Hello guys , i am sorry for my reoccurring posts , but i am a total ignorant when it comes to mechanics and i have no one else to turn to .
as the picture below indicates (poorly drawn , i am sorry) , i want my motor to spin an upper part of a Hand . what will extend from the hand is a simple shaft . in my project i am in need to rotate the hand with average accuracy so there is no need at all for precision , precision is not a factor here . even a loss or gain of 1 or 2 degrees won't hurt (that much!) .
i decided to use a Stepper motor with a planetary track gear box because the hand has to handle a lot of load , and DC motors are a pain to handle in applications like these . i need the gearbox to also be attached to also be attached to the motor . the hand might at some point carry up to 500 to 700 grams of weight .
all the motors i found that seem to be reliable are dedicated to CNC machines and 3D printers .
what i am afraid of is that the maximal speed after the gearbox might be too low . in my project i need relative speed . not nearly as fast as a human hand but also not as slow as a CNC machine (i am sorry if i am being vague ) i also don't seem to be able to calculate the maximal speed of the motor since the load is a variable and i don't seem to be able to get my equations straight .
here is a link for one of the motors i have found : (very expensive ) pretty link
though i fear that the motor might be too slow , unable to carry the load i need or More expensive than what i really need .
to sum up ,i need : -relatively high speed
-ability to carry (rotate) load or to at least hold position in the presence of load
(i am sorry if i sound like i am making demands , i am not , nor do i feel any sense of power , i am merely asking for advice that some of you may be humble enough to present , asking for free advice is rare in my country that's why i am very intimidated by it)
As you already know, the stepper motor rotates one step at a time, depending on your program telling it to make a step. So with all the details you have given, how fast can you make the stepper make a step? Do you require acceleration and deceleration? How are you attaching the load to the hand? The rotation seems to be around the axis of the wrist, so the weight is not a parameter, but the mass is, and it must be accelerated and decelerated by the motor. So, the critical parameter is what acceleration/deceleration do you need?
Many stepper motors can rotate up to about 200 RPM with no problems, with some care you can get quite a bit higher rates, but you lose torque.
The gearbox of the motor linked is 19:1, so a reasonable guess for the output shaft rotation rate is 10 RPM, or a full rotation in about 6 seconds. 3 seconds or less per rotation might be achievable.
That would seem to qualify according to your statement:
in my project i need relative speed . not nearly as fast as a human hand
amine2:
i decided to use a Stepper motor with a planetary track gear box because the hand has to handle a lot of load , and DC motors are a pain to handle in applications like these . i need the gearbox to also be attached to also be attached to the motor . the hand might at some point carry up to 500 to 700 grams of weight .
Your diagram does not give any indication of how the load will impact on the motor and it is not at all clear that a gearbox is essential.
Can you make a simple drawing showing the hand holding the 700g load and showing how the motor is expected to move. The important thing is how much torque the load places on the motor shaft - which depends on how far the load is offset from the centre of rotation.
This is an example of the sort of drawing I have in mind. Approximate dimensions will be useful.
Hello guys, thank you very much for your attention and i am sorry for the original lack of detail in my first post .
thank you Mr Jremington that's a very helpful clarification thank you .
thank you Mr Robin and Mr Paul , and i am sorry i missed some necessary detail at first ,
the Hand is one that will be used by patients and it's oriented for patients with amputated limbs .
the hand on it's upper part has motors that close and open it to hold on to objects , that part is all taken care of by now and it works . the Heaviest thing this hand will probably have to hold is a water bottle .
water bottles are relatively heavy . the Hand will have to hold the bottle the normal way a human does hold it . now the Wrist rotator , which is the part that we are concerned about , has to only simulate the "pronation/suppination" movement of the human hand (i joined a picture that illustrates that) . so the wrist rotator has to only rotate the hand along the longitudinal axis . (the longitudinal axis of the hand) .
the longitudial axis of the hand is fixed here as the degree of freedom that might rotate it which is present in the human hand is eliminated here to simplify the design .
as shown below . with the bottle of water . the biggest load the design will probably have to go through is rotating that bottle of water along the axis indicated in that picture .
also if the wrist rotator wouldn't be able to rotate something or if the load is too high , being able to at least hold it in place against gravity is very important .
(IN that image i also associated something from the CAD design of the upper hand , which contains the exact dimensions in mm , i hope those are the dimensions you were told me about Mr Robin)
Exact dimension from CAD design :
Pronation/Suppination :
Illustration of the Axis and method of rotation here
Only go for stepper motors with a datasheet that includes torque/speed graphs for a range of supply voltages.
Don't be fooled by the stationary torque, you'll get nothing like that much for a moving motor, and it
drops off with speed (although higher voltage supplies will improve matters).
Assume any reduction gearbox has 30 to 40% mechanical efficiency loss per stage too.
A DC gearmotor plus encoder may be a much better option, as the size/power requirements are substantially
less than for a stepper.
And yes, you are being vague using terms like "slow" and "fast". Numbers and units please...
You still need to study the load relative to the axis of rotation as in my diagram.
If the bottle is always held at its centre of gravity then very little torque will be required to rotate it (unless you need to accelerate it quickly). But if it is held at one end the situation will be different. Try this in your own hand with a 2-litre bottle of water of cola.
And a bottle containing liquid has the added complexity that when partially used the liquid can move and substantially change the centre of gravity.
The right thing to do would be make a test device with a freely rotating axis to which the bottle (or any other load) can be attached in positions similar to what the hand would do and then measure the torque. There is a simple system for getting a rough measurement in Stepper Motor Basics.
I would design a system to have at least twice as much torque as seems necessary.
...R
PS... it would probably also be a good idea to motorize the test device (after you have taken the torque measurements) so you can experiment with motors in controlled and simple system.
I am sorry that after yet an other reply i don't seem to be Able to make myself clear .
Hello Mr Mark and thank you for your attention . we are working with amputated patients . the only reason i stated that speed is important , is that the hands will be used by patients in the future . so the motor cannot be as slow as that used in a CNC machine but i don't have an exact speed in mind , and that's actually a good thing , because there is a wide range . the main situation is this , the person has an open bottle of water in hand and will rotate the bottle to poor the water into a cup .all i need is an acceptable rotational speed , you don't want the patient to wait for an entire minute for hand to get to the right angle , do you know what i mean ? i am very sorry if i still seem vague .
thank you very much for your advice about DC geared motors , after more research that turned out to be a better solution for my purpose thank you very much . when it comes to acceleration/deceleration that's not even a factor here . i do not care about that variable .
after some more research , i have found this motor : motor
it is cheap , and it being cheap kind of got me worried .
the supplied voltage in my case is a constant , it's 12V . though i don't know what this guy means with RPM , i don't know if its the RPM of the motor or RPM after the reduction of the gearbox .
what do you think about this second solution ? if you find it convenient i will proceed to buy them now .
Robin , thank you very much for your attention . the problem here is that the situation is fuzzy . the patient might hold the bottle at any point . or might hold any other object of any other shape . what i am aiming at is getting the maximum torque possible (Motor being able to hold highest amount of mass possible) at an acceptable speed (fuzzy) .
i am sorry if i am being vague i really am sorry , if i had a constant speed at hand i would've given it . but all i have in mind is a "Speed" that is fast enough for patients to at least accept .
also i would like to thank you for the simulation solution , but i don't think we can do that .
I understand your vague responses. They come from your lack of experience with such enterprises. So, you will have to experiment with quite a few complete solutions in order to get your needed experience. We cannot be a replacement for your experimentation.
I take exception to your characterization of CNC devices as slow. They are as fast as need be. I have two CNC pick-and-place machines that can pick up more than 2000 SMT components per minute and place them on printed circuit boards. Yes, they are servo operated.
One of the mechanical problems you have not addressed is how to support your "hand" with the weight while the hand is connected to the arm. You cannot use the motor or gearbox shaft to support the weight. So, your support must have some type of bearings that can rotate, while carrying the weight. That probably means two bearings spaced some distance apart, with the "hand" shaft passing through the bearings and attached to the motor shaft with a flexible coupling. The coupling will need to smooth out the rotating impulses from the motor/gearbox, both starting and stopping.
Do you have any mechanical engineering support people?
amine2:
Robin , thank you very much for your attention . the problem here is that the situation is fuzzy . the patient might hold the bottle at any point . or might hold any other object of any other shape . what i am aiming at is getting the maximum torque possible (Motor being able to hold highest amount of mass possible) at an acceptable speed (fuzzy) .
"maximum torque possible" is, unfortunately meaningless. I presume you don't have space for a 1000kW motor from a train. And you probably want something bigger than one of these
Allow for the worst case way in which the bottle is held.
i am sorry if i am being vague i really am sorry , if i had a constant speed at hand i would've given it . but all i have in mind is a "Speed" that is fast enough for patients to at least accept .
That means something definite to you, but not to us. Maybe you could make a short YouTube video of your own hand moving as you envisage?
Again guys thank you very much for your response and attention i highly appreciate it .
here is a video that illustrates exactly what i want .
the rotational speed of that Hand is perfect ! that's exactly what i need right there (speed and load , that's all i need ) :
Paul , thank you very much for your attention sir , once i have the motor in hand , i think i can get some local advice on bearings . but i am at a critical point right now where i have to buy the motors as fast as possible because we are running out of time . when it comes to mechanics , i am a total ignorant , i had programming courses in the old days and wiggled with electronics so i can easily manage . but when it comes to mechanics i am as dumb as they can be . i have been lucky enough to have a pre-made design for the upper part , all i had to do was buy the parts and assemble them . but the hand-rotator is something i have to do on my own . it's alot simpler though
is there anything wrong with the last motor i sent ? why can't i just use that one ?
Robin , again thank you , i hope the video i provided illustrates the Speed ,torque requirements .
the polulu motors are too slow and i think it's impossible for them to carry the weight ( i am sorry if that's a dumb statement) , what's wrong with the last motor i sent ? i like the price , i want to buy it .
would it not work ?
Forget the idea of the motor carrying the weight. It won't and can't. The bearing structure at the end of the arm will carry the weight. And consider putting the motor up inside the upper arm and run a double wound sheathed cable to the hand. Much lighter and better balance. Same torque as if the motor is directly connected.
Mr Paul , Sir the motor will most certainly not carry the weight . i had that decided from the very beginning . the problem here is the motor , is the motor i posted enough ? should i buy that motor or should i look for a stronger one ? that's the main question here sir , even though what you stated is very important and i do admit to that and it's a cardinal point in what we are doing . the question that's on my mind is should i buy that motor or not , at this point in time i must buy a motor .
Paul_KD7HB:
You won't get very far in this world if you can't decide to do something yourself and get it done. Stop wanting others to make your decisions.
Paul
i am very sorry for that .
it's just that i have been under heavy pressure lately .
i am very sorry .
amine2:
Robin , again thank you , i hope the video i provided illustrates the Speed ,torque requirements .
the polulu motors are too slow
I know the motor in my link is not suitable - I was just trying to make you think of the extremes so that you could focus on what you really need.
The rotational movements of the hand in the video do not seem to be fast - my guess is about 2 secs per rev. or 30 rps.
Also I did not see anything that requires exceptional rotational torque - such as would be needed to operate a large screwdriver.
The reality for this project (whether you like it or not) is that you are going to have to experiment. You might be lucky with your first choice of motor. My guess is that the motor in the arm in the video cost at least $1000.
i already bought mine and it's already over but .... 1000$ ? come on . it isn't a batmobile .
what in a motor would cost 1000 dollars ?
+the whole interest of our project is to provide affordable prosthetics for the locals , the cheapest myoelectric prosthetic here is about 30000$ which is to my opinion ridiculous .
Paul_KD7HB:
Forget the idea of the motor carrying the weight. It won't and can't. The bearing structure at the end of the arm will carry the weight. And consider putting the motor up inside the upper arm and run a double wound sheathed cable to the hand. Much lighter and better balance. Same torque as if the motor is directly connected.
Paul
i didn't pay attention to the details you mentioned at first ... but that's really interesting Paul , thank you very much .
aside from that i kinda have a dumb question . if i get a 8mm bearing , which means that it should fit an 8mm shaft . and then i buy an 8mm shaft . is there a risk that the shaft won't fit the bearing because of micrometric differences ?
i am sorry for my reoccurring posts , but i am a total ignorant when it comes to mechanics and i have no one else to turn to .
