Powered VR Shoes

[delta-9]

**broken link removed**

I am currently researching parts to make some powered shoes.
The motors and flexible drive shafts, need to be able to move my weight, 63kg on a hard floor.

**broken link removed**
I will be ordering two of these flexible drives. do you think they will work okay?

What motors would be best for this application?
Anybody with experience with motors or mechanical systems. with advice on components I'd much appreciate it.

 

[dknguyen]

Brushed DC motors would be the easiest to work with since this is a stop-start application. For additional complexity you could go with brushless DC motors instead but for a project like this you probably have your hands full with other issues and brushed DC motors are more straightforward and simpler. Brushed motors do wear more and have lower power density than brushless motors, but if your goal is to get something that works don't worry about brushless for now...you can always swap the motors and motor driver later.

I do not think the typical flex shaft will work for your application. The forces on a rotary tool are high speed, low torque and your application requires high torque, low speed so your flexshaft would break.

This is the strongest flexshaft I know of:
**broken link removed**
and I don't think that one is strong enough either.

I would maybe suggest figuring out (guesstimate calculations based on weight, coefficients of rolling friction, and your desired diameter and wheel material on the particular floor) how much torque and RPM you actually need to determine the power. The main reason for a flex shaft is probably so you can use a motor too large to be mounted on the shoes but if your wheels are very very small so a reasonable linear speed could be achieved with low torque high RPM, you might be able to use it as a form of direct gear reduction and do away with the flex shaft. I'm guessing you could stick a reasonabley powerful motor on the shoe and keep it about the same weight as a steel toed boot.

 

[delta-9]

I understand, I've not found much to order online. If the rotary tool cables wont last It's not worth me spending money.
The prices here are just too much. I could spend £300 max for two cables
**broken link removed**
my only other option is shoe mounted motors.

 

[dknguyen]

Brushed DC motors would be the easiest to work with since this is a stop-start application. For additional complexity you could go with brushless DC motors instead but for a project like this you probably have your hands full with other issues and brushed DC motors are more straightforward and simpler.

I do not think the typical flex shaft will work for your application. The forces on a rotary tool are high speed, low torque and your application requires high torque, low speed so your flexshaft would break.

This is the strongest flexshaft I know of:
**broken link removed**
and I don't think that one is strong enough either.

I would maybe suggest figuring out how much torque and RPM you actually need to determine the power. The main reason for a flex shaft is so you can use a motor too large to be mounted on the shoes but if your wheels are very small you might be able to get away with shoe mounted motors and omit the flexshaft.

I understand, I've not found much to order online. If the rotary tool cables wont last It's not worth me spending money.
The prices here are just too much. I could spend £300 max for two cables
**broken link removed**
my only other option is shoe mounted motors.

Then I would just go for the shoe mounted motors. It won't be a matter of the rotary tool flexshafts not lasting very long, they'll immedietely break. Stronger flex shafts are also stiffer so you may not even want one. As a user, I think I would just have a 1lbs motor on my foot with electrical cables going to my feet over a flexshaft which stiff, bulky, and sensitive to mechanical orientation. Not to mention loud.

Unless....hmmm...do you know of a small child to test things on? lol. They weigh a lot less.

 

[dknguyen]

With 20mm diameter rollers, and a coefficient of rolling friction of 0.3 (which is quite conservative, it should be less with hard wheels on a hard smooth level floor), I get ~5N-m of torque for 163kg.

At 2m/s walking speed, you would need ~1900RPM. 1000W

Hmmm...that doesn't seem doable on a shoe mountable brushed DC motor (a cheap one anyways). But that's also a continuous rating, and when walking the motor isn't spinning continuously. You might have to go brushless for the power density.

It also looks like you'll need to gearbox on it anyways to gear it down even with small wheels.

This is the size of motor you can expect:
https://www.scorpionsystem.com/catalog/aeroplane/motors_1/s-30_v2/SII-3032-690KV/

But you'll need to keep in mind that you need to find a matching gearbox for whatever motor you choose and gearboxes aren't as interchangeable (or as cheap and easily available) as one would like.

 

[dknguyen]

I've been thinking about it some more and are you sure you need to move your own weight? I don't think you do. That greatly reduces torque requirements. After all, the whole point isn't to move the person, but to allow the foot that the person is pushing off of to move so that the person doesn't move forward. All you need is enough force to overcome rolling friction so that it's perceived that there is zero friction when pushing off the shoe. I don't know how much this force is but the fact that it's super hard to walk forward in roller blades indicates that much less torque is required. I think that's why the the flex shaft becomes viable. You're not trying to move the person. You're just cancelling out the rolling friction of the shoe. The rest of the rolling force comes from other person's being off-center-of-gravity and pushing on their foot.

Just curious...how did you plan on dealing with the fact that people don't step with their toes pointing straight forward when walking? Or that the foot rotates as you push off of it? It almost seems like a shoe with balls rather than wheels and drive X-Y rollers and a brake would be more natural. You would also need also need multiple forces gauges around the foot so you could know how the person's weight is distributed on each foot so you know which foot is on the ground and which direction the shoe has to move to counteract their motion. Whenever one foot measures zero force you know that the person might be attempting to push off on the other foot. When the deadband is exceeded (when the person is off-center and falling in a certain direction) you move the planted to shoe to counteract that motion. I guess the tricky part is to know how much force differential on the foot indicates that they are off balance since I am able to balance on leg with the weight distributed on different parts of my foot. Also with balls instead of wheels, you can side step.

Maybe you don't even need a motor. Just really good slick wheels and brakes that fire in accordance to the data from the force gauges should mostly keep you in one place. After all, the whole point is make the planted foot slide so in theory if you had a zero friction shoe, you wouldn't need a motor Motors could also be replaced by standing in a slight semicircle bowl so that the slight incline allows gravity to provide the force to overcome the rolling friction. In that case, the main thing would just be to control the breaks so the inertia when sliding doesn't come into play when it's not supposed to.

EDIT: A simpler way to account for people not walking with their foot straightforward might just be to mount the wheels on a plate that the shoe sits on that can passively rotate slightly, possibly weakly spring loaded. If the plate is made to rotate very slightly off center from the foot the same way a chair caster is, it would automatically align itself in the direction the person is pushing. However, the fact you off-center it means that it probably won't work for both walking forward and backwards, since to align properly when walking forward you need to rotate around a point slightly towards the heel and walking backwards would require rotating around a point slightly towards the toe. Also won't work for side stepping though.

 

[tcmtech]

My vote would be for repurposing some used cordless drill motors and their gearboxes.
Also, methinks somebodies math isn't quite adding up here because by my conversion 2 M/S is ~5 MPH and well above a typical walking speed which is ~ 5 KPH or ~3.1 MPH.

That and I don't think you need 1000 watts to move a smaller sized person at 2 m/s on smooth surfaces. Years ago I built a small go cart and 3 Hp (~2.2 KW ) was more than enough to make it go 15+ MPH (~6.7 M/S) on rough gravel with my 250 pound frame sitting on its near 100# frame. So from that any pair of motors with the correct gearing that can top 100+ watts peak (easily done by any half decent 12 volt cordless drill motor) each are going to be plenty strong enough for what application you have in mind.

I'm thinking a pair of any 12 volt or larger cordless drill motors would be plenty strong to move your 64 KG person at 2 M/S.

They may not be brushless but they would be bonehead easy to work with during the test and tune stage of the design.

 

[dknguyen]

My vote would be for repurposing some used cordless drill motors and their gearboxes.
Also, methinks somebodies math isn't quite adding up here because by my conversion 2 M/S is ~5 MPH and well above a typical walking speed which is ~ 5 KPH or ~3.1 MPH.

That was worst case design speed. I walk at that speed when I have somewhere to be.

 

[delta-9]

I like the idea of small motorized rubber balls in the shape of a shoe sole, would feel more natural than wheels or belts. Utilizing power tools never even crossed my mind. some good information thanks