Autonomous Vehicle Robotic Wheel Drive

The purpose of this project is to create a compact high torque/rpm robotic wheel drive for Bastian Solution's autonomous warehouse robot.

=Problem Definition=

Background
Bastian Solutions has a current WDS (wheel drive system) that uses 30% of the vehicles length and 50% of the vehicles width. The axial length of their current design is approximately 8.8 inches, with the motor contributing to most of the assembly's axial length. It is desired to decrease the WDS assembly to at least 4.4 inches so that other features can be added to the vehicle. For their current motor to meet the required torque/rpm and power conditions, it uses a 4:1 planetary gearbox. A planetary gearbox was chosen because it has advantages over other gearboxes that work well in this application.

Pertinent planetary gearbox advantages:

 Naturally compact, but can supply high reduction ratios (reduces WDS axial length, most motors cannot supply the required torque without modification)   Inline or coaxial design allows input shaft to be inline with gearbox (saves radial space)   Generally have low backlash (vehicle must move with precision)   The nature of their design distributes internal gearbox loads more equally (WDS undergoes high loadings)   Long life and efficiency (gearboxes are expensive to replace and they add to mechanical energy loss)</li> </ul>

For them to add more features and to further optimize the vehicle design, they must decrease the volume of the WDS. They require the WDS to be capable of meeting two primary torque/rpm conditions as well as moving a payload up to 100 lbs.

Specifications
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Volumetric Constraints

The full WDS assembly must not exceed 4.4 inches in axial length nor 8 inches diametric length.

=Iterative Design=

Sub-Systems
Our first step was to break the WDS into 3 sub-systems:  Motor</li> </ul>  Gearbox</li> </ul>  Output Shaft</li> </ul>

Concepts Considered
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Design Procedure For Prototype 1
The biggest constraint is the volumetric constraint of about 8 inches in diameter by 4.4 inches axially. The strategy is to find the thinnest motor who's diameter is at least half an inch smaller than the 8 inch diametric constraint. The motor has to be thin enough to leave room for the gearbox and bracket, while also being capable of providing a reasonable torque/rpm.To figure out what would be a "reasonable" torque/rpm from the motor, one of our members performed an analysis. The analysis showed the different torque/rpms the motor would need to provide using different gear reduction ratios to meet the torque/rpm requirements. Using this method allowed for the gearbox ratio selection process to be easier. The ring gear's outer diameter was constrained to 6 inches to allow for ~2 inches of ground clearance. The torque/rpm vs. gear ratio table and the maximum diameter of the ring gear were used to maximize the pitch diameter of the sun gear to increase its load bearing capabilities.

Dimensional Analysis
Our torque and spatial requirements for our motor led us to investigate pancake-servo motors. Three brushless series pancake-servo motors from three different companies stood out: Printed Motor Work's motors, Koll-Morgen’s motors, and the Thin Gap's motors. Pancake-servo motors have a very high torque constant and a relatively low motor constant which allow for high torque at low voltages. This combination was essential for our needs. We found that a space constraint of less than 7.2” in diameter (referenced to our axle) would be critical to meet our space constraints, as well as an axial length of less than 3.4 inches for the motor and gearbox combination.

Power Analysis
To meet the power constraints, the team created a table in Excel that approximates electrical power required for a given torque, speed, and gearbox ratio. The inputs of the table were the motor constants, mechanical and electrical restrictions of the motor, and the power restrictions from the client. The table used the below governing equations:

$$I=\frac{(T+B\omega)}{k_t}$$,  $$V=\frac{(T+B\omega)R}{k_t}+k_e\omega$$

The selection process involved recording a motor’s specifications into the table and then iteratively changing the gearbox ratio until the motor met every requirement. If the motor didn’t meet all of the requirements within a gearbox ratio range of 2:1-6:1, then a new motor was selected and the process repeated. Eventually, after many iterations, two motors became prime candidates; the TG5152 and the TG5153.



Thermal Analysis
The next step was to perform a parametric study on the heat dissipation (W) of the motor in relationship to power supply and various torque and speed combinations. An electric motor will dissipate heat due to resistance in the coils and damping from lubrication. Considering the gearbox and bearings, the damping coefficient, B, was estimated to be 0.001- 0.003 (Nm∙s)/rad. A Matlab script (below) was written to produce the heat dissipation due to power-loss.

The input values for the script were k_t, k_e, R, and B. To calculate the power-loss, the equations for Voltage and Current were used above along with the below electrical and mechanical power equations.

$$P_E=VI$$

$$P_m=\tau\omega$$

Casing design


Spatial Analysis

For our gearbox design to meet our spatial requirements we need to design our casing to be as slim as possible. We are hoping to incorporate our mounting bracket into our gearbox design so that we can eliminate the need for more bearings and space usage in our final assembly.

Thermal Analysis

The high torque and RPM operating range of our gearbox means that we will need to consider the thermal characteristics of our gear operation. Most planetary gear trains produced today are over 95% efficient, however that does mean that a certain portion of mechanical energy is translated into heat caused by frictional losses. Due to the prototype nature of our design we will simply estimate our inefficiency in order to determine how much energy must be dissipated away from our gearbox. If not managed, the heat generated by the gearbox can lead to damage to our gear teeth and ball bearings, severely reducing the life span of our gearbox. We are looking into using aluminum for the gearbox casing as this gives us much higher thermal conductivity and helps cool our gears and bearings.

Manufacturability Analysis

In order to deliver a cost-effective design we must simplify our part geometry and create a design that can be produced quickly and efficiently. To accomplish this we are focusing on a design that minimizes CNC tool changes and allows for use of larger cutting tools to reduce time spent fabricating. Using aluminum will also speed production as it is a soft metal and will process quickly on a mill for production.

Carrier Plate Design


Fatigue Analysis

Our design must be durable and not allow the carrier plate to crack after years of abuse. To accomplish this we will design for infinite lifespan with a loading safety factor. The primary concerns in designing for this is how we mitigate stress concentrations at the planet gear pins and our output shaft. We also want to minimize weight and rotational inertia so we can maximize our gearbox efficiency.

Material Analysis

Due to the reversed nature of our loading we need a fairly ductile material that won’t crack due to repeated loadings in multiple direction.

Manufacturability Analysis

Low cost and speed of production are critical for this piece, ideally we can create this from a flat cut profile to reduce fabrication costs.

Prototype 1
Prototype 1 features a TG5153 Thin Gap motor and a single stage custom 6:1 planetary gearbox. Its dimensions are 4.5L x 6.1W x 6.1H inches. The neck of the gearbox is designed to accommodate two ball bears to absorb a radial load of 117 lbs. The TG5153 is a high torque density brushless DC motor capable of producing 3.5Nm of torque, and 20Nm with the gearbox. The dry weight of the gearbox is 8.6 lbs which is 30% lighter than Bastian Solutions current gearbox. {| class="wikitable"

Validation
For testing procedures and results refer to the Archives.

Validation


=Team Members=

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