CANopen in ambient intelligence - continued
Plug-and-play
IEEE 1451 is a group of seven specifications, some of them in revision phase, concerned with smart transducer interfaces for sensors and actuators, which will let us have plug-and-play feature on the sensor-actuator side within a distributed control system. Only the following two standards from the IEEE 1451 group has been considered in this work:
IEEE P1451.0: providing a uniform set of commands, common operations, and TEDS (Transducer Electronic Data Sheet) for the family of IEEE 1451 smart transducer standards. This command set allows access to any sensors or actuators in the 1451-based networks. This standard will be used to assure uniformity within the family of IEEE 1451.x interface standards.
IEEE P1451.6: defining a transducer-to-NCAP interface and TEDS using the high-speed CANopen network interface. It defines a mapping of the TEDS to the CANopen dictionary entries as well as communication messages, process data, configuration parameter, and diagnosis information. In any case, IEEE P1451.6 status is currently only in a draft version and it has not been considered overall in this work.
TEDS definition is the key feature of this family of standards and it would be a memory device attached to the transducer having information such as transducer identification, calibration, correction data, measurement range, manufacturer-related information, and so on.
Both developments, the active headrest and ECG sensor, will be integrated into a system with one NCAP (Network Capable Application Processor) and two STIM‘s (Smart Transducer Interface Module). One STIM collects information from four different sensors (Infrareds, Horizontal and Vertical). In the same way, the STIM is connected to the two actuators of the headrest system (horizontal and vertical). One more STIM is dedicated to the ECG sensor getting data from a pre-amplifier and filter device. So, all sensors’ data and actuators’ control are accessible from a NCAP device.
CANopen communication
Focusing on the ECG sensor, the corresponding STIM device works like a NMT slave meanwhile NCAP device is the NMT master device. NCAP (master) and STIM (slave) CANopen devices will follow a minimal boot-up. After power-on or hardware reset, the NCAP completes its initialization process and from its “NMT pre-operational” state will send a broadcast “Start_Remote_Node” message binding itself to go to “NMT operational” state. In the same way, each STIM finishes its initialization process and transits into “NMT pre-operational” state and sends the “Boot-up protocol”. Then, every STIM in “pre-operational” state enters “operational” state, when the NCAP transmits the broadcast message to start all remote nodes. Failing this, the STIM would be started by the NCAP when the last one receives a “Boot-up protocol” from the previous one, once the corresponding STIM has become “pre-operational” and completing its “Initialization”. In any case, the NCAP will be able to start/stop every STIM if necessary through the NMT messages (”Start_Remote_Node” and ”Stop_Remote_Node”).
SDO services are mainly used to read and write TEDS information, mapped in the CANopen object dictionary of each STIM device, from the NCAP. Therefore, it would be sufficient to only consider one SDO object in the “peer-to-peer objects of the pre-defined connection set”. Within the client/server architecture, the following SDO download and upload services are implemented:
- Initiate Block Download,
- Download Block,
- End Block Download,
- Initiate Block Upload,
- Upload Block,
- End Block Upload.
When TEDS in any STIM (server) is to be modified from the NCAP (client), the NCAP will request the STIM (server of SDO and owner of the object dictionary) to prepare for downloading TEDS to the STIM (Initiate Block Download). This service must be confirmed by the STIM. When it is done the NCAP will start sending data-blocks with “Download Block” conformed protocol. Once every block of TEDS has been sent from the NCAP to the STIM, the “End Block Download” protocol is requested to finish TEDS download process.
A similar process is carried out to read (upload) the mapped TEDS in the STIM’s object dictionary from the NCAP, using “Initiate Block Upload”, “Upload Block”, and “End Block Upload” protocols:
This set of SDO services will let TEDS data be read and modified to configure STIM for a particular transducer. In any case, any modification of TEDS should stop the STIM sending the proper NMT service from the NCAP to put the STIM in “Pre-operational” state, then change TEDS, and finally re-start STIM sending “Start_Remote_Node” service from the NCAP.
When the NCAP is in “NMT operational” state it will start sending SYNC messages to define a fixed time window and synchronize PDO messages. So, every STIM connected to sensors will send its data by “Write PDO” protocol according to its PDO mapping information. Through this service the STIM of ECG sensor (PDO producer) sends the data of the mapped application object to the NCAP (PDO consumer) every “SYNC”.
