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) u(V ) t (S) D3D3 D2D2 D1D1 D0D Analog Signal can take infinity values can change at any time Digital Signal can take one of 2 values (0 or 1) can change only at distinct times ADC u(V) t (S) DAC Reconstruction of an analog signal from a digital one (Can take only predefined values)

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)

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 2Ν2Ν D3D2D1D0 Vout (mV)

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/(2 N -1) = 2.55 /(2 8 -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. 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 ºC 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 / ) * 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 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 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.