AB PowerFlex 70 Start/Stop/Reference Sequencing

View attachment 36685Hi all,

Have a quick question about starting and stopping a PowerFlex 70. In my application, I am trying to position the lead of a metal strip via a set of pinch rolls - an encoder is being used to track lead position, am sending a speed reference to the drive (Outer Loop is postion, inner loop is speed) All is working fine, but some times I have issues decelerating, the drive doesn't follow the reference near the end of the reference ramp.

The original programmers of the system basically set the "start" bit whenever the speed reference is <> 0 and pulse the "stop" bit when it is zero again.

What I really need to do is decelerate to zero speed and then set the "stop bit". Currently I am not at zero speed, but my reference is, therefore the "Stop bit" is being set and the drive coasts.

I had seen Ken Roach's solution to start/stop requests in the following post:

http://www.plctalk.net/qanda/showthread.php?t=46728&highlight=powerflex+70+start

Is my best bet to sequence this the following:

a. If "Move Request" Then Set "Start Bit" Until "Active Bit"
b. Release Speed Reference to Drive - Ramped S-Curve
c. Bring Speed Reference to Zero
d. Wait for "Drive Speed Feedback =0
e. Pulse "Stop Bit"


I have attatched a screen shot of the speed/position control, sorry, looks choppy, but will have to do, the reference and speed feedbacks are not really the rough. Yes, you do see a negative speed reference and a positive incremental strip position, that is correct for the roll configuration.

Sorry it is blurry, didn't look like that when I took the screen capture..

Red Boolean = Stop Bit
Green Boolean = Start Bit
Yellow Boolean = Drive "Ready" Status Bit
Blue Boolean = Drive "Active" Status Bit

Green analog Trace = Speed Ref
Red Analog Trace = Speed Feedback
Purple Analog Trace = Integrated Strip position

Does anyone know why the "ready" status bit drops out?? What constitutes the "ready" and "active" status bits. Looked in the manuals and didn't get a good clear explination.

Any advice would be much appreciated.

Thanks,

Andrew Evenson

If ladder logic programming, I'd suggest to issue the Stop command and let the drive ramp down to zero speed using P142/143 (Decel Time 1/Decel Time 2) configured within the onboard processor; the Logix CPU asynchronous ladder logic scan is probably the culprit when experiencing this kind of issues.
If you must wait for zero speed before issuing the Stop command you will probably need to use the Function Block programming method since it is less susceptible to the said scan asynchronicity interruptions.

If I understand correctly, you are issuing a decal to zero speed command and the motor is lagging behind the ramp or taking longer to get to zero speed.

My first reaction to this is to suspect that the drive does not have adequate braking capability and the regenerated power is causing the drive to alter (lengthen) the decal ramp to avoid tripping out on high DC bus voltage. There certainly are other possibilities but I'd check that first.

DickDV has a good possibility.

I'd also throw out:

1: Scan times of PLC / Drive / Trending can make it look worse than it really is.

2: The drive speed loop is not tuned right for what you are a trying to do. I have found with PF70s that the autotuning tends to be very heavy on Integral gain and light on Proportional for the speed loop. This gives good speed stability when at speed, but not the best at following a speed reference change.

Some questions:
1. What PLC are you using?
2. What is the accel/decel of the drive (Params 140-143)
3. What are the values of Ki and Kp (param 445 & 446)