Mechanical characteristics of microswitches adapted for the physically disabled

Mechanical characteristics of microswitches adapted for the physically disabled P.L. Weiss School of Physical and Occupational Therapy, McGill University, 3654 Drummond Street, Montreal, Canada, H3G lY5 Received November 1989, accepted December 1989

ABSTRA(JT The object of this study was to measure the mechanical characteristics of several commonly used adapted switches in order to demonstrate thefksibilityof collecting quantitative &cn$tions of switchperformance. A stepping motor was used to drive a digital microm&r verticah’y toward the tested switch and proving-ring strain gauges recorded the opcratingforce. Measures of activation and deactivation forces, travel and switch com@ance were compared; several clinical examples illustrate tb jkctional application of these results. It ti anticipated that thk information will permit clinicians to prescribe swit&s in a more accurate manner.

Keywords: Severely physically disabled, adapted switches, static mechanical properties, force transducer

INTRODUCTION With the aid of modified alpha-numeric input and cursor control adaptations’“, severely ph sically disabled individuals are becoming increasing ry independent in vocational and avocational activities. Numerous adaptations are currently available54. For example, pneumatic switches controlled by small changes in mouth pressure enable quadriplegics to select items from a scanned sequence of characters and enlarged keyboards can be programmed to be activated by keys of varying size. As the number of adapted switches and alternative keyboards grows, users of micro rocessor-based adaptations must learn to cope wit! a formidable selection process ‘7 lo . Characteristics as diverse as switch size, mass, and compliance, the force required to activate and release the switch, and its o timal position can great1 influence the user’s abi Pity to perform satisfactori ly y”T “. Generally, these characteristics are selected by means of trial-and-error experimentation with clinician and client eventually settling upon a particular input method13. This process tends to be lengthy and there is little assurance that the selected switch is indeed the optimal choice. The scope of this problem is illustrated in the following clinical example. Client RS, a 32-year-old male who has a 6-year history of amyotrophic lateral sclerosis, was referred for microcomputer training. As a result of this degenerative disease of the s inal cord, he has lost almost all voluntary control oP his limbs. His s eech is non-functional due to severe dysarthria and Re is complete1 dependent in all activities of daily living. He is aL le to blink his left eye and to make only, non-specific, poorly controlled movements of the neck. The proximal (PIP) and distal (DIP) interphalangeal joints of the third and fourth 0 1990 Butterworth-Heinemann for BES 0141-5425/90/050398-05

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digits on both hands can be actively flexed to about 90 degrees against gravity and some resistance, but the segments cannot be returned to the extended position when working against gravity. No active movement is discemable at any of the other distal interphalangeal and metacarpophalangeal joints. Due to the extreme1 limited number of active voluntary movements, PPP flexion-extension was the obvious control site for switch activation. However, the selection of the particular adapted switch which would accommodate this client’s abilities, was complicated by the discrepancy between PIP flexor and extensor strength. That is, the weak PIP flexors were most suited to activating touch-sensitive contact switches, but the even weaker PIP extensors were unable to release a switch of this type before its next activation. As illustrated in Figure 7, this dilemma was eventually resolved by lacing a moderate1 sensitive adapted switch, the $ ASH mini cup (4 ash Inc., Ontario, Canada), so that it could be activated and released in the ‘gravity eliminated’ position. Client RS now uses a modified version of the Morse code to generate all alphanumeric and control characters at a speed of approximately eight words per minute by means of successive activations of two mini cup switches. The lack of uantitative data on the mechanical characteristics o9 adapted switches makes the switch selection process an unnecessarily arduous one, since the clinician is forced to experiment with numerous ossibilities which are not readily available. The task Flecomes even more onerous when, in contrast to client RS, additional switch activation sites are also available; yet remarkably little valid and reliable information is available from the manufacturers of these products. The object of the study reported here was to measure the mechanical characteristics of

Mimswitchesjwthephysicallydimbled: P.L. Wetis

mounted

on

Figure 1 Photograph of client RS positioned with the two mini cup switches that he uses with Morse code

several commonly used adapted switches in order to demonstrate the feasibility of collecting quantitative descri tions of switch performance. It is anticipated that tRis information will permit clinicians to prescribe switches in a more accurate manner. MATERLALS AND METHODS Four of the most commonly prescribed adapted switches (the membranes, illow, cup and mini cup switches distributed by T 1 SH Inc.) and one commercially available microswitch were tested. These particular switches were examined because they are in common clinical use, they re resent a functional range of sizes and forces and tl!ey appeared to be appropriate for client RS. Their dimensions, including switch and actuator size and switch mass, are listed in Table 7. Apparatus All switches were tested using the apparatus shown in Figure 2. Only minor modifications to its ori ‘nal design were required for the present applicationTi . A stepping motor was used to drive a Mitutoyo digital micrometer vertically toward the tested switch. The presence of a non-rotating spindle within the micrometer ensured that the rotations generated by the steppin motor were converted into a purely translationa f displacement. A digital output from a Table 1 Morphometric characteristics of tested switches indicating their dimensions and mass Switch schematic

Switch/actuator size (mm)

Switch mass

Switch type Membrane

•l

85.0154.7

30.0

0

64.6

Figure 2 meter-force properties

Block diagram illustrating the stepping motor-microtransducer assembly used to measure switch mechanical

MicroVAX computer controlled the motor rate (400 steps per revolution) such that the 5OO~m per revolution micrometer generated 1.25pm per step of linear movement. A second digital output directed the micrometer displacement. A force transducer was constructed from a 20mm roving ring instrumented with strain diameter gauges (fuR bridge configuration with strain gauge conditioning electronics having a 10m7low Frequency common mode rejection ratio). The force transducer was designed to be linear over a range of forces from 0 to 50N. The force generated throu out the experimental session was sampled by a 1$ -bit A/D converter every 1.25pm for a total of 8OUOsamples. To ascertain the precise position of switch activation and deactivation, the adapted switches were connected to a digital input which was recorded synchronously with the force signal. Testing procedure Each switch was secured to an aluminium support such that the micrometer plunger pointed towards, but did not contact, the centre of the switch. The plun er was driven 5 mm (i.e. 4000 steps) towards the switca and then returned immediately to the initial position. The force generated throughout switch contact as well as the location of activation and deactivation were recorded on the MicroVAX running the VMS operating system. The entire experimental protocol was defined and executed in the VAX realtime Common Lisp environment. A digital multimeter was used throughout the experiment to monitor the switch force output and a modified switch tester was used to ensure that switch activation had occurred. Repeated measures on selected switches were performed while the switch was secured in its original location.

15.4 Mini cup Commercial

Analysis procedures

0

23.U20.6

4.7

0

12.U12.1

0.9

All analyses were carried out using, NEXUS, a language for signals and system analysis 5. The digital

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switch input signal was used to separate the force records into four segments: pre- and post-switch activation and pre- and post-switch deactivation. From these data the following switch characteristics were determined: 1. distance traversed from switch contact to switch activation 2. force required to activate switch 3. distance traversed from switch deactivation to switch release 4. force exerted when switch was deactivated 5. switch compliance.

Table 2 Mechanical characteristics of tested switches. The distance traversed from switch contact to switch activation, the force required to activate switch, the force exerted when switch was deactivated and switch compliance Deactivation

Activation

Total Com-

switch

Force

Travel

Force

We

N

h4

Membrane pillow

0.3 3.3 4.6 2.6 2.6 2.7 3.8

36 436 756 1038 1080 1095 1599

CUP Mini cup 1 Mini cup 2 Mini cup 3 Commercial

(N

Travel (pm)

$ZS)

0.1 1.0 2.1 1.5 1.4 1.5 1.6

379 298 590 985 1024 1033 862

30 21.5 211 471 479 450 280

’

RESULTS The force generated as the plun er was depressed and retracted was recorded and pPotted with respect to micrometer displacement. Typical records for the mini cup switch are shown in Figure 3. Three important features of the switch force-displacement curve are apparent from these data. First, there was a sim le, relatively linear relation between the force an dp displacement signals, but until the plunger contacted the switch, the force signal remained at zero. At the time of contact (normalized to a displacement of Opm) the force increased and continued to increase until the direction of stepping motor action was reversed. The force then decreased until it once again returned to zero. Second, the upward and downward pathways followed by the force record were smooth; two notable exceptions occurred at the point of switch activation (shown by the 1 N ulse on the switch digital input channel record) an dpdeactivation (shown by the - 1 N pulse on the same record). Finally, there was a renounced hysteresis in the data; the force generate 1 during switch activation was significantly larger than that generated during switch deactivation. The mechanical behaviour of this switch was summarized by the five parameters defined earlier. In the example shown in F@re 3, switch activation

occurred with a force of 2.6 N when the micrometer was dis laced by 1038pm. Switch deactivation occurre a with a force of 1.5 N when the micrometer was displaced by 985~m. The compliance of this switch was 47 1pm N-r. The values of the parameters determined for the five tested switches are listed in TubZe2. Activation forces ranged from 0.3 N for the sensitive membrane switch to 4.6N for the cu switch. Deactivation forces, always smaller than tl!e res ective activation forces, varied from 0.1 to 2.1 N. & e range in travel amongst the five switches was approximately 40 fold, varying from the essentially ri ‘d membrane switch to the flexible commercial switca . The mini cup switch was the most compliant (450-479pmN-‘), having approximately twice the compliance of the pillow, cup and commercial switches; the membrane switch was very stiff. The reliability of the testing procedure was verified by conducting three repeated measures of the mini cup switch; these data are lotted in Figure 4 and the parameter values are liste B in Table 2 as results from mini cup switches numbers 1, 2 and 3. It is evident from both the figure and the table that the switch’s mechanical characteristics are reliable as tested here. Results from two additional switches (membrane and cup) are plotted with the mini cup data in Figure 5. Three prominent features of this comparison are a

6 Mini cup

z

4 Mini cup (1,2,3)

J

b

LL

2

0 I

-1000

0

I

I

2000

1000 Displacement

3000

(urn)

Figure 3 Force generated as a function of micrometer displacement for the mini cup switch. The location of switch activation (positive) and deactivation (negative) are shown by the 1 N pulses

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-2

1

I

I

I

I

-500

0

500

1000

Displacement

Figure 4

(elm)

Three repeated tests of the mini cup switch

I 1500

2000

Ah-mwitchesfor thephysical~ dimbltd: P.L. Weiss

cup

-5 ' -500

I 0

I 500

I

I

1000

1500

2000

Displacement(um)

Figure 5 Force-displacement plots for three of the tested switches: membrane, cup and mini cup

the differences in switch activation force, compliance and the magnitude of the hysteresis. The apparent1 large hysteresis of the membrane switch is mislea (ring; as is evident from the travel data listed in Table 7, activation and deactivation of this rigid switch occurred very shortly after contact was made. CLINICAL

APPLICATION

The clinical relevance of these data is illustrated with respect to the switch selection process for client RS. Given his weak flexor digitorum profundus and or the other superficialis muscles, the membrane very sensitive contact switch was first considered. However, the very low deactivation force of the membrane switch made it continue to res ond to the force generated by the mass of RS’s B igit resting against it. His extremely weak extensor digitorum muscles did not permit him to deactivate this switch even when it was laced in the ‘gravity eliminated’ position: as a resu Pt several repetitions of the same character were frequently generated. It was therefore necessary to consider a switch with a deactivation force larger than that due to the digit mass. Of the four remaining switches, the cup switch, with its relatively large activation force, was rejected even though RS was able to activate it; the numerous, consecutive 4.6N switch presses required by Morse code (see below) would lead to muscle fatigue. The final choice from the three remaining switches was the issue of size. The relatively large illow switch ositioned Por access b could not be conveniently finger tips (cf. Figure 7) an s the commercial switc B presented a target area that was too small to be activated reliably. Since client RS had sufficient range of motion to cope with the fairly large travel of the mini cup, and given the suitability of its force activation and deactivation characteristics and its size, the mini cup switch was finally selected for his use.

OTHER CONSIDERATIONS Three additional issues, not applicable to client RS, must also be considered. First, switch hysteresis, the

difference between its activation and deactivation forces, represents a margin of safety for those clients who must maintain the switch in its active state. That is, after exerting sufficient force to activate the switch, it will remain active until the user reduces the force to the deactivation level. This is an important consideration for users of inverse scanning, a method in which switch activation must be maintained until the re uired item is reached by the scanning cursor. 1 econd, although a good match between client abilities and switch characteristics can alleviate many of the difficulties associated with adaptive access data entry, it is recognized that other interventions may also be required. This point is illustrated by the case of client JL, an elderly woman suffering from su ranuclear palsy; she was fitted with two grasp switc x es, 137 x 35 mm rubber cylinders held in a ower grip and activated by squeezing. Although sR e had no difficulty in making isolated activations, her severe akinesia prevented her from using these switches in ap ropriately sequenced patterns. In this case, a time e ay unit designed to compensate for her difficulty dP with temporal sequencing, was required to enable her to use the Morse code. The third issue relates to switch durability. In addition to the data collected in these experiments, information about a switch’s ability to perform uite severe conditions would be reliably under valuable. Durabi ‘tity is an issue of great importance when one considers the number of activations the typical user makes each day. The magnitude of this problem can be estimated as follows. Alpha-numeric and computer control characters are represented in Morse code by one to six consecutive switch activations. A user of this method of data entry could be expected to achieve speeds of ten words per minute. Given an average of three activations per character and an average of six characters per word (including spaces), the number of switch activations over a 4 h period would total 43 200. It is little wonder that switch breakdown is an extremely common occurrence.

CONCLUSION Corn rehensive testing of the mechanical properties of al P the commonly used adapted switches would greatly simplify the selection process provided that the results were made widely available in the clinic. Similar data on the properties of both standard and alternative ke boards and quantitative descriptions of the client’s xm ctional status, includin force and ran e of motion capabilities, would aPso serve to faci Pitate a better match between client and device. The availability of comprehensive information delineating the mechanical properties of both human operators and the devices they use has the potential to he1 severely hysically disabled individuals, who lac K the redun B ancy of abilities which enable others to compensate for poorly designed e uipment, perform vocational and avocational tar,?.s in a more independent manner.

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ACKNOWLJ3DGEMENTS The author is grateful to Dr Ian Hunter for making available the a paratus used to carry out the experiments and for Kis many helpful suggestions. A reciation is also extended to Serge Lafontaine and !z axon August for their assistance. This work was supported by a grant from the Natural Science and Engineering Research Council of Canada.

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