Compactlogix Motion Suggestions

Kev77

Member
Join Date
Mar 2008
Location
Beaverton, Or
Posts
410
Hi,

I have a Compactlogix processor (L33ERM) that is going to sync two lugged convyors and I am looking for some suggestions on the instructions to keep them synced up.

I have a lugged chain that has 15 links between lugs and one revolution of the master axis is 1 lug spacing. The master is not a motion axis that I can control. Basically the master runs open loop at whatever the operator has the speed pot set to and turns the master encoder which is connected to the head shaft of the conveyor. The master encoder is an Ethernet/IP encoder that is configured to give me a position of 0-360 degrees. I could have set it up to be the length between lugs which is about 30".

The slave axis is a 5hp Powerflex 755 with an absolute encoder on the motor and is configured as a CIP motion drive. This axis is a lugged conveyor with 10 links between lugs. With the encoder mounted on the motor, I am working with some ratios between what the encoder sees and what one revolution of the slave is. The slave sprocket is 13 teeth and I need to incrament 10 to match the 15 of the master.

The overall ratio is about 56:1 but is not a round number so I think if I set up the ratios in the motion profile, I am going to accumulate error due to the trailing numbers of the ratio. So I thought to set up the slave to run 0-360 degrees to match the master and then either gear or use an absolute MAM (motion axis move) to issue the slave a target position then an MRP (motion redefine position) each update period to advance the position. The issue I have is the head shaft of the slave is a 13 tooth sprocket so I would need 276.923076923077 degrees of the slave to 360 degrees of master which might cause an issue.


I have most of the program set up as far as fault handling, status and homing of the master axis and have tested the system using jog commands as well as a geared move. The slave runs and tracks the master but I need to make sure they are always synced up.

My questions are:

1. What is the best way to set up the master and slave to ensure that they stay synced up? (Degrees, inches, simulated lug pulses?)

2. Would it be best to use a MAG (motion axis gear) to drive the slave and have it track the master?

3. Would using a MAM (motion axis move) and MRP (motion redefine position) work for gearing these two lugged conveyors?

4. Would a MDAC (master driven axis control) help if I use a MAM to run the slave? It appears that this will give me the ability to start and stop the slave on the fly.

I'm sure that I am over complicating this so I'm hoping someone has some insight on out to do this the correct way.

Thanks,

Kevin
 
Some of this will depend on how you have your base axis positions defined but this doesn't matter a ton for this discussion I don't think.

Logix motion has a couple of tools that might help. At the axis level you define your gear ratio as the relationship between two integers. It is my understanding that the Logix motion engine will enforce this relationship longterm. For example, if you define your gear ratio as 5:3 the Logix motion engione doesn't use 1.666667 as the gear ratio. It makes sure that long term for every 5 revolutions of the motor there are exactly 3 resulting revolutions of the load. At 5000000 motor revolutions you will have exactly 3000000 load revolutions, not 2999999.4 revolutions like my Windows calculator gives me.

This concept also holds with the MAG instruction. You can define the master to slave gearing either as a REAL ratio or as an integer numerator and denominator. If you use the numerator and denominator entry the motion engine will enforce that relationship.

Keep in mind that the Logix motion engine allows superimposed motion. You can perform and MAM on top of an active MAG. So if you have a way to determine that the two conveyors are out of phase you can issue an MAM on the slave axis to get them back into phase.

Keith
 
Thanks Keith,

I wasn't aware that the ratios were handled that way. It makes sense that this is a good way to handle the trailing zeros problem.

So I really have two ratios that I need to handle. The first is the ratio getting from the slave motor encoder to the actual final rotation of the slave drive shaft. So I have a 29:1 Sumitomo Cyclo gearbox and a 2.5:1 between the gearbox and lug chain drive shaft. So if my math is correct that is a 72.5:1 overall ratio. If I understand you correctly, I could set up the ratio in the motion group to an integer value of 725:10, correct? I would also need to set up the scaling to give me 13 teeth per revolution or add some zeros to the integer for some resolution if needed elsewhere in the program. Now I have an accurate lug count of the slave.

As far as the master scaling is concerned, 1 revolution of the master is 15 lugs so I could set up the scaling of the master to a ratio of 1:1 and scaling of 15 lugs to 1 revolution of the master shaft. If I add zeros to the slave for resolution, I would need to add the same to the zeros to the master scaling so they would match.

Then I have a difference of 15 teeth of the master to 10 teeth of the slave to get my correct lug for lug gearing. This is a ratio .6666667 which could be handled in the MAG instruction be selecting the ratio format "Fraction Slave Master Counts" with the master value of 15 and slave value of 10.

Does this sound correct?

Thanks,

Kevin

MAG.JPG
 

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