When Santiago R López Gordo of Airbus Military needed to develop an electrical rig for the development of an unmanned aerial vehicle (UAV), he had two main goals: Create a test environment that allowed him and his team to test the UAV electrical system under different operating conditions and execute tests that showed the integration of the equipment and the electrical system functions.
The UAV electrical system consists of two parts—a power generator and generator control unit, and a power distribution unit consisting of a control unit and load distribution devices. An electric motor is used to mechanically drag the generator in the same way that an engine drags a plane. A load bench system applies load levels to the electrical system in much the same way as loads would be applied to a plane. But a control system was needed to manage all these functions.
A data acquisition system measured certain parameters such as voltage, the status of contactors, and the I/O signals of the control units and simulated the equipment not present in the electrical system but that would interact with it in a plane. This electrical rig has two parts. One is responsible for the excitation of the electric system for testing under the different conditions for which it was designed. The other part monitors the status of the system to determine whether the equipment is correctly integrated and whether the complete system fulfills the design requirements.
The system was executed as follows. An electric engine from Bosch simulates the plane engine that moves the generator. Attached to the electric motor is a frequency variator with a discrete and analog I/O interface managed by a cRIO-based control system. The control system houses the start/finish logic for the driver and engine, as well as the logic for the two functioning modes: dragger and brake, responding to the requirement to test two functions—the electric power generation function itself (where the generator is dragged by the engine) and the start function (where the generator drags the engine). Alarms allow the system to respond safely under unforeseen circumstances.
The simulation of the load environment that receives electric power from the system is done through load benches that dynamically reproduce the load profiles that will be applied to the electric system during the real flight.
As a part of the test environment there is a system based on fault insertion modules from Pickering that can interrupt any signal from the electric system circulating between the equipment, for example the status on a contactor or a voltage measurement. This device is used to automatically simulate fault conditions on the different control software developments, allowing new tests sets with every new software release.
Another part of the application was the data acquisition system, which uses a cDAQ chassis and analog and discrete I/O cards. The parameters that are monitored and recorded are electric measures such as voltage and current, contactors status, system signal status. In addition this infrastructure is used to simulate certain plane elements that interact with the electric system, using models with I/O associated to the I/O of the DAQ system.
Once the control and data acquisition systems were defined, the third pillar of this application was the software developed by the labs department, which has the following functions:
-Analog signal acquisition at 5000 samples per second. This frequency allows for the correct representation of the signals in ac as well as to precisely calculate sophisticated signals such as rms, frequency, power factor, and so on.
-Discrete signal acquisition at 1000 samples per second, which helps Gordo and his team precisely determine the electric systems contactors operating sequences as well as the power transfer happening in the different configurations.
-Hardware signals simulation, such as weight on wheels, operating switches, and so on.
-External equipment simulation connected to the control units of the electric system through serial bus.
-On-line quality analysis of electric power according to the applicable MIL-STD-704 standard is done by generating the signals through software to compare them to the standard, as well as generate the discrete signals to check whether certain part of the standard is fulfilled.
-TDMS Recording, to guarantee compatibility with the analysis software DIAdem. Implementation of synoptics and graphical interface of the control system (cRIO, load benches and Pickering system) and the acquisition system (cDAQ).
-Integration with the automated tests execution environment. Gordo and his team used dynamic libraries (dlls) integrated in NILabVIEW for communication with the Stimulation and Signal Acquisition System, which in turn is managed by a Matlab-based automated test environment.
All the elements described above constitute the electric rig. A vertical rack houses the elements of the plane electric system, as well as the (cDAQ). The engine, driver, generator, control unit and cRIO are located outside the rack connected to it.