COMFORT ANALYSIS IN COMMERCIAL VEHICLE S PASSENGER SEAT TAM WEE KONG A thesis submitted in fulfilment of the requirements for the award of the degree of Master of Engineering (Mechanical) Faculty of Mechanical Engineering Universiti Teknologi Malaysia AUGUST 2006
To my dearest parents and brother iii
iv ACKNOWLEDGEMENT First of all, I would like to express my sincere gratitude to my supervisor, Associate Professor Haji Mustafa bin Yusof and my co-supervisor, Professor Dr. Roslan Abdul Rahman, for their assistance, contribution and valuable guidance through this research, as well as great patience I shall not forget. I want to thank Miss Kartina, Mr. Syam and Mr. Romaizi, for their help with research material collecting, test rig construction, testing and data processing. To those friends who had assisted me as the subjects in all my experiments, thank you for your precious time and tolerance. I would also like to acknowledge the support provided by the technicians; Mr. Fazli from Strength Lab and Mr. Shamsuddin from Composite Centre. I wish to express my appreciation to Miss Deborah Lim Shin Fei for providing me with encouragement, understanding and assistance in the completion of this thesis work. Finally, to my family, especially to my parents who love and believe in me, thank you for all your advice, encouragement and support. I dedicate this thesis to them.
v ABSTRACT A passenger seat is one of the main components to be considered when defining comfort in a moving vehicle. Experience shows that a seat produces different levels of comfort in different conditions. The comfort of automotive seats is dictated by a combination of static and dynamic factors. This research attempts to study the static and dynamic characteristics of a bus passenger seat for comfort through subjective and objective evaluations. The discomfort factors to be studied are the seat structure and pressure distribution at the human-seat interface. Two surveys including a pilot test were carried out to study the subjective evaluation through direct response from local users on seat comfort during their journey on the road. For the objective evaluation, two tests were conducted; SEAT (Seat Effective Amplitude Transmissibility) test and pressure distribution test. An SAE Sit-pad Accelerometer was used to measure vibration on the seat. Whereas, the pressure distribution at the human-seat interface was measured using pressure mapping system. Both tests had been carried out under controlled and uncontrolled conditions. Experimental works in the laboratory were considered as controllable. Uncontrolled condition refers to the road trials or field tests carried out in a moving vehicle which produced random vibrations. The results showed that, besides the postures and size of the passenger, the road conditions also have effects on the pressure distribution and SEAT data. A proposed seat structure with spring and damper properties was used and proved to be more effective in achieving seat vibration comfort. The SEAT values of this proposed seat were lower than the values for the current existing seat. A lower SEAT value means better ride comfort. By improving the seat parameters using the said method, vehicle seats, such as bus seats, could be developed with better ride comfort for local purposes.
vi ABSTRAK Tempat duduk penumpang merupakan salah satu komponen yang perlu dipertimbangkan untuk mendefinasikan keselesaan dalam suatu kenderaan yang sedang bergerak. Pengalaman menunjukkan bahawa suatu tempat duduk memberikan tahap keselesaan yang berlainan dalam keadaan yang berbeza. Keselesaan tempat duduk kenderaan terbentuk daripada gabungan faktor-faktor statik dan dinamik. Penyelidikan ini bertujuan untuk mengkaji sifat-sifat statik dan dinamik pada suatu tempat duduk penumpang bas untuk keselesaan melalui penilaian secara subjektif dan objektif. Faktor-faktor ketakselesaan yang ditumpukan ialah struktur tempat duduk dan taburan tekanan pada permukaan antara manusia dan tempat duduk. Dua kajian soal selidik termasuk ujian pandu telah diadakan untuk mengkaji penilaian subjektif secara langsung daripada pengguna tempatan terhadap keselesaan tempat duduk semasa perjalanan mereka. Dua ujian bagi penilaian objektif telah dijalankan, iaitu ujian SEAT (Seat Effective Amplitude Transmissibility) dan ujian taburan tekanan. Sebuah meter pecut SAE Sit-pad digunakan untuk mengukur getaran pada tempat duduk. Manakala, taburan tekanan pada permukaan antara manusia dan tempat duduk diukur dengan menggunakan sistem pemetaan tekanan. Kedua-dua jenis ujian telah dijalankan dalam keadaan terkawal dan tidak terkawal. Kerja eksperimen dalam makmal dianggap sebagai ujian terkawal. Ujian tidak terkawal dijalankan dalam sebuah kenderaan yang bergerak di mana getaran rawak terhasil. Keputusan menunjukkan bahawa keadaan jalan mempengaruhi data taburan tekanan dan data SEAT, selain kedudukan tubuh dan saiz penumpang. Suatu struktur tempat duduk dengan fungsi pegas dan peredam telah dicadangkan dan dibuktikan lebih berkesan dalam mencapai keselesaan tempat duduk. Nilai-nilai SEAT untuk tempat duduk yang dicadangkan itu adalah lebih rendah daripada nilainilai bagi tempat duduk yang wujud kini. Nilai SEAT yang rendah bererti keselesaan duduk yang lebih baik. Dengan memperbaiki parameter-parameter tempat duduk berdasarkan kaedah yang tersebut di atas, keselesaan tempat duduk kenderaan seperti tempat duduk bas dapat ditingkatkan untuk kegunaan tempatan.
vii TABLE OF CONTENTS CHAPTER TITLE PAGE DECLARATION OF THE THESIS STATUS SUPERVISOR DECLARATION PAGE TITLE PAGE DECLARATION OF ORIGINALITY DEDICATION ACKNOWLEDGEMENT ABSTRACT ABSTRAK TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES LIST OF SYMBOLS LIST OF APPENDICES i ii iii iv v vi vii xi xiii xvii xviii 1 INTRODUCTION 1 1.1 Background 1 1.2 Objectives 2 1.3 Scope of work 3 2 LITERATURE REVIEW 4 2.1 Seat Comfort 4 2.2 Seat Vibration 5
viii 2.3 Pressure Distribution 8 2.4 Seat Depth 12 2.5 Discussion 12 3 THEORITICAL ANALYSIS 13 3.1 Seat Effective Amplitude Transmissibility (SEAT) 13 3.1.1 Frequency Weightings 16 3.2 Static Pressure Analysis 18 4 RESEARCH METHODOLOGY 22 4.1 Subjective Evaluation 22 4.2 Objective Evaluation 24 4.2.1 Pressure Distribution Measurement at the Human-seat Interface 25 4.2.1.1 Apparatus 26 4.2.1.2 Static Pressure Distribution Test 27 4.2.1.3 Dynamic Pressure Distribution Test 28 4.2.2 Measurement of Seat Vibration Transmission Characteristics 29 4.2.3 Experiment Method 31 4.2.3.1 Laboratory Test 31 4.2.3.2 Road Trial 34 4.2.4 Sensor Positioning 35 5 SUBJECTIVE EVALUATION RESULTS AND DISCUSSION 36 5.1 Results 36 5.2 General Information Regarding Respondents Journey 36
ix 5.3 Evaluation of Seat Features 37 5.4 Evaluation of Body Part Discomfort (BPD) 40 5.5 Overall Evaluation 41 5.6 Correlation Analysis 42 6 OBJECTIVE EVALUATION RESULTS AND DISCUSSION 46 6.1 Laboratory Tests 46 6.1.1 Static Measurement 46 6.1.2 Laboratory Dynamic Measurement 59 6.2 Field Tests 67 6.2.1 Pressure Distribution Test 68 6.2.2 SEAT Test 76 6.2.3 Repeatability 83 6.3 Overall Discussion for Subjective and Objective Evaluations 86 7 PROPOSED SEAT DESIGN 88 7.1 Pressure Distribution Test 90 7.2 SEAT Test 94 7.3 Repeatability 102 7.4 Overall Discussion 104 8 CONCLUSION 105 8.1 Recommendations / Suggestions 106
x REFERENCES 107 Appendices A-D 111
xi LIST OF TABLES TABLE NO. TITLE PAGE 5.1 Statistical Summary of Respondents 36 5.2 Mean and standard deviation value for seat feature evaluation 38 5.3 Frequencies (%) of Seat Features Evaluation Result 39 5.4 Mean and standard deviation for BPD 40 5.5 Frequencies (%) of BPD Scale Result 41 5.6 Correlations between variables 43 6.1 Anthropometry of 10 subjects 48 6.2 Pressure distribution test results for 10 subjects during sitting with normal straight posture 49 6.3 Pressure distribution test results for 10 subjects during sitting with 1 st inclination posture 49 6.4 Pressure distribution test results for 10 subjects during sitting with 2 nd inclination posture 50 6.5 Pressure mapping contour of 10 subjects for different postures; subject 1-5: male, subject 6-10: female 51 6.6 Percentage of pressure transmitted to the backrest 53 6.7 Example of the pressure distribution test results for subject 1(male) on the current existing seat 54 6.8 Example of the pressure distribution test results for subject 6(female) on the current existing seat 54 6.9 Peak pressure for 10 subjects during 3 seat backrest inclination 56
xii 6.10 Contact area for 10 subjects during 3 seat backrest inclination 57 6.11 Example of the pressure distribution test results for subject 1(male) on the older seat 59 6.12 Graphs (g rms vs. Hz) showing seat base and seat pan acceleration according to frequencies (from 1Hz until 10 Hz) 60 6.13 Anthropometry of the field trial subjects 72 6.14 Crest Factors for 5 subjects (2 road conditions) 82 6.15 SEAT values for 5 subjects 82 7.1 SEAT values for 5 subjects on the proposed seat 116
xiii LIST OF FIGURES FIGURE NO. TITLE PAGE 2.1 Location of axis system as defined in ISO 2631 6 3.1 SEAT calculation (Griffin, 1990) 14 3.2 Moduli of the acceleration frequency weightings defined in BS6841 (British Standard Institution, 1987a) 17 3.3 Asymtotic approximations to frequency weightings W b, W c, W d, W e, W f, and W g for whole body vibration as defined in BS 6841 (British Standards Institution, 1987a) 17 3.4 A human model on the seat cushion with ENS (erect without back supported) posture. 19 3.5 Pressure distribution at human body-seat interface (Contact area is represented by green hatched area) 20 4.1 The design of the current existing bus seat structure 25 4.2 Xsensor pressure mapping system 27 4.3 Entran Sit-pad Accelerometer 30 4.4 Equipment set up for vibration testing under vertical vibration 32 4.5 Equipment set up for pressure mapping test under vertical vibration 33 5.1 Overall Evaluation 41 6.1 3-D static pressure distribution: male subject 47 6.2 3-D static pressure distribution: female subject 47 6.3 Normal, 1 st and 2 nd inclination of the sitting position 48
xiv 6.4 Comparison of ENS pressure distribution between current existing cushion surface, old cushion surface and wooden surface (from left to right) 52 6.5 Seat position with cushion-added 1 (longer, narrower) and cushion-added 2 (shorter, wider) 55 6.6 Effect of subjects weight onto peak pressure at buttock-seat interface 56 6.7 Effect of subjects weight onto contact area at buttock-seat interface 58 6.8 Vertical (z-axis) seat transmissibility for the existing seat 63 6.9 Contour maps of the dynamic pressure interface between mass system which simulated human buttock and cushion for different frequencies from 1-10 Hz 64 6.10 Characteristics of pressure distribution shown by pressure sensors during laboratory dynamic test (IT Ichial Tuberosities) 66 6.11 Average pressure (onto seat pan) against time for the field test on the bumpy road: 2 subjects with 2 positions each 68 6.12 Average pressure (onto seat pan) against time for the field test on the smooth-surfaced road: 2 subjects with 2 positions each 70 6.13 Average pressure (pink pressure onto seat pan, turquoise pressure onto backrest) with synchronized vibration on the seat base for 1 minute each for 5 subjects on bumpy road 72 6.14 Average pressure (pink pressure onto seat pan, turquoise pressure onto backrest) with synchronized vibration on the seat base for 1 minute each for 5 subjects on straight road 74 6.15 Power spectrum of Channel 1 and Channel 2 for the 5 subjects during bumpy road ride 76 6.16 Power spectrum of Channel 1 and Channel 2 for the 5 subjects during smooth-surfaced road ride 79
xv 6.17 Effect of random vibration onto the seat base (Channel 1) measured with five subjects a, b, c, d, e through the bumpy roads 84 6.18 Effect of random vibration onto the seat pan (channel 2) measured with five subjects a, b, c, d, e through the bumpy roads 85 6.19 Effect of random vibration onto the seat base (channel 1) measured with five subjects a, b, c, d, e through the smooth-surfaced roads 85 6.20 Effect of random vibration onto the seat pan (channel 2) measured with five subjects a, b, c, d, e through the smooth-surfaced roads 86 7.1 Proposed seat structure 106 7.2 Proposed seat structure (with the center of structure and center of movement) 106 7.3 Average pressure (pink pressure onto seat pan, light blue pressure onto backrest)onto the proposed seat with synchronized vibration on the seat base for 1 minute each for 5 subjects on bumpy road 108 7.4 Average pressure (pink pressure onto seat pan, light blue pressure onto backrest) onto the proposed seat with synchronized vibration on the seat base for 1 minute each for 5 subjects on straight road 109 7.5 Power spectrum of Channel 1 and Channel 2 for the 5 subjects during bumpy road ride 111 7.6 Power spectrum of Channel 1 and Channel 2 for the 5 subjects during smooth-surfaced road ride 114 7.7 Graphs showing SEAT values against subjects height and weight on both bumpy and smooth-surfaced roads for existing and proposed seats 118 7.8 Effect of random vibration onto the proposed seat base (Channel 1) measured with five subjects a, b, c, d, e through the bumpy roads 119 7.9 Effect of random vibration onto the proposed seat pan (Channel 2) measured with five subjects a, b, c, d, e through the bumpy roads 120
xvi 7.10 Effect of random vibration onto the proposed seat base (Channel 1) measured with five subjects a, b, c, d, e through the smooth-surfaced roads 120 7.11 Effect of random vibration onto the proposed seat pan (Channel 2) measured with five subjects a, b, c, d, e through the smooth-surfaced roads 121
xvii LIST OF SYMBOLS G ss (f) - Seat acceleration power spectra G ff (f) - Floor acceleration power spectra W i (f) - Frequency weighting H(f) - Transfer function a w (t) - Frequency weighted acceleration time history T - Period of time over which vibration may occur W - Body mass A - Contact area a - Length g - Gravity = 9.81 m/s 2 f - Frequency k - Spring stiffness
xviii LIST OF APPENDICES APPENDIX TITLE PAGE A QUESTIONNAIRES AND RESULTS 111 B SEAT DETAILS 130 C OBJECTIVE TEST PROCEDURES 138 D DATA FIGURES / TABLES 144
CHAPTER 1 INTRODUCTION 1.1 Background Nowadays, comfortable seating in a vehicle is no longer considered a luxury, but as a requirement. A seat that is comfortable in a showroom may have poor dynamic characteristics that make it uncomfortable whilst on road. What is considered comfortable by a user also depends very much on the way a seat is used and how long it has been used. The optimum seat for one vehicle may not be the optimum seat for another vehicle. It is therefore important to consider both static and dynamic comfort when considering the quality of the in-vehicle experience. Until now, there is still no local study on seat comfort for vehicles in Malaysia. Most of the automotive seats, especially commercial vehicle passenger seats, were designed not accordingly to the average size of Malaysian. For a long journey ride, the seat is important because it will affect the comfort feeling of the passenger. These are the reasons that the seat comfort need to be studied in detail. A seat is formed by the seat cushion and the seat structure. The characteristics of the seat cushion can be categorized into three: physical, static and dynamic. The physical characteristics of the cushion include seat contour, softness, and seat inclination. Static pressure distribution is the characteristic to be studied in a static condition (vehicle remains static). Both the seat cushion and structure play important roles in affecting the seat comfort in dynamic condition. Therefore, transmissibility test is necessary for the study on dynamic characteristics of a seat, which will be mentioned in the later chapter. In this research, physical
2 characteristics of the seat cushion were briefly considered during a survey, whereas the seat static and dynamic characteristics were considered and subjected to the objective evaluation. Static comfort can be evaluated using postural assessment, interface pressure distribution and other standard ergonomic techniques. Dynamic comfort is usually assessed by making vibration measurement on the surface of passenger seats using method based on ISO2631-1, ISO10326-1 and other international standards. These dictate that vibration on the seat must be measured using accelerometer mounted in a semi-rigid disk originally specified by the Society of Automotive Engineers (SAE Sit-pad). Besides subjective method, SEAT (Seat Effective Amplitude Transmissibility) test and pressure distribution test had been applied onto the local vehicle passenger seat in this research, by using the subjects of average Malaysian size. The automotive seat aimed for both the subjective and objective analysis is the commercial vehicle (bus) passenger seat. The bus passenger seat was chosen as the commercial vehicles are still the main transportation in Malaysia for the people to travel from places to places. Most of the complaints of body pains after a long journey travel usually come from the bus passengers and not the car passengers. 1.2 Objectives The objectives to be achieved for this research are: a. To determine factors affecting seating discomfort through subjective method. b. To determine human-seat interface pressure distribution and Seat Effective Amplitude Transmissibility (SEAT) values of commercial vehicle passenger seat through objective methods. c. To determine Seat Effective Amplitude Transmissibility (SEAT) values of a proposed seat structure of commercial vehicle for passenger ride comfort.
3 1.3 Scope of work The scope of work for this research includes the following: a. To conduct surveys among public to gain the subjective evaluation towards the design of current existing bus seat. This subjective assessment would be conducted to gather information on existing commercial vehicle seats from public and to evaluate perceived comfort. b. To carry out a pressure mapping test to obtain the pressure distribution at the human-seat interface under static and dynamic conditions. c. To carry out vibration test to obtain the Seat Effective Amplitude Transmissibility (SEAT) values through both laboratory tests and field trials. d. To conduct road trials onto the proposed seat structure to obtain the Seat Effective Amplitude Transmissibility (SEAT) values.