16. Applications and Implications

This week I worked on what will I be needing to complete the final project. Here are some questions to answered and other processes important to ensure that the project won't fail (or at least to reduce the failure modes).

Answering Some Questions

First of all, I had to answer some questions to clear everything to be done:

What will it do?

There are going to be two motors impulsing the wheelchair for the user to be more comfortable, which is going to be controlled thorugh a joystick.

Who has done what beforehand?

There are some examples of wheelchar adjustable motors already in the market, such as: the Power Pack Plus, the Empulse R20, and the JQ-B2 Moving Booster (for what I researched and liked the most). From this ones, only one can change the direction of the whole wheel chair.

What will you design?

A system to let the user control the direction of an extra pair of wheels through a joystick, so the user may have a rest from pushing itself.

What materials and components will be used?

• Joystick part: joystick, board, electornic components, case, battery, battery charger.
• Motorized part: motors, wheels, board, electornic components, battery, battery charger, case.

Where will they come from?

• Electronic components, copper board and joystick: university warehouse.
• Joystick case: 3D printed.
• Battery and battery chargers: a local electornic shop.
• Motors: Amazon.
• Wheels: any local hardware shop.
• Motors case: welded.

How much will they cost?

• Battery and battery chargers: $110 MXN (joystick battery + battery charger) and $1,797 MXN (motors battery and battery charger)
• Motors: $1,314.12 MXN (they had discount at the moment)
• Wheels: $190 MXN

What parts and systems will be made?

The parts and systems that will be made are all the board circuits and cases.

What processes will be used?

3D printing for the joystick case, welding for the motors' case (I don't think wood cutted by the router may hold all the torque), and laser cutting for the top of the motors' case (only for it to look cute).

What questions need to be answered?

The connection from the motors to the board.

How will it be evaluated?

If it pushes a wheelchair with a person in it.

Motors

For the motors I had to know how much torque I'd be needing in order for it to support all the weight, so I made some calculations to know that. These calculations are the same written in the Final project section. I considered gearmotors since they already have the gear system to improve the power. To know how much torque I would be needing from the gearmotor I had to make some calculationsb based on a thesis (from where the free-body diagram is too). These calculations were done considering the following information:

• Desired speed (v): 2 m/s
• Max. ramp angle (θ): 6.88° (0.12 rad)
• Considered wheel radius (R): 0.1 m
• Rolling coefficient (Crr): 0.02
• Wheelchair mass: 15 kg
• Considered person mass: 90 kg
• Approx. prototype mass: 10 kg
• Total mass: 115 kg
• Total weight (W): 1,128.15 N

Then, the free-body diagram is the next one:

The diagram showed that the force needed to move the wheelchair up a ramp obeyed the formula F=CrrN+Wsin⁡θ due to the needed component of the normal force and the component from the weigh. First, the calculation of the normal force:

N=W cosθ
N=(1,128.15)  cos(0.12)
N=1,120.03 N
					

Then the required force to move the wheelchair up the ramp:

F=CrrN+W sinθ
F=(0.02)(120.03)+(1,128.15)  sin(0.12)
F=157.542 N
				

Since each gearmotor would be carrying half the weight, the force would also be distributed among both gearmotors, and each would be exposed to 78.771 N. The torque needed from each motor goes as follows:

T=FxR
T=(78.771)(0.1)
T=7.8771 Nm
				

Finally, the power form the motor obeys the formula P=ωT. Where ω is the angular speed:

ω=V/2πR
ω=2/2π(0.1) 
ω=3.1831 rev/s=190.986 rpm=20 rad/s
				

Once the angular velocity is calculated, the power goes as follows:

P=ωT
P=(20)(7.8771)
P=157.542 W=0.211 Hp
				

In conclusion, I need a motor with 157 W or more.

PCB Controlling the Motors

For the project I need 2 PCBs: one for the joystick that will be sending information and another one to move the motors according to what they receive from the joystick. The joystick PCB would be the one from the input devices week, and, since it already works well, I won't need to create a new one. However, the PCB from the output devices week won't be able to support the voltaje and current from the motors, so I need one that supports it.

For this I entered to a trace width calculator and typed the current and other specifications that I'd be workig with. Then the result was that I need paths with a 7mm width. But, as I wasn't able to put paths that thick in KiCad (the paths intersect with other footprints), I will be having 5m width paths and I will cover them with tin.

Testing

I have to ensure that the motors actually move a wheel chair, so I need some testing. I consider testing the motors with actual wheelchairs (there are wheelchairs in the biomedical engineering laboratory). This would be pretty simple as I am only going to connect the extra pair of wheels to the motors (the ones that are going to be used for the project) and put them in the motor base I'll be doing and observe if they in fact move the wheelchair. I don't think there will be any problems because of the calculations I made previously.

Materials Considered

Here is the list for all the materials needed for each part of the project.

Joystick PCB

Here are listed the necessary parts for the joystick to work as desired, omitting the ones not used in by the project. This PCB was done during the inputs week.

Joystick Case

Motors PCB

Motors Case

This case was done during wildcard week.

Motors' PCB Case and Plugs