Preliminary Detailed Design
Team Vision for Preliminary Detailed Design PhaseDuring this phase we sought to:
- 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.
During this phase we managed to:
- Select all major components and verified they would
fit our needs, as well as revising previous battery
- Still working on verifying with vendors the costs
- Analyzed different scenarios looking at costs vs. degree to which engineering requirements are met.
- Obtained Aileen's wheelchair
- Created CAD drawings showing the parts needed to be manufactured.
- Created electrical wiring schematics
Mechanical System Design
At the moment, the feasibility for our decided motor is ultimately dictated by the fact that similar products such as the Smartdrive MX 2 are using similar rating equipment (a 250 Watt motor at 36 Volts) at a lighter weight point and achieving their goal. As such, we are confident that choosing a motor within the same ballpark will at least achieve the baseline goal, if not meet all our demands.
The theoretical model is still being worked on, as its previous iteration was generating inconsistent results. The initial one was deemed as the 'static problem,' finding the limiting case with regards to frictional forces. The second model which we set our motor decision against took the no slip condition to be a given, and a third proposed by Dat takes into account the frictional forces in a more moment based analysis. During the next phase, we will consult in more detail for these models.
Using the results from the model an excel spreadsheet was set up to input parameters of different motors and quickly determine if the motor would meet our needs.
The motor selected needs to be able to provide enough torque and speed. After analyzing multiple motors, the model we chose is Teknic CPM-MCVC-3432D-RLN. It is a brushless motor, so the lifespan is better than DC brushed motors. The continuous torque for the motor is 120.9 oz*in, and by using a 7:1 gearbox, the amount of torque will be increased to 846.3 oz*in, which is more than the calculated torque needed to keep the wheelchair from rolling down an incline of 6 degrees. The maximum RPM of the motor is also reduced from 1200 to 171.4 due to the gearbox. A wheel of 8 inch diameter rotating at 171.4 RPM has about linear velocity of 4.1 mph.
The decision regarding the gearbox has not been an easy one. The project requires a system that is simple and inexpensive but rugged, durable, quiet, and efficient. Finding the balance between these factors is an ongoing process. Currently we have gotten solid quotes from Designatronics Inc and CGI. Both of these companies quotes significantly exceed the amount originally budgeted for the gearbox. Onvio is slow to provide an accurate quote but has stated that their gearbox would be much closer to the price range. The current plan of action is to present the ideal situation with several backups to determine the best function for the cost.
For the motor assembly to be able to do its job the attachment parts need to provide a certain range of freedom of movement. The assembly should be able to move freely in vertical direction with a limited amount of movement when rotating about that axis. In order to provide the necessary systems that already incorporated similar systems were researched. Requirements for this system were for the parts to be off the shelf, easy to use, and rugged due to the conditions that the system would encounter. The system would also be easily adjustable to different lengths this would mediate enough of the risk in choosing this design to allow flexibility in the wheel and motor assembly in the future. Another factor in the choosing the system was the limited budget available another reason for working with off the shelf parts. To provide for the requirements of the system previous knowledge of the methods used to attach tractor implements was utilized. A part known as a top link is used to control the pitch of the tractor implements. Here is a short video on how top links work: https://youtu.be/6wA--SvjcFE. Top links also come in a variety of sizes which allows for some additional flexibility in the design. Due to their use in outside farm work these parts were determined to be ideal to attach the motor assembly to the wheelchair.
For the parts that attach the top link to the wheelchair and the toplink to the motor assembly exact dimensions have not been firmed out due to other constraints still needing to be met. However, it has been decided that they will most likely be made from 1/8” steel plate which can be found at local hardware stores. This material was chosen for its cost and strength. The idea is to weld several pieces of the plate to achieve the desired shape and then drill holes where necessary. Based on the current results from the gearbox and motor analysis the plate steel will also allow for flexibility to account for several designs pending the final budget of the project.
Below is a preliminary print. Unfortunately without better dimensions from the parts that have not been ordered yet it is not possible to have exact prints at this time
Regarding the omniwheel, only one major option really presented itself, the 8” version provided by Vex Robotics. Vex Robotics parts are typically used in applications ranging up to 100-120 pounds of total weight. Considering that our system will only be bearing ~30 pounds, we believe it to be structurally stable. This also comes as relatively easy to buy and replace should the part wear out in the lifetime of use. At 8 inches in diameter, to achieve a speed of 5 miles per hour, we would need roughly 210 rpm. At 6 inches, this number jumps to 280 rpm. As such, we believe that 8 inches is the smaller end of diameters that we could allow in design.
Included with the omniwheel would be the manufacturing of a hub similar to a Versa hub also supplied by Vex Robotics. This would allow the wheel to attach to the output shaft of the motor gearbox system. Minor physical modifications are planned for the omniwheel to accommodate this manufactured hub, namely the grinding down of the nubs on the inner radius of the wheel. Relative to the stresses involved, we don't believe these pieces to be critical at this time.
Electrical System Design
Things looking for when selecting the battery:
1. Cost: Minimum as possible
2. Voltage: 24V or 36V
3. Max Discharge: 4 Amps or greater
4. Lifetime for our application: 2 hours or more
5. Lightweight: 15 pounds or less
6. Dimensions fit pouch area: 15.5" x 16.5" x 18"
This battery is an ideal choice because it is rated at 12 Volts therefore putting two of them in series will give a voltage of 24 Volts. It has a max discharge rate of 4 Amps and its expected lifetime is 2 hours. The two batteries combined will weigh 3 pounds and the total dimensions are 2.68" x 7.26" x 2.21". There is a charger that can be bought that goes with this battery that will plug into an outlet. The total cost for both batteries will be $128.00 which is one of the lowest costs we were able to find. However this battery is coming from China which is a risk to consider therefore this excel sheet was made comparing different battery options with the requirements looking for in our application.
The Teknic motor has a built in motor controller, so we can connect the power source, or two 12V battery packs in our case, directly to the motor. Teknic provides a software for doing initial setups for the motor. The motor has 6 inputs besides the ones for power and 2 outputs. The 6 inputs are divided into 3 sets: Input A, Input B, and Enable. The software allows us to tell the motor how to use the inputs coming from A and B. The Enable is for determining whether the motor should be enabled or not, and it will be connected directly to one of battery packs because the motor reads any signal from 5 to 24V as logic high.The configuration chosen is to have A determining the direction, and B determining the speed based on the duty cycle of a PWM signal that is generated by a PWM controller.
Here is a simple wiring diagram from Teknic's website.
The PWM controller will be powered directly from one of the battery packs, and it can output PWM signals with magnitude the same as the voltage supplied to the device. The PWM controller has a knob/dim switch that is a potentiometer, and by turning the knob, the duty cycle changes, which will affect the RPM of the motor.
Cost ComparisonsThe mounting methods and controls have little impact on the cost and ability to meet the requirements of the overall system. The main factors are the motor, gear box, and batteries. In order see these tradeoffs, comparisons were done changing the motor and battery selections, then examining the effects it would have on the output of the system.
Updated Bill of Material (BOM)
BatteryBattery Test Plans
MotorMotor Test Plans
Engineering RequirementsEngineering Requirement Test Plans
Design Review Materials
Plans for next phase
- Decide on all remaining components and finalize our BOM.
- Begin ordering some parts (top links, motor) that will definitvely be used.
- Complete CAD drawings
- Make a modest proposal for increase in budget.
- Continue researching backup parts to try and reduce the cost.
- Continue updating risks.
- Continue updating previous documents as more information becomes available.
- Look for conferences our work can be submitted to.