Load shedding PLC having fastest scan time

I am in arequiremnt of a PLC having following I/O counts.
DI:348
DO:102
AI:125

I want to use this PLC for load shedding purpose, which will remove the load from the grid before tripping of the turbine on the grid. Priority will be decided by predefined program.

I need this PLC which should have less than 10 ms Response time.

Hmmmm.... you have quite a bit of IO for the response time you want. Depending on how complex your logic is, you may need to either spread the load over a few PLCS or select a single powerful processor. It may be more economical to use a few smaller processors and network them together using Ethernet or Profinet or similar. You may also find that the distribution of your IO. Either way, 10s for a large and possibly complex system may prove challenging.

I suggest the following things to help you get an estimate. There are not really any general rules of thumb for choosing a processor other than to overestimate at the start and trade down if need be. Be conservative; better to spend extra and be certain the system will work.

1. You need to be able to estimate as early as possible whether your PLC you select is up to the task. Most suppliers offer a range of PLCs with interchangeable processors. Siemens for example offer a range of PLCs where the next model or PLCs processor in the range is close to 10 times as fast as the previous, for example the 317 is nearly 10 times as fast as the 315. Selecting this type of processor allows you to upgrade or downgrade as needed.

2. Generate the I/O device layer first. Write a few generic blocks to interface your pumps, contactors and pressure sensors etc. If your project has relatively simple sequence or cycle requirements this will give you an good early estimate of how loaded your PLC will be if you can get a test PLC to try it out on.

Hello Krupesh

I guess that your requirement is less than 10ms response time for the total system.
If that is so, you have to factor in not just the CPU cycle time, but also the backplane update time, the network update time (if you use Profibus or other bus for distributing the i/o), and the i/o card update time.
As there may be several racks, networks, cards you must find the worst case, i.e. the slowest parts in the system. In particular the analog cards may be slower than the digital cards.

So if we take the slowest of each of the following,
CPU cycle time + backplane cycle time + network update time + analog card conversion time we get the total response time.

If a Siemens S7-319 CPU is used in connection with ET200S i/o on Profibus at 12M, and special fast analog inputs are used, you will get approximately
2-3 ms + 1-2 ms + 1-2 ms + 0.5-1 ms = 4.5-8 ms.

The GE Fanuc RX3i or RX7i will give you the desired response time. You can also 'prioritize' the I/O response with 'Scan Sets' or use "interrupts".
www.gefanuc.com

I agree with RussB on the RX3i. Out of the box with no code, the processor scan time is 4.2 mS. I have recently finished up a project with over 400 rungs of ladder logic, 32 digital inputs, 32 digital outputs, 8 Analog Inputs and 2 Analog Outputs, a high speed counter card, Ethernet and Genius Bus I/O and the scan time is still under 5 mS.

Out of the box with no code, the processor scan time is 4.2 mS. I have recently finished up a project with over 400 rungs of ladder logic, 32 digital inputs, 32 digital outputs, 8 Analog Inputs and 2 Analog Outputs, a high speed counter card, Ethernet and Genius Bus I/O and the scan time is still under 5 mS.

Click to expand...

MMMMM! Appears to have a heavy overhead with no program but obviously gets moving resolving code.

I have just finished a job with Omron CJ1 PLCs using the slowest processor in the range (CPU12) - a cheapy actually. 48 digital in, 24 digital out, 2 analogue in, 2 serial cards - 1 port interrogating a generator controller and a Multilin SR489 protection relay through 1 port, the second port interrogates a Caterpillar generator and there are 4 read requests to different areas of the Cat memory map. The second card - one port is a Modbus RTU slave for the BMS system and the other port runs to a colour touch screen on the door. The inbuilt serial port is used for my Bluetooth serial ports - I have 4 now - running at 115k to 4 PLCs.

Not a lot control logic but a lot of alarms and warnings - 60 complex rungs. Heaps of comms going on for the serial ports and over 90 floating point maths instructions - they take a bit of horsepower. A heap of re-mapping to the Modbus RTU serial port for the BMS system. Scan time is 2.1ms. The high end processor in the range would do the job in under a millisecond.

From what I have seen of late on jobs using the latest hardware from various manufacturers, high end Omron and Siemens APPEAR to be the quickest with Omron holding an edge. Have not tried Control Logix though and they reckon they are pretty quick too. Have used 7 brands in the last 6 months. Several Japanese, Siemens, clunky old SLC etc.

However, a the end of the day it depends on the code required in the processor. Heavy maths take their toll on scan time as do table compares, lots of indirect addressing, it all depends on the code required. Some PLCs will be very quick on digital stuff and very slow on maths. Some quick on everything and some slow on everything. You need to seriously assess your code requirements and then look at the processing speed for the instructions YOU need to use. At the end of the day horsepower and high speed instruction processing will cost you more money.

I have and older PLC running a power station - nearly 20k of instructions. The scan time is 26ms. I transposed the code to the latest Omron CS1 PLC, which will replace these older processors in the next year, and the scan time has dropped below 6ms! Heck of an increase in speed from an old high end processor to a new one.

What happens to the scan time when you start loading the PLC down, monitoring variables, downloading or uploading code? I haven't seen any PLC manufacturer guarantee a scan time under all conditions.

Why cant u just use a contrologix with an event task? This will give you a near immediate response

What happens to the scan time when you start loading the PLC down, monitoring variables, downloading or uploading code? I haven't seen any PLC manufacturer guarantee a scan time under all conditions.

Click to expand...

Absolutely correct but many allow you to set how much service time is allocated for access to the programming port. You can set the time and then calculate, if you wish, the scan time allowing for all functions and digital type processing, programming port access time, overhead time etc. It is an absolute nightmare trying to do it this way of course but it can be done. It did it once and never again.

1 Ms

For load shedding applications you should have Time stamped I/O cards for recording SOE.

For such applications you need I/O cards that can stamp Time at card level at minimum 1 ms resolution.

----
arc