Siemens TI505 PID

I am using Fasttrak Worksop software to programme Siemens TI505. I have setup a PID Loop and am using the Ramp/Soak function. I am also using the PackRS function to allow me to manipulate the Loop Setpoint. I have setup the "Clamp Setpoint High" parameter in the PID Block to a value of 115. My problem is that If I "tell" the loop to ramp to a new setpoint that is greater than my High Limit it will happily do so. I thought (hoped) that the "Clamp Setpoint High" would prevent the setpoint from ever going above (in my case 115). I have a temp. fix in a SFP where I compare my setpoint against LSPH and if it greater I make it equal too. So it's no big deal I was just wondering what exactly does "Clamp Setpoint High" do ?

Hi Consys

This is a tricky one - I've not used a 505 PID for some time, never mind with R/S and clamping etc, so I'm scratching my head to recall this. (I suspect my near namesake Ken Moore will be along any minute to correct any mistakes or expand my explanation so hang on in here. He's always worth reading on TI kit.)

I seem to recall that the Clamp Setpoint Limit only applied if you had specified an address for a Remote SP. I don't know how Workshop s/w displays this configuration, but in Tisoft the Clamp SP was right next to the point where you entered the Remote SP address. That way they were both well away from the point where you specify a R/S Table and confusion was less likely. Anyway, the Clamp would only apply if you had set, say, V200. to act as the remote setpoint. Here the loop would check the value present in V200. before copying it in to the internal LSPxx variable in order to execute the loop. That's what gave it the opportunity to reject out of range values. However a Ramp-Soak Table writes directly to the LSPxx variable on the assumption that as it is a programmed function it should be correctly controlled. Exactly how are you applying a new out-of-limit Setpoint if the loop is controlled by a Ramp-Soak, and also why? I'm assuming you're writing directly to the LSPxxx ?
If your SP adjustment is made after the end of the Ramp-Soak profile, and you now want to restrict changes to the SP, then checking the values in an SFPGM is the correct way.
The only mechanism that would permit use of the Clamp Setpoint limits is to program an address for the Remote SP. Start up the loop in automatic so it operates on the Ramp-Soak as normal, but then, at the end of the Ramp-Soak, switch the loop to cascade mode so it now reads the SP from the Remote SP variable. Now the SP Clamping should be done.
When you need to go back to Ramp-Soak, put the loop back in automatic and retrigger the Ramp-Soak profile.

Ultimately you either need to write the code you've already got, or to write code to change the loop mode at the right stage of the process. Either way the job gets done - the choice is yours.

Good luck and regards

Ken

I thought that might be the case (only the remote SP uses Clamping).I am using the PackRS function in a SFP to allow me to move a variable into the Ramp Setpoint. I have set up the code so that it will NOT move the variable into Ramp/Soak unless the ramp is NOT running. I check the operator setpoint to see if it has chnaged since last time (Operator has entetered a new setpoint) and if it has and if the ramp is OFF then I execute the PackRS function and retrigger the ramp so that if loop is in Auto as the loop will then ramp from old setpoint to new setpoint and reamin in Auto. I "stumbled" on the setpoint issue when I put the loop into Manual , the PV had failed (went to max. scale of 150) then when I put the loop back into Auto the "bumpless" transfer took over and sent the SP to 150. The Ramp then proceeded to ramp from 150 back to my setpoint of 110. It seems to be OK with what I have put in place.

It is used to scale the display graph of PV,CV,SP.

I'm in the middle of a turnaround this week, so I'll have to be brief. I haven't used the ramp/soak feature in a long time, so I'll trust Ken M's answer. Your logic appears to be doing the job, so I would not worry about it to much.

In some of my applications, when going from Manual to auto, I first set the loop output to some value before switching, might be worth considering.
I have one situation where in the beginning of a batch the heating loop is first set to manual with an output of 100%, so that I don't have to wait for the loop to respond (the tuning is very mild) then when the temp. reaches 90% of desired temp, I set the loop to 75%, then switch to Auto.

With the built in PID loops and the SFPGM interface, you can do almost anything you can think of. I love these PLC's for process control.

Gotta run, sorry for the brief response, but I'm pressed this week.

Ken