Lec 5 Sensors for Mobile Robots

Importance of sensorsMost used sensors for mobile robotsInterfacing electronics

An autonomous mobile robot moving around in an unstructured and dynamically changing environment must use various sensors to obtain information about its surroundings to determine its action and accomplish assigned task .

Importance of sensors

From the viewpoint of control, sensor data are required to feedback the actual state of the plant or process, such that the desired state can be achieved. Depending on how the feedback loop is closed, the behavior of robot needs various types of sensor information.

C P

S

Camera

IVaVd△F

Fi

Fa

+

_

Ts = 0.01sec

Ts = 0.5sec

Encoder

Types of robotic sensors

Internal sensors: sensors used for robots to understand the state of itself.e.g. sensors for self-localization (pose estimation),

sensors for motor servo control (shaft encoders), gyros, limit switches, etc

External sensors: sensors used for robots to understand the external world.e.g. ultrasonic range sensors, infrared sensors, laser scanner (range finder), image sensor, microphone, touch sensor, gas sensor, etcOn-board sensors: sensors that are installed on board the robot.

Types of robotic sensors (Cont.)

Intelligent environment: intelligent devices that are installed (deployed) in the environment. These devices can measure the changes of environment as well as robot pose and provide the information to the robot.e.g. Ultrasonic sensor network

Wi-Fi, ZigBee sensor networkRFID systemnetworked cameras

Human friendly room of KAIST, Korea

Elderly-care robot of KAIST

A general connection for sensor inputs

Digital inputs

Switch is released (open)+5v logic1

switch is pressed (closed)0v logic 0

+5v+5v

GND

Sensor signal

+5v

Vsens: connects to either digital input circuitry or analog input circuitry.

Nc No C Touch sensor:Logic high = not pressedLogic low = pressed

Using touch sensors to turn around a corner

Light sensor (Analog inputs)

R2

R1

Vout

Vin

inout VRR

RV21

2+

=

Connecting a resistive sensor

47k

Vsens

Rphoto

+5v

Single photocell circuit1. Brightly illuminated

Rphoto : smallVsens : close to 0

2.DarkRphoto : largeVsens : close to 5V

Connecting a photocell,Analog input, need an A/D converter

Use of light sensors

• Light seeking experiment

Differential photocell sensor

Vsens

Rphoto1

Rphoto2

+5v

12

15

photophoto

photo

RR

RVsens

+=

choose photocell that haves a relatively small dark resistance (i.e. about 10k)

Ω≈+ kkk

25.847

110

1

Wire a pair of photocells to produce a voltage that represents the difference in value between the two.

Resistive position sensors

VL

A

B

d

VsdT

Eth=Vs x

Rth=Rp x(1-x)

RL

xdd

T=

LthL

sL RRRxVV+

=

RLRP

Photo diodes

Photo diode/ photo transistors vs. photocells(CdS) photo resistors

- rapid response time- more sensitive to small level of

light (e.g. from LED element)

Near-infrared proximity sensor

• ~880nm from LED element (Emitter)These sensors typically do not return actual distance to an object, they signify whether or not something is present within the cone of detection.These types of sensors usually have much narrower beam width than sonar range finders.

IR photo diode

Sonar

Laser range finder

Reflective optosensors (active sensors)

Light is reflective off a surface into a detector element, consisting of an emitter LED and a detector photo diode

emitter detectorTransmitter LED and Receiver photo diode

Ambient light is a problem

+5v

i

330Ω47k

Vsens

The more light received by the phototransistor, the more current flows. This creates a voltage drop in the 47K pull-up resistor. This voltage drop is reflected in a smaller voltage on the Vsenssignal line. i =0.01mAV= i R = 0.01*10-3*47*103= 0.47VVsens= 5 - 0.47 = 4.53V

Measurement of light intensity

Imax=100mA

50Ω

+5v

2k

50k

Vcc

2N2222

100Ω

Vo

Vcc

Light sensor interface circuitry

Light emitting LEDReceiving photo transistor

Break-beam sensors

Vsens

+5v

47k

330Ω

GND

+5v

The break-beam device consists of a light-emitting component aimed at a light-detecting component. When an opaque object comes between the emitter and detector, the beam of light is occluded, and the output of the detector changes

Modulated IR signal

IR detectors respond to a modulated carrier sent by the near-infrared LED. The programmer is responsible for blinking the LED in certain pattern such that the detector will respond.

LM538Gndsignal+5v

Stabilized voltage

Modulated signal

40k carrier

Sharp GP1U 52X near-infrared proximity detector

Light is modulated so that intensity of ambient light will not affect the sensitivity of module.Liteon IR330, 38KHz 940nm

Single Infrared SignalTransmit single infrared signal

Receive single infrared signal660μs

26μs

660μs

Continuous Infrared Signal

Transmitted signal660μs continuous pulse

and 1800 μs logic low.

Received signal660μs logic Low and

1800 μs logic High.

660μs

26μs

0V

5V

1800 μs 1800 μs

660 μs

1800 μs

0V 1800 μs 1800 μs

660 μs

1800 μs

5V

Actual

time

600μs 600μs

ideal

(strong)

(weak)

Transmitted signal

Module output

Delay 3 blinks

Delay 8-9 blinks

Pulse width different, depending on the strength of signal don’tknow when to read state of signal Received signal is integrated weak

signal takes longer to integrate to comparator’s trip point

Actual received signal

Start 1 1 1 0 1 0 0 0 stopBit form

Start 0 1

Bit interval

2200μs1560μs1120μs

.. .. .. ..

550μs 550μs 550μs 550μs

How to read a digital code?

Coded Infrared SignalLow interval of transmitted signal

Start bit :1120 μs Logic 1 :1560 μs Logic 0 :2200 μs

Received signal code

550 μs

1120 μs 1560 μs 2200 μs

Start 1 0

26μs

550μs

550 μs

1120 μs 1560 μs 2200 μs

Start 1 0

Decoding of infrared signal

Look at elapsed time between falling edges to determine if a bit is “1” or “0”.

Ex: 1.67ms --> start2.11ms --> 12.75ms --> 0

Sony unit: 1.2ms = 0

1.8ms = 1

Shield of light

IR modules receive light(signals) from wide angles need to narrow angle of received signal to leave it on it.

Window tube (shield )

(Side view)(Front view)

Check point #3Homing: need to recognize a digital code for distinguish robot’s home.Basic Stamp: can use COUNT command to count pulses (1-0-1)COUNT Pin, Period, VariablePeriod: time period that cycles are counted for

variable:where count is stored

During contest, goal will be randomly assigned needto be able to quickly change robot’s goal

Ultrasonic ranging system

SONAR (Polaroid 6500)Measures the time of flight for a sonar “chirp” to bounce off a target and return to the sonar (Echo). More accurate than IR, giving a distance to a direction, possible to make a scanning of the environment.

Echoreceived

Generatechirp

transmitter

receiver

speaker

microphone

countduration

Processed echo

Transmission .. ..

INIT(trigger)

Echo

60ms 40ms

Polaroid 6500 sonar ranging system

Characteristics of Sonar system

A transducer which acts as both the speaker and microphoneA circuit boardchirp frequency 50kHz (49.4kHz), 16 cyclesBlanking of signal in the beginning for 2.38ms (40cm)

d(cm)40

Characteristics of ultrasonic ranging system

22.5°

45°

S0S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

Limitations

Specular reflection

beam angle = 22.5°

Bean angle of the ultrasonic transducer causes measurement error

Beam angle of ultrasonic transducers

Optical distance sensing detector with Sharp GP2D02

PSD(1-D)Position sensitive detector (photo diode plus ckt)Infrared emitter (laser diode)

An IR emitter LED projects a spot of modulated light onto the target surface.

The light is focused on a 1D PSD element.

The angle of the incoming light will change depending on the distance between the target surface and the sensor.

The spot of light will project to different point along the PSD.

The location of the light spot on PSD is a function of distance.

Other types of sensors for environment detection

Pyroelectric sensor can detect the existence of heat generated by human body (1~2 μm

infrared)Cameraa. CCDb. CMOSc. PTZImage frames contain much information of the immediate environment. It’s therefore required to segment useful features and extract information need for task execution.

image acquisition + image processing + recognition algorithm ( visual servoing ,visual tracking )

Laser Sensor for Navigation

Laser scanner Can get accurate environment distance, fast scanning, powerful, but expensive. 75Hz, 81m (10mm resolution), 0~ 180Deg (0.25/0.5/1 Deg resolution)RS 232,RS 422.

Hardware

Laser scanner

IPC

Motion control card

Laser scanner

Laser scanner data

Experimental result

Blue: Robot trajectory recorded by the proposed methodRed: Robot trajectory recorded by odometer only

-300 -200 -100 0 100 200 3000

100

200

300

400

500

600

X(cm)

Y(c

m)

Localization System

Odometer

Experimental result

-500 -400 -300 -200 -100 0 100 200 300 400 5000

100

200

300

400

500

600

700

800

900

1000

X(cm)

Y(c

m)

Robot trajectory recorded by the proposed method

Real-Time Imaging System

TI TMS320C6416 DSK

CMOS Sensor ICM205B Frame Buffer

AL422B

FLEX10K70RC240

Eye-in-Hand ConfigurationThe robot is equipped a 3 degree-of-freedom manipulator to grasp objects.Eye-in-hand configuration for camera mounting.Real-time robotic vision system for image processing.Foot and gripper motion control cards to activate motors.

Foot and gripperMotion control card

IPC

BatteryBattery

CMOS sensor board

Real-time robotic vision system

Power supply (DC-DC converter)