Final Design

According to the statement of work and requirements prescribed by our sponsors, we came up with a design solution as follows. An off-the-shelf pipe would be used as our tank wall where a side opening would be machined, one end of the pipe would be used to install a flexible membrane as the tank bottom and the other would stay open to immerse the overhang LED. A ring bearing would be added to the tank in order for it to stay on the top level of our support table while producing minimal amount of friction during rotation. To plug the transducer into the tank through the side opening, we designed a holder which ensures no water leaks out during operation while the transducer can be adjusted inside the holder to an optimal position for each experiment. For the driving mechanism, our solution was to add a timing belt onto the tank wall and to drive the timing belt with a gear attached to a stepper motor. Finally, a 2-level support table will be machined to integrate all components including a gantry system to support the overhang LED. Both the CAD graph of our initial design and the prototype picture are attached below.


Key Component (1/4): Water tank

  

 Overview   

        

        A water tank that is able to rotate with a gear plate and a sample plate, holding ultrasound transducer during imaging process.

        The rotating water tank has two crucial components one of which is the contacting ring bearing that was press-fit onto the tank and helped stabilizing the tank on the table, and the other is the cover at the bottom of the water tank. For the ring bearing, friction between the bearing and the support table solely determined the difficulty for our drive train to rotate the system. For the membrane which is attached at the bottom of the water tank, it functions as a thin film that isolates water from samples that are pushed up from the bottom of the water tank to the same height of ultrasound transducer while ideally leaving no air gap between the sample and the transducer.

    Functional Requirement:

    Justification of Choices:

For the water tank, we decided to use off-the-shelf PVC pipe as the tank wall due to its convenience and water resistance. The first iteration of the design failed because of the inaccurate measurements and low manufacturing precision of the tank resulting in a failure of the press-fit. The second iteration, however, was successful and the final friction between the tank and the table was diminished to a degree (estimated 0.2-0.3 friction coefficient) where the tank carrying a 5.2-kg load can be driven by our designated motor without difficulties. The maximum load of the water tank during operation was calculated to be 4.8 kg. Overall, this design choice was tested to be successful.

For the bottom cover, there are two different designs where one uses the combination of rubber bands and plastic wrap while the other uses clamp and balloon. Rubber bands performs bad in holding plastic wrap, that sometimes plastic wrap falls out as water poured into the water tank. Meanwhile, plastic wrap always have wrinkles when installed, where massive leakage occurs. On the other hand, a latex balloon perfectly fits the edge of water tank and a hose clamp is more efficient on holding the membrane in position. Thus the final design uses balloon-clamp combination as shown below.

    Final Design:

As shown above, the final design of water tank uses the combination of clamp and balloon. Balloon has very good elasticity that can be pushed all the way to the same height of transducer gate with bottom clamped. During the few tests with our drive train, the water tank rotates smoothly with approximately 3 liters water inside.

Key Component (2/4): Drive Train

    Overview

The Drive Train works to rotate the water tank. The drive train includes gears, shaft, coupling, motor, motor mount, power supply, several wires and converters. From these components, types of gear connection and the selection of motor are two key aspects need to be considered.

    Functional Requirement:

    Justification of Choices:

For the connection of gears, the second choice: the Connection of 1 gear with 1 timing belt works very well. Because it is the easiest one to manufacture and it could satisfy the functional requirement, there is no need to try the third choice and the timing belt-gear connection can be used as the final connection design.

For the selection of motor, due to the limit of size, and minimum torque requirement, a 23MDSI-Stepper motor is selected. It has the maximum torque of 425 oz-in and an encoder is integrated with the motor for the precise position control. Based on the estimation in appendix.calculation1, this motor is powerful enough to rotate a 5 kg cylinder with the gear ratio of 3:1.

    Final Design:

The final version of the drivetrain satisfies all the four functional requirements. The 24volts power supply integrated with the stepper motor can precisely control the motion of water tank. A gear cover which is made of PLA can prevent users’ fingers from hurting. The electronic enclosure at the bottom works as a motor mount which could fix both the power supply and the stepper motor. The electronic enclosure is made of fiberglass which has good quality of heat and electrical insulation.

Key Component (3/4): Transducer Gate

    Overview

The transducer gate mainly functions as a holder of ultrasound transducer which locking its position and a gate prevent water leaking from water tank.  Meanwhile, the transducer gate should also be able to adjust transducer position, that transducer can adjust its focus during experiments.

    Functional Requirements:

    Justification of Choice:

There are three versions of design. The first design of transducer gate is composed of two sliding gates. However, this design performs bad in watertightness; The second design is composed of a transducer lock and connector, which locks transducer well and fits the outer surface of the water tank. But this design does not have the function of adjusting transducer position.Thus the third design is made with good performance of water tightness and adjustable transducer position as shown below.

    Final Design:

As stated in the overview, The final version of design should achieve both goals of watertight and flexible position. It contains two components: gate connector and a tube with holder. The gate connector is made of PVC plate and machined on lathe, with one side curved to fit outer surface of water tank, on which will be glued. The tube contains holders made of 3-D printed soft material, which is compressed when transducer is installed, to achieve the goal of watertightness. The front holder has a cylinder shape with a rectangular hollow in the middle, which fits the transducer’s head without blocking ultrasound signals; While the back holders composes two identical half-cylinder with handles, which locks transducer’s position as well as prevents water leaking from inside water tank.  

Key Component (4/4): LED Gantry

    Overview

The last key component is the LED gantry, which hold and adjust LED device in certain vertical position, as well as combining all other components into a complete system.

    Functional Requirements:

    Justification of Choices:

The combination of gripper and PVC rods is the final design decision since they are easy to manufacture and adjust. Two  PVC rods with large cross section area was used as the vertical supports since they can be fixed onto the base board. The gripper was connected to a horizontal thin steel rod through a joint so that the horizontal and vertical position of LED can be adjusted. This LED gantry can be fabricated by drilling a hole on PVC rod(vertical support) and pressing fit the steel rod through this hole. Then fix the vertical rods onto the base board by using the screw.

    Final Design:

The final design of LED gantry can hold and immerse the LED in the water tank. The gripper with joint provide the adjustable position for the LED which enable the LED light fall on the top of the sample.