EET 2259 Unit 12 Data Acquisition

1. EET 2259 Unit 12Data Acquisition • Read Bishop, Chapter 8. • Lab #12 and Homework #12 due next week.

2. Data Acquisition • The text’s Chapter 8 covers several kinds of material: • Data acquisition hardware (Sections 8.1 to 8.5): Not much on LabVIEW in these sections. • LabVIEWdata acquisition without the DAQ Assistant (Sections 8.6 to 8.8): This is how you had to do it before LabVIEW version 7.0. • LabVIEWdata acquisition using the DAQ Assistant (Sections 8.9 to 8.12): You’re already familiar with much of this.

3. Data Acquisition Hardware • Outline: • Components of a DAQ System (§8.1) • Types of Signals (§8.2) • Transducers and Signal Conditioning (§8.3) • Signal Grounding and Measurements (§8.4) • Analog-to-Digital Conversion (§8.5) • Each of these topics could occupy us for a week, so we’ll just hit the high points. • For lots more detail, see NI’s website: • LabVIEW Measurements Manual • Measurement Fundamentals Main Page

4. Components of a DAQ System (§8.1) Image from NI’swebsite

5. Types of Signals (§8.2) Image from LabVIEW Measurements Manual

6. Transducers and Signal Conditioning (§8.3) • A transducer (or sensor) is a device that converts a physical quantity (such as temperature, pressure, light intensity, …) to an electrical signal (such as voltage or current).

7. Some Common Transducers Image from LabVIEW Measurements Manual

8. Signal Conditioning • In many cases the electrical signal coming from the transducer must be conditioned in some way before it is converted into digital format for use by the computer. • Examples of signal conditioning include: • Amplifying a weak signal • Filtering out noise from the signal

9. Signal Conditioning For Some Transducers Image from LabVIEW Measurements Manual

10. Signal Grounding and Measurements (§8.4) • Signal sources can be either: • Grounded (also called referenced) • Floating (also called non-referenced)

11. Grounded Signal Source • Examples: • Function generator or power supply whose negative terminal is tied to earth ground through the power cord. Image from LabVIEW Measurements Manual

12. Floating Signal Source • Examples: • Our red trainer’s function generator or power supply, whose negative terminal is not tied to earth ground through the power cord. • Battery-powered devices • Transformers • Thermocouples Image from LabVIEW Measurements Manual

13. Measurement System Configuration • Measurement systems can be configured as either: • Differential • Referenced Single-Ended (RSE) • Non-Referenced Single-Ended (NRSE) • The PCI-6221 cards inside our computers can be configured in any of these modes. • The myDAQ can only be configured in differential mode.

14. Differential Measurement System Image from LabVIEW Measurements Manual

15. Referenced Single-Ended (RSE) Measurement System Image from LabVIEW Measurements Manual

16. Non-Referenced Single-Ended (NRSE) Measurement System Image from LabVIEW Measurements Manual

17. Matching Signal Grounding to Measurement Configuration Image from LabVIEWMeasurements Manual. See also NI tutorial on Ground Loops and Returns.

18. Analog-to-Digital Conversion (§8.5) • Before a computer can process analog information, we must first use an analog-to-digital converter (ADC) to transform the analog values into digital binary values.

19. Number of Bits and Accuracy • The ADC periodically samples the analog signal, and converts each sampled value of the analog signal into a binary code. • The more bits that are used in this code, the more accurate is the representation of the original signal. • The following slides (from Floyd’s Digital Electronics) show an example of how using 2 bits results in much less accuracy than using 4 bits.

20. Figure 12.8Light gray = original waveform. Blue = Reconstructed waveform using four quantization levels (2 bits).

21. Figure 12.10Light gray = original waveform. Blue = Reconstructed waveform using sixteen quantization levels (4 bits).

22. Resolution • Several common ways of specifying an ADC’s resolution: • Number of bits, n • Number of output codes, = 2n • Step size (which Bishop calls code width), = Vref / 2n

23. Resolution: Examples

24. Sampling Rate • Sampling rate is another factor determining how well the digitized data match the original analog input. • It’s expressed either as number of samples per second or as a frequency. • Examples: 1000 samples/sec or 1 kHz.

25. Specs for Our Equipment • Let’s find the resolution and sampling rate for the ADCs inside our equipment. • NI myDAQ User Guide and Specifications • NI PCI-6221 Specifications

26. LabVIEW Data Acquisition without the DAQ Assistant (§§8.6 – 8.8) • Before the DAQ Assistant was introduced, LabVIEW programmers had to use the functions on the Measurement I/O > NI-DAQmx palette to perform data acquisition.

27. Express VIs • Express VIs (the DAQ Assistant is one example) were introduced in LabVIEW 7.0. • They provide user-friendly ways to do things that you could also do using other LabVIEW functions and subVIs.

28. Seeing the Code Hidden Inside an Express VI • You can convert an Express VI to standard LabVIEW code by right-clicking and selecting “Open Front Panel.” • In the case of a DAQ Assistant, you can also right-click and select “Generate NI-DAQmx Code.”

29. LabVIEW Data Acquisition with the DAQ Assistant (§§8.9 – 8.12) • You already know much of what is discussed in these sections, since you’ve been using the DAQ Assistant since the start of this class. • Read these sections to review and strengthen your knowledge.