How can Robo Raven “feed” itself in jungles?

Our previous version of Robo Raven needs to be plugged into an electric socket for charging the battery. Ultimately, we envision Robo Raven flying deep into jungles, far away from civilizations, and hence electric sockets. To do this, Robo Raven needs to figure out a way to “feed” itself to keep going during long missions.

Real ravens are omnivorous and are happy to eat whatever is available. Unfortunately, mimicking this feat in Robo Raven is not practical at this point in time because the equipment necessary to convert biomass into 30 W of electrical power would make Robo Raven too heavy to fly. Since it is not practical to build a flying platform that can directly convert the biomass into energy needed to flap wings at the moment, we had to come up with a different option to “feed” Robo Raven.

From an energy perspective, ravens are constantly converting biomass into mechanical energy to flap their wings. Common sources of biomass that ravens consume in the wild are carcasses of dead animals. The dead animals accumulated their biomass by consuming plants which converted solar energy to biomass. Here is a high level summary of the energy conversion process at work behind the flight of ravens. Solar energy is converted to biomass (i.e., plants). One type of biomass (i.e., plants) is converted into another type of biomass (i.e., meat). Finally, ravens convert the biomass (i.e., meat) into mechanical energy needed to flap wings. So, ravens ultimately derive their energy from the sun.

We decided to bypass the multi-step energy conversion process used by ravens and instead have Robo Raven harness solar energy directly. Robo Raven features sufficiently large wings, so we decided to make wings out of flexible solar cells since there would be enough surface area for solar cells to generate a usable amount of power. The underlying material of the flexible solar is different from the material used in the previous version of Robo Raven, so we needed to design new wings. Additionally, we had to develop a new additive manufacturing process for making these wings. Solar cells on Robo Raven do not produce enough power to directly drive the motors (they produce around 3.6 W while we need around 30 W). So we decided to charge the battery using the solar cells. I am happy to report that thanks to the hard work of Savannah Nolen, Ariel Perez-Rosado, and Luke Roberts, students in our lab (co-advised by Hugh Bruck and I), we have developed Robo Raven III, the first flapping wing micro air vehicle that flies with solar cells. Please see below for a video.

So how good is the performance? Solar cells currently cover less than half the wing area in Robo Raven III. These solar cells produce 3.6 W of power during a sunny day. The efficiency of these solar cells appears be around 6%, and the combined efficiency of batteries and motors is somewhere between 25 to 50%.  We hope that the performance will go up significantly as more efficient solar cells become available and we cover more of Robo Raven III’s wing and body area with solar cells in future versions.

So how does this compare with energy conversion efficiency found in the nature? Plants are able to convert less than 10% of available solar energy into biomass. Plant-based biomass to meat-based biomass conversion is not very efficient either. It takes around ten gram of plant based biomass (e.g., corn) to produce 1 gram of meat if you ignore the energy needs of other body parts and metabolism. In other words, you need to eat at least 10 lbs. of corn to gain 1 lb. of body weight. Finally, converting energy stored in biomass into mechanical energy is also not very efficient.  Animals use the aerobic respiration to derive energy from the food, and typically less than one fourth of the energy available from respiration is converted into mechanical energy. Animals also lose a lot of energy due to metabolism.

As described in the paragraphs above, using solar cells to convert solar energy directly into mechanical energy for flapping wings is an order of magnitude more efficient when compared to conversion via the biological path. This advantage will magnify as solar cell technology improves, thus allowing conventional engineering to beat nature in terms of the solar energy conversion efficiency.

However, nature has a significant edge over engineered system in other areas. For example, one gram of meat stores 20 times more energy than one gram of the current battery technology. So in terms of the energy density, we engineers have a lot of catching up to do. In nature, solar energy collection devices (e.g., trees) are not on-board ravens. Hence, ravens ultimately utilize a large collection area to gather energy into highly a dense storage source (e.g., meat), giving them a much longer range and better endurance than Robo Raven III.

We still need to make significant improvements in solar cell efficiency and battery energy density to replicate the endurance of real ravens in Robo Raven III, but the good news is that Robo Raven III has already demonstrated that we can fly with a solar cell and battery combination. Now that we’ve successfully taken this step, swapping new technologies that are more efficient should be relatively simple!

Robo Raven: A Step towards Bird-Inspired Flight

I have always been fascinated by birds. To me they represent beauty, freedom, and a design marvel. I am envious of bird watchers. What a wonderful hobby! Bird watching requires patience and traveling to exotic places where birds like to hang out. Unfortunately patience is not my forte. Also, at this stage in my life I am unable to travel to exotic places. So for now,I have compromised and have settled for the next best thing - creating and watching my own “birds”. I am interested in building robotic birds, not the kinds that look pretty on the shelf, but the ones that can actually flap their wings and fly. 
 
Eight years of experiments have taught me that designing and building robotic birds is hard, despite the apparent simplicity of the idea - flap wings to generate thrust to propel forward and use the moving air to generate lift to stay afloat. I am glad that this looked deceptively straightforward in the beginning; otherwise we would have never started on this adventurous journey. How hard can it be to build two wings and flap them using a motor? It turns out to be quite challenging if you want the bird to actually fly! This requires a long trial and error process due to the absence of accurate simulation tools. Many concepts that look good on the paper lead to a spectacular crash during the flight test, often causing a fatal injury to the robotic bird! So design iterations are painfully slow.

We had the first successful flight in 2007. +Arvind Ananthanarayanan, Wojciech Bejgerowski, +Dominik Müller  were the main architects behind this feat. Hugh Bruck, my faculty colleague at the University of Maryland, offered valuable help with the wing evaluation. We subsequently created three more flying versions using similar ideas. The last one in this series was completed in 2010. John Gerdes played a major role in building the last version in this series. Please click here to see videos of these “birds” in action. We were able to put a tiny video camera on them and get the “bird’s eye view”. We were able to launch them using a small ground robot. They were able to fly in moderate winds of around 10 mile per hour. As I mentioned before, every design flaw led to a fatal crash. But to our surprise, a very successful design also led to a fatal crash for our “birds” for a different reason. A hawk felt threatened by our "bird" and tore it apart in the mid-flight on multiple occasions!

Real birds are able to precisely control each wing during flight which enables them to do all sorts of aerobatic maneuvers. This has been a very difficult feat to achieve in bird-inspired robots. In fact, prior efforts (including our own mentioned above) utilized only simple wing motions where both wings are driven by a single motor. So motions of two wings are coupled. Minor adjustments can be made in wing motions by using small secondary actuators. But two wings cannot move completely independently. In the past, any major change in the wing motion had to be accomplished by doing a hardware change on the ground. Clearly this limited how close a robotic bird came to the real bird in terms of the flight characteristics.

I wanted to build a bird with completely independent wings that can be programmed with any arbitrary motion profiles. We did a preliminary experiment five years ago, but unfortunately it was not successful at that time. So we shelved the idea for few years. Hugh Bruck and I revived the idea again about a year ago. I am happy to report that we finally had a breakthrough last week. The students responsible for this success are Eli Barnett, John Gerdes, Johannes Kempny, Ariel Perez-Rosado, and Luke Roberts. Our new robot is based on a fundamentally new design concept. We call it Robo Raven. It features programmable wings that can be controlled independently. We can now program any desired motion patterns for the wings. This allows us to try new in-flight aerobatics that would have not been possible before. For example, we can now dive and roll. Please see below for the video of Robo Raven. 

 

The new design uses two actuators that can be synchronized electronically to achieve motion coordination between the two wings. The use of two actuators required a bigger battery and an on-board micro controller. All of this makes our robotic bird overweight. So how do we get Robo Raven to “diet” and lose weight? We used advanced manufacturing processes such as 3D printing and laser cutting to create lightweight polymer parts to reduce the weight. However, this alone was not sufficient. We needed three other tricks to get Robo Raven to fly. First, we programmed wing motion profiles that ensured that wings maintain the optimal velocity during the flap cycle to achieve the right balance between the lift and the thrust. Second, we developed a method to measure aerodynamic forces generated during the flapping cycle. This enabled us to quickly evaluate many different wing designs to select the best one. Finally, we had to perform system level optimization to make sure that all components worked well as an integrated system.

Robo Raven will enable us to explore new in-flight aerobatics. It will also allow us to more faithfully reproduce observed bird flights using robotic birds. I hope that this robotic bird will also inspire more people to choose “bird making” as their hobby!

Robotic birds (i.e., flapping wing micro air vehicles) are expected to offer advances in many different applications such as agriculture, surveillance, and environmental monitoring. Robo Raven is just the beginning. Many exciting developments lie ahead. The exotic bird that you might spot in your next trip to Hawaii might actually be a robot!