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THE DrDAq NEW MINIATURE BAROGRAPH - PART 9

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THE DrDAq NEW MINIATURE BAROGRAPH - PART 9

Postby Glovisol » Fri Dec 11, 2015 11:18 am

THEORY OF OPERATION & PICOLOG SETUP WITH THE 4.6V PRECISION PSU OFF THE USB BUS

To avoid mistakes and to simplify the setting up it is best to repeat the entire procedure of Part 3, with the relevant changes. All calculations are carried out in the Excel file, KP-235 Math 6 (4.6V) which is attached & can be downloaded. In this file you will also find the following graphs for 4.6V operation: Vo Vs. P; P Vs. Vo; Error Vs. Baropressure and Factor r Vs. Baropressure.

The sensor transfer function is:

Vo = Vcc*((0,01067*P)-0,32667) [1]

The inverse sensor transfer function is:

P = ((Vo/Vcc)+0,32667)/(0,01067) [2]

where Vo is sensor output in V, Vcc is supply voltage in V & P is Barometric pressure in kPa and must be multiplied by 10 to obtain hPa.

Description of various DrDAQ channels in Picolog now follows.

1. In EXT. 1 (Channel 1 input) : Raw Sensor Output. This is the cell output, reduced by a factor of 2. To read the original voltage in mV we must multiply by a factor of 2000.
Parameter formatting: mV
Digits width: 3
Decimal positions: 2
Min. Value: 0
Max Value: 6000
Scaling equation is: X*2000

2. In Calculated Parameter: Calibrated Sensor Output. Here we correct the reading of the Raw Sensor Output in two ways:
a) Remembering that the sensor reading may be off to a maximum of ± 12.5 hPa, obtaining the actual Baropressure read from a calibrated barometer or from Internet data and using transfer function [1] we calculate a calibration factor k. With the sensor used this was k = 0,988. The procedure to obtain k is described in previous Part 5.
b) In another (following) Calculated Parameter Channel we shall find the variable factor h used to nullify the Ratiomentric Error.
Therefore:
Variable A: Raw Sensor Output
Variable C: PSU Error
(A*0.988)+C where C = h
Parameter formatting: mV
Digits width: 4
Decimal positions: 1
Min. Value: 3000
Max Value: 4500

3. In Calculated Parameter: Raw Baropressure. Here we calculate the uncorrected Barometric pressure from the raw sensor output, using the transfer function [1]. Note that 4600 is nominal voltage in mV and factor 10 transforms kPa into hPa.
Therefore:
Variable A: Raw Sensor Output
(((A/4600)+0.32667)/0.0167)*10
Parameter formatting: hPa
Digits width: 4
Decimal positions: 1
Min. Value: 850
Max Value: 1150

4. In Calculated Parameter: Calibrated Baropressure. Here we calculate corrected Barometric pressure from the calibrated sensor output, using again the transfer function [1]. Note that 4600 is nominal voltage in mV and factor 10 transforms kPa into hPa.
Therefore:
Variable A: Calibrated Sensor Output
(((A/4600)+0.32667)/0.0167)*10
Parameter formatting: hPa
Digits width: 4
Decimal positions: 1
Min. Value: 850
Max. Value: 1150

5. In Calculated Parameter: Baropressure Sensor Error. Here we keep the difference between raw & corrected Baropressures under control. Note that error can be positive or negative. With our sensor is always positive because its raw reading value is above actual value (calibration k = 0.988).
Therefore:
Variable A: Raw Baropressure
Variable B: Calibrated Baropressure
A - B
Parameter formatting: hPa
Digits width: 3
Decimal positions: 1
Min. Value: -12.6
Max Value: +12.6

6. In Calculated Parameter: Calibrated Baropressure mmHg. Here we simply convert hPa into mmHg. Any convenient conversion is possible, e.g. inches of Mercury.
Therefore:
Variable C: Calibrated Baropressure
C/1.33
Parameter formatting: mmHg
Digits width: 3
Decimal positions: 1
Min. Value: 650
Max Value: 850

7. In EXT. 2 (Channel 2 input) : Precision PSU Output. Here we monitor the 4600 mV output of the Precision PSU off the USB Bus. Voltage is reduced by a factor of 3.0667 to 1250 mV by R18/R19, thus 3067 in the scaling equation.
Parameter formatting: mV
Digits width:5
Decimal positions:1
Min. Value: 4585
Max Value: 5015
Scaling equation is: X*3067
Alarm - : 4590
Alarm + : 4610

8. In Calculated Parameter: PSU Error. This is really the calculation of the Ratiometric Error factor h due to deviations in the Precision PSU output and is calculated from a factor r, now derived mathematically, as shown in the Excel file KP-235 Math 6 (4.6V) and in previous Part 6. Because of the stable Precision PSU voltage, we have restricted the range of Vcc to ± 5 mV, but, as was to be expected, the r equation stays the same and it will correct even if the precision PSU output will exceed the ± 5 mV. This is what one would call a belt & braces approach. As we have seen this h parameter is used in above entry 2, Calibrated Sensor Output.
Therefore:

Variable C: Precision PSU output (Vcc)
Variable D: Raw Baropressure
h = (4600 – C)*r = (4600 – C)*((0.001067*D)-0.3265)
Parameter formatting: mV
Digits width: 2
Decimal positions: 1
Min. Value: -20
Max Value: +20

KP-235 Math 6(4.6V).xls
Barograph System Calculation file @ Vcc = 4.6V
(78.5 KiB) Downloaded 44 times
Glovisol
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Re: THE DrDAq NEW MINIATURE BAROGRAPH - PART 9

Postby Glovisol » Fri Dec 11, 2015 4:17 pm

FINAL COMMENTS


11. PK-235-4.6Vcc(2) (Large).jpg
Barograph performance fed from USB Bus with Precision 4.6 V PSU


The above PC screen shows a 24 hour run with the Barograph chip fed @ Vcc = 4.6 V by the Precision PSU taking power from the USB Bus. The USB Bus voltage was 5.06V. Barograph trace is very "clean" and does not show any influence from the short term USB voltage instability that was noted with the chip running barefoot with Vcc error compensation only. The PSU output is monitored by a high precision, 50,000 count voltmeter/recorder. During this test PSU output changed from 4.600, 0 mV to 4.600, 3 mV, while the DrDAQ reading "fluctuated" (by the LSB effect) in a 2 mV range. In other words this set-up shows the natural technical limits of the DrDAQ / PC combination. Never the less the overall system performance is excellent.

To conclude, we have seen that the PK-235 chip /DrDAQ/ PC combination can provide faultless performance as a Baropressure sensor/recorder. We have analysed and tested various set-ups as follows:

- Chip fed by precision voltage regulator and external A.C. / D.C. power supply. Mains back-up necessary, otherwise data loss is possible with mains failure.

- Chip running barefoot from direct USB Bus voltage. Good performance, but Barograph display can be "dirty" from USB Bus voltage transients.

- Chip fed by Precision PSU at 4.6 V, best performance overall at cost of more complex hardware.
Glovisol
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