Optotriac

1
Optotriac

• Inductive loads means voltage and current are out
  of phase and triac never turns off
• Snubber network reduces maximum dV/dt

2
Digital-to-Analog Conversion

• The real world is ANALOG! Computers are DIGITAL!
• Obviously what is needed is an A/D converter
• D/A converters are easier to build
• Operational amplifiers
• Very high-gain differential amplifier (order of
  106 or more)
• Inverting and non-inverting inputs
• Very high impedance input very low impedance
  output
• Use positive and negative feedback to operate
  with controlled gain
• Produce bi-polar outputs

3
Operational Amplifier
Notice that negative feedback is used
Parameters Slew rate Gain bandwidth Settling
time
4
DAC (Digital-to-Analog Converter)
Where 0 ? V0 ? 4.69
Why not 5 volts?
5
A Few Major Problems ...

• Non-precision reference voltage
• Totem-pole output voltage varies with current
  drawn
• Need precision binary-weighted resistors
• 4-bit require accuracy to 1 part in 16 (about 6)
• 16-bit requires accuracy to 1 part in 216 (about
  .0015) and range of 10 K? to 655.36 M?

6
R-2R Resistor Network
Notice that a constant current flows out of each
switch. Also notice the net resistance to ground
at each point, A-D, is R.
7
Commercial DAC Terminology

• Uni- or bi-polar values
• Double buffering
• Nominally, the inp and outp are for 8-bit values
• Two loads to output a 12- or 16-bit value
• Linearity
• Analog output of an n-bit DAC increases in steps
  of 1/2n of the DAC output range
• Two forms
• Integral non-linearity
• Differential non-linearity
• Settling time

8
Terminology (Contd)

• Spiking
• Digital ground line
• Analog ground line
• Current output DAC
• Eliminates the costly high-speed op-amp
• Multiplying DAC
• Works with range of reference voltages
• Output is a the product of DACs digital input
  and reference voltage
• Four-quadrant multiplying DAC
• Handles ? inputs
• Produces ? outputs

9
Analog-to-Digital Conversion (ADC)

• Many different approaches
• Successive approximation
• Dual-slope integrating (based on integrating
  charge)
• Flash (using simultaneous comparisons)
• Sub-ranging (course then finer resolution of
  error)
• Delta-Sigma
• Approach related to
• Speed
• Accuracy
• Cost

10
Successive Approximation ADC
11
ADC Performance Issues

• Aliasing
• Nyguist frequency

12
ADC Performance Issues (Contd)

• Antialiasing filter
• Resolution
• Limited dynamic range
• 12-bit ADC with 0-10 volt range accurate to
  0.024 (1 part in 4096)
• Only 2-bit ADC when sampling a 10 millivolt
  signal
• Use programmable-gain amplifier
• Non-linear
• Ground reference and noise

13
Data-Acquisition Subsystems

• Typical board
• Contains ADC, DAC, and digital I/O
• Antialiasing filters
• Programmable gain amplifiers
• Sample/hold (one multiplexed or one per channel)
• Single-ended or differential input
• DMA control
• FIFO queues and buffers
• Timer/counters
• Software
• Lab Notebook
• Virtual Instrumentation

14
Motor Control

• Two approaches
• Open-loop
• Closed-loop
• Two types of motors
• Stepper
• Reliable unless acceleration, speed or torque
  capabilities exceeded
• Wide range from ¼ revolution per step (rps) to
  1/2000th rps
• Servo
• Cannot be run reliably without feedback
• Based on cheap DC motors

15
Stepper Motor

• Digitally controlled
• Predictable and operated in open loop
• Moves in fixed steps
• Software tracks position
• Mechanically
• Rotor (permanent magnet) and stator
  (electromagnet)
• Rotor consists of two groups of gear-like teeth,
  rotated w/r to each other by 1/2 tooth spacing
• Stator is cylinder with teeth
• Runs either clockwise or counter-clockwise based
  on stator poles activated

16
Stepper Simplified View

• Each step is 30º or 12 steps/revolution
• Opposing winding are always excited together and
  are called a phase
• Example is a two-phase stepper
• Another common design is a 1.8? per step,
  four-phase hybrid stepper
• A bifilar wound stepper motor has two sets of
  oppositely wound stator windings and requires
  only a single power supply

17
Driving Stepper Motors

• For bifilar wound motors, use L/R driver
• Drive current determined by inductance (L) of
  winding and series resistance (R)
• Energize the four phases in proper order and for
  appropriate amount of time
• Single-phase excitation - simplest
• Dual-phase excitation - produces rotor positions
  halfway between single-phase steps and offers
  most torque and smoothest operation
• Half-step excitation - doubles number of steps
• Stepping rate is crucial if steps are not to be
  missed

18
L/R Unipolar Drive Circuit
19
H-Bridge L293D

• An integrated circuit that can supply the
  necessary voltage/current to the stepper or servo
  motor
• For a two coil stepper

20
H-Bridge (Contd)

• The enable pins, 1 9, should be tied to 5 to
  run the motor all the time
• Control pins, 2/7/10/15, are cycled to get the
  four phases for a two-phase stepper

21
Connecting to the BS2

• Very simple
• Can have multiple steppers

22
Servo Motors

• Take the form of a permanent-magnet DC brush
  motor
• Notice the linear speed/torque/current
  relationships

23
Servo Motors (Contd)

• Commutator reverses current through rotor as it
  turns
• Driven by same H-bridge switchers as stepper
  motors
• Pulse width modulation (PWM) used, as before, to
  control voltage/speed without wasting power in
  driver circuit
• Also comes in brushless version
• Avoids brush wear
• Reverses construction of rotor and stator (built
  like stepper) and requires switching current
  through stator
• Unlike stepper, there are not teeth on rotor put
  rotor position sensor is need
• Generates more torque than a stepper at high
  speeds

24
Motor Position Sensor

• Several choices
• Optical encoder
• Incremental uses an encoding disk and counter
• Quadrature uses two encoding disks to counter
  (one to count input and the other to clock input)
• Can sense direction and know absolute count
• Basis for modern mice
• Neither tells initial position
• Normally drive servo to one end of travel limit
  which causes a limit switch to be activated
• Absolute encoder
• Contains multiple concentric tracks to provide
  absolute position within one revolution
• Often uses Gray code (e.g., 000, 001, 011, 010,
  110, 111, 101, 100, 000, )

25
Driving Two DC-Motors

• Can use the same L293D for this but now it will
  control two motors

26
Closed-Loop Motor Control
User Commands
Motor Shaft
Digital Controller
Shaft Position Sensor
Motor Driver
Motor Control Signals
Motor
Load
Shaft Position Feedback
27
PID

• Proportional, Integral, and Derivative control
  algorithm
• Where
• First term is proportional to error signal but
• Second term provides sum of error terms
• Third term offers damping as error grows smaller

28
PID Block Diagram
29
LabVIEW

• An example of virtual instrumentation
• Consists of a computer, software, and data
  acquisition (DAQ) hardware
• LabVIEW provides
• Enhanced functionality over traditional system
  through graphical interface
• User does not write software in traditional
  fashion (using assembly or C languages)
• DAQ is data not control flow driven

30
LabVIEW (Contd)

• Front panel is the window through which the user
  interacts with the VI program
• Must always have front panel window open
• View inputs and output on the front panel
• Front panel made up of controls (knobs and
  switches0 and indicators (numeric and graphical
  displays)
• Drag and drop controls from controls palette
• Source code of the VI is held in the block
  diagram (using functions palette)
• Made up of terminals, nodes, and wires
• Terminal - associated with control on front panel
• Node - program execution element
• Wires - connect nodes and terminals (extensive
  error checking)

31
Simple LabVIEW Example
Control terminal
Wire
Indicator terminal
Node
32
Digital Thermometer

• Front panel

But what does the VI block diagram look like?
33
Other Features

• Rich data structures
• For and While loops
• Shift registers and initializing shift registers
• Sequencing
• Case Structures
• Formulas
• Arrays and clusters
• Operators
• Polymorphism and compound arithmetic
• Display types
• Indicators
• Graphs and charts

34
Features (Contd)

• Instrument simulation
• I/O
• Strings
• File handling
• Many debugging techniques
• Breakpoints
• Single stepping
• Probes
• Reference Learning with LabVIEW 6i, Robert H.
  Bishop, Prentice Hall, ISBN 0-13-032559-7

35
The Software Life Cycle - Chapter 4

• Engineering approach
• Specification, design, construction, testing, and
  maintenance
• IEEE Std 830 for specification
• DOD-STD-2167A for software development
• Software life cycle phase
• Concept
• Requirements
• Design
• Programming
• Test
• Maintenance

Relate to our programming projects
36
Concept

• Purpose
• Define project needs and goals
• Produce a white paper or operational concept
  document
• No...
• formal requirement stated
• hardware/software decisions made
• budgets and schedules are set
• Identify product need and goals
• Produce feasibility studies

37
Requirements

• Decide what the product must do
• Documentation prepared by customer
• What the product does
• Timing, UI, accuracy, etc. specified
• May include schedule and budget
• Testing determined and committed (e.g., formal
  test plan)
• Functional and non-functional requirements (e.g.,
  what can and cannot be tested)
• Functional fire alarm sounds within 2 seconds of
  smoke detection
• Non-functional programmed in C

38
Requirements (Contd)

• Documentation rules
• Must be complete
• Must be correct
• Must be consistent
• Every requirement or design element should be
  testable

39
Design

• Shows how the product will meet the requirements
• Converts requirement into detailed design
• Provides partitioning of the functional features
  into software and hardware modules
• Prepare test cases
• Helps identify conflicts, redundancies, or
  impossible requirements
• Implementation details hidden by using ADTs and
  objects

40
Programming

• Write and debug the software (easy!)
• Fills in details missing in design phase
• Enhanced by tools
• Debuggers
• Version control software
• Simulators
• Code generators

41
Testing

• Verify requirements are met
• Quality assurance
• Automated test generators

42
Maintenance

• Begins after verification
• Product deployment
• Customer support
• Error reporting
• Product enhancement

43
Universal Real-Time Operating System

• Intimately familiar with this RTOS
• Has all the usual components
• Kernel with processes and threads
• Communicates with other units (IPC)
• Can be scheduled and interrupted
• Uses memory management, monitors actions, and
  performs error correction
• Exhibits protection and fault isolation/repair

44
State of the Field

• Real-time systems
• Must meet timing constraints
• Must produce correct result within a specified
  time
• Late (and possibly early) actions are useless or
  harmful
• Not simply a function of increasing system
  throughput
• Requires timeliness and predictability
• Do not have to be fast systems
• Hard real-time
• Critical deadlines to be met
• Soft real-time
• Non-critical deadlines

45
Basic Real-Time Concepts

• Engineering black box approach
• Definition A system has a set of one or more
  inputs entering a black box and a set of one or
  more outputs exiting the black box
• The internal process by which inputs are
  converted to outputs is called the transfer
  function
• Definition The time between the appearance of an
  input and an associated output is call the
  response time of the system

I1 . . In
O1 . . On
46
Real-Time Definitions

• Definition A real-time system is a system that
  must satisfy explicit (bounded) response time
  constraints or risk severe consequences,
  including failure
• Definition A failed system is a system that
  cannot satisfy one or more of the requirements
  stipulated in the formal system specification
• Definition A real-time system is one whose
  logical correctness is based on both the
  correctness of the outputs and their timeliness
• Definition A reactive system is one that has an
  ongoing interaction with its environment

47
Real-Time Definitions (Contd)

• Definition An embedded system is one where the
  specialized control hardware includes the
  computer
• Bottom line
• All practical systems can be said to be
  real-time!
• Response time for some systems is days or even
  weeks (what we called soft real-time systems)

48
Other Terms and Definitions

• Definition An event is said to be synchronous if
  it always occurs at the same time and place
  otherwise, it is said to be an asynchronous event
• Definition A system is said to be deterministic
  if, for each possible state and set of inputs, a
  unique set of outputs and next state of the
  system is known

49
Real-Time Kernels

• Three functions provided
• Task scheduling (scheduler)
• Task dispatching (dispatcher)
• Intertask communications
• Task and process used interchangeably
• Multi-Level Interpretive or layered system
• Applications level
• Programming language level
• Run-time environment
• Operating system level
• Native machine level
• Hardware systems

50
Kernel or Nucleus
51
Reliability - Chapter 11

• For a given system S which fails at time T, the
  reliability of S at time t, denoted r(t), is the
  probability that T is greater than t, that is
• Failure function is the probability that the
  system fails at time t
• We commonly accept the bathtub curve as a
  standard model

52
Fault Tolerance

• The ability of the system to continue to function
  in the presence of hardware or software failures
• Spatial redundancy
• Methods involving redundant hardware or software
• Involves
• Voting
• Checkpointing
• Recovery blocks
• N-version programming
• Built in testing (CPU, memory, etc.)
• Temporal redundancy
• Techniques that allow for tolerating missed
  deadlines
• Hardest of the two to achieve

53
Summary

• This last day has included a great many topics,
  from computer interfaces and motor controls, to
  software development, and real-time systems
• As will all topics, there are the fun parts that
  include building and testing, and the more formal
  parts that result in reliable software and
  hardware
• Students must learn not only the skills and
  techniques but the basic principles that lead to
  newer and better robots/controllers