Time for a Sensor Network?

Readers Ask: Is it Worth Upgrading From Hardwired to Networked Proximity Switches?

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Our machines use a variety of proximity switches to detect presence or absence. These switches have been hardwired to local I/O at our central PLC. We think it might be time to upgrade and move to switches connected to our PLC via a digital network to cut wiring costs and add diagnostics. Our concerns are the complexity of adding a network and, most importantly, real-time performance because we'll be depending on the network for speed of execution and ultimately the safety of our machine. What's been the experience of others in a similar situation to ours—machine builders that have made the upgrade from hardwired to networked proximity switches? Is this a good case for the IO-Link you wrote about?

—From May '11 Control Design

Answers

Make It Simple

The concept of moving your I/O terminal points from the PLC into the field on a digital network can provide many benefits, including lower material cost, enhanced troubleshooting, shorter commissioning time and overall cleaner machine aesthetics. Integrating a bus network between your PLC and discrete I/O can be as simple or as complex as you want.

Networks such as Profibus, DeviceNet and their Ethernet-based counterparts Profinet and EtherNet/IP are quite powerful, and provide many options for device connection in the field, including motor and motion controls. For integrating basic sensors and actuators, such as proximity, photoelectric, solenoids, etc., a simpler network such as AS-Interface could be a great first start.

"Your control panel will slim down without all the terminal points, replaced with one or two bus and power cables running to the field. Your field network will be composed of I/O blocks."

All of these networks and protocols have gateways (backplane cards) that connect to the popular PLCs on the market. These interfaces will look like large I/O cards in your PLC program configuration, and allow you to maintain your current programming software. Therefore, your only learning curve is the hardware and configuration of the bus network. Most of the manufacturers have short, quick-start guides to help ease setup.

Your control panel will slim down without all the terminal points, replaced with one or two bus and power cables running to the field. Your field network will be composed of I/O blocks. These blocks come in a variety of flavors: terminals for cabinets, quick disconnects for easy plug-n-play, IP67-IP69K rated for harsh environments, and so on. Since you can mix and match these on your network, your designs become more flexible.

Once up and running, your PLC program will have full access to your network status, allowing remote troubleshooting down to the I/O block. I/O-Link is a protocol that rides on the back of these bus networks and provides even greater diagnostic capability.

Although specific benefits and ROI depend on the machine function and layout, applications from material handling to parts assembly to bottling have all found great returns in implementing a digital bus network.

Andrew Waugh, Product Engineer,
AutomationDirect, www.automationdirect.com

Distributed I/O Benefits

IO-Link will be most prevalent in sensor devices with analog outputs. At this time, IO-Link is generally not available in discrete position.

Network speed is affected by the length of cable runs and sometimes the number of devices on the network. Using a distributed I/O solution limits the number of nodes and allows the user to maximize the number of inputs per node. A distributed I/O solution gives users more flexibility in selecting a network that better meets their overall needs, and provides the type of diagnostic data they require.

Tony Udelhoven, Director, Sensors Division,
Turck, www.turck.us

Keep It Real

As a manufacturer of I/O devices, I can say that this application is better addressed using a real networking solution instead of an intelligent point-to-point approach.

Going from a hardwired installation to such a network certainly involves learning new things. How much of a learning curve depends on the selected solution. At the sensor level, there is only one real solution (called AS-Interface) that is easy and fast to implement, deterministic, offers millisecond-range update times, and is priced attractively.

AS-Interface is supported by all major PLC manufacturers and—when preferred—directly ties into the most frequently used higher-level networks, including EtherNet/IP. I/O blocks for enclosure and field (up to IP68/69K) are available from a number of providers.

"Going from a hardwired installation to such a network certainly involves learning new things. How much of a learning curve depends on the selected solution."

Although safety was not directly mentioned, it is important to know that AS-Interface is also certified for safety application (Category 4, SIL 3, PLe) without the cost involved in using a safety-rated PLC. The attainable wiring simplification can be as high as a 90% reduction in cable, termination points and labels. Speaking for our company, we have always offered live and/or web-based training sessions to help first-time users get going quickly.

Helge Hornis, manager, Intelligent Systems Group,
Pepperl+Fuchs, www.pepperl-fuchs.us

Reduce Your Wiring

End users have numerous options and flexibilities to reconfigure their machines for cost-saving measures. We offer remote I/O capabilities in both Profibus and Profinet networks. We can migrate the local I/Os to a remote I/O network with benefits to customers from reduced wiring cost and the same reporting and diagnostic capabilities as local I/O.

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