Team Vision for Subsystem-Level Design PhaseDuring this phase we sought to:
- Decide on specific solutions such as the motor, battery, wheels, and switches needed.
- Perform feasibility to ensure solutions selected will actually work.
- Begin creating CAD drawings.
- Begin generating a bill of materials.
- Continue risk management
- Further develop test plans.
- Work better as a team to ensure timely completion of activities.
During this phase we managed to:
- Decide on a preliminary motor to use, battery, casing and mounting method to the wheelchair.
- Begin generating some CAD drawings for the mounting method.
- Create a theoretical model of the forces involved on adding an additional wheel to ensure the motor will produce the power needed to move the system.
- Benchmark different wheels.
- Start a bill of materials.
- Formalize plans for testing against each of the engineering requirements.
Mechanical Subsystem Design
There are some requirements regarding the battery case:
- Compatibility: given the limited space, the case/box's dimensions have to be as close to battery's dimensions (4" x 4" x 2") as possible.
- Lightweight: one of the important customer requirements, meaning that steel box is out of the question. Plastic and its variances are the only feasible choices.
- Safety: the case/box has to protect the user from the potential danger of electrical equipment as well as the battery from the environmental impacts.
Judging the requirements and options, this case/box was chosen  :
Mounting Equipment/Equipment Placement
- 2 bars will be installed to the back of the wheelchair to create another frame for the Omniwheel.
- The Omniwheel and the motor will be mounted to the 2 bars shown in the Solidworks drawing in the Feasibility Section. The Omniwheel will stay in the center of the width-dimension.
- The Battery Case, containing the batteries, will be mounted below the seat, preferably on the frame, in the opposite side of the motor in order to counter the moment generated by the motor's weight (albeit small).
- The Switch will be place on the right armrest (dominant hand) of the wheelchair. It is to decide whether the Switch is drilled into the arrest or directly installed.
- The wires will be cover with plastic tube to prevent environmental effects and other hazards and to protect user.
- The wires will be wired along the frame to preserve the aesthetic of the wheelchair.
Electrical Subsystem Design
Motor Calculations/DecisionsBattery -> Motor Control -> Motor
Motor Control and Motor: Using the EZ lite cruiser as a reference (specs in motor benchmark section), I calculated that the torque needed is about 16.4Nm.
Equation between torque, power, and speed: Power (kW) = Torque (N.m) x Speed (RPM) / 9.5488 (1)
Speed is given, and it is converted to RPM by solving following equation v (m/s) = r (m) × RPM × 0.10472 (2)
The RPM is found to be 210, and the power is 2X180W. Therefore, the torque is about 16.4Nm.
To ensure long lifespan, a brushless motor is chosen, and the specifications for it are taken from the sale website and copied here: “24 Volt 400 Watt MAC® Brushless Motor With Built In Speed Controller Built-in variable speed controller works with any 5k Ohm throttle or potentiometer to vary the speed of this motor from full stop to full forward. 1950 RPM at full speed. Clockwise shaft rotation facing the shaft. 1/2" OD output shaft with two flat spots on end. Fits chain sprockets and belt pulleys with 1/2 ID bore. 3-bolt mounting pattern measures 4-7/8" apart on center. Motor dimensions are 4-7/8" wide x 2-1/4" deep x 4-7/8" deep including shaft. Shaft is 1-3/8" long overall with a 1" long 1/2" OD end and a 3/8" long 5/8" OD collar at its base. Polished aluminum motor case. Manufacturer rated at 1/2 horsepower. Weight: 3.15 lbs.” <http://www.electricscooterparts.com/motors24volt.html#koll>
Here is the link for the picture of the motor and the wiring diagram.
With 400W and max 1950 RPM, the torque that this motor provides is about 1.96 Nm. To achieve the desired torque, we can use an 1:8 gearbox to push the torque up to about 16Nm.
The motor control will be the power relay and the potentiometer. Both the power relay and potentiometer can control either the voltage or the current.
Battery Calculations/DecisionsTo calculate the Maximum Motor Amperage one must use the formula
Where P is the power rated in Watts, V is the voltage, and I is the current rated in Amperes.
From the Motor the Power is 400 Watts, and V is 24 volts when adding two 12 volt batteries together. Therefore the Maximum Motor Amperage is:
When selecting the battery type, Amperage of the battery pack should exceed that of the motor, ensuring that even when the motor is running at 100%, the battery will not hold it back. From this we are looking at a battery with max continuous Discharge Current of around 20 to 30A. This will allow the battery to give 20 or 30A of current to the motor without it harming the battery.
To calculate the Max Continuous Discharge Current if not given you use the equation:
To calculate the running time of the battery and motor combination the equation below calculates that:
In summary we are looking for:
1. Two 12V Batteries Rated at around 20 to 30A of continuous discharge current
2. Lightweight- Lithium Ion, Lithium Iron Phosphate, Nickel Metal Hydride, Nickel Cadmium
3. A running time of around 2 hours continuously
4. Dimensions not exceeding requirements
5. Cost as low as possible because of budget
6. Portable Charger that fits into an outlet
This battery meets are first criteria being rated at 12 Volts and 30A rating This battery meets are second criteria being lightweight weighing at 1.3 lbs per battery so a total of 2.6 lbs. The battery running time calculations will be
Which is a little low for our design specifications so we might need to go with another motor cause of the wattage being so high.
This battery meets are third criteria since the dimensions are 4.52” x 1.75” x 1.84” which is in our range. This will closely meet our fourth criteria being low cost. With the battery being $59.99 per battery and the charger being $22.99 each will give a grand total of $165.96. This will meet our fifth criteria as well because it will be able to charge the batteries from any wall outlet. One questions that came up was if we could use one charger for both batteries and I ended up emailing tech support of battery space and they came back with this response:
So from this we will only need one charger where we thought we might need two of them.
Feasibility: Prototyping, Analysis, Simulation
Wheelchair DurabilityIn an online survey asking how many years do people, on average, replace their wheelchairs, 55.26% answered 5 years, and 18.42% said 4  . While these information cannot be cited as reliable and scientific. They certainly represent the opinions of a portion of wheelchair users.
The most desirable time to replace a wheelchair is approximately 3 years. The reason is because, for new wheelchair, insurance companies insure for a period of 12, 24, or 36 months  . And insurance companies have certainly taken a lot of time to research the durability of a wheelchair.
The tire is an important part that is always in contact with different surfaces and conditions. Depends on how users use the wheelchair and the environment the chairs are used in, the frequency of changing tire can be varied greatly. There are 2 types of tires  :
- Pneumatic tire: air tire, provides lots of comfort and traction but also very delicate. Misusing could lead to flat tire, which can occur up to 3 times a month.
- Airless tire: more durable than the pneumatic tire but more physically demanding. Used widely in power wheelchair. Usually lasts 1 year.
Weight Distribution Calculation
Detailed Calculation - PDF
Theoretical ModelTheoretical Model Setup - PDF
Explained Theoretical Model as word document - Word Document
Theoretical Model Worksheet - Excel
Accessing the Wheelchair
In order to determine if the device could fit into the trunk of a sedan style car, measurements were made on a trunk and compared to the anticipated measurements of the device. The above image shows the measurements of a car trunk. The width at the opening of the trunk is a maximum of 42", once inside the opening, there is a width of 56" available for use, for the first 16" of depth, before returning to 42" wide due to the wheel wells in the remainder of the trunk. The overall depth of the trunk is 36". The height of the trunk is 16". This gives overall dimensions of 42"x36"x16" that the device must fit in.
This drawing shows the rough layout of how the device is to work. It is intended that when stored the two bars can fold up parallel to one another. It also shows the assumed dimensions the bars (1" wide, 12" long), wheels (8" diameter, 2" wide), and motor (4"x4"x5.5").
This shows the calculations for the device when it is stored up and completely assembled. By adding the width, height and length of each component in the final assembly it was possible to determine a final dimensions of 9.25"x8"x12". This is well within the area provided with the trunk (42"x36"x16").
Bill of Materials (BOM)
Updated Risk Assessment
Design Review Materials
Plans for next phase
- Decide on a motor and wheel selection, verifying feasibility with the theoretical model then revisit battery, mounting, and other selections to ensure they still work, or revise as necessary.
- Resolve all remaining items from last phase, decisions on microcontrollers, switches, and remaining mounting method decisions (angle of attack, rod length, etc).
- Obtain either Aileen's wheelchair, or find dimensions/view the wheelchair at a local wheelchair dealer.
- Create CAD drawings of the proposed solution.
- Create a schematic showing the electrical wiring architecture.
- Continue updating the BOM and get more accurate cost estimates
- Continue updating risks
- Continue the improved working together as a team and attempt to get all objectives completed in advance of the deadlines.
- Start looking at ways to reduce the cost once a working system is designed.
- Decide early in the phase if we will participate in the Effective Access Technology Conference and if so create a poster for presentation at the conference.
- ↑ http://www.amazon.com/gp/product/B005T57NX8?keywords=junction%20box&qid=1445440226&ref_=sr_1_10&refinements=p_n_feature_keywords_browse-bin%3A2803809011&s=lamps-light&sr=1-10
- ↑ http://www.apparelyzed.com/forums/topic/15157-how-often-do-you-change-your-wheelchair/
- ↑ http://www.apparelyzed.com/wheelchair-insurance.html
- ↑ http://www2.cruzio.com/~yogi/whchair.htm