{"id":41,"date":"2019-05-28T02:46:37","date_gmt":"2019-05-28T02:46:37","guid":{"rendered":"https:\/\/scholars.spu.edu\/rim\/?page_id=41"},"modified":"2019-06-07T21:36:21","modified_gmt":"2019-06-07T21:36:21","slug":"mechanical-design","status":"publish","type":"page","link":"https:\/\/scholars.spu.edu\/rim\/mechanical-design\/","title":{"rendered":"Mechanical Design"},"content":{"rendered":"\n
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The mechanical design was one of the first focuses of our team when tackling this project. We knew that a significant amount of modeling and fabrication would be needed, and below you will find specific topics that were considered throughout the process. <\/p>\n\n\n\n

RIM’s basic dimensions can be seen below:<\/p>\n\n\n\n

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RIM’s maximum reach is 25.5 inches.<\/strong> <\/figcaption><\/figure>\n\n\n\n
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Why Six Degrees of Freedom?<\/h2>\n\n\n\n

The number of degrees of freedom (DoF) was a major consideration of ours when we were starting our project. DoF refers to the number of directions in which motion can occur; thus, having 6 DoF means we have 6 rotational joints that can move independently. A point in space can be described in many ways, but in order to capture both its position as well as its rotational position (or orientation) there must be at least 6 DoF. Eventually, we wanted the operator of RIM to be able to hold a motion controller and have the arm mimic his or her hand position and orientation. This is why we chose to build a 6 DoF system. <\/p>\n\n\n\n

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RIM’s layout of joints can be seen to the left. Note that there are 6 joints representing the 6 DoF. This allows the arm to reach a specified position and orientation. <\/p>\n<\/div><\/div>\n\n\n\n

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Stepper Motors<\/h2>\n\n\n\n

Like many other robotic arms currently on the market, we decided to utilize the technology of stepper motors over other means of translating electric power into rotational motion. We chose steppers because they offered higher torque at lower speeds and we could get the precision we wanted. These benefits, combined with their relatively low cost, made this an easy choice. <\/p>\n\n\n\n

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Extensive calculations were performed to determine what kind of stepper motors we were going to buy. We needed to make sure the motors could supply enough torque while holding a load and while in motion. We increased the motor\u2019s torque output and their positional resolution by utilizing planetary gearbox systems mounted to each motor. An example of one of these motors with its gearbox can be seen to the left. <\/p>\n<\/div><\/div>\n\n\n\n

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Dividing the Arm Into Sections<\/h2>\n\n\n\n

RIM can be broken down into 7 distinct rigid bodies. Rotational joints are located between each rigid body; thus, there are 6 joints and 6 degrees of freedom. Each rigid body can be thought of as one large part that moves relative to the other sections. This convention was used extensively throughout the design process. <\/p>\n\n\n\n

The sections can be seen below:<\/p>\n\n\n\n

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End Effector (Red)<\/strong><\/p>\n\n\n\n

Wrist (Purple)<\/strong><\/p>\n\n\n\n

Forearm (Blue)<\/strong><\/p>\n\n\n\n

Elbow (Turquoise) <\/strong><\/p>\n\n\n\n

Arm (Green)<\/strong><\/p>\n\n\n\n

Shoulder (Yellow)<\/strong><\/p>\n\n\n\n

Base (Orange)<\/strong><\/p>\n<\/div><\/div>\n\n\n\n

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Building RIM in SOLIDWORKS<\/h2>\n\n\n\n

Once our team decided on the basic layout for the arm and the type of motors we would be using, we then started the process of building RIM in SOLIDWORKS. Our model was built initially as a tool to visualize what the arm would look like, but it quickly became one of our most critical design tools.<\/p>\n\n\n\n

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We used the model to estimate weights of each part. These weights were then used in calculations to help us choose the right motors.<\/p>\n<\/div><\/div>\n\n\n\n

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Another tool of SOLIDWORKS that we utilized is Finite Element\nAnalysis (FEA). This helped us determine whether our parts were properly\ndesigned for the loading we expected them to experience. These analyses also\ninfluenced our decision making with regards to what materials we were going to\nuse.<\/p>\n<\/div><\/div>\n\n\n\n

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Building a 3-dimensional model in SOLIDWORKS allowed us to produce technical drawings for each part that we planned to machine. <\/p>\n<\/div><\/div>\n\n\n\n

After many months of design iterations we arrived at a final assembly of RIM. This assembly of the full arm included 7 sub-assemblies of the sections described above. <\/p>\n\n\n\n

The repository of RIM’s CAD files can be found here<\/a>.<\/p>\n\n\n\n

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The Fabrication Process<\/h2>\n\n\n\n

Once we had a quality CAD model and drawings of our parts, we were able to start our fabrication process. We used numerous techniques to obtain the best results for the project. The majority of the parts were machined out of aluminium; however, there is one steel shaft and one carbon fiber shaft. We also used 3D-printed PLA plastic parts to help save weight and lower cost where we could. <\/p>\n\n\n\n

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We started our project in the fall with a risk reduction prototype. The goal was to go through the process of designing and fabricating one of the six joints that would eventually make RIM. This allowed us to start learning about the stepper motors and it taught us valuable lessons regarding how we should design our next five joints. <\/p>\n<\/div><\/div>\n\n\n\n

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We made our aluminium parts using a traditional milling machine with driven x and y axes. The part shown to the right is a spindle that mounts to the second motor and the main arm component of RIM. <\/p>\n<\/div><\/div>\n\n\n\n

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