Table of Contents

Table of Contents
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Stepper motor assembly V22.1

Render from CAD model September 2022.

Oil system

Introduction

Stepper motor assembly is a rotating actuator for underwater applications. It is used in this project to rotate the wings of the towed ROV. The base design is made by the 2021 ROV group. 2022 group has done some modifications described in this document. I will in this document describe the main design decisions, discuss the problems, and explain ideas and improvements for future designes.

Oil system

This motor assembly has a oil system to keep the water from shorting wires. Oil will help equalize the pressure between inside and outside of the wall. Water will not flow from a lower to a higher pressure. Keeping the pressure inside the tube higher than the outside, will ensure no water can enter. Filling the oil is done through a hole made with a syringe case. A vent is there to let the air out. The hole is pressurized and sealed by pressing down a syringe. A cover/cap is screwed to keep the syringe in place. Sunflower oil is selected because it is cheap, has better expiration date, and has slightly lower viscosity than alternative oils. All food oils will eventually turn rancid and smell bad. Keeping the motor house airtight will slow the expiration, because it is reaction with oxygen that turns the oil rancid. Other non-food oils were considered, but was too expensive, too thick, or a danger to the environment. No oil is good in the environment, but food oil is degradable, and therefore better than petroleum based oils. Silicon liquid/oil/grease can potentially be utilized, but none were found available in stores. 

It is possible for oil to seep inside the cable and into the electronics tube. The cable penetrator on the tube side was filled with epoxy to reduce the issue.The stepper house is filled with sunflower oil. Sunflower oil has lower viscosity and longer expiration date than other super marked oils. Food oil has been used in many previous projects. Other oils was considered but it was either very expensive or an environmental hazard. The main downside with food oil is the expiration date. The oil will turn rancid, smell bad and be sticky and hard to remove. 
Water will get inside the housing if no oil is used, even in shallow water. 
A system for filling oil was made. The design has a syringe house and a vent. Oil can easily be filled inside the syringe when the vent is open. The hole can be covered with the second part of the syringe and a screw cover. The idea behind this design is to make filling easier. Pressing the syringe will also make overpressure inside the stepper house. The overpressure will help keeping the water out. 
Cable is slowly leaking oil into the electronics tube. The cables penetrator is now filled with epoxy. 

Printing material

The stepper motor house is printed with PETG. PETG is stronger and has better water resistance than PLA. (layer adhesion is better). Part cooling fan speed was set to 0 to reduce warping of the part. Infill was set to 100% to increase water tightness and compression strength.
PETG has some problemsdownsides. The main problem is excess material sticking to outside of the printing nozzle. The material will slowly build a clump and then suddenly drop it on the print. The clump can make a gap in the surface. Oil was leaking from the parts with a clump in the surface. Low viscosity super glue was used to fill the gaps. This stopped the oil leak.

Magnets and sensor

Magnets and The design uses a magnet sensor is used and two magnets for calibration. The sensor is fixed to the stepper housingMagnets are placed with a 90 degree offset inside a magnet holder. The magnet holder is screwed mounted to the shaft with the same screws for rotating shaft. A magnet sensor is placed stationary on the bottom of the stepper motor. It houses two magnets with a 90 deg gap between them. The wing is The wing mounted to the shaft, will be level when the sensor is in the middle between the two magnets. When calibrating, the program will find the location of both magnets. Calibration sequence is further explained , and then rotate to the center. More info on the magnet sensor is in the electronics part documentation Detailed description of the calibrating sequence can be found in the StepperWing library ReadME. TODO: link

Note: Magnet holder must be in correct position (calibrated) when assembling and disassembling the stepper assembly. The magnets is conflicting with magnet holder can break the syringe . The port if the position is wrong. Port side syringe is broken because of this. 

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Gears are inaccurate. The wing is not rigid because of small gaps between the gears. It is therefor therefore possible to rotate the wing a few degrees. This could make controlling it harder.

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The rubber end cap is hard to find/buy. It limits the size of the stepper assembly. The assembly could be thinner and lighter. The cap will bend outwards when filled with oil, defeating the whole purpose of using syringe to make overpressure. 

Ideas for future designes

Disclaimer

Currently the two built prototypes are bad and is not ment for reproduction or fixing. Some parts are glued/forced together and disassembling will be difficult. Port side has a broken syring part loose inside the house and can jam the system. One shaft was damaged after an attempt to disassemble the stepper and fix the syringe.

Motor shaft 

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Parts are found under the Materials page.

More info on this design in the Report - Summer 2021.docx