Sensors Joe Ross Team 330 1

What is a Sensor A sensor measures a physical quantity and converts it into a signal which can be read by an instrument. What are human senses? – Touch – Sight – Hearing – Taste – Smell 2

Why we need Sensors A robot is a mechanical device that can perform tasks automatically. In order for a robot to do something automatically, it needs to know its environment 3

What can sensors measure Angle of an arm Speed of a wheel Distance to an object Is something touching Pressure of air in pneumatics system Distance traveled 4

Types of sensors Digital – On / Off only – Two Voltage States (typically 0v and 5v for FRC) Analog – Many values based on variable voltage Digital may be used to encode analog information – bit stream – pulse time

Types of sensors – I2C I2C is a bus that allows multiple sensors to be connected using only 2 wires – SCL – clock – SDA – data Each I2C device is assigned an address – Read the datasheet for your sensor to find out how to assign address I2C Master (cRIO) sends or requests data using device address – only device with that address responds Libraries included in Java/C++/LabVIEW 6

Types of sensors - SPI Another bus, using 4 wires – SCLK – clock (shared) – MOSI -> SDI – Data Input (shared) – MISO -> SDO – Data Output (shared) – SS -> CS – Chip Select (unique per device) Chip Select is used to tell which device is chosen Implemented using DIO pins Libraries included in C++ and LabVIEW – Java will be added for

Switch Digital Sensor Uses a mechanical action to make (or break) an electrical connection – Momentary – electrical connection is maintained only when switch is pressed – Latching – electrical connection is maintained after switch is released Usage: Limit motion of a device, Detect a ball Programming: Digital Input Cost: $1-$5 8

Optical Sensor/Switch Digital Sensor Emits light and senses the emitted light – One Piece – Bounce light off object – Two Piece – Break Beam Key Factors: Distance, adjustable gain Uses: Detect Objects, Detect light/dark (follow lines) Manufacturers: Banner, Rockwell, Sharp Programming: Digital Input Cost: $10 - $50 9

Hall Effect Sensor / Switch Digital Sensor Detects the presence of a magnet – Available in latching and momentary Uses: Detect position of robot parts, measure velocity by measuring time between pulses Key factors: Sensitivity Manufacturers: Allegro Programming: Digital Input or Counter 10

Potentiometer Analog Sensor Uses a variable resistor to provide a variable voltage 11  The position of the knob (wiper) determines the resistance.  Connect the wiper of the potentiometer to something on the robot, and you can measure the position  Key factors: number of turns, linearity, rotation rate  Manufacturers: RadioShack, Vishay, Bourns  Programming: Analog Input

Encoder Digital Sensor with 2 outputs Optical sensors detect transitions from a coded disk Count the number of transitions for distance Compare phase of both outputs for direction Measure length of pulses for speed Programming: Encoder 12 A B  Key Factors: Counts per Revolution (CPR), rotation rate  Manufacturers: US Digital, Grayhill, Bourns

Gear Tooth Sensor Digital Sensor Uses Hall Effect to detect gear teeth – The Hall effect is the creation of an electrical voltage in a magnetic field Must be used with a ferrous gear (eg steel) Requires precise mounting within a few mm of gear One pulse per gear tooth – Count pulses for distance – Measure time between pulses for speed – Only some gear tooth sensors detect direction Manufacturers: Allegro Microsystems Programming: Counter 13

Pot, Encoder, GT Sensor compared PotentiometerEncoderGear Tooth Cost$10$50+$10-$20 Robustness Accuracy Mounting Tolerance Direction Absolute Speed Supported by cRIO Supported by Jaguar 14

Gyro Analog Sensor or I2C/SPI Uses a micro machined vibrating wheel to detect rotation Output is the rate of rotation The rate of rotation is integrated (added) to determine rotation angle Key factors: maximum rate of rotation, drift, sensitivity Usage: Determine angle of robot base Manufacturers: Analog Devices, ST Programming: Gyro Digital Gyros are also available using I2C or SPI. – Not recommended unless they also do motion control 15

Accelerometer Analog Sensor or I2C/SPI Measures acceleration – Due to robot motion – Gravity Can be integrated for Velocity. Double integration for position doesn’t work due to noise Key factors: maximum acceleration, noise, sensitivity Usage: Detect Tipping Manufacturers: Analog Devices, ST, Bosch Programming: Accelerometer or ADXL345_I2C 16

Ultrasonic Uses sound waves to measure the distance to an object (like Bats) Output may be analog, or a pulse Do not know what object is being detected Usage: Detect distance to goal, avoid collisions in autonomous Manufacturers: Vex, Maxbotix, DevanTech Programming: Analog Input or Ultrasonic or Counter 17

Current Sensor Analog Sensor Measures the current flowing Hall effect – Place sensor near wire Resistive – Route current through sensor Usage: – Detect high current draw – Detect changes in current (object in roller, arm at limit) Manufacturers: TI (Jaguar), Phidgets, Allegro, Many others Programming: Analog Channel, CANJaguar 18

Where to buy Sensors Included in Kit of Parts – Encoders, gyro, accelerometer (typically) Local (switches, potentiometers) – Radioshack – Frys – Microcenter Hobby Robotics websites – Sparkfun.com – Pololu.com – Phidgets.com – Vexrobotics.com General Electronics Websites – Digikey.com – Newark.com 19

LabVIEW examples 20

LabVIEW Examples - List 21

330’s Sensors Gyro – AndyMark AM-2067 (Analog Devices) Encoders – Us Digital E4P Potentiometer – Vishay 534B1103JCB10 Hall Effect – Allegro A1101EUA-T IR (optical) – Sharp GP2Y0D805Z0F 22

BACKUP 23

Pressure Switch Digital Sensor Measures the pressure in the pneumatics system Uses hysteresis – Opens at 95 PSI – Closes at 115 PSI Usage: Required for pneumatics Close (Compressor Off) Open (Compressor On)

Camera Find Game piece (2005) Find Fixed Goal (2006, 10, 11, 12, 13) Find Moveable Goal (2007, 09) FIRST provides software to detect target and return size of target and position of target in image – Size of target is proportional to distance from target – Position (left / right) is proportional to angle to target – Position (up / down) is proportional to distance from target 25

Tips for finding retro-reflective targets Use a set of LEDs to illuminate target – Retro-reflective means that light bounces back toward source, will not be confused by multiple Robot’s LEDs – Camera will see the target illuminated in the color of the LEDs Want to minimize the amount of extraneous things the camera sees – Darken image by using hold exposure setting while the camera sees a bright light – Reduces processing because vision algorithm is looking at less data – Reduces bandwidth because images with less clutter compress better 26

Kinect Combines a visible camera, infrared camera and infrared LEDs Provides a visible image and a depth map 27  FIRST provides software to send Skelton data to Robot.  Use on Robot requires a secondary computer