Many of the inputs on our machine automation systems require extensive signal processing for filtering, averaging and other calculations. We know that PC-based control would be a good solution, but our customers require us to use a PLC. Unfortunately, the PLC brands specified don’t include much in the way of signal processing capability at the I/O. This drives us to use a quite expensive model of the specified brand of PLC, even though our I/O counts are low and our other automation requirements are quite simple. However, our customers would accept a PLC with at least some I/O from a vendor of our choice, a vendor specializing in I/O with signal processing and other capabilities. Can we easily mix and match I/O from different vendors with one PLC processor, providing that PLC processor contains industry standard digital network interfaces? Are they any gotchas?
—from December ’08 Control Design
Rev Up Existing Hardware
Today, vendors of modern I/O systems integrate more and more functionalities inside the I/O modules. This means that these modules are no longer just simple analog or digital modules but are capable value averaging, fast Fourier transformation (FFT) and even oscilloscope functionality for signal processing. Digital modules support functions that capture inputs in high resolution and send this data as pattern over a slower bus to a PLC. For the output direction these modules provide drum sequences for the outputs that are set independently by the module over a specified time period.
It is even possible to connect interfaces like RS-232 or RS-485 to a bus coupler that gives you the possibility to attach, for example, a bar-code reader at any place of your machine without running an extra wire to your cabinet. The advantage of intelligent I/O modules is the extreme fast reaction, even if the bus connection or the PLC is relatively slow. This gives you the possibility to combine modern high-speed analysis for analog and digital data with even older existing hardware. In many cases, an independent vendor of I/O systems can provide additional functionalities that the PLC vendor might not support. Many vendors have a modular I/O system that can be assembled exactly for your needs and can be combined with all standard digital bus systems like Profibus, CANopen, DeviceNet, EthernetIP andPowerLink. In any case, open bus systems should be the preferred solution because this will give you the highest flexibility to switch between these bus systems if necessary and make sure that your application is prepared for extensions in the future.
Stephan Stricker, product manager,
B&R Industrial Automation
Connecting I/O modules or subsystems from multiple vendors certainly has been made easier since standardized fieldbus networks such as Profibus, Interbus or DeviceNet found industry-wide acceptance. The same goes for the emerging trend to use Ethernet-based communication protocols like Profinet, Modbus TCP or EtherNet/IP to read and write data to these devices. While use of these protocols standardizes the basic communication, they do not always guarantee a 100% flawless integration of devices from multiple vendors.
Pay special attention on areas such as configuration of the individual modules. Every vendor uses a different set of parameters to configure a module and access the data. For more complicated or configurable devices, you need to make sure that any required drivers or function blocks are available for the specified PLC model.
Other considerations are the connection method and connector style. While a device from Vendor A might use a traditional copper connection, a similar module from Vendor B might use a fiberoptic connector or an entirely different connection method or cable set. If you do your homework before you purchase components from different vendors, you minimize any integration issues and should be in good shape with the startup of your system.
Bjoern Falke, product marketing, automation,
External Help for PLCs
Keeping a PLC’s analog I/O simple, and of course low cost, usually means trying to stick to one multi-channel input card flavor, such as the common 4-20 mA type. This is not too difficult even with a lot of different input sources to be fed in, such as millivolt, potentiometer, frequency, voltage or milliamp signals. Most process field devices give 4-20 mA outputs anyway, but for the rest of those above, linearization and conversion by discrete external conditioning modules into 4-20 mA is a well-used solution. However, for critical measurements for control or safety, the user will want to keep spares for each module type, which then offsets his input card savings a little.
Fortunately, there are conditioners in one configurable module that can handle inputs from any thermocouple, RTD or potentiometer, as well as a range of millivolts, resistance, frequency, current and voltage. These help users minimize their electronics inventory and, through easy-to-use configuration software, enable quick setup of a wealth of parameters, such as input type, range, response time, sensor break action, output action and linearization. Such a broad range is not possible with PLC channel setup. Hence, it could be such universal external discrete modules that act as the major analog functionality element of the I/O interface to the PLC.
The problem is solved then? Not quite.
First, even the non-converted 4-20 mA field device signals could benefit from an isolation module in the signal loop, perhaps due to common powering or where their cables are routed or what grounding/shielding is installed. Discrete isolation modules for these inputs could help here, too.