looking for pwerfull dc motors

[arnoldino]

hi

I want to make some kind of mechanism, that can lock the human arm's motion, form the shoulder down to the hand.
I was looking for the avarage torques of the human arm's joints, and I found that the maximum torque is about 70 Nm at the shoulder and at the forearm.
I'm looking for the smallest motors that I can find, which are capable of such output torque. maybe with a worm gear transmission it is possible to achieve such torque. but I need that the human could move his hand freely (until I want to), and from the motor's encoder to calculate the arm's position. and in this case the worm gear is not good, because the worm gear can not work backward.
is there a solution for my problem?

regards,
arnold

[Soeren]

Hi,

Assuming that you don't want to remote control a human, but just to read and lock arm position (might be dangerous if the person falls over and cannot break the fall!), you might consider magnetic clutches/brakes with an added rotational sensor.

When disengaged, the magnetic clutch will move freely, while a geared down motor won't, as the gear ratio will take some power to move the non-energized motor.

[arnoldino]

amazing idea.
I could have thought on this too.
thank you very much for the advice.

can I make myself such brakes/ clutches, because I had a look at ebay, they are about 20 Euro.
it's a bit way much for my buget.

thanks again.

[Soeren]

Hi,

can I make myself such brakes/ clutches, because I had a look at ebay, they are about 20 Euro.
it's a bit way much for my buget.

Here is what looks like a good deal - a lot of 4 relatively flat 24V brakes going for $99,- (buy now) or bid from $49.
€20 sounds pretty cheap as well - some of these brakes may cost you more than 10 times that.

Can you make them yourself?
I have absolutely no idea of your mechanical savvy, your tool- and machine park or your number of thumbs, so it's kinda hard to answer.

Do you have access to a lathe and stock material?
A solution that's perhaps easier than making an efficient electromagnet might be to use a servo to clutch the two pieces of metal together - a servo with eg. 5 kg-cm where the full 180° is translated into a 2mm movement of the clutch plate would be able to press with a force of 39 kg and if the plates are the right material, this should be ample.

However, even if you're sharp at mech., you'd need to factor in the cost of materials, servos and all the time it would take you to experiment with and finally make a working brake, so it would probably be as expensive, if not more so.

If it's important to make, get the budget sorted out and buy the stuff... Or cut loose before you have any substantial outlay if it isn't.

[arnoldino]

hi

thanks for the suggestions.

I have access to lathe and mill machine, and cheap material too, so I guess I could make something. furthermore I graduated as a mechanical engineer, so would be a shame if I couldn't do such things
My first idea was to use a solenoid to press something to the disc attached to the shaft. or to use a solenoid and a small mechanism to press the disc from both sides, like at the car wheels.
and applying different voltages to the solenoid, the brake would press with different forces.
or the solenoid has not enough power for this application? I had a look on google, and I found solenoids with 5 mm stroke with ~10 Newton force. I need to make calculations to find out that I can use solenoids or not.
any comment would be appreciated.

what materials should I use for the disc and the brake pads?
for brake pads I thought on the actual brake pad material.

regards,
arnold

[Soeren]

Hi,

My first idea was to use a solenoid to press something to the disc attached to the shaft. or to use a solenoid and a small mechanism to press the disc from both sides, like at the car wheels.

I have a mental image of a "disc" with an axle and another disk (with a center hole), like a washer. This could be pressed together from one side via the axle and a "cup" could enclose the active parts, to make it neater and to protect the parts if being bumped into walls and furniture when you suddenly lock up a persons limbs while in motion.

Not sure if a solenoid will do (without a humongous current draw), as they need to be jammed together quite hard - unless you make them both have "splines" fitting together, as that would lessen the demand for holding strength.
A spring of appropriate strength should be fine for holding the discs apart when disengaged.

and applying different voltages to the solenoid, the brake would press with different forces.

If you wanna lock up a joint, there's no real need to vary the braking force - maximum force will do, but if you just wanna "slow down" (like simulating a person with rheumatism,  decreased muscle mass or whatever), a solenoid is best controlled with PWM, just like fuel injectors, as this gives you a roughly linear relation (assuming the PWM frequency is chosen according to the inductance of the solenoid).

But you have to experiment with braking power contra supplied current, to verify whether solenoids will be a viable solution (I have more faith in servos for this).

I had a look on google, and I found solenoids with 5 mm stroke with ~10 Newton force. I need to make calculations to find out that I can use solenoids or not.

How much friction you can get from it will depend on the way it's constructed and the friction in the braking "pad" area, but if made with tight tolerances, you can probably make do with a travel of the discs of around 1mm, so that would make the 5mm travel solenoid 5 times as strong via a lever of sorts (and you can get solenoids with a much longer stroke).

what materials should I use for the disc and the brake pads?
for brake pads I thought on the actual brake pad material.

I think regular brake pad material is too hard for this. You are not trying to stop the inertia of a car and a softer material like urethane rubber will offer more friction, or, if longevity is not the primary concern, an even softer synthetic rubber - one disc with a more or less rough finish and one with a pliable "rubber" will add a lot of braking power for the same energizing force, but is next to impossible to calculate, so experiments will be needed to find the best compromise.

[azy]

OPTIONS


A

Build a metal disc with holes drilled round the edge , get a solenoid to push a pin through the hole(locked), taper and grease the pin and holes so it can be retracted when under heavy load. angle the solenoid slightly so it can slip out when under load.

B

use solenoid or servo with a disk brake set up, use sharp angles and a levers to convert throw into very high gripping torque
use a sanding disc, very grippy and a rubber bycicle chock, hard but sticky rubber. (or use a heavily pitted aluminium disk covered in gritty anti slip paint/super glue, the deep pitting will help the paint and glue bind, use a punch or chisel) trying to mainatin tight tolerences will be impossible....mr grit meets mr hard rubber saves the day

C

use a hydrulic brake system, such as the flexible braided lines they use on motorcycles, metal disc brakes. The BIG ADVANTAGE is that you can have powerfull servos/motors mouted deep with the torso...taking all the complexity weight and bulk out of the arm..just a set of lines and calipers mounted on the arms. result = 300kg + holding power from a 50 buck electric motor sat in the torso. Braided lines are cheap and u can cut them to length, all the fittings u want are out there off the shelf. if u can find a couple of master cyclinders for free from the scrappy this is the way i would go. if u want to lift 10kg this system will do it without batting an eyelid.

I think in the longrun, going for C will not only be the easiest but the best, and u can resuse the system on future projects as most of the key components are not bound up in your current system/mechanism

If u want to be a total cheapskate with system C , use clear plastic tubing wrapped in ductape = braided lines & thin hollow brass rod from the modeling shop as joints. You can use a large weight in the torso and lower it onto a ballon filled with oil/water(bladder) this will give you the hydraulic presure you want. you could use air insted of water, but u will need lots of glue to seal the joints. Air will give you a nice progresive grip. you will need to raise and lower a 5 or 10kg weight onto the bladder..use the battery , you could even use a small bladder to the pads into the disks

You are so asking for a mountain to be moved with a teaspoon if u dont want to spend any money, but my bodge version of the C method will work well, with trial and error.

You could have the sytem reversed, everything always locked by powerfull springs, only when presure is applied do the pads move away from the disks and thus stop the braking action in the event of powerfailure the system naturaly locks, never exposing arm servos etc to damaging loads.....did i tell you i was a genius ?

[arnoldino]

excelent ideas.
thank you guys. it is wonderful how many solutions can be found on a single problem.

azy has very good solutions, thanks, but I try to keep this as small as possibile, and independent form other things.
this brake will be mounted on an easily wearable device on the human arm.
the holed disc I think is not the best solution, because it should work in any position, and on a small disc there is no space for many holes, that would limit the working space.

at this time I'm thinking to Soeren's two disc theory. I'm trying to figure out the forces, dimensions and construction needed to hold on a 70 Nm joint. I'm thinking on a modified construction, with some wedged paths, something similar to the Siemens wedge break technology, but more simplier.

thanks again.
I'll be back as I can achieve something.

[arnoldino]

Hi

I attached the scheme of my idea.


1. The DISC is the actual break disc, and the backward part (actually is in the front on this picture, but I mean the last part, which is held by a back plane), as it is rotated it will push the middle part to the brake disc. As the contact has been made between the middle part and the disc, the backward part should only hold, because the wedge will do his job and give continuous brake force.

2. There is another method to get this mechanism work. I could make the backward part fully grounded, and I could push, or rotate the middle part just a little bit, with low power, until there is contact with the brake disc. From then on, thanks to the friction between the disc and the middle part, the middle part will move together with the disc. Or it would to, because there is the wedged grounded part, and it will lock the middle part, achieving a continuous brake force, until the disc begins to move in the opposite direction. Than I can pull back, or rotate to the initial position the middle part.

Every method has it’s advantages, and disadvantages too, and I can’t decide which one to use.
The first one is a little bit more sympatic for me, because I think I can make full brakes with more precision, and I can change the intensity, so I can use it to slow down the arm's motion, giving a sensation that the arm is moving a heavy object.
I think this I can’t make with the second method, because as there is contact between the disc and the middle part, I should input some power to hold back the middle part to avoid locking.

I made a quick calculation using the first method. If I want a full brake with 70 Nm torque, having a 35 mm disc in diameter, I have to rotate the backward part with 4.678 Nm torque, to be able to hold in position the part. With some gears or worm gear I can lower this to 1, or even less than 0.05 Nm. I think such torque I can get from a very small dc motor. The only problem would be the velocity, that I have to investigate.

Any comments on this would be appreciated.

Regards,
arnold

[Soeren]

Hi,

I'm not quite sure if all of the parts in the drawing is rotating with the joint (or it may lock up)?

Anyway, got me thinking about using two (strong) crown wheels (one "stationary" with one side of the joint, the other rotating with the other side of the joint), keeping them apart with a spring and slamming them together wouldn't require much force, so a small solenoid should do - the "resolution" would depend on the pitch of the crown and will have a natural limit to stay strong of course.

Just a thought from reading your post.

[arnoldino]

hi,

maybe the two crown wheel would be a good solution, but I want to control the brake force, to give the feeling to the user, that he is moving a heavy object. and I can't do this with this method.

the backward part on my drawing won't rotate with the joint, because a motor will hold it in position, and the motor would be connected to the casing, and casing would be fixed to one of the elements of the joint.

thanks
arnold

[VegaObscura]

Sounds to me like you're trying to make a feedback system for a high-realism video game or virtual reality.  I'm very interested in where this is going.

[arnoldino]

you almost have right.

I want this device to manipulate a robotic arm, but of course it will be useful for many other applications, like video games