CAN-MD is simply Machinery Diagnostics processed at the sensor level while using the CAN bus as a way to transmit the processed data. The CAN-MD accelerometers post Condition Indicators (processed features) on the CAN bus and, when instructed, can also collect and transmit raw or spectral data. The CAN-MD tachometer posts speed information which is used by one or all the accelerometers. The CAN-MD tachometer can also read data from a helicopter optical rotor blade tracker and post continuously updated rotor track data on the CAN bus.
Working principle Controller Area Network (CAN) is a broadcast-type bus. A message transmitted by one line replaceable unit (LRU) is received by all the LRUs connected to the bus. Each LRU will have a filter to accept the message relevant to it. Data messages transmitted from any terminal on a CAN bus do not contain source address or destination address. A data message is transmitted as a frame. In each frame, the message is labeled by an identifier that is unique throughout the network. All other LRUs on the network receive the message and each performs an acceptance test on the identifier to determine if the message and its content are relevant to that particular LRU.
CAN-MD is a democratic network, no master/slave relationships or central bus controller required for normal operation. Each sensor has its own processor, thus distributing the processing load across the network and offering additional redundancy. If one sensor fails the others keep on processing data, unlike today's conventional HUMS or VHM systems where if the processor fails, all data recording stops.
CAN found its way into aerospace applications because of its cost effective and efficient networking capability for systems to share data across a common media. The ability of CAN to transmit data, across a shared shielded twisted pair cable, has advantages in terms of significant weight savings along with installation cost and complexity. Additionally, the CAN physical layer protocol specification provides error recovery and protection mechanisms that make this data bus standard attractive to aviation, industrial and automotive applications. CAN bus operates at data rates of up to 1 Mb/sec for cable lengths less than 40 meters.