Backpack Retriever (University of British Columbia)

 Manmohan Daid, Nija Devadas, Eric Moutal, Taruna Thawani, Jensen Yiu


Electric wheelchair users with limited range of motion often struggle to access their backpacks.  The current market does not offer any inexpensive products to help with this issue. The vision of the design team is to create an economical and easily available mechanism that allows these people to effortlessly and independently access their backpacks. The final design accomplishes this by using a 4-bar mechanism that can carry up to 15lbs and it can be easily controlled using a toggle switch. This mechanism can be easily attached to multiple models since the mounting plate is mostly supported by the U-bar present at the back of the chair. Though the biggest advantage of this model is that it does not increase the overall footprint of the wheelchair, hence the users can continue with their daily routines without any trouble.


The Tetra Society of North America in Vancouver, in its ongoing service of providing assistive devices to people with disabilities, has identified a need for wheelchair users to have easy access to the belongings in their backpacks. There are approximately 40,000 electric wheelchair users in Canada and the US (H. Stephen Kaye, Disability Statistics Center: Wheelchair Use in the United States, 2002) who might potentially benefit from this mechanism. This design will create potential economic benefits while improving quality of life for people with disabilities. The project will be designed to assist people suffering from multiple sclerosis, cerebral palsy, partial paralysis and any other disorders limiting motor skills.
Problem Statement

The vision of the design team is to create a mechanism that allows electric wheelchair users with a limited range of motion to reach the contents of their bags independently and effortlessly. This retriever will move a backpack from its initial position of storage behind the wheelchair to a position that is within the reach of the wheelchair user as required.

Design and Development

The main features of the design have been described below.

4 Bar Mechanism

The mechanical arm that transfers the backpack from the back of the wheelchair to the user is designed using the concept of the movement of a 4 bar linkage system. The movement of the 4 bar linkage enables the arm to carry the backpack from the back to the armrest of the chair. Our design shortens the distance the user needs to reach, so it greatly improves the accessibility of several wheelchair users to the contents of their backpacks.
The mechanism has a height adjustable mount for the bag which is convenient to a variety of users who have different preferences. The 4 bar mechanism has a mirror-image design, and this means that the 4 bar mechanism would work for both right-handed and left-handed users.

4 Bar Mechanism


Gears connection between the Motor and the 4 Bar Mechanism


Magnetic Solenoid Latch

The purpose of having a latch in our mechanism is to prevent the 4 bar mechanism from moving freely when it is at the retracted position behind the wheelchair. One advantage of having a gate latch is that the 4 bar mechanism automatically locks itself in place when it is in the retracted position.
The magnetic solenoid is used in order to open the gate and unlatch the mechanism. The solenoid is powered by electricity, and thus, enables the user to easily manipulate the position of the design safely.

Compact Mounting Design

All major components are mounted on a single metal plate, and this will be extremely easy for both installation and removal. The function of the mounting plate is to support as well as position the four bar mechanism, the locking mechanism and the motor relative to each other. This compact design greatly reduces the space required for installation, and thus making it possible to install on different kinds of wheelchairs. Furthermore, the design does not increase the overall footprint of the chair which is highly advantageous since it will not restrict the movement of the user through doorways and tight corners.

Electrical System

The electrical system is designed in way to minimize the complexity of the control interface, while ensuring the safety of the user and the surrounding. Our mechanism is controlled with a single switch. A single switch design greatly reduces the difficulty of the control required to operate the mechanism, so this would allow users with different capability and flexibilities to easily control the device. The design includes an easily removable battery that can be charged in an external charger. Two batteries are provided so that the user can always have one battery charged and perform a simple swap when needed.

Working of the Design
Initially, the user has his backpack hung up on a hook on the four bar mechanism that is sitting up against the back of his wheelchair. The design is activated when the user flips the toggle switch. This triggers the solenoid which pulls up the solenoid rod and opens the gate latch. Simultaneously, the motor is started and the torque is transferred through a spur gear power transmission system to the four bar mechanism that is now unlocked and free to rotate. The trajectory of the four bar mechanism has been carefully chosen such that the backpack is displaced from its initial position at the back of the wheelchair to a position at the side of the user’s wheelchair. The user is then able to access the contents of his backpack. After he has completed his use of the backpack, the user can then flip the toggle switch in the opposite direction which will reverse the direction of the motor and return the load to its initial position and gets locked into place once more.

Backpack Mechanism - Front View

Backpack Mechanism Overview


The tests conducted on the mechanism help ensure that the specifications of the design meet the client’s needs, the requirements and evaluation criteria set by the team. The results of the tests have been provided below:


Test Results

  • Maneuverability: Easy movement of chair through tight corners and doors
  • User Interface :The client can easily control the mechanism
  • Stability : The addition of the mechanism to the chair does not affect the overall stability and functionality of the chair


Test Results

  • Carrying Capacity : Maximum load was found to be 15 lbs
  • Process Time : Found to be 10 seconds per complete cycle (extend and retract)
  • Durability : Mechanism provides over 6 months of operation
  • Battery Lifetime : Theoretical battery life was found to be 171 complete cycles
  • Noise Level : Average noise level was found to be 70 dBA

Further details regarding the test procedures and results can be found in the “Test Protocol” document.

Discussion and Conclusion

The mechanism performs as expected except for a failure to take into account that the deflection of the bars would move the gears away from each other and cause unmeshing/skipping. This has become the limiting factor in the load that can be transferred by the mechanism. Other aspects of the design can satisfactorily support and/or move the load. Hence, we suggest that the user typically carried no more than 10 lbs in his bag when using the backpack retriever.

The battery life provided by a single charge is more than sufficient for multiple days’ worth of use. The user would not need to worry about running out of battery power as long as they occasionally charge the battery. Moreover, the system is provided with two batteries and an external charger which allows the user to swap batteries when necessary and keep one battery fully charged at all times.

Other performance characteristics seen in the tests are listed in the table below and compared to the satisfaction curves that were drawn up based on research and the client’s indicated preferences.
Evaluation Criteria Design Performance (at Recommended Operation Levels) and Corresponding Satisfaction Level

  • Carrying Capacity of 10 lbs  resulted in a 60% satisfaction
  • Cost of 566$ resulted in a 70% satisfaction
  • Design Simplicity of 2 mechanisms resulted in 90% satisfaction
  • Life Cycle of theoretically infinite life resulted in 100% satisfaction
  • Noise level of 70 dBA resulted in a 30%
  • Process Time of 10 seconds resulted in a 90% satisfaction
  • Adaptability of design to be retrofitted onto at least 10 wheelchair models results in a 100% satisfaction
  • Weight of device of 20 lbs resulted in a 60% satisfaction

It should be noted here that the prediction for the life cycle is obtained from simulated tests on a software and so do not accurately represent that actual life cycle of the device. This suggests that the satisfaction level of this criterion might be lower although the team was unable to test the physical prototype due to time constraints.
However, on most other criteria, the team has managed to meet reasonable satisfaction levels.


H. Stephen Kaye, T. K. (2002, May). Disability Statistics Centre. Retrieved October 2013, from University of California (UCSF) Medical Centre:

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