27 June 2008
Improved environmental ruggedness is opening up more applications for on-machine control options.
‘On-machine controls are a truly unique segment of the controls market,’ says Graham Harris, Beckhoff Automation. ‘The idea of moving control devices directly onto machines to reduce or eliminate cabinets and their associated wiring has been around for years. However, technical limitations in designing all components for IP65/67 enclosures have minimised the number of implementations. Devices such as power supplies, modems and safety relays are not commonly available in such device housings.’
Today, Beckhoff and other automation vendors are accelerating the trend towards on-machine controls by introducing ruggedised industrial electronics packages. System integrators are taking advantage of this fact to shorten cable runs and simplify wiring interconnects.
LET’S START WITH THE I/O…
The first and most visible on-machine applications have been distributed I/O. Virtually all major terminal block manufacturers in Europe now offer IP67 versions of their IP20 products. Wago’s Speedway 767, for example, allows clusters of up to 64 I/O blocks to be interconnected directly on the machine. Wiring to them couldn’t be simpler—just connect your sensor cables to the M8 and M12 sockets.
You could be forgiven for thinking the clumsy looking I/O blocks don’t look like a high-speed machine control element, but here’s where Wago has a compelling sales pitch: the IP67 system is just as fast as our IP20 terminals (it uses an internal proprietary signal bus), you can connect to any major fieldbus, and we will eventually have everything included in our system: programmable controllers, safety modules, module and channel diagnostics, power supplies, and CoDeSys V. 3.0 programming. The terminal block manufacturers are telling the machine building world that practically everything they have made over the past 20 years for the control cabinet, they will provide for on-machine mounting. And it will be just as microsecond-splitting fast.
Larger machines achieve wiring savings by placing the I/O close to the sensors and actuators, and running a network cables back to the central controller. The first installations of on-machine controls were small controllers with a fixed amount of I/O, with a low power demand. However, the types of machines that normally need a small control system are also the ones that have less of a need to reduce wiring. At least, with the advent of Ethernet, the controllers can be linked over some distance, so they don’t have to be located in one place.
AND GO ON TO THE PLCS…
PLC manufacturers also want to join the on-machine control party, but they want to do it on their own terms. They’ve got a good story to tell. Instead of building systems around ruggedised I/O blocks, they prefer to transfer their well established PLC portfolios out of the cabinets and onto the machines.
For example, Siemens has built its ET 200pro CPU as an IP67 machine mountable controller, allowing it to be mounted in tough washdown areas and operating in an extended temperature range of -25º C to 55º C. But that’s only the beginning. The controller is designed to work with the equally ruggedised Simatic ET200pro I/O system, which offers an array of IP67 modules for I/O, motor starters, frequency converters, pneumatics, direct connection to the Moby RFID identification system or the integration of fail-safe modules.
And even though the PLC might be well protected in a hostile environment, it does occasionally need servicing. A technician can take his laptop and connect with a local Ethernet port, or, if it’s a part of a local area network—which is usually the case—he might not have to leave the comfortable confines of the control room to talk to the system.
MOTORS AND DRIVES
Most of the major motor and drive manufacturers have learned how to merge their drives into the motor package—both for servo motors and ac induction motors. This is the ultimate on-machine consolidation, but after about 10 years of experimentation with this packaging, the practice is largely confined to the smaller power ranges.
However, confidence in the concept appears to be increasing, especially among machine builders who must deal with multi-axis motion control. Where there are groups of drives in one machine or system component, the price/performance ratio is significant. The compactness of the solution (they are only slightly larger than the motor by itself) not only saves space in the cabinets but make them easier to fit inside machines.
It doesn’t make a lot of sense to talk about distributed I/O and other peripherals mounted directly to the machine without giving some consideration to power distribution. Wieland and Weidmüller are at the forefront of this movement. Weidmüller’s FieldPower is a family of products with a connection concept for decentralised power distribution for systems spread across wide areas. The ‘flexible busbar’ includes pluggable connectors, cables, pre-assembled and terminated cable harnesses, tools and an extensive range of accessories, as well as junction boxes and maintenance switches.
Wieland’s podis system uses a seven-conductor flat cable to distribute both power and AS-Interface (AS-i) bus control signals to large systems, such as conveyors. Fault-free connections are activated at any point along the flat cable by means of penetration bonding.
The system uses podisMOT as a control input system for main and auxiliary power, connecting three-phase current consumers with the podis system. Power can be controlled locally via digital inputs and outputs through the AS-i bus coupler. The main and auxiliary voltages, as well as the binary signals, are conducted together from podisMOT to the consumer with one cable.
Weidmüller had a special stand at Hannover Fair called ‘future. made by Weidmüller.’ which displayed a number of advanced concepts for control system wiring and the components which will be needed to achieve them. The next big step will come when, in the 5 – 10 year time frame, the power and control signals are combined on the same conductor. Bosch Rexroth and SEW-Eurodrive have already plunged into this area, but they use multi-conductor elements inside the same cable housing to keep control and power separate. The technology for merging the two is not yet here, but it is sure to be developed.
—Michael Babb and C.G. Masi
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