3-Axis Center of Gravity Measurement Device

The goal of the project is to create a fixture for measuring the mass and center of gravity for standard Schweitzer Engineering Laboratories' (SEL) devices with complicated geometries and uneven weight distribution in all three dimensions.

=Problem Definition=

Background
SEL designs and manufactures many digital products ranging from transmission protection to control systems around the globe. To better understand the failure modes of the products, a series of mechanical analysis is performed by SEL. But Computer-Aided Design (CAD) models lack detailed information about component density, which leads to inaccurate measurement of the Center of Gravity (CG) of the product. By building a device, which measures the CG of the product automatically, using mass sensors will help to solve the issue of incorrect CG measurement of SEL products.

Deliverables
Proof of understanding of center of gravity measurement techniques Validation of functionality and robustness of the porototype using representative SEL components Proper documentation of the design ideas and measurements used in the project Record of bill of materials A detailed final reoprt of the project with a team presentation

Requirements
Calculates the center of gravity for complex objects in 3 dimensions​ Can measure objects approximately 20" cube down to the size of a cellphone​ Is completely automatic after the sample is inserted​ Displays a result within 3 seconds of measurement concluding​ Has an accuracy within 1% or 0.1 inches​ Hold a static load of 50 pounds​ Operate 1 year without maintenance​ Operate from a standard 120VAC outlet

=Project Learning=

Initial Design Ideas & Prototypes
This project was done by a capstone team in 2018-2019. So, at first, the ideas and works done by the previous team were studied. After brainstorming and research, two methods for the calculation of CG were decided by the team. Two different prototypes were also developed following those methods.

Compression method
The test platform is supported by three or more load cells, and the CG location is calculated from the difference in force measurement at these three points. Since the CG position is determined by small differences in weight measurement at these 3 points, huge CG errors can result from side loads using this method. In order to find the third component of CG, in the z-direction, the object being measured needs to be tilted to a known angle. When doing research on this method many academic papers and the handbook of measurements stated that it was important to tare the scales before placing the object on the test apparatus.

For the prototype development from this method, a square cardboard was used as the platform. A variety of solid objects with known CG such as aluminum cube and acrylic cylinder were used. Cheap amazon mass scales were used as load cells and the platform was glued to three nails and was on top of the scales.

Tension method
Similar to the compression method, a variety of solid objects with known CG were placed on the grid (see Prototype 2: Tension Method), then using the difference of weight on each scale the CG could be calculated. The major benefit of this prototype was having perpendicular forces; as the straps hang straight down. The purpose of this method to was to investigate whether the imprecise force angle on the compression prototype was causing significantly inaccurate results. Our findings suggested that either method could adequately be used to calculate the CG, but due to the clunky nature of the design and shapes of common SEL devices, the suspended tabletop would likely be an inconvenience when placing objects on and off the surface. Additional negatives on the design, deterioration on straps leading to inaccurate results, swaying of table surface, limiting object size due to leg interference.

The prototype design for this method is a piece of square plywood suspended by three luggage scales and the scales are resting on holes on a tabletop surface.

Statics method
Statics was used to derive equations for x-axis & y-axis CG. This was done by summing moments about F1 which was set origin with the coordinate system defined in the left diagram. The last equation for the z coordinate was derived using a second FBD of the table tilted to a known angle and using an object with a known x coordinate. The percent error found in the X & Y axis was within the product requirement but for the z-axis, the percent error was huge.

Vector line method
Based on what was found for the previous method. We came to know that the z coordinate is somewhere above the x and y coordinates. Our idea was to define a vector line that goes through the x, y points perpendicular to the plate. Tilt the table and create a 2nd line and the z coordinate should be the intersection of these two lines.

Kumar et, al. method
This method was based on the paper published in the Journal of Materials Science and Surface engineering by a group from the dept. of Mechanical engineering at the Bangalore institute of technology. This method uses a similar approach that was used previously with the vector line method. This method had taken into account the angle created by a line from the origin to the CG with the table surface while leveled. in this method, the mean offset of both blocks increases and are actually consistent with each other. This method helped us to derive an equation that is able to give consistent answers for multiple block sizes and that moving forward will be important to calibrate the final machine to account for this consistent offset.

=Design Development=

Table lifting mechanism
To achieve the z-axis CG, the table should be tilted/lifted. To achieve that goal, several design ideas were brainstormed and discussed.

Single pushrod
This design idea is based on the previous capstone group. In this design, a linear actuator is used to tilt the table to a desired angle or height.

Double pushrod
Two linear actuators are connected to two sides of the table and using them to lift the table in opposite directions. One linear actuator will lift the table up and the other one down. This would help to increase the overall stroke length of the actuator and the desired tilt will be achieved easily.

Direct torque
In this method, a stepper motor is connected to the edge of the table to tilt it at an angle.



Table top design
When the table is tilted, some objects may not be able to stay stationary on it. This can lead to inconsistency in data measurement and error in final CG calculations. So, to solve that problem a pegboard design was developed to hold multiple shape objects. Lines engraved on the surface also helps to locate the test objects. Pegs will be #D printed in different shapes to create several options to hold objects on the table.

Design Iterations
The final design concept of the team, for now, is an updated version of the previous capstone team design. The table will be made from aluminum and the dimension of the table are according to the product requirement This design is more compact and with the addition of pegboard, the design will be better to hold objects on the table. There are many things that would change in this design after getting some feedback from the concept design review like changing the position of the load cell to the bottom of the table. Use of small linear actuator, and use of different load cells.

Final Design Concept
The final design concept of the team, for now, is an updated version of the previous capstone team design. The table will be made from aluminum and the dimension of the table are according to the product requirement This design is more compact and with the addition of pegboard, the design will be better to hold objects on the table. There are many things that would change in this design after getting some feedback from the concept design review like changing the position of the load cell to the bottom of the table. Use of small linear actuator, and use of different load cells.

=Validation=

=Team Members=

=Additional Documentation=

Project Schedule



Meeting Minutes



Presentations

Client Interview