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Project Number: P15005
STAND-UP MOTORIZED PLATFORM
Gabe LuceMechanical Engineering / Olivia Robertson
Electrical Engineering / Alexandra Silberglied
Mechanical Engineering
George Bishop
Mechanical Engineering / Michael Fleszar
Industrial Engineering / Lawrence Fredsell
Electrical Engineering
Abstract
Spending day after day confined to a sitting position can be very detrimental to the health and well-being of persons suffering from disabilities affecting their lower extremities. The scope of this project was to develop and build a fully-functional standing wheelchair. With this chair our customer, a young woman afflicted with movement and structural impairments, will be able to stand and move about safely. The device has dual functionality: it operates both as an ordinary power wheelchair, and can be lifted into and driven in a standing position. For movement, the customer is able to operate the chair using a regular joystick control. A switch located in the back of the wheelchair operates the transition between sitting and standing mode. While in standing mode, the customer is supported at her torso and legs by a chest bar and shin pad respectively.
Introduction
The vast majority of individuals suffering from disabilities which affect their balance and muscle coordination have no choice but to spend the majority of their lives seated in wheelchairs. The negative effects of spending a life confined to a chair are numerous. A few of the afflictions commonly suffered by individuals in this position are muscle and coordination deterioration, bed sores, digestive problems, and emotional problems, such as depression. The overall quality of life of these individuals could be vastly improved by using modern technology to develop a way to assist in standing.
Being the first of its kind, the scope of this project is to design a safe, reliable, and inexpensive device which would allow an individual with physical disabilities to stand up while moving about. Although this wheelchair was designed for the use of one individual customer, it was designed to be as adjustable as possible for any future growth. In order to do this the team was provided with a two thousand dollar budget from RIT and gifted a fourteen year old retired wheelchair from the customer. Some parts from the old chair were able to be re-purposed and incorporated into the new designs of the wheelchair, the most important of which included the base of the old chair, motor, and control system.
Figure 1: Donated Wheelchair
Design Process
The first step in the design process was to meet with the customer and make a clear and concise list of all of the requirements for our project. These requirements were known as the customer requirements. Once this list was developed, each requirement was then evaluated based upon importance. The most important requirements would we given a value of “9” and a “3” or “1” would be given to the moderately and least importance's respectively. These customer requirements were then used to develop the engineering requirements. Our final list is shown in the Figure below. Customer requirements were translated into measurable engineering requirements which could be easily tested and evaluated .Each requirement will be address as follows:
Adjustable Joystick placement/ Shorter Armrest: The same hardware that was previously used for the Joystick is still being used, which will allow the joystick to swivel. However, the placement of the joystick will be much closer to Megan (< 17 inches) and there will be a shorter armrest that will allow her to reach it more comfortably. The team will be working with Matt McGarvey, Megan’s everyday wheelchair specialist to adjust the sensitivity of the joystick as well as the wheelchair speed.
Must fit In Van/ attach with S clips: The design currently uses the old base of the electric wheelchair in combination with two mechanical actuators that are connected to the base and back of the chair. The overall footprint of the chair has not changed and will still be able to fit within the Downey’s van doorway (52’’x 29.5’’). Since the base of the chair has not changed, the S-clip attachments have also stayed and will continue to be the main securing mechanism to the van. The addition of the actuator support system on the rear of the chair can also be used as a solid structure to secure the chair into the van.
Stabilization, able to reach in all directions, trunk support, able to reach worktable.
To keep the user safe, new features based off of current standing wheelchairs were implemented. These include a custom made chest bar and shin block. They were made using waterproof vinyl and high density craft foam that will take the wear and tear of everyday life. The shin block is attached to the footplate, which is for her feet to rest on. The footplate is attached to the chair via a hitch port located on the bottom of the given wheelchair base. The shin pad’s height and angle are adjustable to maximize support for the user. In order to increase stability of the chair and to alleviate tipping, casters were added beneath the foot platform. Calculations show the center of gravity of the chair in the standing position will not tip on hills with up to a 24 degree slope, however because Invacare specifies that their power chairs should not exceed 12 degrees, the user will be advised to use the chair under 12 degrees. While the chair preforms well on inclines the additional casters provide a solid point of contact beneath the user’s feet.
Long lasting / indoor and outdoor use
Due to the team’s utilization of the previous base and power system, the chair will have similar battery life and durability that it had previously. Elements of the original base were weather resistant and the additions were coated to prevent degradation from weather. .
Adjustable with Growth
Each end of the chest bar is attached to the arm bars and is vertically adjustable. The chest bar can also be moved towards and away from the users body by adjusting the telescoping tubing of the along a track located on the arm bars. The placement of the bar allows upper body support, and the ability to better access a work or dinner table at a comfortable position closer than a normal power chair.
Sit and Stand Functionality
The sitting to standing function is performed using two electric linear actuator that are powered using the same batteries that power the chair’s movement. In line with the customer requirements, the person sitting in the chair cannot access the linear actuator controls that raise and lower the chair. . The assisting person will be able to decide how high or low to position the user. Limit switches cause the actuators to turn off once the chair reaches a certain predetermined sitting or standing position to avoid any injuries.
Figure 2: Sit and Stand Functionality
Neutral Mode
The chair has a neutral mode that cuts power from wheel motors and allows for manual pushing and steering of the chair. This function is achieved by switching two control levels at the rear of the chair, directly behind the wheel, from the drive to the neutral position.
Category / Requirement / Value / Units / Requirement Achieved?General / Speed / 0-5 / Miles per hour / Yes
Total Weight chair can handle / <200 / Pounds / Yes
Size / 52 x 29 / Inches / Yes
Weather proofed / Y/N / Yes
Electrical / Battery Life / >24 / Hours / Yes
Controller distance from body / <30 / Inches / Yes
Neutral Mode / Y/N / Yes
Mechanical / Adjustable arm rest / 20-30 / Inches / Yes
Supports user’s weight by trunk / >200 / pounds / Yes
Θ1 (while Standing) / 120-130 / degrees / Yes
Θ2 (while Standing) / 90-95 / Degrees / Yes
Θ1 (while Sitting) / 0-5 / Degrees / Yes
Θ2 (while Sitting) / <30 / Degrees / Yes
Balance on incline while standing / <30 / Degrees / Yes
Table 1- Engineering Requirements
Benchmarking
The next step in our design process was to research and benchmark other standing wheelchair concepts. We quickly discovered that numerous products with similar applications to our own already existed, however many of these devices were priced far out of the reach of the average consumer.
Figure 3: Invacare Dragon Vertic
With only a two thousand dollar budget, cost was a limiting constraint. To save money, many of our critical design considerations revolved around utilizing our donated wheelchair to the best of its capacity.
After benchmarking other designs we, broke our project down into more manageable subsystems to be designed independently. These subsystems included the Frame, Lifting Mechanism, Torso Supports, Leg Supports, and the Electrical Control System.
Frame/ Drive train
The first major design decisions of our standing wheelchair was the design of the frame and drivetrain. After some light dis-assembly of the donated wheelchair we assessed it for structural and electrical capability. We discovered that with the exception of the batteries, most of the components were in good shape and reusable for our project. This ended up saving us an enormous amount of money and time on the project as we were able to skip the design of components such as the frame, motors and drive train control system.
Figure 4: Salvaged Frame & Drive train
A new pair of 12V lead acid batteries was purchased to replace the original.
Lifting Mechanism
Arguably the most complex and technically challenging subsystem of the standing wheelchair to design was the lifting mechanism. The purpose of the lifting mechanism was to move the occupant from a sitting to a supported standing position, comfortably and at a safe speed. Numerous designs for this mechanism were considered including four-bar linkages and scissor mechanisms. Also, several type of systems were considered including hydraulics, pneumatics, and electrical systems. In the end an electrical linear actuator was selected to propel the device based on cost, safety, and complexity. Electric linear actuators are currently widely used within the industry which helped aid in the team’s decision.
Figure 5: An Example of a Linear Actuator
At the beginning of the project the team was provided with their customer’s previous electric wheelchair and because of the good condition of the components, the drive-train was able to be reused. The team was required to design a lifting mechanism that would be compatible with the existing frame geometry. This constraint combined with limited funds were the biggest factors contributing to the final lifting design. The lifting mechanism used to raise the seat of the chair involved welding a 3.5 inch lever arm to the front center of the seat. This would convert linear motion to an angular change in the seat’s position. A linear actuator was connected from this lever arm to a support structure built on the frame of the rear of chair. The originally geometry of the chair was too small, so the team was required to design an additional frame on the rear of the chair. An actuator mount designed by the team was then welded to this support system to connect the motor end of the larger, bottom actuator. In order to design this system, a free body diagram of the seat of the chair was first constructed as shown below in Figure 6. By summing the moments about the point of rotation at the front of the seat, the force required to lift the seat was calculated. Knowing this force, an actuator which was capable of providing the desired force while also meeting the geometric constrains of the frame was chosen.
While the seat frame is raised, the back of the chair needs to remain perpendicular to the ground. In order to achieve this the team purchased another, smaller actuator. The second “top” actuator connected to a lever arm designed by the team that connects to the back center of the seat frame of the chair. This lever arm unlike the one for the bottom actuator is in a Z shape, which not only extends out horizontally from the seat but also vertically. This was done so the actuator would be at an angle giving it mechanical advantage while keeping the back of the chair upright. The top of the actuator was connected to a pin that was inserted between two cross braces welded between the left and right seat back bars. The pin that the actuator is connected to was chosen for its shear strength and to minimize deflection of the braces. As the bottom actuator retracted and the seat of the chair rose, the top actuator would also be retracting, forcing the back of the chair to remain perpendicular to the ground. There was also a proof of concept model made to demonstrate how the two actuators would move the seat and back.
Figure 6: Force Calculations
Torso Supports
In order to safely lift a user to a standing position, their body needs to be supported throughout the lifting process and while they’re in a standing position. Numerous concepts were considered including harnesses that went through the legs (like a baby bouncer), around the legs (like a rock climbing harness) and separate chest and arm support bars. One of the most important requirements stated that we must fully support the chair’s passenger but also allow for full range of motion with her arms. Our final decision for a torso support was separate chest and arm support bars due to ease of use, cost, and an industry expert recommendation. The existing arm rests were modified and extensions were added to position the chest bar. The extensions are built out of perforated square steel tubing which is not only cost effective but allows for easy adjustments to be made in the forward-backward direction. The chest bar is also vertically adjustable for the user’s comfort
Leg Supports
In addition to her torso, leg supports are also a critical component in our system. Our customer not only suffers from cerebral palsy and scoliosis but also has a malformed hip socket which causes pain in her hips and legs. Putting her into a partial standing position helps to relieve the pressure on her hip. To maintain the standing position we designed a foot plate to stand on and a shin block that would keep her safe while supporting her legs and keeping stress off her knees.
Electrical System
Two parallel systems are used to control the function of the chair. One system controls the existing Invacare electric wheel chair base while the other controls the lifting mechanism to position the chair from sitting to standing. Both systems are powered using two 12 volt, 55 Amp-Hour Duracell AGM electric batteries in series. The drive system uses the entire 24V potentials produced by the batteries while the lift circuit only used 12V .The entire system schematic is shown below in figure 6.
Figure 7. Electrical System Schematic
Due to proprietary constraints the electric wheelchair base was left as is. There were no modifications made to the system since it was still fully functional. The electric wheelchair has two modes; drive and push. To change the mode of the chair, two switches, located near the back casters on the base of the chair, must be flipped. Once the chair is in drive mode, the user has the ability to be in fast mode or slow mode. The user is then able to change which mode they are in by flipping a switch located on the Invacare joystick. The user can then control their movement and speed using the same joystick.
The other electrical subsystem in the chair is the lift circuit that controls the movement of the seat position. This circuit is in parallel with the electric wheelchair base circuit. This circuit controls the two linear actuators mentioned previously that extend and retract when powered to change the angle of the seat. In order to control the actuators, a momentary switch is implemented that will change the actuator’s direction until released or a limit switch is hit. To ensure correct function of this circuit, power diodes were placed within the circuit. These diodes direct the current to the desired actuators based on the direction the actuators are being engaged.