Hyperspectral camera gimbal and lift

Motivations

Capra Robotics in conjunction with Aarhus University wanted a module that mounts on the Capra Hircus robot to control a hyperspectral camera. The camera is used to determine grapes' acidity and sugar content through non-invasive methods. The Hircus robot and camera travel between the rows of the vineyard and wirelessly communicate data about each cluster of grapes to an online database for farmers to monitor the health of the grapes over the growing season.

Project information

  • CategorySummer internship
  • Project dateSummer 2023

System Highlights

The module uses a series of linkages powered by a linear actuator to control the height of the camera to accommodate the changing height of the grapes throughout the growing season. At the top of the lift, there is a 3D-printed structure to maneuver two gimbal motors to control the pan and tilt of a hyperspectral camera. Because our internship was only 8 weeks, we needed every part to be 3D-printed or purchased off-the-shelf and machined in-house, adding a unique design constraint to our process.

My contribution

I collaborated with another intern from MIT to create the camera control module. I focused on the hardware components while she focused on software. Over the summer, I designed, CADed, fabricated, and assembled the lifting and pan tilt mechanisms. I also worked to wire the lift actuators to the robot’s electrical power system and connect the gimbal motors and sensors to the control board.

Reflection

Overall, I enjoyed my experience at Capra Robotics because the project was well-scoped and aligned with my interest in designing electromechanical systems. It was a fun challenge to design with constraints on parts and tools. For example, I initially thought of using a linear slide system with cables and a spool to lift the camera; however, we didn’t have access to a small high-torque motor or linear bearings. So I designed a double parallelogram linkage often seen on articulated booms. I got also more experience iterating and optimizing 3D printed parts to maximize stiffness and load capacity. Initially, the top of the lifting system would sway, but after I modified the geometry of the print to be stiffer, the camera remained stable even on rough terrain. At the end of the internship, we validated this proof-of-concept prototype in a vineyard in the countryside, successfully using the gimbal system to capture photos of the grapes. The internship solidified my interest in pursuing a career in developing impactful electromechanical systems.

Pan tilt gimbal

For the pan tilt rig, we decided to use gimbal motors and take advantage of commercially available controllers for drone hobbyists. Gimbal control allows us to move and hold positions even with disturbances. The motors were designed for a large video camera which is the approximate size of our hyperspectral camera (6 lbs). Because the internship is only about 8 weeks and it takes around 2 weeks for any custom metal parts to be machined, we were encouraged to either 3D all solutions or use easily purchased parts. Therefore everything was 3D printed. The left side of the camera is connected to the tilt motor through a slotted adapter that allows the camera to move slightly to ensure the camera's center of mass is directly on the axis of rotation. To support the right side of the camera a second plate was added to support an axle connected to the camera. A container with lead was added around the axle on the right side so that the system mounted to the pan motor would be balanced. A bar was added to the front to act as a mechanical stop and additional boxes were mounted to store all the electronics.

To control the location of the camera we used the open-source GUI simpleBGC to which takes input from the imu and position input to move the hyperspectral camera to an area of interest. The GUI allows us to input PID values based on tests on the system to hold the set position and correct for disturbances as the robot drives around.

Lift mechanism

The lift consists of only 3D-printed parts and box and sheet aluminum. The system is based on a reverse four-bar system. This is beneficial because the robot can move its center of gravity down to be more stable when it needs to move quickly or make tight turns. The lift has a top and bottom four-bar linkage which ensures that the top of the lift remains parallel to the ground and moves only long one axis (up and down) as long as the top and bottom four-bar linkage have the same angular displacement. For this, we added a few additional bars to constrain movement and a small linear slider to compensate for the change in angle as the height changes. 40mm x 40mm square aluminum was used for all of the linkage and they were all connected using 3D-printed parts or off-the-shelf sheet metal. In order to increase rigidity, all the 3D-printed pieces were printed at high infill. Overall, the lift's height can range from 325mm to 1050mm.

The entire system is powered by a singular linear actuator which is connected to a controller that ramps up and down the speed at desired positions. The linear actuator itself isn't a closed-loop system and it has no feedback on its positions. A linkage system connected to a potentiometer is used so the robot can determine the lift height.