I am a model train guy. I am just getting into this servo stuff for little animations on my layout. How do I figure out how to pick a continuous rotation servo for a drawbridge? I have no idea how to figure out how much torque my bridge requires to operate.
A drawbridge only needs to rotate ~90 degrees - why would you need a continuous rotation servo rather than a normal servo? To “calculate” how much torque you need, which is calculated as force times distance (T = F x R), where R = distance between the axis of rotation and the point of application of the force.
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Weight the entire bridge (the part which you want to pivot), and then find the center of mass. You will assume all the weight is concentrated at this point. Next, measure the distance between the center of mass and the pivot point. This will give you a ballpark idea of the torque required at the pivot point, assuming the servo’s horn is fixed there.
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If the bridge is relatively lightweight, choose a servo which will have no issue lifting it, such as the 645MG:
lynxmotion.com/p-293-hs-645m … servo.aspx -
If you need a continuous rotation servo, but like the idea of rotating to a specific angle, consider the 785HB winch servo:
robotshop.com/productinfo.as … lang=en-US
My drawbridge raises and lowers now with a mechanism I cobbled together. A hobby motor drives a reduction gear box which turns a 1 1/2" diameter cylinder. The cylinder has 2 lengths of monofilament attached to the bridge. The bridge has 2 arms of a toy bulldozer as hinges. The fishing line goes from the rotating cylinder over 2 rollers on towers to the hinge points. I need a continuous servo for this reason. The bridge is a piece of 1/4" plywood with a 10" straight RZ track and a piece of curved track also. My guess is it weighs approx. 2-4 lbs. the pivot point is approx. 8" in from the hinge point. Does all of that give you enough info to recommend a servo?
Calculating the torque in that situation is a bit harder. Your setup seems similar to this:
cnx.org/content/m42172/latest/Fi … 03_10a.jpg
Unfortunately T in that image is tension, not torque.
If you want to be technical, you’d need to do a torque balance about the pivot point, solve for T (tension) then calculate the torque based on torque = tension x radius of the drive pulley.
If you’re not quite prepared to do this, and are not sure if a continuous rotation servo will do the trick (sounds like it might be underpowered) then you should consider a DC gear motor connected to an RC motor controller:
robotshop.com/gear-motors.html
robotshop.com/rc-motor-controllers.html
Something like this would be easy to mount and allow you to adjust the gearing until it moves smoothly:
robotshop.com/tamiya-planetary-gear-box.html
You may want to look into the multi turn sail winch servos like below as they may be what you need. They range from standard size ~13kg to the larger 40kg one.
hobbyking.com/hobbyking/stor … duct=18743
hobbyking.com/hobbyking/stor … g_55g.html
hobbyking.com/hobbyking/stor … g_0_9.html
hobbyking.com/hobbyking/stor … _0_70.html
hobbyking.com/hobbyking/stor … _0_70.html
hobbyking.com/hobbyking/stor … c_64g.html
hobbyking.com/hobbyking/stor … gree_.html
Benson, that’s exactly my set-up! Robert, thanks for the links. They look like just what I need. Either of you want to take a guess as to what one I need based on the specs? Specs: approx. 25" long. Approx. weight 2-4 lbs. Hinges are approx. 6-7" long. I
attached a few photos.
In this instance, it’s better to try to take measurements rather than rely on theory. Is there some way you can measure the tension on the string needed to raise the bridge? Use a weight system attached to the spring - you can even use a water bottle and slowly fill it up (knowing the density of water, you can determine the weight). Once you know that force, and the radius of the pulley, you know the torque.
Wow! That accurate huh? I’ll try…