We deal with voltage signals Sensors convert environment data to electrical signals Output: Voltage Input: Time/Distance/Whatever Move Receiver around

Wall Detection Accurate Turning Cell Counting *Covered in Meeting #3 Powerpoint Rotary Encoder IR LED Sensors Gyro

Used to measure distance traveled Two major flavors LED emitter/receiver pair with optically marked disk Hall effect sensor with magnetically marked disk *Covered in Meeting #3 Powerpoint

One emitter/receiver pair can be used to detect walls in one direction Use infrared light to avoid visible ambient light interference Need to know if there are walls around mouse Emitter emits light Light reflects off wall, if there is a wall Receiver measures light intensity Determine presence of wall and distance to it LEDs emit light with luminance dependent on voltage/current. They work the other way too LEDs emit light with luminance dependent on voltage/current. They work the other way too

Sharp sensors Model GP2Y0A21YK Pre-made and assembled Very easy to use, but they are bulky Provides single analog output to use Slow response time Custom sensors Emitter-Receiver pair required Can be specially chosen for your needs Requires an IR LED Driver circuit: Darlington Driver IC Narrow emission angle is necessary in LEDs Potentially more accurate, but can be harder to calibrate

Two Important Characteristics: Emission AnglePower Density How sharp of an angle the LED emits light at How brightly the LED emits

Look for narrow emission angle Maximizes power efficiency Maximizes signal amplitude received by the receiver Also indicated by “viewing angle” Check datasheet for directivity graphs, which show intensity vs. angle This angle should be small Power Density Measures light intensity/solid angle In data sheets, often measure in mW/sr Higher means more light emitter/better

Match emitter wavelength with the receiver’s most sensitive wavelength Receivers also have directivity – minimize this Reduces interference from other IR light sources Most common wavelengths are 850 nm and 950 nm Check datasheets for this information This emitter emits most at 940 nm This receiver is most sensitive to 950 nm light IR LED emitters and receivers are often sold together and are wavelength-matched already Receivers also have directivity – Look for narrow ellipses

Need at least 3 pairs to detect walls in front and sides 4 or more is recommended for calibration 2 to detect side walls 2 pointing front to detect front walls and front wall alignment Used to straighten the mouse

Get distance to wall by reading the voltage output of the receiver Read the voltage output with MCU Relate Voltage output with distance LEDs are nonlinear Find the relation experimentally

Optional, but highly recommended Encoders can be used to measure angular velocity instead But they are less accurate and susceptible to wheels slipping Used to measure rotation of the mouse Needs stable power source Otherwise, lots of noise generated Gyros output angular velocity about a axis Mouse needs to turn a certain amount and begins turning Gyro reports angular velocity to MCU Integrate to get current turn angle Mouse continues turning until desired turn angle is achieved SMD style SMD on breakout board style

Only need to measure one axis Analog or digital output: MCU can handle either Most important characteristic: Range Typically measured in degrees/second What range you need depends on how fast your mouse spins degrees/second is plenty

Analog output: When not turning, voltage is half of maximum Turning clockwise/counterclockwise will change the output positively/negatively, depending on specific gyro Digital output: Uses a serial scheme such as I 2 C or SPI Same output as analog output, but numbers are encoded digitally (bits) For this analog gyro: Turning clockwise increases voltage Turning counterclockwise decreases voltage

Gyros measure angular velocity Integrate angular velocity to get angular position (which is more useful to know) Gyro output is recorded as discrete samples, so the integration is a summation

Relate voltage output summation with angle Can be done experimentally Non-ideality: Gyro drift Gyros do not measure angular velocity perfectly Integration of the velocity result in an error that increases linearly over time Measure the error and subtract it out We’ll cover how to do this next time

Algorithms! EAGLE tutorial next week Learn how to design printed circuit boards in EAGLE Hosted by our Projects Manager Julian Brown Nov. 14, 6 PM, location TBD