Skip navigation

Correspondence Lesson 6: Distributed Intelligence Controls

Microchip-based electrical control products account for an increasing share of the electrical controls market. How we use electrical controls affects everyone: from the specifier to the electrical contractor to the maintenance electrician. It also affects how you interface with your local utility company, especially in the area of coordinated load management.Computer-based control technologies continue

Microchip-based electrical control products account for an increasing share of the electrical controls market. How we use electrical controls affects everyone: from the specifier to the electrical contractor to the maintenance electrician. It also affects how you interface with your local utility company, especially in the area of coordinated load management.

Computer-based control technologies continue to evolve from central intelligence and control to distributed intelligence and control. Our first generation of computer-based electrical controls focused around a central controller, which received all information, made every decision, and oversaw every operation. Similarly, as computer systems evolved into networks, so have control systems.

In this lesson, we'll look at distributed intelligence control schemes: BACnet and LonWorks. The HVAC industry developed BACnet, which is widely used in many of the more complex "Intelligent building" systems. However, space prohibits us from covering this system. LonWorks is a control network (like a data network), but it operates at lower speeds and increases reliability.

Echelon developed the LonWorks system with a goal to establish a commodity solution to the problems of designing and building control networks. Currently, Echelon has at least 1000 users (mostly large manufacturing and utility companies) for its 100` products. It currently uses LonWorks for process control, building automation, engine control, elevator control, life safety systems, power distribution controls, and similar applications. Many other firms, such as Ameritech, use LonWorks in test facilities before adopting it as a standard.

The LON (Local Operating Network) concept. The LonWorks communications structure is similar to that of a LAN because a number of processors continually exchange messages. But, because the LonWorks system operates under a different set of rules than LANs, they are termed Local Operating Networks, or LONs.

LON technology offers a means for using distributed systems that perform sensing, monitoring, control, and other applications. It also allows intelligent devices, such as actuators and sensors, to communicate with one another through an assortment of communications media. LON technology supports distributed, peer-to-peer communications. Individual network devices communicate directly with one another without requiring a central control system.

Purpose distinguishes LONs from LANs. You design LANs to move data (such as documents, images, and databases) among computers, shared disks, and printers. You view a LAN's performance in terms of its throughput, or the amount of data transmitted and received over the network (measured in megabits transmitted per sec).

You design a LON to move sense and control messages. These are typically short messages that contain commands and status information to trigger actions. You view LON performance in terms of transactions completed per sec and response time.

The critical factor in LAN technology is data speed: in LON technology it's the assurance of correct signal transmission and verification. Control systems do not need vast amounts of data, but they do require correct send and receive messages.

How the system works. An advantage of LonWorks networks is their ability to communicate across different types of transmission media such as powerlines, twisted pair control wiring, coax cable, optical fibers, via radio waves, etc.) in a single network.

For example: A forklift approaches a conveyor belt to pick up a pallet. As it approaches, the conveyor activates to position the pallet properly for loading, then turns off after the pallet is loaded. At the same time, a record enters in the inventory control system to track movement of the pallet from the warehouse to the shipping department.

In this application, the forklift contains a node (a single control location) that uses radio frequency for communication with the conveyor belt node. In turn, the conveyor belt node uses twisted pair media to communicate with power actuators and interfaces to the inventory control system.

Another application might be this: An intruder enters a secured area of a building after hours. A motion detector (connected to other motion detectors with twisted-pair wiring) sends a message to the powerline-based lighting system to illuminate the intruder's area and sends a message by radio frequency to sound the alarm at the security entrance to the building. The security gate receives the message sent over the powerlines by the motion detector and seals off the area so the intruder can't exit.

Although we can accomplish the two examples above with our current home control systems, we can't do them nearly as well. The LonWorks system is almost error-proof.

As all of us who have worked with the most popular home control systems know (referring primarily to X-10 based systems), they require a lot of troubleshooting. There are all sorts of filters, bridges, and associated changes required. The LonWorks system works as reliably as a hard-wired control system. Double redundancy and verifications are built-in.

The Neuron chip. The basic component of the LonWorks system is a special computer chip, called a Neuron chip. Along with a power supply and communications transceiver, this chip makes a fully functional control node. Fig. 1, on page 52, shows the basic LonWorks node. Notice each control location has a physical ID, timer, computation unit (a small CPU), device controllers, power supply, and I/O ports. This makes the node a sophisticated computer. Moving to the center of this drawing, notice the node contains a data storage section and communications transceiver for sending and receiving control signals.

The Neuron chip, which is approximately 1/2-in. square, differs from standard computer chips in that it has several different parts performing several different duties at once (Fig. 2, on page 54). It's a processor and device controller, as well as a memory chip with built-in EEPROM memory. When installing Neuron chips, or even after they are in service, you must use special tools to program them.

Transceiver modules. Like power supplies, you must add transceiver modules to the chips for them to operate correctly with other types of equipment. Since these chips can accommodate virtually every type of communications media, no one type of transceiver works every time. Therefore, basic transceiver functions are built into the chip, and you must connect separate modules to it to complete the transceiver.

The most common types of transceiver modules are twisted pair, power line transmission, and RF transmission.

One of the more important types of transceiver configurations is the power line module. Sending data safely over power lines can be difficult. Echelon's methods uses a spread-spectrum transmission (100 kHz to 450 kHz), error correction, and a variety of other techniques to reach transmission speeds of 10kbits/sec.

Distributed intelligence. One important thing to remember about the LonWorks system is: There is no central controller.Instead of using central intelligence, this system uses distributed intelligence.

When one component in a LonWorks system activates or detects a certain condition (temperature, proximity, light, time, etc.), it informs the system. The installer programs each node with all the information it needs to function. (The programming is modifiable at any time.) You can change, add, or remove parts of the system without major reprogramming. In effect, each node has its own brain and can communicate with every other part of the system.

Message passing. There are a number of trade-offs between network efficiency, response time, security, and reliability. LonWorks defaults to the greatest degree of safety and verification for all communications over the LON network.

The LON TALK protocol offers four basic types of message service:

1.The most reliable service is acknowledged, or end-to-end acknowledged service, where you send a message to a node or group of nodes and expect individual acknowledgments from each receiver. If you don't receive an acknowledgment from all destinations, retry the transaction. The number of retries and the time-out are both selectable. The network CPU generates acknowledgments without intervention of the application. You use transaction IDs to keep track of messages and acknowledgments so the application does not receive duplicate messages.

2.An equally reliable service is request/response, where you send a message to a node or group of nodes and expect individual responses from each receiver. The application on the receiving side processes the incoming message before generating a response. The same retry and time-out options are available as with acknowledged service. Responses may include data, so this service is particularly suitable for remote procedure call or client/server applications.

3.The next reliable is unacknowledged repeated, where a message is sent to a node or group of nodes multiple times expecting no response. You'd typically use this service when broadcasting to large groups of nodes, in which the traffic generated by all the responses would overload the network.

4.The least reliable method is unacknowledged, where you send a message once to a node or group of nodes expecting no response. Typical uses range from when the highest performance is required, network bandwidth is limited, and the application is not sensitive to the loss of a message.

Design. The design approach with LonWorks is: to define the required system nodes; define the connections between nodes; and write application programming (also called code) for each node. With the proper design, the nodes become generic building blocks, applied in various ways to accomplish tasks (to control the lighting on many different buildings using a variety of communications media, etc.). The connection and configuration of each node determines their tasks. Because hardware, software, and network design are independent in a LonWorks-based system, you can program a node's functions without concern about the specifics of the networks.

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.