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65 unmanned systems inside June/July 2016 ENGINEERING. PRACTICE. POLICY. trol system is significantly compromised and recovery of the vehicle is very unlikely. On the other hand, Type 1B failures are those where the UAS control system is only moderately com- promised and a skilled human operator can recover the vehicle with moderate damage. It's apparent the failure rates are or- ders of magnitude higher that what is considered acceptable in manned aviation. Our claim here is not that our target reliabil- ity should be 10 -6 or 10 -9 . Rather what we hope to show is that these poor reliability figures can be enhanced with a modicum of hardware redundancy coupled with analytical redundancy. As Table 1 shows a key contributor to Type 1A failure is the unreliability of rudder and elevator servos. Therefore, in what follows we will narrow our scope and focus on elevator and rudder servo failures. We will start by quantifying the reliability of the key components of the GNC system. Then we describe a somewhat simple re-design of the UAS and its control system to enhance the reliability. Redesign of UAS Control System The purpose of the control system on a UAS is to actuate au- tonomously the various control surfaces and throttle to posi- tion and orient the aerial vehicle in response to control and guidance system instructions. One of the key components of the control system are the servos used to move the control surfaces. The high failure rates of these critical components is what leads to the reliability figures for the Type 1A failures noted above. How unreliable are these components and why are they unreliable? We will answer this question first. Servo Reliability Characterization: Servos are used to actuate the control surfaces in an aircraft and they are one of the most critical components of a UAS. 1 Figure 2 represents the outside and inside of one such servo found on many UAS today. These are off-the-shelf components that are adapted from the hobby- ist (remote control aircraft) community. They come in different varieties ranging from the low cost Futaba S3151 ($30) to the high speed HITEC 9360TH ($180). As seen in the right half of Figure 2, these servos are an assembly of four parts: a DC motor, gear train, control circuit and a position sensor (usually a potentiometer). Regardless of whether they are of the low-end or high-end type, these hobby servos tend to degrade over time and usage. For example, in our work we have observed faults internally in these servos due to slippage of gears, bent linkages and damaged servo driveshaft. 2 Additionally, actuator response may also be faulty due to external factors such as software bugs, unbalanced control surfaces and poor rigging. If any of these High Precision low cost MEMS An all new family of 6 DoF IMUs DMU10 - Silicon Sensing Systems' precision MEMS IMU offering class-leading accuracy, in a small and affordable, yet powerful 6 DoF inertial module. DMU11 - OEM version of DMU10 for high volume applications. DMU30 - High-end MEMS IMU alternative to more costly FOG-grade IMUs for use in exacting motion sensing applications (ITAR - Free). www.siliconsensing.com firstname.lastname@example.org faults occur in flight, it can lead to a failure, which may ultimately result in loss of control of the UAS. In assessing the impact such failures have on a UAS, we need to first determine the frequency of their occurrence. This was done in the study we noted earlier and was the source for the data presented in Table 1. To deal with servo failures in flight, two things must happen in sequence. First, the occurrence of the failure must be detected. Once detected, the failed servo must be neglected and/or the control system must be reconfig- ured for continued flight. References 1. J. Amos, E. Bergquist, J. Cole, J. Phillips, S. Reimann and S. Shuster, "UAV for reliability," December 2013. [Online]. Available: http://www.aem.umn.edu/~SeilerControl/ Papers/Seiler_All.html. 2. P. Freeman and G. Balas, "Actuation failure modes and effects analysis for a small UAV," in American Control Conference (ACC), Portland, Oregon, USA, 2014.