1 ABE425 Engineering Measurement Systems General Characteristics of Measurement Systems Dr. Tony E. Grift Dept. of Agricultural & Biological Engineering University of Illinois
2 Agenda What is a measurement system Validity of measurementCalibration
3 In general, a measurement system comprises a measurand, a sensor, signal conditioning and an indicator/recorder (computer) A measurand is the physical quantity intended to be measured Signal conditioning (amplifiers, limiters, inverters, filters) Often a transducer Nowadays a computer A transducer measures the value of the measurand and converts it to an electrical signal
4 You can only measure objectively what is defined and standardizedMass Distance Time Voltage Current Power Work Luminous intensity
5 How would you go about measuring something like this? What is the unit?Charisma Patriotism Pain Hate Fear Odor
6 How does one measure happiness?Mona Lisa Was 83 Percent Happy AMSTERDAM, Netherlands (AP) —The mysterious half-smile that has intrigued viewers of the Mona Lisa for centuries isn't really that difficult to interpret, Dutch researchers said Thursday. She was smiling because she was happy —83 percent happy, to be exact, according to scientists from the University of Amsterdam. In what they viewed as a fun demonstration of technology rather than a serious experiment, the researchers scanned a reproduction of Leonardo da Vinci's masterpiece and subjected it to cutting-edge "emotion recognition'' software, developed in collaboration with the University of Illinois. The result showed the painting's famous subject was 83 percent happy, 9 percent disgusted, 6 percent fearful and 2 percent angry. She was less than 1 percent neutral, and not at all surprised.
7 How would you measure love? How about pain?
8 Question What is the basic principle of this Mercury in glass thermometer? What is the measurand? What is the signal conditioning subsystem? What is the indicator? How would you calibrate such a device? Is calibration in measurement systems always necessary?
9 Answer The basic principle is thermal expansion of Mercury, relative to its glass container? This is the model that we use for the measurement system. The measurand is the temperature of either fluid, or gas The stem is a very small volume compared to the bulb A scale (in deg C or deg F or both) You calibrate it against a measurement system of higher accuracy or against a standard The far majority of measurement systems needs calibration
10 Errors in measurement Error is the difference between the measured value and the true value of the measurand Types of errors Bias errors (Fixed systematic errors,offsets) Random errors Sources of bias error Calibration error Instrument out of specification Loading error Measurement device itself affects the measurement External influence error Measure a temperature in a gas near a cold or hot wall
11 Terms in dictionary.comac·cu·ra·cy ˈæk yər ə si/ [ak-yer-uh-see] –noun, plural -cies. 1. the condition or quality of being true, correct, or exact; freedom from error or defect; precision or exactness; correctness. pre·ci·sion prɪˈsɪʒ ən/ [pri-sizh-uh n] –noun 1. the state or quality of being precise. 2.accuracy; exactness: to arrive at an estimate with precision. cal·i·brate ˈkæl əˌbreɪt/ [kal-uh-breyt] –verb (used with object), -brat·ed, -brat·ing. 1.to determine, check, or rectify the graduation of (any instrument giving quantitative measurements). 2.to divide or mark with gradations, graduations, or other indexes of degree, quantity, etc., as on a thermometer, measuring cup, or the like.
12 Terminology of measurement systems (Wheeler/Ganji)Accuracy is how close you are to the true value of the measurand. Accuracy is often given as a percentage of Full Scale (FS) Precision is the difference between an actual measurement and the mean among all readings of the same measurand. This error is caused by a lack of repeatability Bias (a systematic error that remains constant)
13 Example 2.1 True value of measurand is 6.11 Volt10 measurements, 5.98, 6.05, 6.1,6.06,5.99,6.02,6.09,6.03,5.99 Volt Estimate the bias error Estimate the maximum precision error
14 Example 2.1 Answer True value of measurand is 6.11 Volt. 10 measurements, 5.98, 6.05, 6.1, 6.06, 5.99, 6.02, 6.09, 6.03, 5.99 Volt Estimate the bias error Average is Bias is average value – true value = = Volt Estimate the maximum precision error Maximum error is 5.96 – 6.03 = Volt
15 An ideal measurement device has a zero offset and a constant uncertainty interval across the span (range) Uncertainty interval often given as percentage of Full Scale (FS)
16 Hysteresis is a constant predictable error and therefore counted as a biasCauses are mechanical friction (play), flex in mechanical parts and capacitance
17 Linearity between the measurand and the measured output is desired, but not required.Terminal-point-based straight line, connects measured values at the extremes of the measurement span. The maximum deviation of the transducer output with respect to this terminal-point straight line represents the linearity.
18 A high sensitivity is desirable for accurate measurements
19 Measurement of Mass (or is it weight?)Mass (kg) is independent of gravity Weight (N) is Mass * Gravity Compare playing pool on the moon and Earth
20 Measurement of Mass and WeightBalance (null method) Analytical Balance (deflection methods) Which one is dependent upon gravity?
21 Calibration of a spring scale can be done with objects of known massWhen calibrating the spring scale one can Go up and down in known increments Choose random ‘weights’
22 Curve fitting is a method of calibrationClassical curve fitting uses Ordinary Least Squares (OLS) estimation to ‘freeze’ the behavior of the instrument
23 The deviation (residual) plot gives the differences among measured values and the fit curve
24 Averaging the values to assess accuracy, precision and non-linearityThe averages of up and downward readings give the hysteresis error The terminal point based straight line is used to express non-linearity Terminal point straight line
25 Dynamic calibration
26 A loading error is caused by the measurement device itself affecting the measurandThe thermometer itself cools the water temporarily, and therefore the initial measured value is lower than the original After a while the temperature of the thermometer is equal to the water temperature (equilibrium) and the true value is read
27 Dynamic response of the measurement device can be zero, 1st or 2nd orderThe graph shows the response of a dynamic system to a step input when the measurement device is brought in contact with the measurand
28 The frequency response is the window in which the measured value is unaffectedThe measurement system has a limited frequency response and acts like a band pass filter
29 Classification of measurement systemsIntrusive / non-intrusive Null method / deflection method Digital / analog
30 Summary of terms (write definition and meaning)Measurand Measured value Accuracy Precision Errors (systematic vs random) Bias (offset) Hysteresis Calibration error Loading error External influence error Drift Calibration (static/dynamic) Linearity Terminal point straight line Step response Zero/1st, 2nd order Frequency response Ordinary Least Squares fitting
31 ABE425 Engineering Measurement Systems General Characteristics of Measurement Systems The EndDept. of Agricultural & Biological Engineering University of Illinois