Author: Nathan Becker

Figure 1: The final claw assembly

The Problem 

With an underwater claw, the main challenge is having a reliable way of transmitting the motion of the motor into the jaws of the claw while keeping the motor dry to not fry its electronics. A mechanical connection from the motor to the claw jaws would require some form of a seal that is rubbing against a moving surface. Having a dynamic seal as described would require very tight tolerances, have substantial friction, and would be prone to leaking. Another possible mechanical connection would involve a rubber bellow, but they can develop cracks over time and would get squished at deep water pressures. To avoid these issues, we just avoided the problem altogether.

 

Our Approach 

To link the motor and the jaws without a mechanical connection, we needed to transmit the torque from the motor across a gap. To do this, the claw uses magnets. The magnets are arranged in a ring with alternating polarity to maximize the torque transmission. This can be more easily visualized by the figure below.

Figure 2: Diagram of the magnetic torque coupler

The amount of torque needed was calculated and the magnets and motor were sized accordingly. This torque coupler has some unique properties that will be important later. First, if there is enough resistance on the output shaft, the input shaft will continue to rotate, and the magnets will slip past each other. Secondly, due to the motion, the magnets are being sheared apart, not pulled, which means the torque transmission is low. Lastly, the gap between the disks can have a non-magnetic material in it and function identically.

 

The Claw Mechanism 

By putting a wall in between the two magnetic disks, the motor can be completely sealed off from the water, which solves the main waterproofing problem. However, now the low torque output of the torque coupler needs to be converted into a useful grabbing motion with high torque. In addition, the mechanism needs to be non-backdrivable, which in this case means that the torque coupler can move the claw jaws but trying to force the jaws apart by hand will not move the torque coupler. The torque coupler needs to be non-backdrivable so that the magnetic coupler disks can slip past each other and so that picking up a heavy object will not be able to force the jaws open. The worm screw has a high mechanical advantage by trading the low torque, high speed of the motor to high torque, low speed needed for the jaws of the claw to operate. Due to the shallow angle of the threads, the worm screw is non-backdrivable and allows for claw jaws opposite each other creating the pinching motion.

Figure 3: Claw worm-screw mechanism

Actuating the Claw 

The claw runs off a DC motor with no encoder or any form of positional feedback. This seems like a problem, however by taking advantage of the fact that the magnetic torque coupler can slip this becomes a useful feature. If the claw closes around something, there will be resistance to the output coupler, and the motor will continue to spin without stalling even when the claw is stopped. So, the motor can simply be driven to open or close the claw for a predefined amount of time and the claw will open or close as far as it can go. And after the set amount of time, the motor can be turned off and the claw will hold its position because the mechanism is non-backdrivable.

 

Making It Real 

After the described design was drafted in SolidWorks, the parts were fabricated utilizing numerous manufacturing methods. Parts with complex geometries that would experience light loads were 3D printed. Aluminum sheet metal profiles were waterjet out and bent to shape for mounting the claw to the attachment panels and to hold the jaws in position. A shaft for the worm screw to rotate on was turned on the lathe. A gasket was cut to seal the static mating surface, and the motor Subcon connector was epoxied in place to the motor housing. Initially, the motor’s magnetic disk was 3D printed, but the strength of the magnets pulled the printed layers apart. This part was replaced with an aluminum machined component that was turned on a lathe and then CNC’d. Lastly, all the parts were assembled, water-tight tested, and can be seen functioning perfectly underwater!

 

To see the claw work, please go to: https://youtube.com/shorts/NatYnBd73fk?feature=share