# ME301: Measurement & Experimentation Laboratory

Unit 4: Computer Assisted Data Acquisition   Current data acquisition methods often employ electronic signal transduction and digital recording for subsequent analysis.  Since these methods are so widespread and since errors in inappropriate implementation may manifest differently than they would if you were to make the same mistake reading a meter, you should understand some of the details of the process and some of the common artifacts of inappropriate implementation.  Consider, for example, the differences between an artifact observed in imperfect digital versus imperfect analog imagery or audio reproduction.

Note: Much of the material in Units 3 and 4 are interdependent and refer you to material in “All About Circuits”; you are encouraged to peruse that resource in full at your own pace if you are not familiar enough with concepts in electrical engineering to understand the material as it is presented here.

This unit should take you 11 hours to complete.

☐    Subunit 4.1: 4 hours

☐    Subunit 4.2: 1 hour

☐    Subunit 4.3: 1 hour

☐    Subunit 4.4: 4 hours

☐    End of Unit Self-Assessment: 1 hour

Unit4 Learning Outcomes
Upon successful completion of this unit, the student will be able to:

• Distinguish between analog and digital signals.
• Apply the concepts of bandwidth, sampling rate, and digital resolution to practical data acquisition problems.
• Gain familiarity with a typical computer-based data acquisition system.

4.1 Analog Signal Processing   - Reading: All About Circuits: “Volume 1, Chapter 9: Analog and Digital Signals” Link:  All About Circuits: “Volume 1, Chapter 9: Analog and Digital Signals” (PDF)

Instructions:  This section should provide you with enough background information to understand analog signals in general and pneumatic and electrical signals in particular.  The analogy between fluid and electrical systems may appeal to those with practical fluid mechanics experience. To view as a PDF, click the PDF link in the top right corner.

4.1.1 Signal Waveforms   - Reading: All About Circuits: “Volume 2, Chapter 1: Basic AC Theory” and “Volume 2, Chapter 7: Mixed-Frequency Signals” Link: All About Circuits: “Volume 2, Chapter 1: Basic AC Theory” (PDF) and “Volume 2, Chapter 7: Mixed-Frequency Signals” (PDF)

Instructions: These sections introduce alternating current (AC) signals.  The material is rich; you may wish to revisit the sections after the initial study.  The main idea is that steady signals permit the communication of only one piece of information: the amplitude of that signal.  By combining that amplitude with measurements of time, we can communicate a new piece of information at each new time.  Schemes for encoding information into signal amplitude and time variation can be quite complex; take, for example, the different communication protocols for radio, television, cellular telephone, etc.  In order to understand these technologies, you must have a firm grasp of the underlying physics and mathematics of time-varying signals.

`````` You should first skim the above sections and flag any topics or
symbols that you do not understand.  You may then revisit these
sections after you have completed both Units 3 and 4 in their
entirety. To view as a PDF, click the PDF link in the top right
corner.

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4.1.2 Voltage and Current Signal Systems   - Reading: All About Circuits: “Volume 1, Chapter 9: Current Signal Systems” and “Voltage Signal Systems” Link: All About Circuits: “Volume 1, Chapter 9: Current Signal Systems” and “Voltage Signal Systems” (PDF)

Instructions: Read these sections; they introduce the ideas and circuit symbols for ideal current and voltage sources and present background material on their utility and non-idealities.  At a minimum, upon completing this reading, you should be familiar with the symbols involved for future study. To view as a PDF, click the PDF link in the top right corner.

Instructions: Read the linked chapter on filters.  The text is sufficiently self-explanatory and contains an introductory discussion.  You might wish to keep in mind the following questions: How can digital signal processors be used to build filters?  Is it necessary to use analog filtering devices? To view as a PDF, click the PDF link in the top right corner.

4.1.4 Bridge Circuits   - Reading: All About Circuits: “Volume 1, Chapter 8: DC Bridge Circuits” and “Volume 2, Chapter 12: AC Bridge Circuits” Link: All About Circuits: “Volume 1, Chapter 8: DC Bridge Circuits” (PDF) and “Volume 2, Chapter 12: AC Bridge Circuits” (PDF)

Instructions: Read the linked sections above and perform measurements as possible.  You needn’t become an expert at bridge design, but rather your aim should be to become familiar with the terminology and use of bridge circuits. To view as a PDF, click the PDF link in the top right corner.

4.2 Digitization   - Reading: All About Circuits: “Volume 4, Chapter 13: Introduction to Digital-Analog Conversion” Link: All About Circuits: “Volume 4, Chapter 13: Introduction to Digital-Analog Conversion” (PDF)

Instructions: Read the linked section above for an introduction to relevant terminology.  You should understand the relationship between analog signals, digital signals, and binary numbers. To view as a PDF, click the PDF link in the top right corner.

Instructions: Read the linked section.  Focus on the two topics (i.e. the headings for subunits 4.2.1 and 4.2.2) listed below.  For example, you might keep the following questions in mind: What is the percent linear resolution in a 24-bit binary encoding? How often must sound be sampled in order to accurately represent 20 kHz signals to the ear? To view as a PDF, click the PDF link in the top right corner.

4.2.1 Number of Bits or Resolution   4.2.2 Sampling Rate   4.3 Common Artifacts from Improper Digitization   Note: The most common artifacts from digitization occur as a result of inappropriate sampling rate and/or signal amplitude.  In principle, these problems are similar to, for example, seeing only every hundredth frame in a video presentation or having the volume knob up way too high on an audio presentation.  There are, however, many other subtle artifacts which may emerge upon close scrutiny.  You may be familiar with the vast difference in the sorts of problems that occur with digital television or cell phone transmission versus analog transmission.  Likewise, problems may manifest differently depending upon the processing that is used to get from the original digital signal to the result.

For a more detailed theoretical discussion of this topic, you may refer to Unit 15 of ME205: Numerical Methods for Engineers. (HTML)

4.4 Tutorial for a Commercial Data Acquisition System   - Reading: National Instruments’ LabVIEW Tutorials: “LabVIEW Basics” Link: National Instruments’ LabVIEW Tutorials: “LabVIEW Basics” (HTML and Adobe Flash)

Instructions: Familiarize yourself with the capabilities of this virtual instrumentation environment.  This is an example of common functionality for such a system in a laboratory.  If you have access to a similar system, take the time to acquaint yourself with it.

`````` Instructions: Please complete the linked assessment.