I am planning to control my bicycle derailleur with an electronic circuit, the advantages being multiple but not topic of this question.
The actuation would be performed by placing the assembly very close to the derailleur (but not on it) and by winding the usual steel cable around a spool placed on axis of a gear, and by using a motor to turn the gear.
This question concerns the alternatives for the spool self-locking mechanism (and eventually the kind of motor to use).
In the literature I found http://nabilt.github.io/diy_eshift/ and other similar ones that directly modify the derailleur with a stepper motor, and then are forced to keep the stepper motor powered up all the time to keep some torque on the shaft. I consider this inefficient, therefore I require a self-locking system to be able to remove power.
I come up with two ideas:
- worm gear operated by a DC motor, where the steel cable is wound around the gear. This system is self-locking or almost self-locking, according to the gear ratio: the gear cannot (easily) drive the worm.
- a motor driving normal gears with an appropriate reduction factor, but with an additional friction element, whose friction force is greater than the strength of the spring mounted on the derailleur (sorry if I mixed the technical terms). This is what normal bicycles already have: the friction along the cable and in the element placed n the handle is high and keeps the derailleur in place.
Both system would be assisted by a position-sensitive element (a trimmer?) to detect the actual position of the gear and/or of the derailleur, all configured in a closed feedback loop.
I don't consider additional options for the gear such as this one: http://www.agma.org/assets/uploads/publications/10FTM17_Kapelevich.pdf that consists of parallel-axis gears, whose teeth are however shaped in a manner to achieve self-locking without the need of a low-efficiency worm system.
From my point of view, I cannot see any clear advantage of worm gear vs friction except for:
- the worm gear may allow me to build a more compact assembly, thanks to the two axes being perpendicular
- speed vs torque: worm gears reduce the torque requirements, but the motor has to spin a lot and I cannot wait 3 seconds for each gear change.
Concerning the choice of the motor type (this is not the main question though), I think that:
- a worm gear allows me to easily use a DC motor, since torque requirements are low and I don't need to detect the position of the shaft. Moreover, DC motors increase torque with decreasing speed, while stepper motors have the maximum torque defined by the stepping frequency.
- DC are more compact and cheaper, important if I decide to offer this assembly as a kit and not unique, personal prototype.
- I am working with 5V supply and I fear most easy-to get DC motors (old printers, scrap electronics) work on 12V, with a significant reduction of the available torque when operated at 5V.
I was looking for a "mechanics" section in Stack Exchange but I couldn't find it, so I opted for Robotics instead of Electronics.