Blog 2 - G10 UH Capstone

           Blog 2



The popular motorized wheelchairs from Action Trackchair have a common shortcoming across most of the models except their newest ones; the motors are not waterproof. For this reason, the company recommends its users to not drive the wheelchairs through water that is 8"-10" (inches) deep and not to use the chair in heavy rain. A waterproof casing around the motor would allow these wheelchair users to go out in the rain, go into shallow water, and fully experience nature. The proposed solution to this problem is to design an enclosure that seals the motor while providing adequate cooling to the motor to prevent overheating. 

The motor, that powers the wheelchair, is a high-torque motor which means that it produces a lot more heat than a low-torque motor due to an increase in the current needed to drive the motor. Optimally, the internal motor temperature should not be more than 10% of the absolute ambient temperature to ensure the motor is cooled properly. A preliminary condution and convection heat transfer analysis will be performed which would allow the team to understand the amount of excess heat that needs to be dissipated to ensure that the motor remains below a certain temperature. Based on this value the team can evaluate potential cooling mechanisms and choose the one that cools the motor the best while being within the other constraints. The main challenge will be providing cooling to a sealed motor since sealing a motor would trap the heat inside the enclosure. This means that the material of the casing must be able to conduct heat effectively which restricts what materials can be used. The casing must be able to cool the motor sufficiently, whether that is through active cooling or passive cooling, when exposed to air or when submerged underwater. 

Another constraint is that the proposed solution must fit on the wheelchair and allow any connections such as the driveshaft or electrical cables through the casing without affecting its other functions. The motor is mounted directly under the seat so there is little headroom above the motor. However, there is ample room to the bottom and sides of the motor to allow the footprint of the motor to be expanded. The motor beneath the seat of the wheelchair is shown below in Figure 1. 




                                                                       Figure 1: Motor Placed on the Wheelchair 

To quantify whether the casing of the motor is impervious, G10 will abide by an industry-standard IP (Ingress Protection) as shown in Figure 2 below that dictates the numeric levels of water resistance. The casing will provide a waterproof rating of (at minimum) IP X7 water resistance (Figure 2 below shows what the digit 7 represents) while ensuring that the motor will be able to function both when exposed to air and water. Thus, the motor with the casing mounted to it must be able to withstand being submerged in 1 meter of water for a minimum of 30 minutes. This rating was chosen because IP X7 was the minimum rating that tests if the device can survive complete submersion underwater. The first number of the IP rating was ignored because dust resistance is not a factor of concern in our project. Since the maximum depth the casing must withstand is known, the team can calculate how much hydrostatic pressure the seals used must be able to withstand to ensure that the casing remains watertight. This information would be used to determine the best method to seal the device based on how much pressure that method can withstand. As far as pressure is concerned, we need to ideally be able to withstand a minimum of 10 kPag of pressure with our enclosure.

                                             Figure 2 - Ingress Protection (IP) Ratings Guide 


Additionally, the team also aims that the casing does not negatively impact the torque output of the motor beyond a certain limit. Ideally, the motor should output the same or a slightly lower torque however, out of all the constraints, this constraint is a lower priority with the water tightness of the casing and the cooling mechanism preventing overheating of the motor being much bigger priorities. 

In ensuring safety, all electrical connections will be insulated against open circuits during testing using various electrical mechanisms to prevent current overload. These mechanisms could be fuses, resistors, and temperature-controlling devices. The table summarizing the major constraints of the project is shown below.


Constraint

Validation Procedure

Measurement Tool

Motor needs to stay cool throughout operation

Using temperature probes hooked to output device motor and casing surface

Temperature probes (RTD, etc.)

IPX7+ waterproof

Using test devices to check for humidity/water content

Arduino Humidity Sensor/Water Leak Test Strips

Dimension of casing

Motor will fit within the ≈ 5 cm of space between the seat and the motor

Ruler, Tape Measure

Motor must not sacrifice power when waterproofed

Output torque and input current will be measured when motor is free and when casing is mounted

Ammeter, Continuous Torque Meter

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