Programming Logical Control

Introduction

This article provides a brief introduction into documenting software programs for Programmable Logic Controllers (PLC). Its aim is to provide clear guidance in the program documentation for PLC's.

This article is based on the Ladder Logic format of PLC Programming, but many of the points also related to Instruction List (IL) and Function Block Diagram (FBD).

PLC Programs should be written and documented so that they can be easily maintained and modified by others than those who originally wrote the program. All PLC Programming Editors have increased documentation capabilities including Siemens, Modicon, Allen Bradley and Mitsubishi.

Documenting PLC Software
Early Programmable Logic Controllers had little or no capacity for the documentation of programs. For technicians to understand the logic the program had to be followed from inputs to outputs. This made fault finding or modifications to programs difficult.

As PLC's have developed the programming packages have also improved, allowing for the software to be clearly annotated.

The programming tools available for most PLC's allow for the following documentation.

Rung Title
Rung Comments
Symbols and Symbol Comments.
This article provides a practical guide to using these documentation techniques.

Symbols and Symbol Comments
The purpose of symbolic addressing is to allow the PLC programmer to write the software using meaningful symbols rather than having to remember addresses.

General rules for Input and Output symbols should be to use the tags from Process and Instrumentation Diagrams (P&ID) or electrical drawing references. Where these are not available use of a simple description should be used.

For the above the following could be used.

Symbol Address Description
VAC01_STPB I 0.0 Vacuum Pump Start Pushbutton
VAC01_SPPB I 0.1 Vacuum Pump Stop Pushbutton
VAC01 Q 0.0 Vacuum Pump Run
VAC01_TRAL M 1.1 Vacuum Pump Trip Alarm (Latched)
VAC01_AL M 1.1 Vacuum Pump Alarm Latch (Failed to Start/Stop)

Although many program editors can allow for more characters within the symbols it is recommended to keep the symbol short and use the comments to provide further details. Our recommendation is to build the symbols from the blocks of two characters to represent the the meaning and to keep the length less than 10.

The numbers tell a tale of programmable logic controller ( PLCs ) firm entrenchment in control applications. For instance, hardware sales figures for 2001 show $80 million spent for open control (PCbased control), with $1.1 billion being spent on programmable logic controller ( PLCs ) solutions. Even as the trend is for both figures to grow by 2004, programmable logic controllers ( PLCs ) still have a sizable advantage over PCs.

Engineers these days are calling for networking capabilities. A programmable logic controllers ( PLCs ) are now low-cost dedicated controllers. They offer more processing power and have added networking capabilities with other programmable logic controllers ( PLCs ) through serial connections. A micro programmable logic controller ( PLCs ) has an Ethernet connection with optional DeviceNet or Profibus modules. So the total system is not just a stand-alone controller, but one that can network to bigger programmable logic controllers ( PLCs ). Also, increased processing speeds allow for any given programmable logic controller ( PLCs ) to do more. Because of these changes, the programmable logic controller ( PLCs ) remains a viable control option.

One trend is towards a so-called hybrid programmable logic controller ( PLCs ). These more common hybrid applications are challenging the capabilities of each control approach. Their number is growing because fewer applications are pure discrete or pure process installations. One example is a programmable logic controller ( PLCs ) on a board with sequence logic-controller functions. It plugs into a PC ISA slot and has the same programming and instruction set as a stand-alone programmable logic controller ( PLCs ). So if you already have a PC on the factory floor, you don't need to buy a programmable logic controller ( PLCs ) in a box, just the board that plugs into the PC. It has a separate power supply so that if the PC crashes, the programmable logic controller ( PLCs ) keeps running. The board also has its own I/O connectors.

Another type of programmable logic controller ( PLCs ) is an industrially hardened gateway device. It comes with a number of ports and can be loaded with a soft programmable logic controller ( PLCs ). It has Ethernet, DeviceNet, and serial communications and serves as a bridge between protocols on the factory floor between legacy systems and current systems. For instance, data from serial devices such as bar code scanners can be connected to the gateway and through the soft programmable logic controller ( PLCs ) program.

After more than a decade of articles touting the benefits of PC-based control over a programmable logic controller and predictions of the PLC's demise, this much is clear: both PCs and programmable logic controllers ( PLCs ) have their place in contemporary control systems. In many cases, they will work as a team.

In lower-end process industries such as food plants where the programmable logic controller ( PLCs ) is strongly entrenched, there is little interest in switching to PC-based control. The reason is the programmable logic controller's ( PLCs ) suitability. Sequential logic control provided by a programmable logic controller ( PLCs ) is best suited for round-the-clock operations common in those industries. In particular, a programmable logic controller ( PLCs ) can offer repeatable, reliable control conducive to high-speed, repetitive processes. Dedicated performance and a high level of security are also benefits of a programmable logic controller (PLCs ). The packaging side of a food plant lends itself well to sequential logic control which makes a programmable logic controller ( PLCs ) the best choice for conveying and sorting applications.

Five years ago, the prediction was that PCs were going to take traditional programmable logic controller ( PLCs ) applications by storm. The reason they didn't is that the programmable logic controller ( PLCs ) has continued to be a good, low-cost control option. For instance, a programmable logic controller ( PLCs ) with highspeed counter inputs, loop controls, analog points, and basic math calculations are available in the $200 to $400 range with 100 or more I/O and with options for expansion. On the other hand, even buying an inexpensive PC requires loading software, setting it up, and making sure it's robust enough to not crash.

However, PC-based control has its merits. Specifically, it has greater number-crunching capability, more communication options, and is geared more toward data logging and trending, plant monitoring, report generation, and documentation. The key reasons to switch to PC-based control include networking the control system to higher-level applications such as ERP, handling complex mathematical applications such as vision inspection systems, and connecting to bar-code scanners, in-motion weigh scales, and other devices. In short, most users believe that when data must be transferred from the controller into a database for analysis, the transfer goes more smoothly when the data is already on a PC-based system.

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10 Lazy Ways To Squeeze A Little Exercise Into Busy Days

There are a lot of us baby-boomers. I think we all recognize that the ease with which we used to discard a few extra kilos, no longer quite works as quickly as it used to in our twenties and thirties.

Many of us also have lifestyles that place greater demands on our time and energy. Some of us are no longer willing to spend the hours at the gym or on the road jogging, like we used to. So here are some ideas for getting movement squeezed into a fast paced schedule. Many of these you may have seen before, however how many have you actually incorporated into your life? Its always good to be reminded.

have music I your kitchen or wear your iPod then dance as you cook or do the dishes. Same with any housework activity such as hanging up the washing go ahead, wiggle those hips! Measure a walking distance in your back garden. Take 4-minute breaks from computer work or any other sedentary activity e.g. my distance measured is 30 metres. Five times each day I drink a 200ml glass of water and directly follow that with 7x30metre walks. This way I get to walk 7 km per week and drink one litre of water daily.

Get a hands-free phone and headset. When having that hour-long conversation with a close friend or family member, walk around the back garden as you chat. Make it a habit for all lengthy calls.

Always take the stairs and always park the car as far from work, as is feasible. Get off the bus or tram a few stops earlier and walk the rest. Tighten your stomach muscles, or raise and lower yourself on your toes to exercise calf muscles, while waiting in a queue.

We used to walk to school by ourselves, several kilometers sometimes. Many parents now drive their children because they say its a safety issue. Could you not walk your children or ride bikes to school together? It has the combined benefit of family fitness and time to speak with each other at length.

Quit making use of the moving sidewalk (travelator) at airports if your work involves a lot of air travel. Use the stairs in large buildings instead of the lifts and elevators, even if only for a few of the floors of that building.

Learn a couple of simple, yet highly effective, yoga, pilates or stretching moves and before leaving your bedroom each morning, use a few of those moves as a 5-10 min. wake up routine. March on the spot while watching television (and you said you dont have time!), or get up during the advert breaks and do a few stretches or laps of the room. Make it a habit whenever an ad begins. Instead of watching your children play in the playground, sitting on a park bench, get up and push their swing or meet them at the bottom of the slide. Kick or throw balls with them. Walk around the track while youre watching your children play soccer or athletic training.

Trade baby-sitting time with your sister or best friend. Get them to mind the children while youre at the gym and vice versa. Is the same opportunity available at work? What could you trade with a colleague to give each other 30 mins. time-out to go for a good walk each day, or few times a week?

Im sure that while reading some of these ideas it may have triggered a few of your own. Any of these activities are better than just giving in and saying that obesity is your lot in life. All the above is a lazy persons guide to exercise. Anyone can create a healthy body habit by habit. You do not have to be Tarzan and Jane with the six-pack, at the local gym. Movement is movement. Stagnation is stagnation. Choose!

2007 Thea Westra is an international life coach who resides in Perth, Western Australia. She is editor and publisher of a free, monthly newsletter at http://www.forwardsteps.com.au Thea also recommends http://www.lightisreal.com and http://freemoneyebook.com

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