Pool Butler Blog #5 Twist & Turn - Messing with Magnets, Flux lines, Hall effect sensor to find home.

In Pool Butler Blog#4 , a statement was made that limit switches or a home switch would be needed to locate the Pool Butler's (airboat) rudder's position after power up.

The control is envisioned to use CW and CCW touch switches to move the rudder to home position  as a run permissive.

Once the control knows where home is , it can step the stepper open loop both clockwise CW and CCW counterclockwise based on the operator's RC steering wheel.

Everytime the homeswitch is detected, the step counters are reset to zero to "know" where the rudder is pointing.

A hall effect sensor for a home switch, and using a magnet mounted on the rudder for the position detection. 

The Hall sensor is from a Rev 1 Arduino Engineering Kit. There are no identifying marks to gather up datasheets. The sensor itself is stamped (really teeny teeny tiny) L353H 0417A , but cant find a reference for that sensor IC. Doing a deep dive on the sensor is thwarted. I've used this sensor successfully on several projects, without details. 

Next is a magnet. These magnets were $2 at Harbor Freight and seemed to be perfect for the job for pairing with the hall sensor to become a proximity switch 

This sketch shows the thickness of the rudder,  magnet dimensions, and sensor dimensions. 

These magnets are like shirt buttons. The top side is the North pole and the bottom is the south pole. You can see in the magnet photo how these magnets stack together. The magnets are pretty powerful, its hard to pry them apart from the stack.

Once one is loose, it tends to latch onto wires or go rolling off to find some attractive metal. 

When using on a rudder whose dimensions are roughly the same as the magnet, putting on the bottom is a bad decision. The hall effect sensor sees the magnet far before it gets to the home position, and the position is dependent on the direction on travel. The sketch should help visualize why this placement cant work. 

The next idea was to mount the magnet vertically on the rudder, find the best spot for identifying the rudder center.

These magnets are really strong. The first try , the hall sensor detected the magnet from 1/2 inch away when mounted this way.

Moving the magnet up and down the rudder, it seemed impossible to locate the magnet in a place to bidirectionally and reliably trigger the hall sensor for the home position. 

What to do next ?  Change the magnet geometry itself , tame those flux lines so they line up better with the hall sensor. 

Using a dual wheel 6 inch Bench Grinder, the magnet was ground down to a semi circle from a circle.

The magnet's flat side is face down towards  the hall effect sensor.

In testing, the magnet/hall sensor is amazingly effective and repeatable in identifying the rudder home position, when traveling from either direction.  

Here is a photo of the modified magnet and it placement on the rudder. 

This magnet and hall sensor are needed for the controls to find "home" since there is no other feedback to make sure the steering rudder is centered.