Investigating Changes in Physical Effort When Operating Aids that Scan at Inappropriate Scan Delays

RESNA 28th Annual Conference - Atlanta, Georgia

Stan Cronk, PhD, Patrick Clerkin, BS, Praveen Aeddy, BS, Walt Besio, PhD

Center for Biomedical Engineering and Rehabilitation Science
Louisiana Tech University
Ruston, LA 71270


Scanning is often used as a selection technique for persons with a severe physical disability. Previous tests showed that users felt that the scan delays at which they achieved their best performance were generally too short for comfort. In this set of tests, we compared measurements of physical effort to determine if operating a scanning device at very short scan delays leads to significant increases in the physical effort expended to operate the control switch.

KEYWORDS: computer access, augmentative and alternative communication, row/column scanning, force


Persons with severe physical disabilities are often only able to operate electronic aids such as communication aids or computers by using the scanning selection technique along with a single switch. Setting the scan delay, or amount of time that the device highlights each selection, is extremely important for successful use of the scanning technique. Users are frustrated whenever the scan delay is either too short or too long; however, no commercial aid currently monitors the user’s performance in order to adjust the scan delay automatically. Though researchers have previously attempted to develop a scanning technique that automatically adjusts the scan delay as the user’s ability to operate the scan changes (1) (2) (3), none have thoroughly investigated the relationships between the user’s perception of the scan delay and measures of their performance.

In previous tests, we had looked at the user’s perception of the scan delay and measures of their performance (4). We found that, in all cases, the users felt that the scan delay at which they achieved their best throughput was either “a little too fast” or “much too fast.” We were concerned that, even though users achieved better performance at shorter scan delays, the discomfort that they experienced at these faster scan delays could lead to inordinately high levels of physical effort.

Research Question

The objective of this study was to evaluate the relationship between the participants’ perception of scan delays and the physical effort they used to operate the control switch.


We converted a simulation of the row-column scan technique previously developed using Microsoft Visual Basic 6 to Visual Basic.NET 2003. We ran the simulation on a Dell Latitude C400 laptop computer running Windows Professional XP. Though the C400 has a smaller (12.1”), dimmer screen than many other laptops on the market, other tests we had conducted demonstrated little effect in the operation of a scan (5).

The left mouse button of the computer trackpad simulated the action of the single switch. The selection set was a six by five matrix composed of 26 English letters in alphabetic order, a period character, and a space character. We designed the simulation so that the activation of the single switch started the scan, which would not stop until the task was completed.

We conducted these experiments with eight non-disabled participants, consisting of five males and three females ranging in age from 18 to 23. The task of each participant was to compose the sentence “The quick brown fox jumps over a lazy dog.” with the row-column scanning aid simulation. To exclude the influence of the time that the participants spent on figuring out the next character in the sentence, a box above the selection matrix displayed the next letter in the sentence the participant needed to enter. Each participant had the opportunity to practice using the scanning simulation until he or she felt ready to begin the tests.

The tests with each participant began with a scan delay of 800ms. When the participant finished the sentence, he or she was asked to judge the scan delay by selecting one of the following five comments: “Much too slow”, “A little too slow”, “Comfortable”, “A little too fast”, “Much too fast”. The test was then repeated with the scan delay decreased by 20%. Testing continued with the scan delay decreasing by 20% from the previous trial until the participant judged that he or she would not able to finish the composing task.

During testing, the scanning aid simulation recorded the time, the number of the currently-highlighted row or column, and details on the accuracy of the selection (correct, incorrect due to anticipation or lag, etc.). The program automatically exported the data into an EXCEL spreadsheet.

During the tests a separate Dell Dimension 2350 PC collected force measurements using LabVIEW 7. We connected a FlexiForce Model A201 force sensor (maximum 1-lb load) to a National Instruments 6036 DAQCard with analog signal measurement and conditioning modules. We mounted the sensor on the left mouse button of the trackpad. A LabVIEW VI (Virtual Instrument) measured the force applied to the sensor at a sampling rate of 40 Hz. The LabVIEW VI stored the force data in a text file.

At the conclusion of the tests, both sets of data files were analyzed using MATLAB 7.0.1 (R14).



Figure 1. Set of Force Measurements for One Experimental Run (Click image for larger view)
Sample set of experimental force measurements showing spikes when the participant activated the control switch

Figure 1 shows a typical force profile for one participant during one run of the experiment. The spikes are the measured forces during switch activation; all other values are sensor measurements while the participant waits for the scan to highlight the next desired selection.

Table 1 shows the average force measurements for the first and last run of the experiment for each participant. Note that the average force measured decreases for six of the eight participants.

Table 1. Comparison of Average Force Measurements for Each Participant for First and Last Tests
Participant Average Force - Longest Scan Delay (g) Average Force - Shortest Scan Delay (g) Difference (g)
1 63.3 80.3 17.0
2 78.6 91.6 13.0
3 101.3 86.1 -15.2
4 74.3 64.1 -10.2
5 89.1 74.5 -14.6
6 75.0 63.7 -11.3
7 119.0 109.8 -9.2
8 87.6 70.4 -17.2


Though we visually observed frustration on the part of the participants as the scanning delay became shorter, the average measured force for the tests increased for only two of the eight participants. We hypothesize that as the scan delay becomes shorter, the participant tries to improve his or her ability to respond by applying a lighter touch to the switch. We did note that, when the participant missed a selection, he or she often hit the switch harder; however, this situation did not occur frequently enough to affect the overall force measurements.

Based on these tests, we conclude that the force used to operate a switch while using the row/column scanning technique does not change significantly as the scan delay decreases. However, a second series of tests with disabled participants will be conducted before we implement an algorithm to automatically adapt scan delays based on user performance.


  1. Cronk, Stan; Schubert, Roy W. (1987). Development of a real-time expert system for automatic adaptation of scanning rates. Proceedings of the Tenth Annual Conference on Rehabilitation Technology. 109-111.
  2. Simpson, Richard; Koester, Heidi H. (1999). Adaptive One-Switch Row-Column Scanning. IEEE Transactions on Rehabilitation Engineering. 7(4): 464-473.
  3. Lesher, Gregory W.; Higginbotham, D. Jeffrey; & Moulton, Bryan J. (2000). Techniques for automatically updating scanning delays. Proceedings of the RESNA 2000 Annual Conference. 88-90.
  4. Cronk, Stan; Wang, Wei. (2002). Investigating Relationships Between User Performance and Scan Delays in Aids that Scan. Proceedings of the RESNA 2002 Annual Conference. 129-131.
  5. Cronk, Stan; Nimmagadda, Puneeth; Clerkin, Patrick. (2003). Effects of Display Characteristics on the Ability of a Disabled User to Operate an Aid that Scans. Proceedings of the 2003 Annual Conference of the Biomedical Engineering Society.


Stan Cronk, PhD
Center for Biomedical Engineering and Rehabilitation Science
Louisiana Tech University
711 S. Vienna
Ruston, LA 71270
(318) 257-4562;