Chapter 17: Sensors
CEG2400 – Embedded system design
CEG2400 Ch17: Sensors (v.4b)
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Contents
1. Sensors for our robot:
2. Touch sensors
3. Range IR proximity sensors: Light sensors
4. Schmitt trigger and IR Path following sensors
5. Sound sensors
6. Ultra-sonic sensors
7. Temperature Sensor
8. Electromagnetic sensors: electronic compass
9. Accelerometers
10. Application example: Self balancing robot building
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1) Sensors for robot
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The classic robot with sensors
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Electronic
compass
•
Our robot
Microcontroller
LPC2131
Obstacle detector:
light-dependent
resistor (LDR), +
Infrared IR_LED
USB
RS232
serial
interface
Bottom
view
Speed encoder for
the D.C. motor:
IR-LED+
IR receiver
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2) Touch sensors
Key switch interfacing
•
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Key switch array: A keypad for PC
keyboard
and
mobile
phones
GPIO output port
•
GPIO input port
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Key scan algorithm
0
1
2
3
4
5
6
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• Key_scan_simple( )
8
9
A
B
• { unsigned char i,x,y;
C
D
E
F
• //handle denounce problem here, such as,
• // check if the previous key press has been released or not
• //scanning for a key press
• for(i=0;i<4;i++) //4 times
GPIO
• { y=1110(B); //Y(3),Y(2),Y(1),Y(0)=1110
4-bit
•
output y to GPIO output port
Output
port
•
X = read in GPIO input port
•
If (X not equal to 1111(B))
•
{ //a key has been depressed and detected,
•
break;
GPIO 4-bit Input port
•
}
•
rotate “y” 1 bit to left, i.e. 1110 will become 1101 etc.
• }
• Find which key has been depressed by current X and Y values.
• }
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Student ID: ___________,Date:_____________
Name: _______________
CENG2400 , Exercises for chapter 17 : sensors
• Exercise 1 :
a) Explain how to scan the key press using x and y?
b) If the key “9” is depressed , what are the values
of x and y when the key is detected?
a) How to handle multiple key presses.
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Tentacles
• Exercise 2: Add “tentacles” to your
project robot. Explain your design and
how to interface the GPIO etc.
• Suggest examples when such “tentacles”
may be useful.
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(3a) Schmitt trigger logic, Exercise 3:
• What is Schmitt trigger logic? Why 7414 inverter
do we need a Schmitt trigger inverter
here?
• What is the value of the input when
the output of 7414 changes from
High to low?
• What is the value of the input when
the output of 7414 changes from Low
to High?
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Schmitt triggered inverter (7414)
http://www.datasheetcatalog.net/de/datasheets_pdf/7/4/1/4/7414.shtml
•
For small frustrations of the
input , the frustrations will
be removed by the Schmitt
triggered inverter
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Example of using Schmitt triggered
non-inverting buffer
• E.g. in IR motor speed encoder
• V1=IR receiver input
V1
IR receiver
Signal with noise
Comparator gives bad result
Unstable region when V1 and
Vref are closed
V0
Better output
Using Schmit trigger
Schmitt trigger inverter
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Diagram for hysteresis (non-inverting Schmitt triggered buffer, same as
inverting Schmitt triggered chip-7414 just reverse the output ), see P.420, S.
Franco, Design with operational amplifiers and analog integrated circuits,
McGraw Hill.
Voltage
•
V0
V1
VTH
VTL
t
Output
Voltage
V1
Switch over
voltage
V0
10V
VTH -VTL=
(Vohigh –Volow)(R1/R2)=2V
-10V
VTL
=-1V
VTH
=1V
CEG2400 Ch17:
Sensors (v.4b)
Vref =0
Input voltage
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(3b) IR proximity range sensor: Frequency modulated range
sensor (less sensitive to surrounding light) used in TV remote
controller
•
Use any MCU with
GPIO
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Using analog-to-digital (ADC)converter
• Using analog-to-digital converter to measure
the distance between the light
sensor and the obstacle.
• ADC=Analog to digital converter
• DAC=Digital to Analog converter
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N channels range measurement using IR,
multiplexer and ADC.
• Used in our classic robot
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Algorithm for channel selection (one of N
channels) and conversion
•
•
•
•
•
Main()
{ Init. System (GPIO , ADC).
Select channel ;
Read sensor reading by ADC.
}
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You may use a commercially available chip: Infrared (IR)
reflective proximity range sensor, from Mondotronics.
Exercise 4 : Draw a block diagram for how to use this sensor in
your robot
•
From: http://datasheet.octopart.com/3337-Mondotronics-datasheet7285303.pdf
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4) IR path following setup
•
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5) Sound sensors
• Types of microphones
• Moving coil microphone
• Condenser microphone
--• Use ADC to convert to digital code
• Input to Speech Recognition system
• Clipping for simple sound detection
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Moving coil microphone
(battery not required)
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Condenser microphone
(more sensitive, battery required)
•
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Microphone with amplifier
•
UA741 or LM741
operational
-amplifier
http://www.ti.com/lit/ds/symlink/lm741.pdf
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A digital sound recorder using ADC and DAC
• Microphone: uses a uA741 op-amp to amplifier the
signal
• ADC circuit –An ADC (ADC0820 or ARM ADC) circuit
is used to convert speech signal input into digital
form.
• Output Digital-to-Analog converter DAC that converts
digital code into analog voltage
• Output is fed to an audio power amplifier (LM386) to
drive a speaker.
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Output power amplifier
using a small LM386 amplifier chip
•
http://www.ti.com/lit/ds/symlink/lm386.pdf
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Algorithm to drive the digital recorder
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xdata unsigned char ram_store[N], i;
record //sampling record and playback at one go
{
for(i=0; I < N ; i++)
{
ram_store[i] = read_in_sound_code_from_GPIO;
output_to_dac = ram_store[i];
delay(); //this determines the sampling rate
}
}
playback() // playback what has been rcordered
{
for(i=0; I < N ; i++)
{
output_to_dac = ram_store[i];
delay(); //this determines the sampling rate
}
}
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Exercise 5
a) Write the program for the interrupt method.
b) Write the algorithm to detect the sound of a
handclap or a whistle sound.
c) What are the elements we need to
implement a speech recognition system?
(advanced question)
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Sound DC level shifter
for sound level detector
•
GPIO cut-off
point (higher than this is
1, lower than this is 0)
With level shifter, all signal
is 1.
After level shifter (shifted
low) sometimes it is 0.
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6) Ultrasonic radar system
• non invasion range detection system
• Batman’s radar system
Ultrasonic
Transmitter | receiver
picture from:
http://szsaibao.taobao.
com/?spm=2013.1.2543493819.1.TCe5j8
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The transmitter circuit
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The receiver circuit
Ultra sound receiver mic
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Method 1 (by polling) : Ultrasonic-radar control using MCU and timer
Ultra sound On/off
control,0=on,1=off
•
Clock input
Courting
on/off
control
Algortithm_radar1() //by polling
Timer clock is 13824 KHz
{
Send out pulses
switch on timer
Wait until echo is received. Stop timer //wait-loop
Read timer,
Convert time into length
}
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Method 2 (by interrupt) : Ultrasonic- radar control using
interrupt and MCU’s internal timer
•
Ultra sound On/off
control,0=on,1=off
Timer clock is 13824 KHz
Video
http://www.youtube.com/watch?v=qHuVhR6-Q1E
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Algorithm_radar2 : Ultrasonic- radar control using MCU
interrupt and its internal timer
• Algorithm_radar2 ( ) // by interrupt and timer method, tested with good
result
• { initialize internal_timer interrupt;
• instruct the GPIO to send out pulses;
• start timer; //32-bit internal timer counting at 13.824MHz;
//loop
• wait a while; // wait a while, the echo is expected to arrive
• read internal_timer value; // result, convert into distance
• }
• //ISR
//////////////////////////////////////////////////////////////////////////////
/////////
• isr2_type2_for_timer1_for_radar2 //executes when echo arrives
• { Stop internal_timer;
• }
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Exercise 6
a) Compare the two methods
Algorithm_radar1(){polling} and Algorithm_radar2( )
{using internal timer interrupt shown above}.
b) For Algorithm_radar2( ), if the object is 3 meters away
and the speed of sound is 330m/s, what is the time
duration that the sound wave travels from the
transmitter to the receiver?
c) What is the result at the count?
d) What is the accuracy (+-meters) of this design? What
factors determine the accuracy of this system?
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7) Temperature sensors
•
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8) Electromagnetic sensor
Electronic compass,
accuracy +/- 1 degree
•
2
perpendicular
coils
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HMC1501 and HMC1512 Linear, Angular, and Rotary Displacement Sensor
• http://www.ssec.honeywell.com/magnetic/datasheets/hmc1501-1512.pdf
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MEMS (Microelectro-mechanical systems )
electromagnetic sensors used in our robot
•
E.g. http://www.st.com/web/en/catalog/sense_power/FM89?icmp=fm89_pron_memsx2_sep2013&sc=mems-x2
•
http://www.st.com/web/en/catalog/sense_power/FM89/SC1449
•
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9) Accelerometers
• An accelerometer measures acceleration,
vibration, and shock.
• Example: car airbag triggering sensor, 3D
mouse -- tilt sensor.
•
From National Instruments Corporation http://www.ni.com/products/
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Example of using accelerometer
From:http://www.csl.sony.co.jp/person/rekimoto/tilt/
Youtube Link:
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Accelerometers in WII
•
http://myskitch.com/keith/wii20070724-011824.jpg/preview.jpg
• IMU (Inertial measurement unit)
camera control / stabilization
Video link
https://www.youtube.com/watch?v=7GVXqNLLH7Q
http://youtu.be/CQ_P5XWkYcI
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Use accelerometer and gyroscope to make a self
balancing robot
2-side wheels
self balancing robot
•
•
Our CUHK robot has one of the
hottest “self balancing robots” in
YouTube.
62637 clicks so far on 17 Nov 2014
Motor cycle
• http://www.youtube.com/w
atch?v=0312BNqIBFI
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Exercise 7: Self balancing robot:
Discuss how to build a self balancing robot using
a) Assume we have an inclination
sensor (accelerometer is an
example) : output 0.1 Volts/degree.
Discuss how to interface the
inclination sensor with LPC2131.
b) Discuss how to control the motor
rotation direction and speed.
c) Discuss how to make a feedback
system for balancing the robot
(Hint: use PID, see the diagram on
the right for speed-control-PID, but
you need to modify it to become a
position control PID)
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Accelerometer
output= *0.1
Volts/degree

A typical speed control PID
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Summary
• A number of different sensors have been
studies
• And examples of how they are used are also
demonstrated
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Appendix: Answer for Exercise 6 for
ultrasonic radar
a) Compare the two methods Algorithm_radar1( ){polling} and Algorithm_radar2( )
{using internal timer interrupt shown above}.
b) For Algorithm_radar2( ), if the object is 3 meters away and the speed of sound is
330m/s, what is the time duration that the sound wave travels from the
transmitter to the receiver?
– Answer: sound traveled 3x2=6m.
– Time_delay (dt)=6m/(330m/s)=18.18ms
– If interrupt is 13.824MHz, each timer clock is 1/13.824M=72.33ns
c) What is the result at the count?
A. so counting result for dt=18.18ms/(1/13.824M)=18.18ms/72.33ns=251347.
d) What is the accuracy (+-meters) of this design? What factors determine the
accuracy of this system?
– Each clock pulse is (1/13.824M)s=72.33ns, that represents dL=330m/s*(72.33ns)=
(330/13.824M)m=0.239mm
– The real accuracy is half of it because it is an echo , so 0.239mm/2=0.119mm
– It is not accurate because temperature may change sound speed.
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