JON SEVERN

Two-thirds of the way to heaven

"Three steps to heaven" was, somewhat ironically, a posthumous hit for the rock singer Eddie Cochran in 1960, and a conversation I had recently started me thinking along the lines of three steps to an automation heaven. For a designer of machinery, the ideal would be to have a single fieldbus network that could be used to convey all information necessary for control. While conventional fieldbuses are fine for transmitting data relating to standard machine I/O, there are limitations - especially when it comes to closed-loop motion control of either position or velocity.

To operate a number of synchronised motor drives, as might be required in, for instance, a high-speed paper mill where a number of sets of rollers have to be controlled simultaneously, determinism is vital, whether the motion or position control function is centralised or distributed. In the example of a paper mill, all the motors need to accelerate or decelerate in unison, otherwise the paper is likely to be torn or be spewed out due to the tension straying outside the acceptable limits.

Most fieldbus protocols either do not have the determinism or speed to control high-speed synchronised drives and, although there are several bus systems that have been specifically developed for operating drives, these tend not to be capable of handling all of the additional general machine I/O data. In many cases, proprietary protocols have been developed for linking drives but, if an OEM chooses to specify one of these, it severely limits the choice of drive supplier and is not a favoured option for those who are keen to utilise open control systems.

Now, ladies and gentlemen, please welcome on stage, the headline act: Profibus! Within Profibus is a profile, Profidrive, that enables the closed-loop control of drives via the same fieldbus that already handles the machine I/O data. This means that the machine builder has less design to do, there is only one cable to route and connect, and the system is likely to be more reliable than a conventional set-up wherein encoders or other feedback devices are wired back to a drive (less wire equals fewer cabling errors during installation and fewer wires to break during normal operation and abuse). With OEMs and end users both working towards reducing the number of suppliers they use today, the concept of a single fieldbus for transmitting I/O and drive data is very attractive.

Although Profidrive has its limitations (typically a maximum of 14 drives on a network and cable lengths that need to be kept short so as to maintain a high data transmission rate, for example), the important point is that it is based on the Profibus standard. As such, the system is open and any manufacturer of drives can design and manufacture compatible products if they desire, which gives fans of open control systems a far broader choice of products.

So far, so good: we are being offered an open fieldbus protocol that is capable of conveying two types of data - which is good progress towards our automation heaven. But Eddie Cochran told us that there are three steps to heaven, not two. So what is the third step? In the June 2002 issue of this magazine I wrote about Profisafe, the profile within the Profibus standard that allows safety-related control data to be transmitted on the same fieldbus as the machine I/O. My question, therefore, is when will Profibus be ready to convey all three types of data (machine I/O, motion control and safety)?

Regrettably, the Profibus expert with whom I was recently conversing would not discuss this topic, but it would be great if such an all-encompassing fieldbus could be created. Meanwhile, as Meatloaf has been singing since 1977, "Don't be sad, 'cause two out of three ain't bad."