Integration of Electronic External Devices for Powered Mobility Systems

Task Leader: Barry Romich
Co-Investigators: Katya Hill, Edmund LoPresti, Donald Spaeth

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Integration of Electronic External Devices for Powered Mobility Systems

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Task Leader: Barry Romich
Co-Investigators: Katya Hill, Edmund LoPresti, Donald Spaeth

RERC on Wheeled Mobility
University of Pittsburgh

Slide 2
Scope of the Project

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Many people who use powered wheelchairs can benefit from using the wheelchair controller to operate other functions. This project is exploring new options for implementing this function with improved performance over traditional approaches.

Slide 3
Distributed and Integrated Controls

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Graphic description: This drawing illustrates the difference between distributed controls and integrated controls. It contains two cartoon drawings of a consumer sitting in a wheelchair. In each drawing, four blocks are drawn representing four different assistive devices. The blocks are labeled wheelchair, AAC device, ECU system and computer. In the first drawing, labeled distributed controls, the consumer is frowning because he must use a separate control for each of his assistive technology devices. In the second drawing, labeled integrated controls, the consumer is smiling because one single control will operate all four of his asssistive technology devices.

Slide 4
Elements of the Project:
Literature review

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Slide 5
Computer Access Proportional v. Switched Control

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Graphic description: This is a diagram labeled Computer Access: Proportional vs. Switched Control. There are two graphs on this diagram. In each graph, the Y axis is labeled cursor velosity and the X axis is labeled joystick position. There are no numbers or tic marks on any of the axes. In the left hand graph,labeled switch control, the graph plot at the origin has a Y value of zero. As we move to the right, along the X axis the plotted value remains at zero for about the first third. then, the Y value jump sharply to a positive value; it then remains at his value for the remainder of the X axis. The graph is intended to illustrate that when you use a switch control, you can only select two states for the cursor velocity: stopped or moving at a fixed speed. This is comparable to a wall switch that turns your lights on and off. The right hand graph is labeled Proportional Control. Here the plot at the origin is also zero but as the plot progress to right, the Y value smoothly increases so that the plot is a rising line. This graph illustrates that when you use a proportional control you may select many different cursor velocities by gradually changing the joystick position. Common examples of proportional controls in the home are the faucet handle on your sink and the volume control on your radio.

One particular application of an integrated control system is to provide computer access through a person's wheelchair controller. Some existing devices already allow a person to operate a computer using a wheelchair joystick. However, these devices translate information about joystick position into four switch inputs. This allows a person to control the direction of cursor movement, but not the cursor's speed of movement; rather the cursor will move at a constant speed. It would be desirable if a proportional wheelchair joystick (one which allows the driver to control both wheelchair direction and wheelchair speed) could also control both cursor direction and speed on the computer.

Slide 6
Proportional Computer Access Position v. Velocity Control

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Position Control: joystick position determines cursor position.

Velocity Control: joystick position determines cursor speed and direction.

Graphic description: This ia a diagram labeled Proportional Computer Access: Position vs. Velocity Control. It contains two graphs. The Y axis on both graphs is labeled Joystick Position. The X axis on both graphs is labeled Time. The plot is intended to represent the response of the screen cursor. In the left graph labeled Position Control we observe the plot is a straight line begining at zero at the origin and rising steadily to the right. A sentence above the graphic reads Position Control: joystick position determines cursor position. A common example of this kind of control is a flashlight or garden hose; as you move the flashligt or hose, the light or water stream moves with you exactly. The right graph is labeled Velocity Control. In this graph the plot the of the cursor first rises then levels out then descends back to zero. This represents a cursor that moves at a rate determined by the joystick position. The more you tilt the joystick the faster the cursor will move. Bring the joystick back to center and the cursor stops immediately. A sentence above this graph reads: Velocity Control: joystick position determines cursor speed and direction.

There are at least two ways in which such proportional control could be provided: position control or velocity control. With position control, the joystick position determines cursor position. With velocity control, joystick position determines the cursor's speed. In order to select an on-screen icon, the user of a position-control system will move the joystick to a position associated with the position of the icon on the screen. The user of a velocity-control system will move the joystick once to select a direction and speed for the cursor. The user will hold the joystick steady until the cursor is close to the target, then move the joystick back to the center in order to stop the cursor on the icon.

Slide 7
Advantages of Position Control

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Position control is faster at higher indices of difficulty

Jagacinski et al. 1978 found

  • Position control is faster for an index of difficulty greater than 4.7 bits
  • 4.7 bits > distance to target > 13 x width of target

Researchers comparing these two methods have found that position control is faster at higher indices of difficulty. In one study, Jagacinski and colleagues found that position control is faster for an index of difficulty greater than 4.7 bits. This index of difficulty corresponds to a situation in which the distance to a target is 13 times the width of the target. These studies indicate that selection of a computer icon with a position-control system is generally superior to selection with a velocity-control system.

Slide 8
Advantages of Velocity Control

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Faster when moving short distances on the screen, or selecting large objects.

Cursor will remain stationary when the joystick is released.

May be less sensitive to a loss of calibration between joystick and cursor.

Velocity control has advantages. Based on these same studies, velocity control is superior for targets with a low index of difficulty. Therefore, velocity control is expected to be faster when moving short distances on the screen, or selecting large objects.

Also, with velocity control a person can release the joystick and the cursor will not move, since the center joystick position corresponds to zero velocity. The person can then concentrate on performing a button click. In a position-control system, it is necessary to hold the joystick at a desired position while also performing a mouse button click.

Finally, a velocity control mode may be less sensitive to a loss of calibration between joystick and cursor, because a particular joystick position does not have to remain mapped to a specific screen location.

Slide 9
Research Goal

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Determine the relative performance of various control modes in a real-world setting.

Informed design of an integrated control system.

While research indicates that selection of a stationary icon with a position-control system is generally superior to selection with a velocity-control system, this superiority could be mitigated by factors affecting real-world computer control. In order to design an interface between wheelchair controllers and computers, it is desirable to determine whether position control or velocity control is most useful in a real-world setting. Therefore, both methods are being implemented and evaluated in a prototype integrated control system.

Slide 10
Elements of the Project: Development

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Slide 11
Mouse Emulation

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Brief Definition:

In an integrated controls environment, providing the option of using the wheelchair joystick as a computer mouse.

Slide 12
Possible Approaches

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RERC D-2 task considered two methods of providing computer mouse access:

1. Directly translating wheelchair analog signals into mouse format.

2. Working with a wheelchair joystick which contains a data port provided by the manufacturer.

Slide 13
Some Existing Options

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Gus software

Penny and Giles mouse joystick

Permobile infrared system

Slide 14
Direct Analog Translation Goals

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Ability to process two channels of analog signals; from wheelchair joystick and possibly other force transducers.

Output should be in a standard mouse format, Serial, PS/2 or USB and work with standard mouse drivers.

Slide 15

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Avoid reinventing mouse controller chip that interacts with the computer’s mouse port.

Rationale for this decision:

  • Public would expect support of multiple formats, PS/2, Serial, USB, Older Mac ADB.

Hardware that can perform this task is readily available at low cost.

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Searched for a commercial mouse controller chip that would accept analog input and translate it into mouse format.

Only prospect located to date is the HulaPointTM by USAR systems. Used with Hall effects transducer on handheld systems.

Slide 17
Investigation Results

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Except for the HulaPointTM, all other mouse controller chips reviewed required quadrature data at the input pins.

Quadrature is the digital data format generated by a mouse's rolling ball driving two optical shutter wheels.

Slide 18
Analog to Quadrature Translation

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Time does not permit a formal discussion of the electronic circuitry used to convert analog signals into quadrature format.

Consult RESNA proceedings or consult with the authors.

Slide 19
Summary of Solution

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Devised a low cost circuit (Radio Shack parts) that translates analog signals into the digital quadrature.

Purchased a mass market mouse and "hotwired" the controller chip pins.

Plug the commercial mouse into a standard computer mouse port.

Commercial mouse "thinks" the digital signals are coming from its own roller ball.

Slide 20

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Circuit built from inexpensive generic technology.

Prototype has been tested with standard serial and Logitech wireless mouse boards. Should work with any rolling ball mouse or trackball: Serial, PS/2, USB, Wired, and Wireless versions.

Target computer runs standard mouse driver.

Possibility of connecting other analog signals in addition to joysticks; i.e., Hall effects transducer, photo sensor, strain gage, piezoelectric.

Slide 21

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Currently a build-it-yourself, experimental solution, requires electronic construction skills, no commercial tech support.

Expect the usual problems motivating tech transfer - costs of tooling a PC board for small volume production.

"hot wiring" commercial products voids warrantees. Mice are cheap, wheelchair joysticks are not. Better if the manufacturer provides access to signals.

Slide 22
Joystick to Mouse Adapter

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Input: Invacare Mk IV controller plus two switches (left and right mouse buttons)

  • Serial joystick position (1 byte per axis)

Output: IR wireless to mouse emulator


  • Position control
  • Velocity control
  • Hybrid

Adjustable gain

Slide 23
Joystick to Mouse Adapter

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Graphic Description: This is a photograph of a rectangular black, plastic box about the size of a deck of cards. Inside this box is a circuit board that can translate the signals from a wheelchair joystick into the signals generated by a computer mouse. Barely visible on the sides of the box are cable connectors, one for the wheelchair joystick cable input and one that connects to the mouse port of the computer.

Slide 24
Elements of the Project: Evaluation

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Slide 25

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  • People who use powered wheelchairs and have good joystick control


  • Position, velocity, and hybrid control
  • Traditional switch system

Tasks and Units of Measure

  • Target acquisition (Ed LoPresti, U of Pittsburgh)
    • bits per second
  • Typing Instructor driven by WiViK
    • words per minute

The End

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Updated: March 12, 2002

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 © Copyright 2006 University of Pittsburgh. All rights reserved.
No quotes from the materials contained herein may be used in any media without attribution to WheelchairNet and the Department of Rehabilitation Science and Technology.

Please note: This information is provided a archival information from the Rehabilitation Engineering Research Center on Wheeled Mobility from 1993 to 2002.

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