Help with Servos for Rocketry Applications

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Richard Dierking

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I don't have previous experience using small servos. I'm learning and gaining an appreciation of what they can do. I have two major applications. The first is small control surfaces on a HP rocket. The second is servos on a TVC gimbal mount for a low to mid-power rocket, up to 38 mm H-impulse, I hope. For this thread, I would like to focus on servos for TVC gimbals.

So, I have a couple makes of small servos (often referred to as micro servos) for a 29 mm TVC gimbal and I would like some guidance (pun intended) on what I should be looking for on servo specs and maybe tips on the best manufacturers. Actually, if I had a recommended manufacturer that would help a great deal. Eventually, I would like to create a 38 mm TVC gimbal after gaining some experience with the 29 mm mount.

Basically, I see two types of construction for these small servos; plastic and metal gears (photo). These are both 90 degree servos.
IMG_2762.jpg
I have two servos (X & Y axis) supplied with a BPS Space Signal R2 kit that are plastic gear type manufactured by Osoyoo, SG90.
I found the following specs for these servos:
Operating Speed: 0.3 sec/60 degrees (voltage not given)
Stall Torque: 1.5 Kg/cm
Voltage: 4.2 to 6 volt
When I check them with a Hiwonder servo tester, they seem a bit "noisy." These are installed in a test stand to check if the TVC mount will be OK using a CTI G33
IMG_2761.jpg
I have another TVC gimbal in a rocket. This gimbal is made with PETG and using TowerPro MG90D, metal geared servos. Using the servo tester, these seem a bit quicker, smoother, and and not so much noise at the center position. Here are the specs:
Operating speed: 0.10 sec/60 degrees (4.8 volts), 0.08 sec/60 degrees (6.6 volts)
Stall Torque: 2.1 Kg/cm (6.6 volts)
Voltage: 4.8 to 6.6. volts.
IMG_2764.jpg
I see many different listings for servos using Amazon and prices vary a lot.
What do you think is best?
Should I be testing these servos in a different way?
I would appreciate your thoughts on this.
 

Richard Dierking

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Thank you watheyak!
Is the accuracy and precision of servos represented in any of the specs? Speed and power are there, but it seems there are important details missing when I see the info for servos. For example, if I would like the TVC gimbal on one of the axis to move to 0.5 degrees accurately, and move back to neutral and do this kind of thing perhaps many times, would there be something in particular to compare in the specs for servos? Or, is the controller the most important thing for this?
 

watheyak

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I that would be a function of the pot in the servo. The higher quality the pot, the higher the accuracy.

What you're asking it to do along with the accuracy is pretty basic. It needs to do that in an R/C plane all the time. Especially when you start to look at applications with active stability where there are many small movements over the course of one second, say on a rudder or a tail rotor.

While those stats may not be measured or available, the MKS servos are going to be some of the best in that area.

Plus, it's only as accurate is your linkage. Any slop there and even the best servo wont matter.
 
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