Lab 4 – Combining 2D and 3D Fabrication to Construct a Mechanism
The first lab in this sequence introduced two-dimensional fabrication. The second lab introduced three-dimensional fabrication. The third lab introduced the development of electromechanical mechanisms using computer-assisted design. The fourth lab in the sequence combines competencies introduced in the preceding labs. The design challenge for this week is to use CAD software to create a mechanism that combines 2D fabrication and 3D fabrication. The fabricated design should be combined with the linear motor to create an electromechanical mechanism.
Animated figures of this kind have a connection to a form of folk art known as a whirligig. Whirligigs are whimsical expressions of American art that first appeared two centuries ago. Whirlygigs were ubiquitous by the 19th century. They are referenced in Washington Irving’s “Legend of Sleepy Hollow” (1819). Whirligigs often portray a figure performing the an action, such as a woodsman chopping wood. The 19th century figure below depicts a ranch hand pumping water from a well.
These articulated figures lend themselves to operation with mechanical linkages and motors, making them well suited to this lab. Animated figures inspired by this historical art can be updated for our time. For example, the figure below portrays a guitar player.
Mechanical Linkages and Connections
The coupling used to connect the motor to a pop-up card in Lab 3 used a direct connection or linkage. Engineering linkages often use leverage to achieve a mechanical advantage. For example, the handles of a pair of bolt cutters move through a wide arc, while the blades move through a short arc. As a result, a much greater force is exerted by the blades than would be possible otherwise.
Similarly, the pivot point of a seesaw affects the range of motion through which each end of the seesaw travels. In the example below, a child on the right side of the seesaw would travel through a larger arc than a child on the left. However, the child on the left would have to exert more force to move the other end of the seesaw than would be the case if the pivot point were in the middle. The principles of operation of levers are typically covered in units on simple and complex machines.
We can use this principle to good advantage to increase the range of motion produced by a mechanism coupled to the linear motor. The design constructed in Lab 2 provides about a three-quarter inch movement. This was enough to open and close the pop-up card when the armature was attached near the bottom of the card. In this lab, a design will be introduced that takes advantage of leverage to achieve a wider range of motion.
The seesaw is an example of a Class 1 lever in which an effort is applied on one side of a fulcrum and resistance (load) on the other side. Archimedes observed that if the distance from the fulcrum to the point at which the input force is applied is greater than the distance from the fulcrum to the point at which the output force is applied then the lever will amplify the input force. Noting this, he famously said, “Give me a place to stand and I will move the world.”
A crowbar and a pair of scissors are both examples of Class 1 levers. A pantograph, a device for copying a drawing on a different scale, makes use of the principle that this type of lever represents. Jefferson’s polygraph, which allowed him to make duplicate copies of letters that he wrote, also makes use of this principle.
Designing a Prototype
In our application in this lab, the linear motor will be connected to a lever that will move about a pivot point. A cardstock figure will be placed at the top of the lever to simulate the action of an animated figure often found in folk art. The figure at the top of the lever represents the load that must be moved. If the armature extension that applies the force is closer to the pivot point than the cardstock figure, the figures’ range of motion will be increased.
This can be verified by allowing a student to attach pencils to the two points of the lever (i.e., at the point at which the armature will be connected and the point at which the cardstock figure will be attached) and manually move the lever around the pivot point. The arc followed by the point at which the cardstock figure will be attached will follow the arc near the point at which the motor will be attached, but the range of motion will be increased.
This real-world example of geometry aligns with the Common Core math standard HSG.MG.A. The drawing exercise offers an opportunity to discuss congruent shapes (HSG.CO.B) and offers opportunities to discuss the role of pantographs in history. This type of project also aligns with the Next Generation Science Standards (MS-ETS1-1, MS-ETS1-2, MS-ETS1-3, etc.).
The first step in the design process is development of a concept tested through construction of a prototype. An Erector set offers a useful set of tools for creating this type of prototype, with a variety of components that can used to explore different approaches to implementing linkages. The Meccano set, predecessor to the Erector set, was invented in 1898. It was designed to encourage exploration of mechanical engineering in informal educational settings. A doctoral thesis at Columbia University explored educational uses of these kits at Horace Mann School in New York in the 1920s. These types of uses represent the equivalent of today’s makerspaces in past eras.
Translating the Prototype into a CAD Design
The prototyping process may require several iterations. Once a design has been tested and verified to work, the next step is to translate the design into a CAD design. The design shown in the figure below is a variant of the linkage connecting the motor to a pop-up card in Lab 2. In this instance, use of a lever allows the motor to increase the range of motion of the animated figure.
The sample design shown in the figure above consists of two components: (1) a pivot arm and (2) a pivot base. The pivot base consists of two parallel rectangles with a hole near one end. The hole provides a point for attaching the pivot arm. A third rectangle provides a base.
In this case, the holes were sized for use with a segment of coat hanger. Once the wire segment was inserted through the two holes, each end was secured with a protective cap similar to the ones used with the wire segments in the armature of the linear motor.
The pivot arm consists of a rectangle with two holes in it: (1) one for the point of attachment to the pivot base and (2) a second used as a point of attachment to the linear motor. After this design was fabricated, it was tested in three phases. First, it was assembled and moved manually to make sure that it moved smoothly and did not bind. After that it was connected to the motor and tested with a nine-volt battery. Finally, it was tested with a waveform generator, with the generator set to a frequency of two back-and-forth movements (cycles) per second. Once the ability of the design to function over an extended period of time was verified, the pivot arm assembly and the linear motor were secured to a common base.
Designing a Cardstock Figure
The next step in the design process involved development of a figure for use with the pivot arm. There are a number of ways to approach this. Those with the artistic interest and inclination can create an original design. Elgin Cleckley, a professor of design in the School of Architecture at the University of Virginia, created the dragon below in a workshop that we conducted at the Smithsonian.
For a number of years a British teacher, Rob Ives, has been designing cardstock animated figures that also could serve as a source of inspiration. The design for the cardstock reindeer in the image below is offered as a free download from the Rob Ives web site.
The reindeer design was combined with the motor and pivot arm depicted in the final version of the design below. The weight of the reindeer revealed one additional adjustment that needed to be made.
When the reindeer reached the end of its arc, the weight of gravity pulled the armature of the motor out of position. Therefore the design of the pivot base needed to be adjusted to restrict the movement of the pivot arm to about 30 degrees on either side of vertical.
The sample design described above is just a starting point for creativity. For example, one group of fifth-grade students designed a golfer, shown below. When the golfer swings his club, the golf ball rolls down a track into a hole.
Lisa King’s students at Hollymead Elementary school took the process a step further. In a language arts activity, they created an original story and developed an animated diorama to illustrate the story. A computer-controlled script, written in the children’s programming language Scratch, controlled the motors and actuators in the diorama.
During the course of the project, students learned a number of key concepts that spanned computer science, engineering, and literacy. Combining making and storytelling offers a natural way to integrate technologies and the humanities. Humans have been using technology to tell stories since the first Paleolithic paintings were created in the Lascaux Caves more than 17,000 years ago.
Placing this work in the context of language and the arts provides a bridge between science and the humanities, supporting introduction of science and mathematics concepts in a context that can be used as a springboard for many related projects.