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Virtual Instrumentation. 9/9/09. Robotics Spring 2002. 14. Motor Control. Two approaches ... An example of virtual instrumentation ... – PowerPoint PPT presentation

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Title: Optotriac

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

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
  • Use positive and negative feedback to operate
    with controlled gain
  • Produce bi-polar outputs

Operational Amplifier
Notice that negative feedback is used
Parameters Slew rate Gain bandwidth Settling
DAC (Digital-to-Analog Converter)
Where 0 ? V0 ? 4.69
Why not 5 volts?
A Few Major Problems ...
  • Non-precision reference voltage
  • Totem-pole output voltage varies with current
  • 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?

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.
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

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

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

Successive Approximation ADC
ADC Performance Issues
  • Aliasing
  • Nyguist frequency

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
  • Use programmable-gain amplifier
  • Non-linear
  • Ground reference and noise

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

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

Stepper Motor
  • Digitally controlled
  • Predictable and operated in open loop
  • Moves in fixed steps
  • Software tracks position
  • Mechanically
  • Rotor (permanent magnet) and stator
  • 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

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

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

L/R Unipolar Drive Circuit
H-Bridge L293D
  • An integrated circuit that can supply the
    necessary voltage/current to the stepper or servo
  • For a two coil stepper

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

Connecting to the BS2
  • Very simple
  • Can have multiple steppers

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

Servo Motors (Contd)
  • Commutator reverses current through rotor as it
  • Driven by same H-bridge switchers as stepper
  • 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

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, …)

Driving Two DC-Motors
  • Can use the same L293D for this but now it will
    control two motors

Closed-Loop Motor Control
User Commands
Motor Shaft
Digital Controller
Shaft Position Sensor
Motor Driver
Motor Control Signals
Shaft Position Feedback
  • Proportional, Integral, and Derivative control
  • Where
  • First term is proportional to error signal but
  • Second term provides sum of error terms
  • Third term offers damping as error grows smaller

PID Block Diagram
  • 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

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
  • 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)

Simple LabVIEW Example
Control terminal
Indicator terminal
Digital Thermometer
  • Front panel

But what does the VI block diagram look like?
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

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

The Software Life Cycle - Chapter 4
  • Engineering approach
  • Specification, design, construction, testing, and
  • 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
  • Purpose
  • Define project needs and goals
  • Produce a white paper or operational concept
  • No...
  • formal requirement stated
  • hardware/software decisions made
  • budgets and schedules are set
  • Identify product need and goals
  • Produce feasibility studies

  • 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

Requirements (Contd)
  • Documentation rules
  • Must be complete
  • Must be correct
  • Must be consistent
  • Every requirement or design element should be

  • 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

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

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

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

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

State of the Field
  • Real-time systems
  • Must meet timing constraints
  • Must produce correct result within a specified
  • Late (and possibly early) actions are useless or
  • Not simply a function of increasing system
  • 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

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
  • 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
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

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

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

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

Kernel or Nucleus
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

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
  • Hardest of the two to achieve

  • 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
  • Students must learn not only the skills and
    techniques but the basic principles that lead to
    newer and better robots/controllers
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