AB Compact Logix and an Absolute Encoder - Advice

antsrealm

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Dec 2010
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Hi,

I am looking at doing a control upgrade for an old packaging machine. It uses an absolute encoder measuring 0 - 360 degrees for each cycle of one of the packs. It has channels in the program that determine certain digital signals such as PE's etc must be on or off at certain degree positions (windows) of the cycle of the pack. It runs reasonably slow 25 cycles per minute on average.

The more critical part is it wraps the packs with shrink film. Some feed rollers with a clutch and a brake as well as cut knife need to operate each cycle at precise positions to maintain the correct tension on the film to be able to cut and feed for each pack.

It's a 30 year old machine so it's nothing ground breaking. I have only dealt with incremental encoders on a HSC card on an SLC so integrating an absolute encoder in a compact logix system is new to me.

Can someone make a recommendation to me or offer some advice as to whether the AB Ethernet absolute encoders would be a good option and can I do all the encoder related functions in the normal ladder / scan time?

I have not purchased any hardware yet so I am open to suggestions as to which processor etc to go for.

Thanks for the advice.
 
I would recommend choosing the encoders and controllers that support CIP Motion, so that you get the power of the motion instruction set.

There are two flavors of EtherNet/IP encoders. The ordinary 842E that run a cyclic I/O connection, and the 842E-CM that act like a feedback axis in the CIP Motion systems.

You can run instructions like gearing, and camming, and position-based interrupts using that feedback axis. I would want that if I was controlling a packaging machine.

Similarly, the CompactLogix controllers have variants that support CIP Motion as well as ordinary EtherNet/IP I/O connections. These typically have an "ERM" in the part number, for "Ethernet Ring and Motion".

They can be as small as the 1769-L18ERM that supports just two CIP Motion axes, or you can choose one of the larger ones that has more memory and I/O capacity and more motion axes.
 
Ken Roach said:
I would recommend choosing the encoders and controllers that support CIP Motion, so that you get the power of the motion instruction set.

There are two flavors of EtherNet/IP encoders. The ordinary 842E that run a cyclic I/O connection, and the 842E-CM that act like a feedback axis in the CIP Motion systems.

You can run instructions like gearing, and camming, and position-based interrupts using that feedback axis. I would want that if I was controlling a packaging machine.

Similarly, the CompactLogix controllers have variants that support CIP Motion as well as ordinary EtherNet/IP I/O connections. These typically have an "ERM" in the part number, for "Ethernet Ring and Motion".

They can be as small as the 1769-L18ERM that supports just two CIP Motion axes, or you can choose one of the larger ones that has more memory and I/O capacity and more motion axes.
Click to expand...

Thanks everyone for the replies. Ken, this looks interesting. I'm not sure of the additional costs of going this way but I can appreciate the benefits. I have done a little bit with some kinettix drives and motion groups but more on a fault finding level. I just read through the encoder manual regarding setting up the motion group etc. Once that is done and I have the actualposition from the encoder how would I use that information to actuate outputs on the PLC. Can I simply use it in the ladder with comparators or is there a special motion instruction I should be using? For eg there is a brake and clutch I need to actuate for each revolution of the encoder at specific angles / position windows. How would you recommend I go about this assuming I get the motion supported hardware.

Thanks.
 
The best way to get discrete outputs to run based on motion positions is with a Motion Axis Output Cam instruction. This lets you set up specific outputs on specific modules to fire at specific positions for specific durations.

Here's a nice readable programming booklet on the MAOC feature:

https://literature.rockwellautomation.com/idc/groups/literature/documents/at/1756-at017_-en-p.pdf

I use this in a fluid dispensing application where I have to fire micro-dispensing valves only when they are over a little 3mm diameter hole, while moving along at about 500mm/sec. It's VERY accurate.

For the best performance, it requires both motion axes and the MAOC instruction, and a Scheduled type output module with a method to connect to the controller. I use the 1756-OB16IEFS Fast Scheduled output modules in the local chassis with a 1756-L71.

The smallest CompactLogix controllers use a fast DeviceNet (on the 1769-L1 with POINT I/O) or the proprietary 1769 data bus (based on SPI), neither of which support Scheduled I/O.

So you could use one of the fancier new CompactLogix with the 5069-series I/O modules (like the 5069-L306ERM and 5069-OB16F), or you could use a standalone Scheduled output block (1732E-OB8M8SR) that goes on EtherNet/IP and participates in the CIP motion system.

Note that I said "for best performance". The MAOC can still drive an ordinary output module at the Coarse Update Rate of the motion group, which can be as low as 2 or 4 or 8 ms.

If it were my machine, I'd be itching to use one of the newest CompactLogix because they're so shiny. With an -ERM version you would have access to the motion axis position of the Ethernet-connected encoder with CIP Motion, as well as access to 5069-OB16F modules for local fast outputs.
 
Ken Roach said:
The best way to get discrete outputs to run based on motion positions is with a Motion Axis Output Cam instruction. This lets you set up specific outputs on specific modules to fire at specific positions for specific durations.

Here's a nice readable programming booklet on the MAOC feature:

https://literature.rockwellautomation.com/idc/groups/literature/documents/at/1756-at017_-en-p.pdf

I use this in a fluid dispensing application where I have to fire micro-dispensing valves only when they are over a little 3mm diameter hole, while moving along at about 500mm/sec. It's VERY accurate.

For the best performance, it requires both motion axes and the MAOC instruction, and a Scheduled type output module with a method to connect to the controller. I use the 1756-OB16IEFS Fast Scheduled output modules in the local chassis with a 1756-L71.

The smallest CompactLogix controllers use a fast DeviceNet (on the 1769-L1 with POINT I/O) or the proprietary 1769 data bus (based on SPI), neither of which support Scheduled I/O.

So you could use one of the fancier new CompactLogix with the 5069-series I/O modules (like the 5069-L306ERM and 5069-OB16F), or you could use a standalone Scheduled output block (1732E-OB8M8SR) that goes on EtherNet/IP and participates in the CIP motion system.

Note that I said "for best performance". The MAOC can still drive an ordinary output module at the Coarse Update Rate of the motion group, which can be as low as 2 or 4 or 8 ms.

If it were my machine, I'd be itching to use one of the newest CompactLogix because they're so shiny. With an -ERM version you would have access to the motion axis position of the Ethernet-connected encoder with CIP Motion, as well as access to 5069-OB16F modules for local fast outputs.
Click to expand...

Thanks Ken, great information. I'm getting the 5069-L306ERM quoted as you suggested. I'll allow enough money for the fast output card option but I have a feeling I don't need it. The fastest they run is 25 packs per minute so that's 2.4 seconds per pack at full speed. The encoder is doing 360 degrees for each pack. So that's 6.6ms per degree of rotation if I have calculated it correctly. Usually the window it needs to trigger in is several degrees wide. That should give the MAOC with a standard digital out using the CUR of 2ms plenty of time I presume. If I found for some reason that is not enough then I can just order the fast output card and get the faster operation. I feel like that's a good place to be.
 
Ken Roach said:
The best way to get discrete outputs to run based on motion positions is with a Motion Axis Output Cam instruction. This lets you set up specific outputs on specific modules to fire at specific positions for specific durations.

Here's a nice readable programming booklet on the MAOC feature:

https://literature.rockwellautomation.com/idc/groups/literature/documents/at/1756-at017_-en-p.pdf

I use this in a fluid dispensing application where I have to fire micro-dispensing valves only when they are over a little 3mm diameter hole, while moving along at about 500mm/sec. It's VERY accurate.

For the best performance, it requires both motion axes and the MAOC instruction, and a Scheduled type output module with a method to connect to the controller. I use the 1756-OB16IEFS Fast Scheduled output modules in the local chassis with a 1756-L71.

The smallest CompactLogix controllers use a fast DeviceNet (on the 1769-L1 with POINT I/O) or the proprietary 1769 data bus (based on SPI), neither of which support Scheduled I/O.

So you could use one of the fancier new CompactLogix with the 5069-series I/O modules (like the 5069-L306ERM and 5069-OB16F), or you could use a standalone Scheduled output block (1732E-OB8M8SR) that goes on EtherNet/IP and participates in the CIP motion system.

Note that I said "for best performance". The MAOC can still drive an ordinary output module at the Coarse Update Rate of the motion group, which can be as low as 2 or 4 or 8 ms.

If it were my machine, I'd be itching to use one of the newest CompactLogix because they're so shiny. With an -ERM version you would have access to the motion axis position of the Ethernet-connected encoder with CIP Motion, as well as access to 5069-OB16F modules for local fast outputs.
Click to expand...


Hi Ken,

I have gone with the following hardware:-

5069L320ERM
5069OB16F
842ECMSIP4BA

This encoder will operate with units of 0 to 360 degrees continuously rotating and then feeding and cutting film at certain points in the rotation.

Can you give me an example of the MAOC intruction being used. I've created the axis and I'm probably pretty close but an actual example would be nice to have a look at.

Thanks,
 

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