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THE DrDAQ NEW MINIATURE BAROGRAPH - PART 7

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THE DrDAQ NEW MINIATURE BAROGRAPH - PART 7

Postby Glovisol » Mon Nov 30, 2015 3:51 pm

PERFORMANCE OF THE NEW r PARAMETER FOR THE RATIOMETRIC ERROR CORRECTION

First of all a mistake was found in the calc. Excel File KP-235 Math 5(5V). Luckily this mistake did not impair the relevant computations. The amended File KP-235 Math 6(5V) is enclosed.

KP-235 Math 6(5V).xls
Amended calc. file
(74.5 KiB) Downloaded 45 times


The performance of the proposed correction method works unbelivably well, reducing the ratiometric error to less than 1 mmHg over the full barometric pressure range, while meeting a power supply variation Vcc from 4,75 to 5,25 V! The following table compares the different situations.

Correction....Vcc = 4.75V............Vcc = 5.00V......................Vcc = 5.25 V
......... Max. error hPa/mmHg..... Max. error hPa/mmHg.......Max.error hPa/mmHg
No corr................± 64,1/± 48,2..........± 69,6 / ± 52,3.............± 75,1/± 56,5
With r = 0.75.........+29,34/+14....................0.....................-29,34/-14
With r = f(P)..........±1,33/±1......................0.....................±1,33/±1


In fact I am of the opinion that performance is even better, because the test was done while continuous changes in baropressure where happening. The PLW file with all measurement data is enclosed.

PK235- Wide range corrected2.PLW
Test data file
(28.78 KiB) Downloaded 47 times


PK235-Wide range corr.2.jpg
PC screen of PLW file


To eliminate PSU transients produced while changing Vcc and their effect on recorded baropressure, data has been copied and processed in an Excel file which is enclosed along with the generated graph. Of course no transients will be experienced with the chip fed by the USB Bus, as variations there will be extremely slow.

Error analysis 1.xls
Error analysis file
(81.5 KiB) Downloaded 43 times


Error analysis graph.jpg
There area few gliches with this graph. Pse download the Excel file & browse it there.


This work has shown the great potential of contemporary software like Picolog in efficient handling of ratiometric type sensors, as the correction methods shown are of general interest and can be profitably applied to a great variety of devices. In the next instalment the second solution mentioned in Part 6, namely the application of a special minimal dropoff PSU will be described. This solution provides even better performance, zero error throughout, at the cost of more hardware.
Last edited by Glovisol on Sat Dec 05, 2015 12:59 pm, edited 1 time in total.
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Re: THE DrDAQ NEW MINIATURE BAROGRAPH - PART 7

Postby Glovisol » Fri Dec 04, 2015 12:30 pm

FINAL CONSIDERATIONS ON FEEDING THE BAROGRAPH WITH THE USB BUS VOLTAGE

Another analysis of the dynamic r parameter correction method is shown below, which again shows that error is negligible and not additive.

5. PK235 Wide range 3.jpg
Dynamic r parameter correction analysis


Please carefully note the "cleanness" of the Baropressure waveform, in view of what is coming next. Up to the PC screen shown above, the Barosensor has been fed with variable voltage (4.75 to 5.25 V) coming from a lab PSU. But our aim is to feed the system from the USB bus. Here is the result.

6. PK235 - USB Bus feed.jpg
Here at last, Barograph working off the USB Bus!!


Precision and correction are excellent, but unfortunately the USB Bus voltage is "dirty". In other words the short term stability of the PC derived USB voltage is far from good (contrary to what I had previously written " PSU Bus voltage variations will be extremely slow") so that the system records the small voltage change, then corrects: therefore the Barograph trace is not smooth, but has some "lumpiness", which however does not impair the precision of the reading.

Finally the "barefoot" schematic diagram of the Barograph, with direct, buffered connection to the DrDAQ (which eliminates the need of the Universal DrDAQ Interface) is shown below.

Baro USB Barefoot 1.bmp
Simplest Barograph schematic diagram
Glovisol
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Re: THE DrDAQ NEW MINIATURE BAROGRAPH - PART 7

Postby Glovisol » Sat Dec 05, 2015 10:17 am

QUALITY OF THE USB BUS VOLTAGE

It came to my mind that perhaps the short term instability I had found was peculiar to the laptop PC I was using (IBM / LENOVO X61 equippend with Windows XP O.S.). I connected the Barograph to another X61 and found a much better short term stability of the Bus voltage (as was perhaps to be expected). Will post the results here soon.

PK235 - DrDAQ USB FEED 3.PLW
(28.96 KiB) Downloaded 41 times
Last edited by Glovisol on Mon Dec 07, 2015 5:00 pm, edited 3 times in total.
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Re: THE DrDAQ NEW MINIATURE BAROGRAPH - PART 7

Postby Glovisol » Mon Dec 07, 2015 8:53 am

QUALITY OF USB BUS VOLTAGE - NEW DATA

The PK235 Sensor was connected to a different IBM/Lenovo SX 61, equipped with Windows 7 O.S.
The short term stability of the USB Bus voltage was found to be notably better, 0.3 mV pk compared to 1 mV pk, as shown below:

8. PK235 - USB Bus feed 11(1).jpg
PC screen using exactly the same time & amplitude scales as USB FEED 3


The "lumpiness" of the Barograph display is barely noticeable here. The next PC screen shows Barograph operation going from Mains to Battery supply and vice-versa. There is no significant error caused by the transitions.

9. PK235 - USB Feed 11 (2).jpg
A.C. Mains feed & Battery feed compared


To conclude, each PC has an USB Bus voltage with a different short term stability. Therefore with Parameter r compensation only, the Barograph display quality is somehow influenced by the characteristics of the PC used. In any case this effect does not noticeably impair the precision of the measurement.

PK235 - DrDAQ USB FEED 11.PLW
(28.91 KiB) Downloaded 43 times
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