P19305 Preliminary Detailed Design

Team Vision for Preliminary Detailed Design Phase

Over the Preliminary Detailed Design Phase, Team 19305 will complete the initial rendering of the electric longboard's folding mechanism, submit our grant application to IEEE, maintain open communication and utilize consultation with our guides, and develop expectations for our detailed design phase. Additionally, the initial parts list and BOM will be created and verified by the team. Updates to necessary documentation, criteria, etc. will be made as necessary and preparation for the detailed design will begin.

Prototyping, Engineering Analysis, Simulation

A method to individually calibrate the response curve of the force sensor circuit through software was envisioned via adjusting the op-amp reference voltage using a digital to analog converter (DAC).
In order to verify the effectiveness of such a strategy, a mock circuit (image on the right) was created in LTSPICE. Then a DC sweep for the DAC input voltage and a parametric sweep for a resistor (which substitutes for a force sensor with a changing force on it) were run simultaneously.

The results of the simulation clearly demonstrate that software calibration of force sensor range through usage of a DAC is a viable and effective method.

Folding Mechanism

Initial Design

The initial design firm mechanism works for having two pieces that slide against each other linearly, and then translate.

Version 1

To accomplish this, we first sent out by trying to design a mechanism out of commercial off-the-shelf components. Version one of the system used 4 to 1x1" aluminum tubes with a trolley using eight bearings and one linkage to accomplish this task. The mechanism of the hole needed to fit inside a box which measured 10 inches wide by 15 inches long by 2.5 inches deep. Because of this rather small size constraints, version one of this design was not exactly practical. shafts that trolley bearings rotated on were 2 mm in diameter. It was clear after some initial modeling, that this design would have to be iterated quickly.

Versions 2 and 3

Versions two and three of the design focused on sacrificing with of the rail, to decrease the death of the mechanism into the body. This as a whole would increase the volume inside the board giving us more space for electrical components and batteries. To accomplish this we first considered using rounded bearings inside the channel but found that parts for this style of mechanism were not available commercially. From there we then moved to using V groove bearings and rails.

This mechanism was further simplified by using 3 linkages and rails instead of 4. The resulting assembly was modeled using entirely available commercial off-the-shelf components, and can be manufactured as a proof concept moving forward. This design below was intended to be made out of a 2x10" piece of wood for cost effectiveness. Realistically this is shallower in depth and thinner in width than the actual depth of our board, but it does demonstrate how the mechanism would work.

Feasibility: Prototyping, Analysis, Simulation

Deck Design and Protyping

Initial Sketch and Michelin Mobility Submission

Our initial sketch of the board showcased a few of his features as well as the intended folding mechanism in a coherent design.

Initial CAD Modeling

From there the board was modeled to somewhat parallel the sketches, but ultimately the modeling strayed away from the design aspects that were most important. The model intended to use a deck lid and under tray system for attaching the electronics. This type of system would not work with the folding mechanism so the design had to be reverted back to its original sketches to focus on a unibody design.

Foam Modeling

After drawing a few sketches on paper, we began cutting and foam. A foam model gave us a tangible part and test some of the early ergonomics of the board. The board was thickened to accommodate for internal electronics, and the sculpted for an athletic form factor.

Next Steps

Once this foam model is finished, we will begin modeling it in CAD. Our intention is to build this board as one uniform piece, as a testbed for the electronic systems. Following this we will then build a second board in multiple parts that incorporates the folding mechanism. The intention for this plan is a risk mitigation to avoid the bottleneck of workflow. Since the folding mechanism is relatively complex and can be proven on a different testbed, it makes sense to separated from the design for now with the intention to add it in later. If the folding mechanism does not function properly it can be replaced by simpler mechanisms such as hinges.

Between now and the end of the semester, your team will continue to add detail to your design, which will require more detailed analysis and prototyping to answer feasibility questions. Consider:

Iterative activities to demonstrate feasibility, including assumptions you made in your analyses, simulations, or prototypes.
Have you completed sufficient preliminary analysis or prototyping to ensure that your design is likely to satisfy requirements?
Have you included all usage scenarios in your modeling?

Purpose

Confirm that the selected concepts can deliver functionality defined by the System Architecture.
Define the optimal values of the most sensitive design parameters.
Support the evaluation of your team's concepts with quantitative information.
Make decisions about design drivers.

Instructions

Instructions and EXAMPLE must be deleted before the Preliminary Detailed Design Review AND Identify an owner for this document.
This document will be inspected at all project reviews during MSD1.
At the subsystem design level, feasibility analysis and prototyping may still be relatively coarse. First order analysis, estimation, and rough physical mockups are to be expected. Detailed analysis on critical subsystems or components may be complete.
Select the concepts that are expected to be most sensitive and/or problematic. These are usually concepts that will not be satisfied by COTS components.
Define the transfer function that converts the concept/subsystem inputs into the desired outputs as a function of specific design parameters. This can be done via theoretical analysis or empirical prototyping. it is often useful for two or more team members to work together so that the work is checked and a clear understanding of these models is captured. Get help here from SMEs who should be able to alert your team to potential difficulties and possible solutions.
Use the models to quantitatively specify the design parameter targets and acceptable ranges.
Confirm that the concepts selected and the designs anticipated will generate the desired performance.
Considering the purpose, the team should anticipate potential failure modes associated with construction and use of this document.

Inputs and Source

Engineering Requirements
Concept Selection
Results of preliminary prototyping, analysis, and simulation

Outputs and Destination

A list of Design Parameters, Quantified Targets, and acceptable tolerances
Sensitivity analysis
Refined concept Selection
Drawings, Schematics, Flow Charts, etc.

Drawings, Schematics, Flow Charts, Simulations

Concept Design Development:

Rolling Cam:

Center of gravity of the truck insures that when you release a turn, the cam will return to natural flat resting position.
Flat top surface of cam reduces speed wobbles at high and low speeds
Cams can be tailored to fit specific riding dynamics by changing geometry

Base Plate:

Mounting positions for New and old school mounting patterns.
Adaptable for different hanger mounting angles
Minimal complexity allows for 3-D printing and injection molding

Input and Sour

Selected Concepts
Feasibility Models
System design and interface definitions

Output and Destination

Complete hierarchy of design files from system level down to components
Parts list (DEIRDRE)
Software design that specifies coding requirements
Test plans, including expected performance vs. requirement and any applicable test standards (e.g., ASTM)

Bill of Material (BOM)

A link to the live Parts List and Bill of Materials can be found here.

Test Plans

Calculation of BLDC Motor Parameters:

In order to model a BLDC motor with good accuracy, the armature self inductance per phase, the armature resistance, the moment of inertia of the rotor, and the viscous friction coefficient must be known. The back emf constant is already known as it is the inverse of kv which is given and the torque constant is also known as it is equal to kv, a necessary result of the conservation of mechanical and electrical energy. The armature mutual inductances per phase, eddy and hysteresis losses, as well as the variation in inductance with respect to rotor phase angle are beyond what is necessary for the given project goals. As such the following test plan to materialize the motor(s) will be implemented upon acquisition.

Armature resistance will be determined via usage of an ohmmeter applied to both ends of the Armature.
A small AC voltage supplied by a function generator will be applied to the armature, the current and voltage will then be measured using an ohmmeter. These measurements will be used to calculate the impedance of the armature, which will, when combined with the armature resistance found in the previous step, enable discovery of the armature's reactance. Inductance
=reactance/(2*pi*frequency), where frequency in this case is the frequency of the AC signal that was applied to the circuit.
The moment of inertia for the rotor can be found via suspending the motor on a pendulum made up of two wires by its rotor, as shown in the diagram below. The formulas at the bottom of the image are the manner by which the rotor moment of inertia will be obtained.
Current is directly proportional to torque in a DC motor. Torque is also inversely proportional to the motor speed. At full speed with no load, the torque and thus the current should be zero. Thus if the current through the armature is measured when the motor is running at full speed, unloaded, the measure value represents the current generated by the torque needed to overcome friction. Solving for the viscous friction from there is straightforward.