Use of a Touch Screen and Force Plate System to Quantify Touch Characteristics During a Timed Reciprocal Tapping Test
Curt B. Irwin, PhD, Robert H. Meyer, MS, Thomas Y. Yen, PhD, David P. Kelso, MS, Mary E. Sesto, PT, PhD
ABSTRACT
Increasingly, people are required to interact with touch screens at places ranging from grocery stores to airport kiosks. To date, most of the usability research related to touch screens has included young, healthy subjects. Using novel instrumentation consisting of a force plate and a touch screen, a reciprocal tapping study examined finger-touch screen interaction by participants with Cerebral Palsy, Multiple Sclerosis, and non-disabled controls. Timing data as well as forces and impulses in three dimensions were collected for each touch. The results indicate that, although force vector magnitudes are similar between the groups, there are significant differences in force impulses and timing.
KEYWORDS
Touch Screen, Force, Impulse, Tapping, Usability
BACKGROUND
As more touch screens are installed for use in public places, the inability of individuals with disabilities to effectively use this input device may hamper access to products, services and opportunities. Thus, research to improve touch screen usability and accessibility is important to the effective design of these devices for public use.
Enhancing touch screen technology requires a mechanism to characterize performance for individuals with varying disabilities. To date, usability studies on touch screens have typically focused on timing and accuracy. Research investigating the touch characteristics of the user including kinetic measures of force, impulse and force direction data is lacking. Comparisons of touch characteristics including force-related information for disabled and non-disabled people may provide insight into motor control strategies used by people with disabilities that can be used to improve touch screen accessibility.
In the past, tapping tests have been used to investigate possible motor control strategies of diverse groups of individuals. Young and elderly participants, people diagnosed with cerebral vascular accident (CVA), cerebral palsy, Alzheimer’s disease, or Parkinson’s disease have been studied while performing a tapping-style test. From a movement standpoint, researchers have used tap tests to help characterize the functioning of an individual’s motor control system. The resulting kinetic and tap test information is important to understand how people with varying movement disorders interact with technology.
For example, in tap testing of children with spastic hemiparesis it was found that dwell times on the key were longer while tapping with the impaired hand than with the non-impaired hand (1,2) or compared to controls (3). Other researchers have shown that finger tapping by children with developmental coordination disorders in either the basal ganglia or cerebellum slower and has more force variation than tapping by children with no disabilities (4, 5).
In this study we examine the timing, force, and impulse (area under the force-time curve) characteristics to determine if differences or similarities exist between individuals with cerebral palsy (CP), multiple sclerosis (MS) and non-disabled controls. Participants who were right handed performed a five second reciprocal tapping test using their right hand as part of a larger number entry experiment. Whereas other reciprocal tapping tests use physical keys, digitizing tablets, or simply paper and a marker pen, this study uses instrumentation that consists of a touch screen connected to a force plate. Although forces normal to keys have been examined in the past, this study expands upon our understanding of tapping characteristics by quantifying forces and impulses in directions other than directly normal to the key.
METHODS
Subjects
Seven adults from each category (CP, MS, and Control) were recruited. The control subjects were age and gender matched to the disabled group. Table 1 presents the gender, age, and diagnosis of the subjects. Informed consent was obtained in accordance with the University of Wisconsin guidelines for the protection of human subjects.
| Control | Subject | Age | Gender | CP | Subject | Age | Gender | MS | Subject | Age | Gender |
|---|---|---|---|---|---|---|---|---|---|---|---|
1 |
41 |
Male |
1 |
42 |
Male |
1 |
43 |
Female |
|||
2 |
40 |
Female |
2 |
60 |
Male |
2 |
53 |
Female |
|||
3 |
71 |
Male |
3 |
80 |
Male |
3 |
50 |
Female |
|||
4 |
59 |
Female |
4 |
47 |
Male |
4 |
51 |
Female |
|||
5 |
42 |
Male |
5 |
47 |
Male |
5 |
59 |
Female |
|||
6 |
53 |
Male |
6 |
37 |
Male |
6 |
48 |
Female |
|||
7 |
42 |
Female |
7 |
48 |
Male |
7 |
42 |
Male |
|||
| Average | 49.7 | Average |
51.6 |
|
Average |
49.4 |
|
||||
| SD | 11.8 | SD |
14.4 |
|
SD |
5.9 |
|
Instrumentation
The kiosk-style data acquisition system consisted of a touch screen mounted to a force plate. The force plate was oriented horizontally in an extruded aluminum frame and the touch screen was rigidly affixed at an adjustable angle to the force plate (6). Touch location information was obtained from the touch screen (Elo-touch by Tyco Systems, inc.) and the force and impulse (force x duration) data was measured by the force plate (Bertec model NG4060-10). The force plate-touch screen system was mounted in an adjustable frame that, for the current experiment, positioned the touch screen with adequate knee clearance of at least 69 cm as specified in the ADA accessibility guidelines. Forces and impulses in the x-direction are to the user’s left and right in the plane of the touch screen. Forces and impulses in the y-direction are up and down and the z-direction refers to forces and impulses normal to the touch screen. The data acquisition system and setup is described in full in (6).
User interface
Figure 1: Tap test layout and vector direction definitions (Click for larger view) The experiment consisted of a tap test imbedded into a larger number entry task. The tap test was designed to be an indicator of participant fatigue. Additional self-report fatigue measures were also used. The number entry task used a 4x 4 numerical keypad on the touch screen with the addition of a “Go” button on the upper left side and a “Done” button on the upper right side of the button array. Participants were required to enter sixty 4 digit numbers using an array of button sizes and spacing. Before the number entry trials began and after every twelve number entry trials a five second tap test was performed resulting in six separate tap tests per subject. The tap test itself involved two 60 x 60mm buttons on the touch screen separated by 90mm (Figure 1). Participants were instructed to tap in a reciprocating manner between the two buttons as fast as possible. A white circle appeared on the button to be touched and moved to the alternate button following a successful touch. The buttons were activated with a “land-on” strategy and the touch screen required an activation force of 0.98 N. Practice sessions for all tasks were provided.
Data Analysis
Force and impulse data was collected for every button push event occurring during the tap test but for the purposes of this study the information from the first button press of each tap test session was eliminated as this button push only served as a timing marker to begin the 5 second test window. The force and impulse information was averaged across all six tap tests for each individual as there was no statistically significant difference for timing or force between tests. Tap data was eliminated for three subjects that were either unable to complete all 60 number entry trials or were unable to complete all six tap tests.
Data were collected for all three dimensions relative to the touch screen, x, y, and z as defined in Figure 1. The forces reported for the three dimensions are the instantaneous maximum forces measured during the entirety of the touch event but may not all be maximized at the same moment in time. For instance, the maximum value for the x-direction force may occur early in the touch event and the y and z-direction maximum forces may occur later in the touch. The overall maximum force vector is calculated when the resultant vector of the x, y, and z-direction forces is at its maximum. This may not occur at the same instant as any of the constituent force maximums. The directional impulses are calculated by integrating under the force curve for the entire duration of the touch event. The time spent pushing each button, or the dwell time, was also collected for each button touch. The dwell times link the force and impulse data because impulse is roughly calculated as force multiplied by dwell time.
Analysis of variance was used to determine significant effects of force, impulse, and dwell time. Post hoc Bonferonni analysis was used to identify differences between CP, MS, and control groups. Pairwise t-tests were used to identify differences in the measurements between the left and right buttons.
RESULTS
The number of total taps per 5 seconds for each group is displayed in Table 2. The difference in number of taps between the groups was found to be significant (p<0.01). Post-hoc testing revealed that the control group was significantly faster than both the CP or MS groups (p<0.01). The dwell times for both the right (p<0.01) and left p<0.01) buttons were also significant between groups. Post hoc testing demonstrated the dwell time for the right button was greater for the CP group as compared to both the MS and control groups (p<0.05). For the left button, the CP group had a significantly greater dwell time than the control group (p<0.01) and the dwell time was greater for the CP group than the MS group (p=0.051).
|
N |
|
Number of Touches |
Left Dwell Time [s] |
Right Dwell Time [s] |
|---|---|---|---|---|---|
Control |
7 |
Mean |
22.61* |
0.095* |
0.089* |
(SD) |
(6.83) |
(0.012) |
(0.008) |
||
CP |
4 |
Mean |
6.45 |
0.222 |
0.216* |
(SD) |
(2.34) |
(0.084) |
(0.080) |
||
MS |
7 |
Mean |
9.63 |
0.136 |
0.126* |
(SD) |
(3.71) |
(0.059) |
(0.050) |
Force and impulse data for every direction and subject group are presented in Tables 3 and 4. No significant differences were found for the peak forces Fx, Fy, Fz, or Fv (p>.05) for either the right of left buttons between subject groups.
| Force [N] | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Right Button | Left Button | ||||||||
| Peak X | Peak Y | Peak Z | Peak Vector | Peak X | Peak Y | Peak Z | Peak Vector | ||
| Control | Mean | -1.317 |
-1.651 |
5.708 |
6.133 |
0.626 |
-1.434 |
5.377 |
5.688 |
| (SD) | (0.494) |
(0.853) |
(1.888) |
(2.039) |
(0.714) |
(0.688) |
(1.263) |
(1.389) |
|
| CP | Mean | -0.850 |
-0.588 |
5.400 |
5.763 |
-0.515 |
-0.200 |
5.161 |
5.351 |
| (SD) | (1.683) |
(0.730) |
(2.499) |
(2.706) |
(0.713) |
(0.468) |
(2.443) |
(2.536) |
|
| MS | Mean | -0.925 |
-1.575 |
5.138 |
5.533 |
0.648 |
-1.205 |
4.875 |
5.214 |
| (SD) | (0.848) |
(2.088) |
(4.596) |
(5.112) |
(1.844) |
(1.365) |
(3.887) |
(4.412) |
|
| Impulse [N-s] | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Right Button | Left Button | ||||||||
| X | Y | Z | Vector | X | Y | Z | Vector | ||
| Control | Mean | 0.049 |
0.047 |
0.210 |
0.224 |
0.046 |
0.048 |
0.234 |
0.248 |
| (SD) | (0.012) |
(0.022) |
(0.0612) |
(0.065) |
(0.014) |
(0.015) |
(0.049) |
(0.049) |
|
| CP | Mean | 0.170 |
0.141 |
0.665 |
0.714 |
0.125 |
0.163 |
0.720 |
0.763 |
| (SD) | (0.116) |
(0.120) |
(0.473) |
(0.510) |
(0.087) |
(0.152) |
(0.527) |
(0.565) |
|
| MS | Mean | 0.053 |
0.058 |
0.290 |
0.305 |
0.057 |
0.060 |
0.328 |
0.345 |
| (SD) | (0.037) |
(0.056) |
(0.217) |
(0.228) |
(0.068) |
(0.044) |
(0.252) |
(0.263) |
|
| Total | Mean | 0.077* |
0.072 |
0.342* |
0.365* |
0.068* |
0.078 |
0.379* |
0.400* |
| (SD) | (0.074) |
(0.073) |
(0.300) |
(0.323) |
(0.064) |
(0.084) |
(0.331) |
(0.351) |
|
The impulses for directions and buttons that were significantly different between groups are indicated with an asterisk in Table 4. Post hoc testing showed, with one exception, all of the significant impulse differences were between controls and CP participants, with the impulses for the CP group being larger (p<0.05). No significant differences were found for impulses between the control and MS groups except for the impulse in the x-direction for the right button, which was significantly different between all three groups (p<0.05).
To determine if the corresponding measurements between the left and right buttons were different, paired t-tests were performed. The results indicated that only the peak force in the x-direction and the peak force in the y-direction were significantly different between the two buttons (p<0.05).
DISCUSSION
Similar to the results reported in van Roon et al., the CP group demonstrated significantly fewer button pushes (29%) than the control group for the five second tap test. This is expected as it has been reported that people with movement difficulties are less accurate in aiming and have longer deceleration phases in pointing movements (7). However, the amount of time spent aiming and moving, without touching the button, is but one component of the reciprocal tapping motion. Similar to the findings from other experiments (1, 2, 3) disabled subjects in the current experiment demonstrated longer dwell times than non-disabled subjects. The MS and control groups in the current experiment had substantially shorter dwell times, 60% and 42% respectively, than the CP group. Van Roon et al. postulate that CP participants may have longer dwell times because of their difficulty in relaxing muscles. This combination of longer aiming and movement times and longer dwell times sum to produce fewer button touches for the CP group.
Prior research with individuals with spastic hemiparesis has noted that the impaired hand generated less force than the unimpaired hand during reciprocal tapping activities (2). In our study, the peak force vector magnitudes for both the MS and CP groups were smaller than the control group, although the differences are not significant. The lack of significant findings may be due to the small sample size. Our results did show that button location may have an effect on peak force in the x and y-directions.
Impulse in this experiment is calculated by integrating under the force-time curve for each button push. As such, it is force multiplied by time. Since the forces between the groups are not dramatically different but impulses are significantly different, the dwell time on the button appears to be the reason why the impulses differ.
CONCLUSION
This is the first in a series of studies examining interface behavior within and across disability groups using instrumentation that allows us to better understand the exact physical characteristics at the human – interface point of contact. This research may help to improve our understanding of how people with varying movement disorders interact with technology. As a corollary, we can examine motor control strategies and compare and contrast these strategies across disability types and the effects on interface use. Understanding these forces and movements may help designers engineer better interfaces that will work with a wider range of user abilities.
REFERENCES
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- Williams, H.G., M.H. Woollacott, and R. Ivry, Timing and motor control in clumsy children. Journal of Motor Behavior, 1992. 24(2): p. 165-172.
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- Yen, T., Meyer, R., Vanderheiden, G., Irwin, C., Kelso, D., Sesto, M. Incorporating force-related information to improve characterization and functional assessment of touch screen use. Manuscript submitted for publication, 2007.
- Lough, S., et al., Measurement of recovery of function in the hemiparetic upper limb following stroke – a preliminary report. Human Movement Science, 1984. 3(3): p. 247-256.
ACKNOWLEDGEMENTS
The contents of this paper were developed under grant H133E030012 from the National Institute on Disability and Rehabilitation Research (NIDRR), U.S. Department of Education. However, these contents do not necessarily represent the policy of the Department of Education, and you should not assume endorsement by the federal government.
Author Contact Information
Curt Irwin, PhD, University of Wisconsin-Madison 1550 Engineering Drive, 2107 ECB
Madison, WI 53706 Phone 608 262-6966 Email: Irwin@cae.wisc.edu