Electromechanical Design | Hanna Schlegel Engin

Gasket Project

This project was done to introduce essential learning objectives for any mechanical engineer. These objectives were to make accurate measurements of a mechanical component, produce dimensioned sketches to base CAD models on, and to gain experience with GibbsCAM software and a CNC mill.

A Gasket is a mechanical seal used to fill the space between mechanical components to prevent leaks. Gaskets are often made of a softer material that will create a perfect seal when compressed. For this project, we were tasked with manufacturing a gasket for on an aluminum block.

The specific task was to produce a 1/16"-thick PVC gasket that would fit on top of an aluminum block with several machined features. The gasket should allow unrestricted access to all of the blocks features and should be within 0.005” of the blocks measurements.

Before creating the gasket, we made a plan for crucial steps to the design process. The steps were to take all measurements of the aluminum block needed, create a CAD model of the gasket based on these measurements, and machine the part using GibbsCAM.

First, we used a caliper to take precise measurements of the aluminum block we wanted the gasket to fit. For the four threaded holes, we used a screw size chart to determine the clearance hole size needed for the screws used in the aluminum block. For the two locating pins, we made the gasket's holes' 0.005" larger to allow for a free fit clearance. Using all of these measurements, we created a dimensioned sketch on paper and then used out sketch to help build a CAD model using SolidWorks. We then used GibbsCAM to translate the CAD model into G-code and used a CNC mill in EPIC to machine the gasket out of PVC.

My dimensioned sketch, CAD model, and physical prototype can be seen in the images below. I have also included images that show the fit of my gasket on the aluminum block. There is a close fit between the components but the gasket does not restrict access to any of the block's features, meaning that this project was a success.

Through this project I was able to gain more experience working with calipers, adjusting tolerances depending on the type of fit desired and screw hole standards, creating detailed sketches, and converting CAD models to G-code for CNC machines.

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The Claw Machine: A Cartesian Motion System

This project was done to apply concepts and methods that we have learned in class to our own electromechanical design. The goal of the assignment was to work with a team to design and build the mechanical and electrical sub-systems of a motion system with 2.5 degrees of freedom (DOF).

Our learning objectives included:

To devise different ways to embody ideal joints such as revolute, prismatic, spherical and/or universal joints.
To design mechanical components with rational shapes, considering their function, mechanical loading, required accuracy, method of fabrication and assembly.
To integrate and test a product prototype with mechanical and electrical components.

A motion system with 2.5 DOF is one that can move freely in two dimensions but is limited to only two positions in the third dimension, either "on" and "off" or "up" and "down". Without having freedom of movement in the third dimension, a system doesn't have a full 3 DOF but it has more motion than a system with 2 DOF, hence the 2.5. Examples of 2.5 DOF systems include printers and arcade claw machine games. One version of arcade claw machine games is the Japanese bridge-style machine which has a similar mechanical system to the traditional claw machine, with the exception of having a different goal. The goal of the Japanese version is to knock the prize off of support beams, or to knock the prize of the "bridge". For this project, we were tasked with designing a cartesian motion system with 2.5 DOF, similar to the products outlined above.

Our specific task was to design and build a 2.5 DOF system to perform a task of our choice. The design needed to be unique to some extent, or require the design of a purpose-built end effector. The work volume where the end effector moves around was constrained to a cube of about 2.5 in x 2.5 in x 2.5 in. The prototype system should also be small and light enough to be easily carried in a backpack and needed to be totally or partially disassemblable for transportation by one person in no more than 10 minutes.

Before deciding on our chosen task of the cartesian motion system, we made a plan for crucial steps to the design process. The device would be comprised of structure and motion elements. The structure would be designed around the provided 25 mm x 25 mm 8020 aluminum extrusions while the linear cartesian stages would use provided stepper and servo motors, pulleys, linear guides and belts. The actuator and end effector used would need to be 3D printed to fit our designs needs and would be based on our chosen task. Additionally, non-standard components, such as the carriages for motion and housing of additional stepper motors, would need to be 3D printed as well since they depended on the nature and needs of our unique design.

To achieve linear motion in the x and y directions, we used a belt and pully system on the 8020 extrusions. We fixed a gear on each side of the 8020 and looped a belt around them that connected the gears with the 3D printed carriages. We used two linear stages in the x-direction and for the y-direction, we fixed one linear stage's ends to each of the x carriages. For the z-direction however, we designed a unique belt and pully system to extended our claw into the "down" position and the back into the "up" position. Lastly, we used a smaller servo motor to control the gears that open and close the claw mechanism.

The carriages, end caps for the 8020 that help the gears in place, as well as the claw mechanism were all unique designs that we made CAD files for. The x-direction carriages include a platform for the y-direction linear stage to rest on and one of them includes an additional platform to hold the motor for the y-direction movement. Similarly, the carriage for the y-direction movement contains a platform for the motor used in z-direction movement, but since the motor is oriented so that a moment force is applied by the claw mechanism, we also included support walls for the motor in this carriage. Images of each CAD part can be found below in the Project Gallery.

The remaining structure was made by measuring, cutting, tapping and screwing together the frame out of 8020 extrusions and attaching the linear stages with the carriages. The last structural component is a laser cut box that encloses all electrical components and provides users with a sleek controller box that a joystick pops out from for claw control. The joystick is used to read user input into an Arduino Uno. The output from the microcontroller then relays the user input to a servo motor as well as a CNC shield connected to the stepper motor drivers and the stepper motors. The final circuit diagram can be found below in the Project Gallery and the Arduino code can be found here.

The code uses the user input to activate the motors through the CNC Shield to move in x and y-directions. The claw's starting position is defaulted to the top left corner and will return back after being deployed. This will be done by moving the claw in the x and y-directions simultaneously, which is a unique feature to this design. To deploy the claw, users will press down on the joystick which will extend the claw mechanism down while the servo housed inside the claw mechanism activates to close the arms and then open them again once the claw mechanism begins returning to the "up" position. Images of the final product as well as a video demonstration can be found below in the Project Gallery.

The device performs to the expected function since it is an entertaining game to play that incorporates the use of at least 2.5 degrees of freedom. While the concept of a claw machine is not original to our project, the claw mechanism used in our product is a unique end effector that makes the fun and portable game not a reproduction of existing systems. This design meets all requirements indicated on the assignment sheet by being disassemblable by a student in under 10 minutes and being compact enough to find into a backpack. While the design might feel simple, it is an efficient use of materials and components that leads to a robust and reliable product.

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Finite Element Analysis

This project was done to apply concepts and methods that we have learned in class to a simulation of a multibody dynamic system coupled to a flexible structure.

Our learning objectives included:

To models physical systems with complex interactions using multibody dynamics simulation.
To implement a control strategy to maintain constant rotor speed, when the rotor is subjected to a time-varying friction torque.
To determine the effects of rotation-induced friction forces on the vibration of flexible structural components.

The first task was to create a multibody dynamic model with motion simulation of rigid components. The rigid rotor system with friction coupling to an elastic structure that we modeled:

Using SolidWorks, we were able to reproduce the stick-slip friction behavior exhibited by a neoprene rubber block when it is in contact with a cylindrical rotating shaft running at constant speed around a fixed horizontal axis.

The block is a 1 inch cube and is mounted on a rigid holder whose dimensions and material are arbitrary for the motion analysis we are interested in. Because of this, there is no holder in the CAD file above, and we chose to override the mass properties of the cube component to be 0.25 pounds as this will be the combined cube and holder mass. The diameter of the shaft is 1 inch and is 12 inches long. It is coated with a neoprene rubber layer and therefore all friction coefficients used throughout the analysis will correspond to a rubber-rubber contact. The shaft is being suspended by a rigid holder solely for the purposes of the motion analysis therefore the material and dimensions are arbitrary.

The face of the block that is in contact with the shaft is assumed to be perfectly flat. The block is constrained to move along a linear path, tangentially to the shaft rotating at 30 revolutions per minute. Contact between the block and shaft is ensured through a constant force (1 pound-force) acting on the block, which will push it in the direction of the center of rotation of the shaft. A spring applies a tangential force on the block to prevent it from sliding away due to the friction forces and the spring constant (8.20 N/m) derivation will follow in the second section. A video of the motion study can be seen below along with a graph of the block's displacement in the y-axis as a function of time.

The second task was to create a finite element model of a prismatic beam. The purpose of this is to be able to use finite element model to determine the tip stiffness of the cantilevered steel beam with a cross section of 1 inch by 0.125 inch and a length of 12 inches. At the end of the beam we added a mass of 0.25 pounds (representing the mass of the cube and rigid holder from above) and found the following static displacement model:

Using the maximum displacement from the static model above with Hooke's law (F=-kx), we were able to derive the experimental stiffness of the cantilever beam as follows:

We are also able to derive the theoretical value that finite element analysis should approach with decreasing element size by using the equation , where E is the Young’s modulus of the material, I is the second moment of area of the cross section and L is the length of the beam.

Comparing the two cantilever beam stiffness's, we are able to conclude that the theoretical and experimental values are similar, indicating the finite element analysis did an accurate job at predicted the true stiffness. Moving forward in analysis we will be using the theoretical stiffness as this is the value we expect to approach as we make the finite analysis mesh finer.

The third task was to manipulate the model to compare the torque of a constant rotational speed motor with that of a closed-loop controlled torque based on using velocity as the feedback parameter to control it.

Running the shaft at a constant speed of 30 revolutions per minute using a rotational motor, we can create the following graph of the motor torque:

According to the SolidWorks Motion engine, Adams, this plot shows the theoretical torque that is required to maintain a constant speed in response to the changing magnitude of the friction force exerted by the block on the shaft. This ensures the shaft moves with a constant angular velocity:

Using this plot of angular velocity as the feedback parameter to control the torque, we can replace the rotational motor with a closed-loop controlled torque. We will design the controller using the instantaneous angular velocity to correct the torque using the equation kp(wf - w), where kp was found to be about 1. This will ensures that this velocity is kept constant. Comparing the torque of the rotational motor with the torques of the controlled system, we see the following relationship:

The torque of the rotational motor and that of the controlled system are very similar and follow the same trend. The only noticeable difference is that the closed-loop control system begins with a very high torque. This can be explained by the angular velocity used to control this system. The torque is very high to bring the angular velocity up to the 180 degrees per second desired and then almost immediately returns to decrease and match the torque of the motor.

The fourth and final task was to use the closed-loop speed control to collect data of the friction force between the rubber block and the shaft so that we can use them to simulate the vibration of the shaft in the finite element model created earlier.

The results plot of the friction force between the block and the shaft is shown below.

Using this friction data, we were able to create a curve that drives the vibration of the cantilever beam in a finite element model. We then ran the FEA simulation to plot the displacement of the center of the mean to visualize the vibrations experience here:

From the plot of the vibration of the midpoint of the beam over time, we see that the amplitude of vibration is around 0.005 inches, but the peak amplitude occurs at about 0.5 seconds at 0.006 inches. After performing a Fourier transform on this data to translate the results from a time domain to the frequency space, we find the resulting plot:

The main frequency components of the vibration are around 15 Hz and 23 Hz. The vibration of the beam is related to the vibration of the block in the motion model because the cantilever beam oscillates at the same vibration of the block in the motion model. From the friction force, we can also see that this repeats about 3 times in 0.2 second, resulting in a 15 Hz frequency. This frequency is close to where the peak of the beam vibration occurs. From the block's friction, we can also see that it changes about 12 times in 0.5 second, resulting in a 24 Hz frequency. This frequency is also close to where a peak of the beam vibration occurs.

To test this relationship for different rotational velocities of the shaft, the analysis was repeated for both 15 and 45 r.p.m. The resulting plots for friction force, beam vibration and Fourier transforms are seen here:

We see that the relationship found at 30 r.p.m. stands for different rotational velocities of the shaft, indicating that the beam vibrates at the same frequency as the block oscillates.

Finite element analysis was successfully used to model the stick-slip relationship between a rubber coated shaft and a block. For advanced analysis, I would recommend analyzing the beam vibration and block oscillation for even more rotational velocities to test for limits. I would also recommend collecting results for longer than one second to gain a fuller understanding of changes over time.