Remote DAUs
Instrumenting an aircraft can require the installation of a significant number of sensors. The data from these sensors must be gathered to a single point i.e. the Data Acquisition Unit (DAU) that is performing the core function of the data acquisition. This requires cables, typically shielded twisted pairs, that can contribute a significant amount of weight to the aircraft. This weight gain is undesirable on any platform, particularly for spacecraft and small vehicles such as UAVs and rotorcraft. One solution is to use a small lightweight Remote Data Acquisition Unit (RDAU) close to a cluster of sensors and transport the acquired data over Ethernet to the unit performing the core function of the application.
In addition to removing significant weight, replacing a long loom with a single Ethernet cable can remove many installation problems. Any bore holes required for cabling will have a much smaller cross section for an Ethernet cable than for a loom containing multiple cables. Ethernet cables are also very flexible – they have a bend radius of typically 4 times their diameter which allows routing the cable in such a way as to avoid obstacles or potential noise sources. Being transformer-coupled, Ethernet nodes are inherently well protected against lightning strike and other sources of over-voltages and transients. ACRA CONTROL equipment is further protected through the use of voltage suppression diodes on Ethernet ports.
Reduced cable length can also have an effect on accuracy. The RDAU captures and digitizes the analog data much closer to sensors. Running the analog signals over longer cables makes the signal more susceptible to noise and the larger resistance of the longer wires will vary more with temperature and this can degrade accuracy. For a long loom it may not be easy to segregate the loom from a strong noise source – e.g. motors, actuators or antenna.
An RDAU can support many different types of sensors e.g. accelerometers, thermocouples, RDT’s, LVDT’s, potentiometers, microphones etc. Time correlation of sampled data in the RDAU is straightforward. Parameters sampled at the same rate are sampled at exactly the same point in time – referred to as isochronous sampling. Parameters that are sampled at different rates are all sampled together at start of the acquisition cycle and thereafter at equidistant sampling intervals.
This isochronous sampling holds true for all modules in all chassis for a system, thus it is unnecessary to realign or interpolate samples during analysis. Since the RDAU is effectively an Ethernet network node which is IEEE 1588 compliant, and all sampled data is time stamped with Precision Time Protocol (PTP), all of the data across all of the DAUs is synchronized to better than 100ns using the PTP. The open metadata standard XidML is used to ensure ease of setup for the RDAU.