Download Lecture 4

Sensor Technology
Dr. Konstantinos Tatas
Outline
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Introduction
Sensor requirements
Sensor Technology
Selecting a sensor
Interfacing with sensors
Integrated sensors
Nanosensors
Case studies
Introduction
• A sensor is a device that converts a
physical quantity into a signal (typically
voltage) that can be measured
• Typical sensors:
– Temperature
– Humidity
– Pressure
– Acceleration
– Light intensity
Sensor requirements
• Sensitivity: The smallest change in quantity it
can detect
• Linearity: The range of detection should be
mapped to the output value range ideally in a
linear or logarithmic function
• Must not disturb the measured quantity
• Must not be sensitive to other properties of the
environment
• Power consumption: Sensors vary significantly
in power consumption depending on their
materials
Selecting a sensor
• Appropriate dynamic range:
• Sufficient sensitivity:
Interfacing with a sensor
• Sensors may be:
– Standalone: analog output, require an ADC to
read them
– Digital output: The ADC is integrated, the
digital value can be read
– Integrated in an MPSoC: The sensor, the ADC
and the processor and memory elements are
in a single chip
Signals (Analog - Digital)
u(V
16)
1111
1110
14
1100
12
10
101
0
1001
8
100
0
0110
6
4
Analog Signal
• can take infinity values
• can change at any time
0101
0100
2
1
2
3
4
5
6
7
8
9
ADC
Digital Signal
• can take one of
2 values (0 or 1)
• can change only
at distinct times
Reconstruction of
an analog signal
from a digital one
(Can take only
predefined
values)
u(V)
t (S)
16
1111
1110
14
1100
12
1010
D0
D1
0
0
1
0
0
1
1
0
0
1
0
1
1
1
0
0
0
0
1001
10
1000
8
DAC6
011
0
0100
D2
1
0
1
1
0
1
1
1
0
D3
0
1
0
0
1
1
1
1
1
0101
4
2
1
2
3
4
5
6
7
8
9
t (S)
QUANTIZATION ERROR
• The difference between the true and quantized value
of the analog signal
• Inevitable occurrence due to the finite resolution of
the ADC
• The magnitude of the quantization error at each
sampling instant is between zero and half of one LSB.
• Quantization error is modeled as noise (quantization
noise) u(V)
Analog signal value at
sampling time: 4.9 V
16
Quantized Analog signal
value: 5.0 V
14
12
Quantization error:
5.0 - 4.9 = 0.1 V
10
8
6
4
2
1
2
3
4
5
6
7
8
9
t (S)
SAMPLING FREQUENCY
(RATE)
• The frequency at which digital values are sampled from the analog
input of an ADC
• A low sampling rate (undersampling) may be insufficient to represent
the analog signal in digital form
• A high sampling rate (oversampling) requires high bitrate and
therefore storage space and processing time
• A signal can be reproduced from digital samples if the sampling rate
is higher than twice the highest frequency component of the signal
(Nyquist-Shannon theorem)
• Examples of sampling rates
– Telephone: 4 KHz (only adequate for speech, ess sounds like
eff)
– Audio CD: 44.1 KHz
– Recording studio: 88.2 KHz
Digital to Analog Converters
• The analog signal at the output
of a D/A converter is linearly
proportional to the binary code
at the input of the converter.
– If the binary code at the input
is 0001 and the output
voltage is 5mV, then
– If the binary code at the input
becomes 1001, the output
45mV
voltage will become ......
• If a D/A converter has 4 digital
inputs then the analog signal at
the output can have one out of
16 values.
……
• If a D/A converter has N digital
inputs then the analog signal at
the output can have one out of
2Ν values.
…….
D3
D2
D1
D0
Vout
(mV)
0
0
0
0
0
0
0
0
1
5
0
0
1
0
10
0
0
1
1
15
0
1
0
0
20
0
1
0
1
25
0
1
1
0
30
0
1
1
1
35
1
0
0
0
40
1
0
0
1
45
1
0
1
0
50
1
0
1
1
55
1
1
0
0
60
1
1
0
1
65
1
1
1
0
70
1
1
1
1
75
Characteristics of Data
Converters
1.
2.
3.
4.
Number of digital lines
– The number bits at the input of a D/A (or output of an A/D) converter.
– Typical values: 8-bit, 10-bit, 12-bit and 16-bit
– Can be parallel or serial
Microprocessor Compatibility
– Microprocessor compatible converters can be connected directly on the microprocessor bus
as standard I/O devices
– They must have signals like CS, RD, and WR
•
Activating the WR signal on an A/D converter starts the conversion process.
Polarity
– Polar: the analog signals can have only positive values
– Bipolar: the analog signals can have either a positive or a negative value
Full-scale output
– The maximum analog signal (voltage or current)
– Corresponds to a binary code with all bits set to 1 (for polar converters)
– Set externally by adjusting a variable resistor that sets the Reference Voltage (or current)
5.
6.
Characteristics of Data
Converters (Cont…)
Resolution
– The analog voltage (or current) that corresponds to a change of 1LSB in the
binary code
– It is affected by the number of bits of the converter and the Full Scale voltage
(VFS)
– For example if the full-scale voltage of an 8-bit D/A converter is 2.55V the the
resolution is:
VFS/(2N-1) = 2.55 /(28-1) 2.55/255 = 0.01 V/LSB = 10mV/LSB
Conversion Time
– The time from the moment that a “Start of Conversion” signal is applied to an A/D
converter until the corresponding digital value appears on the data lines of the
converter.
– For some types of A/D converters this time is predefined, while for others this
time can vary according to the value of the analog signal.
7. Settling Time
– The time needed by the analog signal at
the output of a D/A converter to be within
10% of the nominal value.
0.1Vo
Vo
ADC RESPONSE TYPES
• Linear
– Most common
• Non-linear
– Used in telecommunications, since human
voice carries more energy in the low
frequencies than the high.
ADC TYPES
• Direct Conversion
– Fast
– Low resolution
• Successive approximation
– Low-cost
– Slow
– Not constant conversion delay
• Sigma-delta
– High resolution,
– low-cost,
– high accuracy
Sensor sensitivity vs ADC resolution
• Sensor sensitivity (accuracy) and ADC resolution
are not the same thing!
• The TCN75A is rated for an accuracy of +/-1ºC
and has selectable resolution from 0.5ºC down
to 0.0625ºC
• What is the maximum error when reading a
value of 24.63ºC with a resolution of 0.5ºC?
• What is the error upper bound for any
temperature?
Case study 1: Generic sensor with
analog
output
const int potPin = 0; // select the input pin for the
potentiometer
void loop() {
int val; // The value coming from the sensor
int percent; // The mapped value
val = analogRead(potPin); // read the voltage on the pot
//(val ranges from 0 to 1023)
percent = map(val,0,1023,0,100); // percent will range
from 0 to 100.
EXAMPLE: Temperature sensor
const int inPin = 0; // analog pin
void loop()
{
int value = analogRead(inPin);
float millivolts = (value / 1024.0) * 3300; //3.3V analog
input
float celsius = millivolts / 10; // sensor output is 10mV
per degree Celsius
delay(1000); // wait for one second
Case study 2: PIR motion sensor
Using PIR motion sensors
const int ledPin = 77; // pin for the LED
const int inputPin = 2; // input pin (for the PIR sensor)
void setup() {
pinMode(ledPin, OUTPUT); // declare LED as output
pinMode(inputPin, INPUT); // declare pushbutton as input
}
void loop(){
int val = digitalRead(inputPin); // read input value
if (val == HIGH) // check if the input is HIGH
{
digitalWrite(ledPin, HIGH); // turn LED on if motion
detected
delay(500);
digitalWrite(ledPin, LOW); // turn LED off
}
}
Case study 3: ultrasonic sensors
• The “ping” sound pulse is generated when the
pingPin level goes HIGH for two microseconds.
• The sensor will then generate a pulse that
terminates when the sound returns.
• The width of the pulse is proportional to the
distance the sound traveled
• The speed of sound is 340 meters per second,
which is 29 microseconds per centimeter. The
formula for the distance
• of the round trip is: RoundTrip = microseconds /
29
Using ultrasonic sensors
const int pingPin = 5;
const int ledPin = 77; // pin connected to LED
void setup()
{
Serial.begin(9600);
pinMode(ledPin, OUTPUT);
}
void loop()
{
int cm = ping(pingPin) ;
Serial.println(cm);
digitalWrite(ledPin, HIGH);
delay(cm * 10 ); // each centimeter adds 10 milliseconds delay
digitalWrite(ledPin, LOW);
delay( cm * 10);
}
Using ultrasonic sensors
int ping(int pingPin)
{
long duration, cm;
pinMode(pingPin, OUTPUT);
digitalWrite(pingPin, LOW);
delayMicroseconds(2);
digitalWrite(pingPin, HIGH);
delayMicroseconds(5);
digitalWrite(pingPin, LOW);
pinMode(pingPin, INPUT);
duration = pulseIn(pingPin, HIGH);
// convert the time into a distance
cm = microsecondsToCentimeters(duration);
return cm ;
}
long microsecondsToCentimeters(long microseconds)
{
// The speed of sound is 340 m/s or 29 microseconds per centimeter.
// The ping travels out and back, so to find the distance of the
// object we take half of the distance travelled.
return microseconds / 29 / 2;
}
Case study 4: Temperature sensor
void setup(){
IOShieldTemp.config(IOSHIELDTEMP_ONESHOT |
IOSHIELDTEMP_RES11 | IOSHIELDTEMP_ALERTHIGH);
} //oneshot mode, 11-bit resolution and alert
void loop()
{
float temp;
int celsius;
char sign, msd_char, lsd_char;
//Get Temperature in Celsius.
temp = IOShieldTemp.getTemp();
}
Case study 5: Gyro sensor
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•
Gyro sensors measure angular velocity in a device (typically in degrees/s)
Example: Analog devices ADIS16266
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Yaw rate gyroscope with range scaling
±3500°/sec, ±7000°/sec, and ±14,000°/sec settings
2429 SPS sample rate
Start-up time: 170 ms
Sleep mode recovery time: 2.5 ms
Calibration temperature range: −40°C to +70°C
SPI-compatible serial interface
Relative angle displacement output
Embedded temperature sensor
Digital I/O: data ready, alarm indicator, general-purpose
Sleep mode for power management
DAC output voltage
Single-supply operation: 4.75 V to 5.25 V
3.3 V compatible digital lines
Operating temperature range: −40°C to +105°C
Case study 6: accelerometer
Case study 7: Resistive
touchscreen
• A uniform voltage gradient is applied to one
sheet. Whenever the second sheet touches the
other sheet, the second sheet measures the
voltage as a distance along the first sheet. This
combination of voltage and distance provide X
coordinate.
• After the X coordinate is located, the process
repeats itself by applying uniform voltage
gradient to the second sheet in order to find the
Y coordinate. This entire process happens in a
matter of milliseconds, oblivious to human eye.
Reading XY coordinates from
resistive touchscreen sensor
const xres = ;
Const yres = ;
const int xPin = 0; // analog input pins
const int yPin = 1;
void loop()
{
int xcoord, ycoord;
int xres, yres;
xres = analogRead(xPin);
yres = analogRead(yPin);
xcoord = map(xres,0,1023,0,xres);
ycoord = map(yres,0,1023,0,yres);
delay(100);
}
Example
const int xPin = 0; // analog input pins
const int yPin = 1;
void setup()
{
Serial.begin(9600); // note the higher than usual serial speed
}
void loop()
{
int xValue; // values from accelerometer stored here
int yValue;
xValue = analogRead(xPin);
yValue = analogRead(yPin);
Serial.print("X value = ");
Serial.println(xValue);
Serial.print("Y value = ");
Serial.println(yValue);
delay(100);
}
Question 1
• A temperature measurement system uses
a sensor that operates in the -20 to 32.5
degrees Centigrade. The system requires
a resolution of 0.05 degrees. Choose an
appropriate ADC between 8, 10 and 12 bit
options.