Small electric in-wheel motors

[G00s]

Hello there!

I'm really only proficient when it comes to software, but this idea (slightly influenced by various sources) sprung to mind, and it would involve the use of small electrical motors.

So my experience with electric motors only stretches to a fairly unsubstantial physics education, so I would appreciate if things were kept simple! If this idea is brought any further, I would have to acquire help manufacturing anyways. The things that I'm concerned about is only the physical possibility of such a project with today's technology.

Having said that, here's the general idea: A pair of [heavily modified] rollerblades would be driven by four small electrical in-wheel motors. These motors will not power all the propulsion of the user, but only help to accelerate with "bursts" of speed. (quotation marks due to the choice of words, these bursts wouldn't be short enough to make the user fall over, but short enough not to drain the batteries and let the user rely solely on the motors)
The rest is still reliant on the user's own power.

The magnitude of the motors' acceleration would be determined by how much force is applied to the "kick" which one normally use to accelerate on a pair of rollerblades.

So that's the general idea of how the motors would work. The questions I have are the following; Can electric motors rotate both backwards and forwards? With a pair of rollerblades, you would sometimes want to go backwards. The thing I would want to do is that the motor checks which direction the wheels are spinning in, and accelerate that rotation. It would also be useful to have a brake that worked towards the opposite direction of the wheel, or something similar.

And; Would it be possible to generate power out of these motors by the rotation that occurs which is not powered by the motors? And would this be possible to fit with an in-wheel motor where the wheel itself would have a diameter of <10cm?

Lastly; Would four in-wheel motors of that size be possible to produce juice enough to achieve any significant speeds? The goal would be >30km/h.

Thanks in advance! I know the idea might sound strange, but I just want to know if the technology exists to make something along the lines.

 

[G00s]

Another question; Reading around a bit about ultracapacitors, would it be possible to use an ultracapacitor instead of a battery? I'm not sure if it's needed in such a small application, but it seems as if the higher power output/input would be needed for quick acceleration, and to be able to take advantage of the regenerative features of an electric motor.

 

[LionelHutz]

I'd think every question can be answered yes.

If the motors are very small then there will be thermal limits on their output. In other words, producing the output to achieve a certain speed will cause them to get hot. Do it too often or try for too high a speed and they will fail. Many drive systems take this into consideration - Radio controlled cars to golf carts can all limit the power to the motor to protect it - so considerations for this issue are nothing new.

 

[BobM3]

As a former blader and current old mechanical engineer I thought this was an interesting problem. I took a look at the physics only. I used the following:

Person mass = 150 lbm
Go from standing start to 5 mph in 10 seconds.
Assume a rolling resistance factor of .03 (3%)

I then get the following:

- Ignoring the wheel resistance, it takes 2050 joules to linearly accerate the 150 lb person from a standing stop to 5 mph. The constant force needed to do this (again, ignoring rolling resistance) would be 3.45 lbf.

- The rolling resistance force (=150 lbm x .03) would be 4.5 lbf. The power to keep you moving at 5 mph (no acceleration) would then be 396 watts.

So for sizing your motor and battery and drive you would need to add the effects from accelerating and rolling resistance.

You also may need to look at gyroscopic forces with the additional rotating inertia. Another problem would be if a person tries to do the traditional drag of the rear skate to brake. You would have to make sure the wheels on the rear skate weren't rotating.

 

[BobM3]

And, for those turns and braking events where the blade skate would be angled relative to the ground you would have to make sure the drive mechanisms didn't hit the ground. That might be tough considering the wheel diameter is only about 3 inches.

 

[G00s]

Thanks for the interest and the quick answers! Regarding the last reply Bob, which drive mechanisms are you referring to? The image I was having in my head would have more of a closed-type frame to hold the battery parts, and possibly some sort of system communicating and calculating the speed between the wheels. One fear I'm having is that the extra components in addition to the normal boot would add too much weight. And as for batteries, I suppose Li-po batteries hold the best power- to weight ratio? (power density?)

 

[G00s]

Does anyone have any ideas as of which type of motors would be best to use? As far as I've gathered there's brushed, brushless and AC / DC motors. What I haven't really gathered was the advantages / disadvantages of them all.

And perhaps slightly off-topic or in the wrong forum-section (I'm not really sure where else to put it, but it concerns this topic)
I was thinking of a magnetic shock absorber, as simple as having two magnets (permanent or electro, I suppose permanent due to the lack of effect to the batteries' lifetime) repel one another, placed between the frame and the boot. The force created by the magnets would increase exponentially the closer they got to eachother, so the shock applied from landing etc. would push them together, repelling with greater force than before, hopefully diminishing a part of the shock.
Does anything similar exist anywhere? Or am I just way off and better off with conventional spring/hydraulic shock absorbers?

 

[BobM3]

I think I better understand how you want to mount the motors. You're proposing using a motor integral (inside) the wheel? I had assumed you would mount them in-line with the wheel axles.

I'm surprised the motor guys haven't responded. Is this for a "hope-to-sell-it-someday" product or just a "one-of" hobby project?

You'll need a motor capable of developing full torque at zero speed and, assuming 3" diameter wheels and a top speed of 20 mph, would have a maximum speed of 2240 RPM.

I think you're looking at a very expensive system.

For a high power/size ratio I would think the Brushless DC motor would be what you want but there are others on this forum better qualified to give you direction there.

 

[imcjoek]

A few random comments...

The hobby brushless motors of a size that will fit within a roller blade wheel are typically sensor-less. This means they struggle at low speed starts and so they wont be suitable. I suspect even industrial sensored motors may not have enough torque (since the motor length will likely be very short and diameter is restricted). Better to opt for some kind of gear reduction and a higher RPM motor. Also be aware that brushless motors require individual speed control. You cannot just daisy chain them. So this drives up your cost with 8 total motors.

The capacity li-po batteries you would need to make this work (probably 14.4V+ and 1500+mAh) are inherently dangerous. This may not be a problem for a do-it-yourself hobby project, but for a consumer item...not so good.

The regenerative coasting you suggest is supported by most hobby radio-control car speed controllers.

 

[G00s]

Ah, I see. Right now this is just a hobby project. I just think it would be ridiculously fun to ride a pair.

imcjoek, could you clarify what sets apart a sensored and sensorless motor? And thanks for your concern, what is it exactly that makes these batteries unsafe? And since the application would only require bursts of energy, and would regenerate from various sources, would it be possible to use an ultracapacitor alone? I'm not sure how much total power they can store in comparison, but they have an inherently high output right?

 

[G00s]

Just another "request"; is there anyone who knows where one could find wheelmotors for purchasing? If anyone had a link or something that sells just something along the lines of this, I would be thrilled.

 

[imcjoek]

imcjoek, could you clarify what sets apart a sensored and sensorless motor

A sensored motor uses an encoder or similar feedback to the speed controller to tell where the poles are at any given time. This allows the motor to have very good torque and speed control down to almost zero speed.

A sensorless motor relies on the controller guessing the position of the poles based on back-EMF or similar means. They will jitter, stall, try to go backwards, etc when loaded at low speeds.

On the subject of Li-po batteries they are safe when handled properly. However exceeding their discharge rating or an error in charging can result in... rapid expansion.

I would suggest you try a hobby related forum for further questions. Rcgroups gets a lot of traffic and there will probably be a section where your project would fit.

 

[G00s]

Alright thanks for the clarification. I'm really just throwing the idea around to get some feedback, so everything is appreciated