P16007: Motor-Assisted Wheelchair

Subsystem Design

Table of Contents

Team Vision for Subsystem-Level Design Phase

During this phase we sought to:

During this phase we managed to:

Mechanical Subsystem Design

Battery Casing/Box

There are some requirements regarding the battery case:

Judging the requirements and options, this case/box was chosen [1] :

public/Subsystem Design Documents/BatteryCase.jpg

Mounting Equipment/Equipment Placement

Electrical Subsystem Design

Motor Calculations/Decisions

Battery -> 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.

http://www.electricscooterparts.com/images6/MOT-B24400.jpg Motor

http://www.electricscooterparts.com/images/MOT-K24300WIRING.jpg 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/Decisions

To calculate the Maximum Motor Amperage one must use the formula

public/Power Equation.png

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:

public/Power Equation1.png

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:

public/Max Amperage Equation.png

To calculate the running time of the battery and motor combination the equation below calculates that:

public/Running Time Equation.png

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

We have decided on the following battery: public/Battery Chosen.png

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

public/Running Time Equation1.png

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: public/Email of Battery Company.png

So from this we will only need one charger where we thought we might need two of them.

Feasibility: Prototyping, Analysis, Simulation

Mechanical Feasibility

Wheelchair Durability

In an online survey asking how many years do people, on average, replace their wheelchairs, 55.26% answered 5 years, and 18.42% said 4 [2] . 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 [3] . 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 [4] :

Weight Distribution Calculation

Wheelchair External Force Free Body Diagram

Wheelchair External Force Free Body Diagram

Detailed Calculation - PDF

Theoretical Model

Theoretical Model Setup - PDF

Explained Theoretical Model as word document - Word Document

Theoretical Model Worksheet - Excel

Accessing the Wheelchair




Stored Dimensions


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)

public/Bill of Materials.png

Risk Assessment

Updated Risk Assessment


Design Review Materials

Plans for next phase

Team Vision

Team Vision

Individual Plan


  1. 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
  2. http://www.apparelyzed.com/forums/topic/15157-how-often-do-you-change-your-wheelchair/
  3. http://www.apparelyzed.com/wheelchair-insurance.html
  4. http://www2.cruzio.com/~yogi/whchair.htm

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