Now that the design for the external and inner wiper are nearing completion, it is time to determine the magnets required for coupling the movement. The unknown that has to be answered is: How many magnets will be needed, and what are their dimensions?

We can narrow down the possibilities with calculations.

Constraints:

• Stick with 1/4" outer diameter / outer size
• N52 is the strongest
• K&J magnetics is less expensive than McMaster-Carr
• Using the standard size servos from Adafruit as a baseline

kg-cm to lb-in calculator

Torque means the motor can push X amount force at 1 unit of distance of its lever arm. To find the force, we divide the torque rating by the actual distance of the lever arm.

The servo arm distance with the metal piece is ~35 mm = 1.37 in. For the standard servo at 5V, this means 3.4 lbs of force. Looking at a range of values, if the torque ranges from 4.2 lb-in to 4.7 lb-in, this means the force ranges from 3.0 lbs to 3.4 lbs. Now we can move on to figuring out the magnet portion.

(By the way, in case this is confusing, lbs is an imperial unit of both mass and force.)

The factors for the actual pulling force of the magnets in the wipers are:

1. Strength of the magnet
2. Gap between the magnet
3. Number of magnets

Additional forces acting are the friction between the external wiper and enclosure, and inner wiper and enclosure. Since this is operating under water, there is drag force from the coupled external wiper as well.

The distance between external and internal wiper is 8.4 mm = 0.33 in.

The goal is to have a combined magnet pull force be within the force from the servo’s torque rating. This will need to be verified experimentally.

Using the calculator on K&J magnetics, this determines the pull force between two magnets of the same size given a certain gap distance. The constants will be grade (N52), width (1/4"), and gap (0.33”). This means length, shape, and thickness will change.

Here are the results:

The results for the 0.25” thickness disk are surprising, as these were the magnets used last year and it worked with < 26 magnets.

From the servo calculations earlier, the force is in the range of 3.0 - 3.4 lbs. It would be ideal to achieve this range with ~5 magnets or so. This narrows down the results to the 0.25” thickness block, at 0.5” and 0.75” length.

There is 81.90 mm (3.2 in) to play with on the external wiper magnet holder piece. Both 6x 0.5” length or 3x 0.75” length magnets can fit into.

To verify these results experimentally, we’ll need 12x of the 0.5” magnets, 6x of the 0.75” magnets, and 24x the original disk magnets to serve as a baseline. (It will also be important to not forget that we also need magnets for the servo module before ordering!)

The magnet holder piece for both the external and internal wiper is 3D printed (can be milled during mass production). For adapting to the block magnets instead of disks, dogbones will need to be added to the corners of the pockets. 3D printing will allow for rapid experimentation of combinations of magnets, as well as fine-tuning the alignment in real life.