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IndustryArena Forum > CNC Electronics > Stepper Motors / Drives > Stepper Motor Driving Torque
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  1. #1
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    Stepper Motor Driving Torque

    Hello. I am working on my first project with stepper motors. It is about the design of a robotic arm.

    I am using the two stepper motors with the links mentioned below.

    Item # PK244-04AA, Stepping Motor on Oriental Motor U.S.A. Corp.

    Akcesoria CNC - silniki krokowe, serwo, przek³adnie - czyli wszystko co ci potrzebne

    My question is, would it be possible for any of these two motors to pick up a load of around 1 Kg?

    I tried to drive these motors and pick the load using gears but the problems does not seem to work out. Is there any gearbox that I can use to pick up this 1Kg load with any of these motors?

    I have to make this robotic arm work with these motors, please help, what can I do to make it work right?

    If anyone wants, I can send a jpg image of the gears that I am using right now. Please try to reply as soon as possible.

  2. #2
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    I'll assume your motor has 36 oz-in of holding torque. That means it could hold (not necessarily lift) a 36 oz weight on a string wrapped around a 1" radius drum on the motor shaft. If your "drum" was a radius of 36", then you could only hold 1 oz of weight.

    In your case, I'd guess you have a robot arm several inches in length and most of the torque the motor generates is needed just to hold the arm, let alone lift anything.

    You need to gear down the motor to get a higher torque capacity. Pictures of your robot and it's drivetrain would be useful to make further suggestions.

    Matt

  3. #3
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    Pictures of robotic arm and gears attached

    Thank you Matt.

    I am attaching the images of the setup that I have for the motors and robotic arm. You can see two motors in image1 (actually there are three but the third one is inside the robotic arm jaws). The motor on the left most side is motor1, which has a holding torque of 36 oz-in. The gear attached to it is also shown (details in images 2 and 3). I want to concentrate on motor1 now. When I understand the concepts behind it, I think I will be able to fix motor2 (the one on the right) on my own.

    The motor does not have to hold the arm, the arm is held itself without exciting the motors. The motors just have to move the arm up and down from the stationary position (as shown in image1).

    I have tried to take the pictures as clearly as possibly. Hope that helps.
    Attached Thumbnails Attached Thumbnails Image1.jpg   Image2.jpg   Image3.jpg  

  4. #4
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    Worm gears are inefficient due to the sliding friction. Have you considered using regular gears?

  5. #5
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    About gears

    Actually I got this robotic arm arrangement from my supervisor and he assigned me the task of making it work without making a lot of changes to the hardware. So, with this gear arrangement, it is not possible to move my robotic arm? Can I do anything to reduce friction to make it work? What can I do to make it work with this gear arrangement? Please help.

  6. #6
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    Maybe it does. As I said, this is my first project and I am not a mechanical student. So I do not know much about gears and stuff that is why I posted here to get help, not to read sarcastic comments. I am sure there are a lot of other students/learners who would be willing to know the same things I am trying to ask. So please try to provide some help.
    Thanks

  7. #7
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    I think most of us can agree the design of the robot is quite poor. With that said, it could still be made to work.

    There is a huge amount of weight sticking out and a tiny motor expected to move it. If you design like this, you need the motors to get bigger and bigger as you move closer to the base of the robot. It's just like a tree, the branches get bigger as they are asked to support more weight.

    Some of the bad:
    Large overhung weight
    Axial loads on motors will kill the bearings
    Frame should be beefy at the base and thinner toward the end
    Base motor should be much bigger and then progressively smaller as you move along.

    Probably the easiest fix would be to add a counterweight to the back of the main arm. This weight should be sized so that the arm is balanced. This will minimize the torque required by the first motor. It may be possible to counterweight the second arm as well for the same reason, I am not sure what range of motion you need but it may be limited by a counter weight.

    Matt

  8. #8
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    One thing that bothers me is that you haven't said how much it can lift as it is. I think testing the current arm is the first step. How much can it lift when fully extended? by moving each joint?

    Matts suggestion of a counter weight is a very good one. The only downside is that rotation of the base will be more difficult. A more difficult but better solution would be to move the "elbow" motor back just behind the "sholder" gear, turned 90' with a shaft extension coupled to it, to take the motion back to the worm gear drive at the elbow. That both reduces the hung weight and uses the motor as the counter ballance. It would require: 1) a new mounting bracket for the motor, 2) the shaft extension and couplers 3) a new mount for the drive gear in the elbow worm gear.

    A larger motor at the shoulder might help, but I think a worm gear with a finer pitch would make more of a difference. The motion would be slower, but stronger. As noted, friction would increase, but I think it would still be worth trying. The other option is a gear box between the motor and the worm gear, but the problem there is that it introduces additional slop and friction. Replacing the worm gear with a gear box is probably best, but the most difficult.

    I do hope you will keep us in the loop and show us what changes you make and how it changes the design.
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  9. #9
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    Thank you for your replies James and Matt. About having a counterweight; well I think it might not work.
    I forgot to mention that I was using half stepping. Tomorrow I will try full step sequence as it provides higher torque. But the problem I find with adding a counterweight is that the motor is not able to move the robotic arm downwards as well. The counterweight will help when I try to move the afm upwards.

    When I run the motor, it tries to move and produces a ticking sound but is unable to rotate. Let me perform a set of experiments tomorrow and I will ask for more help.

    James your suggestions are really helpful, I will try them for sure after performing tomorrow's experiments.

    Thanks
    Ema

  10. #10
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    If it is perfectly counterbalanced, the torque to raise or lower the arm (move it) will be virtually identical no matter if it is going up or down. You only want to counteract the weight you have hanging there, nothing more.

  11. #11
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    Quote Originally Posted by EmaBlunt View Post
    ...the motor is not able to move the robotic arm downwards as well. The counterweight will help when I try to move the afm upwards. When I run the motor, it tries to move and produces a ticking sound but is unable to rotate.
    Ah! This is new information and VERY helpful! Now I think what you need is a more powerfull driver and motor.

    What driver are you using? A good driver will provide full power even in half step mode. E.g. the open source www.linistepper.com driver does so.

    What are the specs on your /existing/ motors? Is that what you listed in your first post? If those are your current motors, then you need bigger motors and a driver to match.

    The best way to figure out what motor / driver you need is to power off the motor, then turn the shaft with a torque wrench. The calculator at:
    Stepper Motors
    will allow you to find what you need given that torque and the minimum RPM at which you want to turn it.
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  12. #12
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    The shoulder motor (the left most motor in image1 of my previous post) is:
    Item # PK244-04AA, Stepping Motor on Oriental Motor U.S.A. Corp.

    The elbow motor (the one on the right) has the following specs:
    Komocon
    16H40H-1006B
    1.8 deg,
    1.0A
    6 ohm/phase (measured from the ohmmeter).
    I could not find any information about the elbow motor on web, all these are written on the motor.

    I made the driver myself. The image attached (driver.jpg) shows the design. I am using IRFZ44N in my designs which has a current of 49A.

    I tried a few experiments today. I used full stepping. I tried the counterweight method on shoulder motor by holding the arm with my hand and even pulling it upwards and downwards with my hand but it still was not able to move.

    On the elbow, I tried to add a little weight near the hand of the arm (100 grams maybe) and tried to move it downwards with a little push of my hand, and it started moving downwards. It just needed a little momentum to move the elbow joint downwards. I havent tried to move it upwards. I know it will be a problem if I am adding a little weight as I did. But I want to move it downwards for now, I will deal with the upwards movement of elbow later.

    There is no progress I could make on the shoulder.

    Now I have a few questions:

    My shoulder motor has the following specs:
    24V DC
    120 ohm/phase
    0.2A (200mA)

    I set the voltage on the power supply to 24V but when I connect the supply to the motor, the voltage does not go above 14.7V. The current drawn from the supply is 210mA. Why is the voltage not increasing above 14.7V? Similar thing happens with the elbow motor, I supplied 6V from the power supply but the voltage does not go above 3.7V. the current is around 1A (the current rating is 1A for the elbow motor).

    Secondly, finding bigger motors is not an easy thing for me, do not have a lot of budget. Maybe I can do something about the design of the robotic arm and gears. But I will go for changing the motors too if there is no other possibility.
    Attached Thumbnails Attached Thumbnails Driver.jpg  

  13. #13
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    As I've asked before, is there a reason why you cannot use a high ratio gear instead of a worm drive? The latter waste too much power.

    I assume you are using a lab power supply with current limiting. That is why your voltage does not go higher.

    If you want better performance from that motor, you will need to use a better drive. They will put out much more power than the circuit you are using. What is your budget?

  14. #14
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    There is no reason behind using the worm gears. My adviser/supervisor gave me the robotic arm as shown in the images. My task is to fix it with minimal changes to the design. I talked to him about changing the worm gears and he said that he got it manufactured by spending a lot of money and time and he is not in favor of changing the worm gears. Please suggest me a gear for my design and I will talk to my adviser about it.

    So you are suggesting that my drive circuit is not good? What drive circuit do you propose?

    Secondly, if my motor is a 24V, 0.2A motor, can I supply it with more current and more voltage than the rated voltage and current? Won't it be unsafe for it to provide more than the rated voltage and current?

    My budget is less than $100. You might laugh at this, but that is true.

  15. #15
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    I would /strongly/ (strongly) advise you to take the time to measure the actual torque required to turn the motor shaft before spending ANY money on anything. Then use the calculator at
    techref.massmind.org/techref/io/steppers.htm#Estimating
    to estimate how many watts you need to move that at a reasonable speed. I forgot to mention that you should do that measurement with your desired load /attached/ to the arm so you can see what it will actually take in real practice to lift that load.

    Doing anything before you know what is actually required to spin the shaft is silly. I would have said that from the start, but we all thought the motor was already spinning, and was just stalling out when you tried to life a load.

    My guess is that you will need a larger motor more than a better driver. A better driver would get you more speed. H500 might be right that a better power supply would help.
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  16. #16
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    The simplest mechanism is 2 gears, with one having a much larger diameter. You can get them at a hobby shop. A cheap $10 cordless screw driver from Walmart would have a very high torque planetary gear drive that can be used.

    If you use a better drive, you will get several time more power out of your motor. If you can solder, this kit will work well. I just assembled and tested one.
    http://users.skynet.be/ldt/CNC%20ele...4_pricing.html

    Others might be good also, but I have never tried them. A 6560 Chinese drive from Ebay is the cheapest, but most of them are incompetently design and will self destruct quickly.

    The proper way to drive a stepper is to use as high of a voltage as possible and then limit the current to the rated value. The better drives does that electronically.

    $100 is plenty. You could order a 400-500 oz-in motor and a better drive. That will give you enough torque to drive that worm gear.

  17. #17
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    H500 is right about all of that. The planetary gear drive from the cheap screw driver is a very good idea. I'd still measure the torque first and figure out how much power you need, then if it's more than you can provide with a good motor and driver, you know you must take the step of replacing the worm gear drive first, then measure the torque required again.
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  18. #18
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    Thank you guys. I am going to measure the torque required using torque wrench first and will see which motor will be required. But how would I know that my motor will be able to work with worm gear or not? I mean for example the wattage turns out to be 2 watts and my motor is a 24V DC with 0.2A current. That would make 24*0.2=4.8 watts. Would my motor work fine with worm gears or not?

    I talked to a person on phone who deals with stepper motors and he said that he has geared stepper motors and as far as I think, these geared motors have enough torque no matter they are small. I am going to visit him in a couple of days. What do you guys think?

    H500 do you have the schematics of the drive circuit that you posted? I cant find the schematics. If I have the schematics I can solder it myself.

  19. #19
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    Given your example, then yes, your motor /should/ work. If that were the actual finding, then I would say the problem is in the driver and one that regulates the current as H500 said might do much better. But if the torque measurement shows that you need 10 watts, then you will need to replace the motor, and maybe a bigger motor will be ok with your current driver, but a better power supply. If the wattage is up around 50 or 60 then you should look at a new motor AND driver. As H500 said, all of that is in your budget. If the need is several hundred watts... then you might be in trouble, $$ wise.
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  20. #20
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    The problem with worm gears is that the friction is hard to determine. A rough calculation would be as follow:

    Friction force = (length of lever arm) / (radius of gear) * load * (coefficient of friction)
    Coefficient of friction = .2 to .8 for steel on steel. Let's assume .5

    friction torq = (friction force) * (radius of worm)

    Your motor would need that amount of torque just to overcome the friction on the worm drive. More would be needed to lift the load.

    The schematic and assembly instruction can be found here:
    http://users.skynet.be/ldt/CNC%20ele...THB6064AH.html

    A 40v supply with that driver might be able to move your mechanism, but keep in mind that 35 oz-in is VERY weak for a stepper. Look here
    Stepper Motor

    for some better ones. I'm using KL23H276-30-8B. With the proper gearing, it is able to lift a 115lb milling head upwards at 100ipm.

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