2007-10-10 by Roy E. Burrage
It appears that most of the scale circuits you've seen are the result of
some digital guy messing around in my world...analog. Do it your way.
It's unfortunate that specsmanship is becoming the norm rather than the
exception any more, and an understanding of both conversion techniques
and interface circuitry is becoming a lost art.
Spec for class 1 weights is 25 mg. at 1 kg, that's true and you did your
math properly. However, yours may be calibrated to tighter tolerances
but how well do they maintain that accuracy with changes in temperature
and humidity? There's a reason we certify equipment to tighter
tolerances than what the specification requires.
Thermal emf errors are not necessarily line impedance related. How much
lower resistance can a line be than 2 hunks of wire welded together?
There are a lot of methods that can be used to get around a potential
noise problem. It sounds like most of your "trade approved" scales are
just using an expensive A/D converter to avoid a little analog design work.
Good luck on your project.
REB
kernels_nz wrote:
>Hi Rob,
>
>Wow, I see to be generating a bit of interest with what I viewed as a
>pretty simple problem. Firstly, in practice, every trade-approved
>weigh-scale that ive seen so far has used at least a 24-bit A2D
>converter. The load-cells are completely passive, they are merely a
>wheatstone bridge made up of four strain guages. Thus, to get 10 000
>divisions from a 3mv/v load-cell with 5v excitation (15mv full scale)
>output, you need 15mv / 10000 = 1.5uv resolution in your measurement
>system. In reality im only looking for 3000 divisions (for trade
>approved weighing), but as you stated, I need some head-room.
>
>I would imagine that weighing systems get around the thermocouple
>junction effects by running a low impedance system, load-cells are
>only 300 OHM impedance. Im not sure what the reason is, but I worked
>in the weighing industry for a couple of years and never had any
>problem with "junctions". The 15mv can be measured at the input to the
>ADC chip, most good digital weight indicators dont use any
>amplification of the signal anywhere, I guess the more amplification
>you have along the way the more likely you are to introduce noise.
>
>Yes, the number of noise free bits required would be way less if my
>full-scale output voltage was closer to the reference voltage, but in
>all of the scale circuits ive studied, the excitation voltage for the
>load-cell is fed straight into the reference of the ADC, this then
>compensates the ADC for any changes in excitation voltage. This also
>means a reference voltage of 5V with a full scale signal of around
>15-20mv!
>
>The Class 3 test weights used for weights and measures tests are
>accurate to 10mg in 1kg, thats way more than 0.25%, I think a
>weights-and-measures inspector would laugh at me if I suggested he
>used 0.25% test weights to check our equipment. Im not even sure 0.25%
>weights could be sold, thats 2.5g in 1kg, ridiculous.
>
>But alas, designing the analogue part of this system will merely be a
>copy and paste operation from earlier model weigh-scales - no problem.
>The bit thats got me thinking is what the best way is to do the
>software calibration on a 8-bit micro controller. The one currently
>used is 32 bit with a proper floating point module, and deals with the
>math no-problem. I was really fishing for some integer-math-only way
>of doing the calibration code and luckily one guy on the Atmel ARM
>forum has given me some good pointers, so it looks like it should be
>possible. The other option of course is biting the bullet and just
>doing the floating point math!
>
>Cheers
>Hein B
>Auckland, New Zealand
>
>
>
>--- In AVR-Chat@yahoogroups.com, "Roy E. Burrage" <RBurrage@...> wrote:
>
>
>>We do a little bit with measurement here, Hein.
>>
>> www.measuretech-tn.com
>>
>>My experience has been that a scale, most mechanical measuring devices
>>for that matter, aren't very accurate below 10% to 20% of their full
>>scale range. A set of ANSI Class 1 calibration weights is only
>>
>>
>accurate
>
>
>>to something on the order of 0.25%, so how many bits does that equate
>>to? Transfer accuracy is, at best, 2:1 with 4:1 recommended...and 10:1
>>optimum. So where does that leave your bit resolution to accuracy
>>ratio? 10 bit accuracy is better than this 0.25% and averaging can
>>easily give you 12 to 14 bits of resolution if your A/D converter is
>>linear enough.
>>
>>You may be trying to resolve a couple of microvolts, but that certainly
>>is only going to be right at the load cell. Most load cell outputs are
>>something on the order of 10mv to 20mV. I suspect your 5 volt
>>
>>
>signal is
>
>
>>after amplification local to the load cell to prevent thermal emfs from
>>swamping out load cell output changes. Look at the mV/degree change of
>>a type T (copper/constantan, constantan being a copper alloy)
>>thermocouple some time. How many thermocouple junctions would you have
>>from a load cell, a 20 foot run of cable, then some electronics? Don't
>>miscount your junctions or get dissimilar metals in there.
>>
>>Make sure you aren't confusing resolution and stability with accuracy.
>>There is a big difference in the 2.
>>
>>
>>REB
>>
>>
>>
>>
>>kernels_nz wrote:
>>
>>
>>
>>>Hi Roy, generally you don't need a particularly accurate reference,
>>>just one that doesn't vary with time and environmental factors. All
>>>measurement is done relative to a "calibrated test weight", so scales
>>>are calibrated and do not rely on the accuracy of the reference.
>>>
>>>In practice the weigh scale manufacturers are trying to look at 1.5uV
>>>changes in strain-bridge output with a 5V reference, so unfortunately
>>>you do require around 3 million counts or 22 bits.
>>>
>>>At low sample rates (10 s/s) and good analogue design 22 bits is not
>>>too hard to achieve, it's all about using the right parts for the job.
>>>
>>>All that stuff is comparatively easy, the bit thats got me a bit
>>>worried is migrating the code from the existing 32-bit processor to a
>>>baby AVR. I was hoping someone had done something similar and could
>>>give me some advice on how to do the calibration routines without
>>>using floating point math!
>>>
>>>Cheers
>>>Hein B
>>>Auckland, New Zealand.
>>>
>>>--- In AVR-Chat@yahoogroups.com, "Roy E. Burrage" <RBurrage@> wrote:
>>>
>>>
>>>
>>>
>>>>Is it 24 bit accuracy or 24 bit resolution? At 24 bits, 1 part in
>>>>almost 17 million...where do we get a reference that accurate with a
>>>>reasonable cost suitable for a weigh scale product?
>>>>
>>>>I suspect 12 bit accuracy would be plenty accurate for your
>>>>
>>>>
>application
>
>
>>>>and you wouldn't have the thermal emf problems as with 24 bits. Just
>>>>throw the lower bits away after you average them into your equations.
>>>>
>>>>On a regular basis here, we take data points and store them in a
>>>>
>>>>
>moving
>
>
>>>>average filter. There's a very good application note both from Atmel
>>>>and at AVRFreaks which describes the process. If it were me I'd use
>>>>
>>>>
>>>>
>>>>
>>>the
>>>
>>>
>>>
>>>
>>>>upper 16 bits of your A/D converter, store those in 2 8 bit
>>>>
>>>>
>registers,
>
>
>>>>add that to the 2 registers you have already set up for your
>>>>
>>>>
>averaging
>
>
>>>>filter, add the overflow bit to another register set up for that
>>>>purpose, and then after your predetermined number of sample points
>>>>
>>>>
>take
>
>
>>>>an average. It's as simple as that.
>>>>
>>>>You'll also have to adjust the limits of your filter to allow for
>>>>
>>>>
>wind
>
>
>>>>gusts or other vibration sources so they don't drive your gizmo wild.
>>>>
>>>>
>>>>REB
>>>>
>>>>kernels_nz wrote:
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>>Hi there, this one is out of left field a bit (Baseball phrase ???)
>>>>>but im working on a weigh-scale project that uses a 24-bit ADC and I
>>>>>was just wondering whether anyone had ever written a weigh-scale
>>>>>module for a 8-bit micro controller.
>>>>>
>>>>>Im just interested to see how all the calibration etc. for
>>>>>
>>>>>
>finding the
>
>
>>>>>actual displayed weight from having 32 bit numbers representing the
>>>>>Calibration Zero and Span values and the current ADC value. Is
>>>>>
>>>>>
>there a
>
>
>>>>>way to do all the calculations without using floating point math?
>>>>>
>>>>>Cheers for any information!
>>>>>Hein B - Auckland, New Zealand
>>>>>
>>>>>
>>>>>
>>[Non-text portions of this message have been removed]
>>
>>
>>
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>Yahoo! Groups Links
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