FSD – Fan Speed Detector

ME 102A Spring 2010May 4, 2010

Jennifer LewStephen Tu

Motivation

Fans cool a computer

Rotational Speed Cooling Capacity

CPU would almost instantly melt

CPU: http://hardforum.com/showthread.php?t=1423185Fan Grid: http://forums.legitreviews.com/about12716.html

How It Works

Idea: Use fan blades as an Optical Encoder By measuring the number of pulses in a fixed period of

time, can determine speed of blades

Rotary: http://hades.mech.northwestern.edu/index.php/Rotary_EncoderFan: http://computershopper.com/shoptalk/components/silverstone-fm83-cram-more-fan-into-the-same-space

Layout

Measurement

Power

Breadboard

Results – Characteristic Curve

Results – Calibrated Fans

Calibrated Fan

Vrequired

(V)Nominal RPM

ToleranceVsupplied

(V)sv RPM sRPM

1 5 735 ±5% 4.95368 0.0002340 726.415 0.843447

2 12 4300 ±1% 12.0584 9.18E-05 4305.82 5.83136

Measured range

Nominal RPM

95% Confidence Interval

Challenges

Issue Solution Drawback

Current limit on power supply

Use analog lab equipment that does not impose as stringent current limits.

Makes the UI less robust, due to less automation

Fluctuations in ambient light

Limit RPM readings to phototransistor voltages of magnitude greater than 1.5 mV. Below 1.5mV-> RPM=0

The threshold voltage is not appropriate for all conditions

Precision error Allow user to specify number of iterations before displaying the mean value of iterations

Increases accuracy but increases processing time