I just installed both SV42 servos in my Rutan Defiant. Haven't completed my panel upgrade yet so have not powered them up. But I can feel "pulses" in the stick when moving it for either pitch or roll. These pulses feel like they might be magnets on the servo drive motors, as they rotate. (not sure if these are even pm motors) I feel it in both servos, but the pitch servo is more pronounced. Is this normal?
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SV42 roughness
- Thread starter hmanvel
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- Mar 23, 2005
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Actually, feeling the notches when the servos are completely unpowered is not normal and can indicate a failure of a component in the servos. Please give our tech support a call if you can so we can discuss and see if you need replacements.
If the servos are powered and engaged, then the notches can be felt, but the servo should be smooth when not engaged or unpowered. They do offer some resistance to motion, but it shouldn't have any notches in it.
If the servos are powered and engaged, then the notches can be felt, but the servo should be smooth when not engaged or unpowered. They do offer some resistance to motion, but it shouldn't have any notches in it.
Not sure who at Dynon responded above but I can assure you that your servo will have some notchy feel to it even when not powered. Steppers are PM motors that have many poles in em and you will feel it if you spin them. Grab one out of the box and turn the shaft....
This is not noticeable in flight.
If you cause the servo to slip under power, this is a much more noticeable condition and may sound like a bang or jerk.
This is not noticeable in flight.
If you cause the servo to slip under power, this is a much more noticeable condition and may sound like a bang or jerk.
- Joined
- Mar 23, 2005
- Messages
- 13,226
We need to be careful about what we are talking about here to make sure that people know if their servos are good or not, so let me describe what is normal and what is not with our servos:
The servo motor is a stepper, and when unpowered, it has some very small variation in resistance as you rotate the shaft. This occurs about 800 times per revolution of the output shaft. If you rotate the output shaft of the servo by hand slowly, you should feel a very small amount of of "notchiness" which is almost like a bad bearing or fine sand in a geartrain. It's very fine, very repeatable, and it gets smoother as you rotate faster because the inertia of the system hides it.
What is not normal is if you move the servo quickly and feel very big notches that have a huge amount of variation, almost like the servo is fighting you now and then. This only happens when you move quickly. This is indicative of a failed component inside the servo. We specifically test for this on every servo before it ships and have never seen this fail in the field, but we know it to be technically possible.
I admittedly think of the "normal" notches as so smooth that I don't notice them, but it is true it's not as smooth as a DC motor, and I was over-assuming that the original poster was referring to the larger, problematic ones.
The servo motor is a stepper, and when unpowered, it has some very small variation in resistance as you rotate the shaft. This occurs about 800 times per revolution of the output shaft. If you rotate the output shaft of the servo by hand slowly, you should feel a very small amount of of "notchiness" which is almost like a bad bearing or fine sand in a geartrain. It's very fine, very repeatable, and it gets smoother as you rotate faster because the inertia of the system hides it.
What is not normal is if you move the servo quickly and feel very big notches that have a huge amount of variation, almost like the servo is fighting you now and then. This only happens when you move quickly. This is indicative of a failed component inside the servo. We specifically test for this on every servo before it ships and have never seen this fail in the field, but we know it to be technically possible.
I admittedly think of the "normal" notches as so smooth that I don't notice them, but it is true it's not as smooth as a DC motor, and I was over-assuming that the original poster was referring to the larger, problematic ones.
Okay, after a return call from Dynon asking me to verify things: I removed the pushrod and confirmed that when moving the output arm more rapidly, the "notches" smooth out. Makes a very slight whirring noise. Reinstalled and tried with the control stick, still seems like the "notches" are magnified somehow, but only when moving the stick very slowly. When moving the stick faster it also smooths out. Also, this is just hooked to an elevator torque tube, not connected to elevators right now.
Thanks for the clarification Dynon....
Harry, you will not notice it in flight, heck you most likely will not notice it once the engine is running. The reason it gets magnified is in the geometry of the linkages and the fact that your stick movement to servo movement ratio is such that you are moving the servo less per stick movement. This translates into more response at the stick to these poles in the servo.
Harry, you will not notice it in flight, heck you most likely will not notice it once the engine is running. The reason it gets magnified is in the geometry of the linkages and the fact that your stick movement to servo movement ratio is such that you are moving the servo less per stick movement. This translates into more response at the stick to these poles in the servo.