Design and Performance Evaluation of 4 Wheeled Omni Wheelchair with Reduced Slip and Vibration
Abstract
Holonomic wheelchairs are being popular for their ability to move in constrained spaces due to their omnidirectional mobility. In this paper we have presented design and development of a 4 wheel driven omni wheelchair suited for indoor navigation with reduced wheel slippage and vibration. The design has been evaluated with wheel load measurement from current consumption and vibration measurement with a 3 axis accelerometer mounted on the chassis. From the result and analysis, it is evident that our proposed design shows less wheel slippage and vibration than existing designs. The system can find its application as an assistive aid for geriatric population or as a smart indoor mobility vehicle.
1.Introduction
Powered wheelchairs have been developed over the years for locomotion disabilities and for geriatric assistance. Smart electric wheelchairs are special class of powered wheelchairs, which are becoming a natural substitute of the conventional wheelchairs as an assitive device. Moreover, due to the ease of control, application specific human machine interface and smooth mobility, electric wheelchairs are becoming a popular indoor navigation vehicle. One of the first prototypes of smart wheelchair was proposed by Madarasz et.al 1 in 1986 which presented a wheelchair designed to transport a person to a desired room within an office building given only the destination room number. Since then, many such smart wheelchairs have been developed and few have been commercialized. Most of the developed smart wheelchairs are modification over existing commercially available powered wheelchairs with add on facility to enhance maneuverability, navigational intelligence and multi-modal control interfaces. To name a few, NavChair 4, Office wheelchair with high Maneuverability and Navigational Intelligence (OMNI) 5, Mobility Aid for elderly and disabled people (MAid) 6, Smart Power Assistance Module (SPAM) 7,
TinMan8, etc. provides controlled indoor navigation. Among the wheelchairs developed with omnidrive or omnidirectional mobility, the OMNIis a mecanum wheeled wheelchair developed for individuals with severe mental and physical disabilities. Another example of an omnidirectional wheelchair is iRW9, which provides a telehealth system with easy-to-wear, non-invasive devices for real time vital sign monitoring and long-term health care management for the senior users, their family and caregivers. In this paper we have presented design and development of a 4 wheel driven omni wheelchair with reduced wheel slippage and vibration. All the wheelchairs or indoor transporters with holonomic drive are developed with mecunum wheels or are a three wheeled omni platform. Mecunum wheels are inherently suitable for handling high load but its turn rate is slow compared to omni wheels. 4 wheel platform with omni wheels are difficult to design, mainly because of its unequal ground reaction force. If designed properly, 4 wheeled omni platform provides better performance than platform developed with mecunum wheels. We propose a unique wheelchair design with omni wheels and proper suspension mechanism to provide enhanced mobility in indoor environment. The design has been evaluated with wheel load measurement from current consumption and vibration measurement with a 3 axis accelerometer mounted on the chasis.
Methodology
2.1. Omnidirectional Wheelchair Platform Development
Omni directional wheelchairs 14,15,16 posses special maneuverability due to the omni wheels which allows translational as well as lateral mobility. Unlike differential or steering drive, omni drive systems does not possess holonomic constraints, allowing motion in both the body axis possible. Moreover, translational movement along any desired path can be combined with a rotation, so that the robot arrives to its destination at the correct angle 17, 18. In order to achieve this, the wheel is built using smaller wheels attached along the periphery of the main wheel. Each wheel provides traction in the direction normal to the motor axis and parallel to the floor. The forces add up and provide a translational and a rotational motion for the robot. Holonomic drive system is usually designed with mecanum wheels (4 wheeled configuration) and omni wheels (3 wheeled configuration). 4 wheeled omni driven wheelchair are not common, but if designed properly, 4 wheeled omni drive provides better traction force compared to mecanum while turning. Design of a 4 wheel driven Omni Wheel based platform needs special attention. Regardless the surface type, all four wheels should receive equal ground reaction force (GRF) or else there are chances of wheel slippage. A Omni wheelchair is designed to support 120 Kg including the platformrsquo;s own weight and payload with proper suspension mechanism to provide equal GRF in all wheels. Wheelchair design comprises designing of the motor wheel assembly, a suspension mechanism and a chasis with sufficient load bearing capacity. Fig.1 shows different parts of the omni wheelchair. Motor wheel assembly (Fig.1a) consists of a lsquo;Lrsquo; shaped part called lsquo;Main Lrsquo; which holds the dual omni wheels of 8 inch diameter and ball bearings. The upper side of the lsquo;Main Lrsquo; houses two slots for attachment of another smaller lsquo;Lrsquo; shaped mild steel part labeled as lsquo;secondary Lrsquo;. The lsquo;secondary Lrsquo; houses a pair of vertical slots which connects a motor attach plate. The motor (Buhler, 24V) is firmly affixed with the motor attach plate. The horizontal slot present in the lsquo;Main Lrsquo;, vertical slot present in the lsquo;secondary Lrsquo; provides the ability to align the motor shaft co-axially with the wheel rotational axis. This minimizes jamming of the motor shaft due to misalignment or manufacturing defects. Finally, a flange is designed to couple the motor shaft to the dual omni wheels. This motor-wheel arrangement prevents the system weight to be transferred to the motor shaft as radial load. Four of such assemblies are connected to the platform chassis. The next most important issue to consider is the suspension system to ensure equal ground reaction force on the four omni wheels. Suspension system has been designed using rear shock absorber. The hydraulic damping present in the shock absorber reduces the oscillation in the system. A pair of rear shock absorber is selected on the basis of required spring stiffness, travel length. Fig.1b shows the designed front motor-wheel assembly with suspension mechanism, attached with the chassis. The suspension mechanism is intentionally connected in the front motor-wheel assembly to maintain the symmetry. The chassis is designed with mild steel lsquo;Lrsquo; of 20 mm width and 3 mm thickness. Finite Element Analysis is done in SolidWorks to optimize the width of the material at the given load. The chassis measures 540 mm x 440 mm and is rectangular to accommodate the suspension mechanism in the front. In the suspension design, two lsquo;Ursquo; shaped mild steel plates are used as lsquo;Hold platesrsquo; and named as lsquo;Upper Hold Platersquo; and lsquo;Lower Hold Platersquo;. The lsquo;Upper Hold Platersquo; accommodates adjustment slots to adjust the suspension mechanism to work with a varied payload. The motor-wheel assembly is attached with the lsquo;Lower Hold Platersquo; with 6 screws. Two linear guides are placed between the both hold plate to restrict the relative motion between them to only one axis. This arrangement prevents tilting of the front motor-wheel assembly with varied load conditions. Two shock absorber along with the two hold plates form a 4 bar system. With changing loads, the effective length of the shock absorbers would change. Thus the angle between the members of the 4 bar will also change. To accommodate this, the shock absorbers are connected with the hold plates as joints with ball bearings. Fig.1c shows the view of the CAD model of the assembled omnidirectional robotic platform. Along with the front motor-wheel assembly mounted with suspension mechanism, the rear and the side motor-wheel assemblies are directly fixed with the chassis. The motors are positioned in the motor-wheel assembly horizontally with offset. In the rear side of the platform, a housing for the electronics drive and controller made with acrylic fibre sheets is placed. All associated specifications for wheelchair development are tabulated in Table 1.
Electronic components of the wheelchair, shown in Fig.2 consists of four Buhler DC motors (77W) and their drivers (40V, 20A) from lsquo;Rhino Motion Controlsrsquo; to drive the motors. These motor drivers accept PWM/DIR input from the controller. The output velocity is proportional to the input PWM duty cycle. The driver measures the speed of the motor from current harmonics and regulates it by close loop PD control. The PWM/DIR inputs are connected to STM32F103, a 32 bit arm cortex M3 micro controller. All the PWM/DIR interface lines between microcontroller and the motor drivers are opto-isolated. 4 PWM channels of lsquo;timer 1rsquo; of the microcontroller is connected to the 4 motor driverrsquo;s PWM input. PWM resolution is kept as 2000 and frequency is kept as 50KHz. The motor drives also have current limit feature which restricts the start-up current to a user-set value. The Buhler motors are equipped with 50 CPR taco generator. These are connected with the 4 Input capture channels of timer 2 of the STM32 microcontroller. These enables the controller and the NUC to measure the actual RPM of the individual motors. ACS712-20A current sensor modules are connected with individual motors to monitor the current consumption. These sensors analog voltage outputs are connected to the analog input pins of STM32 microcontroller. Finally the STM32 microcontroller communicates with the master controller (NUC) via serial port. FT232R USB to serial converter is used to interface the serial port of the STM32 and USB port of the NUC. A MATLAB script running in the NUC generates the wheel velocities and sends to the STM32 microcontroller. The same script also monitors the actual velocity and current feedback from the motors through the STM32 microcontroller.
Result and Discussion
5 healthy male subjects were asked to control the wheelchair using conventional joystick control to traverse a predefined path. Fig.4 shows snapshot during wheelchair navigation and the traced path by 5 different subjects. Mechanical design of the system has been validated in terms of performance of the suspension-wheel mechanism. For 4 wheeled omni drive system, main design consideration is a proper suspension mechanism to distribute equal ground reaction force in all four wheels. Load on a particular wheel is estimated from the amount of current drawn by the particular motor. Current consumption in individual wheels is measured by the current sensor module (ACS712-20A) connected in series of the individual motors. The other issue to be considered is the vibration generated in the system. A 3 axis mems accelerometer ADXL 345 is mounted with the chassis of the system. To evaluate the performance of the system, a trajectory is fed into the system and current consumption and vibration data are recorded. This experiment is done without the suspension mechanism attached, and with the suspension mechanism attached. Fig.5a shows the figure of eight path traversed by the wheelchair during the experiment. Tracked path data has been calculated from odometry. Fig.5b and Fig.5c shows the generated velocity, current consumption in individual wheels and the 3 axis vibration data obtained from experiments without the suspension mechanism and with the suspension mechanism respectively. Table(2) lists the average values of the current and vibration data obtained during the experiment. The Z axis data of the accelerometer includes gravitational pull ( 1g). From the graph and the tabular data, it is evident that the current drawn by the the individual motors are symmetric and follows the trajectory pattern. If any of the wheels had lost ground contact, the corresponding motorrsquo;s current data had shown significant reduction. Now the graph also shows the nature of the vibration without and with the hydraulic suspension mechanism. From the table, it is evident that average vibration in all the axis has been reduced upto 50% after installing the new hydraulic suspension mechanism. Also it is evident that due to reduced vibration, the current consumption of the system is also lowered by an amount of 15% with the improved design.
References
- R.L.Madarasz, L.C.Heiny, R.F.Cromp and N.M.Mazur,1986, “The design of an autonomous vehicle for the disabled”, IEEE Journal on Robotics and Automation, vol.2(3), pp: 117-126. 2. R.C.Simpson,2005, “Smart wheelchair: A literature review ”, Journal of Rehabilitation Research amp; Development, vol.42(4), pp: 423-436. 3. R.H.Krishnan and S. Pugazhenthi,2014, “Mobility assistive devices and self-transfer robotic systems for elderly, a review ”, Intelligent service Robotics, vol.7, pp: 37-49. 4. S.P.Levine, D.A.Bell, L.A.Jaros, R.C.Simpson, Y.Koren, J.Borenstein, “The NavChair assistive wheelchair navigation system ”, IEEE Transaction on Rehabilitation Engineering, vol.7(4),pp: 443-51. 5. U.Borgolte, H.Hoyer, C.Buehler, H.Heck, R.Hoelper,1998, “Architectural concepts of a semi-autonomous wheelchair ”, Journal of Intelligent Robotic System, vol.22(3) pp: 233-53. 6. E.Prassler, J.Scholz, P.Fiorini,2001, “A robotic wheelchair for crowded public environments ”, IEEE Robotic and Autonomous Magazine, vol.8(1), pp: 38-45. 7. RC.Simpson, E.F.LoPresti, S.Hayashi, S.Guo, D.Ding, R.A. Cooper, 2003, “Smart Power Assistance Module for manual wheelchairs. Technology and Disability: Research, Design, Practice and Policy ”, 26th International Annual Conference on Assistive Technology for People with Disabilities (RESNA), Jun 1923, 2003; Atlanta, GA. Arlington (VA): RESNA Press. 8. D.P.Miller, M.G.Slack,1995, “Design and testing of a low-cost robotic wheelchair prototype ”, Autonomous Robots, vol.2(1), pp: 77-88. 9. Po Er Hsu, Yeh Liang Hsu1, Kai Wei Chang and Claudius Geiser, 2012, “Mobility Assistance Design of the Intelligent Robotic Wheelchair”, International Journal of Advanced Robotic Systems, Vol. 9, pp:244-53. 10. L. Fehr, W.E. Langbein, and S.B. Skaar, 2000, “Adequacy of power wheelchair control interfaces for persons with severe disabilities: A clinical survey ”, Journal of Rehabilitation Research and Development, vol.37(3), pp. 353-60. 11. S. Nakanishi, Y. Kuno, N. Shimada, and Y. Shirai, 1999, “Robotic wheelchair based on observations of both user and environment”, IEEE/RSJ International Conference on Intelligent Robots and Systems, Kyongju, Korea, pp. 912-7. 12. N.I. Katevas, N.M. Sgouros, S.G. Tzafestas, G. Papakonstantinou, P. Beattie and J. M. Bishop, 1997, “The autonomous mobile robot SENARIO: A sensor-aided intelligent navigation system for powered wheelchairs ”,IEEE Robotics and Automation Magazine, vol. 4(4), pp. 60-70. 13. I. Moon, S. Joung, and Y. Kum, 2002, “Safe and Reliable Intelligent Wheelchair Robot with Human Robot Interaction ”, Proc. of IEEE Int. Conf. on Robotics and Automations (ICRA02), 2002. 14. Kundu, A.S., Mazumder, O., Chattaraj, R., Bhaumik, S. “Close loop control of non-holonomic wmr with augmented reality and potential field. ”In: Engineering and Computational Sciences (RAECS), 2014 Recent Advances in. 2014, p. 15. 15. Mazumder, O., Kundu, A.S., Chattaraj, R., Bhaumik, S. “Holonomic wheelchair control using emg signal and joystick interface.”In: Engineering and Computational Sciences (RAECS), 2014 Recent Advances in. 2014, p. 16. 16. Kundu, A.S., Mazumder, O., Chattaraj, R., Bhaumik, S. “Door negotiation of a omni robot platform using depth map based navigation in dynamic environment. ”In: Contemporary Computing (IC3), 2014 Seventh International Conference on. 2014, p. 176181. 17. Keigo Watanabe, Yamato Shiraishi, Spyros G. Tzafestas, Jun Tang, and Toshio Fukuda. “Feedback control of an omnidirectional autonomous platform for mobile service robots”, Journal of Intelligent and Robotic Systems, vol.22(3), pp:315-330. 18. A. S. Conceic ao, A. P. Moreira and P. J. Costa,2006, “Model Identification of a Four Wheeled Omni-Directional Mobile Robot”, Controlo 2006, 7th Portuguese Conference on Automatic Control, Instituto Superior Tecnico, Lisboa, Portugal.
译文：
四轮式全方位轮椅减少滑动和振动的设计和性能评估
摘要
完整的轮椅被流行的能力进入受限空间由于其全向移动。在本文中,我们提出了设计和开发的4轮驱动的泛光灯轮椅适合室内导航与减少车轮滑移和振动。设计评估了轮载荷测量从当前消费和振动测量3轴加速度计安装在底盘。从结果和分析,很明显,我们提出设计车轮滑移和振动显示低于现有的设计。系统可以找到应用程序作为一个辅助帮助老年人口或智能室内移动车辆
- 介绍
多年来电动轮椅已经开发为运动障碍和老年人提供援助。智能电动轮椅作为电动轮椅的特殊类,替代传统的轮椅作为辅助性设备。此外,由于易于控制,特定于应用程序的人机界面和平滑的移动,电动轮椅成为一个受欢迎的室内导航的车辆。第一个提出的智能轮椅原型的是Madarasz出版社，他在1986年提出了一个轮椅设计，在一个办公大楼里运输人所需的只有目的地的房间号码。从那时起,许多这样的智能轮椅已经在发达国家发展起来并且有了一定的商业化。大多数发达的智能轮椅在现有商用电动轮椅的基础上添加设施来提高可操作性,比如航行情报和多模式控制接口。举几个例子,NavChair 4,高机动性和运行情况收集轮椅帮助老年人和残疾人6、巧实力帮助模块7、TinMan8等提供室内导航控制。轮椅的开发和全向移动,全方位轮椅发展对有严重的心理和生理障碍的人有帮助。全向轮椅的另一个例子是iRW9,为远程医疗系统提供容易穿戴、无创的装置实时监测生命体征和长期医疗管理。在本文中,我们提出了设计和开发的4轮驱动的泛光灯轮椅能减少车轮滑移和振动。所有的轮椅或有运输能力的完整体随着mecunum轮子而发展或者就是三轮完整平台。与Mecunum轮子天生适合处理高负载相比,它的周转周期是omni轮子慢的。4轮平台全方位车轮很难设计,主要是因为其不平等的地面反作用力。如果设计得当,4轮式全方位平台比3轮平台开发mecunum轮子提供更好的性能。我们提出一个独特的轮椅设计与全向轮和适当的悬挂在室内环境机制来增强的灵活性。设计从安装在底架的加速器计测量当前振动3轴加速度计评估轮载荷测量。
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