We discussed limit and home switches recently and I suggested that magnetic reed switches would be a suitable alternative because of the long lifetime and sealed switch contacts. It was questioned if the repeatability of actuation would be sufficient to serve as a home switch and I opined that it would given the use of a very small but powerful magnet.
This is so because the strength of the magnetic field surrounding a magnets falls off in strength by the inverse third power of distance from the magnet as measured in radii of the magnet. So, the smaller the magnet the more quickly the field diminishes with distance. This is what is needed to provide a sharp and repeatable actuation of the switch.
I decided to quantify this tonight. The setup is a very small rare earth magnet from the optical tracking mechanism in a CD Rom drive glued to the end of an aluminum rod held in the mill spindle. This is adjusted to positively actuate the garden variety burglar alarm magnetic reed switch sold by Radio Shack.
The measuring device is an Interapid .01mm per division dial indicator. The protocol is to use the home axis function in Mach 3 to have it home the Y axis, taking note of the indicator reading. Then the magnet is moved well off several inches for perhaps 30 seconds and the process is repeated.
The result was very good with a repeatability error band of +0.01/ -0.03 mm over many cycles. It has a hysteresis of 1.1mm which is just right to deal with any backlash that may be present. The backlash compensation at the time of the test was disabled and the measured backlash of the axis was ~.01mm.
The measured backlash isn't due to looseness in the leadscrew but is due to the total amount of system compliance (flex) present.
I didn't bother to test it with a button sized supermagnet because it would actuate it from a distance of several inches which wouldn't be at all useful.
The numbers translate to +.0004/-.0012 inches. If the home operation is repeated one second after the initial homing the error band reduces to +0.0/-0.01mm or -0.0004 inches. The measured error band may well be due entirely to system compliance instead of variability of the switch actuation point.
Conclusion: If used properly with a very small but strong magnet a magnetic reed switch is a viable option for system home and limit switches.
This is the setup I used.

These are the magnets. Two are used in every optical drive read/write head assembly.
This is so because the strength of the magnetic field surrounding a magnets falls off in strength by the inverse third power of distance from the magnet as measured in radii of the magnet. So, the smaller the magnet the more quickly the field diminishes with distance. This is what is needed to provide a sharp and repeatable actuation of the switch.
I decided to quantify this tonight. The setup is a very small rare earth magnet from the optical tracking mechanism in a CD Rom drive glued to the end of an aluminum rod held in the mill spindle. This is adjusted to positively actuate the garden variety burglar alarm magnetic reed switch sold by Radio Shack.
The measuring device is an Interapid .01mm per division dial indicator. The protocol is to use the home axis function in Mach 3 to have it home the Y axis, taking note of the indicator reading. Then the magnet is moved well off several inches for perhaps 30 seconds and the process is repeated.
The result was very good with a repeatability error band of +0.01/ -0.03 mm over many cycles. It has a hysteresis of 1.1mm which is just right to deal with any backlash that may be present. The backlash compensation at the time of the test was disabled and the measured backlash of the axis was ~.01mm.
The measured backlash isn't due to looseness in the leadscrew but is due to the total amount of system compliance (flex) present.
I didn't bother to test it with a button sized supermagnet because it would actuate it from a distance of several inches which wouldn't be at all useful.
The numbers translate to +.0004/-.0012 inches. If the home operation is repeated one second after the initial homing the error band reduces to +0.0/-0.01mm or -0.0004 inches. The measured error band may well be due entirely to system compliance instead of variability of the switch actuation point.
Conclusion: If used properly with a very small but strong magnet a magnetic reed switch is a viable option for system home and limit switches.
This is the setup I used.

These are the magnets. Two are used in every optical drive read/write head assembly.

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