EXPERIMENTAL MEASUREMENT METHODS AND METHODOLOGY Strategies and Tactics for Measurements in 16.62X 16.621 Experimental projects Lab I
EXPERIMENTAL MEASUREMENT: METHODS AND METHODOLOGY Strategies and Tactics for Measurements in 16.62x 16.621 Experimental Projects Lab I
BASIS FOR MEASUREMENT Whatever exists, exists in some amount Our goals: Measure the appropriate quantity, Measure with the appropriate accuracy to allow us to assess our hypothesis Some questions: What data do we need? What do we measure? How do we measure? How well do we need to measure? What do we do with the data?
BASIS FOR MEASUREMENT • Whatever exists, exists in some amount • Our goals: – Measure the appropriate quantity, – Measure with the appropriate accuracy, to allow us to assess our hypothesis • Some questions: – What data do we need? – What do we measure? – How do we measure? – How well do we need to measure? – What do we do with the data? 2
ERROR AND UNCERTAINTY IN MEASUREMENT This is the subject of a future class, but the idea is so important that I mention it today as well In engineering the word"error", when used to describe an aspect of measurement does not necessarily carry the connotation of mistake or blunder(although it can!) Error in a measurement means the inevitable uncertainty that attends all measurements We cannot avoid errors in this sense We can ensure that they are as small as reasonably possible and that we have a reliable estimate of how small they are [Adapted from Taylor, J.R,An Introduction to Error Analysis; The Study of Uncertainties in Physical Measurements"]
ERROR AND UNCERTAINTY IN MEASUREMENT • This is the subject of a future class, but the idea is so important that I mention it today as well • In engineering the word “error”, when used to describe an aspect of measurement does not necessarily carry the connotation of mistake or blunder (although it can!) • Error in a measurement means the inevitable uncertainty that attends all measurements • We cannot avoid errors in this sense • We can ensure that they are as small as reasonably possible and that we have a reliable estimate of how small they are [Adapted from Taylor, J. R, “An Introduction to Error Analysis; The Study of Uncertainties in Physical Measurements”] 3
SOME GENERAL MEASUREMENT CONCEPTS The measurement exists in some context We want to do this measurement because The context (and objective shapes the measurement strategy and tactics [ Begin with the end in mind"Steven Covey You generally do not directly measure the quantity you want Most measurement devices are systems which translate the measured quantity(voltage, for example) into the quantity you want (pressure) All measurements have some uncertainty, or error, connected with them It is important to know not only the result of the measurement but also the fidelity of the result (how big is the uncertainty, how much can I trust the answer?)
SOME GENERAL MEASUREMENT CONCEPTS • The measurement exists in some context – We want to do this measurement because……. • The context (and objective) shapes the measurement strategy and tactics [“Begin with the end in mind”- Steven Covey] • You generally do not directly measure the quantity you want – Most measurement devices are systems which translate the measured quantity (voltage, for example) into the quantity you want (pressure) • All measurements have some uncertainty, or error, connected with them – It is important to know not only the result of the measurement but also the fidelity of the result (how big is the uncertainty, how much can I trust the answer?) 4
ELEMENTS OF A MEASUREMENT SYSTEM Sensing Signal Conditioning Variable‖ Measured Primary Variable Conversion Medium Sensing Manipulation Element Element Element Data Storage/Playback Readout Element Data Transmission Presentation Observer Element Element Adapted from Ernest O Doebelin, Measurement Systems: Application and Design, McGraw Hill
ELEMENTS OF A MEASUREMENT SYSTEM Measured Medium Observer Variable Manipulation Element Primary Sensing Element Variable Conversion Element Data Transmission Element Data Storage/Playback Element Data Presentation Element Sensing Signal Conditioning Readout Adapted from Ernest O. Doebelin, Measurement Systems: Application and Design, McGraw Hill 5
MEASUREMENT ELEMENT DEFINITIONS Primary Sensing element Retrieves energy from the measured system Produces some form of output Strain gage, thermometer tip Variable conversion element Changes data from one physical form to another Elongation to resistance, temperature to volume change Variable manipulation element Performs mathematical operation on data Amplifier, filter Adapted from ErnestO. Doebelin, Measurement Systems Application and Design, McGraw Hill
MEASUREMENT ELEMENT DEFINITIONS – Primary Sensing element • Retrieves energy from the measured system • Produces some form of output ¾ Strain gage, thermometer tip – Variable conversion element • Changes data from one physical form to another ¾ Elongation to resistance, temperature to volume change – Variable manipulation element • Performs mathematical operation on data ¾ Amplifier, filter Adapted from Ernest O. Doebelin, Measurement Systems: Application and Design, McGraw Hill 6
MEASUREMENT ELEMENT DEFINITIONS Data transmission element Gets data between measurement elements Wire, speedometer cable, satellite downlink system Data storagelplayback element Stores data for later retrieval hard drive, RAM Data presentation element Gets data to form detectable by human Indicators, alarms, analog recording, digital recording Adapted from Ernest O. Doebelin, Measurement Systems: Application and Design, McGraw Hill
MEASUREMENT ELEMENT DEFINITIONS – Data transmission element • Gets data between measurement elements ¾ Wire, speedometer cable, satellite downlink system – Data storage/playback element • Stores data for later retrieval ¾ Hard drive, RAM – Data presentation element • Gets data to form detectable by human ¾ Indicators, alarms, analog recording, digital recording Adapted from Ernest O. Doebelin, Measurement Systems: Application and Design, McGraw Hill 7
OBSERVATIONS ABOUT THE ELEMENTS Two primary functions Extract data Present data Two points of intended contact with the"outside world Functional blocks Can have more than one of each type Not all measurement systems have all functions May be in any sequence(except primary sensing and data presentation)
OBSERVATIONS ABOUT THE ELEMENTS • Two primary functions – Extract data – Present data • Two points of intended contact with the “outside world” • Functional blocks – Can have more than one of each type – Not all measurement systems have all functions – May be in any sequence (except primary sensing and data presentation) 8
ERROR ANALYSIS Error is inherent in experimental process Uncontrolled Inputs (Co-Factors) Discrete error Controlled Outputs nputs Experiment (Factors) (Responses) coO Continuous error
ERROR ANALYSIS – Error is inherent in experimental process Uncontrolled Inputs (Co-Factors) Discrete Error Controlled Inputs (Factors) ls Humans Experiment Amb Cond Inst Drift Materia Machine Variation Outputs (Responses) Continuous Error 9
16.622 MEASUREMENT GOALS Our goals Measure the appropriate quantity(DO THE RIGHT THING) Measure with the appropriate accuracy(O THE THING RIGHT) This is not only strategy(selecting the needed accuracy) but also implementation(being able to carry out the measurement) To do this, you may have to learn something about differerent specific measurement methods 10
16.622 MEASUREMENT GOALS Our goals: • Measure the appropriate quantity (DO THE RIGHT THING) • Measure with the appropriate accuracy (DO THE THING RIGHT) – This is not only strategy (selecting the needed accuracy) but also implementation (being able to carry out the measurement) – To do this, you may have to learn something about differerent specific measurement methods 10
