CHI 97 Electronic Publications: Late-Breaking/Short Talks
The Tactile Touchpad
I. Scott MacKenzie
Dept. of Computing & Information Science
University of Guelph
Guelph, Ontario, Canada N1G 2W1
+1-519-824-4120
mac@snowhite.cis.uoguelph.ca
Aleks Oniszczak
Dept. of Computing & Information Science
University of Guelph
Guelph, Ontario, Canada N1G 2W1
+1-519-824-4120
aoniszcz@uoguelph.ca
ABSTRACT
A prototype touchpad with embedded tactile feedback is
described. Tactile feedback allows the touchpad to mimic the operation of a
mouse for basic transactions such as clicking, double-clicking, and dragging.
A button click is achieved by increasing the finger pressure applied to the
touchpad, instead of using a lift-and-tap strategy or by pressing separate
buttons. The result is more natural and less error prone. Pressure thresholds
for the button-down and button-up actions are under software control and
include hysteresis to minimise inadvertent selections.
Keywords
touchpads, pointing devices, tactile feedback
© 1997 Copyright on this material is held by the authors.
INTRODUCTION
Touchpads have recently become the pointing device of choice for
notebook computers. Since notebooks are often operated in constrained spaces,
mice are generally not used. Until recently, most notebook computers included
either a trackball or an isometric joystick as a pointing device. Apple
introduced the touchpad (the "TrackPad") in their PowerBook 500 series notebook
computer. A "lift-and-tap" method of clicking became a later addition.
TOUCHPADS VS. MICE
Although touchpads are also available for desktop computers, most people
prefer a mouse for their desktop system. So, why is a mouse a better pointing
device than a touchpad when space is not an issue? The answer lies in the
separation of selection from positioning. Using a mouse, the cursor is
positioned by moving the mouse on a mousepad. The device is gripped between
the fingers and thumb and movement occurs via the wrist and forearm. With a
touchpad, pointer movement is accomplished by sliding a finger along the
touchpad's surface. Both are generally used as "relative positioning" devices,
where the pointer moves relative to its previous position when the device or
finger moves.
For a mouse, selecting (or clicking) is the act of pressing and releasing a
button while the cursor is over an icon or other screen object. Double clicking
and dragging are relatedoperations that also require pressing a button. There
are two common implementations for selecting with touchpads: (a) using
lift-and-tap, or (b) using physical buttons. Both inherit problems which we
are attempting to correct in our tactile touchpad.
Physical Buttons
Most touchpads include physical buttons that are typically operated with
the index finger or thumb. If an index finger is used, the finger must move
frequently between the touchpad and the buttons and this impedes performance.
If the thumb is used, then positioning and selecting proceed in concert, as
with a mouse; however, the result is sub-optimal because of interference
between the muscle and limb groups engaged. A similar problem exists for
trackballs , wherein high error rates (particularly for dragging tasks) are
attributed to the "closeness" of the muscle and limb groups required for the
separate acts of positioning and selecting. With a mouse, on the other hand,
positioning occurs primarily via the wrist and forearm, while selecting occurs
primarily through the fingers. Thus, the limbs and muscle groups are separate
for each task and do not interfere.
Lift-and-Tap
Because of the problem noted above, most touchpads also support
"lift-and-tap" as an alternative to pressing buttons. However, this is simply
replacing one problem with another. We'll illustrate this by considering the
basic transactions with computer pointing devices. According to Buxton's
three-state model of graphical input , these can be modelled by three states:
- State 0
- out-of-range (the device/finger is elevated)
- State 1
- tracking (cursor movement)
- State 2
- dragging (movement with button depressed)
For touchpads and mice, pointer motion occurs in state 1 - the tracking state.
The comparison becomes interesting when we consider clicking, double clicking,
dragging, and clutching. (Clutching is the act of lifting the mouse or finger
at the edge of the mousepad or touch surface and repositioning it. Clutching
is a requirement of relative pointing devices.) compares dragging for a mouse
and touchpad. Two observations follow: (1) lift-and-tap necessitates extra
state transitions when compared to a mouse, and (2) the use of state 1-0-1
transitions is confounded with clutching (not shown) which uses the same state
transitions.
Figure 1. State transitions for dragging. (a) mouse (b) touchpad using
lift-and-tap. The first horizontal bar is the cursor positioning before
dragging.
THE TACTILE TOUCHPAD
In view of the preceding, it worth exploring alternate, perhaps better,
implementations for state transitions. One possibility is to implement state
transitions by pressing harder with the pointing/positioning finger. A mouse
button provides aural and tactile feedback when it is pressed, and this is an
important component of the interaction. Similar feedback may be elicited from
a touchpad by means of a mechanical solenoid or relay positioned under the pad
and activated with an electrical signal to create a "click" sensation in the
fingertip. Since a mouse button clicks both when pressed and when released,
the same response is desirable for a tactile touchpad.
To prevent spurious clicks, the transitions should include hysteresis; that is,
the state 1-2 pressure threshold should be higher than the state 2-1 pressure
threshold. This is illustrated in . The correct thresholds must be determined
in user tests.
Figure 2. Pressure-state function. A click is generated for state 1-2
transitions and for state 2-1 transitions.
There is some prior work on embedding a solenoid under a mouse button to create
tactile feedback . We feel the combination of spatially-placed aural and
tactile feedback at the finger tip is preferable to spatially-displaced
audio-only feedback using the system's loudspeaker, although the latter can be
investigated as a simple alternative.
Our tactile touchpad is illustrated in . For our prototype, we cut a hole in
the bottom of a Synaptics T1002D touchpad and installed a Potter &
Brumfield T90N1D12-5 relay. A wooden platform attached to base provides space
for the relay. The relay is controlled by signals sent from the host's
parallel port.
The Synaptics touchpad includes an x-y-z mode in which the z-axis
information is the applied pressure. Our software uses z-axis
information to determine when to energise and de-energise the relay. In
informal tests, one user noted that the tactile sense in the finger tip gives
the impression that the touchpad moves slightly at the pressure thresholds.
Formal experiments are planned to determine the correct pressure thresholds and
to compare the speed and accuracy of basic point-select tasks with other
pointing devices.
Figure 3. The tactile touchpad. (a) top view. (b) bottom view.
Acknowledgements
We thank Joe Decker of Synaptics for providing the touchpads and
technical documentation for our prototype. This research is funded by NSERC of
Canada.
References
- Akamatsu, A., and MacKenzie, I. S. Movement characteristics using a
mouse with tactile and force feedback, International Journal of
Human-Computer Studies 45 (1996), 483-493
- Buxton, W. A. S. A three-state model of graphical input, In
Proceedings of Interact '90. Amsterdam: Elsevier
Science, 1990, pp. 449-456.
- MacKenzie, I. S., Sellen, A., and Buxton, W. A comparison of input
devices in elementatl pointing and dragging tasks, in
Proceedings of the CHI '91 Conference on Human Factors in
Computing Systems. New York: ACM, 1991, pp. 161-166.
CHI 97 Electronic Publications: Late-Breaking/Short Talks
