Harvesting the sun

CAN and Kvaser’s CAN PCI boards are at the heart of BrightSource's solar field communications system.

The number of Controller Area Network (CAN) nodes is growing exponentially. Tens to hundreds of sensor nodes are linked on the CAN network of every modern car nowadays, and the same can be said for trucks, buses and machinery. But imagine a CAN network encompassing some 60,000 nodes or more over a distance of several thousand acres?

Some of the largest CAN networks today can be found in power generation, most notably in solar fields. Here, CAN is being used as a means of communication between thousands of ground-mounted mirrors used to concentrate sunlight and the solar field’s control centre.

BrightSource has used CAN to control and monitor its ‘heliostats’ – a term used to describe the mirror, its support structure, pylon and tracking system. Each heliostat has a 2 axis controller that allows the mirrors to be tilted up and down and from side to side to track the sun. Their reflected solar energy is focused onto a massive boiler where water is heated to more than 1,000 degrees Fahrenheit to create super-heated steam, which drives turbines in the conventional way.

With each CAN bus capable of controlling approximately 100 heliostats, the average solar field can have as many as a few hundred CAN networks running simultaneously These are linked using ‘gateways’ that connect one or two Ethernet ports to a set of between four and eight CAN buses.

A recent pilot project conducted by BrightSource in southern Israel provided an excellent demonstrator of CAN versus RS-485’s functionality, as one half of its solar field was controlled using RS-485 and the other half using CAN. There are a number of ways in which CAN and RS-485 differ, but one of the most notable is that CAN is a multi-master bus, whilst RS-485 works in a single-master fashion.

Notably, Israel-based LogiCAN provided CAN engineering design and support to BrightSource for the project, designing a pc-based gateway that provided two-way communication between the control centre and each heliostat. The gateway acts as a hub, converting Modbus/TCP messages to Modbus/CAN and vice versa. Four of Kvaser’s CAN PCI boards were used for the implementation. “These proved faultless and were very easy to accommodate, thanks to Kvaser’s easy to use driver software,” said Ran Weiss, engineering manager of LogiCAN.

Commenting on the project, Elad Goldberg from BrightSource explained: “CAN provided us with a smarter bus, enabling the heliostats to initiate their own status report messages in a prioritised order, without waiting for the master to ask for it, as is the case when using RS-485. In addition, each CAN node only speaks when it needs to, resulting in far less communication on the bus in any given moment and saving a lot of bandwidth.”

Weiss cited other advantages of CAN over RS-485 in this application: “CAN error confinement makes it virtually impossible for any node (or heliostat in this case) to miss a message. Many bus activity handling issues are also embedded into the CAN ‘UART’ hardware, relieving the software and assuring higher system integrity.”

The sheer size and scale of a solar field does pose certain challenges for CAN. Amongst CAN’s drawbacks is the differential-voltage-drop issue, which comes into force when bus lengths of over 1km are employed. To counteract this, LogiCAN specified the use of an active repeater in all cases where the bus is more than 600m long. Another challenge that the repeater partly overcomes is the CAN transceiver’s ‘weak’ pull-up resistors, which cannot pull the ‘dominant’ level to ‘recessive’ as fast as a RS-485 transceiver does. This results in a deterioration of the signal relative to bus length and capacitance, meaning the longer the bus, the lower the bit-rate. Concluded Weiss: “This is a CAN protocol limitation, but is also what makes CAN’s arbitration scheme work so well, and thus makes CAN so attractive in this application.”

Since the pilot project, LogiCAN has remained the Gateway keeper and embedded developer for projects CAN-related for BrightSource. Its most recent project has been to develop a new CANopen based protocol for the next generation of BrightSource’s heliostats and an embedded gateway, which LogiCAN has also designed.

Explaining some the new gateway’s features, Weiss said: “Using CAN, the gateway can handle a mix of heliostat vendors, each with its own little differences. Also, different heliostat versions can co-exist on the same network, allowing for drop-in replacement in almost all cases.” An additional capability is autonomous heliostat addressing on the network, whereby the system automatically assigns each heliostat an address, allowing the installer to plug in as many heliostats as required (up to the 100 limit), without worrying about bus address set up. The new gateway also boasts autonomous bus parameter calibration. Weiss explains: “In solar field installations, CAN bus cable length, node number and topographical conditions vary between buses and also in time. Since overall throughput is a key issue, we have introduced an adaptive mechanism which modifies CAN bus key parameters (e.g. bit rate) dynamically, to the higher acceptable values. The heliostats are fitted with mechanism to automatically detect the actual CAN bit rate at start-up and can synchronise to any running network.”  Confirmed Goldberg: “LogiCAN’s embedded gateway has provided a real upgrade to our system capabilities.”