Download Measurements (final)

Computing for Scientists
Measurements
(Sep. 14, 2010 – Sep. 29, 2010)
Jie Zhang
Copyright ©
CDS 130 - 003
Fall, 2010
Measurements
1. How sensors measure scientific data?
2. Issues with sensor?
• Sensor is not perfect; it has limitations
3. Issues with data?
• Signal and noise
Sensors
(Sep. 14, 2010)
Objectives
•How does a sensor work?
•Different types of sensors
Sensor
A sensor is a device that measures a
physical quantity and converts it into a
signal which can be read by an observer
or by an instrument.
http://en.wikipedia.org/wiki/Sensor
Thermometer
Thermometer
biosensor
Electric Measurement
Most sensors are electric-based, converting
physical signal into electric signal: electric
voltage and/or electric current
R: the resistance of the
conductor in units of Ohms
V: the potential difference
measured across the
resistance in units of Volts
I: the current through the
resistance in units of
Amperes
Electric Measurement
V
I
R
Ohm’s Law
I = V /R
V=IR
R = V/I
Ohm's law states that the
current through a conductor
between two points is directly
proportional to the potential
difference or voltage across the
two points, and inversely
proportional to the resistance
between them
Electric Measurement
Water Flow Analogy
Question?
In a clear day, a 10 inch by 8 inch solar panel
can generate a voltage potential of 20 volts at
the output. If a 40 Ohms resistance is
connected to it, what is the electric current
flowing through the circuit?
CCD
Charge-Coupled Device: A sensor of
electromagnetic radiation
Biosensor
for imaging
CCD sensor
for imaging:
the eyes of
digital world
CCD
http://en.wikipedia.org/wiki/Charge-coupled_device
•CCD was invented in 1969 at AT&T Bell Labs by
Willard Boyle and George E. Smith
•In 1969, the CCD was a simple 8-bit shift register
•In 1974, a 2-D 100 X 100 pixel device
•Sony managed to mass produce CCDs for
camcorders
•In 2009, Boyle and Smith were awarded the
Nobel Prize for Physics for their work on CCD
CCD - Operation
•In a CCD for capturing images, there is a photoactive
layer (region) and a transmission layer (region).
•The photoactive region is an array of capacitor
(pixel); each capacitor accumulate an electric charge
proportional to the light intensity striking at that
location
•After the exposure to the array, a control circuit
causes each capacitor to transfer its contents to its
neighbor (operating as a shift register). The last
capacitor in the array dumps its charge into a reader,
which is converted to a readable voltage and saved.
CCD - Operation
Charge Accumulation: more photons, more
charges during the exposure time
CCD - Operation
Charge Transfer: The charge packets (electrons, blue)
are collected in potential wells (yellow) created by applying
positive voltage at the gate electrodes (G). Applying
positive voltage to the gate electrode in the correct
sequence transfers the charge
http://en.wikipedia.org/wiki/Charge-coupled_device
CCD - Operation
Color CCD: Digital color cameras generally use a Bayer
mask over the CCD. Each square of four pixels has one
filtered red, one blue, and two green. The result of this is
that luminance information is collected at every pixel, but
the color resolution is lower than the luminance resolution
Biosensor
http://en.wikipedia.org/wiki/Biosensor
A biosensor is an analytical device for the detection of an
analyte that combines a biological component with a
physico-chemical detector component. It consists of 3 parts
1. the sensitive biological element (biological material (e.g.
tissue, microorganisms, cell receptors, enzymes,
antibodies, nucleic acids))
2. the detector element that transforms the signal resulting
from the interaction of the analyte with the biological
element into another signal that can be more easily
measured and quantified
3. associated electronics or signal processors that are
primarily responsible for the display of the results in a
user-friendly way
Exp: blood glucose biosensor
Sensor Types
•Imaging: CCD detectors
•Temperature
•Pressure
•Chemical Sensors:
•Carbon Monoxide Detector
•Motions Sensors
•Biosensor
•………
Summary
•Nearly all sensors use or can use electrical measurements
•Ohm's law governs electrical circuits
•Sensors can be broken into different classes, depending
on what things they measure, e.g., CCD, glucose sensor
•Good sensors measure what they are suppose to
measure, and not disturb the experiments
Sensor Limitations
(Sep. 16, 2010)
Objectives
•What is lower and upper limits?
•In terms of magnitude or intensity
•What is resolution?
•In terms of space and time
•Calibration issues
•linearity
Lower Limit
•Any measurement below this point is recorded
as just zero or noise
•All sensors have lower limits
•Temperature sensors have minimum
temperature
•CCD has a minimum level of light, which is
considered “not a noise”
Upper Limit
•Any measurement beyond this point is recorded as
the same value, or zero, or as corrupted
•It causes saturation, which should be avoided for
most of data collection
•Examples
•Temperatures larger 140 degrees are recorded as 140
degrees
•Wind speeds larger than 250 mph cannot be recorded
•Voltages greater than 10,000 volts cause the voltmeter
to fail
•SOHO satellite records the magnetic field on the Sun
larger than 3000 Gauss as zero, a big headache for
scientists.
Resolution in Time
•Also called “temporal resolution”
•Sensors take time to measure a signal
•Time for the signal to built up
•Electric charges to build up in CCD pixels,
e.g., the exposure time
•Time to read the data from the censor
•CCD readout time, e.g., 1 second
Resolution in Space
•Also called “spatial resolution”
•It describes the detail an image holds. Higher
resolution means more image details, e.g.,
number of pixels, dots per inch
•Monitors
•CCD cameras and camcorders
•Projectors
•Printers
Resolution in Levels
•Limits the precision of measurements
•It is limited by the sensitivity of sensors
•It is also limited by the resolution of the Analog to
Digital Converter (ADC)
•Number of bits used to store data
•8 bits: 256 levels
•10 bits: 1024 levels
•12 bits: 4096 levels
ADC
http://en.wikipedia.org/wiki/Analog-to-digitanl_converter
•An analog-to-digital converter (abbreviated ADC
is a device that converts a continuous quantity to a
discrete digital number
•Typically, it converts an input analog voltage (or
current to a digital number proportional to the
magnitude of the voltage or current.
Resolution in Levels
A typical home outdoor electrical temperature sensors
has an operational range between 140 and -40 degree in
Fahrenheit
, and use 12 bits
to convert(abbreviated
the data. WhatADC
is
•An
analog-to-digital
converter
thea resolution
of the
detector?
is
device that
converts
a continuous quantity to a
I max  I min
R
N
I max  140
I min  40
N  212  4096
R  0.044  F
What is the resolution if the ADC is 8 bits?
Linearity
•A desirable feature of a sensor is Linearity: the
measured signal is proportional to the input signal
Linear
Non-Linear
Data Limitations
-Signal and Noise
(Sep. 29, 2010)
Motivation
The data we get from sensors is not perfect. Every
measurement has noise and limitations, both from
the sensor and other random factors. We need to
understand noise, in order to fully understand
scientific measurements
What Happens to Stars?
Constellation Orion
in the Country
Constellation Orion
in the City
Signal to Noise Ratio (SNR)
http://www.dspguide.com/ch25/3.htm
Objectives
•Understand what are signal, noise and background.
•Do calculation of SNR (Signal to Noise Ratio)
Signal, Noise and Background
A Measurement
Signal, Noise and Background
•The background is an average of the values when
there is no signal
•The noise describes variations in the background
•The signal is the value of the peak MINUS the
background
Signal to Noise Ratio (SNR)
The signal to noise ratio (SNR) is the ratio
between the signal and the noise
In previous figure:
•The peak is about 27500
•The background is about 25000
•The signal is 2500 = 27500-25000
•The noise is about 150
•The signal to noise ratio = 2500/150 = 16.6
Signal to Noise Ratio (SNR)
•Most noise in scientific measurements follows
Poisson distribution
•Poisson noise is equal to the square root of the
background level
•A SNR of 3 is often required as a minimum to
ensure that the measurement is a true signal
The End
(Sep. 28, 2010)
Note: In the week of Sep. 20 to Sep. 24, 2010,
Joe made a tutorial on Matlab.