Analog Interfaces, Sensors, and Actuators

Transcription

1 Analog Interfaces, Sensors, and Actuators Minsoo Ryu Department of Computer Science and Engineering Hanyang University

2 Topics Covered Sensors Analog-to-Digital Converters Actuators Digital-to-Analog Converters 2 2

3 Sensors A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument For example, a mercury thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube A thermocouple converts temperature to an output voltage which can be read by a voltmeter For accuracy, all sensors need to be calibrated against known standards 3 3

4 Sensing Phenomena Piezoresistivity/metal strain Metals and semiconductors change resistance with strain Piezoelectricity Crystals produce voltage with applied pressure Capacitive Electrical energy stored between two plates changes with separation Tunneling Current changes with separation between two nearly touching conductors. Photonics Current in semiconductor changes with light intensity Magnetic induction Current changes with changing magnetic field Magneto-resistive Resistivity of semiconductors depends on magnetic field 4 4

5 Sensing Phenomena Field effect Electric field modulates transistor gate bias Hall effect Magnetic field produces bias in current carrying semiconductor Phase change Temperature changes state of matter, resulting in conductivity change Voltammetry Oxidation of metal by reactive species affects current Ion velocity Ionic property affects flow rate in external electric field. Seebeck effect Different metals in contact produce voltage dependent on temperature Thermo-resistive Resistivity of metals and semiconductors depends on temperature 5 5

6 Strain gauge Stress and Strain Sensors that can convert strain into electrical signal 6 6

7 Pressure Piezoresistive pressure sensor Capacitive pressure sensor 7 7

8 Acoustic Sensors A microphone is basically a high speed pressure sensor 8 8

9 Types of Microphones 9 9

10 Example Circuit 10 10

11 Accelerometer Accelerometer is a sensor that detects change in velocity Accelerometers originally developed for inertial guidance systems (ICBMs) 11 11

12 Spring-mass device Basic Principles Accelerometer with strain gauge 12 12

13 Accelerometer Applications Monitoring vibrations Brake sensor, swerve sensor, bounce sensor Airbag deployment 13 13

14 Gyroscopes Gyroscope is a device that can measure angular motion or displacement Applications Aerospace: Inertial guidance systems Automotive: Angular rate sensors (for traction control, etc.) Entertainment/consumer: Virtual reality sensors, pointing devices, etc. Industrial automation: Motion control, robotics 14 14

15 Gyroscope Principles The simplest gyroscopes use a high speed, rotating inertial disk that is loosely coupled to the frame holding it A rotation in the frame imparts a torque (rotation) on the spinning disk, which rotates as a result (conservation of angular momentum) 15 15

16 Topics Covered Sensors Analog-to-Digital Converters Actuators Digital-to-Analog Converters 16 16

17 Mixed-Signal Device Analog Input Digital Output What is an ADC? Most ADCs convert an input voltage to a digital word, but the true definition of an ADC does include the possibility of an input current An ADC has an analog reference voltage or current against which the analog input is compared The digital output word tells us what fraction of the reference voltage or current is the input voltage or current So, basically, the ADC is a divider 17 17

18 What is an ADC? May be considered to be a divider Output says: Input is What Fraction of V REF? Output = 2 n x G x A IN / V REF n = # of Output Bits (Resolution) G = Gain Factor (usually 1 ) A IN = Analog Input Voltage (or Current) V REF (I REF )= Reference Voltage (or Current) 18 18

19 A 3-Bit ADC For a 3-bit ADC, there are 8 possible output codes In this example, if the input voltage is 5.5V and the reference is 8V, then the output will be 101 More bits give better resolution and smaller steps A lower reference voltage gives smaller steps, but can be at the expense of noise 19 19

20 Resolution The Resolution of an A/D converter is the number of output bits it has In the previous example, 3 bits Resolution may also be defined as the size of the LSB (Least Significant Bit) In the previous example, 1 Volt 20 20

21 Quantization Error 21 21

22 Adding ½ LSB Offset 22 22

23 Sampling Rate The analog signal is continuous in time and it is necessary to convert this to a flow of digital values It is therefore required to define the rate at which new digital values are sampled from the analog signal The rate of new values is called the sampling rate or sampling frequency of the converter 23 23

24 ADC0801/0802/0803/0804/ Bit μp Compatible A/D Converters From National Semiconductor Compatible with 8080 μp derivatives no interfacing logic needed (access time is 135 ns) Easy interface to all microprocessors, or operates stand alone Key specifications Resolution: 8 bits Total error ±1 4 LSB, ±1 2 LSB and ±1 LSB Conversion time: 100 μs 24 24

25 Typical Applications 25 25

26 Write (start conversion) Timing Diagram 26 26

27 Timing Diagram Read 27 27

28 Topics Covered Sensors Analog-to-Digital Converters Actuators Digital-to-Analog Converters 28 28

29 Actuator Actuator is a device that converts a control signal (usually electrical) into mechanical action (motion) 29 29

30 Electromagnetic Actuators Electromagnetic actuators use magnetic fields to move components Moderate force and moderate displacement Relay (electrical switch) Motors (rotary and linear) Speaker (voice coil) 30 30

31 Pneumatic Actuators Pneumatic actuators use air pressure to move components High force and moderate displacement Pneumatic valve Pneumatic cylinders Pneumatic motor Pneumatic drill 31 31

32 Hydraulic Actuators Hydraulic actuators use water pressure to move components Highest forces and moderate displacement. Hydraulic valve Hydraulic disk brake Hydraulic motor Hydraulic lift 32 32

33 Piezoelectric Actuators Piezoelectric actuators use electrostatic pressure of crystals to move components Moderate forces and small displacement. Printer head Precision Actuator 33 33

34 Thermal Actuators Thermal actuators use heat to move components Small forces and small displacement Thermometer dial Inkjet Printer head 34 34

35 Electrical Motors An electric motor is a device using electrical energy to produce mechanical energy, nearly always by the interaction of magnetic fields and current-carrying conductors The reverse process, that of using mechanical energy to produce electrical energy, is accomplished by a generator or dynamo 35 35

36 A Brushed DC Motor Stator: stationary outer housing, permanent magnet or wire coil Rotor: rotating inner part, wire coil or permanent magnet (armature: rotor and its winding) Brush (graphite) and commutator for a DC motor The commutator and brushes work together to let current flow to the electromagnet, and also to flip the direction that the electrons are flowing at just the right moment. The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the magnet. The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator 36 36

37 A Brushed DC Motor 37 37

38 DC Motor Characteristics Background from physics Torque = F x r Force that acts in a rotational manner (aka moment) Angular speed = w Power = Torque x w Watts newton-meters per second Motor characteristics Torque is inversely proportional to the speed of the output shaft 38 38

39 Motor Control A microcontroller cannot drive a motor directly because it cannot supply enough current Instead, there must be some interface circuitry so that motor power is supplied from another power source and only the control signals derive from the microcontroller This interface circuitry can be implemented with many technologies, such as relays, bipolar transistors, power MOSFETS (metal oxide semiconductor field effect transistors), and motor-driver ICs In all cases, the basic topology of the circuit is the socalled H-Bridge, four switches connected spatially like an H, where the motor terminals form the crossbar of the H Each switch is implemented by a relay or transistors 39 39

40 H-Bridge Switches control direction A switches closed for clockwise B switches for counter-clockwise PWM for speed control Pulse Width Modification Control duty ratio = On/Off ratio A s duty cycle for clockwise speed B s duty cycle for counter-clockwise speed 40 40

41 Advantages of DC Motors Easy to understand design Controlling the speed of a brushed DC motor is simple The higher the armature voltage, the faster the rotation This relationship is linear to the motor's maximum speed The maximum armature voltage which corresponds to a motor's rated speed The smallest industrial motors are rated 90 VDC and 180 VDC Larger units are rated at 250 VDC and sometimes higher Specialty motors for use in mobile applications are rated 12, 24, or 48 VDC. Other tiny motors may be rated 5 VDC Torque control is also simple, since output torque is proportional to current 41 41

42 Advantages of DC Motors Simple, cheap drive design Varying the speed of a brushed DC motor requires little more than a large enough potentiometer (variable resistor) In practice, these have been replaced for all but sub-fractional horsepower applications by the SCR and PWM drives, which offer relatively precisely control voltage and current Common DC drives are available at the low end (up to 2 horsepower) for under US$ and sometimes under US$50 if precision is not important Large DC drives are available up to hundreds of horsepower However, over about 10 horsepower careful consideration should be given to the price/performance tradeoffs with AC inverter systems, since the AC systems show a price advantage in the larger systems But they may not be capable of the application's performance requirements 42 42

43 Disadvantages of DC Motors Expensive to produce Can't reliably control at lowest speeds Physically larger High maintenance Periodic replacement of brushes and springs Dust 43 43

44 An AC Motor (Squirrel Cage Rotor) 44 44

45 An AC Motor (Squirrel Cage Rotor) 45 45

46 Advantages of AC Motors Simple design Low cost for applications requiring more than about 1/2 hp (325 watts) of power Overwhelmingly preferred for fixed speed applications Extremely reliable, low maintenance operation The wide use of the AC motor has resulted in easily found replacements Many manufacturers adhere to either European (metric) or American (NEMA) standards 46 46

47 Disadvantages of AC Motors Speed control is expensive (The electronics required to handle an AC inverter drive are considerably more expensive than those required to handle a DC motor) Standard AC motors should not be operated at speeds less than about 1/3rd of base speed (due to thermal considerations) Positioning control is expensive and crude 47 47

48 Step Motors A step motor is a device that converts electrical pulses into mechanical Movements Conventional motors rotate continuously, but a step motor, when pulsed, rotates (steps) in fixed angular increments Step size, or step angle, is determined by the construction of the motor and the type of drive scheme used to control it Traditionally, step resolution has ranged from 90 degrees (four steps per rev) to a fraction of a degree, though 15 degrees (12 steps per rev), to 1.8 degrees (200 steps per rev) has been most common More recently, however, microstep motors have been introduced that are capable of.0144 degree steps (25,000 steps per rev) 48 48

49 PM Step Motor A permanent magnet step motor 49 49

50 Servo Motors Servo motors are used in closed loop control systems in which work is the control variable The digital controller directs operation of the motor by sending velocity command signals to the amplifier, which drives the motor 50 50

51 Topics Covered Sensors Analog-to-Digital Converters Actuators Digital-to-Analog Converters 51 51

52 What is a DAC? A digital-to-analog converter is a device for converting a digital code to an analog signal such as voltage, current or electric charge It performs the reverse operation of ADC ANALOG REFERENCE SOURCE CPU OR PIO D0 D1 D2 D3 D4 D5 D6 D7 DIGITAL SIGNAL D0 D1 D2 D3 D4 D5 D6 D7 DAC ANALOG SIGNAL OUTPUT 52 52

53 DAC0800/ bit high-speed current-output digital-to-analog converters (DAC) From National Semiconductor 53 53

54 Generation of 0V ~ -10V Typical Application

55 Typical Application 2 Generation of +10V ~ -10V 55 55