I am building a small/miniature electrical cylinder which will be used in scale models, the dimensions of the cylinder(s) are .5" diameter by 3" stroke. A small micro gear motor in the cylinder will be rotating a lead screw which will then extend the piston/ram, the gear motor will be proportionally controlled via a continuous rotation RC servo board (cheap form of a speed controller). My issue is that I don’t know how or what I can use as a limit switch to stop the ram once it has reached its stroke, my obvious constraint is size and I want it to still look like a ram. Could I use a membrane pot on the inside wall of the cylinder? Any thoughts ideas etc would be very much appreciated. Thanks
If an external switch will do, you can buy very small microswitches, e.g. http://www.allproducts.com/ee/switronic … h_ssm.html
Similar switches can be salvaged from junked floppy/CD/DVD drives or cassette recorders. Just about any device that detects a change of physical position of a part uses either a switch or photointerruptor.
Small hall effect sensor comes to mind as well. Would it be possible to design in the switch? Could a metal endcap of a plastic ram/piston make contact with a metal cylinder to make the switch?
Wow these small hall effect sensors seem great! I am just confused how I would be able to retract the cylinder once it has reached its stroke. Correct me if I am wrong but once the stroke is reached the sensor will cut the power to the gearmotor, but with the power cut how can I get it to reverse/retract with out somehow introducing another magnetic field to switch the sensor? Could the magnetic field be created by the signal wire of the rc servo board?
In modern days, the limit switch does not normally interrupt the power, but rather it just sends a signal back to a micro that stops the motion. The micro can then implement the logic that would allow the motor to turn the other way when up against a stop.
In the old days, with fancy wiring, a mechanical implementation was performed that did pass the power of the motor through the limit switch. If the power was too much for a switch, a relay was used. Now unless your motor is tiny, I suspect that the power from the motor could not be switched by the Hall effect sensor directly. If a micro is not in your future (a small PIC would be enough), then you will need some FETS to replace the relays and you will need o implement some clever wiring. I would Google limit switches and try to find an example of an old fashioned 'mechanical" implementation.
To be more specific … what controls the motor (beyond the speed controller) ? How does it get told to run/stop and in what direction ? Is this supposed to be like your windshield wipers, just a momentary push on a switch and it completes a full (or in your case half) cycle ?