Limited Data Rate in Control Systems with Networks
Data rate governs the speed of data transmission. A very important consideration in data communication is how fast we can send data, in bits per second, over a channel. Data rate depends upon 3 factors:. Two theoretical formulas were developed to calculate the data rate: one by Nyquist for a noiseless channel, another by Shannon for a noisy channel.
In the above equation, bandwidth is the bandwidth of the channel, L is the number of signal levels used to represent data, and BitRate is the bit rate in bits per second. Bandwidth is a fixed quantity, so it cannot be changed. Hence, the data rate is directly proportional to the number of signal levels. Note — Increasing the levels of a signal may reduce the reliability of the system.
Input1 : Consider a noiseless channel with a bandwidth of Hz transmitting a signal with two signal levels. What can be the maximum bit rate? Input2 : We need to send kbps over a noiseless channel with a bandwidth of 20 kHz. How many signal levels do we need? In the above equation, bandwidth is the bandwidth of the channel, SNR is the signal-to-noise ratio, and capacity is the capacity of the channel in bits per second.
Input1 : A telephone line normally has a bandwidth of Hz to Hz assigned for data communication. The SNR is usually The data field is restricted to 8 bytes. The standard primarily covers layers 1 and 2 the physical layer and the data-link layer in the OSI model. CAN is an event-driven versus clock-driven protocol, so there is no specified cycle time.
Message-level jitter is the maximum length of the message minus one bit. The number of nodes for the network varies depending on device and network topology. Practically speaking, a linear topology can easily support 64 nodes but with careful design, it can go higher.
The data-link layer now supports data rates faster than 1 Mbps, and a byte data field. When only a single node is transmitting, there is no need for synchronization between nodes, this enables a temporary speed boost. The trade-off is that nodes need to be resynchronized before the transmission of the acknowledgment ACK slot bit.
It is a hybrid master-slave and peer-to-peer architecture in which any node on the network can communicate with and control other nodes, while itself acting as a slave to a different master.
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Each node in the network can be assigned a priority so that in the event of data collision, the most important node prevails. CANopen is based on an object-oriented data model that features standardized communication objects COBs. Each CANopen device incorporates three elements: a protocol stack to handle CAN communications over the network, application software for internal control and interfacing to process hardware, and the CANopen object dictionary. The object dictionary defines the datatypes it uses and contains all relevant data about the device, including application-oriented parameters and communications parameters.
The object dictionary does not simply contain static values. Entries can be written to the object dictionary of a device to tell it to turn on, for example. Conversely, the object dictionary can be updated with the latest operating parameters.
The SDO enables other nodes to read or write data to a single node at a time. Other components in the protocol stack include network management tools, error control functions, and special functions like synchronization and time stamping. CANopen operates at data rates as high as 1 Mbps over a nominal bus length of up to 25 meters. Dropping the speed to 10 kbps stretches length to meters.
Motion Control Resources
CANopen supports up to nodes; if each node becomes a master, the total number of nodes possible is 10, It is based on a modified version of the RS physical layer and can connect up to 64 nodes on a network over a bus as long as meters. It delivers a top speed of 10 Mbps and supports cycle times as low as 10 ms.
Based on a full-duplex copper or fiber bus, CC-Link IE Field Motion delivers synchronous communication over a master-slave architecture. Only the node possessing the token at any one time can put data onto the network.
As the physical layer suggests, the protocol operates at a maximum data rate of 1 Gbps. It can connect up to nodes per network with cable runs of up to meters over copper and meters over fiber.
It is compatible with star, line, mixed star and line, and ring networks. It defines comprehensive communications services for automation, including controls, motion, safety, synchronization, etc. CIP is based on a producer-consumer model rather than a source-destination approach. Producers are the field devices that generate data while the consumers are the devices that use that data; some devices can be both. The chief benefit is that if multiple consumers need the same data, the producer only needs to transmit one time, no matter how many consumers are on the network.
This improves bandwidth efficiency and enables the use of a variety of hierarchies such as master-slave, slave-multi-master, target-to-originator, and peer-to-peer. CIP is an object-oriented protocol in which each device is associated with a collection of objects that details its characteristics, performance, and how it communicates across the network.
The protocol encompasses three types of objects: required objects, application objects, and vendor-specific objects. Among the required objects are the identity object sort of an electronic nameplate for each device , the message-router object which directs object-to-object communications , and the network object which describes the network details required to connect an object in the network. Application objects describe the way a device encapsulates data. The application object for a motor might include size, current rating, and speed.
A device might require multiple objects that combine to create an object profile. To simplify implementation, the CIP protocol includes a library of predefined application objects and profiles. Objects can be grouped into assemblies that make it possible to establish rules for different behaviors; for example, one assembly might be created to monitor the output of a current meter for a drive at ms intervals while another might be established to only capture the values when they exceed some trigger point.
The user just picks the assembly that best suits their needs. Vendor-specific objects enable manufacturers to customize objects beyond what is available in the libraries. CIP Motion features motion profiles encompassing drive control. Examples include drive configuration, status, and diagnostic attributes and services; unicast control-to-drive communications; multicast peer-to-peer communications to synchronize position and velocity in drives slaved to multiple distributed controllers, and more.
It protocol can support master-slave architectures for conventional centralized control or for distributed control over peer-to-peer connections. It can link up to 64 nodes with a top speed of kbps. Although it is not designed for motion, it can be an economical solution for tasks like connecting encoders in simple motion systems. CIP handles layers 5 through 7 while DeviceNet takes care of the network- and transport-layer tasks.
The physical layer delivers both communications and up to 24 VDC, 8 A of power on the same bus. It is an open standard covered by IEC EtherCAT is a layer 1 and layer 2 protocol. In layer 2, it only uses the MAC layer in order to identify node locations and transmit to them. Designed for real-time applications, it allows exact scheduling of node-to-node communications. At the same time, it can carry standard Ethernet traffic to, for example, communicate with a supervisory control and data acquisition SCADA systems or manufacturing execution systems MESs.
To accomplish standard Ethernet communications, the protocol embeds the Ethernet frame with headers inside of an EtherCAT frame, where it gets unpacked at the receiving device. EtherCAT is based on a master-slave architecture. The master transmits a telegram that goes to each slave node in sequence. Each slave device features a dedicated controller able to process the frames in hardware. When the telegram arrives, the slave device reads the applicable data, inserts data of its own in real time, and passes it to the next node.
At the end of the bus, the signal returns, courtesy of the full-duplex bus.
Control Engineering | Optimizing communications for embedded machine control systems
Having each data packet passed through each node before return does introduce a small amount of latency at each node and must be compensated for in the scheduling. Timing is based on a distributed clock. The first slave node acts as a master clock, distributing it to the other nodes in the network. The protocol corrects for propagation delay to maintain the jitter specification.
Each master can support 65, slaves.
source link EtherCAT requires a specialized processor in slave devices. This enables it to not only use standard Ethernet equipment but to carry Internet traffic alongside machine data.