SOFT PLC Vs Conventional PLCs

[electricshock]

i want to know that which type of control solution is better for safety interlocking .industrial PLCs with SCADA and logic programming facilities ie soft plcs or slot PLCS OR CONVENTION PLCs

 

[ctolbert]

Everyone's definition of a soft plc varies.

The concept of a scada system interfaced with processor based plc's is common and very reliable. The redundancy is the plc itself. You can lose the scada pc and the plc's will still control the process.

If you are speaking about PC based control and dumb I/O, however, it is another story all together. The logic is controlled in the pc, not the plc. Same situation as above; the dumb I/O will not continue the process if you lose the pc running the system.

If you are speaking about a plc that is Windows CE based (like the winplc from

http://www.automationdirect.com

,) well, the jury is still out. The unit was meant to bridge the gap between both worlds. It is a great controller in a plc chasis, but sometimes it is cost restrictive. As far as redundancy, it works just like a plc.

I'm curious about other points of view on this topic.

Thanks,
Carl

 

[electricshock]

hello carl
thanks for your inputs.the origin of plcs was to replace the conventional relay based logic ckts.moreover right from the begginning they have been made to withstand harsh envirnomental conditions and disturbances like EMI.as u have rightly mentioned it is the plc which provides redundancy in sacda interfaced with plc. or for that matter if it is other controller, that has to withstand the conditions because pc will be most likely in the control room only.now in pc based logic with dumb i/os in the field i fell u will also agree with me ,reliability will suffer as we are putting our logic in the pc which is more prone to failures because of its operating system.i dont know whether todays industrial pcs with some lean operating systems can match the reliability of scada+plc based system

 

[rwill19050]

You should never, ever rely on software only for safety interlocking, especially if human operators could be endangered by the system being controlled. Always use an approved 'stand alone' system.

 

[PhilipW]

I totally agree with the above comments.

1. Soft PLC's have a place in processor intesive applications where their number crunching abilty is needed and their ability to transfer data to other PC apps is unsurpassed...but any Emergency Stop or safety shutdown interlocking MUST be in a separate system. I would NEVER use a SoftPLC in any safety related function.

2. Even the "Hard PLC's" although a significantly more reliable...they too should never have E-Stop logic passing through their program. Certainly it is likely that you will have inputs advising the logic and operators which device has tripped, but the actual interlock must still be hard-wired.

A look at for example Rockwell's tech notes on this subject will confirm that this has always been the rule. Some industries have "fudged" this issue and put E-Stop shutdowns into their "Hard PLC'S" but the suppliers never approved of this.

The solution is to look at the range of "Safety PLC's" and related products that are now available. Rockwell again (I'm just familiar with them) now have an excellent range of safety solutions that are TUV certified.

Safety systems have become a specialty in their own right, and the various pressures on industry to improve thier conformance to international codes will raise its profile even further.

 

[skills]

Most of the statements above rather skirt the issue of latency. The robustness of a SOFT PLC as most people think of one is primarily a function of the operating system and has little to do with the computer hardware. A system with windows is probably using the HAL that Microsoft provides. The Hardware Abstartion Layer effectively limits the Real Time Control and Response one has and Windows C.E. "Control Edition" was made romable to address that very issue. Several good companies have rewritten the HAL for safety; but, non-standard HAL may impair your compatibility with stardard software. A operating system like N.T. or 95 should not be relyed upon to safe 'life and limb'. I might consider using XP because of the maturing they brought to its design. The WINPLC is great; but, Microsoft still usurps a little to much control of the backplane.