August 2008

Factory Automation

It's all in the LANs

Manufacturing remote control over a local area network

Fast Forward

  • Manufacturers' challenges-products' smooth transition from engineering.
  • Capturing information between machines, devices essential.
  • University project integrates PLC control, data transmission with local area network.
  • Retrieved real-time information builds case for future research.
 
By Fereshteh Fatehi and Yuqiu You

The most data-intensive and dynamic part of any manufacturing organization is the factory floor. When manufacturing is distributed among different vendors, the factory floor becomes a virtual realm, and gaining control becomes even more challenging. Some of the challenges today's manufacturers face include smoothly transitioning products from engineering to manufacturing, managing dynamic product code and feature sets, and optimizing complex multi-step, multi-vendor production cycles.

Manufacturers know the first step to building a reliable global manufacturing network is to gain visibility and control on individual machine stations on the plant floor. The solutions should be able to provide complete details of products and machines in process beginning in engineering, and to capture all information, both sensor-derived and manually collected. Manufacturing remote control helps manufacturing networks on the plant floor communicate remotely among different machines and devices.

Programmable logic controllers (PLCs) see use in almost every segment of manufacturing that requires automation on the plant floor. They represent one of the fastest growing sectors of industrial electronics. With the development of computer and networking technology, you can process all kinds of data over different networks. Integrating manufacturing automation with computer and networking technologies improves the way of data processing and information transmission in a manufacturing environment.

In one of our latest projects, we integrated the PLC control process and data transmission by applying the local area network (LAN) technology in PLC control. We set up the communication of the remote PLC control by establishing a LAN connecting computers and PLCs. Through the LAN, any of the computers connected to the network can program all PLCs and monitor them online. We used local input data of a PLC to control remote output devices through programming and I/O configurations. In this way, we could establish an efficient integrated control system in the LAN. We can retrieve real-time information and share it over the network in the formats of data tags or documents compatible with Microsoft Office applications, such as Excel and Access. We also used this project to prepare the network for future research projects on implementing PLC control remotely over the Internet.

Manufacturing remote control

You can use manufacturing remote control applications with PLC and related technologies and control the motor remotely by different on-off switches, limit, and photoelectric switches. This application also provides our students with opportunities to explore devices and technologies for implementing computer-integrated manufacturing. It helps them gain skills in process design, programming, and system integration.

In this project, we established a LAN to connect 12 laptop computers and 12 controllers in the PLC control lab of the School of Technology. The architecture provides a wide range of input and output modules to applications from high-speed digital to process control. The programmable automation controller (PAC) architecture uses producer/consumer technology, which allows input information and output status sharing among multiple PACs.

We established the LAN using the star topology and transmission control protocol/Internet protocol (TCP/IP) addressing. TCP/IP is a transport-layer protocol and a network-layer protocol, respectively, commonly seeing use in a business environment to communicate within and across internetworks. The IP address identifies each node on the IP network (or system of connected networks). Each TCP/IP node on the network (including the Ethernet module) must have a unique address. The star topology consists of a central hub with spokes extending out from it, and it terminates in nodes. It is one of the lowest cost-per-node topologies. All you need to do is add another node to a hub port. Star topology has low overhead and high throughput, and it is easy to build and maintain. The reliability of the star is directly related to the hub. In the PLC network in this project, hosts include all the laptop computers and PLCs. Each host gets a unique IP address. We assigned each laptop's IP address to its network interface card, and we assigned each PLC an IP address to its 1756-ENET Ethernet module. TCP/IP uses a binary method of addressing at the network layer. You can use it to identify each host connected to the network.

A TCP/IP address includes four octets and can break down into two parts. The first two octets, Net ID, determine the network on which the host resides. The last two octets, Host ID, refer to the host.  In this LAN, all hosts have the Net ID 192 168, and unique Host IDs.

Each of the 12 control stations has one laptop and one PLC. The laptops are installed with software running on Windows 2000 operating system, programming PLCs, monitoring the status of network nodes, and managing PLC programs. We wired each PLC with its local input/output devices including start/stop pushbuttons, one sensor, one limit switch, and one motor through input/output modules. The PLC looks at its inputs, and turns on/off its output, depending on the input information and the program running inside. 

To function, a network requires a service to share or access a common medium or pathway, and thus connects the computers. We connected the 12 laptops to an SMC multi-segment stackable hub by Category 5 twisted pair cables. The PLCs have 1756-ENET Ethernet communication modules that plug into the back plane of the chassis. They are connected to a catalyst switch by Category 5 twisted pair cables through the Ethernet module. The hub and the switch are connected together to work as a central hub of the star network. We used two network devices for troubleshooting.

Remote control process

We achieved remote control process by creating data tags to carry signals from one PLC station to another station and configuring the Ethernet modules for communication. When a signal from a local input device needs to control an output device in a remote station, the input signal will save in a local data tag and transfer to a remote data tag through the LAN connection between the two Ethernet modules on both PLCs. You can create a data tag and configure it in the data tag property window.

For the remote control between two PLC stations, the Ethernet port on the local station performs the role of an output device to get data from the controller processor, while the Ethernet port on the remote station performs the role of an input device to send data to the controller processor. When we configure the communication through the Ethernet modules, we need to specify IP addresses of the local and remote stations in the module configuration window.
In order to clearly explain the system configurations for the remote PLC control, we demonstrated a simple remote motor control application. Different on/off switches, limit, and photoelectric switches will control the remote motor. A limit switch controls the speed of the motor. In this process, PLC stations 83 and 81 serve as a remote motor control. Station 83 works as a produce controller, and station 81 works as a consume controller. Pushbuttons on station 83 can control a motor in station 81.

We developed two PLC programs for the remote motor control process. The program installed on station 83 is to save the input signals from local pushbuttons to two data tags and send the data tags to station 81. Two created data tags send signals to the remote station as pushbutton on-off switch, Pushbutton_for_station_81.1 and Pushbutton_for_station_81.0. The program installed on station 81 is to retrieve the data carried by data tags from workstation 83 and control the status of the local motor. In implementing the remote control on these two stations, we must configure all instructions and data tags in each program to be consistent with each controller's name. You can edit the instructions and data tags of the programs from their properties windows. It is important to designate the local instructions and data tags to their local station's number, while designating the remote instructions and data tags to the remote station's number.

In order to realize the tag transferring, you need to configure the local Ethernet module and the remote Ethernet module on both stations. Specify IP addresses for the local Ethernet module and the remote Ethernet module in the Edit window so the controller processors can identify them for proper communication. The configuration establishes a path for data tag transmission in the remote control process between the station 83 and the station 81. 

After configuring the system for the remote control, we download programs to the two control stations respectively. While two PLC stations are in online mode and programs are running, you can control the motor located in station 81 remotely from station 83. With correct programs and configuration, you can control output devices located in one station on the LAN and monitor them from any station connected to the network.

ABOUT THE AUTHORS

Fereshteh Fatehi (fatehi@ncat.edu) is a professor in the department of Electronic and Computer Technology at the School of Technology at NC A&T State University in Greensboro, N.C. Yuqiu You (yu.you@morehead-st.edu) is assistant professor in the department of Industrial and Engineering Technology Morehead State University.

Consider wireless I/O

Historically, regardless of the industry, hardwiring has been the only option available for users to connect remote instrumentation assets in the field. However, new technology enabling greater use of spread spectrum radios gives companies the ability to connect remote instrumentation in the field without the need for a costly wired infrastructure. In fact, asset information is now available from applied and embedded sensory points enabling sophisticated diagnostics, remote monitoring, and control and plant optimization.

Wireless I/O is a mechanism by which analog (4-20mA, 1-5VDC, etc.), discrete, and other raw signals are transmitted via radio to and from a central processing device, such as a distributive control system, programmable logic controller, or other remote terminal unit. Transmitted data includes level, pressure, flow, temperature, alarms, and signals generated to actuate final control elements, such as valves.
 
Why wireless I/O?

Frequently, a company has geographically scattered assets, and they need sensor data at a central point. In the past, the only available option included digging trenches and/or running conduit and pulling wire to acquire the signals.

Advantages

Installation costs: The most intuitive of all the advantages is the reduction of labor and material costs required to hardwire the remote assets. Wired systems can take days or weeks to be properly installed, isolated, and commissioned. Wireless I/O networks generally require only the end points to be installed and configured, saving substantial time for projects with aggressive schedules.

Economies of sale: Any network, wired or wireless, should scale gracefully as the number of endpoints increases. Deploying additional points in a wireless I/O network is incremental. Instead of installing spare conductors, additional I/O slaves may share a common I/O master.
 
Fail safe: No system is completely immune to signal loss. Wired systems are prone to having wires cut during construction or even routine maintenance. Rust, corrosion, steam, dirt, dust, and water all can affect a wired instrumentation system. The difference is wire cannot alert a user of a problem.

Flexibility: Wireless I/O users are not required to replace existing legacy infrastructure. They can implement it slowly and integrate it into existing systems. Should the need arise to relocate instruments, there is no expensive conduit to be demolished, relocated, or added. Moreover, if mobile instrumentation is to be used within the company, wireless I/O offers an attractive solution.
 
Reliability: Depending on the specific application, corrupted data can result in anything from a disruptive glitch to a devastating failure. Three factors determine the signal reliability: path loss, RF interference, and transmit power. In order to identify and ultimately maximize signal reliability, we recommend performing an RF site survey or path study.

Diagnostic monitoring: The diagnostic activity occurs outside the normal transmission of I/O data and can feed into a diagnostics software package, which will notify the system user of any abnormal operation of the system. In the case of wireless I/O, an additional signal is extracted and analyzed during the course of normal operation of the sensor. As the sensor operates, the signal is monitored for abnormalities in terms of signal, noise, voltage, temperature, reflected power, etc.

Low power consumption: Although not necessarily a specific advantage over wired alternatives, but one of the most important considerations for remote site operation, low DC power consumption translates into smaller batteries and solar panels, making remote site deployment feasible in areas previously considered impractical for monitoring and control.

SOURCE: Brent McAdams of Freewave Technologies in Baton Rouge, La. (bmcadams@freewave.com).

 

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