Evaluation of the Force Sensitive Application hand force measurement system

Evaluation of the Force Sensitive Application HandForce Measurement SystemKihyo Jung,1 Heecheon You,2 and Ochae Kwon3

1 Department of Industrial and Manufacturing Engineering, The Pennsylvania State University, University Park,Pennsylvania, USA2 Department of Industrial and Management Engineering, Pohang University of Science and Technology, Pohang,Kyungbuk, South Korea3 Samsung Electronic Co., Ltd., Soonhwa-dong, Jung-gu, Seoul, South Korea

Abstract

The Force Sensitive Application (FSA) system is used to measure forces exerted by the hand on anobject. The present study evaluated the FSA system in terms of stability, repeatability, accuracy, andlinearity at the sensor and system (pulp press, pulp pinch, and power grip) levels. At the sensor level,the FSA sensor showed high performance in stability (coefficient of variation [CV] ≤2%) and linearity(coefficient of determination [R2] = 0.95), but low performance in repeatability (CV =11∼19%) andaccuracy (14∼26% overvaluation). At the system level, the FSA system showed a decreasing accuracy(12∼30% undervaluation) as the number of sensors involved in the test increased, but retained a highlinearity (R2 = 0.95∼0.98) to measurement from a dynamometer. These evaluation results indicatethat the FSA system should be used for relative, not absolute, comparison in force evaluation due toits lack of accuracy and that the accuracy of the FSA system can be improved due to its good linearityby calibrating FSA measurements with a dynamometer having a high level of accuracy. C© 2010 WileyPeriodicals, Inc.

Keywords: Force glove measurement system; Performance evaluation measures; System-level eval-uation; Sensor-level evaluation

1. INTRODUCTION

Use of excessive force by the hand in the workplace hasbeen identified as a risk factor of work-related mus-culoskeletal disorders (WMSDs). Ayoub (1990) andPutz-Anderson (1988) indicate that biomechanicalstresses and musculoskeletal injuries can be caused byexertion of excessive manual force. A comprehensiveliterature review by the National Institute for Occu-pational Safety and Health (NIOSH, 1997) indicates

Correspondence to: Heecheon You, San 31 Hyoja-Dong,Pohang, Kyungbuk, Republic of Korea, 790-784. Phone:+82-54-279-2210; e-mail: [email protected]

Received: 16 March 2009; revised 27 May 2009; accepted 26June 2009

Published online in Wiley InterScience(www.interscience.wiley.com).

DOI: 10.1002/hfm.20177

that hand force is positively associated with WMSDsat the upper limb, such as carpal tunnel syndrome andtendonitis.

Direct and indirect measurement methods are avail-able for evaluation of the biomechanical stress due tohand force, having trade-offs in terms of directness offorce measurement and interference with hand mo-tion. Surface electromyography (sEMG) is commonlyused as an indirect measurement method by estimat-ing exerted forces from electrical muscle activities. Al-though the EMG method is of use in evaluating muscleactivities, it cannot directly measure hand forces ap-plied to an object. In contrast, a force glove systemsuch as the Force Sensitive Application (FSA) system(Verg Inc., Winnipeg, Canada; Figure 1) has been in-troduced to directly measure hand forces exerted on anobject during work. With force sensitive resistor (FSR)sensors attached to the palmar side of the glove, the

226 Human Factors and Ergonomics in Manufacturing & Service Industries 20 (3) 226–232 (2010) c© 2010 Wiley Periodicals, Inc.

Jung, You, and Kwon Evaluation of the FSA Hand Force Measurement System

Figure 1 FSA hand force measurement system.

force glove system can measure the contact forces be-tween the hand regions and an object manipulated bythe hand. The sensors and wires on the glove may dis-turb motions of the hand, however.

A force glove system has been used in er-gonomic evaluation of hand tools. Kong and Freivalds(2003) used a force glove system incorporating thin(0.127 mm), flexible, conductive polymer pressure sen-sors (FlexiForce A101-25; Tekscan Inc., Boston, MA)to evaluate seven meat-hook designs by comparingforces applied on the hook grips. Also, Kong and Lowe(2005a,b) used the force glove to identify an optimalcylindrical handle diameter for a grip task and the ef-fects of handle diameter and orientation on the maxi-mum torque capability for a rotation task.

A lack of understanding exists, however, regard-ing the performance of a force glove system. NexGen(2008) claimed that the coefficient of variation (CV)of the FSA system is less than 10%. Kong and Lowe(2005a,b) reported that the coefficient of determina-tion (R2) between measured values and applied forcesis 0.986. An in-depth evaluation, however, has not beenreported regarding various performance characteris-tics such as reliability, stability, and accuracy for theforce glove systems.

The present study evaluated the performance of theFSA system in terms of stability, repeatability, accuracy,and linearity at the sensor and system levels. Duringthe sensor-level evaluation, a sensor was tested on aflat surface by placing various weights on it. Duringthe system-level evaluation, one or more FSA sensorswere tested by exerting forces over the sensors on a dy-namometer. Then, the measurements of the FSA sys-tem were compared with corresponding weights anddynamometer measurements.

Weight

Block

(size: 1 cm3;

weight: 0.6 g)

FSA sensor

FSA sensor

Sensing area:

0.64 cm × 0.64 cm

1.7 cm

1.5 cm

wires

Figure 2 FSA sensor evaluation.

2. MATERIALS & METHODS

2.1. Apparatus

Weights and NK dynamometers (precision = 0.098 N;NK Biotechnical Corporation, Minneapolis, MN) wereused to evaluate the FSA system at the sensor and sys-tem levels, respectively. In the sensor-level evaluation,three different weights (0.5 kg, 1 kg, and 2 kg) wereplaced one at a time on a sensor of the FSA system(Figure 2). For uniform contact between a weight andthe FSA sensor (size: 1.7 cm × 1.5 cm; sensing area =0.64 cm × 0.64 cm), a 1-cm3 steel block (0.6 g) wasplaced between them. In the system-level evaluation,pulp press, pulp pinch, and power grip NK dynamome-ters were pressed by the index finger alone, pinchedbetween the thumb and index finger, and gripped bythe hand, respectively (Figure 3). To compare measure-ments from the FSA and NK systems, the time clocksof the two systems were synchronized by AboutTime(AboutTime, 2008). A voltage output from each FSAsensor generated by a force applied to the sensor wasconverted to a pressure (unit: psi) by a proprietary cal-ibration equation embedded in the FSA system andthen to a force (unit: N) by considering the unit con-version factor 1 psi = 0.69 N/cm2 (Sensorsone, 2008)and the sensor’s sensing area (0.4 cm2). The FSA systemprovided both measurements of the individual sensorsand their total.

2.2. Evaluation Criteria

Four criteria (stability, repeatability, accuracy, and lin-earity) were considered in the evaluation as shownin Table 1. To quantify the four criteria, measuressuch as CV, mean difference (MD), standard error(SE), and R2 were selected by referring to the stud-ies of Kessler, Hodges, and Walker (1995), Wise andcolleagues (1990), and Quam and coworkers (1989).

Human Factors and Ergonomics in Manufacturing & Service Industries DOI: 10.1002/hfm 227

Evaluation of the FSA Hand Force Measurement System Jung, You, and Kwon

Pulp press (1 sensor) Pulp pinch (2 sensors)

Measurement

motion

FSA sensor

attachment

Classification Power grip (18 sensors)Pulp press (1 sensor) Pulp pinch (2 sensors)

Measurement

motion

FSA sensor

attachment

Classification Power grip (18 sensors)

Figure 3 FSA system evaluation.

Although the four criteria could be applied to thesensor-level evaluation, stability and repeatabilitycould not be evaluated at the system-level evaluationbecause hand forces exerted to a dynamometer contin-uously changed over time. Thus, only accuracy and lin-earity of the FSA system were examined at the system-level evaluation.

2.3. Experimental Procedure

At the sensor-level evaluation, each of the weights(0.5 kg, 1 kg, and 2 kg) was placed on the steel blockover an FSA sensor, and then measurements from theFSA system were collected for 10 seconds at a samplingrate of 10 Hz. After unloading the weight, a break of

2 minutes was taken before the next trial to minimizethe possible effects of creep and hysteresis on measure-ment. Next, at the system-level evaluation, wearing theFSA glove, one participant exerted forces on each ofthe three NK dynamometers (pulp press, pulp pinch,and power grip) while the FSA and NK systems were insynchronization at 10 Hz. With the upper arm hang-ing down naturally, the elbow flexed at 90 degrees, andthe forearm neutral in sitting, the participant gradu-ally increased a force exertion to a designated force level(30 N for pulp press, 60 N for pulp pinch, and 300 Nfor power grip), then gradually released force exertion.The force levels were selected by considering the maxi-mum force measurement limit (35 N) of an FSA sensorrecommended by the manufacturer and the number of

TABLE 1. Criteria Applied to Sensor-Level and System-Level Evaluations

Criteria Applieda

Criteria Description Measures Sensor Level System Level

Stability The extent of fluctuation in measurementalong time when a constant force is applied

CV = sx̄ O ×

Repeatability The degree of variation betweenmeasurements in repeated trials

CV O ×

Accuracy The discrepancy (e) of a measurement from thetrue force

MD =∑n

i =1 ei

n O O

SE =∑n

i =1 e2i

n−1

Linearity The degree of a linear relationship betweencollected measurements and applied forces

R2 O O

aO: applied; ×: Not applied.

228 Human Factors and Ergonomics in Manufacturing & Service Industries DOI: 10.1002/hfm


TABLE 2. Sensor-Level FSA Evaluation Results

Accuracy

Weight Stability Repeatability MD MD% SE(x ; unit: N) (CV, %) (CV, %) (unit: N) (MD/x ) (unit: N) Linearitya

4.9 1.4 19 0.9 18 1.2FSA = 1.23x(R2 = 0.95)

9.8 0.9 14 1.4 14 1.819.6 1.2 11 5.1 26 5.1

aFSA = FSA sensor measurement.

sensors involved in each grip condition. Three forceexertions were made in each trial, and a break of2 minutes was provided between trials. Both the sensor-level and system-level evaluations were conducted with10 repetitions.

3. RESULTS

3.1. Sensor-Level Performance

The FSA sensor showed good stability (CV < 2%)and linearity (R2 = 0.95), but low repeatability(CV = 11∼19%) and accuracy (MD% = 14∼26%)(Table 2). The FSA sensor produced stable measure-ments (CV = 0.9∼1.4%) along time for all the weightconditions. A significant linear relationship [r = 0.98;t(28) = 23.1, p < 0.001] was found between weightand FSA sensor measurement. The FSA sensor pro-duced inconsistent measurements in repeated test con-ditions, however. Last, the FSA sensor produced over-rated measurements (slope > 1), and the magnitude oferror in FSA sensor measurement increased as weightincreased—the MD and SE of FSA sensor measurementincreased as weight increased.

3.2. System-Level Performance

The FSA system underrated forces as compared to mea-surements from NK dynamometers as illustrated inFigure 4, and this trend became worse as the numberof sensors involved in measurement increased. Table 3shows that the MD and SE of the FSA system mea-surement increased as the number of sensors involvedincreased: MD from 1.3 to 14.4 N and SE from 1.5 to25.3 N. The linear regressions between the FSA andNK dynamometer measurements had high R2 valuesand were statistically significant for all the three exer-tion conditions [F (1, 37) = 2,417, p < 0.001 for pulppress; F (1, 51) = 1,260, p < 0.001 for pulp pinch;F (1, 15) = 315, p < 0.001 for power grip]. The slopes(<1) of the regression equations indicated, however,that the FSA system produced measurements smallerthan the NK dynamometers.

4. DISCUSSION

The measurements of the FSA system should be care-fully interpreted as more sensors are involved in mea-surement. At the system-level evaluation, the MD and

TABLE 3. System-Level FSA Evaluation Results

Accuracy Linearity (FSA = b1 × N K )a

Grip Type MD SE Slope(# of sensors) (unit: N) MD%b (unit: N) (b1) R2

Pulp press (1) −1.3 12 1.5 0.89 0.99Pulp pinch (2) −4.5 20 5.3 0.82 0.98Power grip (18) −14.4 30 25.3 0.76 0.95

aNK = NK system measurement; FSA = FSA system measurement.bMD%: percentage of mean difference relative to corresponding NK system measurement.



0

5

10

15

20

25

30

35

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Time (sec)

Force (N)

DynamometerFSA

(a)

0

10

20

30

40

50

60

70

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Time (sec)

Force (N)

Dynamometer

FSA

(b)

0

50

100

150

200

250

300

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Time (sec)

Force (N)

Dynamometer

FSA

(c)

Figure 4 Comparison of FSA and NK measurements: (a) pulp press, (b) pulp pinch, and (c) power grip.

SE of FSA measurements increased as the numberof sensors involved in measurement increased. Thisdiminishing performance of the FSA system mightbe caused by propagation of errors—the error of an

individual sensor is small, but the summation of errorsof many sensors becomes significantly large. The mea-surements of the FSA system were closely correlatedwith those of the NK system, however. Therefore, the



FSA system should be used for relative, not absolute,force comparison, especially when many sensors areinvolved in measurement.

A contradictory result was found in the evaluationof the FSA system: overrated force measurement atthe sensor-level evaluation and underrated force mea-surement at the system-level evaluation. During thesensor-level evaluation, a steel block was used to en-sure that force was applied evenly and normally to theFSA sensor on a flat, hard surface, which is the re-quired condition for FSA system calibration. At thesystem-level evaluation, however, forces could be ap-plied neither evenly nor normally to the FSA sensorsattached on the glove due to difficulty in controllingthe contacts between the hand regions and FSA sen-sors. This incomplete contact condition between thehand and FSA sensors could have caused underratedforce measurements of the FSA system.

The evaluation results of the present study indicatedthat the performance information of a force glove mea-surement system should be provided both at the sensorand system levels. The performance results of the FSAsystem were quite different at the sensor and systemlevel evaluations. The FSA system manufacturer, how-ever, provides only partial performance information(repeatability) regarding the sensor used in the system.

The experiment results of measurement error in thepresent study showed that a nonlinear equation wouldbe more appropriate than a linear one for the calibra-tion of the FSA system. At the sensor-level evaluation,the FSA sensor overrated forces more at the low andhigh force levels (18% overvaluation at 4.9 N and 26%overvaluation at 19.6 N) than at the medium force level(14% overvaluation at 9.8 N). This trend of measure-ment error implies that use of a nonlinear calibrationequation in the FSA system would reduce measurementerror by achieving better fit.

Furthermore, the measurement-error results in thepresent study indicate that use of a calibration equationspecific to an application context of the FSA system isnecessary for better accuracy. As shown in Tables 2and 3, the regression coefficients explaining the re-lationships of FSA measurements with correspondingreference values are high but vary largely (ranging from0.76 to 1.23) depending on measurement conditions. Apost hoc analysis of MD conducted in the present studydemonstrates that the accuracy of the FSA system canbe improved by 48–110% by applying the regressionequations obtained at the sensor-level and system-levelevaluations to FSA measurements (Table 4).

TABLE 4. Post Hoc Analysis Results on MD

MD (N)a

Category Before After � MD%b

Sensor-levelevaluation

4.9 N 0.9 0.2 799.8 N 1.4 0.7 48

19.6 N 5.1 −0.5 110System-level

evaluationPulp press −1.3 −0.2 87Pulp pinch −4.5 −0.5 89Power grip −14.4 −3.6 75

aMDs before and after applying the regression equationsobtained from the sensor-level and system-level evaluationsof the FSA system to FSA measurements.b�MD% = (Before MD – After MD) / Before MD × 100.

To obtain more reliable and accurate measurementsfrom the FSA system, it is recommended to use a glovewith a proper fit to the hand. If a loose glove is worn,the sensors can slip over or lose their proper contactsto the object during grip measurement. Conversely, ifa glove that is too tight to the hand is worn, motionsof the fingers and hand can be disturbed or overratedforce measurements can be obtained. To avoid theseundesirable effects due to the use of a glove that doesnot fit to the hand, FSA sensors can be attached di-rectly to the bare hand. Sweat from the hand and directcontact of the bare hand with the object can, however,reduce the friction between the sensors and the objectbeing gripped, thus increasing the possibility of slip onan object.

The experimental protocol of the present studyneeds to be upgraded for more valid, informativeevaluation. The number and range of the weightconditions (4.9 N, 9.8 N, and 19.6 N) selected inthe sensor-level evaluation need to be extended byconsidering the maximum force measurement limit(35 N) of the FSA sensor suggested by the manufac-turer. Also the creep and hysteresis of the FSA sensorneed to be examined by a sophisticated experimentalprotocol.

Last, evaluation of various force glove measure-ment systems is necessary. The present study pro-vides a comprehensive understanding of the perfor-mance characteristics of the FSA system. In-depthevaluations of other force glove measurement sys-tems would be helpful to compare their strengths andlimitations.



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Evaluation of the Force Sensitive Application hand force measurement system