Oscillating PSI

[ptonsparky]

View attachment 17776

My goal is 50 PSI but it is not so critical that it has to be adjusted 4 times a second. Is this a reasonable oscillation to expect?

FWIW:Non inverter duty rated motor within 30' of drive with a Load reactor installed.

 

[jumper]

Fixed title. PI to PSI.

 

[Smart $]

...My goal is 50 PSI but it is not so critical that it has to be adjusted 4 times a second. Is this a reasonable oscillation to expect?

FWIW:Non inverter duty rated motor within 30' of drive with a Load reactor installed.

I believe we need more information to determine whether the oscillation should be expected. Adjustment interval is in accordance with the reset ["I" of PID] setting.

 

[Sahib]

May be excessive gain in control loop. Can you reduce gain?

 

[gadfly56]

It looks like there is a ~2.5 Hz oscillation in the output pressure. It's been a long time since my controls course, but IIRC that suggests the response is under damped and you may want to increase the time constant for "D". That assumes that any perturbations are also relatively slow, and you'll probably have to change the reset (windup) parameters.

 

[Ingenieur]

doesn't look bad considering the control element is a vs pump
how does it respond to a disturbance?
put set point at say 35, let stabilize, then change to 55
plot the response
that will show over or under dampened or if it needs some integral gain, etc.

I might reduce P a bit, and increase I a bit

 

[ptonsparky]

I believe we need more information to determine whether the oscillation should be expected. Adjustment interval is in accordance with the reset ["I" of PID] setting.

This is on a Pivot that will have a pretty consistent flow of water through the sprinkler heads with the only flow difference being when more heads and/or the end gun turn On. Time would be in the hours range between off/on of either event.

Is that the info you want?

 

[ptonsparky]

I=10, P=.5 as of last week.

What is worse on the motor, tight correction as it is now or letting it range a pound or two in a minute or so? The sprinklers won't care.

 

[gadfly56]

I=10, P=.5 as of last week.

What is worse on the motor, tight correction as it is now or letting it range a pound or two in a minute or so? The sprinklers won't care.

How fast does the pivot turn, degrees per minute? The pressure oscillation will change the coverage, but as long as it's pretty much along the path of travel that shouldn't matter. I'd vote for longer period of pressure change as it might be a little easier on the motor.

 

[Jraef]

One thing you can do to even out the pump speed if the oscillations bug you but you don't want to mess with the PID loop, is to simply extend the ramp time on the drive. Has a similar effect because regardless of what the PID loop tells it, you are dampening the response time in the Control Variable (the drive). It's kind of a unique situation.

 

[GoldDigger]

Possibly either the sensor or the VFD control input are too coarsely digitized for the dead band programmed into the control loop.
Increase the frequency one unit and the pressure is too high, decrease it one unit and the pressure is too low?

Sent from my XT1585 using Tapatalk

 

[Smart $]

I=10...

10 what? msec sec min?

The integral setting is the amount of time per reset. If your I value is 10 msec, that's way too often. If it's 10 seconds, the oscillation is in the system, not the PID control loop.

 

[gadfly56]

10 what? msec sec min?

The integral setting is the amount of time per reset. If your I value is 10 msec, that's way too often. If it's 10 seconds, the oscillation is in the system, not the PID control loop.

That's a really good point. There's a reason I didn't go into controls after school...

 

[GoldDigger]

10 what? msec sec min?

The integral setting is the amount of time per reset. If your I value is 10 msec, that's way too often. If it's 10 seconds, the oscillation is in the system, not the PID control loop.

Regardless of what I is set to, the graph shows approximately 7 changes in control state every two seconds. Possibly it takes that long for the digitized pressure transducer output to change a full unit, triggering a change in control output. It seems never to find a stable value, again probably because the minimum pump speed change is too large to allow the set point to stay within the deadband of the control.

Notice that the graphed pressure seems to lag about 1/4 cycle behind the change in pump speed setting.

And, more importantly, that the displayed pressure seems to jump back and forth between "too high" and "too low", suggesting that the granularity of the pressure sensor output is too large. Possibly scaled incorrectly at an analog PLC input?

 

[Cow]

What kind of drive?

Is the PID loop in the VFD or a PLC?

I can't tell from the graph, how much is the pressure oscillating?

 

[Ingenieur]

Looks like 50 +/- 1 psi
the vfd output looks very stable

 

[ptonsparky]

What kind of drive?

Is the PID loop in the VFD or a PLC?

I can't tell from the graph, how much is the pressure oscillating?

Yaskawa VFD. About 1 lb but several times a second. Amperage varies about 1 amp as well. I think the HZ change is less than .1, IDR.

 

[ptonsparky]

10 what? msec sec min?

The integral setting is the amount of time per reset. If your I value is 10 msec, that's way too often. If it's 10 seconds, the oscillation is in the system, not the PID control loop.

I value is seconds.

 

[ptonsparky]

One thing you can do to even out the pump speed if the oscillations bug you but you don't want to mess with the PID loop, is to simply extend the ramp time on the drive. Has a similar effect because regardless of what the PID loop tells it, you are dampening the response time in the Control Variable (the drive). It's kind of a unique situation.

The ramp speed is 30 sec IIRC, coast to stop. There is another ramp option that works with the PID that is set at 0 sec. I'll take a closer look at that when we get back to it in a day or two.

 

[Dzboyce]

The default setting for the yaskawa IQ1000 is 2.00 for P(b5-03) and 3.0 seconds for I(b5-03). If I were you, I'd call the tech line at yaskawa and get their advice. They've always been very helpful for me.

i see your using the drive wizard. I save all my modified parameters for each installation on my laptop with it. I haven't used the trend recorder before. I'll have to try it out. I usually just use the signal monitoring mode while I have the laptop hooked up to the drive.