Motors and Drives
Structured text resembles C programming and is useful if complex algorithms need to be brought into play, so it is quite common to write one or two ST routines inside a function and otherwise mix and match routines. Ladder still has a loot of attraction from and maintenance
and troubleshooting point of view, whereas instruction list is similar to machine code for computer processors and very powerful because of its low-level instructions, although it needs a high skills set. Daniels adds: “Rockwell Automation does not support
IL because we do not believe that engineers need to go to so low a level of machine programmability, and that you can achieve everything in IL by using a combination of the other methods. SFC is used in pharmaceutical and batching applications where you have small individual chunks of code and you want to interlink them, so you can write small chunks of LD or FBD and then link them together in SFC. This works well with the state change model and makes it easy to write code in a very structured way to meet all the procedures that pharmaceutical operations need to comply to. “If you have too much prescriptive behaviour around a
language, it can stifle innovation so the IEC 61131 standard has specified a minimum set of functions as rules for engineers to play by, but PLC manufacturers offer additional functions over and above those specified.
Add-on instructions
“Rockwell Automation provides add-on instructions to package up multiple standard-compliant instructions into a chunk of code. Customers are doing this because of productivity and flexibility benefits, for example a in process which involves many pumps that need controlling. “You can basically build an instruction called ‘pump
control’, which might have a couple of PID loops, and which gives you a level of capability beyond the original scope of the IEC 61131 standard. It also provides enhanced security
because you can lock down the intellectual property created.” IEC 61131 covers data structures and types and Daniels
says Rockwell Automation provides for user-defined data structures, for example where there may be several process variables associated with a pump. Data types include integers, Boolean and floating point which may be combined into a user-defined data type called ‘pump’. The online forums provide
an interesting insight into the preferences of various engineers for particular IEC 61131 programming methods. Maurice Daniëls, a control
engineer in the Eindhoven Area of the Netherlands, favours ST, “Because it is easy to read. You can create simple programmes, but also more difficult ones with the same language, and some PLC development tools let you build classes, do file handling and string commands - some functions other language do not support.” Patrick van Eybergen, an Australian industrial automation specialist likes FBD, “to layout the logical components of my project. Inside the function blocks I either use ST or SFC, depending on the complexity of the logical component inside. When I use SFC, I use structured text inside the actions. Again depending on how complex the requirements are, I may use case statements inside my actions, to break the action into even smaller sequential code blocks. I’ve been programming with IEC61131 for more than 10 years, and am still finding new and better ways to do things.” ●
Grinding machine produces quality gears O
ndrives Ltd has recently taken receipt of a new gear grinding machine which
enables them to gear grind to precise measurements.
This new machinery will, says Managing Director, Andy Higgs, take the company to the next level in terms of precision for demanding applications such as aerospace. The Niles – Kapp ZE400 profile grinding machine produces gears to quality grade 3, DIN 3961/3962/3963 (ISO 3/AGMA 14+) gears from 0.5Mn to 15Mn (50Dp– 1.69Dp), with 0-400mm tip diameters available as standard. This machine is also equipped for process measurement/ inspection.
Ondrives manufactures spur gears,
parallel and crossed helical gears, anti- backlash spur gears, pinion shafts, racks, spline shafts, clamps and bushings, worms, wheels and bevel gears.
These are all currently produced to DIN 7 (ISO 7/AGMA Q10) within the standard catalogue range.
Ondrives Ltd says it will be exhibiting a range of these gears – along with its in-house produced gearboxes, which include worm gear reducers, right angle helical reducers and bevel and spur gearboxes – at several different exhibitions throughout the year. These include Offshore Europe in Aberdeen, UK, on 3-6th September; Motek 2013 on 7-10th October in Stuttgart, Geermany; and Aero Engineering at the NEC, Birmingham, UK, on 12-13th
November. ● Enter 47 or ✔ at
www.engineerlive.com/ede
Ondrives Ltd is based in Chesterfield, Derbyshire, UK.
www.ondrives.com
www.engineerlive.com 47
Fig. 2. Mark Daniels: “Function block is really taking off.”
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