All of our documents pertaining the detailed design can be found in the DDR_Documents directory.
Engineering Requirements/Analysis/Testing Summary
The following matrix summarizes and ties together the Engineering Requirements, the relevant Analysis to prove the design will meet these requirements and then the Tests to finally verify that the physical design meets the requirements.
(Please see the Systems Design Page for information about how we determined the bump height range and roller speed ranges)
Detailed Design Documentation
(All dimensions meet requirements S8 and S9)
Part and assembly drawings can be found in Drawings PDF Folder
Fabrication and Assembly
See PMS_3.pdf below for an electrical schematic of the wires running thoughout the handle assembly.
Power Management System
- PCB Layout
Although not complete, the layout image above verifies all the components will physically fit on the printed circuit board. The power management system will prototyped first to ensure functionality, ultimately saving money.
Software Design Flowchart
Motor Selection and CharacterizationThis analysis is relevant to S1, S2, S3, S6 and S14
We were able to find a series of motors that met the torque, rotational speed range, size and voltage requirements determined during the Sub-Systems Design phase. The picture and drawing below depict the motor that will be used in our prototype. It is a High Power Pololu Micro Metal Gearmotor with a 298:1 gear ratio.
Before continuing with the motor selection, we were able to estimate the Torque-Speed and Torque-Current curves. This theoretical analysis gave us confidence that the motor would be able to achieve the rotational speed requirements (S2, S3) under the grip load (S1).
Theoretical Motor Characteristic Curve
- We are also able to use the Torque-Speed curve to estimate the torque of the motor at a known speed, especially at speeds further away from stalling. This is where most DC motors are more efficient at converting electrical energy to mechanical energy. This is important because we did not have a small DC motor dynamometer available for the motor testing (described below) and allowed us to use rotational speed to relate to Power consumption instead:
We proceeded to test the motor in order to determine its Speed vs. Power curve. We then used the data to later help prove this motor within our design will meet the battery life requirement (S14).
The rig we used to test the motor was comprised of a motor mounting fixture, a plate to mount the motor, a hub attached to the motor shaft, and an adjustable friction brake applied to the motor hub. Using a PWM generator to drive the motor, the current/power drawn by the motor at various speeds was recorded. The rotational speed was changed by adjusting the applied load (friction brake). It is pictured below.
Motor Speed vs Power Curve
To ensure the motor is able to start from no rotation to the minimum rotation speed under the specified grip pressure, the motor was loaded to the grip pressure and then run for 0.500 seconds. The amount of rotation over the 500 ms was then used to find the average RPM speed. Because this average speed exceeded the minimum requirement, we know that the rotational speed exceeded the requirement before the 500 ms were complete. This gives us confidence that the requirement will be met in our final design and we can proceed. Future testing during MSDII will estimate the actual rotational speeds and over the 0.5 second time period for more data.
Battery LifeThis analysis is relevant to S14
Reasons for selecting battery:
Two Lithium Ion battery cells were chosen to supply our project. With a high energy to size ratio, Lithium Ion batteries were perfect for our high power, compact design.
Dimensions per cell: 69.5mm (H) x 19mm (D)
Voltage per cell: 3.6
Each cell is approximately 30% larger than a typical Alkaline battery, however it has twice the potential and double the current capacity.
Assumptions to calculate Battery Life:
Passive components power dissipation used to bias the integrated circuit were ignored.
When the H-Bridge is supplying the motor with power, it is consuming 40uA supplied by the 5V node, and the Op-Amp is consuming about 94uA, so there power consumption was neglected.
The inverse of the efficiency was multiplied by the power consumption of the two switching supplies to take the inefficiencies of the supplies into consideration.
The worse case current draw without stalling the motor was estimated to be 200mA, which was obtained during our load testing.
Battery Life: About 15 hours at room temperature
Bump Roller DeflectionThis analysis is relevant to S4 and S5
In order to be confident the bump height will not deflect outside the specified range (S4, S5), critical areas were evaluated under static loading with a grip pressure of 5 psi (which is very conservative, as 5 psi is excessive and unlikely during normal use). Both deflection and stress were considered to ensure there are no major concerns.
Fatigue and Deflection of Motor Shaft
We performed stress & fatigue analysis on the motor shaft to ensure that it will not fail under excessive loading. Deflection was also in the analysis, as to meet our bump height engineering requirements(S4,S5). Our motor shaft deflected less than a ten thousandth of an inch, which doesn't hinder meeting our engineering requirement. Also, stress was determined to be safe.
Roller Pin Deflection & Fatigue
We performed theoretical stress & fatigue analysis on the roller pins to ensure that they will not break under loading. Deflection was also in the analysis, as to meet our bump height engineering requirements(S4,S5).Our roller pins deflected less than a ten thousandth of an inch, which doesn't hinder meeting our engineering requirement. Also, stress was determined to be safe.
Total Bump Deflection
By adding the deflection from the motor shaft and roller pins, we were able to estimate the total bump deflection. Our theoretical bump deflection is less than a thousandth of an inch, so there is no concern about realizing our engineering requirement.
Motor Support Plate Buckling
One concern was the possibility of buckling the motor mount plate under the excessive grip. Our results prove that buckling won't occur.
We determined the worst case theoretical handle deflection. The deflection is minimal and the handle assembly is rigid as expected.
Bump Slot Stress Analysis
One of the parts of the design we were worried about is the stress in the bump slot section. We determined that stress is not an issue.
Motor Mount - Tolerance Stack-up
Impact ConsiderationsAfter discussions with SME's, we can make several safe assumptions to avoid over-analyzing and over designing. Assuming the cane is accidently dropped from 3 ft and the total cane weight is about 1 lb, the fiberglass case can absorb the impact energy without significant risk of failure due to the robustness of fiberglass.
Therefore we can continue to evaluate critical components without over designing:
The bumps avoid direct impact due to the size of the ABS collar and the end cap outer diameters. The energy is absorbed by the fiberglass casing and ABS, and therefore, impact is not a risk to damaging the internal components, especially when the lightweight cane is dropped from 3ft
According to the manufacturer, the batteries are able to withstand the impact of a 20 lb mass dropped onto the sample from 24 (+-1) inches, which far exceeds the impact energy we expect even our cane/handle to absorb.
There are 8 screws connecting the handle adapter to the cane and handle, along with epoxy to fill any gaps. Each screw provides more than 100 pounds of holding force, so screw shear out is not an issue. Also, breaking by bending of the adapter cross section isn't likely. The adapter has a much larger cross section and inertia value, a tensile strength half of that of fiberglass, and experiences a lower applied moment in buckling and normal loading cases. It is unlikely that the adapter will break as compared to the bump slot section of fiberglass.
To ensure heat dissipation will not be a concern in the design, we made very conservative assumptions, ran the motor for an extensive period of time and measured the temperature change every 15 minutes.Temperature was measured by infrared reader. If the temperature of the motor was near it's rated maximum temperature, we would have continued with some more in depth analysis, however; the results proved this to be unnecessary:
- Motor excessively loaded: ~57 oz-in at 20 RPM
- Initial Temperature: 75 F
- Insulated: Covered with 1" OD PVC tube
- 6V PWM, 80% duty cycle
Final Steady State Temperature: 82 F
Low current under normal use, even when motor is running, so heat generation is very low.
Temperature will be checked by resistor in Power Management System circuit to protect the battery and system
Handle Weight EstimateThis analysis is relevant to S12 Weight Estimate
Manufactured Cost Estimate & Prototype Budget
This analysis is relevant to S10 and S11Budgets
- 1st sheet is the project cost
- 2nd sheet is the prototype cost
For the prototype cost, we are assuming a manufacture lot size of 10,000 cane handles.
Manufactured cost for the handle is $89.72
Prototype cost is $668.75
In the linked document the following test procedures can be found:
- Bump Deflection Test
- Handle Diameter Test
- Bump Loading vs RPM & Motor Ramp up Test
- Motor Ramp Up Camera View