What is the Cycle time in ms / total number of process tasks for the s-series controllers (SD Plus, SX, and SQ).
Could not manage to find as manual or datasheet mentioned the Cycle time in ms / total number of process tasks
The SIMATIC PCS 7 CPU 410-5H (New versatile Controller) Cycle time is up to 10 ms/9 process tasks. Is the s-series controller (SD Plus, SX, and SQ) having faster cycle time and no of process tasks?
Esther,
Different control system processorts use different metrics to express capacity/capability. The metric of Cycle time in ms/total nubmer of process tasks may make sense for PCS 7, but I don't see it applying to a DeltaV controller. I have to ask, what does 10 ms/ 9 process tasks mean to a process engineer concerned about control strategy performance?
DeltaV executes control strategies in control modules. Each module has a configurable execution time from 100 ms to 60 seconds. This is how frequent the module executes. The content of each module willd determine how long it takes to execute, which is displayed in the EXECTIME parameter of the running module. When you download the controller, the modules are automatically scheduled to balance the load over time. The scheduler then executes the modules on their configured frequency.
If a process task is a control strategy, then you could calculate how many 1 second modules can you execute in 1 second, divide by 100 and get to a 10 ms/ X control modules.
The actual process clock speed is not really meaningful either when comparing systems. The software being executed on a particular processor results in a given capability. The same processor on a different system could potentially provide more or less capability based on the software. What is important is the capability of the system. Whatever the processor, the capability of the controller in terms of control strategies, will determine how many controllers the system needs.
DeltaV uses DST count to give uses and idea of perforamnce/capacity. The SD Plus and SQ are 750 DST's, and the SX is 1500. This tells you the SQ and SDPlus are equivalent, and the SX is double. The complexity of each module will determine how many control modules can be executed before the CPU is exhausted, but given X number of modules in an SDPlus, SQ, you can expect 2 X in an SX. Basic control modules typically execute in less than 1 ms. If the modules have a lot of supplementary logic for data management and take 2 ms, then you would be able to get half as many into a given controller.
In the end, CPU loading is typically the limiting resource in a control system, and the rate of consumption is entirely dependent on the user configuration (complexity and execution rate). If you use an established configuration library, you know the load per module type and can accurately size controller loading.
Andre Dicaire
In reply to Andre Dicaire:
Thank for the advise.
Consider a process task is a control strategy, then if I require 10 modules with 100ms and execute in 100 ms, does that meant 10 ms / 1 control modules?
In reply to Esther:
I think your going about this back wards. You cannot dictate how much CPU a control strategy uses. You can make decisions to optimize/reduce excessive features, but if you add a PID block to a module, it will consume CPU. You need to evaluate how much CPU load your strategy takes, and then you can determine how many of these you need. A control module for an AI signal, vs a PID loop, vs a simple Start/STop motor, vs a motor with interlocks, vs an Equipment module or a start up sequence, all have different function blocks and will require varying amounts of CPU to execute. The average execution time of basic modules is typically under 1 ms per module, but I've seen a PID module consume 5 ms because of supplementary logic designed to drive behavior at the operator interface. a module with more than 10 ms execution time is not normal, though might be justified. In general, one should assume module exectime is less than 2 ms.
A PID module with AI and AO blocks is about 0.750 ms in an SDPlus controller.
Basically, trying to apply the performance measure of an PCS 7 processor to a DeltaV controller is comparing an Apple to a bunch of grapes. (not an orange) A DeltaV Module is a grape, and the bigger the grapes, the fewer there are in a bunch. You can slice up the apple to make it easier to eat. The point is the two are not directly comparable.