1. ECB4042 ISDP Semester I SENSORS and interfacing
may 2015 semester

2. LESSON outcomes At the end of this session, you should be able to:
Describe the various types of sensors commonly used in mobile robots.
Select appropriate types of sensors to be used to achieve the desired goals.
Design and implement appropriate strategies for mobile robot navigation and path planning.

3. Autonomous Mobile Robots
The three key questions in Mobile Robotics
Where am I ?
Where am I going ?
How do I get there ?
To answer these questions the robot has to
have a model of the environment (given or autonomously built)
perceive and analyze the environment
find its position within the environment
plan and execute the movement

4. General Control Scheme for Mobile Robot Systems
Knowledge,
Mission
Data Base
Commands
Localization
Cognition
"Position"
Map Building
Path Planning
Global Map
Environment Model
Path
Local Map
Information
Path
Extraction
Execution
Perception
Raw data
Actuator Commands
Motion Control
Sensing
Acting
Real World
Environment

5. SENSORS FOR MOBILE ROBOTS
Ultra-sonic radar
Transmitter
receiver
touch sensors by switches
Approximate sensors by infrared red light
Path following sensors
Sound sensors

6. Sensor/Actuator Systems
Peripheral Processing
Actuator
Sensor
Peripheral Processing
Fuser
Central
Processing

7. What is a Sensor?
A device that captures information about the environment and converts it into a usable data signal
Human example:
Temperature information is captured at the posterior root of a sensory nerve and passed via an electro-chemical signal to the brain
Computer Example:
A LM34 chip converts the ambient temperature of its case into an analog voltage (0-5 volts) which an ADC turns into a binary signal usable by a computer

8. What is an Actuator?
A device that converts data signals into a useful modification of the environment
Human example:
The brain sends an electro-chemical signal down a motor pathway to a peripheral nerve causing a muscle to contract
Computer Example:
A processor generates a PWM signal (and a set of related logic signals) to an LMD18200 chip which converts these to a control voltage causing a DC motor to turn at a set speed in a set direction

9. Sensors… Detect what? Use what? Obstacle detection (without contact)
Ultra Sonic
Infra red
Distance
Proximity Sensors e.g. IR.
(Also Electromagnetic, capacitive, etc.)
Optical Sensors
Laser Sensors
Color detection
Color Sensor, Optical Sensor, Camera
Boundary Detection… ??

10. Sensors Primary Infrared beacons Infrared range detectors
Ultrasonic range detectors
Secondary
Momentary contact sensors
Reed switches
Mercury switches
Others …

11. Sensor Types Strain and pressure Applications Load Pressure
Floors, doors, windows
Scales

12. Sensor Types Position, direction, distance and motion Applications
Accelerometer
Gyroscopes
Compass
Motion
Applications
Security
Locator
Tracking

13. ACCELEROMETERS adxl202 2-axis accelerometer
Mems technology provides precision mechanical electrical devices
ADXL202 outputs convenient PWM output whose duty cycle is proportional to acceleration
Cost about 30$ - easy to interface to PIC

14. Sensor Types Light, radiation and temperature Applications Camera
Light sensors
Radiation sensors
Temperature sensors
Applications
Security, location, tracking
Health safety
Energy efficiency

15. LDR
Also known as Light Dependent Resistors / photoconductor / photoresistor / photocell Component that has a resistance that decreases as intensity of light hitting the surface increases

16. LDR Are typically made from cadmum-sulphide (CdS)
Cheap but not accurate
Good at detecting changes in light levels whether it is ‘light’ or ‘dark’.
Calibration for each unit required to accurately measure light levels
The material is a high resistance semiconductor that has the same spectral response as the human eye.
It is a photoconductive material

17. LDR
As a photoconductive material, the photons (from light) hitting the surface of the material with sufficient energy would release electrons from its atomic bonds. With the release of an electron, a positive ‘hole’ would be created. Therefore, when a voltage is applied across the terminals of the LDR the free electrons would move in one direction and the holes in the opposite direction. The higher intensity of light, the more electron-hole pairs are created and thus lowers the resistance of the LDR and allows higher current to flow through.

18. Analog Distance Sensor
Analog distance sensor , as by its name suggests, is a sensor that is able to measure the distance of an object in front of the sensor. This sensor is actually an IR (infra-red) sensor that detects the amount of IR radiation.

19. IR Sensor Working Mechanism
An IR sensor is a device that consists of a pair of IR LED and phototransistor where
The LED emits IR radiation
The amount or intensity of the IR radiation received by the transistor controls the output of the sensor.

20. IR Sensor Working Mechanism
There are two primary configurations of IR sensor
Direct incidence
LED and phototransistor are opposite
each other.
Almost all radiation received by
phototransistor received.
Indirect incidence
LED and phototransistor placed side by
side.
Radiation received is dependent on
the reflected radiation
Surface type affects such as reflective
or non-reflective surface

21. Typical Applications Proximity or Distance Sensors
Uses the reflective indirect incidence configuration
Line Follower Robots
Detects the colour of the surface beneath to find the line beneath the robot.
Item Counter
Uses the direct incidence configuration.
Where if an item obstructs the line of IR radiation, a signal could be used to increment a counter.

22. Analog Distance Sensor
Sensor only provides an analog voltage output which is given by the (sample) graphs detailing the voltage vs distance characteristics.

23. Sensor Types Sound Applications Microphone Security Volume control
Speech recognition

24. Ultrasonic
Also known as ultrasonic transceivers or transducers where it both sends and receives signals. Works on the principle similar to radar or sonar that works by interpreting the echoes of sound waves

25. Characteristics of Ultrasonic Sensor
4 pin unit
Vcc – 5 V DC
Trig – Trigger (input signal)
Echo – Echo (Output signal)
GND – ground
Non- contact range detection
Operation not affected by sunlight or black material
But a soft material such as cloth can make detection difficult

26. Operation of Ultrasonic Sensor
To initiate measurement
A trigger signal with the following specification is needed :
High signal of 5V
Pulse width of 10 micro-seconds.
Internally , the sensor will transmit an 8 cycle ultrasonic burst of 40 kHz and wait for the reflected burst.
When sensor detects the reflected signal, output is set high.
The duration of that signal is dependent on distance of detected object.

27. Sensor Types Solids, liquids and gases Applications Humidity Rain
Carbon dioxide pH
Flow
Applications
Pool maintenance
Sprinkler efficiency
Security and health monitoring

28. Sensor Interfacing and Control

29. Motion control Power the wheels Speed control
Power method 1: Mechanical relay
Power method 2: Single direction Power Transistor
Power method 3: Dual direction H-bridge circuits
Speed control
Speed control 1: pulse code modulation
Speed control 2: by photo interrupters and Feedback
Optic isolation
legged robot by servo motors

30. Power method1: Direct Current DC (1A) motors by mechanical relay
driver
e.g.
ULA2001A
DC motor
Controller
relay

31. Power method 2: Direct Current DC(1A)motors by transistor (one direction)
Using a single power transistor 5V
High power
resistor
5 , 5W
560, 1W
Controller
output
Current
driver
or optical
isolator
TIP3055 +
DC motor -

32. Power method 3: Direct Current DC motors by H-bridge transistor circuit (Two-direction)5V
motor A
JP11 -> Motor
Forward : A=1, B=0
Backward:A=0,B=1
Stop :A=B B
GND

33. Speed control 1: Approximate speed control by Pulse Width Modulation
E.g. 2.5KHz
Off time
By switching the motor on/off at a fixed frequency (e.g. 2.5KHz) with different mark/space ration,
The power transmitted to the motor depends on the mark/space (on/off time) ratio.
Mark (motor on)
Space (motor off)
time
On-time

34. Positional control: Servo motors: e.g. Futaba FP-S148 RC

35. Interfacing circuits
Open collector is a common output found on IC devices
But it does not provide an output.
The output of this circuit forms an open circuit or a connection to the ground
The output consists of an external pull-up resistor to raise the output voltage when the transistor is turned off.
When the transistor is turned on, the voltage is driven to nearly 0 V.
In MOSFET circuits it also known as open-drain.
Applied in interfacing different voltage circuits

36. Inter-integrated Circuit (I2C)
Defined by Phillips for intercommunicating between devices in television sets and monitors.
Also known as IIC / I2C
Has capability of 3 speeds
Slow (under 100 KBps)
Fast (400 KBps)
High-speed ( 3.4 MBps)
Developed to allow microcontroller to communicate via a simple, synchronous, serial interface to devices such as ADC, DAC, temperature sensors, LCD displays, etc. etc.
Bus topology is multi-point.

37. I2C Bus Configuration (I)
Has a Master / Slave Relationship.
Master provides clock signal for data transfer
Has ability for multi-master bus
A synchronous serial data link operating at half duplex.
Physically, the interface consists of only 2 wires
Data signals
SDA – Serial Data
Control Signals
SCL – Serial clock
No chip select / slave select required to select slave device.

38. I2C Bus Configuration (II)
Typically , the connection for a I2C bus configuration is as follows :
I2C is a bi-directional protocol where data can be sent either direction on the bus by the slave or master.
SDA – data is transferred to and from the master.
SCL – synchronous clock controls when data is transmitted. Master controls the SCL

39. I2C Bus Configuration (III)
Electrical Signal level, the I2C interface has only two levels
Float ‘high’
Works by utilizing pull up resistors
If no device, pulling it low, it would ‘float high’.
Drive ‘low’
The microcontroller would drive the line to a low state
If no pull up resistors, Electrical state would float to an unknown state.
Number of devices are limited by bus capacitance where limit is at 400 pF

40. I2C Bus Configuration (III)
The value of pull up resistors are recommended for various speeds

41. Protocol
Master device sends a start condition (S) and controls clock signal
Master sends unique slave device address (7 bits)
Master sends read/write (R/W) bit
0 for slave to receive
1 for slave to transmit
Receiver sends acknowledge bit (ACK)
Transmitter (slave or master) transmits 1 byte of data

42. Protocol Receiver issues an ACK bit for byte received
Steps 5 and 6 are repeated if more data needs to be transmitted.
For write operation (master transmitting), master issues stop condition (P) after last byte of data.
For read operation (master receiving), master does not acknowledge final byte, just issues stop condition (P) to tell slave the transmission is done.