by By integrating sensors into rotation mechanisms, engineers can build smart hinges that know when a door is open, or gears in a motor that tell a mechanic how fast they are turning. MIT engineers have now developed a way to easily integrate sensors into these types of mechanisms, using 3D printing.
While advances in 3D printing allow rapid fabrication of rotational mechanisms, integrating sensors into the designs is still notoriously difficult. Due to the complexity of the rotating parts, sensors are usually embedded manually after the device has already been produced.
However, integrating sensors manually is not an easy task. Put them in a device and wires could get caught in the rotating parts or impede their rotations, but mounting external sensors would increase the size of a mechanism and potentially restrict its movement.
Instead, the new system the MIT researchers developed allows a maker to print 3D sensors directly into the moving parts of a mechanism using conductive 3D printing filament. This gives devices the ability to sense their angular position, rotational speed and direction of rotation.
Their system, called MechSense, allows a maker to manufacture rotational mechanisms with integrated sensors in one go using a multi-material 3D printer. These types of printers use multiple materials at once to fabricate a device.
To streamline the manufacturing process, the researchers built a plug-in for computer-aided design software SolidWorks that automatically integrates sensors into a model of the mechanism, which can then be sent directly to the 3D printer for fabrication.
MechSense could enable engineers to quickly prototype devices with rotating parts, such as turbines or motors, while incorporating sensing directly into the designs. It can be particularly useful in creating tactile user interfaces for augmented reality environments, where sensing is critical to tracking a user’s movements and interacting with objects.
“A lot of the research we do in our lab involves taking manufacturing methods that factories or specialized institutions create and then make them accessible to people. 3D printing is a tool that many people can afford to have at home. So how can are we providing the average maker with the tools needed to develop these kinds of interactive mechanisms? Ultimately, this research is all about that goal,” said Marwa AlAlawi, a mechanical engineering graduate student and lead author of a paper on MechSense.
AlAlawi co-authors include Michael Wessely, a former postdoc in the MIT Computer Science and Artificial Intelligence Laboratory (CSAIL) who is now an assistant professor at Aarhus University; and senior author Stefanie Mueller, an associate professor in the MIT Departments of Electrical and Computer Science and Mechanical Engineering, and a member of CSAIL; as well as others at MIT and employees of Accenture Labs. The research will be presented at the ACM CHI Conference on Human Factors in Computing Systems.
Built-in detection
To incorporate sensors into a rotation mechanism in a way that would not interfere with the device’s motion, the researchers used capacitive sensing.
A capacitor consists of two plates of conductive material with an insulating material in between. If the overlapping area or distance between the conductive plates is changed, for example by turning the mechanism, a capacitive sensor can detect the resulting changes in the electric field between the plates. That information can then be used to calculate speed, for example.
“With capacitive sensing, you don’t necessarily need to have contact between the two opposing conductive plates to track changes in that particular sensor. We took advantage of that for our sensor design,” says AlAlawi.
Rotational mechanisms usually consist of a rotating element located above, below, or next to a stationary element, such as a gear rotating on a static axis above a flat surface. The revolving gear is the rotating element and the flat surface below it is the stationary element.
The MechSense sensor contains three patches made of conductive material pressed into the stationary plate, with each patch separated from its neighbors by non-conductive material. A fourth piece of conductive material, which has the same surface area as the other three pieces, is pressed into the rotating plate.
As the device rotates, the patch on the rotating plate, called a floating capacitor, alternately overlaps each of the patches on the stationary plate. As the overlap between the rotating patch and each stationary patch changes (from fully covered, to half covered, to not covered at all), each patch individually detects the resulting change in capacitance.
The floating capacitor is not connected to any circuit, so wires do not get caught in rotating components.
Rather, the stationary patches are connected to electronics that use software the researchers developed to convert raw sensor data into estimates of angular position, direction of rotation and rotational speed.
Enabling rapid prototyping
To simplify the sensor integration process for a user, the researchers built a SolidWorks extension. A maker specifies the rotating and stationary parts of their mechanism, as well as the center of rotation, and then the system automatically adds sensor patches to the model.
“It doesn’t change the design at all. It just replaces part of the device with a different material, in this case conductive material,” says AlAlawi.
The researchers used their system to prototype several devices, including a smart desk lamp that changes the color and brightness of the light depending on how the user turns the bottom or center of the lamp. They also produced a planetary gearbox, like those used in robotic arms, and a wheel that measures distance as it rolls across a surface.
While prototyping, the team also conducted technical experiments to refine their sensor design. They found that as they reduced the size of the patches, the amount of errors in the sensor data increased.
“In an effort to generate electronic devices with very little e-waste, we want devices with a smaller footprint that can still perform well. If we take the same approach and maybe use a different material or manufacturing process, I think we can downsize while accumulating fewer errors with the same geometry,” she says.
In addition to testing different materials, AlAlawi and her collaborators plan to explore how to increase the robustness of their sensor design for external noise, as well as develop printable sensors for other types of moving mechanisms.
This research was funded in part by Accenture Labs.
