1 Chapter 9: Controls System Interface (Mouse, touch screen, joystick) Response expectancy Compatibility Speed accuracy tradeoff Decision complexity Feedback

2 General principles of control design n Decision complexity (Simple choices have faster response than complex, but few complex choices are faster than many simple) n Response expectancy (RT much smaller for expected events) n Compatibility (Location and movement compatibility should match mental model) n Speed-accuracy tradeoff (More errors with speeded response) n Feedback (Display of system response)

3 Decision complexity and Hick-Hyman law Number of alternatives (N) Reaction time RT=a+b log 2 (N) Does NOT imply many simple decisions are faster than a few complex decisions Decision complexity advantage (typing vs Morse code; deep vs. shallow menus)

4 Display control compatibility Location compatibility Movement compatibility

5 Speed accuracy tradeoff n Possible to be fast and error prone OR slow and precise n Due to strategies that reflect different payoffs between errors and response speed n Due to control devices that induce faster but less precise control

6 Feedback n Feel of button (deflection of key and click of keyboard vs. membrane keyboard) n Feedback and delays: less than 100msec to avoid disrupting motor control less than 1.0 sec to avoid disrupting thought 100msec to 1.0 sec user feels directly connected to data less than 10 seconds to keep user’s attention focussed on the dialog. Feedback regarding magnitude of delay is critical

7 Types of controls and input devices n Discrete (toggle switch, buttons, keyboards) n Positioning devices n Text and symbol n Voice input n Continuous control

8 Discrete control devices n Feedback: Physical feel of device and the problems with touch screens compared to toggle switches n Speed accuracy tradeoff: Button or key size and errors vs. response speed n Decision complexity: Many similar controls promote confusion n Location compatibility: Poor labels, poor positioning of labels

9 Positioning control devices n Speed accuracy tradeoff: Fitts Law Movement time=a+b log 2 (2A/W) n A=movement amplitude n W=target width n Very general law that applies to microscopic movements to foot pedal control n a and b depend on device characteristics W A

10 Positioning device characteristics n Direct controls: Light pen and touch screen n Indirect controls: mouse, trackball, tablet, touch pad n Indirect velocity controls: joystick (speed of cursor proportionate to deflection) n Mixture of position and velocity controls (most mouse designs)

11 Positioning device characteristics n Feedback: System lags disrupt performance n Speed accuracy tradeoff: Gain (change of cursor/change of control position) High gain = fast movements Low gain = precise movements Gain of direct position controls equals 1 n Appropriate device type and gain are task dependent

12 Text and symbol entry n QWERTY and Dvorak keyboards n QWERTY persists even though Dvorak offers a 5-10% advantage n Chording keyboards benefits No requirement for visual feedback Less susceptible to repetitive stress injury More rapid data entry (decision complexity advantage) n Chording keyboard cost= extensive training

13 Voice input n Benefits of voice input Natural Supports timesharing (dialing a car phone) n Costs of voice input Limited vocabulary size Word confusion (“cleared to” vs. “cleared through”) Constraints on speed Interference of noise and stress Compatibility with spatial tasks (steering) Introduces some costs in timesharing (talking on cell phone while driving)

14 Continuous control and tracking n Make system output correspond in time and space to a time-varying input n Examples include Baseball batter Driving Pilot Oil tanker pilot Process control operator Company safety record

15 Elements of the basic tracking loop Display Human Control device System or plant Disturbance input id(t) o(t) u(t)f(t) e(t) Each transfer function receives time-varying input and produces a time-varying output Command input ic(t)

16 Inputs to transfer functions Display Human Control device System or plant Disturbance input id(t) o(t) u(t)f(t) e(t) Error Force/action Control output System response Command input ic(t)

17 Control and system dynamics Display Human Control device System or plant Disturbance input id(t) o(t) u(t)f(t) e(t) Control dynamics u(t)=f(f(t)) Command input ic(t) System dynamics o(t)=f(u(t)) Human limits f(t)=f(e(t))

18 Characteristics of control systems n Input Bandwidth Type (sine wave, step function, square wave) n Control dynamics Gain Control order Time delays n Stability =f(time delay, control order, gain, bandwidth) Pilot induced oscillations Speed accuracy tradeoff

19 Control order n Position control (zero-order) Movement of control specifies position Much effort n Rate or velocity control (first order) Movement of control specifies rate of change Little effort to initiate, more effort in prediction Requires a clearly defined neutral point n Acceleration control (second order) Movement of control specifies rate of change of rate of change Poor control

20 Cursor response to control inputs Zero order (position control) First order (rate control) First order (rate control) Second order (acceleration control) Sine input Step input Ramp input Gain=2 Input Output

21 Zero order (position control) First order (rate control) First order (rate control) Second order (acceleration control) Sine input Step input Ramp input Input Response Operator response to match inputs

22 Human limits as a controller n Processing time is not zero Delay for 0 and 1st order = msec Delay for 2nd order = msec n Bandwidth Maximum frequency Hz (two corrections/sec) Maximum frequency for predictable targets 2-3 Hz n Anticipation and higher order systems Poor at detecting acceleration and prediction

23 Selecting an appropriate gain n Selection of gain depends on task and system n High gain: Large movements Fast movements Less stable and prone to overshoot n Low gain Precise movements More stable More effort and time

24 Control and system dynamics Display Human Control device System or plant Disturbance input id(t) o(t) u(t)f(t) e(t) Command input ic(t) Pursuit: Target and cursor independently (road) Match output to goal Compensatory: Error only, cursor-target (flight) Minimize error

25 Selecting the appropriate display n Pursuit displays generally outperform compensatory displays: Less ambiguity of compensatory information Compatibility of motion (tendency to move toward stimulus) n Whenever some tracking input is a command, pursuit displays will benefit: ic(t) not constant n Benefit of pursuit displays decline with fixed command input: ic(t)=constant Landing on a runway vs on an aircraft carrier

26 Open and closed loop control Display Human Control device System or plant Disturbance input id(t) o(t) u(t)f(t) e(t) Closed loop control Display Human Control device System or plant Disturbance input id(t) u(t)f(t) e(t) Open loop control o(t) Command input ic(t) Command input ic(t)

27 Limits of open loop control n Ignores disturbance inputs n Susceptible to mismatches between mental model (internalization of system dynamics) and system n Requires high degree of skill or reliable procedures Baseball player Pianist Process control operator Cook following recipe

28 Mitigating problems of controlling 2nd and higher order systems n Predictive and preview displays n Teach strategies Perceive trend information (look down the road to see heading of car) Internalize system dynamics Appropriate eye scanning patterns n Automate task Open loop control by person Closed loop control by technology

29 Mitigating instability n Lower gain n Reduce lags (e.g., less complex graphics) n Change strategy to ignore high frequency input n Change strategy to seek predictive cues n Change strategy to adopt open loop response

30 Control theory and safety management

31 Safety management as feedback control and its limits

32 Safety management as open loop control and its limits

33 Key points of continuous control n Generality of the control theory description of system performance n The following concepts and their effect on performance: Gain Control order Bandwidth Stability Feedback Compensatory and pursuit displays n Requirements and characteristics of open-loop and closed loop performance