SkyStopper uses an innovative reference mechanism
For the skystopper to operate correctly, it is important for the controller to take into account the exact position of the carriage on the threaded shaft.
Rather than use a limit switch and an index sensor, the SkyStopper uses a large disk with a pin on it, which contacts a pin on the carriage body.
Seeking the reference position is done at partial motor power. When the carriage has rewound to the start point, further rotation of the shaft is prevented.
The process of seeking the index position steps backwards quickly for the full range of the shaft. When the carriage reaches the start position, there's a buzzing sound, but the motor is not strong enough to cause damage. You can short-cut the buzz by pressing the red button on the remote, or the button on the control unit, or it will simply buzz for a while and complete the process without aid.
When the controller is sure that the carriage has travelled to the start position and has hit the pin, it then moves the carriage forward 20 steps, and slowly back 40 steps. 20 of those steps will be prevented by the pin. Finally, it moves the carriage forward 5 steps from the absolute stop and designates that the reference position.