Building not an eBike.

[heavylift]

A bit off topic, but the knowledge here looks great, hope this is OK to post here! I have had a thought to build a motor assisted weight lifting machine for many years, and seeing now that (very expensive) commercial machines like the Tonal exist makes me think my idea has merit.

Basic overview is modifying a cable weight machine I have so the weight can be dynamically controlled, and even fancy things like using more weight on the eccentric vs concentric etc.

I'm looking to target upper limit at roughly 100kg at 1m/s, I calculate that to require just under 1000 watts.

I looked around large servos like Clearpath, Chinese servos and steppers etc. All of these while I'm sure very nice to work with get rather pricey when that big and also would require some sort of gearbox, all $$.

I basically only need to control the torque, have position sensing, and use a motor to pull a wire with that varying force.
Looking for alternatives, I came across ebike hub motors that are much cheaper. Aliexpress has ebike hub motors listed as 48V, 1500w, 414-487 RPM and 71-74.6N.m. This looks workable, without any extra gearing from my calculations, that'd life 100kg at at 1m/s (130RPM) with a 145mm diameter cable drum. (Thinking I might even be able to get away with using the hub itself to spool the cable)

I'd use a 48v 1500W mains power supply, and looks like an odrive would give me all the control I require (I'm still not sure if hall sensors would be accurate enough to give smooth control I'd want, but looks like just adding a position sensor if not is easy)

The problem is I don't know a lot about any of this so far (ChatGPT has been helping me a bit ). Small details, like I believe the motor while I'm lifting should be braking rather than pulling, but does need to pull while I'm lowering. Motor should rarely be in a stalled state, it should always be moving likely average around 0.5m/s but I don't know about stalling / overheating. With normal weight lifting sets, the duty cycle should be something like 1min on /3mins off. Aliexpress specs I'm sure are wrong, but how wrong are they. Can the motor run smoothly at very low RPM. Listing says direct drive, but I'm sure this has a planetary gear in there. Are these gears up to this task. Things like this, and things I don't know I don't know.

Please poke holes in any of the stuff I'm thinking above, before a drop a few hundred dollars on things that wont work for me Or I'd love ideas for better suited (but still cheap ) motors if people know.

 

[Papa]

Would a vehicle/boat winch suffice for the drive?

Thinking something like this:

2500 lb. ATV/Utility Winch with Wire Rope and Wireless Remote Control

Amazing deals on this 2500Lb Atv/Utility Electric Winch at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

 

[amberwolf]

The ebike/etc motors you're looking at are intended to be spinning "all the time" to do the work they're designed for. They *could* be used for this job, but the controllers probably aren't that easy to adapt to the way you want them to work. Unless they have a dyno (dynamometer) chart for the motor available that uses them in the same loading/supply/etc you're going to, the "ratings" they provide may not apply to your usage, which is going to be at "stall" much of the time. You can experiment with the simulator at ebikes.ca for motors they have tested to see how those behave in your type of setup.

Things like the winch motor Papa posted, or if you really want to DIY there are brushed powerchair motors with rightangle gearboxes you could put on the cable reels, easy to control, but you'd need to work out a position sensor, like an optical wheel or potentiometer on the shaft, and some form of MCU (arduino, etc) to do this work for you--essentially building your own giant servo motor.

But the already-existing large servo motors run by an Arduino or the like would be simplest if you want fine control over them; they already have position sensing built into them so you just tell them what position to orient in and they do that on their own. Here's an example on how they work

How to Control Servo Motors with Arduino - Complete Guide

Learn how servo motors work and how to control servo motors using Arduino. Also, learn how to control multiple servos with Arduino and...

howtomechatronics.com

You can probably find relatively cheap scrapped equipment with servos in it you can use, if you get creative enough in the places you look.

 

[heavylift]

I did look at winch motors, but not suitable as far as I can see. I need fast accurate torque and position control and most of these are just 12vDC brushed (or cheap AC for non car ones). This one is only 1hp = 750 watts, 980w is the minimum electrical power physically required, if 100% efficient, to raise the 100kg I need (1.5kw would be pulling 125 amps @12v!) But the killer is their gear ratio, they are designed to pull thousands of lb's, but very slowly. I need around 1 meter per second, this is doing 4 meters _per minute_ at zero load, 15x too slow unfortunately!

Yes, servo would be ideal, those Clearpath ones for eg are amazing, I would love to replace my 3d printer steppers with them
Problem is just their cost. I have many years of experience with hobby electronics, micro controllers, small servos / steppers etc. It's easy just to overspec a hobby servo without worrying too much about cost, but I need minimum 1000w likely 1.5-2kw to be safe, I have never worked with such large motors. Even second hand these industrial servos are many hundreds / thousands of dollars.

Because of these large AC servo's higher speed and lower torque compared to eg these geared hub motors, they would require another strong $100+ gear reduction box on top of their $700US+ price point (super hard sell over an $90 hub motor!)

I was thinking of all sorts of things like running a 2m ball screw as you can get them quite cheap out of China also, but those numbers don't work out either unfortunately unless I get expensive custom one with very large lead.

These hub motors being 70ish N.m if they can pull that at 130ish RPM look almost perfect, that is on paper anyway. But alwasy the problem that I don't know what I don't know!

 

[amberwolf]

Yes, servo would be ideal, those Clearpath ones for eg are amazing, I would love to replace my 3d printer steppers with them
Problem is just their cost. I have many years of experience with hobby electronics, micro controllers, small servos / steppers etc.

Ah, then my apologies for pointing you at the noob tutorial ...I just started working with the MCUs and RC servos for a totally non-EV project elsewhere on the forum.

But I have long been pretty good at finding things other people don't want and converting them into things I can use, for budget (or lack thereof) reasons. I haven't looked into them in a long time, but the universities here (ASU, etc) and the large industrial companies and the airforce bases used to have auctions and public sales for all sorts of stuff that would have had this type of stuff in it. Back when I went to them, I didn't bid on things, I noted down who bought the lots with things I wanted in them, and talked to them about buying jsut the bits I was after. Sometimes they would sell, sometimes they wouldn't, sometimes they'd just give me the part(s) becuase the lot was for 30 pallets of electronic junk.... :lol:

Because of these large AC servo's higher speed and lower torque compared to eg these geared hub motors, they would require another strong $100+ gear reduction box on top of their $700US+ price point (super hard sell over an $90 hub motor!)

True; but it depends on the motor's size and design--the actual work it can do generally has a lot to do with it's size vs it's speed, so if the hubmotor isn't proportionally larger than the other motor, it's probably going to still have to spin at faster speeds to do the work without excessive current draw and potential overheating, depending on the duty cycle of your system and what kind of cooling you might be able to implement.

These hub motors being 70ish N.m if they can pull that at 130ish RPM look almost perfect, that is on paper anyway. But alwasy the problem that I don't know what I don't know!

I know I already mentioned it, but I do recommend experimenting with the motor simulator at ebikes.ca to see what some of the ones they have actually tested can do under your conditions. It's meant to emulate a vehicle or bike, so it's likely to take some poking at to configure for your setup to give reasonable guesstimates.

 

[Papa]

Sorry,.. I'm just not crystal clear on the design specifics & physical parameters expected (drawing, pics?) Might look at:...

Surplus Center

Warehouse distributor for many top name manufacturers. Parker Hydraulics, Hypro Pumps, Prince Hydraulics. Direct from the factory. Surplus equipment. We buy large quantities of manufacturer's surplus or excess stock.

www.surpluscenter.com

 

[heavylift]

Ah, then my apologies for pointing you at the noob tutorial ...I just started working with the MCUs and RC servos for a totally non-EV project elsewhere on the forum.

Ha, no problem, I didn't qualify myself in any way, I appreciate all input! Always better to learn together!

Re second hand industrial motors, while I'm sure I could scavenge up a good deal eventually, I think I'm just getting too old for that sort of thing! lol. I feel like with the correct new, cheap, likely Chinese gear, I can get something working adequately. My main concern is buying something like this hub motor that works on paper and having it not work IRL because of a shortcoming of my knowledge.

True; but it depends on the motor's size and design--the actual work it can do generally has a lot to do with it's size vs it's speed, so if the hubmotor isn't proportionally larger than the other motor, it's probably going to still have to spin at faster speeds to do the work without excessive current draw and potential overheating, depending on the duty cycle of your system and what kind of cooling you might be able to implement.

So as I see it, non negotiable is the wattage, it must be 1kw+, not combination of torque or rpm is going to lift the weight without that. Second is the gearing. So without a gearbox, 20N is sort of the minimum, with more being better. duty cycle will be standard weight lifting sort of thing, so 5-15 x 3 second reps = 30s - 1 minute on. Then 2-3 minutes off. My gut says these hub motors could handle that, but I have no real experience.
Adding extra active cooling is relatively cheap and easy (although these units look pretty sealed, claim to be waterproof etc)

I know I already mentioned it, but I do recommend experimenting with the motor simulator at ebikes.ca to see what some of the ones they have actually tested can do under your conditions.

Yes, I appreciate the link, and will for sure have a play a bit later on today when I have time. Hopefully get some idea even if the budget motor I plan to get is not specifically listed. Cheers!

 

[heavylift]

Sorry,.. I'm just not crystal clear on the design specifics & physical parameters expected (drawing, pics?) Might look at:...

Surplus Center

Warehouse distributor for many top name manufacturers. Parker Hydraulics, Hypro Pumps, Prince Hydraulics. Direct from the factory. Surplus equipment. We buy large quantities of manufacturer's surplus or excess stock.

www.surpluscenter.com

I can't do a drawing right now, but for practical purposes it's the same as:

A motor attached to the roof, pulling a cable that can lift 100kg up at 1 meter per second, while knowing accurate (+-1cm) position and accurate (+- say 500 grams at the effector / handle) torque sensing

For now I can attach an image to give basic idea. A cable machine like this uses stacks of adjustable weights. I will replace those weights with a motor, pulling down, providing the same force as the metal weights would have (That can be dynamically adjusted, even mid exercise)

View attachment 1705373469343.webp

 

[amberwolf]

Re second hand industrial motors, while I'm sure I could scavenge up a good deal eventually, I think I'm just getting too old for that sort of thing! lol. I feel like with the correct new, cheap, likely Chinese gear, I can get something working adequately.

That's where I've gone with the Snuggles The Wolf robot project; I'd rather DIY everything from recycled junk no one else wants (been kind of a principle with me, though it started as a necessity). (also with some of my other projects in recent years, whenever I could afford it...too much stuff to get done, too little time and energy)

My main concern is buying something like this hub motor that works on paper and having it not work IRL because of a shortcoming of my knowledge.


So as I see it, non negotiable is the wattage, it must be 1kw+, not combination of torque or rpm is going to lift the weight without that.

Keep in mind that while the types of motors you'd prefer to use are generally rated specifically for continuous duty or some duty cycle for some wattage, the stuff you're looking into is not. Any ratings they have are created by marketing people, not engineers. It's pretty likely that under nominally ideal conditions they could handle the rating they are given indefinitely, this is not always true (same for controllers, batteries, etc).

Second is the gearing. So without a gearbox, 20N is sort of the minimum, with more being better. duty cycle will be standard weight lifting sort of thing, so 5-15 x 3 second reps = 30s - 1 minute on. Then 2-3 minutes off. My gut says these hub motors could handle that, but I have no real experience.

My guess is that there's plenty of them out there that will do what you want for not much money.

The cheap controllers, however, may not respond in the way you want. I expect you're going to need an FOC controller so that you can use the throttle input as a torque control.

I think you probably should look into the VESC project, as it's open source and there are many variations of it, and even interactive displays for it (at least a couple here on ES) that you could modify as your control panel for it without starting from scratch.

Adding extra active cooling is relatively cheap and easy (although these units look pretty sealed, claim to be waterproof etc)

they're generally not actually waterproof or even truly water resistant, but there are exceptions. However, for cooling purposes there are numerous modders here on ES and elsewhere pushing a motor well beyond it's actual capabilities, at least short term, and so many cooling possibilities have been explored and tested in various threads, if you run into the need.

Yes, I appreciate the link, and will for sure have a play a bit later on today when I have time. Hopefully get some idea even if the budget motor I plan to get is not specifically listed. Cheers!

yes, you can create custom motors if you know anything about it's various parameters, or simply pick one that's similar to whatever you're after. Sometimes you can use the model listed in the sim to do an image search to see what it looks like (wish the sim had a chart like this already but it doesn't--it should), and compare that to the one(s) you're considering.

A cable machine like this uses stacks of adjustable weights. I will replace those weights with a motor, pulling down, providing the same force as the metal weights would have (That can be dynamically adjusted, even mid exercise)

As long as it doens't get mad at you and suddenly decide to engage full torque.

 

[heavylift]

Any ratings they have are created by marketing people, not engineers. It's pretty likely that under nominally ideal conditions they could handle the rating they are given indefinitely, this is not always true (same for controllers, batteries, etc).

Yeah, and when they are listed on Aliexpress, half the time they are probably just made up!

they're generally not actually waterproof or even truly water resistant, but there are exceptions. However, for cooling purposes there are numerous modders here on ES and elsewhere pushing a motor well beyond it's actual capabilities, at least short term, and so many cooling possibilities have been explored and tested in various threads, if you run into the need.

I did see another post someone drilling holes into the casing with a step bit, likely something I can look at if it does get too hot. Sounds like there is lots of good information to search around this point anyway, good to know!

As long as it doens't get mad at you and suddenly decide to engage full torque.

Something that can move 100kg 1 meter in one second, this is a very legit worry. I will not be doing anything where my body is in any way in between the end handle and the pulley pulling it! I have thought about this and likely a very good idea after proving out that it can generally work, to have something like dual micro controllers, one maybe measuring load with a load cell in a different place, that communicate what they see to each other. If they ever disagree outside some bounds, or don't hear a ping from one another, they turn of a SSR that powers the whole deal. Some sort of fail safe like has been on my mind for sure, maybe a grip button, or a foot pedal that requires you to apply pressure for it to work, so if you release grip or step off the button it dies instantly, but anyway, I'm rambling, step one is to see if I can even get it moving

If no one comes along and replies to this post with some game killing reason this is not plausable in the next couple days, I'll order some gear. Most of the fun is in trying anyway!

Cheers for all the input.

 

[amberwolf]

Yeah, and when they are listed on Aliexpress, half the time they are probably just made up!

I wish that weren't as likely to be true as it is.

Still, AE has some very good deals if you are careful who you buy from.

I did see another post someone drilling holes into the casing with a step bit, likely something I can look at if it does get too hot. Sounds like there is lots of good information to search around this point anyway, good to know!

Holes are one method (of which there are many variations including those with fins placed to force airflow, etc, ferrofluid / statorade and hubsinks are another, oil-filled cooling, etc.

Something that can move 100kg 1 meter in one second, this is a very legit worry. I will not be doing anything where my body is in any way in between the end handle and the pulley pulling it! I have thought about this and likely a very good idea after proving out that it can generally work, to have something like dual micro controllers, one maybe measuring load with a load cell in a different place, that communicate what they see to each other. If they ever disagree outside some bounds, or don't hear a ping from one another, they turn of a SSR that powers the whole deal. Some sort of fail safe like has been on my mind for sure, maybe a grip button, or a foot pedal that requires you to apply pressure for it to work, so if you release grip or step off the button it dies instantly, but anyway, I'm rambling, step one is to see if I can even get it moving

Some form of failsafe is good. Even my wolfy-bot will have failsafes even though it isn't very powerful (I expect most of it can be done with mg996r servos or similar), hopefuly including some sensing to help it not poke eyes out with waving paws and the like.

 

[rafalg]

I think a geared hub motor would be able to provide the needed speed and torque (but would have to be modified to be engaged in both directions, not freewheeling). But controlling that would be a fun project (however, there are many awesome open source projects dealing with various control methods)
Wont contribute much here, i'm just intrigued by the idea and don't know anything about cable machine design. Would all forces be generated by the motor, or would it use some constant mechanical force (like a built-in weight or a spring mechanism) only adjusted by the motor action? Maybe instead of a motor, consider using some spring mechanism with tension regulation?

 

[heavylift]

(but would have to be modified to be engaged in both directions, not freewheeling)

The motor only actually needs to drive in one direction (like the machine weights only ever pull down). Me pulling moves the other way. I brought a hub motor from Aliexpress, it says gearless but also says 400rpm and 70N.m Torque... so we'll see what I get!

Would all forces be generated by the motor, or would it use some constant mechanical force (like a built-in weight or a spring mechanism) only adjusted by the motor action?

Yeah, if there is a known lower limit on required weight, I could add weight plates to that level, and then anything over that the motor produces, it would reduce load on the motor. But one of the advantages I see is quickly changing the perceived weight, eg swap from one exercise that requires 10kg, to one that requires 60kg with a button push, rather than 30 seconds of moving weights each time.

A common thing done in lifting is things like drop sets, where the weight is incrementally dropped, or pyramid sets where the weight goes up and down, or just generally dropping the weight as the muscle starts to fatigue. Not doing the same thing every time seems to be good for hypertrophy, for sure good for boredom anyway!

Maybe instead of a motor, consider using some spring mechanism with tension regulation?

This was my first thought also, but the physics don't really work, you could only ever decrease the weight during a single lifting action. Increasing the weight would require more energy from somewhere. (Like pouring water our of a jug while you're lifting it up is OK, but adding more water, unless you lift it also, means the water has to already be above the level of the jug)


As I'm a nerd I also have all sorts of ideas with measuring strength using the applied torque to calculate correct weight for x number of reps, or measuring the actual force-length relationship of a certain movement, and dynamically changing the load to maintain better consistent muscle tension. Recording my workouts with calculated total watts produced instead of just number of reps

We'll see! Now I wait for Aliexpress speed shipping.

I did stumble across this yesterday, which gives me hope (while also reinforcing the fact that none of my ideas ever seem to actually be unique! lol) This looks to be using only a 750w motor (but the guy also looks to be pushing those handles around way too easily!)

 

[nicobie]

Would a electric linear actuator work?

 

[amberwolf]

Most of those are pretty slow.

Some are faster as you get fairly big (huge) on the motor part...but then the cost goes way up.

Most of the typical LA's are just like your 3D printer's Z-axis stepper motor's screwpost--the motor spins the screw and the nut on the actuator moves the actuated part linearly along the screw. To move something like that quickly takes more power and is harder on the screw and nut, usually requiring enclosure and lubrication. (some serious aircraft failures have been because of insufficient lubrication on what amount to LA jackscrews)

 

[Karl Ebike]

The ebike/etc motors you're looking at are intended to be spinning "all the time" to do the work they're designed for. They *could* be used for this job, but the controllers probably aren't that easy to adapt to the way you want them to work. Unless they have a dyno (dynamometer) chart for the motor available that uses them in the same loading/supply/etc you're going to, the "ratings" they provide may not apply to your usage, which is going to be at "stall" much of the time. You can experiment with the simulator at ebikes.ca for motors they have tested to see how those behave in your type of setup.

Things like the winch motor Papa posted, or if you really want to DIY there are brushed powerchair motors with rightangle gearboxes you could put on the cable reels, easy to control, but you'd need to work out a position sensor, like an optical wheel or potentiometer on the shaft, and some form of MCU (arduino, etc) to do this work for you--essentially building your own giant servo motor.

But the already-existing large servo motors run by an Arduino or the like would be simplest if you want fine control over them; they already have position sensing built into them so you just tell them what position to orient in and they do that on their own. Here's an example on how they work

How to Control Servo Motors with Arduino - Complete Guide

Learn how servo motors work and how to control servo motors using Arduino. Also, learn how to control multiple servos with Arduino and...

howtomechatronics.com

You can probably find relatively cheap scrapped equipment with servos in it you can use, if you get creative enough in the places you look.

Someone I know built an ebike using a winch motor and salvaged forklift controller. I will try to get some details.

 

[heavylift]

Most of those are pretty slow.

Some are faster as you get fairly big (huge) on the motor part...but then the cost goes way up.

Most of the typical LA's are just like your 3D printer's Z-axis stepper motor's screwpost--the motor spins the screw and the nut on the actuator moves the actuated part linearly along the screw. To move something like that quickly takes more power and is harder on the screw and nut, usually requiring enclosure and lubrication. (some serious aircraft failures have been because of insufficient lubrication on what amount to LA jackscrews)

Yeah, I was thinking at one point to use a 2m ballscrew, but the lead on the cheap Chinses ones I would buy is 5mm, or less common 10mm (I'd also be paying for micrometer accuracy I have no need for!), and those numbers didn't work out with the servo specs I found for cheap.
That along with more friction and limited I assume freewheeling etc ruled that out.

My Aliexpress motor hasn't even shipped yet, and has a estimated delivery date of March, so, I should have read that listing a bit more closely!

 

[heavylift]

Still waiting for my motor on the slow boat, but just seen this, basically what I'm planning to make, but in a nice small battery powered package lol.

Still, nothing beats doing it yourself.. right? Actual difference is this thing is 2kUSD, and my Aliexpress kit was only $300. Rich suckers!