the activity is purely academic; quite an
>> interesting puzzle.
>>
>> Thanks.
>>
>> Mark
>>
>>
>>
>>
>> > On 24 July 2021 at 09:39 Martin Towers <tinytowers at xtra.co.nz
>> <mailto:tinytowers at xtra.co.nz>> wrote:
>> >
>> >
>> > Hi Mark,
>> > This may not be a full bridge, it may have 1, 2 or 4 active arms and
>> > precision resistors elsewhere. In the 2 active arm, he dummy arm may
>> > use different values to the active arm.
>> >
>> > There may also be a trim to bring the reading to zero.
>> >
>> > Try feeding voltage in across black and yellow and see what you get.
>> >
>> > As Andrew says it does look like there may simply be 33 Ohms each in
>> > series with black and yellow. Maybe???
>> >
>> > Martin
>> >
>> >
>> > On Sat, 2021-07-24 at 06:35 +1200, Mark Atherton wrote:
>> > > Hi Andy,
>> > >
>> > > What you say is true, but then looking into R1 from the outside
>> > > would
>> > > result in a different value than R2.
>> > >
>> > > I am wondering if there are extra components in the network
>> between
>> > > RED-WH and BLK-YELL that aren't obvious from black-box testing.
>> > >
>> > > -Mark
>> > >
>> > > On 24/07/2021 12:42 AM, Andrew Dachs wrote:
>> > > > Hi mark, it’s a bit late and I need a piece of paper but I think
>> > > > the essence is R1 must equal R3, R2 must equal R4 but doesn’t
>> > > > follow that they all have to be the same value. R1 might be
>> > > > different from R2 and R3 different from R4. You’ve done enough
>> > > > measurements to work them all out.
>> > > > Andy
>> > > >
>> > > >
>> > > > Sent from my iPhone
>> > > >
>> > > > > On 23/07/2021, at 8:37 PM, Mark Atherton
>> <markaren1 at xtra.co.nz <mailto:markaren1 at xtra.co.nz>>
>> > > > > wrote:
>> > > > >
>> > > > > Just acquired a small pile of 250kg load-cells, and
>> everything
>> > > > > was going swimmingly until I looked a little too closely.
>> > > > >
>> > > > > A) 5.0V DC excitation voltage is applied between Red and White
>> > > > > wires. At rest, and with respect to White, both Black and
>> Yellow
>> > > > > are at 2.50V. And yes, I have checked that there is 0V between
>> > > > > BLK and YEL. Apply some force to the unit and the bridge
>> > > > > unbalances just as it should and a 100kg or so produces a 12mV
>> > > > > imbalance which sounds about right from previous experience.
>> > > > >
>> > > > > B) With the unit completely disconnected, the resistance
>> looking
>> > > > > between YEL-RED, YEL-WH, BLK-WH and BLK-RED are each 600
>> ohms ±
>> > > > > sod-all.
>> > > > >
>> > > > > C) I was hoping that I wouldn't be pushing my luck too far by
>> > > > > assuming that each network leg would be 800 ohms. (600R
>> external
>> > > > > = 800R internal in parallel with (3 * 800R internal).
>> > > > >
>> > > > > D) But alas no, BLK-YEL is 700R and RED-WH is 765R. This
>> rather
>> > > > > implies that the upper resistor pair are different values
>> to the
>> > > > > lower resistor pair. However if that was true, then test A
>> would
>> > > > > not indicate a 50% voltage division.
>> > > > >
>> > > > > Also, if each internal limb was 800R, then there should be
>> 800R
>> > > > > in both cases of D).
>> > > > >
>> > > > > So clearly applying 5V excitation is different from using
>> an ohm-
>> > > > > meter, so I dropped excitation voltage to 260mV and the
>> mid-point
>> > > > > voltage was still 50%. I also heated the unit from 17c up to
>> > > > > around 27c without any significant change.
>> > > > >
>> > > > > Swapping polarities of just about everything does not affect
>> > > > > results, so it looks like a purely resistive network (unless
>> > > > > there is something weird about strain-gauges I am unaware of).
>> > > > >
>> > > > > Also tried exciting BLK-YEL which made no difference.
>> BLK-RED and
>> > > > > BLK-YEL still measured 2.5V each (per A above).
>> > > > >
>> > > > > I am running out of things to poke at from the outside, and am
>> > > > > quite intrigued about the fundamental (and incorrect)
>> assumption
>> > > > > I have made with all of these tests.
>> > > > >
>> > > > > The units appear to work as expected, but can't find a
>> datasheet
>> > > > > (units are marked 250kg 1420230).
>> > > > >
>> > > > > Suggestions please.
>> > > > >
>> > > > > -Mark
>> > > > >
>> > > > > <load cell.gif>
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