Design and Fabrication of Motorized Stairs Climbing Vehicle

Transcription

1 Design and Fabrication of Motorized Stairs Climbing Vehicle Suraj Bhatta Department of Mechanical Engineering The Oxford College of Engineering Bangalore, Karnataka, India Saujanya Shah Department of Mechanical Engineering The Oxford College of Engineering Bangalore, Karnataka, India Sudha Panthi Department of Mechanical Engineering The Oxford College of Engineering Bangalore, Karnataka, India Raja Reddy Department of Mechanical Engineering T John Institute of Technology Bangalore, Karnataka, India Abstract To overcome problems faced in conventional wheel chairs which only move on plain surfaces, researches are going on to replace them with sophisticated models of wheel chairs, which can move both on stairs and plain surfaces. Rapidly increasing accidents and withering injuries are the evidence of requirement of a vehicle which can both move on stairs as well as on the plain surfaces. As per the requirement, in this current work we are designing and fabricating a wheel chair which can move both on stairs and plain surfaces. The present investigation has been focused on manufacturing a highly sophisticated and expensive wheel chair. Here, we are designing the front wheel with additional changes in the frame so that it ensures movement on any surface. Keywords frame, front wheel, front subwheel, rear wheel, seat. I. INTRODUCTION The wheel chair is a device providing wheeled mobility and seating support for a person with difficulty in walking or moving around. It is one of the most commonly used assistive devices for enhancing personal mobility, which is a precondition for enjoying human rights, living in dignity and assists people with disabilities to become more productive members of their communities. Stairs are frequently encountered obstacles in daily living. Although healthy persons climb stairs quite easily, this movement task is quite demanding when motor functions are reduced, for example, elderly or obese subjects, women during pregnancy, people with different neuromuscular skeletal impairments, subjects with joint or limb replacements. The current work is a model in a scale of 3:1 to the real model of a stair climbing system and was designed, fabricated and tested. The work describes modified design of a traditionally existing wheel chair with its versatility of climbing the stairs in addition to rolling on the ground. Planetary wheel mechanism is a main concept used to enable the vehicle to climb the stairs in this project work. It is constituted by several small wheels that are equally distributed on a tie bar with shapes such as Y or +. The additions of small wheels which revolve around the central axis provide an additional movement on plain surfaces. Every small wheel rotates on its own axis, when the wheelchair moves on the ground and every small wheel revolves round the central axis, when the wheelchair goes up or down the stairs. Various measurements of stairs were taken from public areas like temple, hospital, colleges to design the front and the rear wheel. Anthropometric data from [1] were used to determine the length and size of the body parts of human being as well as the body weight. The measurement properties that can be used to assess the propulsion technique of people who use hand and/or foot propulsion can be obtained from [2]. II. LITERATURE REVIEW Various wheel chairs are designed to overcome daily obstacles. Some of the wheel chairs are with triangular shaped wheel as in [3], where the triangular arms rotate to move the vehicle on stairs as well. Some even do use tracks as in [4] and [5]. The use of tracks is to cover more surface area, so that the vehicle doesn't fall during accent and descent. As we know that, more the surface area we cover, more balanced we are. This kind of vehicle may, notwithstanding, welcome problems during start and finish of the accent or descent. The use of expensive materials and technologies as used in [6] and [7] may not be well accepted because of the price involved. Other vehicle such as [8] can be better, but vibrations produced while encountering obstacles are unavoidable. III. DESIGN AND FABRICATION A. Frame 1) Design of frame: A complete new design is proposed for the frame which sums up to various advantages. The frame has been designed in such a way that it enables the smooth climbing of the entire vehicle on encountering the stairs. The side view of the vehicle looks like a 'Human Shoe' with the curve at the front end. The arrangement for the placement of the rear and front axles including the seating arrangement is carefully done. The figure 1 depicts the 3D view of frame using solid edge

2 TABLE I. RESULT OF ANALYSIS Particulars Configuration 1 (Vertical Element) Configuration 2 (Inclined Element) Load Applied 100 N 100 N Maximum Displacement 2.55E E-07 Fig. 1. 3D view of frame. As the vehicle starts climbing the stairs, the whole body makes an arc about the point of contact of the rear wheel with the ground; this could bring out the probability of toppling the vehicle because of the weight acting at an offset from the rear axle. To overcome this, the frame has been designed in such a way that the angle of inclination of the line passing through the rear axle and the point of application of main load is more than the angle of ascent of the stairs as in the figure 2. This assures the line of action to be towards the front axle and avoids the reverse torque. Fig. 2. Angle of inclination. Analysis of frame, using ANSYS, was done for two different configurations to check for the best condition to support the load, which is as shown in figure 3. One configuration had vertical element connecting the chair fulcrum to the horizontal base element while the other involved the use of an inclined element. It was found that the load bearing capacity of the first configuration was more as the displacement of the elements shown was less in the first configuration than in the second under the same force condition as shown. The favorable result of the first configuration triggered its selection. Another major part of the design was the anterior. Placement of the front axles, which holds front wheels, played an important role in this project. The front part was curved as shown in the figure 2. This curved part of the vehicle not only makes it convenient to place the front axles, but also prevents the vehicle from head-on collision of the bottom part with any obstacle. Again, the other benefit of placing the front axles at this position is the uplift of the vehicle by the reaction force produced when the vehicle hits the stairs. Also, this placement gives the vehicle a negative inclination which makes the C.G. shift forward resulting in equal distribution of the load on the front and rear axles. Also, while climbing, the line of action of weight should not pass through or behind the rear axle. Although the line of action of weight passes through between the axles under normal conditions, while climbing, the vehicle assumes inclined position which shifts the C.G. of the vehicle backwards. This increases the chances of somersaulting of vehicle, due to backward rolling caused by the moment produced due to the weight. Hence, a negative inclination given to the vehicle overcomes this problem and ensures that the line of action of weight falls between the axles. 2) Fabrication of frame: The entire frame was prepared using hollow cast steel metal pipe of 2 cm outer diameter and 2.5 mm wall thickness. The frame structure was outlined and metal pipes were cut to dimensions, which was followed by bending of pipe. After bending, pipes were welded and the structure of the frame was made. The frame was drilled with the drill bit of 3 mm diameter so that the cover plates could be fastened to it using nuts and bolts of 2.5 mm diameter. B. Front Wheel 1) Design of front wheel: Various designs of the front wheels were proposed, such as triangular, rectangular, pentagonal and hexagonal as shown in the figure 4. Fig. 3. Analysis of frame

3 TABLE II. CALCULATION OF WHEEL TRAVEL DISTANCES Wheel Shape Angle between Centre of Sub Wheel to the Tread of Stairs A ( ) Radius of Rotation (mm) Total Distance {(A/360) * Circumference } (mm) Triangular Rectangular Pentagonal Hexagonal Fig. 4. Designs of front wheels. All the configurations seem convincing on the fact that they are able to make an ascent over the stairs, notwithstanding, different configurations come with various drawbacks. The triangular armed wheel, for an example, as tabulated below has the highest travel distance and the axle is nearly at the same height as the near edge of the obstacle which makes the rotation of the axle difficult when it hits the obstacles such as stairs. It is clear from the table that the pentagonal arm shaped wheel gives smoother ride than other designs. Finally, a design, as in figure 6, was proposed to the front wheel which looks like an incomplete wheel having five arms projecting outwards, each with five sub wheels at one end. Fig. 6. Front axle with wheels fixed at ends. Fig. 5. Analysis of wheel travel distances. Also, since the angle of separation between the arms of the wheel is very high i.e. 120 degrees as shown above, this type of wheel produces a high jerk when it moves through the stairs. Similarly, rectangular armed wheel has a reduced travel distance, but is more than the travel distance given by the pentagonal armed wheel. Reduction of jerk is seen by the use of pentagonal arm shaped wheel. Use of hexagonal arm shaped wheel is shunned because as in the figure 5, the subwheel 1 already rests on the stairs and any extra rotation rolls the subwheel 1 horizontally, adding on the extra jerk which is less in case of other configuration. Also, if in any case the subwheel 1 doesn't roll, since the subwheel 1 is already at rest, the distance traveled by subwheel 2 is more than the other cases. 2) Fabrication of front wheel: The arms for the working model each of length 6 cm were welded on the circular disc and a hole was drilled near the tip of each arm at a distance of 8 cm from the center of the wheel falling on the pitch circle to accommodate the front subwheels. Each subwheel is 3.25 cm in diameter and 1.25 cm in width. The front wheel is designed in a way that it can ascend and descend the stairs without any problem. The arms were made by cutting metal pieces into required length and then the ends were ground to make it semicircular. Five such arms were fixed in the washer and an element with five arms from a circular disc was prepared. Two such elements were welded with a 1.3 cm spacer in between and sub wheels were fixed in all the five arms and the assembly was then fixed to the shaft. C. Rear Wheel 1) Design of rear wheel: Rear wheels used on the model were circular in shape with a friction material on the circumference of the wheel. The wheel is 20 cm in diameter. The wheel is supported on the axle using bolts welded on a solid shaft with the help of washer. The wheel of the model is of plastic material with metal fitted in the center through which the bolts pass, which in turn is tightened using washer and nut. This makes wheel fixed on the axle. Figure 7 shows rear wheels fixed on the rear axle

4 TABLE III. AMPLIDUDE OF VIBRATION FOR DIFFERENT DIAMETER WHEELS S. No. Rear Wheel Diameter (cm) Amplitude of Vibration (cm) Fig. 7. Rear axle with wheels locked at both ends. The wheel should be optimized for better functioning of the vehicle. Whenever a round object encounters rectangular obstacle like stair step, to climb over the obstacle, it revolves about the instantaneous center. When it does so, the maximum amount of displacement of the center of the wheel can be found out by finding the point of intersection between the arc and the normal to the stair's inclination, as shown in figure 8 with various proposed wheel diameters. Fig. 9. Plot of amplitude of vibration vs. rear wheel diameter. The selection of wheel diameter to be used in the rear axle has to be done in such a way that the vibration is minimum, as Fig. 8. Analysis of rear wheel. The table and graph for amplitude of vibration normal to the stair's surface for different wheel diameters is given below. Fig. 10. Problem faced using 70 cm and 80 cm diameter wheels

5 it invites a lot of problems. To reduce vibrations, the optimum wheel diameters have to be used. Wheel of 30 centimeters cannot be used because it produces high amount of vibration because the point of contact about which the wheel revolves lies in the same level with the center of the wheel. Again, the wheels with 70 and 80 cm diameters cannot be used since the center of the wheel lies at a negative offset to the edge of the stairs as shown in figure 10, which could topple the vehicle. Hence, 60 cm is the optimum wheel diameter for the vehicle, which for our working model is 20 cm. 2) Fabrication of rear axle: Axle was made by turning EN 9 rod to 2 cm diameter and then cut to required length. A sprocket which drives the rear axle taking power from the motor shaft was fixed on the axle. The axle is 2 cm in diameter with bearings attached on it and is fixed to the frame by the help of C-Clamps. At the end of axle, a 2 cm inner diameter hollow tube was inserted, which was welded to washer on one end and welded to the axle on the other. Since the vehicle undergoes a lot of vibrations during ascent, use of belt drives is not recommendable. The belt may shift from the pulley due to vibration and hence may disengage, causing the vehicle to roll backwards. On the other hand, belt drive is susceptible slippage, which may be disastrous while climbing the stairs. Again, use of gear is also not recommended as it requires number of bearings and shafts. The large scale speed reduction is difficult, since it causes the increase in the overall weight of the vehicle because of the increase of the number of gears. Also, proper lubrication system has to be incorporated for smooth functioning of gears, which is another problem. Solution to all these problems was the chain drive system. It just requires one driving sprocket, one driven sprocket and the chain of required length for given center distance. Thus, it does not add much load on the vehicle for same power transmission compared to gear drives. On one hand, this requires very little lubrication and maintenance compared to gears and on the other hand, if the chain used is of proper length, there is no slip, unlike in the belt drive system. Power transmission efficiency for chain drive system is also very good and is more suitable for the center distances in our work. D. Seating Arrangement 1) Design for self alignment: In most of the available stair climbing wheelchairs, it was found that the horizontal alignment of the chair while climbing the stairs was done by employing pneumatic/hydraulic lifters coupled with sensors and microprocessors. A very simple solution to this was found by using the self-aligning property of hanging bodies by the virtue of gravity. Any hanging body's weight through C.G. is always directed downwards. Using this principle the selfaligning chair was designed, by providing a rotatory joint to the chair with the frame. When the vehicle is normally running on plain ground, the chair, obviously by the virtue of gravity, is horizontal. Even when the vehicle is in the middle of ascent, weight of the chair along with a mass on it, tries to continue to be directed vertically downwards, which brings the chair back to its original horizontal alignment. This completely eliminates the use of sophisticated devices like microprocessors and sensors. 2) Design of seat: The self-aligning chair, although, seems to be very simple, the chair in the first place aligns itself to gain equilibrium. For the chair to be in equilibrium in the first place, the line of action of weight through C.G. should pass through the point of pivot. On doing this the chair reclines in such a way that it faces downwards making it difficult for sitting purpose. The chair should be such that, whatever position the entire vehicle assumes, the chair comes to its mean position as shown in figure 11. To take care of this, the frame of the chair itself was modified into a curve such that some portion of it goes backwards from the point of pivot which itself acts as the counterweight and hence eliminates the necessity of using counterweight. Fig. 11. Analysis of chair. Finally, a drawing of chair was prepared. Fig. 12. Design of chair. E. Motor A motor used for medium load applications is used in the current work. The motor rated 0.1 HP drives the rear shaft using a chain drive and in turn drives the vehicle. F. Calculations 1) Calculation for the minimum force required to move the vehicle: The calculation for minimum force required to move the vehicle on stairs is given below. Fig. 13. Calculation of force required on the stairs

6 We know that, to increase the altitude, we require some amount of work. This work done is stored in the form of potential energy of the body. Amount of work done on the object = Potential Energy gained by the object Where, (Joules) m is the mass of object being raised in Kilograms g is the acceleration due to gravity in N/m 2 h is the height through which object is to be raised in Meters E is the energy gained by the object in Joules Taking into account the frictional losses and the overload case, let us take the amount of work to be done for lifting the object through height h be three times the normal work done. Fig. 14. Hence the amount of work to be done E act = 3 E If total time of ascent for the height h = t seconds Then power required, P = E act / t (Watts) Calculation of circumferential force required to roll over the step. When the wheel is about to turn over the curb/step, the contact with the floor is lost and hence there is no reaction from the floor C. The body is in equilibrium under the action of three forces, namely Applied force F ( in Newton) Self-weight W (in Newton), which is vertically downwards acting through the centre of roller and Reaction R (in Newton) from the edge of the step. Since the body is in equilibrium under the action of only three forces, they must be concurrent. It means the reaction at edge A of step passes through the point B as shown in figure. Referring to the figure above, Since the wheel radius is double the step height, and let Considering all vertical forces in equilibrium, Considering all horizontal forces in equilibrium, This gives the required amount of force to pull the wheel over the obstacle which tells us that amount of force depends on the point of application of force which in turn determines the value of ɵ. As the value of ɵ increased, so does the value of force to be applied. If force is applied on the circumference as shown in the figure above, i.e. ɵ =30, gives This equation suggests us that force equal to almost 60% of the total weight has to be applied at the circumference (generally manually propelled wheelchairs) to overcome the obstacle. Using the sprocket on the rear axle for propelling the vehicle shifts the point of application of force near the centre. In this case the reaction passes through the centre of the roller. Fig. 15. Calculation of central force required to roll over the step. (1) (2) (3) 362

7 Considering all the vertical forces in equilibrium, Considering all the horizontal forces in equilibrium, Combining equations (4) in (5) we get, Differentiating equation (6) to get the minimum force required, Therefore, Substituting this in (6) we get, F min = (4) (5) (6) F min = (7) 2) Calculations for the model: If the vehicle is carrying the load of 20 kg up to the height of 3 meters in 60 seconds then, Weight of the empty vehicle = 20 kg Weight carried by the vehicle = 15 kg Total weight carried by the vehicle W = 35 x 9.81 = Total time of ascent for the height h = 60 seconds Energy required for the climb, E = 35 x 9.81 x 3 Joules E = J Considering overload and frictional losses, E act = 3 x E i.e. E act = x 3 = J P = / 60 = 51.5 Watts i.e. P=0.069 HP. Hence, the model should easily run on any motor rated above H.P, if losses encountered are 3 times the work done. IV. LIMITATION Turning mechanism has not been considered during fabrications. However, the regular turning mechanisms can be applied to the front wheel without hassle, as the modification has been done only on the rim. The front axle and steering setup can be done in the conventional manner, which will in turn make the planetary wheels turn, and thus turning the wheelchair. The friction required between the rear wheel and the stairs, to exert considerable forward force is not calculated, which poses a threat to the operation of this wheelchair in highly polished/slippery stairs. The wheelchair cannot traverse along spiral stairs, as they need specialized turning mechanism to move along spiral stairs. V. FUTURE SCOPE The work is a basic model of wheel chair without the use of electronic devices, use of use of which can definitely bring out changes for disabled people. The modified wheel can be added in the rear (without providing rotational joint to the planetary wheels) on a tandem axle. This tandem axle can be made active during stair manoeuvre, as it can easily provide lift and thrust to move up the stairs. Brakes can be provided on the front planetary wheels as well as the wheel assembly to assist down-stair movement. Swinging of the chair during movement, ascent and descent can be minimized by employing dash-pots. Suspension systems on the axles or the seat can dramatically increase user comfort and minimize the impact loading on the chassis. VI. CONCLUSION This wheelchair depicts that not only does critical thinking push the boundaries of science and technology, but can assist mankind in making life easier in ways that are cheap, durable, efficient and thus feasible. Little efforts in understanding the milieu can give wonderful ideas to improvise the technology that hasn't been upgraded or considered by significant others. The model was made employing the concept and was tested for the climbing features over the model stairs. The ascent was successfully achieved except the vibrations which couldn't be eliminated because the shock absorbers were not installed. During the descent, controlling of vehicle was found to be difficult which necessitated braking system on wheels. The main purpose of this project was to check the validity of the concept in making the cheap and affordable simple wheelchair. For general population it is very difficult or almost impossible to afford very expensive modern wheel chairs where the current work could be the substitute. Hence, Power needed P = E act / t 363

8 VII. REFERENCES [1] Vikash Sharma, "Anthropometry of Indian Manual Wheelchair Users: a validation study of Indian accessibility standards", Accessability, India. [2] "Wheelchair Skills Program (WSP)", [3] Giuseppe Quaglia, Walter Franco and Riccardo Oderio, "Wheelchair.q, A Motorized Wheelchair with Stair Climbing Ability" in Mechanism and Machine Theory, 2011, Vol. 46, No. 11, pp [4] Watkins, Baxter R., "Stair-climbing wheelchair with stair step sensing means", patent: , [5] Lawn, M.J., Sakai, T., Kuroiwa, M. and Ishimatsu, T., "Development and practical application of a stair climbing wheelchair in Nagasaki", in International Journal of Human-friendly Welfare Robotics Systems, 2001, vol.2(2), pp [6] Lawn, M.J. and Ishimatsu, T., "Modeling of a stair-climbing wheelchair mechanism with high single-step capability", IEEE Transactions on Neural Systems and Rehabilitation Engineering, vol. 11(3). [7] Johnson & Johnson (2009), ibot, URL: access on 03/05/2010. [8] Wellman, P., Krovi, V., Kumar, V. and Harwin, W., "Design of a wheelchair with legs for people with motor disabilities, IEEE Transactions on Rehabilitation Engineering, 1995, vol. 3(4), pp