Pid scaling for compact loigx help

[snakes79]

Hello all. I am on site trying to scale my rotors controlled with a compact logix through analog inputs and outputs. My brain is racked.

The rotors are to be ran between 30 - 60 Hz.

I do a SCP with my input min / max set at 30 and 60 respectively.

I do the scaled min / max at 4000 and 20000 respectively.

It works, but not in all ranges. If I do 30 or 60 in my scp I get the correct speed output, but if I do 45, my counts go to 12000 and it is a little off.

It seems not to be linear. If I do my SCP input min / max to 0 and 60 respectively, the middle range is accurate, (like 45 Hz), but the minimum speed is 37 instead of 30.

I cannot linearly scale them with a scp block. Are there any tricks I am missing?

Thanks

 

[OkiePC]

The non-linearity you are seeing most likely exists in the drive (assuming you're signal ends up in a VFD). You might need to tell us more details about the end device you are sending this signal to.

Because of the values you describe, I am assuming the results of the SCP go to an analog output card. If your programmed minimum and maximum in the VFD are clamping the output to those limits, even if the analog signal is outside that range, you won't see that by looking at the VFD command speed. An offset or scaling error in the VFD parameters could account for this.

The VFD may have a "boost" setting or some other parameter that causes its output to be non-linear even if the analog signals are sent and received with perfect scaling at both devices.

If it is possible to view the analog value being "seen" by the VFD, that may help determine which device to study closer for the root cause.

 

[daba]

I do a SCP with my input min / max set at 30 and 60 respectively.

I do the scaled min / max at 4000 and 20000 respectively.

It works, but not in all ranges. If I do 30 or 60 in my scp I get the correct speed output, but if I do 45, my counts go to 12000 and it is a little off.

With 30-60 input range, and 4000-20000 output range of your SCP, with 45 input you will get 12000 output. You say it is "a little off" - how much is it off ?

What SCP are you using ? CompactLogic doesn't have a built-in SCP, are you using an AOI, or have you used a CPT instruction ?

 

[proof]

It looks like you are trying to scale an analog output card using scaling where 30 hz corresponds to a 4000 on the card for 4 ma. and 60 hz corresponds to 20,000 for 20 ma. 45 hz would be 12000 and which should be 12 ma. When you say 'it' is wrong, what is the 'it'? Is your analog output not at 12 ma? You say that the 30 and 60 is correct, an go on to say that 45 is a little off, then you contradict yourself, and say the 45 is OK, but he 30 is 37. So which is it? Are your 'rotors' scaled so that 4 ma gives 30 hz and 20 ma gives 60 hz? What are the analog inputs that you are using?

 

[daba]

If you are using the SCP AOI from the Samples, you will get the result on the left.

If you have "rolled-your-own" AOI using integer parameters and maths, you will get the result on the right. A similar error will occur if you've used a CPT with integer maths in the expression.