Chem 454 january 14, 2004

Sensors and transducers prior notes: statistics/curve fitting

Start with brief project (covered in class)

The proverbial "needle in a haystack" -- device a plan to find it

Discussion-- if possible, interview first and get the parameters

(the difference, in my experience, between architects and engineers)

archeologist might be looking for a bone needle or a bronze needle

size of needle and haystack will be important

is it really a needle or is this a metaphor (e.g., wire in the feed)

is there a needle (perhaps confirm none present) or more than 1 (scan all)

one haystack? quality control issue handled by sampling vs test all

do you want the needle (burn they hay?)

do you want the hay needle free (cattle feed)

would you have space to sift and move the haystack?

is it cheaper to buy new, safe hay?

how much time, money is available (may be cheaper to buy new feed)

Obvious tools-- X-Ray, magnets, ultrasound, sift, visual, tender fingers (the ouch test)

Mars Exploration (covered in class)

Difficult project, limited budget

Choose parameters to study

compromise with cost, weight, ruggedness, maneuverability, lifetime to failure

properties that we'd like to study

political and PR choices (need to make an impact vs. pure science issues)

not likely to analyze with classical glassware and solution

Data Domains (will cover Friday 1-16)

analog continuous time | digital discrete time

tiny changes limited changes (fixed digits)

resolution (set by device)) resolution (set by digits allowed)

temp, pressure, volume, voltage, wavelength, current, concentration, pH, mass ....

time (plays a more complex role, continuous but becomes discrete when measuring)

electrical properties are easily interconverted (voltage, current, charge, resistance)

Transducer maps data from one domain into another

temp to voltage (thermocouple), intensity to current (pmt), mass to position (use spring)

devices often convert A --> position (distance, angle)

thermometer, needle type volt meter, calipers, graduated cylinder

devices that transform A --> electrical quantity are often called Sensors

thermocouple, IC pressuregauge, phototransitor, ...

Machinists Calipers, Micrometer --Exercise-- (covered in class)

(length-->position; perhaps position to angle, some to LCD display)

measure diameter of the ball, both devices (use inches)

Resolution (readability)

Precision (reproducibility)

device, sample, technique

Accuracy (set zero? actual accuracy )

rely on manufacturer, measure test standards, compare with others

Accuracy-- absolute vs relative

+ 0.001 " absolute 6 inch range

5.012 + 0.001 = 1 part in 5000 or 0.02% certainly 4 significant figures

1.022 + 0.001 = 1 part in 1000 or 0.1%

0.012 + 0.001 = 1 part in 12 or about 10%

usually specified as % of FS (full scale)

Analog to Digital Conversion -- analog to digital/numerical domain (cover on Friday 1-16)

We generally can't store or compute using analog property (we use digits)

Calibrated scale, eye, brain can convert distance/angle to a numerical value

we can interpolate for last digit

scale is generally linear (not always-- Spec 20 meters have log scale)

We have devices (multiple parts) that function as analog to digital converters

digital watch, electronic balance, digital voltmeter, digital calipers, digital therm.

direct transformation of analog property to digital display

Otherwise we can use sensor and an electronic Analog to Digital Converter

the simplest is a digital voltmeter or multimeter (dvm, dmm)

typically the result is a visual display (LCD screen typically)

DVM output--

you may be able to select the range (internal amplifier or divider)

the digits are fixed ... typically +1.99 +1.999 +1.9999

these are called 2-1/2 3-1/2 and 4-1/2 digit displays

3-1/2 dmm available now $2.99

ADC module

inside computer or other electronic device (heart of DVM)

converts voltage (analog) to digital electronic signal (chapter 2)

typically 8, 10, 12, 16 bits (signal wires)

can count to 0,1, .. 2N-1

to 255, to 1023, to 4096, to 65,535 (often 1K, 4K, 64K)

speed varies -- some convert in 0.1 seconds, others in <10 microseconds

PIC-- I'm working with some microprocessor chips

limited computer, on board program and data memory (burn program)

has 12 bit ADC, 10 input channels; 2 DAC output channels

4 separate timers, up to 64 binary input/output lines

standard computer communications ports .............$8-15 each

Digital to Analog Conversion

setting a dial, printed scale -- rotate a potentiometer -- serves as a voltage divider

numerical value becomes an analog voltage

DAC module -- computer software computes a value, converts to voltage, send it out

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