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The purpose of the project is to design and build a pool lift that can safely lower the child into the water while being seated. Furthermore, it must allow all necessary life support equipment to travel with the child in need.
The problem we are solving is to build a motorized pool lift the child can safely get on while on solid ground, then be moved over water and lowered down. The child’s name is Kiera who has a ventilator and trache at all times. In order to allow her to safely enjoy their pool, we must design a swiveling pool lift where she can get in the chair while on the deck, then be rotated into the water while buckled in. Furthermore, we must ensure she is not able to be lowered too far into the water or too far above the deck.
Design a Pool Lift to safely raise and lower a child into an above ground pool.
Possible Methods of Lift:
Possible Chair Implementations:
Metal/Fixture Usages:
Possible rotation implementation:
Additional Safety:
This design model replicates the structure of a crane. It incorporates a secondary arm which acts as a stabilizer as the child is being lifted in and out of the water, keeping here at a level position. The design incorporates a swivel base, which keeps the child away from the pool edge while getting on. Least amount of force required within the actuator. However this will have a large moment created at the mounting base. can also implement a slotted chair arm to allow for growth of the child.
This design follows a similar concept to design one with the difference being the placement of the lift cylinder. The lift cylinder for this lift is placed on the opposite side of the mast as the chair. In order to move chair, the main arm is extended past the back allowing for the cylinder to connect. This idea allows for the arms to be able to fold up when not in used to allow for a more compact system when not in use. The extend support arm will also have handle on the backside to make it easier for the parents to manually turn.
Pros:
Cons:
This chair lift design is the most most compact, however it requires the child to be near the pool edge to get in the seat. Additionally, this design has a shorter radius which would position the child close to the pull edge. This would allow the child to have access to toys and pool items without the addition of trays or carriers to the lift itself. Placement of the child’s medical equipment would also not be an issue due to the proximity and distance in the pool. On the other hand the family would like the child to be placed further out into the pull to allow her to not feel like she is confined to the pool side. Also the seat is in a fixed position meaning she is facing the side of the deck rather then facing towards the center of the pool. Other accommodations that the family request would also be harder to incorporate with this design such as an umbrella. A draw back to this design is that the lifting cylinder will have to pull to lift, which decreases the working force of the cylinder since they are made to push.
For our final selection, we will be doing concept design one. This will allow minimal linkages interfering with the pool area. Furthermore, the child will be farther away from the water when getting on and off the lift. This design does not need as strong of an actuator as the others, and will be able to withstand the forces. In order to keep the chair arm level we will keep the stabilizer arm. Therefore, this will be the safest option while having the least interference with pool space while still allowing the child to enjoy the water.
For the design of our lift, we follow the same structural design as an engine hoist/bed crane. The design features two arms with the top arm being used to keep the chair at a level position as it is articulated up and down. The lift cylinder would be mounted so that when the cylinder is fully closed its at the correct water level. This creates a physical safety that would keep the child from being placed too far down. The arm supporting the chair would be made up of 2 aluminum sections, the main arm and a sleeve that would allow for seat adjustment over team. The chair would feature a waterproof cushion, arms that fold on the left side (furthest from pool), a cup holder and a small removable tray. The chair support also has a bracket to mount a tree stand umbrella to help protect her from the sun (if time permits). The bolts located on the main mast to the support arms would be greaseable to help with wear and corrosion resistance. The mounting tabs from mast to the arms and arms to the chair beam would be made out of 1/4 plate that overlapped the structure 2/3 of its total distance. To rotate our structure a pipe would follow a sleeve pipe through the deck that is welded to a 1.5′ X 1.5′ X 1/2″ plate. The pipe section would be connected to the bottom of the mast by mating flanges that are welded to each. The through pipe connected to the mast would extend past the bottom of the deck supporting’s and would feature a lever arm. This lever arm would have a secondary actuator attached to it that would then be mounted to the deck. The lift would attach to the deck using U-bolts that straddle the 2X6 planks underneath the deck and attach to the plate. The rotating actuator will be mounted to a the 2×6 underneath and will incorporate a brace as well to support. Our electronics will be separate from the lift and will utilize a box to incase the electronic controls and battery. Regarding the chair, we plan to take apart a school chair and bolt it onto the sleeve. The chair we have chosen has uv and heat protection. In order to keep our child safe we will implement a 5 point harness seatbelt to keep her safe in the water. This will be bolted onto the seat beforehand. A hook will be attached to the back of the sleeve to hold her ventilator.
Motion Modeling:
Lift raise and lower: Lift rotation:
Force analysis:
To determine our forces acting throughout our system, we performed hand calculations. We tripled the load that would be put on the end of the lift to account for any growth and additional part placed on the lift. Likewise for our material itself we used a distributed load based on the length and weight per foot, and simplified it to a point load acting in the center of each member. We preformed several analysis to help determine the forces and stresses by treating the lift as a static system. For the first calculation we set the lift at the 90 degree mark. This step showed how the forces acted through each member. This told us if our actuator was adequate for the weight that would be placed on it when at rest. Next we placed the lift at the lowest position. The reasoning for this is that this position is where the actuator will receive the most force placed on. Our next set of calculation would tell use the moment being placed at the best of the lift. This will help account for the twist our base plate will endure as well as our tube sleeve for rotation.
Weight:
Calculations of the dry weight of our pool lift since there is concern about the weight. This is purely calculating the lift material since all other controls will be located independent of the lift.
Material:
2 – 4×4 1/4″ steel tubing 3′ @ 12.4 per foot = 74.4 lb
1 – 4×4 1/4″ steel tubing 5′ @ 12.4 per foot = 62.0 lb
8 – 1/4″ 4×9 steel plate @ 2.60 lb each = 20.8 lb
1 – 12″ x 12″ x 1/2 Steel = 20.50 lb
2 – 8″ diameter steel disks @ 8 lb each = 16 lb
1 – 4×4 1/4″ Aluminum tubing 5′ @ 2.4 per foot = 12.0 lb
1 – Linear actuator = 7 lb
Total Weight = 217.7 lb
When adding a 2.0 times safety factor to the weight to account for material differences and fixtures weight: the lift weight on the deck will be 435.4 lb
Electronics:
Overview:
For our chair lift, we are incorporating two electronic lift cylinders to raise/lower and rotate the mechanism. Our system will be powered by a 12V 21.04Ah battery. The battery and control components will be located remotely from the lift itself. The actuators we’ll be using will be IP68 and require a 6A draw at 12V DC.
Controls:
We originally looked at a control box from the manufacturer of the lift cylinder, however it will not work for our application. The other control boxes on the site use wireless remotes to control them. Our team feels this might make it easier to lose or more apt to fail.
Our control box will be made up of fuses, relays, and switches. The controller will utilize two switches that are used to control trim motor positions on boats. These buttons are water proof and have a 3 position state to control each actuator. Since there are 2 individual switches, we will look at having a 3D printed switch holder made. Our current design for our remote is to have a box that will house the 2 trim controllers for the lift. We will incorporate a flap that will keep the operator from using more then one button at a time to save battery run time. Additionally the flap will have the direction of travel listed for each button. In order to keep high current from going through the controller, we will incorporate relays. The lift cylinder manufacturer recommends using a single pole relay in the control system. The relays will help shuttle higher current from the battery to the actuator without going through the controller. Fuses will be placed before the wire going to the remote as well as a fuse between the relay solenoid and actuator. To help eliminate congestion in the battery box, distribution blocks would be put on the side to help connect terminals. Additionally, a fuse box similar to one in a car can be made to condense and make it easier to work with.
(Revision)
We decided to go a different route with our controller once we figured out that the fuse box would need to be rewired. Instead of 2 separate sets of trim switches, we ordered a toggle switch. The new controller would resemble a winch controller and would use a trim switch and the new toggle switch. The trim would be used to control the motor of the actuator while the toggle switch would swap between the actuators. This new wiring would make it impossible to run both actuators at the same time. This will help with overloading system and draining the battery. This would be achieved by having 2 sets of relays. The first would control direction and feed the output to the next set of relays that would be connected together. When the toggle switch is off the closed connection will power a certain actuator and then when the switch is flipped, the connection will be made to swap actuators.
Battery Box:
To help with keeping the system isolated from the water, the control box and battery will be located inside the shown toolbox. This toolbox features a weatherproof seal around the lid. Additional holes will be placed in the bottom of the box to help with air circulation. The box is big enough to fit our electrical components, while allowing added protection from weather. Additional vents can be added to the side with hoods and screens. Wires for the solar panel, actuator, and remote can be ran through the box and sealed with a grommet and silicone. The box also features 2 dividers that can be altered to help secure wires and components such as the battery and fuse box.
(Revision)
Since we were unable to install the lift due to setbacks caused by time. We decided to run the actuator connections out a singular side of the box to help with storage and added less wire and connections.
Connection:
To run the wire from the control box to the actuators, we plan to further protect our wires by incasing them in a rubber hose. Codes state that wires should be placed in PVC or conduit, however since our system will be rotating it will be better to have it incased in the flexible material. The house will be secured the the deck with slack left at the transition from the deck to lift. The hose connected to the lift will then be secured by rubber insulated clamps. Similarly the wires that run to the rotating actuator will be secured to deck with slack being given at the actuator. All additional wire splicing and mating will be soldered with heath shrink added to splices and connection ends to help corrosion resistance. Wires and connections within the box will be suspended so if the water possibly pools within the box, it will be unable to reach connections.
(Revision)
Due to being unable to install we elected to add a pigtail to the actuators to make it easier to store. Additionally, in the event an actuator becomes defective, the new actuator will just need the new connection made without having to go into the control box.
Additional:
A solar panel will be added to our system to trickle charge the battery. The flow of charge will allow for the family to have to worry about charging. The flow will also help maintain the battery and increase its longevity. Since there is a chance that the solar panel will allow a flex in voltage, a voltage regulator will be installed to keep the voltage from exceeding 12v. The solar panel will be mounted to the railing of the deck and the wires will run through hose that is sealed and connected to our battery box. To help monitor the battery state, a volt meter will also be added to either the box.
Safety Factor:
In general 12V DC is a safer alternative then other means of power. We have added additional precautions which have been listed, such as: fuses, additional insulation, waterproof environment, low amp source, etc. Other safety features discussed in codes are bonding, which is grounding the system to the earth to keep from going through people. However, for a DC source, the grounding is not needed since DC looks for its opposing force and isn’t strong enough in this configuration.
There are also examples in everyday life that show the voltage in the system is relatively safe, such as;
Backing a boat into the water with the trailer lights being active. Trailer lights that are none LED general pull 10 A per light at 12V.
Touching a car in the rain. Vehicles have their batteries grounded to the chassis and carry current throughout the vehicles body. General car batteries are 12V 650A.
Metal bottom boats are also similar to vehicles since the engines are mounted, even isolated systems become active in the boat.
Codes:
The codes pulled for this project come from ‘ARTICLE 680—SWIMMING POOLS, SPAS, HOT TUBS, FOUNTAINS, AND SIMILAR INSTALLATIONS’ by Mike Holt. The codes lay out the ground work for some of the electrical safety. Most of the coding references AC circuity and the safety implementations, but does have some relevant in relation to water resistance. The article calls out DC as general being safe under 30V with additional precautions being needed at 12.4V if the circuit experiences Hz waves.
4/1
Order request submitted.
4/4
Received 4×4 metal, base plate and tubing. Cut 4×4 sections to length for mast and arms.
4/5
Received main electrical components. Began mock up of electrical box with fuse box and battery. Drilled entry for solar panel and wired it to the battery with its inline fuse. Added extra insulation and sealed the box with a grommet. Took measurements for remote, fuse box mount to begin design.
4/7
Completed prints for the first version of the remote and fuse box stand.
4/8
Received both actuators and sleeve pipe. One actuator had damage and was sent back. The sleeve pipe was cut to the correct size and a key way was milled into it. Checked fitment of remote and designed improvements.
4/9
Mounted fuse box to our holder and trimmed excess material out of battery box. Drilled holes into aluminum chair arm and cut to length. Water jet was broke so we were unable to have mounting brackets cut to continue the fabrication.
4/10
Drilled holes into lift arm and had brackets water jetted. Received more electrical parts. Installed voltmeter and battery cut off. Drilled holes for remote lead connection into bottom of box. Tested actuator with pigtails and ran main power cable. Additionally the sleeve we were given by the vendor was incorrect. Picked up correct rotational sleeve.
4/11
Drilled stabilizer mounting hole. Sandblasted end caps and mounting brackets. Rewired broken actuator, since vendor gave a lead time of end of May. Soldered pigtails to the actuators ans tested. Finalized wiring for voltmeter. Combined power wired for fuse box.
4/12
Welded end caps to the stabilizer arms. Began rewire of fuse box since the factory wiring ran the power supply from fuse to the 30 slot where our lift cylinder needs to be. Upon this discovery we decided to ditch the original idea of 2 sets of switches and immediately ordered a toggle. Rerouted wires from 30 to 87 on 2 of the relays for the motor controls. Then connected the 30 from control relay to second set of relays so we can control which cylinder it goes to. This in-series system will make the toggle switch system work.
4/15
Started design modeling for new remote housing, and wiring schematic following new system design. Fixed welds on one of the caps (not structural, for aesthetics)
4/17
Welded tabs for first stabilizer arm. Jeff worked on the flange for bearings, sleeve for bushings, and base plate. The solid shaft was milled for better movement in bushings. Received the parts for electrical. Worked routing wires in control box. Fuse box had its final connections formed and started on running wires for remote.
4/18
Began running pigtail wiring for the actuators into the control box and placed the wires through the extra insulation. Soldiered wires for toggle switch.
4/19
Began welding the rotational sleeve to the base plate, setting the root pass, making sure not to get the bushing too hot. Welded first flange to the mast pole. Finished fuse box and began testing the box y rigging temporary controls to make sure wiring was correct as well as connections were good.
Initial control system testing:
4/20
Printed remote handle.
4/22
Finished control box. Placed switches into the remote and ran wires through the hose. Sealed all holes in box and hoses. Added wire loom to clean up look of interior.
4/23
Finished welding brackets to the mast and completed both stabilizer arms. Welded actuator brackets to the lift. Added welding pass to bushing sleeve and base plate.
4/24
Welded final pass for the sleeve and base plate. Mocked up left without pivot shaft. Tested the actuator on lifting boom. Welded 3.5 inch sleeve to solid rod. Welded flange to the pivot shaft. Ground down excess from pivot shaft and welded assembly to the mast. Cleaned up welding around arms. Started mount for chair.
4/25
Assembled chair lift. Welded backet to chair and mount to lift.
Initial structure testing:
After every use the control box should be turned off to prevent any type of draining and accidental activation. The pool lift is meant for children and should not have a person or object of higher weight then specified (to be determined once chair is mounted). The person controlling the lift should be a trusted individual and needs to pay attention to lift positioning and make sure the lift destination is clear of obstruction and individuals. Additionally the lift has been made so that the lift arms are at a 90 degree (or just over) orientation from the mast when coming over the deck.
The pool lift project was a very involved project that pulled from many aspects of engineering; from structural analysis, electrical analysis, background research, problem solving, and even manufacturing. We were tasked with creating a safe apparatus that would place a child into a pool and provide her with a place that would allow here to play with others. We looked through several different designs and considered many concepts. We researched several ways to move the lift while going through all the pros and cons. For our structural analysis we went to Eric James, P.E. for guidance. For electrical we went to Chris Mills who was extremely knowledgeable and offered alot of guidance and help when figuring out controls. Once we got passed our analysis and research, we ordered the parts. As shown in section titled document fabrication process, we spent the next 5 weeks working on as much of the project as we could when we could. From the first day parts arrived we began getting parts ready. Weekdays were spent working on everything we could while the weekends were spent reviewing what was done, assessing any problem that arose and finding solutions, as well as working on electronics since those were able to be worked on at home. All of the fabrication could not have been done without the help of Jeff Randolph who kept the shop open passed close to help students. He also helped guide us on many of the fabrication steps where we were unsure. Additionally he made some of the more complex parts, as while as some parts we were having trouble getting to due to exams. The pool lift structure and electrical was finally finished and a mock up assembly was made to test. As of now the pool lift was sent to powder coating during finals and was finished on graduation day. We will be working with Dr. Canfield, Chris Mills, and the family over the summer to complete the installation after reinforcing the deck to ensure it can withstand all the load of the lift, not just people on the deck.
