Sensor Technology

Dr. Konstantinos Tatas

Outline• 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)

2

4

6

8

10

12

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16

u(V)

1 2 73 4 5 6 8 9 t (S)

D3

D2

D1

D0

0

0

0

1

0

0

1

1

0

0

1

1 0

0

1

1 0

1

1

1

0

0

1

1 1

1

1

1

0

0

1

1

0

1

0

0

Analog Signal • can take infinity values • can change at any time

0100

1001

0110

0101

1010

1110

1111

1100

1000

Digital Signal • can take one of

2 values (0 or 1)• can change only

at distinct times

ADC

2

4

6

8

10

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14

16

u(V)

1 2 73 4 5 6 8 9 t (S)

DAC

Reconstruction of an analog signal from a digital one

(Can take only predefined values)

1001

0110 0101

1010

11111110

1000

1100

0100

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)

2

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16

u(V)

1 2 73 4 5 6 8 9 t (S)

2

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u(V)

1 2 73 4 5 6 8 9 t (S)

Analog signal value at sampling time: 4.9 V

Quantized Analog signal value: 5.0 V

Quantization error: 5.0 - 4.9 = 0.1 V

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 voltage will become ......

• If a D/A converter has N digital inputs then the analog signal at the output can have one out of ……. values.

• If a D/A converter has 4 digital inputs then the analog signal at the output can have one out of …… values.

45mV

16

2Ν

D3 D2 D1 D0Vout(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. 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

2. 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.

3. Polarity

– Polar: the analog signals can have only positive values

– Bipolar: the analog signals can have either a positive or a negative value

4. 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)

Characteristics of Data Converters (Cont…)5. 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

6. 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.

0.1Vo

Vo

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.

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 potentiometervoid loop() {int val; // The value coming from the sensorint percent; // The mapped valueval = 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 pinvoid loop(){int value = analogRead(inPin);float millivolts = (value / 1024.0) * 3300; //3.3V analog inputfloat celsius = millivolts / 10; // sensor output is 10mV per degree

Celsiusdelay(1000); // wait for one second}

Case study 2: PIR motion sensor

Using PIR motion sensorsconst int ledPin = 77; // pin for the LEDconst int inputPin = 2; // input pin (for the PIR sensor)void setup() {pinMode(ledPin, OUTPUT); // declare LED as outputpinMode(inputPin, INPUT); // declare pushbutton as input}void loop(){int val = digitalRead(inputPin); // read input valueif (val == HIGH) // check if the input is HIGH{digitalWrite(ledPin, HIGH); // turn LED on if motion

detecteddelay(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 sensorsconst int pingPin = 5;const int ledPin = 77; // pin connected to LEDvoid 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 delaydigitalWrite(ledPin, LOW);delay( cm * 10);}

Using ultrasonic sensorsint 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 distancecm = 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• Gyro sensors measure angular velocity in a device (typically in degrees/s)• Example: Analog devices ADIS16266

– 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 pinsconst 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);}

Exampleconst int xPin = 0; // analog input pinsconst int yPin = 1;void setup(){Serial.begin(9600); // note the higher than usual serial speed}void loop(){int xValue; // values from accelerometer stored hereint 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.