A little help with Torque measurements?
Ravenkallen
Posts: 1,057
First off i hope i am posting in the right forum for this question, seeing as it pertains more to mechanics i thought that this would be the best place to ask it??
So, my understanding of a torque measurement is this. Lets say a servo has 11 Kg/ cm of torque, which equates to roughly 156oz/in? Does that mean that if the servo had a "arm" that was one inch long, that it could conceivably lift a weight that was 156oz's? Or if the arm was 10 inches long, it could lift 15.6oz's?... I am working on a robot arm and i want the arm to be able to extend as far as a human arm and to have a few degrees of freedom, while still being able to lift 5 or 6 ounces? The servos i bought are rated for a stall torque of 11 Kg/cm which is quite a bit, but i don't know if it will quite do it. Should i double up on the servos?..... Thanks in advance guys:)
So, my understanding of a torque measurement is this. Lets say a servo has 11 Kg/ cm of torque, which equates to roughly 156oz/in? Does that mean that if the servo had a "arm" that was one inch long, that it could conceivably lift a weight that was 156oz's? Or if the arm was 10 inches long, it could lift 15.6oz's?... I am working on a robot arm and i want the arm to be able to extend as far as a human arm and to have a few degrees of freedom, while still being able to lift 5 or 6 ounces? The servos i bought are rated for a stall torque of 11 Kg/cm which is quite a bit, but i don't know if it will quite do it. Should i double up on the servos?..... Thanks in advance guys:)
Comments
I think you have the general idea correct.
The units of torque aren't Kg/cm and lb/in. They are Kg*cm and lb*in.
So your 11Kg*cm servos should be able to lift 110g with a 1m arm. That's less than the 5oz you're after. I think 110g is between 3oz and 4oz.
Let's call 1oz 30g (I think that's close). So you want to lift 150g with your 11Kg*cm servo.
11,000g*cm / 150g = 73.3 cm (over 28 inches).
I don't think you'd want to have the servos running at max torque all the time though. It's looking like doubling up is a good idea.
BTW, I used to be high school chemistry amd physic teacher. Units are your friends. They can help you see if you set the equation up correctly or not.
Duane
I won't even get into N-cm vs. kg-cm.
While that might be theoretically correct, keep in mind that the shaft itself will be susceptible to shear forces, and whatever bearings it has will be susceptible to the friction. In other words, mathematically it works out that such a system should be able to lift a weight of 1560 oz on a 0.1 inch arm, or 15600 oz (975 pounds) on a 0.01 inch arm, etc, etc. but in reality such a force would likely shear off the shaft or deflect it beyond usefulness. Or at least create so much friction on the bearing that the shaft would not turn. So the only time these torque numbers really mean much is when the "arm" length is of reasonable size.... but if the arm is too long, then you must take into consideration the weight of the arm, too. You can try to balance out an arm with a counterweight so the torque needed is just the difference, but again you have to consider what your total shear forces are, etc.
Also note these torque ratings are generally for quasi-static loads, so if you're really swinging things around, then you must take into account dynamic effects (moments of inertia, etc.).
I looked at force gauges but wow they where expensive.
I have a Halloween project that I what to move.
http://www.facebook.com/video/video.php?v=10150269867955550
Been looking for non pneumatic options.
I think for most electric motors the torque rating is the stalled torque, which means the motor will be providing the maximum torque it can provide, but the motor will be stalled - in other words, the motor won't be moving at all at its stall torque. At stall torque, motors are generally sucking up the maximum amount of current they can use, so that means the motor windings are generating their maximum heat. So stall is the most likely scenario at which a motor might overheat, if it can overheat. So running a motor at or near stall is, generally speaking, probably not the wisest thing.
Note that stall torque is not the same thing as maximum power, which is achieved at a peak that is related to the product of speed and torque. At stall torque you are theoretically getting zero power because you are achieving zero speed.
Keep in mind that these are ideal numbers, so you will want to take friction into account or wind, etc. if such is the case.
To Ragtop: not everyone has been subsumed into the Facebook borg. Also, you're somewhat hijacking Raven's thread here, so how about starting a separate thread of your own and post your project in a way that does not require a Facebook account. That would help make suggestions on your project easier to manage. :-)
Save your money, hobby servos for what you want to do will not work for the project you have in mind.
First off a company wants their rating to look good. So if the servo is rated at 156oz/in it means that it will stall at that torque; which in turn means your servo will break with in secs!
Also "double up on the servos" may sound good but it is not. Each servo takes small steps as it turns. Not all servos are going to take the same size step. So the servos will fight each other just a little. This will waste power or if they are really out of step by a lot; break a servo.
You are going to need more than hobby servos if you want to build an arm that extends as far as a human arm with human like joints. The only way you can get an arm that extends as far as a human arm using hobby servos is to build a linear arm. They don't use human like joints.
Al
It seems similar enough to me. Before buying a motor, how do you determine how much torque your application is going to need?
To what does your it refer here? Your motor spec? your configuration? Your topic?
One way to do it would be to construct an arm of the desired length, then figure out how much force is applied at the end of that arm tangential to the circular movement. To figure out the force, you could use a spring scale similar to one of these:
http://www.carolina.com/product/pull-spring+scales%2C+economy%2C+100+g+x+2+g.do?keyword=spring+scale&sortby=bestMatches
http://www.mcmaster.com/#spring-scales/=cj2oap
If you don't want to use a spring scale, you could use a weight at the end of the arm, but make sure your test has the arm in the horizontal position and the force of that weight is acting straight down, at a right angle to the arm. Water makes a good weight because you can make a container from a lightweight cup and easily adjust the weight by adding or subtracting water. You can easily look up the density of water so all you need is a good way to measure the volume of water.
Under circumstances like the above, your torque = (the length of the arm) x (the weight at its end).
I don't see any difference talking about understanding torque measurements and determining how much torque you need.
But thank you that is what I was looking for.
11 Kg-cm = (24.25 pounds)(0.39 inch) = about 9.5 in-lb. = 152 oz-in. In other words, to hold a weight of 5 ounces, the arm could be 152/5 = about 30 inches long. But remember this is the stall torque, so nothing is moving at that torque. Also, you must consider the weight of the arm, whose weight will appear to act at its center of gravity. Assuming the arm is of constant cross section all along its length, and the arm is operating in the horizontal position, and let's say the arm weighs 8 ounces, then a 30 inch arm will require (8)(30/2)= 120 oz-in of torque just to lift it. You can use a counter weight to balance things better, but keep in mind how much total shear force that starts to add to the shaft and how that might start to bind the bearings, flex the motor housing, etc.
I agree with al1970 that you can't just "double up" a motor to get more strength. Generally speaking it's just not a good idea. I'm sure it's been done in some circumstances but the two motors would need to be precisely coordinated so they truly work together, and that isn't easy to do.
Ragtop,
because you posted your own project and were talking about options to pneumatics and so forth, I thought it would be more courteous to Raven and more helpful to you for you to start a new topic here so people could look at your specific project and provide inputs.
http://www.hobbypartz.com/kahaoubrmo14.html
So i guess i am in another pickle. Couldn't someone just use a little clever programing to make up for the slight difference in the servos? Like if one is to far to one direction could you just tell that one to go back slightly? Linear actuators are nice, but they are really pricey. I am trying to do this on a budget:)
Thanks for all the help!!
Heck, for that price I'd say go ahead and try doubling them up. If you could monitor the current to each servo you should be able to see when they are fighting each other. I'm pretty sure those are analog servos so if their position is off a little they wont draw a lot of current trying to move.
Just make sure and post your progress somewhere on the forum (if you want to). I'd like to see what you do.
Duane
I am using a servo with specs similar to the ones you've got for the elbow of a small arm - I'm using a Hitec HS-645MG.
The shoulder of my arm is a Hitec HS-755HB. This has a max torque at 6V of 183 oz-in (according to specs).
My arm is only a little over 1 foot long but it does have 3 smaller servos at the wrist/gripper adding weight out at the end.
I expect that you will find you do not have enough torque for your arm without doubling the servos.
There are a number of companies selling kits for servo-based robot arms. I suggest you take a look at them - not to purchase but to get an idea about what kind of servos are required.
Lynxmotion (www.lynxmotion.com) has a range of arms and also discussions in their forums about arms and servos.
I have one of these. There is Propeller code for controlling this arm.
The Dynamixel servos are really cool. They cost a lot more though.
I'll save you some time. There is nothing you can do in code that will work. Giving a hobby servo the same timing data will not put the servo in the same place all the time. That is why they heat sometimes and sometimes not. "So i guess i am in another pickle" , not really. You can build a Linear arm like I said the 1 st time and no you don't need Linear actuators to do that. You would have to make the servos continuous rotation servos and make wood slide parts. The arm will not look anything like a human arm.
The other way to go about it is to use rubber band power. When I was a kid we used rubber bands to build everything from guns to planes! What you need to do is make a joint with a servo on one side of the joint and a wheel and shaft to the servo and the other side of the joint. Cut the rubber band and glue it well to the wheel, wrap the rubber band around the wheel. Put the other side of the rubber band to somewhere where it can be held in place. The power of the rubber band will offset the weight on that joint. A note here. When a arm that is 30" long (human size arm) drops a load there will be a kick back force to the grears of the servos, so your servo life may be less.
Try bungee cords?
A similar approach might be to use constant force springs. Keep in mind the force vectors change as the arm moves through space but sometimes springs like this can provide enough boost to prevent your needing a larger motor. Gas springs might also help in some circumstances, but they are expensive unless you can score some used or surplus. You often see gas springs used on the hatchbacks of cars, for example, so they can be useful for heavier loads.
Have you built a smaller arm or are you just jumping into the large scale arm you have talked about previously? If it's your first arm, I suggest you start simple - shorter arm, lower required torques, etc. Starting in with needing to double servos is adding a lot of extra work.
My 12" arm (that I referred to previously) is using fairly beefy servos (but not doubled) and is also using some springs to counter the torque at the elbow).
If you want to go larger, you may want to forget servos altogether. You could contact OBC and Gramps for advice:
http://forums.parallax.com/showthread.php?128683-Update-on-Gramp-s-Arm
http://propellerpowered.com/?p=329
Jim