Effect of Masking on Vertical Vibration - MDPI

actuators Article

Ride Comfort Control System Considering Physiological and Psychological Characteristics: Effect of Masking on Vertical Vibration on Passengers Keigo Ikeda 1 , Ayato Endo 1 , Ryosuke Minowa 1 , Takayoshi Narita 2 1

2

*

ID

and Hideaki Kato 2, *

Course of Mechanical Engineering, Tokai University, Kitakaname 4-4-1, Hiratsuka-shi 259-1292, Japan; [email protected] (K.I.); [email protected] (A.E.); [email protected] (R.M.) Department of Prime Mover Engineering, Tokai University, Kitakaname 4-4-1, Hiratsuka-shi 259-1292, Japan; [email protected] Correspondence: [email protected]; Tel.: +81-463-58-1211

Received: 15 June 2018; Accepted: 20 July 2018; Published: 23 July 2018

Abstract: Active seat suspension has been proposed to improve ride comfort for ultra-compact mobility. Regarding the ride comfort of passengers due to vertical vibration, the authors have confirmed from biometry measurements that reduction of the vibration acceleration does not always produce the best ride comfort for passengers. Therefore, heart rate variability that can quantitatively reflect stress is measured in real time, and a control method was proposed that feeds back to active suspension and confirms its effectiveness by fundamental verification. In this paper, we will confirm the influence of the vibration stress on the psychological state of the occupant by the masking method. Keywords: voice coil motor; active seat suspension; electrocardiogram

1. Introduction Because the demand for advanced product design in transportation machines such as automobiles, railways, and aircraft is increasing, there is a strong desire to improve quality from the psychological point of view of a passenger, such as mental stress and physical ride comfort. Degradation of ride comfort in transport machines has various influences such as vibration, noise, and ambient temperature, but there are also survey results which show that vibration affects comfort, especially in trains [1]. In addition to improving ride comfort against vibrations in automobiles, evaluation of comfort using the response characteristics of the human body to vibration [2,3], grasp of psychological effects by sensory evaluation [4], quantification of psychology, and fatigue state using physiological measurement, a study on the establishment of a vibration mitigation method is proposed [5]. Against this backdrop, the authors focus on the vibration of ultra-compact mobility vehicles, which have attracted attention as a result of the deterioration of ride comfort as a major issue. Although it has been widespread in recent years, we continue to study the countermeasures from several viewpoints. First, for ultra-small vehicles characterized by a compact and lightweight body, a voice coil motor is placed at the bottom of the seat with an active seat suspension that only controls the occupants, not the entire vehicle. Although it is technically difficult to install an actuator in a limited space and to exhibit high thrust, it is possible to obtain reasonable control [6]. In addition, we investigated a control method that can obtain a high vibration damping performance even when occupants of different weights occupy the vehicle, such as during car sharing, and have demonstrated its usefulness experimentally [7]. Conversely, in order to quantitatively and continuously evaluate the psychological state of an occupant, a system is proposed in which physiological measurement and vibration control

Actuators 2018, 7, 42; doi:10.3390/act7030042

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its usefulness experimentally in order to quantitatively and continuously are performed in real time [8]. In[7]. theConversely, early stages of control system construction, theevaluate psychological the psychological state of an occupant, a system is proposed in which physiological measurement state was evaluated by physiological measurement of the autonomic nervous system activity index and vibration control are performed in real time [8]. In the early stages of control system construction, calculated from heart rate variability and fed back in real time, and by switching the vibration control the psychological state was evaluated by physiological measurement of the autonomic nervous according to the psychological statefrom of the occupant, results were continuously To improve system activity index calculated heart rate variability and fed back in real time,improved. and by switching comfortthe forvibration vibrations, we compare not only the conventional vibration characteristics of the human control according to the psychological state of the occupant, results were continuously improved. To improve comfort for vibrations, compare not only the conventional vibration body, but also psychological characteristics based we on physiological measurement. characteristics of the human body, can but be alsoobtained psychological physiological Physiologic measurement devices easily,characteristics so they havebased madeonremarkable progress measurement. in recent years. However, a system that constantly controls, measures, evaluates, and reflects the heart Physiologic measurement devices can be obtained easily, so they have made remarkable rate during vehicle operation may be a burden on the driver. Therefore, it is necessary to determine progress in recent years. However, a system that constantly controls, measures, evaluates, and the psychological and physical characteristics of each passenger construction of reflects the heart rate during vehicle operation may be a burden in onadvance. the driver.Thus, Therefore, it is a ride comfort control systemthe(Figure 1) thatand automatically selects vibration control isindesired. necessary to determine psychological physical characteristics of each passenger advance. That is, Thus, construction of athe ridepsychological comfort controlcharacteristics system (Figure 1) automatically vibration it is necessary to determine of that the occupant forselects various vibrations in is desired. That is, it is necessary to determine the The psychological characteristics of the advancecontrol to construct a system that provides vibration control. simple evaluation of psychological occupant for various vibrations in advance to construct a system that provides vibration control. The characteristics for vibration has already been studied by the authors. Previous experiments confirm simple evaluation of psychological characteristics for vibration has already been studied by the that vibration affects psychological characteristics [9]. Furthermore, we evaluated the authors. frequency Previous experiments confirm that vibration frequency affects psychological characteristics influence control to reduce vibration psychological and iton has been [9].ofFurthermore, we evaluated theacceleration influence of on control to reduce characteristics, vibration acceleration psychological characteristics, and it has confirmed that continuing apply only the control confirmed that continuing to apply only thebeen control system that reduces to the acceleration amplitude of system that uncomfortable reduces the acceleration amplitude of vibrations uncomfortable frequency vibrations having frequency characteristics does having not always psychologically improve characteristics does not always psychologically improve ride comfort [10]. Therefore, to improve ride ride comfort [10]. Therefore, to improve ride comfort, it is necessary to construct a new vibration comfort, it is necessary to construct a new vibration control method according to psychological control method according to psychological characteristics. characteristics. Driver’s information  Physiological characteristic  Psychological characteristic

Driver Vehicle

Selecting of ride comfort control considering driver’s information Control A

Control B

Control C

･･･

Active seat suspension Figure 1. Ride comfort control system.

Figure 1. Ride comfort control system. In the case of using this system for a vibration control system, the performance of the actuator is limited terms of stroke, thrust, for response frequency, etc. An inexpensive actuator whichofcannot In the caseinof using this system a vibration control system, the performance the actuator secure a sufficient response and thrust at the next stage of popularization was used. When a vibration is limited in terms of stroke, thrust, response frequency, etc. An inexpensive actuator which cannot that is psychologically uncomfortable is input as a disturbance, reduction of this disturbance is being secure a sufficient response and thrust at the next stage of popularization was used. When a vibration studied from various viewpoints, as described above. If it is possible to establish a method to change that is psychologically uncomfortable issuperimposing input as a disturbance, of band this different disturbance is the comfort of the ride by aggressively the vibrations reduction of a frequency being studied various viewpoints, described above. If itmaking is possible establish for a method from thefrom inputted disturbance to adjustasthe driving environment, it moretocomfortable passengers, it will be possible to propose an innovative ride comfort improvement method. In this to change the comfort of the ride by aggressively superimposing the vibrations of a frequency study, wefrom propose method of masking which relatively theenvironment, sensitivity of undesirable band different the ainputted disturbance to adjust thereduces driving making it more vibrations by aggressively inputting vibrations of other frequencies against the vibration of comfortable for passengers, it will be possible to propose an innovative ride comfort improvement frequencies which is input as disturbance and is perceived as discomfort. Conversely, though the method.evaluation In this study, we propose a method of masking which relatively reduces the sensitivity of of physical characteristics and comfort by sensory evaluation has been performed on

undesirable vibrations by aggressively inputting vibrations of other frequencies against the vibration of frequencies which is input as disturbance and is perceived as discomfort. Conversely, though the evaluation of physical characteristics and comfort by sensory evaluation has been performed on composite vibration obtained [11] by superimposing a plurality of vibrations having different frequency characteristics, evaluation of psychological characteristics based on physiological measurement has not been performed; it is necessary to verify this experimentally.


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Based on thevibration above contents, in this paper, as a fundamental on masking, we verified composite obtained [11] by superimposing a plurality ofstudy vibrations having different its effectfrequency using thecharacteristics, Vibration Test by simulating the vibration environment the masking. evaluation of psychological characteristics based applying on physiological measurement has notwith been performed; is necessarywith to verify thisfrequency experimentally. Experiments on vibration sinusoidalit vibration single characteristics were first Based on the above contents, in this paper, as a fundamental study on masking, we verified its conducted, and the vibration used for masking was investigated. Next, we simulated the situation effect using the Vibration Test by simulating the vibration environment applying the masking. masked by using the frequency selected by this experiment and carried out the vibration test with Experiments on vibration with sinusoidal vibration with single frequency characteristics were first a small number ofand subjects. At this time, the relative acceleration amplitude of the conducted, the vibration used for masking was investigated. Next, we simulated thevibration situation to be superimposed on the disturbance was also examined. In addition, we carried out a demonstration masked by using the frequency selected by this experiment and carried out the vibration test with a small of subjects. At thisof time, the relative amplitudethat of the vibrationride to be experiment bynumber increasing the number subjects furtheracceleration using a condition improves comfort, superimposed on the disturbance was also examined. In addition, we carried out a demonstration which was confirmed by a small number of subjects. From these investigations, we clarify the influence experiment by increasing the number of subjects further using a condition that improves ride of masking in vertical vibration on the comfort of riders and present the prospects for the design of comfort, which was confirmed by a small number of subjects. From these investigations, we clarify a new ride comfort control system. the influence of masking in vertical vibration on the comfort of riders and present the prospects for the design of a new ride comfort control system.

2. Masking Method

2. Masking Method In this paper, we propose a masking method as a new type of vibration control for improving the paper, we propose a masking method as a new type vibration control for improving ride comfortInofthis vehicles. Figure 2 shows schematic drawings ofofthe masking method. The masking the ride comfort of vehicles. Figure 2 shows schematic drawings of the masking method. The masking method is a type of vibration control considering the vibration frequency sensation of a human. method is a type of vibration control considering the vibration frequency sensation of a human. The The purpose of general vibration control is the elimination of unpleasant disturbance. However, purpose of general vibration control is the elimination of unpleasant disturbance. However, the the masking method decreases sensitivity disturbance by means of superimposing masking method decreases the the sensitivity of of disturbance by means of superimposing vibration. Itvibration. is It is advantageous in that it is available even though the disturbance is unknown or complicated. advantageous in that it is available even though the disturbance is unknown or complicated.

Vibration transmitted to driver (Disturbance + Masker)

Driver

Active seat suspension

Masker vibration (Superimposing vibration on disturbance using actuator)

Disturbance vibration Figure 2. Masking method.

Figure 2. Masking method. 3. Evaluation of Psychological State Using Heart Rate Variability

3. EvaluationInof Psychological State Using (ECG) Heartwhich Rate can Variability this study, an electrocardiogram perform a simple measurement and can continuously acquire data was used as a physiological measurement for evaluating the psychological

In this study, an electrocardiogram (ECG) which can perform a simple measurement and can stress of the participant. The ECG represents the electrical activity of the heart and measures the continuously acquire datainwas used physiological measurement psychological potential difference heart rateas viaa an electrode directly attached to for the evaluating chest. Figurethe 3 shows an stress ofexample the participant. The ECG represents the electrical activity of the heart and measures the of an electrocardiogram waveform obtained during measurement. In the same figure, the potentialpeak difference in heart rate appearing via an electrode directly the chest.contraction Figure 3of shows in the positive direction periodically is calledattached an R wavetoand reflects the left The time from the R wave toobtained the next Rduring wave is measurement. called the R to R interval (RRI). figure, an example of ventricle. an electrocardiogram waveform In the same When this RRI is organized as a time history, it becomes as shown in Figure 4. Further, frequency the peak in the positive direction appearing periodically is called an R wave and reflects contraction analysis is performed on RRI time history to calculate the power spectral density, as shown in Figure of the left ventricle. The time from the R wave to the next R wave is called the R to R interval (RRI). 5. The integrated value in the range of 0.04 to 0.15 Hz with this power spectral density is referred to When this RRIfrequency is organized as athe time history, it in becomes 4. Further, frequency as low (LF), and integral value the rangeasofshown 0.15 to in 0.4 Figure Hz is referred to as high analysis frequency is performed on RRI time history to calculate the power spectral density, as shown in Figure 5. (HF). LF and HF are widely used as indicators of activities of the autonomic, sympathetic, and parasympathetic systems. this Hz paper, a comparison acquired density data is simple, and to as The integrated value in thenervous range of 0.04 toIn0.15 with this powerofspectral is referred therefore autonomic system activity the occupant was by low frequency (LF), and thenervous integral value in theofrange of 0.15under to 0.4the Hzexperiment is referred to evaluated as high frequency LF/HF, which is an indicator of the activity state of the sympathetic and parasympathetic nervous (HF). LF and HF are widely used as indicators of activities of the autonomic, sympathetic, and system. It is considered that there is a psychological state affected by the autonomic nervous system parasympathetic nervous systems. In this paper, a comparison of acquired data is simple, and therefore autonomic nervous system activity of the occupant under the experiment was evaluated by LF/HF, which is an indicator of the activity state of the sympathetic and parasympathetic nervous system. It is considered that there is a psychological state affected by the autonomic nervous system activity; LF/HF is predominant in the sympathetic nervous system—if the value is high, the parasympathetic nervous system dominates. If it is low, it can be evaluated as a relaxed state.

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activity; is Actuators 2018, 7,LF/HF 42 activity; LF/HF is predominant predominant in in the the sympathetic sympathetic nervous nervous system—if system—if the the value value is is high, high, the the4 of 13 activity; LF/HF is predominant in the sympathetic nervous system—if as the value isstate. high, the parasympathetic parasympathetic nervous nervous system system dominates. dominates. If If it it is is low, low, it it can can be be evaluated evaluated as aa relaxed relaxed state. parasympathetic nervous system dominates. If it is low, it can be evaluated as a relaxed state.

[mV] Voltage [mV] Voltage [mV] Voltage

1.5 1.5 1.5 11 1 0.5 0.5 0.5 00 0 -0.5 -0.5 -0.5 -1 -1 -1 0 0 0

R R wave wave R wave RRI RRI RRI

0.5 0.5 0.5

R R wave wave R wave

R R wave wave R wave RRI RRI RRI

11 1.5 1.5 [s] 1 Time Time 1.5 [s] Time [s]

22 2

2.5 2.5 2.5

Figure Electrocardiogram Figure (ECG). Figure3.3. 3.Electrocardiogram Electrocardiogram (ECG). (ECG).

[ms] RRI [ms] RRI [ms] RRI

Figure 3. Electrocardiogram (ECG).

11 1 0.8 0.8 0.8 0.6 0.6 0.6 0.4 0.4 0.4 0.2 0.2 0.2 00 0 0 0 0

30 30 30

60 60 60 Time Time [s] [s] Time [s]

90 90 90

120 120 120

Figure 4. Time history of the R-R interval (RRI).

density spectral Power density spectral Power density spectral Power 2 /Hz] [ms 2 2 /Hz] [ms /Hz] [ms

Figure 4. Timehistory history of of the the R-R interval (RRI). Figure R-Rinterval interval (RRI). Figure4.4.Time Time history of the R-R (RRI). 3 ×10 ×103 40 40 ×103 LF LF 40 35 LF 35 35 30 30 30 25 25 25 20 20 20 15 15 15 10 10 105 5 50 0 0 0 0.1 0 0.1 0 0.1

HF HF HF

0.2 0.3 0.2 0.3 Frequency [Hz] 0.2 0.3 Frequency [Hz] Frequency [Hz]

0.4 0.4 0.4

0.5 0.5 0.5

Figure Figure 5. 5. Frequency Frequency analysis analysis of of RRI RRI time time history. history. Figure 5. Frequency analysis of RRI time history.

Figure 5. Frequency analysis of RRI time history. 4. 4. Experimental Experimental Apparatus Apparatus and and Method Method 4. Experimental Apparatus and Method 4. Experimental Apparatus and Method 4.1. 4.1. Active Active Seat Seat Suspension Suspension 4.1. Active Seat Suspension In this study, 4.1. ActiveInSeat thisSuspension study, in in order order to to confirm confirm the the change change of of riding riding comfort comfort by by masking, masking, the the vibration vibration was was In this study, in order to confirmmobility the change of riding comfort by masking, theseat vibration was carried out using an ultra-compact vehicle (Figure 6) where the active carried out using an ultra-compact mobility vehicle (Figure 6) where the active seat suspension suspension In this out study, in an order to confirmmobility the change of riding thesuspension vibration was carried using ultra-compact vehicle (Figure comfort 6) whereby themasking, active seat

carried out using an ultra-compact mobility vehicle (Figure 6) where the active seat suspension (Figure 7) was installed as the experimental vehicle. In the active seat suspension, aluminum plates are used for the seat portion, and when not driving, the subject is supported by four coil springs. The movement direction of the seat is restrained by the four linear sliders installed in the surroundings in the vertical direction. For the control actuator, a voice coil motor, which is a type of linear motor that can be highly

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(Figure 7) was installed as the experimental vehicle. In the active seat suspension, aluminum plates (Figure was as theand experimental vehicle.the In subject the active seat suspension, plates are used7)for theinstalled seat portion, when not driving, is supported by fouraluminum coil springs. The are used for the seat portion, and when not driving, the subject is supported by four coil springs. The Actuators 2018, 7, 42 5 of 13 movement direction of the seat is restrained by the four linear sliders installed in the surroundings movement direction of the seat is restrained by the four linear sliders installed in the surroundings in the vertical direction. For the control actuator, a voice coil motor, which is a type of linear motor in thecan vertical direction. For the a voice whichsince is a type of linear motor that be highly responsive tocontrol thrust, actuator, was adopted. In coil this motor, experiment, the vibration test is responsive to thrust, was adopted. In this experiment, since the vibration test is performed using that can be highly responsive to thrust, wasthere adopted. In this experiment, since the vibration testactive is performed using active seat suspension, is a possibility that vibration different from the seat suspension, there is a possibility vibration different from the objective maydifferent be input in thethe state performed using active there is a possibility that vibration from objective may be input inseat the suspension, statethat when the vehicle is parked. For this reason, we conducted an when the vehicle is input parked. this reason, wevehicle conducted experiment with the jacked objective may be in For the state when is parked. For this reason, wevehicle conducted anup. experiment with the vehicle jacked up. the Furthermore, toanprevent the psychological state from Furthermore, to prevent the psychological state from changing due to influences other than vibration, experiment with the vehicle jacked up. Furthermore, to prevent the psychological state from changing due to influences other than vibration, it was carried out in a state of the vehicle parked changing due than vibration, it was carried out in a state of the vehicle parked it was carried outtoininfluences a state of other the vehicle parked indoors. indoors. indoors.

Jack Jack

Figure 6. Photograph of the ultra-compact vehicle. Figure 6. Photograph of the ultra-compact vehicle. Figure 6. Photograph of the ultra-compact vehicle.

Accelerometer Accelerometer

Seat surface Seat surface

Actuator Actuator (Voice coil motor) (Voice coil motor) Figure 7. Photograph of active seat suspension. Figure 7. Photograph of active seat suspension. Figure 7. Photograph of active seat suspension.

In this experiment, the subject was seated on the active seat suspension, and vibration was In this experiment, wasthe seated on order the active seatthe suspension, and vibration was applied to the subject by the onlysubject actuating seat. In to verify effect of masking, the loaded In thistoexperiment, theonly subject was seated on the activeto seat suspension, and vibrationthe was applied applied the subject by actuating the seat. In order verify the effect of masking, loaded vibrations set several vibrations with different frequency characteristics, and the details are described to vibrations the subjectset byseveral only actuating the seat. In order to verify the effect of masking, the loaded vibrations with different and thedriving details are described in each chapter. Also,vibrations to make the posture offrequency the subjectcharacteristics, closer to the actual environment, setin several vibrations with different frequency characteristics, and the details are described in each each chapter. Also, to make the the steering posture wheel of the and subject closer to the actual driving environment, subjects were directed to grasp assume a driving posture. This is because the chapter. Also, todirected make the posture of thebody subject closer to theaactual driving environment, subjects subjects were to of grasp steering wheel and depending assume driving posture. This is because the vibration characteristics the the human change on the posture when exposed to were directed to grasp the steering wheel and assume a driving posture. This is because the vibration vibration characteristics of the human body change depending on the posture when exposed to vibration, as shown by Mansfield et al. [12]. vibration, as shown Mansfield al. [12]. depending on the posture when exposed to vibration, characteristics of the by human bodyetchange as 4.2. shown by Mansfield al.Method [12]. of Electrocardiogram Measuring Device et and 4.2. Measuring Device and Method of Electrocardiogram Bio Amp ML132, Power Lab 8/35 PL 3508, MLA 2340, and MLA 2503 shield leads (AD 4.2. Measuring Device and Method of Electrocardiogram Bio AmpSydney, ML132,Australia) Power Lab 8/35 PLfor 3508, 2340, andand MLA leads (AD Instruments, were used ECGMLA measurement, the 2503 heart shield rate variability of Instruments, Sydney, Australia) were used for ECG measurement, and the heart rate variability of Amp ML132, Power PL 3508, MLA and MLA 2503 shield leads (AD Instruments, theBio company was used as Lab the 8/35 analysis system. The2340, method of affixing the electrode is not easily the company waswere used as the analysis system. The method affixing electrodeofisthat notcompany easily Sydney, Australia) used for ECGthan measurement, and theofheart ratethe variability the affected by action potentials other myocardial activity, and NASA induction can be affected action other than myocardial activity, and NASA that can be was used asbystably the analysis system.In The method of affixing the electrode is not easily affected by action measured waspotentials adopted. addition, since the ECG makes it difficult to induction accurately estimate the measured stably was adopted. In addition, since the ECG makes itcan difficult accurately estimate the potentials other than myocardial activity, NASA induction thatrhythm, be measured stably was not adopted. autonomic nervous system activity by and changing the breathing wetodirected subjects to autonomic nervous system activity by changing the breathing rhythm, we directed subjects not to speak during In addition, sincemeasurement. the ECG makes it difficult to accurately estimate the autonomic nervous system activity during byspeak changing themeasurement. breathing rhythm, we directed subjects not to speak during measurement.

4.3. Experimental Method Figure 8 shows the flow of each vibration experiment. In the vibration test, first, a mental calculation task was given to the subject for one minute, and then the subject was made to ride the vibrating vehicle for three minutes. The ECG was acquired for a total of four minutes. The mental


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Figure 8 shows the flow of each vibration experiment. In the vibration test, first, a mental Actuators 7, 42 6 of 13 4.3.2018, Experimental calculation task wasMethod given to the subject for one minute, and then the subject was made to ride the vibrating vehicle for three The vibration ECG wasexperiment. acquired for total of four test, minutes. mental Figure 8 shows the minutes. flow of each Inathe vibration first, The a mental calculation task was carried out with the object of matching the psychological state of each calculation task was given to the subject for one minute, and then the subject was made to ride the calculation task was carried out with the object of matching the psychological state of each experiment experiment by loading the subject with a certain stress before the experiment. The subject was vibrating three minutes.stress The ECG wasthe acquired for a total four minutes. The mental by loading thevehicle subjectforwith a certain before experiment. Theofsubject was instructed to add calculation tasktwo wastypes carried out with values the object of matching the psychological state ofmonitor each instructed to add of numerical (two digits) intermittently displayed on the two types of numerical values (two digits) intermittently displayed on the monitor installed in front experiment by of loading the subject with a certain before experiment. The instructed subject wasnot installed in front the vehicle for five seconds each.stress At this time,the subjects were also of the vehicle for five seconds each. At this time, subjects were also instructed not to answer the add two types of numerical (two digits) intermittently to instructed answer thetocalculations aloud because values the influence on the heart rate displayed variation on by the themonitor utterance calculations aloud because the influence on the heart rate variation by the utterance was a concern. installed in front ofvibration the vehicle for five seconds each. this time,the subjects alsostate, instructed not was a concern. In the experiment, in order to At reproduce actualwere driving we directed In thetovibration experiment, in order to reproduce the actual driving we directed the subjects answer the calculations aloud because influence on the with heart the ratestate, variation by the utterance the subjects to watch the monitor in the the driving direction driving attitude (Figure 9). to watch monitor invibration the driving direction with to the driving the attitude (Figure 9). Considering the was athe concern. In the experiment, in order reproduce driving we directed Considering the fatigue and physical and psychological burden on actual the subject bystate, the vibration test, fatigue and physical and psychological on the subject with by the test, we setheart a sufficient to watch monitor inburden the driving direction thevibration driving attitude (Figure 9). wethe set subjects a sufficient breakthe time between experiments in each vehicle. Blood pressure and rate Considering the experiments fatigue and physical and psychological burden on theheart subject bywere the vibration test, break time between in each vehicle. Blood pressure and rate measured before were measured before starting the experiment in each vehicle, and it was confirmed that the values we the set aexperiment sufficient break time vehicle, between and experiments in each vehicle. Blood pressure and heart constant, rate starting in each it was confirmed that the values were nearly were nearly constant, so that the psychological state of the subjects before the vibration experiment were before starting in each the vehicle, and it was confirmedwas thatapproximately the values so was that themeasured psychological stateRegarding ofthe theexperiment subjects before experiment approximately the same. this experiment, itvibration was approved by the ethics committee of were nearly constant, so that the psychological state of the subjects before the vibration experiment theTokai same.University’s Regarding“Research this experiment, it was approved by the ethics committee of Tokai University’s targeted at humans”, and the subjects were given prior explanation of was approximately the same. Regarding this experiment, it was approved by the ethics committee of “Research targeted at experiment. humans”, and the subjects were given prior with explanation of the contents of the theTokai contents of the Subjects agreeing on cooperation the experiment signed and University’s “Research targeted at humans”, and the subjects were given prior explanation of experiment. agreeing on cooperation with the experiment signed and sealed the consent form sealed the Subjects consent form approved by the committee. the contents of the experiment. Subjects agreeing on cooperation with the experiment signed and approved by the committee. sealed the consent form approved by the committee.

Mental arithmetic Mental (1 min) arithmetic

Vibration test (3 min) Vibration test (3 min)

(1 min)

0

1 0

1

2 Time2 [min]

3 3

4 4

Time [min]

Figure the experiments. experiments. Figure8. 8. Flow Flow of of the Figure 8. Flow of the experiments.

Figure9.9.Photograph Photograph of the experiment set up. Figure Figure 9. Photograph of the experiment experimentset setup. up.

5. Vibration ExperimentofofSingle SingleFrequency Frequency with with Constant 5. Vibration Experiment ConstantAcceleration AccelerationAmplitude Amplitude 5. Vibration Experiment of Single Frequency with Constant Acceleration Amplitude Vibration Condition 5.1.5.1. Vibration Condition 5.1. Vibration Condition In this section, to investigate the vibration frequency applied to the masking, which is the In this section, to investigate the vibration frequency applied to the masking, which is the In this section, investigate the vibration frequency appliedfrequencies, to the masking, whichexperiment is the vibration vibration controltowhich superimposes the vibrations of multiple a vibration vibration control which superimposes the vibrations of multiple frequencies, a vibration experiment with a single frequency sinusoidal wave was performed as a basic experiment. The authors have control which superimposes the vibrations of multiple frequencies, a vibration experiment with a single with a single frequency sinusoidal wave was performed as a basic experiment. The authors have frequency sinusoidal wave was performed as a basic experiment. The authors have already reported that the change in the LF/HF occurs depending on the frequency when the displacement amplitude is constant, but in the ride comfort assessment for vibration, in general, since the acceleration is used, the vibration condition with a constant acceleration amplitude was chosen. We decided to measure the psychological state of the subjects.


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For sensory curves ofPEER ISO 2631 [13] and ride comfort limit curves of Janeway [14], it is said to Actuators 2018, 7, x FOR REVIEW 7 of 13 be an unpleasant vibration as humans feel vibration sensitively at a 4 to 8 Hz vibration frequency. already reported that the change in the LF/HF occurs depending on the frequency when the In addition, when steady vibration wasbut input etassessment al. [15], itforwas demonstrated that subjects displacement amplitude is constant, in theby rideTakei comfort vibration, in general, since the acceleration used, the vibration condition with a constant acceleration was chosen. could express a feelingisfor vibration with a fluffy feeling at 0.2 to 3 Hzamplitude and a feeling of fluttering We decided to measure the psychological state the subjects. at 8 to 20 Hz. Therefore, we investigated this byofadding 3 Hz, 9 Hz, and 10 Hz, which differ in the For sensory curves of ISO 2631 [13] and ride comfort limit curves of Janeway [14], it is said to be sense of vibration from oscillation frequency 4 to 8 Hz which is unpleasant, with vibration at a single an unpleasant vibration as humans feel vibration sensitively at a 4 to 8 Hz vibration frequency. In frequency addition, of 3 to when 10 Hz. Acceleration amplitude set to 1.5demonstrated m/s2 under conditions, taking steady vibration was input by Takeiwas et al. [15], it was thatall subjects could express a feeling for vibration with a fluffy feeling at 0.2 to 3 Hz and a feeling of fluttering at 8 to 20 Hz. into consideration the worst vibration input assumed to be a steady input during driving and the Therefore, investigated thisnotation by adding 3ofHz, 9 Hz, and 10in Hz,this which differindicates in the sense a of single vibrationamplitude performance of thewe actuator. The amplitude paper from oscillation frequency 4 to 8 Hz which is unpleasant, with vibration at a single frequency of 3 to value. In this chapter, since the objective is to determine changes in psychological state by LF/HF, 10 Hz. Acceleration amplitude was set to 1.5 m/s2 under all conditions, taking into consideration the the subjectworst was vibration a male university student 22 years old). driving and the performance of the input assumed to be (age a steady input during actuator. The notation of amplitude in this paper indicates a single amplitude value. In this chapter, since theResult objective is to determine changes in psychological state by LF/HF, the subject was a male 5.2. Experimental university student (age 22 years old).

Figure 10 shows the LF/HF calculated from the ECG data during the vibration time at each Experimental Result frequency. 5.2. From the figure, it can be confirmed that a change appears in LF/HF, which is a stress index according to frequency. As described LF/HF can be data evaluated as being in time a higher stress state Figure 10 shows the LF/HF above, calculated from the ECG during the vibration at each frequency. From the figure, it can be confirmed that a change appears in LF/HF, which is a stress as the value becomes higher. It can be confirmed that the subject was most stressed at 7 Hz. It was index according to frequency. As described above, LF/HF can be evaluated as being in a higher stress confirmed that there was approximately a two-fold difference at the highest value of 7 Hz compared state as the value becomes higher. It can be confirmed that the subject was most stressed at 7 Hz. It to 3 Hz, which has the lowest LF/HF. In addition,a3two-fold Hz (fluffy feeling) andhighest 10 Hzvalue (feeling was confirmed that there was approximately difference at the of 7 of Hzfluttering) decrease LF/HF compared to 4hastothe 8 Hz required improve comfort. It suggests that subject compared to 3 Hz, which lowest LF/HF. Into addition, 3 Hzride (fluffy feeling) and 10 Hz (feeling of fluttering) compared to 4the to 8results Hz required to improve ridechapter, comfort. Ititsuggests was relaxed by 3 anddecrease 10 Hz LF/HF vibration. From obtained in this was possible to that subject was relaxed by 3 and 10 Hz vibration. From the results obtained in this chapter, it was determine by the LF/HF the psychological state changed according to the vibration frequency in the possible to determine by the LF/HF the psychological state changed according to the vibration vertical direction. on thisdirection. result, we will the we influence ofthe masking, frequency Based in the vertical Based on study this result, will study influencecombining of masking, multiple combining multiple vibration frequencies state, in the next chapter. vibration frequencies on psychological state,on inpsychological the next chapter.

6

LF/HF

5 4 3 2 1 0 3

4

5 6 7 8 Frequency [Hz]

9

10

Figure 10. LF/HF values in each vibration frequency (amplitude: 1.5 m/s2).

Figure 10. LF/HF values in each vibration frequency (amplitude: 1.5 m/s2 ). 6. Fundamental Study on Superimposed Vibration

6. Fundamental Study on Superimposed Vibration 6.1. Vibration Condition

6.1. Vibration Condition Based on the results in Section 5, this chapter confirms what psychological change is caused in passengers by overlapping vibrations having different frequency characteristics. When masking

Basedcontrol on the resultswhen in Section 5,on this chapter confirms what psychological is caused in is applied traveling an actual road, various patterns such as frequency,change amplitude, passengersand by overlapping vibrations having different frequency masking control phase are generated between the vibration input from thecharacteristics. road surface andWhen the vibration is applied when traveling on an actual road, various patterns such as frequency, amplitude, and phase are generated between the vibration input from the road surface and the vibration generated by the actuator. In addition, there are few driving environments in which vibration with a single frequency occurs. At the time of this experiment, there was no report on the influence on the mental state by the superimposition of vibration to date. We conducted a simple vibration experiment simulating the driving environment with the goal of confirming whether masking improves ride comfort as acquiring the basic data. In this study, we assumed single frequency vibration assuming the worst driving


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generated by the actuator. In addition, there are few driving environments in which vibration with a single frequency occurs. At the time of this experiment, there was no report on the influence on the mental state by the superimposition of vibration to date. We conducted a simple vibration experiment Actuators 2018, 7, 42 8 of 13 simulating the driving environment with the goal of confirming whether masking improves ride comfort as acquiring the basic data. In this study, we assumed single frequency vibration assuming the worst driving environment for the vibration characteristics ofas the human body so as we a disturbance, environment for the vibration characteristics of the human body a disturbance, confirmed so we confirmed the change instate the when psychological masking with a singleagainst frequency the change in the psychological maskingstate with when a single frequency vibration the vibration against the disturbance vibration. disturbance vibration. In this this experiment, experiment, a sinusoidal wave In wave of of 55 Hz Hz was wasset setas asthe thevibration vibrationinput inputtotothe thevehicle vehicleasasa the human human adisturbance. disturbance.This Thisisisbecause becauseMatsumoto Matsumotoetetal. al.[16] [16]reported reported that that the the natural frequency of the body at the sitting position is 5 Hz, and it is considered to be the vibration which should reduce body at the sitting position is 5 Hz, and it is considered to be the vibration which should reduce sensitivitymost most when when traveling. traveling. The The vibration vibration to to be be superimposed superimposed on on the the disturbance disturbance does does not not affect affect sensitivity the human body, and it is necessary to set a vibration which can obtain a different feeling from the the human body, and it is necessary to set a vibration which can obtain a different feeling from the disturbance. In In this this experiment, experiment, as as explained explained in in Section Section 5, 5, the the two two types types of of vibration vibration frequency frequency bands bands disturbance. having different different feelings, and two levels having levels of of sinusoidal sinusoidal vibrations vibrationsof of33and and10 10Hz, Hz,which whichproduced produceda comfort in inthe thevibration vibrationexperiment experiment with a single frequency, were used aminor minorinfluence influenceon on the the ride comfort with a single frequency, were used for for masking. In addition, the vibration used for masking is considered affect the vibration masking. In addition, sincesince the vibration used for masking is considered to affecttothe vibration sensed sensed by a acceleration relative acceleration with respect to external disturbance, types of by a relative amplitudeamplitude with respect to external disturbance, two types oftwo acceleration 2 are set for each frequency. 2 acceleration amplitude condition with an amplitude of 1.5 and 3.0 m/s amplitude condition with an amplitude of 1.5 and 3.0 m/s are set for each frequency. Table 1 shows Table 1 showsconditions the vibration conditions set in this experiment.the Additionally, acceleration response the vibration set in this experiment. Additionally, accelerationthe response and frequency and frequency characteristics in each experimental condition are shown in Figure 11. In addition, characteristics in each experimental condition are shown in Figure 11. In addition, these waveforms thesecalculated waveforms were calculated by the MATLAB. Regarding the several phase,patterns when preparing several were by MATLAB. Regarding phase, when preparing preliminarily and patterns preliminarily and performing experiments with several subjects, we used that the condition performing experiments with several subjects, we used the condition that answers the feelingthat of answers that the feeling of riding most changed by masking was obtained. riding most changed by masking was obtained. Table1. 1. Vibration Vibrationconditions conditionsin inexperiments. experiments. Table

4.5 3 1.5 0 -1.5 -3 -4.5

Acceleration [m/s²]


Vibration 1 (Disturbance) Vibration 2 (Masker) Vibration 1 (Disturbance) Vibration 2 (Masker) Case 2 Case Frequency (Hz) Amplitude (m/s ) Frequency (Hz) Amplitude (m/s2) 2 Frequency (Hz) Frequency (Hz) Amplitude (m/s ) Amplitude (m/s2 ) I ― ― I II — 1.5 — 3 5 1.5 3.0 1.5 II III 5 1.5 3 III IV 1.5 3.0 10 V 3.0 1.5 IV 10 V 3.0

0

1 Time [s]

3 2 1 0

2

0

5 10 15 Frequency [Hz]

0




(I) 4.5 3 1.5 0 -1.5 -3 -4.5 0

1 Time [s]

2

(II) Figure 11. Cont.

3 2 1 0

Actuators 2018, 7, 42 Actuators 2018, 7, x FOR PEER REVIEW

(III)

0 -1.5 4.5 -3 -4.5

1 Time [s]

2

(III)

1 Time [s]



0 0 -1.5 -3 4.5 -4.5 3 01.5

2

(IV)

-3 -4.5

1 Time [s]

2

(IV)

1 Time [s]


3 0 1.5 0 -1.5 4.5 3 -3 1.5 -4.5 0 0 -1.5

3

Acceleration [m/s²] Acceleration [m/s²]

2

2

2 1 9 of 13

0 0

3


2


1 Time [s]

1 0

3 2

0


1 3

0 0

2


1



3 1.5 0 4.5 -1.5 3 -3 1.5 -4.5





4.5 3 1.5 0 -1.5 Actuators 2018, 7, x FOR-3 PEER REVIEW -4.5 04.5

9 of 13 9 of 13

0 3 0

5

10

15

2 Frequency [Hz] 3 1 2 0

0 1


0 (V) 0 5 10 15 1 2 Frequency [Hz] (I) 5 Hz (amplitude: 1.5 Time [s] histories on masking. histories and and spectra spectra oneach eachvibration vibrationcondition conditionofof masking. (I) 5 Hz (amplitude:

0

Figure 11. 11. Time Figure Time 2 2 + 3 2Hz (1.5 m/s2); (III) 25(V) 2); (IV) 5 Hz 2) + 2 ); 5(II) m/sm/s ); (II) Hz5(1.5 (1.5 m/s2(1.5 ) + 3m/s Hz 2(3 1.5 Hz m/s (1.5 )m/s ) + 3 Hz (1.5 m/s );Hz (III) 5 Hz ) +m/s 3 Hz (3 m/s2 );(1.5 (IV)m/s 5 Hz 2 2 2 2) + 2 );and 2 ) +).10 condition 2 ). Figure Time spectra each of masking. (I) 5 Hz (amplitude: 1.5 10 Hz (1.5 m/s (V) 5histories Hz m/s + 10on Hz (3vibration m/s (1.5 m/s 1011.); Hz (1.5 m/s(1.5 (V) 5) Hz (1.5 m/s Hz (3 m/s m/s2); (II) 5 Hz (1.5 m/s2) + 3 Hz (1.5 m/s2); (III) 5 Hz (1.5 m/s2) + 3 Hz (3 m/s2); (IV) 5 Hz (1.5 m/s2) + 10 Hz (1.5 m/s2); (V) 5 Hz (1.5 m/s2) + 10 Hz (3 m/s2). Experimental Result

6.2. Experimental Result 6.2.

6.2.LF/HF Experimental The in condition when when LF/HF LF/HF at Hz amplitude amplitude is is set set to to 11 is is shown shown in in The LF/HF in each eachResult condition at the the reference reference 55 Hz Figure 12. In addition, the subject in the experiment conducted in Section 5 is subject C in this figure. LF/HF inthe eachsubject condition LF/HF at theconducted reference 5 Hz to 1 is shown in figure. Figure 12. InThe addition, in when the experiment in amplitude Section 5isisset subject C in this 2, the 2 Figure 12. In addition, the subject in the experiment conducted in Section 5 is subject C in this figure. When the amplitude of the vibration to be superimposed is 1.5 m/s LF/HF became higher than When the amplitude of the vibration to be superimposed is 1.5 m/s , the LF/HF became higher than When the for amplitude ofAthe vibration to beCsuperimposed is stressed 1.5 m/s2, the LF/HF became higher than the disturbance subject and B. Subject only became when the superimposed vibration the disturbance for subject A and B. Subject C only became stressed when the superimposed vibration disturbance22for subject A and B. Subject C only became stressed when the superimposed vibration is 10 10 Hz Hzthe and 1.5m/s m/s . From this, itit was confirmed that when masking with the the same amplitude amplitude as the is 1.5 From this, was thatwhen when masking 2. From isand 10 Hz and 1.5 .m/s this, it wasconfirmed confirmed that masking withwith the samesame amplitude as the as the disturbance, the subject becomes stressed. disturbance, the subject becomes stressed. disturbance, the subject becomes stressed.

4 3 2 1 5Hz 5Hz 5Hz (1.5m/s² m/s² + 3Hz (3.0) m/s²) 0 43210 5Hz ++ 3Hz 3Hz (1.5 ) ) 5Hz5Hz + 3Hz (3.0 m/s² A10Hz + 10Hz m/s² 5Hz ++A 10Hz (3.0 ) ) B B 5Hz +5Hz 10Hz (1.5(1.5 m/s² ) ) 5Hz (3.0m/s² m/s²

3.5 3.5 LF/HF ratio

LF/HF ratio

3 2.5

3 2.5 2

2 1.5 1.5 1 1 0.5 0.5

0 A

0 A

B Subjects

B

C

Figure 12. LF/HF values in masking of vibration.

C

Figure 12. LF/HF values in masking of vibration. Subjects

Figure 12. LF/HF values in masking of vibration.


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Conversely, when the usedfor formasking maskingwas wastwice twice Conversely, when theacceleration accelerationamplitude amplitude of of the the vibration vibration used thethe disturbance, LF/HFshowed showed a low value case where three subjects werewere exposed to disturbance, LF/HF valuecompared comparedtotothe the case where three subjects exposed the disturbance alone and showed a relaxed state. In the ride comfort evaluation conventionally to the disturbance alone and showed a relaxed state. In the ride comfort evaluation conventionally indicated ISO,the thecomfort comfortwill will be be impaired impaired as increases, butbut wewe confirmed that that it indicated byby ISO, asthe theamplitude amplitude increases, confirmed possible to to mask mask unpleasant unpleasant vibrations vibrations by of of it isispossible by superimposing superimposingother othervibrations vibrationsininconsideration consideration frequency. Based on this result, we will increase the number of subjects thesection next section and frequency. Based on this result, we will increase the number of subjects in thein next and conduct conduct a more detailed verification. a more detailed verification. 7. Examination RideComfort ComfortImprovement ImprovementEffect Effect by by Masking Masking 7. Examination ofof Ride Experimental Conditions 7.1.7.1. Experimental Conditions 2 2 In Section 6, by masking witha avibration vibrationofof33Hz Hzand and10 10 Hz Hz and and the In Section 6, by masking with the amplitude amplitudeof of3.0 3.0m/s m/swhich which is twice amplitude forinput inputdisturbance disturbanceof of 55 Hz, Hz, three three subjects is twice thethe amplitude for subjects confirmed confirmedthat thatthey theyexperienced experienced a relaxed state. thischapter, chapter,we werecorded recorded results results from from the to to a relaxed state. InInthis the three threesubjects subjectsininSection Section6 6ininorder order verify the effect and the cause of the masking only condition, which superimposes twice the verify the effect and the cause of the masking only condition, which superimposes twice the amplitude amplitude at a 3 Hz and 10 Hz frequency against 5 Hz disturbance. We conducted an experiment at a 3 Hz and 10 Hz frequency against 5 Hz disturbance. We conducted an experiment with 12 subjects. with 12 subjects. (Male university and graduate students, average age 22.1 years old). (Male university and graduate students, average age 22.1 years old).

7.2. Experimental Result 7.2. Experimental Result Figure 13 shows the ratio of LF/HF under each condition with LF/HF set at 1 at a 5 Hz vibration Figure 13 shows the ratio of LF/HF under each condition with LF/HF set at 1 at a 5 Hz vibration frequency. Figure 13a shows subjects who were relaxed by masking (subject A, B, C, D, E, and F), and frequency. Figure 13a shows subjects who were relaxed by masking (subject A, B, C, D, E, and F), and Figure 13b shows those who classified themselves as stressed by masking (subject G, H, I, J, K, and Figure 13b shows those who classified themselves as stressed by masking (subject G, H, I, J, K, and L). L). In addition, subjects I and L who became stressed by masking became relaxed only when In addition, subjects I and L who became stressed becameriding relaxed only when superimposing a vibration of frequency 10 Hz.by Formasking these reasons, comfort may superimposing be improved a vibration of frequency 10 Hz.onFor these reasons, ridingmay comfort may bedepending improvedon bythe masking, and by masking, and depending occupants, ride comfort be different occasion; depending on occupants, ride comfort may be different depending on the occasion; we confirmed we confirmed that experiencing a relaxed ride varies by individual. In addition, when the acceleration thatamplitude experiencing a relaxedthe ride varies by individual. addition, whenatthe acceleration amplitude is constant, displacement amplitudeInbecomes larger 3 Hz than 10 Hz. In this is constant, the displacement amplitude larger at 3 Hz than 10 Hz. thisso experiment, all subject experiment, all subject groups who becomes relaxed by masking were relaxed at In 3 Hz, displacement may groups who relaxed by masking were relaxed at 3 Hz, so displacement may affect ride comfort. affect ride comfort. 5 Hz + 3 Hz (3.0 m/s²)

5 Hz + 10 Hz (3.0 m/s²)

5.0

5.0

4.0

4.0

LF/HF ratio

LF/HF ratio

5 Hz

3.0 2.0

3.0

2.0 1.0

1.0

0.0

0.0 A

B

C

D

Subjects (a)

E

F

G

H

I

J

Subjects

K

L

(b)

Figure Comparison subjectsby byLF/HF LF/HF values values on becoming Figure 13.13. Comparison ofof subjects on masking maskingof ofvibration: vibration:(a) (a)Subjects Subjectsofof becoming relaxed by masking; (b) subjects of becoming stressed by masking. relaxed by masking; (b) subjects of becoming stressed by masking.

In order to further pursue the differences in vibration sensation by subjects, the vibration In order torate further pursue vibration sensation by subjects, vibration transmission to each part ofthe the differences human bodyin was measured, and an evaluation wasthe made as to transmission rate to each part of the human body was measured, and an evaluation was made whether or not the cause of vibratory sensation differs depending on subjects. Here, the seat surface as to whether orwith not the cause of vibratory depending on subjects. Here, the seat was vibrated white noise using the sensation active seatdiffers suspension, and the transfer function was surface was vibrated withsurface white acceleration noise usingmeasured, the activeasseat and the calculated from the seat wellsuspension, as the acceleration of transfer each partfunction of the washuman calculated the seat surface acceleration measured, as well as measured the acceleration each part body.from Incidentally, the acceleration of the human body was at five of positions, including head, shoulder, arm, waist, and thigh. 14 body showswas the measured measurement position of of the humanthe body. Incidentally, the acceleration of theFigure human at five positions,


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including the head, shoulder, arm, waist, and thigh. Figure 14 shows the measurement position of the acceleration. At the of measurement, the acceleration lead was affixed directlydirectly to the skin. Subject the acceleration. Attime the time of measurement, the acceleration lead was affixed to the skin. A was relaxed masking, and subject was stressed by masking. Table 2 shows highest of Subject A was by relaxed by masking, andGsubject G was stressed by masking. Table 2the shows thevalue highest transfer function and frequency of each part of the human body in this experiment. value of transfer function and frequency of each part of the human body in this experiment.

Head

Shoulder

Arm

Belly

Thigh

Figure 14. Measuring points of acceleration. Figure 14. Measuring points of acceleration.

Subjects Subjects A A G G

Table 2. Peak of the transfer function in each part of the human body. Table 2. Peak of the transfer function in each part of the human body. Measurement Points Measurement Head Shoulder Arm Points Belly Frequency (Hz) 3.0 6.0 5.5 5.5 Head Shoulder Arm Belly Transmissibility ((m/s2)/(m/s2)) 2.1 4.2 3.1 2.2 Frequency (Hz) 3.0 6.0 5.5 5.5 Frequency (Hz)2 3.0 5.0 5.5 5.5 2.1 4.2 3.1 2.2 Transmissibility ((m/s )/(m/s2 )) Transmissibility ((m/s2)/(m/s2)) 2.0 5.4 3.0 3.6 Frequency (Hz) 3.0 5.0 5.5 5.5 2.0 5.4 3.0 3.6 Transmissibility ((m/s2 )/(m/s2 ))

Thigh 12.5 Thigh 2.2 12.5 13.0 2.2 1.7 13.0 1.7

In the experiment described in the previous section, Subject A who was relaxed by masking and two subjects who were stressed by masking were selected. It is confirmed in the same table that there In theno experiment in the previous Subject A who was relaxed by masking and is almost differencedescribed in the vibration frequencysection, at which the transfer function reaches the highest two subjects who were stressed by masking were selected. It is confirmed in the same table that there value in each part of the human body. The difference in ride feeling sensation that can be evaluated is no difference in the frequency at which the transfer of function reaches the highest byalmost biological information is vibration not the physical vibration characteristic the human body of each value in each part of the human body. The difference in ride feeling sensation that can be evaluated by subject, and psychological conditions and other causes are considered to be significant. biological information is not the physical vibration characteristic of the human body of each subject, and psychological conditions and other causes are considered to be significant. 8. Conclusions In this study, we focus on masking to reduce the sensitivity of the disturbance vibration, which 8. Conclusions contributes to a psychologically uncomfortable feeling, to improve ride comfort. We conducted this thisthe study, we focus on masking to reduce the control sensitivity of the disturbancephysiological vibration, which studyInfor purpose of designing a ride comfort system considering and contributes to a psychologically uncomfortable feeling, to improve ride comfort. We conducted psychological characteristics. Since the acceleration and frequency of vibration are greatly involved this study for the purpose of designing a ride comfort system in the comfort of the occupant, basic experiments werecontrol conducted on considering the effect of physiological ride comfort and psychological characteristics. Since the acceleration and frequency of vibration aremasking greatly improvement by masking. Furthermore, the number of subjects was increased based on the involved in the comfort of the occupant, basic experiments were conducted on the effect of ridea control condition in which improvement of riding comfort was observed, and we conducted comfort improvement by masking. Furthermore, the number of subjects was increased based on the demonstration experiment. In this thesis, we clarified the following: masking control condition in which improvement of riding comfort was observed, and we conducted 5 Hz sinusoidal vibrationInagainst superimposing and 10 Hz sinusoidal vibrations with twice a(1)demonstration experiment. this thesis, we clarified3the following: the acceleration amplitude by masking results in greater ride comfort than with 5 Hz sinusoidal vibrations which are susceptible to vibration. This was confirmed by the psychological state evaluation index LF/HF obtained from the ECG.


(1)

(2)

(3)

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5 Hz sinusoidal vibration against superimposing 3 and 10 Hz sinusoidal vibrations with twice the acceleration amplitude by masking results in greater ride comfort than with 5 Hz sinusoidal vibrations which are susceptible to vibration. This was confirmed by the psychological state evaluation index LF/HF obtained from the ECG. Based on the above section, the effect of masking by sinusoidal vibrations of 3 and 10 Hz with twice the acceleration amplitude in twelve subjects was evaluated from the vibration test, and it was confirmed that the effect of masking varied by subject. To confirm the physical difference between subjects with different effects of improving riding comfort by masking, the vibration transmission rate of each part of the human body was evaluated from the vibration experiment, but no clear difference was confirmed.

Although the results obtained from the above investigation are limited, it was possible to confirm the effect of improving the ride comfort by masking. However, it was revealed that there was a difference in the effect from a psychological point of view, even among subjects with little physical response. Therefore, in order to perform vibration control by masking, it is necessary to select the vibration by sufficiently considering the psychological characteristics of the participant against the vibration. Conversely, vibration with various acceleration amplitudes and frequency bands is considered to be disturbance. In addition, conditions such as acceleration amplitude, displacement amplitude, phase, and frequency characteristics are assumed for the vibration to be superimposed. Furthermore, the psychological state changes from moment to moment, and depending on the psychological state, it is assumed that its effect will change even when the same vibration control is applied. Therefore, in future work, we will systematically summarize the psychological characteristics of passengers and the psychological response by masking control and aim to construct a system that can apply masking control that matches the participant’s condition. Author Contributions: K.I., T.N. and H.K. wrote the paper; A.E. and R.M. conducted the experiments; A.E. contributed through analyzing the data; T.N. and H.K. conceived experiments. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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Kato, H.; Oshinoya, Y.; Hasegawa, S.; Kasuya, H. Ride comfort Evaluation of Active Seat Suspension for Small Vehicles Using Psychology and Physiology—Fundamental Consideration by Analysis Heart Rate Fluctuation and Salivary Amylase Activity. Proc. Sch. Eng. Tokai Univ. 2011, 36, 29–34. Kobori, T.; Kajikawa, Y. Ergonomic Evaluation Methods for Bridge Vibrations. J. Jpn. Soc. Civ. Eng. 1974, 230, 23–31. [CrossRef] Mansfield, N.J.; Griffin, M.J. Effect of posture and vibration magnitude on apparent mass and pelvis rotation during exposure to whole-body vertical vibration. J. Sound Vib. 2002, 253, 93–107. [CrossRef] International Organization for Standardization. Guide for the Evaluation of Human Exposure to Whole-Body Vibration; ISO 2631; ISO: Geneva, Switzerland, 1978. Janeway, R.N. Human Vibration Tolerance Criteria and Application to Ride Evaluation 750166; SAE Technical Paper; SAE International: Warrendale, PA, USA; Troy, MI, USA, 1975. Takei, K.; Ishiguro, M. Evaluation of Ride Comfort on the Basis of Subjective Judgement. R&D Rev. Toyota CRDL 1995, 30, 47–56. Yasunao, M.; Griffin, M.J. A Study on Dynamic Response Characteristics of Human Body upon Vertical Vibration Exposure. Jpn. Soc. Civ. Eng. 2002, 703, 185–201. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).