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Solenoids

Off the shelf options

This solenoid comes from Adafruit. It costs $14.95, weighs 144 grams, lists a 6N starting force with a 10mm throw. Each square piece in the current phenolic-paper based composite system weighs 6.7g meaning that this motor is an order of magnitude larger in mass.


There are also a category of solenoids designed for door latches. This one also comes from Adafruit. It costs $14.95, it's only designed for 1-10 second long activation time. It weighs 148g.


This one comes from Amazon and costs $10.02, weighs 68g and exerts a force of 150g with a stroke of 7.5mm.


The basic design consists of a coil wrapped around a tube with a plunger fitted in the center. The coil sits inside a box and outside the box is a spring. This spring keeps the plunger out of the coil in the powered off configuration, then when power is applied the electromagnetic force generated pulls the plunger inside. This means that it only draws power when the plunger is pulled inside.

Maths and physics

From Wikipedia - The force applied to the armature is proportional to the change in inductance of the coil with respect to the change in position of the armature, and the current flowing through the coil (Faraday's law of induction). The force applied to the armature will always move the armature in a direction that increases the coil's inductance. $$B=\mu_0 \frac{Ni}{l}$$ $$\phi = BA$$

What happens when you put a pearmenant magnet into a solenoid?