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Aerospace

CAN we find life on Mars?

The successful launch of Exomars 2016 is the first step towards bringing CAN to the Red Planet. CAN networks are also used in other aerospace projects. Chipmakers provide radiation-resistant CAN transceivers and FPGAs.

(Photo: ESA/ATG medialab)

On Monday, March 14, 2016 ESA started the first part of the Exomars (Exobiology on Mars) mission. After seven months the Red Planet will be reached. A Russian rocket carried the Trace Gas Orbiter (TGO) sub-systems and the EDL (entry, descent, and landing) demonstrator module, also known as Schiaparelli. The main objectives of this mission are to search for evidence of methane and other trace atmospheric gases that could be signatures of active biological or geological processes.

The Trace Gas Orbiter and the Schiapralli used in the Exomars mission 2016 (Photo: ESA/ATG medialab)

Both parts of the Exomars satellite, the TGO as well as the EDL demonstrator, are using embedded CAN networks. Thalis Alenia Space (France) has developed the modules. In the CAN conference organized by ESA, which took place a few days before the Exomars launch, the company presented some details about the CAN communication. It makes use of the CANopen IP Core developed by Sitael. The physical layer is based on EIA 485 variant due to the fact that there were no rad-hard transceivers available when the development was started. For the second part of the Mars mission, it is planned to implement rad-tolerant CAN transceivers.

Radiation-tolerant silicon

Since many years, CAN is a candidate for avionics applications. Volume-wise aerospace is not a big business. But there are still some chipmakers willing to provide rad-hard transceivers and other semiconductors including CAN controllers. Cobham, Intersil, and Texas Instruments have already developed CAN transceiver suitable for outer space applications. The transceivers must feature a low dose rate, and apply single event transient (SET) mitigation techniques to reduce system level bit error rates. Because most of the aerospace CAN networks use redundant bus-lines, Intersil will provide two transceivers in the same enclosure. The chipmaker also plans to develop transceivers with galvanic isolation integrated into the silicon.

The Schiaparelli – without heat shield and back cover: The embedded CAN network connects several scientific sensors (Photo: ESA/ATG medialab)

Intersil’s ISL72026SEH transceiver is qualified for Airbus’s MPIU (Modular Payload Interface Unit) used in the E3000 Satcom platform. Texas Instruments will also release officially rad-hard CAN transceivers by end of 2016. The company already has shipped engineering samples, which are used for example by Vector in piggy-backs to connect its CANanalyzer and CANoe tools to aerospace CAN networks. The 3,3-V chips feature a short-circuit protection to ±36 V, and an ESD protection exceeding 16 kV.

Most the offered CAN transceivers are available as 3,3-V powered versions. They are as far as possible ISO 11898-2 compliant, some of them will also feature a low-power mode (e.g. the UTCAN333x series by Cobham), because energy saving is an important issue. Another challenge in aerospace applications is the reduction of weight. “Once we adapt a total CAN protocol,” said Gianluca Furano, on-board computer engineer at the European Space Agency, “we expect satellites will achieve sensible mass and power reductions and manufacturers will have the ability to add several millions of dollars of functional capability." Cobham investigates in plastic packages and plans to provide also radiation-tolerant CAN micro-controllers in 2017. Atmel is another chipmaker being able to provide radiation-tolerant CAN silicon including micro-controllers.

Evaluation board for the SN55HCD233 3,3-V CAN transceiver made for aerospace applications (Photo: Texas Instruments)

Modified CANopen

As application layer, ESA researchers have chosen CANopen and modified it for their needs. It is specified in the ECSS-E-ST-50-15V document. This approach uses mainly asynchronous and synchronous PDOs as well as the Heartbeat protocol for managing the bus-line redundancy. One PDO is used to transmit information in broadcast time. The redundancy approach is ‘cool’ stand-by approach. The missing Heartbeat message causes the switching to the redundant bus-lines. In addition, SDOs (expedited, segmented, and block transfer) are used to download software during the development process. The NMT slave devices do not implement an Object Dictionary. In order still to use off-the-shelf CANopen configuration and diagnostic tools, EDS (electronic data sheet) files are used to feed the tools. The entire CANopen protocol of the NMT slave devices is available as VHDL core by Sitael and is implemented in an FPGA. This allows implementing NMT slave nodes without CPUs.

This modified CANopen seems to become a standard in aerospace applications. During the CAN conference at ESA, Thales Alenia Space (TAS) introduced the Spacebus 4000 Avionics Architecture making use of redundant CAN/CANopen sub-networks. The requirements include a 500-kbit/s data-rate and up to 120 nodes. The TM/TC (telemetry/tele-command) modules produce 8-byte every 32 s. The host-controller sends one command message per second. This approach is used for the Satcom study of future satellites. For this purpose, acyclic PDOs are evaluated.

More space missions

Also the Spacebus Neo architecture by TAS for telecom satellites will implement CAN networks. Based on the Cast IP core a dual CAN controller has been developed. On the attached ASIC (Mega CAN protocol manager) a CANopen protocol stack has been implemented. There are two applications: CAN as payload data bus and CAN as backplane bus within the modules.

Airbus Defense & Space (France) also counts on CAN. The company uses CAN as telecom payload serial network saving TM/TC budget and harness. CAN replaces the proprietary LSSB serial bus. The Eurostar Neo platform based on CAN will support data-rates up to 1 Mbit/s at a length of 15 m connecting up to 44 nodes. The system validation will be finished in 2016. First launch is planned for end of 2019.

Tesat-Spacecome (Germany) has introduced at ESA’s CAN conference a CAN-based TM/TC interface for payload equipment. The launched programmable Microwave Power Modules (MPM) uses the CAN network to communicate between sub-units, for programming purpose during the manufacturing process, and interface TM/TC modules for housekeeping purposes. The CAN interfaces are redundant and feature data-rates up to 500 kbit/s. They are based on the Inican IP-core by Inicore (USA) and uses a CANopen-based application layer as specified in ECSS-E-ST-50-15C. It implements a strict master/slave communication using just the PDO and Heartbeat protocols. The PDOs are requested by PDOs, not by remote frames. The nodes IDs are assigned by means of connector pins. It is a geographical addressing, so-to-say. The redundant application masters use node-IDs 1 and 2, node-IDs 112 to 126 are reserved for multicast messages, and node-ID 127 is reserved for broadcast messages. The first version was developed in 2013, and will be implemented also in future projects.

CAN in smaller satellites

Besides the larger satellites weighing more than 500 kg, there are also smaller once. They are categorized in mini- (100 kg to 500 kg), micro- (10 kg to 100 kg), and nano-satellites (1 kg to 10 kg). There are more than 500 of such smaller satellites to be launched between 2015 and 2019. Researchers at Delft University of Technology evaluate to use CAN communication in nano-satellites. CAN has already been in three missions between 2004 and 2015.

Luxspace (Luxemburg) as supplier of ESA’s Automatic Identification System (AIS) will use CAN in two 100-kg satellites. They will be launched in 2018. Redundant CAN networks will be used as command and control bus running at 1 Mbit/s. A daisy-chain topology is intended. For future design, only rad-hard ISO 11898-2 transceivers will be chosen. As in many other projects, a CANopen-based application will be implemented. Some interfaces will provide SDO communication for firmware downloading. The PDOs are not configurable. The more simple hard-coded interfaces do not support SDO. Nevertheless, all CANopen-based devices come with an EDS.

The second step of the Exomars mission

Schiaparelli, the Exomars EDL demonstrator module will provide Europe with the technology for landing on the surface of Mars with a controlled landing orientation and touchdown velocity. The design of Schiaparelli maximizes the use of technologies already in development within the Exomars program. These technologies include: special material for thermal protection, a parachute system, a radar Doppler altimeter system, and a final braking system controlled by liquid propulsion. Not to forget the embedded CAN network.

The EDL demonstrator is expected to survive on the surface of Mars for a short time by using the excess energy capacity of its batteries. The science possibilities of Schiaparelli are limited by the absence of long term power and the fixed amount of space and resources that can be accommodated within the module; however, a set of scientific sensors will be included to perform limited, but useful, surface science.

The 2018 mission of the Exomars program will deliver an European rover and a Russian surface platform to the surface of Mars. The Exomars rover will travel across the Martian surface to search for signs of life. It will collect samples with a drill and analyze them with next-generation instruments. The rover embeds also a CAN/CANopen network. The locomotion is achieved through six wheels. Each wheel pair is suspended on an independently pivoted bogie (the articulated assembly holding the wheel drives), and each wheel can be independently steered and driven. All wheels can be individually pivoted to adjust the height and angle with respect to the local surface, and to create a sort of walking ability, particularly useful in soft, non-cohesive soils like dunes. In addition, inclinometers and gyroscopes are used to enhance the motion control robustness. Finally, sun sensors are utilized to determine the absolute attitude on the Martian surface and the direction to Earth.

The Dreams (dust characterization, risk assessment, and environmental analyzer on the Martian surface) sub-system co-developed by Temis is also based on CAN/CANopen communication. The connected CEU boards have different functions. The CAN interface is realized by means of the CCIPC core from Sitael. The boards come with EDS files generated with the CANeds tool by Vector.

Happy faces

In general, all researchers were satisfied with the robustness and reliability of the CAN communication. The chosen CANopen functional subset and enhancement (redundancy management) kept the system design simple. Of course, the missing radiation-tolerant transceivers required an intermediate solution (modified EIA 485 approach). But this disappeared with the availability of rad-hard ISO 11898-2 transceivers. For future European aerospace mission, CAN networks using CANopen functionality are set. The aerospace industry is very conservative: Once a technology has been successfully approved, it will be used for a long time. Many participants in ESA’s CAN conference in Katwijk (Netherlands) confirmed this.

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