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I have a camera mounted in an Adafruit pan-tilt mini kit with 2 servos (SG92R).
Installed python-smbus and i2c-tools.
From raspi-config I enabled 5 Interfacing options P5 I2C.
Also installed Adafruit_Python_PCA9685.
Executing a python script I position the camera.

I only use pan-tilt to fine tune the camera position just at startup for timelapse.
I found that despite I added a @reboot line in crontab to run that script with the same two parameters (horizontal and vertical servo positions) I used the first time, just in case of restart after power failure or manual reboot, the position of the camera is slightly different than the original one.

I tried disabling I2C interface option but the result is far away the original position of the camera.

Then only way I guess I can accomplish that is to disconnect the power of the pan-tilt controller (Adafruit) after getting the best position but this could be a mess for me.

Is there another way to get servos quiet in case of reboot?

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  • maybe a disk brake that is clamped by a spring and released by another servo
    – jsotola
    Feb 20, 2018 at 19:42
  • Sounds good but tricky. No idea how to accomplish that.
    – dstonek
    Feb 20, 2018 at 20:05
  • something like this, but the servo would pull the brake open ... youtu.be/mOGItAg7tmY?t=39
    – jsotola
    Feb 21, 2018 at 2:17

2 Answers 2

Reset to default
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Perhaps you need more expensive servos? They are more likely to go to the same position when given the same commands (pulse widths).

Also remember that servos will only maintain a position against a force if they are powered and are continually receiving the same commands (pulse width).

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  • This is my kit: cdn-shop.adafruit.com/product-videos/1024x768/1967-02.mp4 cdn-shop.adafruit.com/1200x900/1967-01.jpg I understand what you say about more reliable servos but I also would need the assembly structure. I found one larger (metal) compatible with these servos: MG995 or HS322,HS422,Hitec,Parallax,Futaba, etc 40×20×36mm. I don't know if they would serve my needs. They also must be compatible with my PCA9685 16-Channel PWM Servo Motor Driver I2C Module. I remember I tried a couple of EMAX ES08MD servos but they didn't work
    – dstonek
    Feb 20, 2018 at 23:54
  • 1
    @dstonek Perhaps there is simply too much slack in the system to get repeatable results. I think the astronomy guys would probably use stepper motors on their mounts for a variety of reasons. You may need to find a more specialized site to ensure that the proper eyes see your problem and can offer a good solution.
    – joan
    Feb 21, 2018 at 8:24
  • Yesterday I've replicated this question at forums.adafruit.com/… and I got interesting answers.
    – dstonek
    Feb 21, 2018 at 16:02
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I finally solved this issue thanks to Adafruit techs.
I added a non-latching relay to control the power output from the RPi to the servo controller PCA9685. For example GPIO6 is set to INPUT by default so when I need to move the servos I change the state to OUTPUT and the servos get power.
After adjustments are made I turn the state to INPUT to cut the power.
As said, after reboot, servos don't get power to move because I choose a GPIO pin with an INPUT state (LOW/OFF).
Here's the wiring schema I used enter image description here

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