7.5.1 Device Selection
The main control unit requires large amounts of processing power to carry out complex control calculations, high level decision making and data processing from audio or video sensors. A 64-bit architecture with a clock speed of at least 2 GHz and a minimum of two CPU cores would be sufficient. Additionally, a small form-factor is desirable to ensure it is compact, lightweight and low powered for a mobile robot. HADES moves over rough terrain, therefore the device should avoid mechanical hard drives that are susceptible to vibration and shock damage, and instead include a Solid State Drive (SSD). The control unit must include at least 4 GB of RAM for multitasking and sufficient storage space for data that cannot be transmitted over the network. Finally, USB 3.0 is required to interface with RGB-D cameras. A range of small form factor PCs are compared, as displayed in Table 7.7.
Table 7.7: Comparisons of PC’s
PC Most Suitable CPU RAM Storage Features Size (mm) Price USD
Dell Inspiron Micro
2.67 GHz, Quad Core, Pentium
2 GB 32 GB 52.5 × 131 × 131 $229
Intel NUC Intel i5 and i7 ranges 16 GB >1 TB USB 3.0, M.2 32.7 × 111 × 115 ~$350 Gigabyte BRIX Intel i5 and i7 ranges 16 GB >1 TB USB 3.0, M.2 62 × 111.4 × 114.4 ~$400 HP Stream Mini 1.4GHz, Dual Core,
Celeron
137 Asus Chromebox 1.7 GHz, Dual Core, i3 4 GB 16 GB 43.2 × 124.5 × 124.5 $300 Samsung Chromebox 2.5 GHz, Dual Core, Intel i5 4 GB 16 GB Speaker 32.5 × 190.5 × 190.5 $329
Many available small form factor PCs are designed primarily for cloud operation. Therefore, their RAM and on-board storage is limited, embedded and not upgradeable. As internet access is not available during deployment these devices are unsuitable.
The remaining two non-cloud devices (Intel NUC and Gigabyte BRIX ) are capable of 4 GB of RAM and large storage space. They are both bare-bone PCs, which allows hardware customisability; specifically, their memory and storage can be increased. The M.2 standard that Gigabyte and Intel include allows integration of SSD cards as small as 12 × 16 mm, which reduces the overall motherboard size. Both the NUC and the BRIX are available with any mobile i5 or i7 Intel processor and include USB 3.0 ports to interface with RGB-D cameras. The biggest differentiating factors between the two PCs are size and price. The BRIX costs more with slightly larger dimensions. Low cost and small size is desirable, therefore the Intel NUC is most suitable for HADES.
7.5.2 Implementation
The NUC5i5MYHE model, shown below in Figure 7.22, has chassis dimensions 32.7 × 111 × 115 mm, making it small and compact. Common robotic functionality such as computer vision, localization and mapping as well as image and audio processing are not implemented in this project, but may be desired in future. An i5 fifth generation dual core 2.3 GHz processor with 2.9 GHz turbo and 8 GB of RAM is included to enable this. Four USB ports are available on-board, but five are required for four cameras as well as communication with the embedded controller. A USB hub is added to provide more options.
138 Figure 7.22: Next Unit of Computing (NUC) with 3D printed mount
Video File Storage
As mentioned in Section 5.1, not all the gathered information can be transmitted over the wireless network due to bandwidth constraints. The solution is transmitting crucial compressed information and storing the remaining high quality data on-board. This means retrieval of HADES is not mission critical as only additional high detail data is retrieved when HADES exits the mine. This data includes detailed sensor information, high quality audio, 3D depth data and high resolution video. Video files consume the most storage space due to four cameras with RGB, IR and depth data. Two hours of total operation and therefore video storage is required.
The Kinect-v2 and the RealSense can output RGB images at 60 fps, but only 30 fps is needed to detect salient features in a video feed. Using the H.264 video file format [111], data rates of 8.4 MB/s are produced. With an operating time of 7200 seconds, each RGB camera requires 60 GB of storage. The infrared and depth images are 360 × 480 pixels at 30 fps which are stored at a rate of 0.7 MB/s. These images produce 2.5 GB video files each. Three video types from four cameras totals 260 GB of storage space. The SSDs are available in
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capacities of 2n, therefore 256 GB is insufficient and 512 GB is required. This provides 252 GB for the operating system as well as other sensor and audio streams.
7.6 Summary
HADES is powered by six Li-Po batteries. To monitor and control the power supplied by the batteries, a distribution board is required. The board routes power to the ESCs and the DC- DC converters. The ESCs are powered directly from the batteries and are controlled by relays. The DC-DC converters are supplied by the input stage, which provides protection and switching capabilities. The output power for all components is arranged into modules which measure current and utilize high side switches for digital control. The entire power distribution board fits on a 250 mm × 95 mm PCB which is mounted against the main access hatch.
The embedded control board is mounted above the distribution board and provides low level control for power management, motor operation and sensor interfacing. Three distributed ATmegas are used for each of the aforementioned functions, communicating between each other via I2C. Intrinsically safe sensor ports are designed on the embedded control board to allow environment sensing in an explosive atmosphere. The embedded control board communicates over Rosserial with the Main Control Unit, which carries out all the high level processing. The device is an Intel NUC with specifications that endeavour to provide future- proofing.
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