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  • Torque

    im basically creating a pan tilt head for a light and the light weighs about 329oz would a servo with the torque of 330 be sufficient or do i need something more powerful


  • #2
    Torque is "twisting force" thru an arm. The units represent force (ex. lbs) x distance (ex. foot). How much of an arm is the light on? (distance from center of motor shaft to center of weight?
    ADEN ENGINEERING
    Automation & Electronic Controls

    Design, Build, Repair, Training

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    • #3
      The arm is 13in long

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      • #4
        Minimum torque then would be 329 x 13 = 4277 oz. inches or 23 ft. lbs. If you need to do rapid moves, acceleration force would be added to that.
        If speed requirements allow, a gear or pulley reduction would reduce torque requirements proportionately.
        ADEN ENGINEERING
        Automation & Electronic Controls

        Design, Build, Repair, Training

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        • #5
          You're going to need to somewhat match the inertia between the load and the motor as well. Servos typically perform best with a match ratio of 1:1 to 10:1 (the lower, the better). (ratio = load inertia / motor inertia)

          You're definitely going to need some kind of reduction.

          To get a rough idea, you need to find the moment of inertia of the lighting unit.

          The equation would be I=m(r^2) for a point mass type where m is the mass in Kg and r is in meters.

          I=(9.33)(.3302)^2 = 1.017 Kg m^2

          To give you an idea of motor inertia, a 1Kw medium inertia motor has an inertia of .000598 Kg m^2, quite the difference.

          (ratio = 1.017 / .000598 = 1700 (remember we want 10 or below)

          The good news is any reduction ratio you use effectively reduces that mismatch by the square of the ratio - a 10:1 gearhead reduces the mismatch by (10^2) 100.

          Square root of 1700 = 41.23 (ideal reduction ratio)

          That would be a good ballpark. I'm not taking into account the inertia of the gearhead gearing though - lets ignore that for this example.

          If you had a 40:1 gearhead, you mismatch would be (1700)/(40^2) = 1.06

          Keep in mind your motor speed will also be reduced by the same ratio. A 3000 rpm motor would now be a (3000/40) 75 rpm motor.

          The torque the motor produces will also be affected (example):

          Motor torque x gearhead ratio x gearhead efficiency (%) = usable torque

          That is where the balance lies.

          It's also going to depend on your move profile. The more aggressive the acceleration, the more torque you're going to need.

          There are some free sizing programs out there, I've used 'Visual sizer' in the past with excellent results. I like it because it's not tied to any particular brand of motor.

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          • #6
            So i have decided to go with a stepper motor instead of a servo and use a 4:1 gear ratio would this be a good stepper to go with or should i look elsewhere
            http://www.ebay.com/itm/CNC-Router-M...c09b91#Product

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            • #7
              Originally posted by cmbasse View Post
              So i have decided to go with a stepper motor instead of a servo and use a 4:1 gear ratio would this be a good stepper to go with or should i look elsewhere
              http://www.ebay.com/itm/CNC-Router-M...c09b91#Product
              "would this be a good stepper [for this application, using a 4:1 gear]"
              No it would not, not even close. It looks like you took the 4277 oz. inches number johnaden gave and divided it by 4 (= 1069.25) for the 4:1 gear. Now while 1205 is indeed greater than 1069, that is the maximum standstill holding torque of the step motor, on a good day with no vibration the motor may hold your light in a fixed postion (working against gravity with the lever arm horizontal) but you will be unlikely to actualy be able to move it. (on the other hand if you are using an orentation where you will not be fighting gravity you will be in better shape, but it will be still be quite difficult to accelerate the load)

              I recomend you read Adisharr's excelnent post again. A motor similar to the one you selected would probably work well with something like the 40:1 ratio Adisharr sugessted, probably even 20:1 if you need a bit more speed.

              Also since this is Automation Direct's forum, you might consider a STP-MTRH-34127 only a few dollars more and you don't have to wait for it to come from Hong Kong (unless you are in Hong Kong yourself)
              Since you seem to be trying to do this on the cheap, you might consider something like one of these Worm Gearboxes, However, this is far from an ideal option for this sort of application, its just relativley cheap, one big problem is that the step motor will not couple directly to that sort of gear box, a custom adapter will be needed, if you have the skills and tools (an 8" to 12" engine lathe and a drill press would probably be enough) it could be cost effective. But if you have to pay someone to make an adapter you might be better off buying a gear box made for the job (might cost 2 to 5 times as much) You might have been considering a toothed belt for the 4:1 ratio, you might try a countershaft and 2 x 4:1 (= 16:1) probably the easiest and least expensive

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