Automobile Headrest Design
By: William HutsonAdvisor: Dr. Rana Mitra
Instructor: Dr. Cris Koutsougeras Senior Design, ET 494
Southeastern Louisiana UniversityEngineering Technology Department
Mechanical Concentration
Abstract
Whiplash is a common problem in accidents. The positioning, design, and materials of most headrests are very simple and do not provide a maximum level of safety for drivers. The goal of this design is to altar the design of a headrest in order to provide greater safety, as well as increase comfort. This can be done with a combination of springs, materials, and shape changing ability incorporated into a headrest. These changes can decrease impact forces, and in turn the likelihood of whiplash, as well as provide support.
Introduction of ProblemIn most vehicles, there is a gap between them and the back of the driver’s head. This gap is generally around 2”. The headrest design is very simple and doesn’t incorporate many safety features. One common problem is that, while on a long drive, the head is not supported. Due to this lack of support, the driver can become tense and their neck can begin to hurt. Another issue is that due to the placement and shape of headrests, whiplash is a common occurrence.
SolutionThe solution is to create a headrest that can support the driver’s head, while maintaining the current safety standards. In addition to current safety specifications, other features can be added to provide more safety and better protect the driver. Also, this headrest will be made to have the ability to partially conform to different drivers’ heads in order to provide a plausible solution for everyday families.
Several factors that have been considered in the design of this headrest are the following: The gap between the head and posts is about 6 inches. Design must incorporate vertical forces. Shock must be absorbed. It must conform to different drivers. It must be relatively cost effective.
OverviewThe project will be split into two phases. Phase 1 will be to focus on all of the physical design features and material selection. This portion of the project is what was done in this spring semester. Phase 2 will be the part in which construction of the headrest will be finished and electrical components will be applied. Phase 2 will be completed in the fall semester. Parts of the headrest will be built over the summer to avoid spending time in time consuming areas so that more time will be available for design and to be able to budget cost over a period of time, rather than all at once in the fall semester.
Deliverables Analyze the forces acting upon the headrest, Design the shape and dimensions of the headrest
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Determine the method by which the headrest will attach to a standard seat Provide possible movement and sensing methods Determine materials to be used
TimelineTask Finish Date Measurements from head to headrests 3/25 Study Current Works and Safety Standards 4/1 Study Sensors and Motion/Force Distributions 4/8 Study Materials, Shape, General Design 4/15 Determine Movement Methods, Materials, and Design 4/29 Prepare and Give Report 5/4 Redesign Mounting System 9/8Study Actuator Control 9/29Determine Motor to be Used 10/6Determine Switches to be Used 10/13Build Shape 10/20Incorporate Switches and Control in Design 11/3Complete Build 11/10Testing 11/17Create Final Report and Present 12/1
Head Injury Criteria
HIC=[ 1t2−t1
∫t 1
t 2
adt ]2.5
( t2− t1)
According to US Motor Vehicle Standard 202a, the Head Injury Criteria, or HIC, must be less than 1000 in order to provide reasonable protection against whiplash, but the smaller the HIC, the less chance there is of head and neck injury. The difference of (t2 — t1) is a sample time selection, usually being about 15 milliseconds. The acceleration is the acceleration of the head. The goal of this phase of the project is to lower this number as low as possible.
When examining the equation, it is fairly easy to determine that the way to lower the HIC is to lower the acceleration over that given timeframe. The formula for acceleration is:
acceleration=∆ velocity∆ time
The initial velocity, about 56 ft/s once all other safety features from a vehicle are added, and the final velocity, 0 ft/s, will be the same regardless of what changes as far as the design. The normal relationship between a solution and a fraction is the greater the denominator, the smaller the
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solution. The only method by which acceleration is lowered is to lengthen the time over which this speed change occurs.
The method I am using to increase this is by placing springs to absorb the force. By adding springs, resistance is added to the force. This resistance causes the time to increase. The method by which the springs are selected will be discussed later.
Force AnalysisForce was determined using Newton’s Second Law:
Force=mass∗acceleration
According to US Safety Standard 202a, the force used in analysis for headrests should be around 820 lbs. The average weight of a human head is about 16 lbs, so this places the acceleration at about 51 ft/s. Since the force from impact on a headrest design that is located further forward should be a little larger, since the time from where the initial position and acceleration is and the impact will be smaller, 900 lbs. was used for this. This change gives the acceleration a value of around 56 m/s2.
The force directly on the headrest will almost always be at some sort of angle, which would change depending on different factors. The force in the horizontal force isn’t as important because the forces are easily accounted for, but the vertical force is very important. So, in order to find the maximum likely force in the vertical direction, the maximum likely angle must be used. The maximum angle at which the force could be applied is about 60°. To find the forces, simple trigonometry is used.
Horizontal force at 60°: 900cos(60°) = 450 lbs.
Vertical force at 60°: 900sin(60°) = 780 lbs.
Here is a simple illustration of the forces:
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450 lbs
780 lbs
900 lbs
60°
Friction will be neglected in the calculation because it will be small and will not cause the vertical force to be greater.
CompositionThe composition will be split into several parts:
Shape and Buildo “Pillow”o Motor Housingo Spring Housing
Rod Diameters Spring Selection Miscellaneous
Shape and Build
Pillow: The “pillow” headrest, when sitting inactivated, will be 12”W x 10”H. There will be a 5”x5” area at the top, center that will be fixed. On either side of that, there will be a 3.5”W x 5”H section that will be fixed on the inside, but will be able to pivot outward up to 1.75”. The bottom section will be 12”W X 5”H, which will be the entire bottom half. This section will be able to pivot up to 2.5” outward. The “pillow” will be made of 1” thick Memory Foam. The four corners will be rounded at some symmetrical radius, but this radius is not required to be any specific size.
Figures 1 and 2 in Appendix
Motor The Motor housing will be located directly behind the pillow. This area Housing: is 5”W x 5”H x 4”D. This is where the motors that will used to push the
extending sections of the pillow outward will be located. There will be rods at each of the 4 corners that connect two plates. The plates are made of 1/8” thick 2024-T3 Aluminum. The rods are bolted on each end to the plates. These rods are made of 5/8”-13 stud bolts. The bolts are made of A193 B7 Steel.
Figures 3 and 8 in Appendix
Spring The springs will be located behind the motor housing area. 3 springs Housing: will be used to absorb the force. Two bolts are used to support the vertical
weight of the headrest, as well as to guide the direction of the force straight back onto the springs. By doing this, the bolts can also hold a small force against the springs so that they can be more easily fixed in place. Upon impact, the head end of the bolts will be able to slide into the motor housing area, passing the horizontal force directly to the springs.
Figure 4 in Appendix
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Rod DiameterThe rod diameters were determined by using the following formula:
Diameter=(√ (2.94 ) ( K t ) (F )( NS' n ))/x
Where, Design Factor, Kt = 1 F = 780 lbs. * distance Safety Factor, N = 3 x = number of rods S’n = Sn*Cm*Cst*CR*CS ; where,
Sn = ½ Ultimate Tensile Strength of Material Material Factor, Cm=depends on material Type-of-stress Factor, Cst = 1 Reliability Factor, CR = 0.9 Size Factor, CS = 0.8
Note: Factors were picked in correlation to types of load and general information. Most were determined from charts in chapter 5 of Machine Elements in Mechanical Design by Robert L. Mott.
The formula was put into an Excel spreadsheet in order to more easily compare diameters of different materials. By doing this, is became easier to evaluate the needed diameter. Upon ordering materials, available materials’ properties will be input in order to determine which material will be the best fit and best cost. The Excel sheet is shown below.
Also, a 1.25 Safety factor was added to the diameter found in this spreadsheet due to the bolts being threaded. Using this, the diameters are derived.
Spring Selection
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The springs are going to be arranged in parallel, so the formula needed to find the spring rate is the following:
F=( kn )∗x k=( F
x )∗n
Where, k = spring constant of each spring x = change in distance = 2 n = number of springs
3 springs will be used in the making of the headrest. The springs are made by Century Spring Company. The specifications are as follows:
CSC Stock # S-92OD (in) 1.953
ID (in) 1.579
Free Length (in) 2.750
Rate (lbs/in) 139.000
Suggested Max Deflection (in) 0.600
Suggested Max Load (lbs) 83.000
Solid Length (in) 0.750
Wire Dia. (in) 0.187
Total Coils 4.000
Material Stainless
Ends Closed Ground
Finish None
Note: The Max Load and Max Deflection will be ignored due to the way the force will be applied.
MiscellaneousThere are various other specs that need to be mentioned. The posts that are used to attach a headrest into a seat are not standard across all makes and models. So, a slot is placed behind the housing for the spring. The rods can be placed into the seat holes and the slot can be placed over the top of them. This solves the issue of being able to accommodate different widths between posts. A cover made of a stretchable material will cover the entire headrest.
SensingSmall roller switches will be placed between the foam and the elastomer. These will control the motion of the motor and act as a sort of “off button” when a small force from the driver’s head pushes the switch. Another switch will be placed on the back supporting part of the driver’s seat to determine when the driver is sitting in the seat.
Figures 5 and 6 in Appendix
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Time/Spring RelationshipThe time interval at which a spring oscillates can be directly used to determine the change in time in can create. By using the formula for the time interval and incorporating the relationship between force equations for a spring/distance and mass/acceleration, we find:
1T
=1
2 π √ km
→ F=m∗a=k∗x→ km
=ax
→ 1T
=1
2 π √ ax
After solving the two formulas, we find that the time interval with the spring is 0.127 seconds, while the interval without the spring is 0.037 seconds. This shows us that the time is increased by about 4 times by adding the springs.
HIC=[ 1t2−t1
∫t 1
t 2
adt ]2.5
( t2− t1)
When using the information we have learned, we find that the normal HIC would have been around 700, but by adding the springs, it is dropped to around 400. At 400, the chances for head and neck injury in an accident drops to much less that 5%, compared to around 15% at 700.
Electronic ComponentsThe electronic components include:
Microcontrollero The microcontroller than will be used to run the program is an Arduino Uno.
This system has 14 places to send and receive signals from and is fairly simple to operate. This system is also relatively inexpensive. A program will be written using the Arduino software that will control the motion of the motors. After the driver is seated and ready, the motors will receive a “High” signal in order to turn. As they turn, they will push the flaps forward (the system is explained in more detail later). When the flaps are in position, a signal will be sent to stop turning. When the driver gets up, a signal will be sent for the motor to spin in the opposite direction, causing the flaps to return to their original positions. This will be accomplished by use of an H-Bridge to change the direction of current flow. An external power source will be used due to the size of the motors.
Figure 9 in Appendix
Roller Switches o The roller switches will be used as sensors. When the switch it pressed, it will
send a signal to the Arduino. These are much less expensive and complicated than the low pressure load cells that were originally going to be used. Specs are located in the appendix.
Figure 10 in Appendix
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DC Motorso Low RPM DC motors will be used. These are much less expensive and are
smaller than the coinciding linear actuator that would have to be used to meet the requirements. The RPM of the motors are 110 at 12V and 250 at 24V.
Figure 11 in Appendix
Motion of “Flaps”A screw system will be used to move the “flaps”. A threaded bolt will be attached to the end of the DC Motor. This will turn as the shaft of the DC motor turns. A coupling nut will be attached to the flap with matching threads. As the threaded bolt is turned, it will cause the coupling nut to push forward, pushing the flap forward. The threads per inch required are determined by using the following formulas:
rpm60 =rps
(rps )(desired time)(desired length)
=threads per inch
ConclusionAfter completing these analyses and incorporating these safety features, a safe, effective prototype can be created. By adding the springs, the headrest gives the driver better peace of mind by allowing them to not have to worry about whiplash in an accident. Also, since the headrest will be interactive and can support the driver, it will provide a greater level of comfort to the driving experience.
Appendix
83.5” 5” 3.5”
9
5”
5”
1” ½”
2.5”
Figure 1 Side View of Pillow
Figure 2Top View with One Side and Bottom Extended
Figure 3 Side View of Motor Housing
4”
2”
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Figure 4 Side View of Spring Housing
Figure 5 Slot for Inserting Posts Figure 6
General Placement for Sensors
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Figure 8 Bolts/Shafts for Housing Figure 9
Arduino Uno
Figure 10Roller Switch
Figure 1112V/24V DC Motor
