Optimizing communications for embedded machine control systems

Opposite to factory automation, embedded machine control systems need to be optimized to design better products than those of competitors. This requires configuring the communication, in particular the scheduling and content of control and status messages. The process data object (PDO) protocol transmits, in real-time, control and status information as well as measured values.


Figure 1: Configuring PDOs in order to reduce busload as much as possible is supported by tools available from different providers. Courtesy: EmtasCANopen has been designed for embedded machine control systems. Opposite to factory automation, embedded machine control systems need to be optimized to design better products than those of competitors. This requires configuring the communication, in particular the scheduling and content of control and status messages.

CANopen uses the process data object (PDO) protocol to transmit, in real-time, control and status information as well as measured values. PDOs are mapped to a single CAN data frame, which is normally send asynchronously. Asynchronous transmission includes trigger by means of a change-or-state and by means of elapsing of the event timer. Many system designers use a periodical transmission of PDOs, which cause high busloads. Most of the standardized CANopen profiles pre-define a change-of-state scheduling. In the best case, this causes just one transmission when entering the operation state. Depending on the change-of-state configuration any change triggers a PDO or the corresponding message is only transmitted when a specific threshold is reached (above or below). Of course, if the value changes frequently this can cause a lot of traffic. To reduce the busload, the system designer can configure the inhibit timer, which forbids to send the message again for a specified time. Of course, this doesn't effect the automatic transmission of faulty messages.

Figure 2: The synchronous PDO communication is a unique feature of the CANopen application layer. Courtesy: Vector CANtech

In some applications, synchronization between different CANopen devices is required. For example, several sensors need to measure at the very same moment the sensor values or several actuators need to start operation simultaneously. For those applications, CANopen uses the SYNC message, which triggers the capturing of current values and enables already received commands. It is also possible to configure a PDO in the transmitting device as synchronous and in the receiving device as asynchronous meaning the process data is immediately processed. Or vice versa, the Transmit-PDO (TPDO) is sent asynchronously (change-of-state) and the corresponding Receive-PDO (RPDO) is configured as synchronous and performs the command when the next SYNC is received.

Figure 3: The CANopen tools hide the internal addresses (16-bit index plus 8-bit sub-index) and show the mapped process data in more or less clear text. Courtesy: IxxatSynchronous PDOs also can cause high busloads. In order to reduce it, CANopen allows to configure them in a way that the TPDO serves only each second, third, etc. SYNC. The "slowest" synchronous TPDO serves every 240th SYNC message. Another option to reduce busload is to send synchronous TPDOs acyclic.

Besides the scheduling of messages, CANopen allows to optimize the PDO communication by means of the assigned priority. In CANopen, each device may support up to four TPDOs and up to four RPDOs with pre-defined CAN identifiers, which determines the bus access priority. This means each device may have one high-prior, one middle-prior, one lower middle-prior, and one low-prior PDO. The system designer can be lazy, not reconfiguring the PDO priority. But it is also possible to reassign the pre-defined priorities to optimize the time delay for the more important and critical data. In some applications, a dedicated device may require more than one high-prior PDO.

To tune the PDO communication, the system designer can also assemble those process data into one PDO, which have the same priority and scheduling requirements. This is known as PDO mapping: you pack together what fit together. In classical CAN this is limited to 8 byte. The upcoming CAN FD (flexible data-rate) protocol, so-to-say the next generation CAN protocol, supports data length up to 64 byte. Of course, it transmits the data with a higher speed. It is intended to use for embedded machine control networks an arbitration bit-rate of 500 kbit/s, 250 kbit/s, and 125 kbit/s depending on the bus length (125 m, 250 m respectively 500 m). The data-phase speed can be increased by the factor two, four, or even eight. Considering a data-phase bit-rate of 4 Mbit/s and an arbitration bit-rate of 500 kbit/s seems to be realistic. Running a bit-rate of 1 Mbit/s is also possible, but an increase in the data-phase to 8 Mbit/s is a challenge for the physical layer design, when the same robustness and reliability as in classical CAN is required. The lower the speed the less are the challenges. By the way, this is true also for other communication technologies including Ethernet-based solutions.

Figure 4: With this tool all the PDO communication parameters can be set and optimized. Courtesy: Kvaser/PortIn both protocol variants, classical CAN and CAN FD, the mapping of process data is configures by means of tables providing pointer to the process data to be mapped into a dedicated PDO. The mapping lists of a length of 64 entries meaning you can assemble up to 64 process data. In case of the classical CAN protocol, you can map bit-wise data. Using the CAN FD protocol, you are limited to an 8-byte granularity when using the maximum payload of 64 byte.

To summarize, CANopen is one of the most flexible network technologies in respect to optimize process data communication. On the other hand, the PDO optimization requires configuration. Software tools support this. Configuring CANopen networks by hand it is not state-of-the-art.

- Edited by Jessica DuBois-Maahs, associate content manager, CFE Media, jdmaahs@cfemedia.com.

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
Strategic outsourcing delivers efficiency; Sleeve bearing clearance; Causes of water hammer; Improve air quality; Maintenance safety; GAMS preview
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role of plant safety and offers advice on best practices.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me