Chapter 10. Manual Control

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Chapter 10. Manual Control

• OVERVIEW
• the analog form or the time-space trajectory of
  the response the domain of continuous control
• human performance in manual control
• skills approach
• primarily involves analog motor behavior, in
  which the operator must produce or reproduce a
  movement pattern from memory with little
  environmental uncertainty
• with little environmental uncertainty, skills in
  theory may be performed perfectly and identically
  from trial to trial
• open-loop control because once the skill has
  developed there is little need to process the
  visual feedback from the response
• focuses on skill acquisition and practice
• dynamic systems
• examines human abilities in controlling or
  tracking dynamic system to conform with certain
  time-space trajectories in the face of
  environmental uncertainty
• because of the need to process error signals ?
  closed-loop control
• focuses on mathematical representations of the
  humans analog response when processing
  uncertainty
• generally addresses the behavior of the
  well-trained operator

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• OPEN-LOOP MOTOR SKILLS
• Discrete Movement Time
• Fitts Law
• when movement amplitude (A) and target width (W)
  were varied, their joint effects were summarized
  by a simple equation (Fig 10.1 Fig 10.2)
• speed-accuracy trade-off in movement movement
  time and accuracy reciprocally related
• index of difficulty (ID) of the movement
• higher b indicates less efficient movements, a
  reflects the start-up time for a movement
• Models of Discrete Movement
• typical trajectory or time history as the stylus
  approaches the target (speed-accuracy tradeoff)
• an exponential approach to the target with an
  initial high-velocity approach(initial ballistic)
  followed by a smooth, final, homing phase
  (current control)
• velocity profile ? control is not continuous but
  discrete corrections, acceleration and
  deceleration
• feedback processing assumption (Fig 10.3)
  sample-and-correct process until the target
  boundary is crossed closed loop behavior --
  generally exponential approach
• Motor Schema
• with highly learned skills under minimal
  environmental uncertainty, visual feedback is not
  necessary ? open-loop fashion ? visual feedback
  may be harmful

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• general characteristics of well-learned motor
  skills
• they may well depend on feedback, but the
  feedback is proprioceptive
• the patterns of desired muscular innervation may
  be stored centrally in LTM and executed as an
  open-loop motor program without visual feedback
  correction or guidance
• motor program and motor schema
• highly overlearned skills that do not depend on
  guidance from visual feedback
• low attention demand, single-response selection,
  and consistency of outcome
• Low Attention Demand
• the motor program tends to be automated
• Single-Response Selection
• single-response selection required to activate or
  load a single motor program even with a number
  of separate, discrete responses ? resources are
  demanded only once, at the point of initiation,
  when the program is selected
• Consistency of Outcome Programs Versus Schemata
• a motor program is assumed to generate very
  consistent space-time trajectories from one
  replication to another ? what is consistent is
  not the process of muscular innervation but the
  product of response ? signature of ones name
  meets the criteria of motor program
• certain characteristics of the time-space
  trajectories remained invariant ? whatever is
  learned and stored in LTM cannot be a specific
  set of muscle commands but must represent a more
  generic or general set of specifications of how
  to reach the desired goal motor schema
• once a schema is selected, the process of loading
  requires the specific instance parameters to be
  specified to meet the immediate goals at hand

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• TRACKING OF DYNAMIC SYSTEMS
• when describing human operator control of
  physical systems, research moves from the domain
  of perceptual motor skills and motor behavior to
  the more engineering domain of tracking
• this shift in domain from three nonhuman elements
  on the performance
• the dynamics of the system itself
• the input to the operator
• the display
• The Tracking Loop Basic Elements (Fig 10.4)
• control dynamic the relationship between the
  force, f(t), applied and the steering wheel
  movement, u(t)
• system dynamic the relationship between control
  position, u(t) and system response, o(t)
• error sources
• command inputs, ic(t) changes in the target to
  be tracked
• disturbance inputs, id(t) applied directly to
  the system
• input (Fig 10.5) may be transient or continuous
  (predictable, periodic, random)
• display the source of all information necessary
  to implement the corrective response
• pursuit display independent movement of both
  the target and the cursor
• compensatory display only movement of the error
  relative to a fixed 0-error reference
• tracking performance in terms of error

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• Transfer Functions
• the mathematical relationship between the input
  and the output of a system (Fig 10.6)
• Pure Gain -- (the output)/(the input) -- low-gain
  system sluggish
• Pure Time Delay transmission lag
• delays the input but reproduce it in identical
  form T seconds later
• no effect on gain, nor does gain have any effect
  on time delay
• Experimental Lag
• gradually home in or stabilize on the target
  input
• defined by time constant Ti the time the output
  reach 63 of its final value
• Velocity-Control, Integrator, or First-Order
  System
• constant velocity, time integral of the output
• closely related to exponential lag
• Acceleration-Control, Double-Integrator, or
  Second-Order System
• combines two integrators in series constant
  acceleration
• unstable, or difficult to control inertia
• Differentiator
• minus-first-order produce an output position of
  a value equal to the rate of change of the input
• theoretically step response is a spike
• Frequency-Domain Response

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• Human Operator Limits in Tracking
• Processing Time
• effective time delay perceived error translated
  by operator after a lag
• zero and first-order system 150 to 300 msec
  second-order 400 to 500 msec
• time delays due to human processing or system lag
  are harmful to tracking (Fig 10.7)
• any lag will cause output to no longer line up
  with input
• with periodic or random inputs, delay produce
  instability, oscillatory behavior
• Bandwidth
• tracking involves the transmission of information
  common or disturbance-induces error
• the limit of information transmission in tracking
  is between 4 to 10 bits/sec (preview)
• frequency limit determines the maximum bandwidth
  of random inputs -- 0.5 to 1.0 Hz
• max. frequency with which corrections are exerted
  in tracking 2times/sec
• Prediction and Anticipation
• must anticipate future errors on the basis of
  present values to make control correction after a
  considerable lag
• Processing resources
• difficulty in anticipation related to the
  resource demands of spatial working memory
• mental model of the systems dynamics ? working
  memory ? disrupted by concurrent tasks
• Compatibility
• tracking is primarily a spatial task -- spatial
  compatibility affects tracking performance

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• Effect of System Dynamics on Tracking Performance
• Gain
• U-shaped function of system gain (system output /
  control input)
• intermediate gain the lowest error and easiest
  to track
• high gain
• minimal control effort to produce large
  corrections
• overcorrections and oscillations
• instability resulted in lags in system
• optimal gain the crossover point of the
  instability at high gain and effort at low gain
• Time Delay
• pure time delay harmful in tracking
• worse tacking performance with greater delays
• System Order
• zero-order and first order roughly equivalent
  successful tracking requires position and
  velocity to be matched (Fig 10.8) economy of
  movement and space
• the orders above first error and subjective
  workload increase dramatically
• second order control unequivocally worse
• generating lead higher order derivatives be
  perceived as a basis for correction
• ? longer effective time delay ? increased lag ?
  respond smoothly
• bang-bang double impulse or time-optimal
  control optimal control (Fig 10.9)

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• Instability
• oscillatory and unstable behavior
• positive feedback systems
• once an error is in existence, feedback works to
  add the error in the same direction
• negative feedback systems
• more typical
• humans and systems function to reduce rather than
  increase detected error
• high gain and long phase lag instability
• Tracking Displays
• Preview
• command input and system output determine the
  future error
• the future input will be most accurately
  available when there is preview it enables the
  operator to compensate for processing lags in the
  tracking loop
• no preview predict the future course of the
  input (Fig 10.10)
• precognition mode the input is nonrandom or
  contains periodicities easier prediction
• Output Prediction and Quickening
• output prediction a combination of its present
  position and its higher derivatives ? displays in
  which a computer estimates error derivatives and
  future position ? predictive display
• (Fig 10.11 Fig 10.12)
• accuracy of any prediction sluggishness
  (inertia) of the system and the frequency of
  control or disturbance inputs

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• quickening where the system error likely to be
  in the future if is not controlled --no
  indication of current error because current error
  provides no information that is useful for
  correction
• Pursuit Versus Compensatory Displays
• the goal of tracking to match the output to the
  input, or minimize error
• pursuit display views the command input and the
  system output moving separately
• generally superior performance to compensatory
  one for two major reasons -- ambiguity of
  compensatory information and the compatibility of
  pursuit displays
• cannot distinguish among the three potential
  causes of error command input, disturbance
  input, and the operators own incorrect control
  actions -- error is ambiguous and control is more
  difficult
• a changing command input advantage with greater
  S-R compatibility compensatory display
• CONTROL DEVICES
• the light pen, touch screen, trackball, mouse,
  cursor keys, or joysticks
• implication of different control devices for
  simple positioning tasks, characterized by
  Fittss law
• Manual Control
• manual control devices anthropometric and
  biomechanical factors
• The Speed-Accuracy Trade-off
• direct point devices (light pen or touch screen)
  -- very rapid but less accurate than indirect
  devices (mouse)
• the slope of the Fittss law the overall
  effectiveness of a control device, (lower,
  better)
• the slope by the mouse lower than other control
  devices

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• Control Order Compatibility
• most control devices displacement produce a
  constant displacement (zero-order) or constant
  rate of movement (first-order) of the cursors
  across the screen
• only the light pen and touch screen must be
  zero-order controllers
• natural affinities or compatibilities mice
  for zero-order control dynamics, and joysticks
  for first-order control dynamics
• mouse
• best captures the natural eye-hand coordination
• the optimal gain is between one and three the
  cursor should travel between one and three times
  the distance traveled by the hand
• if first-order control abandons the natural
  eye-hand coordination and the zero-velocity
  resting place
• spring-loaded joysticks
• spring loading maximize proprioceptive and
  kinesthetic feedback
• natural resting state by snap back
  automatically recovered
• problems as a zero-order device the lack of
  precision with which any position off of the
  center can be maintained, lack of available
  movement range

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