SLC5/05 and Powerflex 700 VC

Hi guys,
I am having few issues which I would like assistance on:
I have a SLC5/05 that is communicating to a Powerflex 700 drive. I am new to both these equipment. I need help with controlling the drive. The drive has an encoder input.
What I am having issue with is that when I give it 12mA (from PLC AO) signal to run at 50% speed, and when the encoder input is not linked then the drive just takes off and goes to maximum. I would like for it stay at 12mA instead of ramping up to max. What parameter would this be associated to?
When the encoder input is linked then the drive controls itself at 50% speed or whatever the PLC is asking for it do.
I need to be able to change this on-demand so is there a parameter that I can change over Ethernet/IP from PLC that when the parameter is set it looks at the analog input into the drive and when the parameter value is not set then it looks at the drive's PID control output?

The drive is communicating over Ethernet/IP using messaging. I do not currently know how to bring back the encoder data to the PLC.
Any help would be greatly appreciated.
Thanks!

After reading the manual for a bit, here is what I found so far:
Parameter 124 is set for exclusive mode, and P125 bit 0 has PI Enable checked in the drive program.
So I think, this is where it is configured to take the setpoint from the analog input and then use the encoder to control the speed of the drive.
Am I correct in this assumption?
Page 137 is where I found this:
http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/pflex-rm001_-en-e.pdf

Thanks!

Do you know exactly which variant of PowerFlex 700 you have ? Some of the functions changed significantly between Series A and Series B, and on the VC (Vector Control) and 700S models.

What exactly do you mean when you say the encoder input is "linked" to the drive ?

Are you saying that the encoder is intentionally disconnected from the drive during some modes of operation ?

Switching between speed references is relatively easy to do, both with discrete selector inputs and with messaging.

But switching between closed-loop control with analog reference and onboard PI loop control might be more difficult.

I'm not sure if you're asking to switch between closed-loop and open-loop control, or if you're switching in and out of onboard PI lop control.

Ken Roach said:

Do you know exactly which variant of PowerFlex 700 you have ? Some of the functions changed significantly between Series A and Series B, and on the VC (Vector Control) and 700S models.

What exactly do you mean when you say the encoder input is "linked" to the drive ?

Are you saying that the encoder is intentionally disconnected from the drive during some modes of operation ?

Switching between speed references is relatively easy to do, both with discrete selector inputs and with messaging.

But switching between closed-loop control with analog reference and onboard PI loop control might be more difficult.

I'm not sure if you're asking to switch between closed-loop and open-loop control, or if you're switching in and out of onboard PI lop control.

Click to expand...

We have the VC version.

With regards to your question on encoder, I was testing today with the encoder disconnected and seeing what the VFD was doing and it was ramping up to max speed as it did not have a speed feedback.

The process essentially is that - there is a lower VFD drive motor which is raised slowly to 'touch' a upper wheel (not on drive). The encoder is hooked onto the upper wheel and goes to the drive that controls the lower wheel. When they're touching, we start the drive and both the upper and lower wheel begin to turn. The encoder then picks up speed from the upper wheel and controls the speed from the setpoint provided. The 'touching' process is currently manual and we want to automate that.

We now would like to change the process a little in where, the lower drive wheel will start rotating really slow and just when it touches the upper wheel and encoder picks up speed is when we would like to stop the lower wheel. Operation can then start the lower wheel when they need to.

This is why when I say encoder is not linked that means the lower wheel is not touching the upper wheel for it to pick up speed. When I start the lower wheel while the wheel not being touched, it just goes to max speed (because no speed feedback in the drive). I would like to control the speed at min speed when the wheel is not touching until the wheel touches and speed is picked up by the encoder. When this happens, I would like to stop the wheel, switch to PI control and then let operations start the process. The speed feedback from the encoder now to control the speed at the desired setpoint.

What do you mean by switching between open loop control or closed-loop control? Would my going from analog out from PLC to analog input of VFD to control the speed of the wheel be considered open loop? This will be hard setpoint and constant minimum speed always. And the encoder tying into the VFD and controlling the speed a closed-loop? If yes, then this is what I would like to. Control the speed using analog output and then switching back to PI control to be controlled by the encoder.

When you say swithcing between speed references then is that swithcing between PI control and analog input control?

Encoder feedback is for closing the speed loop regulator...in your application you don't want to use this encoder for that....the encoder on the wheel is a speed reference not speed feedback. So I don't have the 700 manual sitting in front of me but you should be able to use a pulse train from the encoder as a direct speed reference. Then it's just a matter of switching references between the analog and pulse train. The other way would be to use a counter card in the SLC and wire the encoder to this to produce a rate. Then just use the analog from the plc to control the drive based on an internal PLC reference or the pulse train....that's what I would do which gives the most control over when to switch the reference and make it bumpless.

Thanks Robert.
Would you off hand know what parameter it is to switch the reference speed on the drive? Or do I have to switch off the 'PI Enable' in P125 bit0 and it will automatically take the ref speed from analog input.
P90 which is Speed Ref A Sel is currently on selected Analog input.

The PI block is for adding/subtracting speed through an integrator....a trim speed based on an external signal. For example, speeding up or slowing down a motor to maintain web tension based on a load cell. I don't think the PI is going to do you any good in this scenario.

I checked the 700 VC manual and you can't have a pulse train as a reference, so your best bet is to bring that capstan encoder into the PLC and determine rate there. Then you would spin the drive up using an analog reference from the PLC, move the capstan into place, and once you see the rotating speed of the capstan in the PLC you change the analog in the PLC to vary the speed of the drive to keep the rate of the capstan. Probably through a PID block in the PLC, where SP is speed you want, the PV is the Capstan speed, and CV is the analog signal to the drive. There might be a way to massage the PI block in the drive to do that but I would be afraid of a big slip in the capstan causing huge errors and runaway conditions in the drive. With the PLC in control you have lots of massaging you could do and monitor performance and not make huge changes in speed.