Types of Endstops
3D printers are extremely accurate and precise positioning systems, but the way 3D printers track their position is different than many people think. The way a 3D printer controller tracks the position of the print head is much different than how a human might do the same thing. A 3D printer does not move the print head to a location in absolute coordinates. Rather, movements are made relative to the current position of the print head. So, for example, you cannot tell a 3D printer to move the print head to the coordinates (15mm, 10mm, 12mm). You can only tell the 3D printer to move 15mm up, 10mm right, and 12mm forward.
Therefore, at the beginning of every print, the 3D printer must move the print head to a starting position, which the 3D printer firmware considers to be the origin, (0, 0, 0). The origin point for most 3D printers is on one corner of the build platform, at a height above the build platform equivalent to one layer height. This origin point is called “home.” Starting every print from the same home position allows the 3D printer to reference its movements.
So, at the beginning of every print, each of the 3D printer’s axes backs up until each axis reaches its zero position. Some kind of switch or trigger is placed at the zero position of each axis so that the 3D printer knows when the zero position is reached. These switches are called “endstops.”
There are three different types of endstops: mechanical, optical, and Hall effect.
Mechanical Endstops
Mechanical endstops are the simplest and most commonly-used type of endstop. Mechanical endstops are simply switches placed on each axis. At the start of each print, the 3D printer moves each axis until the carriage hits the switch.
The PrintrBot Simple uses mechanical endstops.
Mechanical endstops are used on many scratch-built printers.
The actual switches used for mechanical endstops are micro switches. Micro switches have a thin metal lever that presses on a small button to actuate the switch. Micro switches are a good choice because they require very little force to actuate. This means the endstops can be triggered without the carriages crashing into them too hard.
Mechanical endstops are a popular choice in 3D printers because they are very inexpensive and simple to use. Mechanical endstops connect to the 3D printer motherboard with just two wires. Therefore, in terms of setup, one mechanical endstop is mounted to each axis and wired to the motherboard.
Often it is desirable, sometimes even necessary, to adjust the actuation distance for the endstops. This is usually accomplished by installing a bolt on the carriage such that the bolt contacts the mechanical endstop to trigger it. By screwing the bolt in or out, the actuation distance for the endstop can be adjusted, thus adjusting the home position.
A bolt allows the home position to be easily adjusted.
Optical Endstops
The second type of endstop is the optical endstop. Optical endstops are the least popular type of endstop because, honestly, they are more expensive and more complicated than mechanical endstops and offer few benefits to offset these disadvantages. The only real advantage to using optical endstops is that optical endstops can be triggered without any physical contact between the carriage and the endstop. This can increase the life of the endstop and decrease the chance of any components being broken from bumping into each other.
The most important part of an optical endstop is a U-shaped optical sensor called a photointerrpter. The U-shaped photointerrupter has an infrared emitter on one side, and an optical detector on the other side. The endstop is triggered with the light beam between the two sides is broken.
3D printers using optical endstops typically have some kind of thin blade mounted on the carriage. The blade breaks the photointerrupter beam to trigger the endstop.
The optical endstop is triggered when the photointerrupter beam is broken.
Hall Effect Endstops
The final type of endstop is the Hall effect endstop which is a fairly popular option, but not quite as popular as mechanical endstops. Hall effect endstops consist of two main parts. The switch itself uses a magnetic sensor called a Hall effect sensor, which is triggered in the presence of a magnetic field. The second part is a magnet mounted on the carriage. When the carriage nears the endstop, the Hall effect sensor detects the magnetic field and triggers the endstop.
A Hall effect endstop. The black square on the top is the actual Hall effect sensor.
Hall effect endstops detect the proximity of a magnet.
Hall effect endstops, like optical endstops, have the advantage of being non-contact endstops. This means the endstop will be triggered without physical contact from the carriage, which can reduce wear on parts. Also, because Hall effect endstops do not make physical contact with the carriage and work over a distance (unlike an optical endstop), they can be calibrated to trigger at an exact distance from the carriage. The main advantage of Hall effect endstops is that they allow the home position of each axis on the 3D printer to be precisely controlled and they allow the home position to be reached with a high degree of repeatability.
The downside to using Hall effect endstops is that they are more complex and somewhat more expensive than mechanical endstops – but they are less expensive then optical endstops. Hall effect endstops require a PCB with a few other components in addition to the Hall effect sensor itself, like an operational amplifier and a few resistors. That said, simple Hall effect endstops can be constructed for under $2, so the endstops only account for a very small portion of the total cost of a 3D printer.
This is the Hall-O endstop, the most popular Hall effect endstop design.