Sensors and Actuators

Sensor and Actuator Notes

Introduction to Sensors and Actuators

  • Sensors and actuators are crucial for implementing process control in embedded systems.
  • The system collects data from the production process using sensors and transmits signals via actuators.
  • Key components include:
    • Sensors: Measure continuous and discrete process variables.
    • Actuators: Drive continuous and discrete process parameters.
    • Devices for Analog-to-Digital Conversion (ADC) and Digital-to-Analog Conversion (DAC).
    • I/O devices for discrete data.

Definitions

  • Transducer: A device that converts one form of energy into another.
  • Sensor: A device that converts a physical parameter to an electrical output.
  • Actuator: A device that converts an electrical signal to a physical output.

Sensors

  • A sensor responds to an input quantity by generating a functionally related output, usually an electrical or optical signal.
  • Examples include ultrasonic range sensors, precision temperature sensors (-40 to 100 degrees Celsius), and flex sensors.
Sensor Components
  • Physical Medium
  • Sensing Element
  • Conditioning
  • Target Handling
  • Example: Temperature (physical medium) -> Resistance (sensing element) -> Voltage (conditioning) -> Information (target handling)
  • Transducers and micro-sensors (10^-6 m) are also key elements.
  • Stimulus (s) leads to Signal (S).
Types of Sensors
  • Acoustic, sound, vibration
  • Automotive, transportation applications
  • Chemical sensors
  • Electric current, electric potential, magnetic, radio sensors
  • Environment, weather, moisture, humidity sensors
  • Flow, fluid velocity sensors
  • Ionizing radiation, subatomic particles sensors
  • Navigation instruments
  • Position, angle, displacement, distance, speed, acceleration sensors
  • Optical, light, imaging, photon sensors
  • Pressure sensors
  • Force, density, level sensors
  • Thermal, heat, temperature sensors
  • Proximity, presence sensors
Analog Sensors
  • Output a voltage or current (e.g., 0-5V, 4-20mA).
  • Pros:
    • Usually cheaper.
    • Easy to use with Microcontroller Unit (MCU) ADCs.
  • Cons:
    • Vulnerable to noise.
    • Careful signal routing is required.
  • Example: IR proximity sensor (voltage output is non-linearly proportional to reflected infrared light).
Digital Sensors
  • Output:
    • PWM (Pulse Width Modulation) - Duty cycle, Pulse Width
    • Quadrature
  • Pros:
    • Less vulnerable to noise.
    • Easy to use with MCU Input Capture Pin.
    • No complicated protocol.
  • Cons:
    • Strong noise can still malform the signal.
  • Example: Ambient Light Sensor (frequency output with fixed 50% duty cycle; frequency changes).
Active vs. Passive Sensor Classification
  • Active Sensors
    • Require external power (excitation signal).
    • Emit energy into the environment and measure the reaction.
    • Examples: Tactile sensors (optical barriers, non-contact proximity sensor), Wheel/Motor sensors (optical, magnetic, inductive, and capacitive encoders), Active Ranging (reflectivity sensors, ultrasonic sensors, laser rangefinder, optical triangulation), Motion/Speed Sensors (Doppler radar/sound).
    • Generally require more power.
  • Passive Sensors
    • Do not need an additional energy source.
    • Generate an electrical signal in response to an external stimulus.
    • Measure ambient environmental energy entering the sensor.
    • Examples: Tactile sensors (bumpers, contact switches), Wheel/Motor sensors (brush encoders, potentiometers), Heading Sensors (compass, gyroscopes, accelerometers), Vision based sensors (CCD/CMOS cameras, visual ranging packages), Temperature, microphones, pyroelectric IR (PIR).
    • Generally require very little power.

Analog to Digital Converter (ADC)

  • An electronic integrated circuit that transforms a signal from analog (continuous) to digital (discrete) form.
  • Analog signals are directly measurable quantities.
  • Digital signals have two states: “0” and “1”.
Why Use ADC?
  • Microprocessors can only perform complex processing on digitized signals.
  • Digital signals are less susceptible to additive noise.
  • ADC provides a link between the analog world of transducers and the digital world of signal processing and data handling.
ADC Overview
  • Microcontrollers commonly use 8, 10, 12, or 16 bit ADCs.
ADC Process
  • Sampling and Holding (S/H)
  • Quantizing and Encoding (Q/E)
Sampling and Holding (S/H)
  • Holding the signal benefits A/D conversion accuracy.
  • The minimum sampling rate should be at least twice the highest data frequency of the analog signal.
Quantizing and Encoding (Q/E)
  • Resolution: The smallest change in analog signal that results in a change in the digital output.
  • Equation for Resolution: AV=V2NAV = \frac{V}{2^N}, where V = Reference voltage range, N = Number of bits in digital output, and 2^N = Number of states.
  • Resolution represents the quantization error in converting the signal to digital form.
  • Quantizing: Partitioning the reference signal range into discrete quanta and matching the input signal to the correct quantum.
  • Encoding: Assigning a unique digital code to each quantum and allocating the digital code to the input signal.
ADC on Arduino
  • ATmega 328 microcontroller has a 10-bit ADC microprocessor.
  • Arduino divides the range of 0 to 5 volts into 1024 different voltage levels or intervals.
  • 0 volts is in the interval 0, and 5 volts is in the interval 1023.
  • 2.5 volts would be in the interval 511 (as well as 2.52 volts or 2.5103 volts).
ADC Characteristics
  • Not every pin on a microcontroller can perform analog to digital conversions; on Arduino, these pins are labeled with an ‘A’ (A0 through A5).
  • ADCs vary greatly between microcontrollers.
  • Arduino has a 10-bit ADC, detecting 1,024 (2^10) discrete analog levels.
  • Some microcontrollers have 8-bit ADCs (2^8 = 256 discrete levels) and some have 16-bit ADCs (2^16 = 65,535 discrete levels).
10-bit ADC Transfer Function
  • TF=(V<em>ref+−V</em>ref−)/1023TF = (V<em>{ref+} - V</em>{ref-}) / 1023
  • Example: 5115/1023 = 5 mV wide steps
10-bit ADC Resolution Range
  • Transfer Function: 5115 mV1023 steps=5.0mVstep\frac{5115 \text{ mV}}{1023 \text{ steps}} = 5.0 \frac{\text{mV}}{\text{step}}
  • Resolution = 1 step = 5.0 mV
  • Resolution % = 1 step1023 steps≈0.00098=0.10%\frac{1 \text{ step}}{1023 \text{ steps}} \approx 0.00098 = 0.10\%
  • General n-bit A/D Resolution %: 12n−1\frac{1}{2^n - 1}
Analog Input Range
  • Given an n-bit ADC with a digital output D, the analog input range is:
  • Analog Input Range = (D+12±12)(V<em>ref+−V</em>ref−)2n−1(D + \frac{1}{2} \pm \frac{1}{2}) \frac{(V<em>{ref+} - V</em>{ref-})}{2^n - 1}
  • Example: 10-bit ADC, V<em>ref+=5115V<em>{ref+} = 5115 mV, V</em>ref−=0V</em>{ref-} = 0, D = 1021
  • Analog Input Range = (1021+12±12)5115−0210−1=5107.5±2.5(1021 + \frac{1}{2} \pm \frac{1}{2}) \frac{5115 - 0}{2^{10} - 1} = 5107.5 \pm 2.5 mV
Voltage Reference (Vref)
  • Example: (V<em>ref+−V</em>ref−)=5115±1% mV(V<em>{ref+} - V</em>{ref-}) = 5115 \pm 1\% \text{ mV}
  • Digital output: D = 210
  • =5115±51.15 mV= 5115 \pm 51.15 \text{ mV}
  • Analog Input = (D+12±12)(V<em>ref+−V</em>ref−)2n−1(D + \frac{1}{2} \pm \frac{1}{2}) \frac{(V<em>{ref+} - V</em>{ref-})}{2^n - 1}
  • =(210.5±12)(5.000±0.0500)=1052.5±2.500+10.53 mV= (210.5 \pm \frac{1}{2}) (5.000 \pm 0.0500) = 1052.5 \pm 2.500 + 10.53 \text{ mV}
  • Conclusion: Vref stability is more important than the number of ADC bits.
Improving ADC Accuracy
  • Increase the resolution: Improves accuracy in measuring the amplitude of the analog signal.
  • Increase the sampling rate: Increases the maximum frequency that can be measured.

Actuators

  • Actuators: Convert electrical signals to physical outputs.
Types of Actuators
  • Servo Motor
  • H-Bridge Motor
  • Pulse-Width-Modulation (PWM)
Actuator Construction
  • Electrical motors
  • Pneumatics and valves
  • Solenoids, valves, cylinders
  • Hydraulics, pneumatics
  • Motors
  • Heaters
  • Lights
  • Sirens/Horns (audio)
Servo Motors
  • Components: servo input (electronic control signal) -> electronic control -> servo motor -> gear train system -> output position sensor -> servo output (physical shaft movement).
  • Motor speed determined by supplied voltage.
  • Motor direction determined by polarity of supplied voltage.
  • It is difficult to generate analog power directly from a microcontroller, requiring an external amplifier (pulse-width modulation).
Brushed Motor: H-Bridge
  • Allows a motor to be driven in both directions.
  • Drive forward: Close 1 and 4.
  • Drive backward: Close 2 and 3.
Controlling Brushed Motor: H-Bridge
  • States: right, left, free, brake
  • Configurations: S1 S2 S3 S4
  • right: 1 0 0 1
  • left: 0 1 1 0
  • free: 0 0 0 0
  • brake: 0 1 0 1 or 1 0 1 0
Stepper Motor
  • Converts electrical pulses into discrete mechanical movement.
  • Shaft rotates in discrete step increments.
  • Full torque at standstill.
  • Precise positioning and repeatability.
  • No brushes.
  • Low-speed possible.
  • Cons:
    * Resonance can occur.
    * Not easy to control at high speed.
Motor and Encoder
  • Motor speed determined by supplied Voltage.
  • Motor direction determined by Polarity of supplied voltage.
  • Difficult to generate analog power signal (1A…10A) directly from microcontroller.
  • External amplifier (Pulse Width Modulation).
Pulse Width Modulation (PWM)
  • A/D converters are used for reading analog sensor signals.
  • Instead of using D/A converters for motor control (too expensive and needs power circuitry), PWM is done by software by, e.g., switching power on/off in intervals.
How PWM Works
  • The supplied voltage did not change.
  • Power is switched on/off at a certain pulse ratio matching the desired output power.
  • Signal has very high frequency (e.g., 20kHz).
  • Motors are relatively slow to respond.
  • The only thing that counts is the supplied power (integral/summation).
  • Pulse-width ratio = t<em>on/t</em>periodt<em>{on}/t</em>{period}
PWM Terminology
  • Period = t<em>on+t</em>offt<em>{on} + t</em>{off}
  • Frequency = 1Period\frac{1}{Period}
  • Duty Cycle = t<em>ont</em>on+toff∗100\frac{t<em>{on}}{t</em>{on} + t_{off}} * 100
PWM Details
  • A pulse with finite length in time is called ‘time on’ – how long the electrical signal was present.
  • Once the electrical signal is removed and until the signal is applied again, it is called ‘time off’.
  • If the electrical signal changes to ‘on’ and ‘off’ with continuous and equal intervals of ‘time on’ and ‘time off’, the pulse is periodic.
  • Its period is equal to ‘time on’ + ‘time off’.
  • The pulse period must remain constant in any change of time.
PWM with Arduino
  • With Arduino's PWM frequency at about 500Hz, the green lines would measure 2 milliseconds each (period).
  • A call to analogWrite() is on a scale of 0 - 255, such that analogWrite(255) requests a 100% duty cycle (always on), and analogWrite(127) is a 50% duty cycle (on half the time) for example.
Arduino Code Example
const int motorPin =
    9;  // PWM pin connected to the transistor base
void setup() {
  pinMode(motorPin, OUTPUT);  // Set motorPin as an output
}
void loop() {
  analogWrite(motorPin, 204);  // Set duty cycle to 80% (204 out of 255)
}
  • motorPin is the PWM output pin connected to the base of the NPN transistor.
  • Setup Function: Sets the motorPin as an output.
  • Loop Function: Uses analogWrite(motorPin, 204) to control the PWM signal. 204 is 80% of 255 (255 * 0.8 = 204), which means the pin will be on for 80% of the time and off for 20% of the time.
RC Pulse Width Modulation
  • Remote control/ RC PWM is different to PWM signal used to control DC motor speed.
RC Servo Motors
  • RC Servos has designed to accept the RC PWM signal which is periodic pulse with 1.0 ms to 2.0 ms width (standard).
  • The idea behind this position protocol is that 1.5 ms commands the servo to go to the center position.
  • A 1.0 ms pulse commands the motor to attempt to reach its leftmost position and 2.0 ms to its rightmost position.
  • Any pulse measuring in between 1.0 ms and 2.0 ms is decoded as a position in between leftmost and rightmost.
  • RC PWM signal can be generate using microcontroller. The positions available are dependent on timer resolution.
DC Motor Speed Control PWM
  • In DC motor it is important to decode the position information (RC Pulse Width) and generate a speed magnitude signal.
  • This is done by achieving maximum reverse speed when input is 1 ms.
  • While, the motor will stop at 1.5 ms.
  • At 2.0 ms, DC motor will move forward at maximum speed.
  • Any other pulse width is then decoded to partial speed on the corresponding direction.
Controlling DC Motor : H-Bridge
  • H-Bridge motor utilize PWM to control DC motor speed.
  • In H-Bridge motor, the duty cycle is proportional to voltage applied into the load –means that voltage applied is directly proportional to motor speed.
  • The H-Bridge is chopping the amount of time the motor is receiving energy.
  • By varying duty cycle, an infinite amount of voltages can be applied to the motor. This will result, an infinite DC motor speed ranging from stopped running at full speed.