Distance Sensors

Kyle Zhang

Jeff Peil

Kristian Kalaj

Agenda

Ultrasonic Sensors• How They Work• Interfacing• Things to Be Careful About• Examples

IR Sensors• How They Work• Behavioral Problems• Examples

Ultrasonic vs. IR Sensors

How Ultrasonics Work

• Uses a transducer to convert electrical energy into higher frequency sound waves (ultrasonic)

• When the echo of the emitted waves returns, it converts the sound back into electrical energy and evaluates

http://www.education.rec.ri.cmu.edu/products/nxt_video_trainer2/resources/helpers/nxt_sensors/ultrasonic.html

How Ultrasonics Work (cont..)Calculates the time interval

between the sound wave being transmitted to when it is received

and outputs a matching pwm signal to an FPGA

Distance = (T * speed of sound) / 2

Speed of sound varies depending on environment

How We Used Ultrasonics In Lab

• A signal is raised high when the sound is sent

• The same signal is lowered when the sensor receives the echo of that sound

• Counted the # of clock cycles the signal was high to calculate distance

Another Way to Interface with Ultrasonics

• An ultrasonic sensor uses the IN/OUT port of an FPGA

• Requirements of a typical sensoro 5 volt power supply and ground

o One pin from the FPGA that has the ability to work as input and output (example: GPIO)

Things to Be Careful About

• Limited Rangeo ex: 28015-ND sensor is 20mm-3m

• Mechanical Echoo Must be filtered out for data to be

accurate

Things to Be Careful About (cont.)

• Since we are reflecting a sound wave, certain materials may not reflect as wello Density

o Sound absorption

• Shape of a surface may affect reflection/detectiono Sphere

o Angles

http://www.societyofrobots.com/member_tutorials/node/71

Examples of Ultrasonic Sensors

• RB-Dfr-11o Price: $14.50

o Range: 4 cm to 5m http://www.robotshop.com/media/files/pdf/datasheet-sen0001.pdf

• 28015-NDo Price: $29.99

o Range: 20 mm to 3 mhttp://www.digikey.com/product-detail/en/28015/28015-ND/1774419

• US-T50/R25o Price: $300

o Range: 0 - 50 cmhttp://www.wolfautomation.com/assets/15/US-T50R25.pdf

Ultrasonic Shortcomings

Deficiencies

Can be quite expensive (~$25)

Must be careful of what you are trying to

detect

Reflection, echoes, materials

Absorbent materials (ex: sponges) don’t

work well with Ultrasonic sensors Wave propagation

property

How IR Sensors Work

• Emits beam of infrared light, detects reflection

• Susceptible to quantity of light in environment

• Measures angle of reflection, not time

• Outputs an appropriate voltage to represent the distance, but not always

http://www.societyofrobots.com/member_tutorials/node/71

IR Behavior Problems

• Bad behavior for very close objects

• After ‘initial spike,’ non-linear behavior then linear behavior

https://engineering.purdue.edu/ece477/Webs/S11-Grp03/nb/myersar.html

How to Deal with Bad Behavior

• Make it stand in the corner!o But seriously……

o Reposition sensor to compensate

Types of IR Sensors

• Different Types

• Binary/Proximityo Outputs either high or low

o Cheap

o Good at detecting proximity/threshold

• Multiple Bit/Rangingo Output actual distance from sensor

Analog or digital format

o More expensive but still relatively cheap

Sharp IR Sensor Graph

Examples of IR Sensors• TSOP38238

o Price: $1.95

o Range: N/Ahttp://www.adafruit.com/products/157#Description

• SHARP GP2Y0A21YK0Fo Price: $11.75

o Range: 10cm - 80cmhttp://www.robotshop.com/en/sharp-gp2y0a21yk0f-ir-range-sensor.html

• SHARP GP2Y0A02YKo Price: $15.95

o Range: 20 cm-150 cmhttp://www.adafruit.com/products/1031?gclid=CKemybT7t7oCFeYWMgod1XgAHQ

IR Pros and Cons

• Pros○ Cheap○ Outputs voltage based on distance○ Precise detection of objects

○ Cons○ Outputs voltage based on distance

■ Non-linear behavior○ Susceptible to light influences

Conclusion Ultrasonic vs. IR

• Use Ultrasonics when:o You care about

distance accuracy

o There will be light influences

o You have money to spend

• Use IRs when:o You want to detect

movement or thresholds

o You are on a budget

o You need precision in what you are detecting

Further Reading

• Infrared vs. Ultrasonico http://www.societyofrobots.com/member_tutorials/n

ode/71

• Parallax Sensor and General Ultrasonic Usage Tipso http://dc254.4shared.com/doc/fiS_Vzyu/preview.htm

• IR Tutorialso http://www.intorobotics.com/use-infraredir-sensors-t

utorials/

Questions?