Motors and Drives
Fig. 1. The Allen-
Bradley ControlLogix L7 programmable automation controller (PAC) from Rockwell Automation has predefined add-on instructions.
4 There is more to motors than just switching them on or off. The IEC 61131 standard provides for five general methods, some more textual and some more graphical, according to Boris Sedacca.
4 Il y a plus à faire sur un moteur que simplement l’allumer ou l’éteindre. La norme CEI 61131 fournit cinq méthodes générales, certaines plus textuelles et certaines plus graphiques, selon Boris Sedacca.
4 Motoren bestehen aus mehr als nur dem An/Aus-Knopf. Die Norm IEC 61131 enthält fünf allgemeine textliche und graphische Methoden, so Boris Sedacca.
Motor efficiency depends on logic control
I
EC 61131 specifies the syntax, semantics and display for PLC programming methods: ladder diagram (LD), sequential function charts (SFC), function block diagram (FBD), structured text (ST) and instruction list (IL).
IL and ST are text-based, while SFC, LD and FBD uses graphical programming techniques. Whereas textual languages originate in
computer programming, graphical representation of applications is more typical in PLC programming. Of the graphical development environments, ladder is probably the most familiar to electrical engineers, using elements like ‘normally open’ or ‘normally closed’ relay contacts. The engineer can instinctively see two switches in series as providing the logic AND function, while two in parallel provide the OR function. Large scale logic processes lend themselves more readily to FBD, where function blocks correspond to complex logic circuits. Before the days of programmed logic, motors
were controlled by relays or coils that were hardwired into an electrical control circuit. The hand drawn schematics were known as ladder diagrams. Mark Daniels, UK & Ireland field business leader for architecture and software at Rockwell Automation, has seen his company’s traditional strength in LD for PLC development evolve over the years to encompass the other IEC 61131 programming schemes, allowing the company to enter new markets like process industries in addition to its traditional base in discrete manufacturing.
46
www.engineerlive.com
“We started by adding an SFC capability and a little bit of structured text, but now we support all methods and the one which is really taking off is function block,” Daniels states.
Multi-disciplinary control
“Rockwell Automation now provides multi- disciplinary control on its hardware, from ladder for high speed motion control problems to more complex distributed control systems (DCS) where FBD is very much the configuration of choice, particularly for proportional integral derivative (PID) loop control on analogue systems for flow, pressure and temperature. “The function block editor is optimised for
things like instrumentation readings, and the native format of a drawing in FBD is pretty much based on a process control closed loop. The editing environment maps out the way in which inputs, processes and outputs are connected as blocks in the PID loop, in keeping with the way in which an engineer would conceive the control algorithms for a particular application.” Daniels claims that Rockwell Automation’s
Logix PLC provides a common hardware platform for all operations ranging from high speed canning lines at 2,000 per minute to slower moving processes incorporating multiple PID loops. He argues that most engineers would not want to work on all IEC 61131 development environments, and would want to maintain a skill set on one hardware environment to keep control of cost.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60