EtherCAT EZ-ZONE RMZ4, Руководство пользователя

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W
ATLOW
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EZ-ZONE
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RMZ4
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E
THER
CAT
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A
DAPTER
6 T
HEORY OF
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PERATION
EtherCAT® is a protocol that runs on standard Ethernet hardware – CAT5 cable, RJ45 jacks and standard
physical layer transceivers. The Ethernet Media Access Controller (MAC) is a slight variation of a standard MAC
implemented with an ASIC. It allows data to be read and modified on-the-fly as the bits pass through the MAC.
This implies EtherCAT® is a ring topology with each device being part of a single chain. Data frames are not
received and responses formulated. It is more like a train of bytes that does not slow down at the station, data
jumps on and off as the ‘train’ passes though. This allows cycle times around the network in the microsecond
range, even with thousands of data points. This is the PDO (Process Data Object – real-time data) mechanism.
Responsiveness is very important in a large system to keep scan times short. This means products should not
introduce latencies in the system’s data cycle. Therefore the data of interest to the larger system must be
maintained close to the EtherCAT® access port. It is not acceptable to have long latencies while data is
retrieved via the backplane. To assure the required speed, the EZ-ZONE architecture has been modified slightly
to maintain critical data objects in the EtherCAT® module itself rather than each RM module.
EtherCAT® models data as CoE objects. CoE is CANOpen over EtherCAT®. This is the object model developed
for the CAN Open protocol and reused as the EtherCAT data model. It uses objects defined by indexes (0x0000
to 0xFFFF) each containing up to 255 sub-indexes or data elements of simple or complex data type.
EtherCAT® transports data using PDO and SDO methods. PDO is Process Data Object. It is the regular data
shared to and from the master to all the slave devices. The network PDO scan times are very fast, 0.5 to 4ms
typically. The data in the PDO is a selected set of Object:SubIndex parameters. Typical PDO parameters are
process temperature, set point, output power, and errors. PDO data is only exchanged when the system is in
the operational mode (OP mode). As a default, the RMZ4 controller only controls temperature in the
operational mode. The Safe State parameter configures this behavior.
SDO is Service Data Object. This is an asynchronous, on-demand mailbox service that provides access to all the
CoE objects in the device. The SDO method is used to configure the device or tweak settings. Setting PID,
Proportional Integral Derivative control loop parameters or starting an auto-tune would be typical uses for an
exchange. SDO exchanges can occur during normal operation but are secondary to PDO traffic. SDO exchanges
can occur in all modes except boot mode.
The EtherCAT® module holds all the data that is part of the Semi TWG (Technical Working Group) Standard for
all the loops being controlled locally in the EtherCAT® module. The control loops are hosted by the EtherCAT®
module. The EtherCAT® module works in conjunction with RM modules to act as sensor inputs and heater
outputs only. The PV (Process Value Temperature) is produced by an RM and consumed by the EtherCAT®
module via data sharing. The loop power is produced by the EtherCAT® module and consumed by an RM
output via data sharing. The values accessed by the EtherCAT® module are the values held internally. This
includes PV, SP (Set Point), PID values, control mode, percent power, etc.
In legacy product configurations (ST,PM,SL-10) the pertinent data is held locally within the EtherCAT® adapter
for instant access but the control loops still execute in the individual devices due to the lower bandwidth
buses. There means parameter changes made over EtherCAT® can take up to 200ms to be written to the
remote legacy device since they need to be sent by proxy on the legacy communications bus. Input values are
continuously scanned by the RMZ4 and the latest value is available for PDO access.