Pid what is tieback

Find More Posts by Ron Beaufort. June 30th, , PM Kron. Thank you Ron for the help. I've been reading all of your quotes about PID and they are really helpful. Best Regards, Kron. Join Date: Aug I've been reading this thread; very helpful information so far, by the way, so thanks everybody for the discussion. I did have a question about a specific implementation that I'm considering: I'm looking to implement a bumpless transition for a PID loop in an enhanced PLC5.

The loop is used for temperature control on a valve. The system has experienced a "kick" in the CV upon the transition of the PID loop from manual back to auto, due to the error between the PV and SP at the time of that transition. I'm trying to eliminate that kick. The way the logic works is that in a startup condition it "ramps up" the output by gradually incrementing the PDXX:XX.

SO while the block is in manual mode, to open the valve to a desired position automatically. I'm hoping to use the "tieback" feature currently set to "0" on the PID block to mitigate this problem.

My hope is that upon the transition from manual back to auto, the tieback value will be retained as the control output, and an instantaneous large change in the CV can be avoided.

Does this seem like a sound approach? Ideally I'd like to have a test system to play with this on, but that may not be feasible. Thank you all in advance for your time and consideration of this issue. Your input would be most appreciated. September 6th, , PM Mispeld. There is no operational difference between the two: both methods work equally well in any situation. We recommend you use dependent gains for three reasons: it is more commonly used, it is the ISA Standard, and it is what was used in the Logix platform.

If you should choose to use independent gains, be aware that Integral Reset operates as Repeats per Second instead of Minutes per Repeat also known as time constant in minutes and that Derivative Rate operates in seconds rather than in minutes. The following information assumes, however, that you will configure your PID block for dependent gains. The next selection is Control Action. This means reverse-acting versus forward-acting control action.

In most cases the choice for Derivative Of will be PV. If your process can tolerate significant overshooting and you are looking for very fast response, choose Error. Loop Update Time will not be critical in most applications. Try a setting of 0. Keep in mind that the faster the update time is, the more overhead time burden will be imposed on the processor. Leave Deadband Value at zero unless you have a good reason to use one, such as avoidance of short-cycling a compressor or oil burner, although our recommendation even in those cases is to use a zero deadband, but instead limit short-cycling by manipulating the discrete output of a time-proportioning routine such as the one we suggest in our sample downloads.

That way you keep the actual PID tuning nice and clean. We recommend the five check-options on the right side be left unchecked unless you are certain you will need any of them. The Alarm tab is self-explanatory. This can be a great convenience if needed. Next, select the Scaling tab. There are actually two significant issues associated with the PV field: scaling of data if necessary , and the defining of the control range absolutely critical to the tuning of your PID loop. Posted 4 Apr edited.

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Sign in to follow this Followers 0. Hello folks, I am trying to access the AB online Knowledge Base, chasing up TechConnect numbers etc and assuming that our toolkit is eligible for this, but I am also interested to know if there are examples of code elsewhere using PID loops to regulate pump drives and flow and level control valves with RSLogix Pumps are using pressure setpoints in Auto and speed setpoints in Manual.

I have used PIDs before to control linear axes in SoftLogix but I can't seem to make head nor tail of the current format. What is a tieback?

Does anything equate to these? From browsing here I have found one tip, ie to put the PID block on its own rung. Share this post Link to post Share on other sites. Greetings MattS ACD and TemperatureControl.

Ron, Thank you for the comments. I believe we have a full development toolkit but I have not found the files you mentioned, though I will keep looking. I will leave the structured text out for now, though it should be part of my repertoire and it should be available somewhere.

Perhaps it is easier than I think. If I label the PID, fill in the Process Variable with the pressure setpoint and the Tieback with the manual speed setpoint zero at first and feed the Control Var to the drive, with correct scaling, then something should happen.

It may be that the gains etc are configured on the drive. I will start with a small one, not the kW ones, and perhaps assume that the default settings are OK. By then I'm sure someone will know what to do. What is a bit frustrating is the search for documentation on the topic, which must be there somewhere, just buried amongst a lot of other stuff. The AB sample code facility requires a title, for example, but I only have a topic.

It will sequentially scan through all the assigned programs, as scheduled by the programmer. Once the task has finished executing, the whole process will be repeated. The speed at which this occurs may gradually decrease over time, as the project expands or may even drastically increase when the controller is upgraded to a faster controller. This will have an adverse effect on PID loops.

When the speed at which a continuous task is scanned changes, the frequency of how often the calculations inside the PID instruction is executed also changes. This, in turn, affects the frequency of output changes that is sent to the final control element and thus the control loop may not react as to how it was initially configured. To eliminate the chances of this happening, a different type of task may be created to host the program where all the routines implementing PID instructions are assigned to be executed.

This type of task is called a periodic task. The icon for this task, as depicted above, looks like a watch. This type of task only executes after a pre-set time period has elapsed.

Executing the PID instructions inside a periodic task ensures that the frequency remains the same, overcoming the possible problems that may be experienced with the continuous task. With the fundamental understanding of why and when a PID instruction will be used, together with the where it should be implemented, the how part will now be discussed.

There are a few steps that must be completed before continuing with this tutorial:. The Engineering unit scaling range for both inputs and outputs are 0 — , as can be seen in the images below. As discussed previously, it is advised to execute the PID instructions from within a Periodic task. The first step is to create the needed periodic task. The main routine has been created that will be used to call the different subroutines that will be created throughout the project life for all the different types of PID control implementation.

The first control subroutine will now be created.