"The seamless interface between the CompactRIO and the NI LabVIEW development environment offers a turnkey hardware/software solution with very little learning required."
The Challenge: Providing portable renewable energy to remote villages and other areas that do not have access to the power grid. Remote villages without access to the power grid must rely on diesel or petroleum generators for electricity, which means they have power only when they can get fuel.
The Solution: Developing a portable 12 kW airborne wind energy system that uses a tethered, flexible airfoil to replace the blades and tower found on traditional wind turbines. Because the system has no tower, it does not require a heavy reinforced concrete foundation, so it can be mounted to a trailer and can
provide portable renewable energy in remote off-grid areas.
Windlift was founded in 2006 to develop portable airborne wind energy (AWE) technology for postconflict reconstruction, disaster relief, and third-world development. This technology has the potential to also benefit military operations by displacing diesel generators as the primary source of electricity for forward operating units.
Our company has four employees and is funded by $1.3 million USD of private investment and government grants to date. The company has built and is currently testing two 12 kW (rated) prototype systems and has developed a conceptual design for a 23 kW Humvee-towable system. Windlift is currently funded under a contract from the U.S. Department of Defense to develop the AWE technology for postconflict reconstruction in Afghanistan. When compared to other renewable energy solutions such as solar, Windlift's AWE technology offers higher power density.
System Operation
The AWE technology uses a flexible airfoil to capture power from the wind. The airfoil is tethered to a base station and the tethers are spooled onto a large drum. The system, which is mounted to a trailer, operates as a long-stroke reciprocating engine. During the generate phase of the cycle, the airfoil is actively flown in a cross-wind manner downwind of the base station, which maximizes the tension in the tethers. As the airfoil moves away from the ground station, the tethers unspool from the drum and drive it to turn a motor/generator. Electrical power from the generator is transmitted to a battery bank that is also mounted to the trailer. When the maximum tether length is reached, the airfoil is “depowered” (that is, oriented into the wind to minimize tether tension) and retracted. The net energy gain per cycle is the energy generated during the outgoing stroke minus the energy consumed during the retract stroke.
We control the system by using an AC motor/generator, two servo motors to steer the airfoil, and two stepper motors to operate the levelwind, which neatly stacks the tethers onto the drum. All of these devices interface with an NI CompactRIO embedded system through a controller area network (CAN) bus interface. Additionally, we use two analog joysticks and a number of digital I/O to interface with the system. Currently, the AWE system is manually operated, but future designs will be automated.
In addition to the user controls and actuators, the CompactRIO also interfaces with sensors that monitor the horizontal and vertical angle of the airfoil with respect to the base station, the tension in the tethers, the amount of tether remaining on the drum, and the flow of power to and charge state of the battery bank. Data from all of these sensors is used to control the cyclic operation and maximize generated power and stability.
From Prototype to Production With CompactRIO
We chose the CompactRIO platform for this project for several reasons. First, the seamless interface between the CompactRIO and the NI LabVIEW development environment offers a turnkey hardware/software solution with very little learning required. Second, the wide availability of modules for the CompactRIO means that all of the varied sensors and protocols can be integrated with a single modular system (load cells, temperature sensors, fieldbus, analog, digital, and so on). Third, the power and flexibility of the combined field-programmable gate array (FPGA) and real-time processor architecture offers functionality that would not be possible with either component alone. Fourth, and perhaps most importantly, National Instruments illustrated a clear development pathway with the CompactRIO from prototype to production with the same hardware and software. We also considered using dSPACE hardware for this development, but the anticipated hardware cost was substantially higher than the CompactRIO system and the transition from prototype to production was less clear.
The FPGA backplane in the CompactRIO system was particularly useful in our development. The ability of the FPGA to run tasks in a fully parallel manner at high speed (40 MHz clock cycle) enabled us to offload time-critical tasks from the real-time processor. One example of a task that is well-suited to the FPGA is monitoring the proximity sensors (configured in quadrature) that act as an incremental encoder to measure the drum's rotation. A code segment running on the FPGA counts each sensor pulse at up to 800 pulses per second and communicates the incremental drum position to the real-time controller.
During this development, we used an NI Green Engineering Grant award for a seat of the NI Developer Suite, which included NI DIAdem data management and analysis software; the LabVIEW Real-Time, LabVIEW FPGA, LabVIEW NI SoftMotion, and LabVIEW Control Design and Simulation modules, and the LabVIEW PID and Fuzzy Logic Toolkit, among others. One of the most useful tools for this project outside of the LabVIEW development environment was the DIAdem data analysis software. We used the shared variable engine feature of the CompactRIO to port data over a TCP/IP connection to a remote laptop for data logging.
The large volume of data generated during the test program was invaluable to the design and development process. Each data file, which represents 10 minutes of operation time, is approximately 4 MB and contains almost 70 individual channels. DIAdem was highly useful for processing and analyzing this large volume of data. An additional functionality that proved to be invaluable was the ability to synchronize the data with video of the system in operation.
We are currently in the final stages of testing the portable AWE system prototype. The CompactRIO embedded system has been a valuable asset during this development process and we anticipate that it will continue to be an integral part of the system. One important aspect of this approach is that the transition from the current manually operated system to an automated system will be streamlined, and will involve only a software update. This is possible because the manually operated system is flyby-wire and the CompactRIO has the functionality and performance to replace the user in an automated system.
National Instruments provides a graphical system design platform for test, control, and embedded design applications that is transforming the way engineers and scientists design, prototype, and deploy systems.
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