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# A little help with Torque measurements?

Posts: 1,057
edited June 2011 in Robotics
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:)
I got the best hair around...Guaranteed!

• Posts: 10,002
edited May 2011
Ravenkallen,

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
• Posts: 7,619
edited May 2011
Torque is force x distance.
Leon Heller
G1HSM
• Posts: 741
edited May 2011
It is not unusual to find official servo documentation from the manufacturer showing torque units written kg/cm and oz/in. Yes, it's wrong and it bugs me every time I see it.

I won't even get into N-cm vs. kg-cm.
• Posts: 2,702
edited May 2011
Generally the posted torque spec is stall torque or holding torque. My rule of thumb with servos is to multiple the rated torque spec by 2/3.
Any fool can make something complicated. It takes a genius to make it simple. Woody Guthrie
• Posts: 4,561
edited May 2011
...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?...

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.).
• Posts: 1,057
edited May 2011
Thanks for all the replies:)... I didn't know the units of measurement(kg/cm and oz/in) were wrong? I saw it on datasheets all the time so i assumed the servo manufacturers knew what they were talking about:). There are many other variables that i didn't include such as the weight of the arm, so i will have to factor those in to. I think doubling up will be my best bet. The quoted torque is the maximum amount of force that can be applied before the quoted rotation speed will begin to degrade, right? Like the servo will still be able to rotate, but just not as fast? Could that cause damage?
I got the best hair around...Guaranteed!
• Posts: 406
edited May 2011
On the other end is there a way to measure how much torque you are going to need?

I looked at force gauges but wow they where expensive.

I have a Halloween project that I what to move.

Been looking for non pneumatic options.
Thomas Fletcher
Ocala, FL
• Posts: 4,561
edited May 2011
...The quoted torque is the maximum amount of force that can be applied before the quoted rotation speed will begin to degrade, right? Like the servo will still be able to rotate, but just not as fast? Could that cause damage?

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. :-)
• Posts: 64
edited May 2011
Hi:

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
• Posts: 406
edited May 2011
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. :-)

It seems similar enough to me. Before buying a motor, how do you determine how much torque your application is going to need?
Thomas Fletcher
Ocala, FL
• Posts: 4,561
edited May 2011
Ragtop wrote: »
It seems similar enough to me....

Ragtop wrote: »
...Before buying a motor, how do you determine how much torque your application is going to need?

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).
• Posts: 406
edited May 2011
ElectricAye, I don't know what is up your crawl, but READ title of post.

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.
Thomas Fletcher
Ocala, FL
• Posts: 4,561
edited May 2011
...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?.....

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.
• Posts: 4,561
edited May 2011
Ragtop wrote: »
ElectricAye, I don't know what is up your crawl, but READ title of post.....

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.
• Posts: 10,002
edited May 2011
Ravenkallen, I'm not sure what kind of servos you have. I personally haven't tried doubling up on normal hobby servos but I have doubled up with Dynamixel AX-12 servos. The AX-12s aren't to hard to use in pairs.
• Posts: 1,057
edited May 2011
These are the servos i bought. They were really cheap and they had a high torque rating, so i thought they would be ideal:)
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!!
I got the best hair around...Guaranteed!
• Posts: 10,002
edited May 2011
Ravenkallen,

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
• Posts: 1,057
edited May 2011
@Daune.... Yeah, i am going to try it out on two of my three dollar servos first, just to see if it will work... The finished project is not going to be perfect, but it is going to be really cool. Think wearable third arm:). I will definitely be posting it up once i finish it
I got the best hair around...Guaranteed!
• Posts: 282
edited May 2011
I'd imagine using multiple servos would be ok if you used some form of mechanical mixing. Or you might be better off removing the position feedback circuit in your servos, and implement your own with sensors somewhere on the arm (or you can still use the potentiometer in the servos).
• Posts: 741
edited May 2011
I have doubled up servos for the shoulder of a robot arm. It worked, but I wasn't entirely happy with it.

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.
• Posts: 10,002
edited May 2011
You might also want to look at the arms at CrustCrawler.

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.
• Posts: 1,057
edited June 2011
Well, i tried to double up two servos(9 gram) and the result was exactly what some said would happen. The servos were fighting each other to the point where a lot of heat built up. BUT it does work sort of and the heat only builds up sometimes. The behavior is erratic though? It heats up a little and then stops working... I will try add some code that will make the one servo not fight the other. I will post some videos and pics soon...Thanks for all the help:)
I got the best hair around...Guaranteed!
• Posts: 64
edited June 2011
Hi Ravenkallen:

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.
• Posts: 4,561
edited June 2011
al1970 wrote: »
...

The other way to go about it is to use rubber band power. ....

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.
• Posts: 741
edited June 2011
Well, i tried to double up two servos(9 gram) and the result was exactly what some said would happen. The servos were fighting each other to the point where a lot of heat built up. BUT it does work sort of and the heat only builds up sometimes. The behavior is erratic though? It heats up a little and then stops working... I will try add some code that will make the one servo not fight the other. I will post some videos and pics soon...Thanks for all the help:)

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://propellerpowered.com/?p=329
• Posts: 1,057
edited June 2011
Actually late last night i figured out the source of the problem. The way i had the servos connected made them come together at a odd angle and that caused the servo to power up and try to fight the other servo. I tried connecting them differently and there does not seem to be as much noise/ heat... Further experimentation is needed. Thanks for all the comments, i will consider every option:)... Oh and this is a smaller arm. I didn't want to start constructing my bigger arm until i had a working mini prototype.
I got the best hair around...Guaranteed!
• Posts: 1,106
edited June 2011
You could use a mechanical differential to interconnect your servos. With that, you can have different motion on each of the servos with a single unified output result. Should be available from hobby stores that support RC Cars!.
Jim
• Posts: 18,732
edited June 2011
Capital idea, RS_Jim! A real differential OR a generally compliant mechanical coupling between the two servos would allow some angular mismatch without too much servo infighting. I mean, can't we all get along? :)
"When you make a thing, a thing that is new, it is so complicated making it that it is bound to be ugly. But those that make it after you, they don’t have to worry about making it. And they can make it pretty, and so everybody can like it when others make it after you."

- Pablo Picasso