Final Design

Description of Final Design Solution

Early prototype of final wheelchair design featuring 4-wheel mobility design with linear actuated sit-stand system.

The final design incorporates two systems to address the primary objectives of mobility and a sit-stand feature. The mobility system includes the main outer frame and the four wheel struts mounted to it. Its primary function is mobility, and was designed with comfort and stability in mind for the user. The sit-stand system includes the inner frame that hinges, the cam lock, the comfort components (frontal harness and rear saddle), and the linear actuators and electrical components (power supply, wires, switch). Its primary function is to provide motorized assist that enables lifting and lowering of the rear. It was designed with safety, interface design, and electrical maintenance in mind for the owner and pet in mind.

Mobility System

- - Outer frame. The main outer frame is fabricated from perforated aluminum tubing. Aluminum was selected for its lightweight strength. The perforation allows user to adjust positioning of wheels and comfort components.
  - Fixed back wheels. The wheels are fixed directionally to allow for stable, controlled steering.
  - Swivel front wheels. The wheels are swivel to allow for full range of motion and immediate response to direction change. Angled struts are used in the back to increase stability across uneven surfaces.

The mobility system.

Sit-Stand System (Hardware)

- - Comfort components. This consists of the components that are in contact with Bodie’s body: the front harness and back saddle.
  - Inner frame. The isolated inner frame is fabricated from perforated tubing with a raised back bar for waste disposal, and mounted inside the outer frame.
  - Brake lock. The brake lock is mounted to the inner frame’s pivot axis, and secures the retractable front harness when walking, and releases it when sitting.

Sit-Stand System (Electrical Components)

- - Linear actuators. An actuator is mounted on either side of the frame, each one supplying 667 N (150 lbs) of force with a 10.16 cm (4 inch) stroke. These provide the controlled linear force that lift and lower the inner frame to allow standing and sitting.
  - Battery. Two 12VDC/1Ah batteries supply power to the linear actuators and DPDT switch.
  - Relay switch. A double-pole, double-throw toggle switch is used to control the linear actuators.

Comfort components that are in contact with Bodie’s body.

The sit-stand system (brake lock and switch not pictured).

Design of Key Components

Front Swivel Wheels

Overview

Two front wheels were incorporated into the design of the wheelchair in order to stabilize the cart and remove the issue of uncomfortable weight distribution. The front wheels feature swivel caster wheels with 360° mobility that are mounted on outward angled struts, which provide greater stability particularly over uneven surfaces.

Functional Requirements

- - Support the weight of the frame
  - Cannot interfere with quality and range of mobility afforded by 2-wheel design

Choice Justification

The previous 156B wheelchair is a 2-wheel design that collapses to enable sitting. With this design, the wheelchair does not stand on its own which makes it difficult to lift and load Bodie, and it also rests on Bodie’s body which then requires precise harness positioning to prevent uncomfortable pressure points. As seen below, the 2-wheel design’s forward tilt adds load to Bodie’s shoulders. Even when the frame sits parallel to the ground, half of the frame at minimum is loaded onto Bodie’s front. As seen below, 4-wheel design uses the two front struts to take the load of the frame off of Bodie’s body to alleviate uncomfortable pressure points. It also stands alone which aids the owner in loading and unloading the dog. Swivel caster wheels with 360° mobility offer immediate response to direction changes, and are good for following the steering motion of the back wheels. They are mounted to outward angled struts rather than vertical ones in order to provide greater stability, particularly when running over uneven surfaces.

The inner frame mounts inside the outer frame.

Previous 156B team 2-wheel design applies load to Bodie’s harness.

Mount design that achieves larger rear displacement using actuators with smaller stroke lengths

The frame weight distribution without (left) and with (right) two front wheels.

Linear-Actuated Sit-Stand System

Overview

The linear-actuated sit-stand system is used to assist Bodie in transitioning from standing position to sitting and laying down position. Two linear actuators each provide 667 N force along a single-axis distance (stroke) of 10 cm. They are mounted to a hinging inner frame (see next section Isolated Inner Frame with Brake Lock). The mount design allows the frame to lower by a vertical displacement greater than the 10cm stroke of the actuators. Bodie’s rear weighs approximately 11 kg which was taken to be the minimum force requirement.

Functional Requirements

- - Support the weight of rear
  - Enable controlled vertical motion of rear

Choice Justification

The previous wheelchair model pursued a mechanical sit-stand system that folded into sitting position via brake stops and springs. The issue with this design is that it did not offer easy transition between positions. Bodie exerts visible physical effort and strain to get up. To sit, Bodie must walk backwards to engage the wheelchair’s sitting system. He then “falls” a great height before landing into a resting sit position. The effort required to operate this uncontrolled motion is excessive and exhausting for a feature that was intended for daily use.

In mechanical assist systems, Bodie’s heavy rear requires a large mechanical advantage to resume standing position without external forces. Motors were researched to supply external force that can be integrated into an assist system to provide controlled vertical motion for sit-stand. Electric motors had high costs for the target force range beyond the budget scope. Linear actuators are a less expensive but equally robust and reliable source of constant applied force. Another advantage is linear actuators do not require a microcontroller for motion control. Internal limit switches program circuits to open after the actuator stroke has fully extended or retracted. This key feature eliminates the need to program, wire, and mount an additional electrical component like a mini Arduino onto the wheelchair.

Linear rod actuators of 10.16 cm stroke supplying 667 N of force were selected. In deciding linear actuator specifications for the wheelchair, the greatest restriction was size. As seen in the figure below, smaller actuators were used and mounted angled near the pivot point in order to create a geometric advantage to lowering the back end of the frame with a smaller stroke. Though this increases the necessary amount of force supplied, it enables a lighter, more compact actuator without affecting the price or required voltage supply.

Linear actuated system in up (left) and down (right) positions.

Rocker Switch Control System

Overview

The rocker switch control circuit is the electrical component of the sit-stand system. It consists of a power supply, a linear actuator, and a DPDT switch. The non-momentary rocker switch has an on-off-on switch operation that allows a simple interface: pushing the button either extends or retracts the actuator fully, to transition the wheelchair between sitting and standing positions. The circuit will be wired as in the following diagram:

Basic rocker switch control of linear actuator.

Functional Requirements

- - Activate sit-stand system
  - Provide a simple interface for Bodie to learn and use

Choice Justification

The previous wheelchair model requires Bodie to walk backwards to engage the wheelchair’s folding into sitting position. A problem with this model documented in the previous project report is that walking backwards is a non-intuitive motion for dogs, so training this command was challenging.

A switch still requires training but has a simple interface that requires less strenuous effort from Bodie. The switch also activates the linear-actuated assist system that provides vertical motion of rear in a controlled manner. A tradeoff between an electronic switch and mechanical cue-trigger is that it is not integrated into the frame, and requires mounting in an easily accessible location.

Adding to the usability is the use of a DPDT switch, which was recommended via consultation with industry engineer Frank at Progressive Automations (1-800-676-6123). When the DPDT switch is activated, it closes the circuit. After the actuator fully extends, or retracts, the internal limit switch automatically opens the circuit until it is activated again by the switch. This prevents passive draining of power to extend battery life and also reduces the risk of overheating.

Isolated Inner Frame with Brake Lock

Overview

The inner frame is the hardware of the sit-stand system. It supports the front harness and back saddle, and moves independently of the larger one. This frame hinges at a pivot point near the front of the wheelchair to lower in the back. The brake lock features the lock which controls the retractable straps attached to Bodie’s harness. The lock is located at the pivot axis and pivots with the frame to unlock Bodie’s frontal harness when his rear is lowered. The inner frame and brake lock are complementary features integrated together to allow for independent and total up/down motion in the front and back.

Functional Requirements

- - Support the weight of Bodie and saddle
  - Allow independent vertical motion in front and in back
  - Secure harness in place when user is in wheelchair standing position
  - Release harness when user is in wheelchair sitting position

Choice Justification

Earlier motorized design solutions featured sit-stand systems integrated into the main wheelchair frame. A viable model features a fixed flat-angle frame that is lifted/lowered up/down the back struts with actuators, while the front struts hinge forward to accommodate the shifting height. Results from the hardware testing of this model are detailed below in Chapter 4, but a key finding is that moving the entire frame required moving the front and back in unison, restricting Bodie to either standing (front up, back up) or laying down (front down, back down). The flat frame test revealed the front and back needed to be able to move independently of each other. The figure below shows the relative harness and saddle positions required for Bodie to stand, sit, and lie down.

Brake lock system locking and unlocking retractable harness.

Due to the nature of the brake lock system, precision in spacing is crucial when mounting the brake lock assembly to the outer frame. To avoid interference from other components and ensure that the harness retracts with a consistent motion, the team created a casing to house the assembly with the appropriately placed spacers and guide the retractable leash back into place each time. As seen in the figure below, the leash retracts back through the guide at the top of the casing, and everything else is housed underneath inside the casing.

Positional requirements of harness and saddle

The front harness had to have a single axis degree of freedom: it must move up and down but cannot move forward or backward. One solution considered featured retractable cables (similar to those for clippable employee ID cards) but a concern was its durability of holding the harness in a single-axis, and securing it in place when running. Gate latches are strong, but also bulkier and require string or an additional method to unlatch/latch it exactly when the system went to sit/stand.

The brake lock mounted to a hinging inner frame integrates the two solutions for lowering the front and back. With the brake lock attached and rotating at the hinge, the frame and brake lock rotates downward, unlatching the retractable harness--as seen in the figure below. This enables Bodie the freedom in his front half to either sit (front raises) or lie down (front lowered).

Brake lock casing guides the retractable harness.

Testing of Final Design Prototype

Linear Actuator Load Testing

Sit to Lay Down Test

Uneven Terrain Test (grass and small curbs)

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