Multidisciplinary Senior Design ConferenceKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: P16007

MOTOR ASSISTED WHEELCHAIR

Brad ShumwayElectrical Engineer

Jason ZhongElectrical Engineer

Nick BrownMechanical Engineer

Alan BurgerMechanical Engineer

Dat LeMechanical Engineer

ABSTRACT Current renditions of fully electric wheelchairs involve high expenses and heavily strain the impaired party to

allow higher mobility of the patient. This project aimed to develop a low weight, low cost, removable attachment to attain similar benefits while still allowing a flexibility of usage. The end result would aid users and facilitators while minimizing customer upkeep. Utilizing an electric battery-motor system, a fifth wheel would be driven to provide supplemental power to the user. This assists with mobility in uneven terrain both apparent and unapparent to the rest of the population. Control of the system would be managed through a set of hand controls in close proximity to the arm of the wheelchair.

In a manual wheelchair the user must spend significant physical energy to maintain control when traversing an incline. A motor assisted wheelchair supports the user in everyday use, giving the operator the option to utilize supplemental power. An electric wheelchair completely eliminates the need for a facilitator as the user is in complete control. However, the use of one is burdensome, has significant cost, and can result in muscle atrophy of the user due to inactivity. An electric wheelchair is not the most desirable solution when the user does not have access to a wheelchair-supported vehicle and/or wants to utilize upper body strength. Therefore, the motor assist system developed should not impact the manual usage of the wheelchair. The design for the addition must satisfy these requirements while avoiding the excessive price and inconvenience associated with a fully electric wheelchair.

INTRODUCTION The primary incentive for pursuing this project came from one member’s relationship with a wheelchair user.

This individual is not permanently confined to a wheelchair, though a significant amount of time is spent in one. As it was they were unable to cover the monetary cost of a fully electric wheelchair even with the help of insurance. Even if they were able to, the other drawbacks present in using a fully electric wheelchair would place a significant toll on the family. Due to the large relatively large footprint, mobility indoors is a concern. Transporting the electric wheelchair is also a problem as that requires a wheelchair enabled van, another significant expense. This project looked to develop a full prototype including mechanical, electrical and controls subsystems. This would cost less than a fully electric wheelchair while providing similar benefits in terms of mobility, and giving the user more flexibility as far as choice and independence in using the system.

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DESIGN PROCESS The requirements for the design involved several interrelated criteria. For assisting mobility, the system was

expected to operate at a speed of 4 miles per hour for at least 3 hours, resulting in 12 miles of total travel. This was thought to be enough to operate for the course of a full day for the average user on a single charge. This in turn set a relationship between the motor chosen as well as the batteries, which will be explained in more detail later. Another aim was to keep the overall weight of the system below 25 pounds. The wheelchair weighed approximately 35 pounds by itself, and adding another factor of 100 percent weight was not intended. This tied into the battery and motor considerations, as increasing the overall weight increased the power requirement, which typically required a bigger battery, and the problem repeats itself. This was balanced against the need for traction on the ground with additional weight allowing better grip. An additional requirement was to minimize the footprint increase of the wheelchair. For wheelchair users, it is already difficult enough to navigate indoors and through doorways. We did not want to escalate this issue. The controls were intended to be both easy to use, as well as safe. The safety of the user is paramount to anything else, and care was taken to analyze and ensure safety in our use cases.

The motor-wheel-gearbox assembly was mounted on a welded steel plate piece in the rear of the wheelchair. The battery box with contained batteries was secured on the top face of the welded assembly. Using a set of top links, this assembly was then attached to two mounting brackets fixed to tubing already present on the wheelchair.

Figure 1: 3D Model of Assembly

BatteryThe batteries were intended to be rechargeable so the user could recharge in between using the system,

primarily at school. This limited the choice to nickel cadmium (NiCd), nickel-metal hydride (NiMH), lead acid, lithium ion, and lithium polymer batteries. Another criteria was for the batteries to be as lightweight as possible in order to minimize the added weight to the system. Lithium ion batteries were generally the lightest, followed by lithium polymer, nickel-metal hydride (NiMH), nickel cadmium (NiCd), and finally lead acid. The lifetime for the battery was important as well to meet the requirement for 3 hours of continuous usage.

The battery choice was also driven by the motor specifications. The motor chosen required a voltage between 24 and 72 volts and a discharge of 4 amps. Initially, the battery was intended to fit under the wheelchair seat or in a pouch behind the user, however this was not a viable option after further research.

After consideration we settled on a setup of two 12 volt rechargeable nickel-metal hydride batteries. The total cost was $359.90 and they weighed 11 pounds. The max discharge for each battery was 13 amps, and was predicted to run for at least 3 hours continuously. Using two batteries, these features met customer/engineering requirements.

MotorThe motor selected needed to be able to provide enough torque and speed during our projected run time of 3

hours. After analyzing multiple motors, the model chosen was the Teknic CPM-MCVC-3432D-RLN, a 24 Volt DC brushless motor with a continuous torque of 120.9 oz-in. There were options at other voltages such as 36 or 75 Volts. This was a major point of discussion as with the 36 Volt motor, it was possible to attain our original speed requirement of 5 mph as well as increasing the drivable range and capabilities of the system. However this would have required the purchase of an additional battery ($180), increasing weight and cost. After discussion this trade off it did not seem a worthy consideration. By using a 7:1 gearbox, the applied torque would increase to 846.3 oz-in.

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This was more than the calculated torque required to keep the wheelchair from rolling down an incline of 6 degrees. The speed of the motor through the gearbox would be reduced from 1200 rpm to 171.4 rpm. A wheel of 8 inch diameter rotating at 171.4 RPM has a linear velocity of 4.1 mph. This was a change to the initial requirement of 5 mph, a required compromise though this was still within the nominal value laid out in the beginning of the project.

Omni wheelThe wheel used for this project was intended to be ‘dumb’ with no direct

steering capabilities as this would have been significantly more difficult to implement in the overall design. That being said a normal ‘rigid’ caster wheel was not a good option because of the shear force at the point of contact whenever the user would turn. This was thought to require extra effort on the user while needlessly wearing down the wheel.

The idea of the Omni wheel (pictured in Figure 2) is that along with normal operation, it can translate side to side with minimal resistance due to perpendicular oriented casters along the circumference of the wheel. This allows the user to operate in any direction with minimal additional resistance.

GearboxAs mentioned in the motor section, the desired gear box ratio was 7:1 to achieve the desired operational speed.

The gearbox also had to be compatible with the frame size of the motor (NEMA 34). Though the focus was on minimizing cost, the most dependable vendor was more expensive. Ideally the gearbox would have been a right angled version to allow the weight of the system to be centered, but the price was simply too high, $900 compared to $420. This is a one of the biggest points of improvement on the current design. With an inline gearbox the motor/gearbox assembly imparts a significant torque on the wheel and its mount due to the distance it protrudes from the front to back centerline of the wheelchair.

Motor-Wheel-Gearbox AssemblyThe order of assembly is as follows from Figure 3: The

output of the motor [4] leads to the input of the gearbox [3], which is secured by a set of 4 Allen bolts. The gearbox then attaches to the welded assembly [1] in similar fashion, using 4 Allen bolts. On the output shaft of the gearbox, a keyway is set using external pressure. The omniwheel [2] interfaces with a hub which contains a female key connection. This hub is connected to the wheel using supplied Allen bolts. The wheel and hub is aligned with the key on the gearbox output and a hex bolt is secured perpendicular to the location of the keyway. On the end of the gearbox, a hole was placed to allow a cotter pin to be inserted. This prevented the wheel from falling off the end of the gearbox shaft.

Top LinksTop Links are an industrial version of tie rod ends found on tractors and used to attach heavy pieces of

equipment. For the initial prototype these were chosen because they were inexpensive and much stronger that the system required. Several other methods of connecting the wheel to the wheelchair were brought up and discussed. These ideas included permanently attaching the system by using a bar across the underside of the chair. This was not desired as one customer requirement was to keep the elements of the attachment removable.

After reviewing our engineering requirements, some of the other proposed solutions had flaws. For example, using a bar on the underside would have prevented the wheelchair from being folded and stored away, a feature our customer specifically requested. The top links were versatile; they provided stability as well as a degree of freedom necessary for the wheelchair to fold while the system was attached. There were some concerns about the added weight (approximately 10 pounds), but this added an extra benefit of increasing traction.

Copyright © 2016 Rochester Institute of Technology

Figure 2: Omni Wheel

Figure 3: Motor-Wheel-Gearbox Assembly

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Mounting BracketsKeeping with the intent of having each element being removable, the

mounting brackets had to be easy to attach and remove, simple and quick to interface with, and robust. Because there was surplus plate steel available, a set of two brackets were made. On this bracket, 3 sets of holes were made to accommodate a set of perpendicular U-bolts. These would inhibit extraneous degrees of freedom between the wheelchair and the attachment, making a relatively rigid connection.

There was also a hole drilled through the half circle flanges of the bracket to allow the top links pin joint to pass through. This pin joint is secured by a quick release pin that is placed towards the outside of the wheelchair. The mounting brackets are detailed in two views in figures 4 and 5.

User InterfaceThe power going into the entire system would be controlled by two

switches. The first one is an on/off rocker switch, and the second one is an emergency stop. The emergency stop is a push button that the user can press on to immediately cut off the power going into the motor. This might be used when the user feels they have lost control of the system. Afterwards the user can pull the emergency button up to reestablish the connection. As such, the emergency stop acts as an inherent enable for the On/Off switch. The On/Off switch is a Double Pole Single Throw (DPST) rocker switch with a protective cover placed to prevent accidental activation.

Speed of the wheelchair would be regulated by a Pulse-Width-Modulation (PWM) controller. This allows the user control over the motor output. Software was provided that allowed the user to control their max speed or maximum applied torque. Using this software an appropriate speed/torque can be selected as desired. Figure 6 shows the control panel. This panel was attached to a plate already present on the wheelchair underneath the armrest. Two belt clips were used to secure the controls to the wheelchair plate.

While going up a steep slope it is possible that the motor could stall where there is no total motion though torque is still being applied. In this case the motor controller would force the motor into emergency shutdown mode to prevent damage to the motor. When this situation occurs, an audible buzzer activates. This alerts the user to turn off the system to prevent possible damage to the motor or gearbox. However in order for the buzzer to work it must be supplied with five volts. This required a voltage regulator to be implemented in the circuit as well. The circuit is shown in figure 7 below.

Battery BoxThe battery box had to be waterproof and have

enough room to fit two of the chosen batteries. Being lightweight was a large concern as well as being durable. Plastic was the first choice since it’s a material that is appropriate for electrical applications. It’s a light material with high electrical resistance, isolating the batteries and electric circuit from the metal mounting piece. For dimensioning, the battery’s height (3.7 in.) was used to further optimize the battery box. Ideally the dimensions of the box would be only slightly bigger than the volume taken up by the batteries as well as the electric circuit.

Copyright © 2016 Rochester Institute of Technology

Figure 4: Mounting Bracket

Figure 5: Mounting Bracket Alternate Angle

Figure 6: Control Panel

Figure 7: Circuit Diagram

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EXPERIMENTAL PROCEDURE Battery, Gearbox, Omni Wheel, and Motor

The first experiment done used the batteries, gearbox, Omni wheel, fuse, rocker switch, and motor combined together to test basic system connectivity. Using 14 gauge wire, all the components were connected with the motor held fixed by a clamp. Observed behavior was consistent with what was expected; when the rocker switch was switched on the motor and Omni wheel would spin. Likewise, when turned off the motor and Omni wheel would stop spinning. When connecting the motor via computer software from the motor controller, the speed and torque programmed was seen to be consistent with the physical output.

Pulse Width Modulation (PWM) ControllerThe second experiment performed added the PWM controller to the first

experiment in order to see if the speed could be controlled manually. Observed behavior was consistent as adjusting the PWM controller corresponded to a change in motor output. An oscilloscope probe was put on the PWM output to see what the output would be at the minimum and maximum speed corresponding to the desired RPM and torque. The minimum speed and the maximum speed software screen and oscilloscope capture are shown below. At minimum speed the RMS voltage was 4.26V, while at maximum speed, the RMS voltage was 26.8V. These are both reasonable values to obtain given the PWM specifications.

Top Links and Mounting Piece Strength AnalysisStrength analysis was performed on the mounting pieces using ComSol. The first analysis was done using

hypothetical stresses that would frequently be applied on the system during daily usage. These stresses/forces included weight from the batteries, motors, and gearbox. The second analysis was an extreme case, where forces were multiplied until the respective parts were under ultimate stress, in order to calculate factor of safety. An example of the simulation is shown in figure 11 below.

Battery Box Strength AnalysisTwo scenarios were simulated to test the battery box. The first scenario tested how much weight and force the

box can idly handle in its nominal position. The second scenario tested how much force the box could withstand when it was lifted up with the mounting piece to store away.

TEST RUNS SET-UP Unfortunately while attempting to assemble the entire system

for engineering requirement tests, some problems manifested. First the motor would perpetually go into stall mode when going up even the slightest incline. After root cause analysis and trial and error, the root of the problem seemed to be the motor setting. Initially it was set to ‘Constant Speed’ and not ‘Constant Torque.’ Changing this setting had several effects.

The motor no longer stalled on nominal wheelchair inclines, and the user could propel themselves along as they normally would while the motor was engaged. This were significant improvements as enabling the user to propel themselves under any circumstances was a feature desired from the very beginning.

The next obstacle came in a form of mechanical alignment. With the wheel mounted on the axle of the gearbox, there was nothing restricting motion of the wheel along the axle. There was a cotter pin installed on the end of the axle, though this only prevented the wheel from falling off the end of the assembly. Because of the persistent vibration during usage, the wheel would dislodge itself from its seating on the gearbox key. This would result in a free spinning wheel with no transfer of power. This was fixed by drilling a hole perpendicular to the key at the axial point central to the key. A bolt was then put through this hole to secure motion side to side. This problem was an

Copyright © 2016 Rochester Institute of Technology

Figure 8: Initial Motor Battery Test Setup

Figure 9: Mounting Piece Strength Analysis

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initial oversight of the design. These fixes allowed some of the more demanding engineering tests like battery life to be done.

RESULTS AND DISCUSSION Per Engineering Requirements

Table 1 lists the results of tests for individual engineering requirements. As a first pass prototype of the tested engineering requirements were passable. Having said that test results like the weight of the addition should be improved or streamlined. There is a substantial section at the end of this document about potential ways to improve the performance of the design.

Spec. Number Importance Engineering

RequirementUnit of

Measure Marginal Value Ideal Values

Test Values

S1 3 Weight of Addition lbs < 50 25 40.05S2 15 Noise Level dB <60dB 50 29

S3 6 User Interface Usability Standard >8 10 Pendin

gS4 11 Attachment Length ft < 3.5' 3.5 2.41S5 4 Attachment Width ft < 2.67' 1.33 1.33

S6 7 Max Climbing Incline degrees > 4.76 6 Pending

S7 4 Max Speed mph >4 5 4.48S8 2 Driving Range miles >10 12 10.125

S9 1 Life of Mobile Energy Source (1 use) time (h) > 2.5 3 2.66

S10 10 Cost $ USD < $3000 500 1380

S11 12 Stored (Folded up) Volume ft*ft*ft < 36"x42"x16" 10.5 2.291

Table 1: Engineering Requirements Test Results

Cost was a fairly big issue, as the 500 dollar ideal value was exceeded almost by a factor of 3. However the nominal value for this was $ 3,000 USD. At this upper limit, the cost of manufacturing would have been forecasted to be approximately twice this amount. This scenario would have been the equivalent of ‘breaking even’ in the market. Most of the cost of this prototype comes from the motor, gearbox, and batteries at a total of $1,140 USD. All told, these three components account for approximately 82% of what was spent.

Though the attachment length falls within accepted values, when looking at the final assembled system, a problem that was noted involved the ability for another person to push the wheelchair. Because the assembly sticks so far behind the wheelchair, it is awkward for another person to push, which would be helpful should the batteries die during the day. Because of this, a section of the recommendations goes over shortening this length, and increasing the handling capability as a result.

It was seen that the system had little to no trouble going up regulation ramps. That being said, we were not able to find the maximum threshold that the motor stalled at. This maximum incline would be tested with solely the motor providing power, with no human interaction.

Experimentally it was seen that 4.5 miles per hour was significantly faster than human walking speed, so this requirement was possibly set too high, though not knowing for sure the maximum incline possible, the motor might still have been sized well. The drive range was barely over the nominal value required. This is not a big issue, as this test was isolated to the motor; in the case of the average user, the usable range would be extended by taking some of the load off the motor and propelling themselves.

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Proceedings of the Multidisciplinary Senior Design Conference Page 7

The noise level test was surprising. We were initially under the impression that we would need to muffle the buzzer and system to prevent hearing damage. Ambient conditions normalize to approximately 60 dB SPL (decibels with reference to sound pressure level), and with the system engaged, this increased to 89 dB, which was within safe limits. A user can be in these conditions for approximately 2 hours without damage. The buzzer however, was barely audible when it went off. This is a concern for the fail-safe mechanics of the motor.

Top Links and Mounting Piece Strength AnalysisFigures 9 shows an example of the strength analysis of the mounting plate. It shows that the most impactful sum

of stress will be at the connecting joints of the piece. This is to be expected based on the behavior of a concentrated connection force. The factor of safety of this mounting piece compared to nominal loading is 300, meaning it would take a couple of 13200 N (2967.5 lbs.) forces to break it. This was a significant avenue of overdesign, and future materials could probably be less heavy duty than ¼” plate steel.

Battery Box Strength AnalysisFigures 10 and 11 show the mounting piece and battery box (with batteries) under hypothetical worst case

stresses in two scenarios. When the box is idle or moving with constant speed, the factor of safety is 28, meaning that the box could carry and withstand up to 1900 N (427 lbs.) until failure. In the case where the box is lifted to carry, the factor of safety is 26, meaning that the box can withstand 575 N (130 lbs.) maximum.

CONCLUSIONS AND RECOMMENDATIONS Conclusions

The end result of this project was a proof of concept. Parts were chosen that worked together, while allowing enough leeway for potential mishaps in design. As it is now, many elements are not optimized; however there is good material and a working basis for another team to pick up the project. Overall the project was a success, but was not at a good enough state to hand off to the end user. This project will be handed off to another team to act on some of the recommendations and optimizations suggested.

RecommendationsThe gearbox chosen for this project was not ideal. Since it was an inline gearbox the torque applied to the

gearbox and wheel was significant and produced a visible tilt in the operating line. In the ideal scenario, a right angle gearbox would have been used, but with the budget allotted, this was not possible. For a comparison of cost, the quote for a right angle gearbox was approximately $900 USD compared to the $420 USD of the current gearbox. This rotational torque also is relevant to options concerning the top links. Concerning the final behavior, speed did not seem to be an issue for our system. As such, the ratio of the gearbox could probably also be increased. The 7:1 ratio is awkward and not readily available. A 10:1 gearbox might be a more convenient possibility.

Top links were used as a convenient piece known to work far past the demands of our application. An alternative to using top links would be to use tie rod ends which accomplish similar goals. They are lighter and have

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Figure 10: Idle Box Strength Analysis Figure 11: Box Deformation during Lifting

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more freedom of motion. Due to the lack of freedom of movement in the original top links the wheelchair was unable to completely unfold. Another downside of the top links were the coarse threads that unscrewed under the weight of the system. This is important because the top links were able to rotate in dimensions which introduced a tilt in the system. In an attempt to correct this welds were placed on the top links closest to the motor to prevent rotation of the top link. To achieve a better result other methods of attachment or a more precise set of tie rod ends should be used. The top links used also protruded significantly behind the back plane of the wheelchair, which inhibited the ability of someone else to push the user and increased the footprint of the wheelchair as a whole. A better solution would be to relocate the attachment brackets and reducing the size of the top links so that the omniwheel does not extend beyond the rear tires of the wheelchair. This would be useful when a person is used for an additional aid rather than the system.

For mechanical assembly of the system there were a large number of small improvements that could be made to increase the ease of assembly. To make welding the flat pieces of metal together a simpler task the pieces could be designed with indexing notches so that they fit together like a puzzle. This would keep the parts in the correct place when the pieces are welded together and will ensure accurate placement. Due to the ambiguity of some of the parts that were ordered the sizing of the components that were designed in Solidworks would be subject to change.

Another improvement could be in the choice of material. To the aim of making this ‘disaster proof’ 1/4 inch plate steel was used. This introduced a significant amount of weight which made the addition sturdy at the expense of making it harder to handle. The factor of safety from simulation was significant, which implies that a less durable material could have been used. It is unknown if the welded plate piece would be viable as a collection of hardware-attached machined parts, but that would be another step to take with regards to this element.

An observed difficulty was traversing significant hills, well beyond ADA standards. This indicates that the torque applied by the motor was not quite enough. This might require a more powerful motor which would entail either compromises to the engineering requirements or significant weight reduction, possibly both.

The controls system was not made to satisfaction, partially due to budget constraints. The safety of the controls would have to be improved, as currently it is not sufficiently weather proof. The attachment method for the panel is also not ideal. Currently the plate that the controls attach to must be taken off the wheelchair, which is detrimental to the ‘removable’ requirement of the project. It may be worth a look to consider attaching to the actual armrest of the chair, though this would be a slight intrusion on the user’s space.

While the dynamics of the wheel were satisfactory, durability was a serious question. The ability to withstand normal wear and tear was overestimated, though finding another durable 8” omniwheel might be a difficult task. This wear was exaggerated by the tilt imparted by the gearbox. Even considering this the wheel was expected to last longer. It does not seem capable of performing well off road as was initially thought. Another option might be to get a second free rotating omniwheel to support some of the weight of the attachment. While this is appealing in the symmetry, there may be problems in adjusting the angle of attack of the powered wheel.

ACKNOWLEDGMENTS Thanks to project guide Sarah Brownell as well as program coordinator Elizabeth DeBartolo. Thanks to former

member Brenden Hoff for his help in the first section of MSD as project manager. Thanks to the professors that were referenced as subject matter experts, including Dr. Hany Ghoneim, Dr. Timothy Landschoot, George Slack, and Lynn Fuller,

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