Continuous Servo for Projector Screen

I believe I bought a continuous rotation servo ( However I have some questions about the servo's capabilities.

I'm working on creating an automated projector screen. The screen will roll up and down. The servo will roll the screen up and down connected to a long metal rod the screen is attached to. At the bottom of the screen is another rod that keeps the screen taught and straight. The weight of screen+metal rod is ~5lbs, however the top rod is only 1inch, so its about 2.8kg-cm in torque pulling the top rod down.

The servo can do 12kg-cm, which is great when pulling the screen up. In addition the natural torque of the servo is good enough to hold the rod in place when off.

When I set pulsewidth to ~1500ms, the servo it stops in its location (yay). However 1500ms pulsewidth leads to the servo speed to be slow, but also seems as though no juice is going through it because I can manually turn the servo when pulsewidth = 1500ms.

In addition, when I lower the screen with my servo slowly moving VERY slowly forward (pw=1502ms) the weight of the rod/screen unravels as though it were in a free-fall. I thought since setting speed to 1520, the servo would slowly turn, but also hold the weight.

  • Is this the way a continuous servo should operate?
  • Any suggestions for how to move my screen up and down at a controlled rate and also keep it from moving when the motor/servo is off?

use a motor with gear and low RPM. microswitches to detect ends.
The servomotor have no holding torque when off.

I think your problem is that a continuous rotation servo is not really a servo. When you send a 1500usec pulse it thinks it is at the correct location and that there is no need to apply force to stay there. With a proper servo it immediately knows it is not at the correct location when you try to move the servo arm - so it resists the motion.

If the servo is fast enough you could have it drive the screen through a worm gear which will prevent slippage. The servo has the advantage that it has the motor driver inside it.

Otherwise you will need a small DC motor, worm gearbox and a motor driver board.