Hi, I’m an artist working with bird wings that I connect to servos, making them move again. I control them with arduino nano copy and homemade shield. Right now I have a wing weighing about 150g being 80 cm long that I attach to the servo from the bottom. I gave ChatGPT a go at calculating and suggesting a suitable servo and gave me Hitec HS-645MG which turned out too weak. I have a budget for more expensive, I just don’t want to hunt any longer. What should I think about and do you have any suggestions?
At 7.7 to 9.6kg-cm I would think it would be strong enough.
Either your measurments are off (150g, 80cm) or your servo power supply could not supply enough current.
Best to find out which before you buy another servo
from a physics point of view, that's not enough information.
The distribution of weight along the wing is key for calculating the torque needed to move the tip of the wing up or down because torque is defined as the product of the force applied and the distance from the pivot point (the servo acts as the pivot point) to the point of application of that force.
When the weight of the wing is evenly distributed, the center of mass is located at a specific point along the length of the wing, which affects how much force is exerted due to gravity at that point.
If the center of mass is closer to the servo, the torque required to lift the tip of the wing will be less because the effective distance from the pivot to the center of mass is shorter. Conversely, if the weight is concentrated towards the tip of the wing, the center of mass will be further from the pivot, resulting in a greater torque requirement to achieve the same upward movement of the tip.
Looking at your picture, the center of mass is likely close to the Servo.
You could approximate everything and say the wing is like a 150g mass at say 30cm of the servo.
The force exerted F is the mass x gravity = 0.15 kg x 9,81ms-2 = 1,4715 Newton
The torque required to lift the weight can be calculated using the formula F x distance, so 1,4715N x 0.3m = 0,44145 Nm or in units you usually find for servos, 4.5 kg.cm (0.15kg x 30cm)
Your Hitec HS-645MG offers 9.6kg.cm under 6V so it should be plenty unless there is something else at play.
A possible issue is that it's challenging to model the wing as a simple weight force applied at the center of mass because the weight distribution is uneven; for instance, there may be more weight at the front of the wing compared to the rear. This uneven distribution can create a torque that causes the wing to attempt to rotate around the pivot point, exerting pressure on the axis and the shaft, which may lead to friction that hinders the proper functioning of the servo.
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Wow, that's such an informative answer. Thank you so much for taking the time!
The servo is moving the wing, but it's not smooth and it's clearly too heavy at critical points when it's fighting gravity on it's way down and up again. It's definitely unevenly balanced, I've been thinking about adding weights to the other side to make the pivot point closer to the center. Would that make any difference?
I will double check my power supply in the studio tomorrow, would be amazing if that's the issue!
Yes it would make it worse
It should be 5 or 6V and at least 3A
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The further away from the servo the center point is and the heavier the wing is, the more torque you need …
So no don’t add weight.
Yes, do add counterweights on the other sid if you can. If your application and space permits then counterweights on the opposite side of the pivoting servo shaft as versus the side the wing is on can cancel out almost all the torque needed to lift the wing against gravity. The only torque the servo then needs to provide is enough to handle the peak acceleration you want the wing to achieve. Where they can physically be eprmited counterweights are very effective.
This: Falkirk Wheel - Wikipedia is powered by a series of motors not much more powerful than washing machine motors specifically because it is a perfectly balanced counterweight.
The key is in ‘if you can’. I don’t think it’s possible.
There is no other side according to the pictures… the servo is at one end of the wing where the articulation with the body is. The bird’s body should stay where it is when the wings flap and both wings go up and down at the same time so you can’t tie them so that one would act as the counterweight for the other.
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