Basic Voltmeter Test

INTRO

Finally! After a safe Initial Bring-Up, it's time to test drive our complete Digital Voltmeter. Two key test categories are:

• FULL FUNCTIONAL TEST
• ACCURACY TEST

The Full Functional Test adds code to toggle between the 4V and 20V Ranges using a Pushbutton. (For the Initial Bring-up, the voltage range was selected manually by setting a variable in the code.)

The Accuracy Test answers this critical question - does our instrument meet our Accuracy Spec as chosen in our Requirements?

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ANALOG + ARDUINO SCHEMATIC

Here's the schematic of the Solderless Breadboard and connections to the Arduino UNO Board.

 

PROTOBOARD + ARDUINO LAYOUT

Check out a suggested physical layout of the voltmeter.

 

FULL FUNCTIONAL TEST

The Basic Voltmeter code verifies that many hardware and software functions operate correctly.

• Switches SW1,2 and R-Divider R1,2 operate as expected
  •  4V Range (Vrange=0):  SW1 passes signal directly (Kdiv=1.0).
  • 20V Range (Vrange=1): SW2 passes signal from divider tap (Kdiv=0.2)
• The ADC outputs the correct value with Vin=3.3V
  • Expected Value:  ADCword = Vin*Kdiv*1/Vref*1024
  • 4V Range:  ADCword = 3.3V*1.0*1/5V*1024 = 676
  • 20V Range:  ADCword = 3.3V*0.2*1/5V*1024 = 135
• The Code computes the correct Vin given ADCword
  • Expected Value:  Vin = ADCword/1024*Vref*1/Kdiv
  • 4V Range: Vin = 676/1024*5V*1/1 = 3.3V
  • 20V Range: Vin  = 135/1024*5V*1/0.2 = 3.3V
• The code correctly detects a pushbuttonstate change from 1 to 0 and toggles the Vrange.

 

How can we verify all of the above functions? By applying an input signal Vin=3.3V and displaying key variables in the Serial Monitor, you get an excellent view into the voltmeter's inner workings.

• Add wire jumper from the 3.3V pin (Arduino UNO) to the Vin signal (Solderless Breadboard).
• Open the sketch: DVM1_Basic_Voltmeter.ino
• Add code to print the variables Vinput, ADCword, buttonState, Vrange to the Serial Monitor
• Run the sketch: Click Upload and Serial Monitor.
• Press the Pushbutton and verify a 1 to 0 transition on the buttonState variable.
• Verify that the Vrange toggles from between 0 and 1 at each button press.
• Verify the ADCword toggles between 676 (Vrange=0) and 135 (Vrange=1).
• Verify the code calcution for Vin remains at 3.3V for either range.

TEST TRACKER

• Record your test results and comments.
• Excel File:  DVM1_Prototype_Test_Tracker.xlsx
  Right Click on the filename, select "Save link as...".
• Use as template for your own tests.
• Note any unexpected behavior, issues or lessons learned.

 

ACCURACY TEST

How can you verify the accuracy of the voltmeter? We'll apply the Comparison Test which compares the voltmeter's reading to another instrument that serves as a Reference Standard.

COMPARISON TEST

Just a few steps accomplish the test:

1. Choose a stable voltage source to measure.
2. Measure the voltage with the voltmeter under test (Vm).
3. Measure the voltage with the reference voltmeter (Vm_ref).
4. Calculate the Accuracy.
5. Apply the Decision Rule.

ACCURACY CALCULATION

The measured Accuracy must include the uncertainty of the reference voltmeter.

  Accuracy = |Error Measured| + |Uncertainty of Reference Standard|
                   = |Vm - Vm_ref|      + |Offset_err + Gain_err*Reading|

REFERENCE STANDARD

• We'll use a medium-cost ( <$170 ) Multimeter as our Reference Standard.
• FLUKE 17B+ Multimeter Specifications.
• 0 to 3999 Digits Display
• DC Volts Accuracy:
  • Offset_err + Gain_err * Reading = 3 digits + 0.5%*Reading
  • 4V Range (0.001 resolution): 3 digits = 3mV
  • 40V Range (00.01 resolution): 3 digits = 30mV

 

DECISION RULE

The instrument will PASS or FAIL based on a simple decision.

   If Accuracy < Accuracy_Spec_Limit, then PASS, else FAIL

The Accuracy_Spec_Limit was previously determined in the Requirements Phase of the project.

• 4V Range: 15mV + 1.5%*Reading
• 20V Range: 75mV + 1.5%*Reading

 

NUMERICAL EXAMPLE (4V Range)

• Choose a stable voltage source:  3.3V Supply on the Arduino
• Measure Voltage using voltmeter under test: Vm = 3.286V
• Measure Voltage using reference voltmeter (Fluke 17B+): Vm_ref = 3.309V
• Calculate Accuracy
  • Accuracy = |3.286V - 3.309| + |3mV + 3.309V*0.5%|
                   = |-23mV| + |20mV|
                   = |43mV|
• Apply Decision Rule
  • Calculate Limit
      Accuracy_Spec_Limit = 15mV + 3.286*1.5% = 64mV
  • Decide P/F using limit
      If Accuracy = 43mV < Accuracy_Spec_Limit = 64mV
      then PASS!

 

TEST TRACKER / CALCULATOR

• Check out this handy Accuracy / Spec calculator.
• Excel File:  DVM1_Prototype_Test_Tracker.xlsx
  Right Click on the filename, select "Save link as...".
• Use as template for your own test.
• Record your results and comments.
• Note any unexpected behavior, issues or lessons learned.

 

 

NEXT UP

Congratulations on completing a journey down the long and winding road of a project design! The last step brings us full-circle back to our original driving mission - measure the health of common household batteries.

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