Plastic Extruders: Separate Controllers for Heat & Pressure?

[AutomationTechBrian]

I'm trying to work through a question I have about using a PLC, plus separate temperature and pressure controllers on a plastic extruder rebuild. I'm hoping to get some thoughts from here.

I helped put together a second-hand plastic extruder for a customer back in 2012/13. It was a used PTI machine. Back then, they had a SLC 5/05 for a PLC, and separate loop controllers for the temperature and pressure loops. That was a very successful project, and it has been producing great plastic sheets ever since. Now, they just asked me if I'd like to help them do it again.

I'm not going to use the SLC. I'm probably going to upgrade/update to a Compactlogix. My question is, would you still look at separate controllers for the 36 temperature, and 2 pressure loops? I know it made sense back then, but I know the processors are far superior these days. Also, there might be other considerations to having separate controllers that go beyond the processor's ability to handle the load. However, having all the values in the one PLC would sure simplify the HMI. Does anyone have experience or thoughts to share?

 

[Steve Bailey]

The largest number of PID loops I've ever personally encountered in a single PLC was 28 on a furnace controlled by a GE Rx3i PLC. My impression was that the PLC was not overloaded and could have handled more zones. The GE Rx3i (now Emerson) is similar in capability to the Compact Logix.

Having all of the loops in the PLC definitely made the HMI task easier by eliminating the need to communicate with 28 stand-alone controllers. At the same GE facility I had to set up Modbus TCP communications between a Quick Panel and 33 Watlow controllers which was pushing the limits of the HMI.

 

[Peter Nachtwey]

The real problem is the pressure loops since pressure can change rapidly.
I agree with Steve, the temperature loops should be OK but I wonder how much it will affect the pressure loops that should run at as fast a loop time as possible. A lot depends on how compliant the plastic is when extruding.

 

[AutomationTechBrian]

The real problem is the pressure loops since pressure can change rapidly.
I agree with Steve, the temperature loops should be OK but I wonder how much it will affect the pressure loops that should run at as fast a loop time as possible. A lot depends on how compliant the plastic is when extruding.

Well, having 2 separate pressure controllers would not be a big deal, compared to the temperature loops. I'd be happy with that. The pressure controller loops control the screw speed in automatic mode, through the 4-20mA analog out. Otherwise, I'd use Ethernet I/P for the VFD speed reference, with the PLC.

 

[drforsythe]

Our latest control scheme for our twinscrew extruders utilizes Gefran GFX4 controllers with Ethernet/IP comms. Each GFX4 can control 4 zones (electric heat with water cooling) and up to 4 controllers can be put on one network for a total of 16 zones. Multiple 16-channel systems can be added to a Logix controller. The GFX 4 has one fuse per zone and current monitoring. I add an enable contactor ahead of the power connection in order turn allow a disconnection point if the controller runs away. The ControlLogix PLC handles all of the machine and feeder controls but the GFX4s control the heat zones. I used to do all of the heat control using PID in the PLC, but can save a lot of I/O and real estate on the backplane by using the GFX4 system.

 

[AutomationTechBrian]

Our latest control scheme for our twinscrew extruders utilizes Gefran GFX4 controllers with Ethernet/IP comms. Each GFX4 can control 4 zones (electric heat with water cooling) and up to 4 controllers can be put on one network for a total of 16 zones. Multiple 16-channel systems can be added to a Logix controller. The GFX 4 has one fuse per zone and current monitoring. I add an enable contactor ahead of the power connection in order turn allow a disconnection point if the controller runs away. The ControlLogix PLC handles all of the machine and feeder controls but the GFX4s control the heat zones. I used to do all of the heat control using PID in the PLC, but can save a lot of I/O and real estate on the backplane by using the GFX4 system.

Wow, thanks. I'll have to look into this deeper when I get home. So where does the thermocouple get connected? ...On the GFX 4 terminal block?

Just for clarity... your setpoint, PV, output, values are tags in the Compactlogix? So setting up your HMI, you only have to communicate with the Logix cpu to display the values? I'm contemplating using an Ignition Edge HMI for this project (partly to get experience), and they only allow connections to 2 controllers without paying for more (I believe $200 per additional connection).

 

[drforsythe]

Wow, thanks. I'll have to look into this deeper when I get home. So where does the thermocouple get connected? ...On the GFX 4 terminal block?

Just for clarity... your setpoint, PV, output, values are tags in the Compactlogix? So setting up your HMI, you only have to communicate with the Logix cpu to display the values? I'm contemplating using an Ignition Edge HMI for this project (partly to get experience), and they only allow connections to 2 controllers without paying for more (I believe $200 per additional connection).

The thermocouples get connected to the GFX4. The setpoint, PV, output, alarm settings, current feedback and other tags are all tags in the Logix. You only have to communicate to the CPU to read/write them.

 

[AutomationTechBrian]

The thermocouples get connected to the GFX4. The setpoint, PV, output, alarm settings, current feedback and other tags are all tags in the Logix. You only have to communicate to the CPU to read/write them.

Cool! At first I was put off by the price per zone, compared to using the components they already have, and adding Eurotherm Mini-8 controllers. But it's looking better, once you factor in that the tags are in the Logix processor.

 

[AutomationTechBrian]

The thermocouples get connected to the GFX4. The setpoint, PV, output, alarm settings, current feedback and other tags are all tags in the Logix. You only have to communicate to the CPU to read/write them.

While you're here... how do you handle pressure control? Do you use a separate controller, or the PLC?

I attached part of my HMI "Main Control Screen" in case you had questions about the "pressures" I was referring to. ...Maybe your system is different.

 

[drforsythe]

For the stuff we make, pressure is not critical, so we only monitor pressure. We do not control based on pressure. Gefran does make a GFX unit that does not include the fuses and solid state relays for other PID control. Gefran makes pressure transducers just like Dynisco, so I am sure they have used their controllers for closed loop pressure control.