Kinetix 300 accel decel in position mode

I am trying to set up a Kinetix 300 to do position moves. I am controlling the drive with a Micrologix 1100 over Ethernet.

I am looking for an explanation of user units, velocity, accel limit, and decel limit. How do they work together to make up a position move that works without faulting but also works without over taxing the system.

I understand that user units is a representation of one rotation of the servo motor for a position command of 1.

I have found in my experiments that my accuracy increases if my user units increase. For example a user unit of 1 and a position command of 1 gives me a linear movement on my ball screw of 0.017" A position command of 2 should give me 0.034" but instead gives me 0.040" The error gets worse as I expand.

A user unit of ten gives much better results as I climb in position. I tried increasing my user units even more (100) and I faulted the drive. I realize that my accel and decel limits need to be changed along with velocity. I just do not understand how they are reflected so I do not know what to change it to.

I also noticed in my experiments that the higher my accel limit the faster it climbs to speed. The lower the limit the slower it climbs.

The manual does not go into much detail about this. I guess they assume that I should already know it? Can someone explain it or send to a manual that can?

Thanks

I'll take a stab at this.

For "User Units" for a linear ball screw I would convert this to inches.

So for example if your ball screw spec is 10 rotations = 1", I would set the "User Units" to 10.

Now your accel, decel, velocity and position is all in inches. Velocity = inches/sec, Accel/decel = inches/sec2, Position = inches.

If you are getting high position error, you can adjust the "Dynamics". I believe there is an auto tune, but I don't think it works very well. The error you have of 0.006" is like two sheets of paper, which in my opinion is very good.

d21x posted a good basic explanation and I'll confirm and emphasize.

I understand that user units is a representation of one rotation of the servo motor for a position command of 1.

Click to expand...

The user manual phrases it as "revolutions of motor per user unit". It's used internally to calculate the scaling factor between user units and encoder counts.

You described your ball screw as moving 0.017 inches for one revolution of the motor, so that's probably a 60 revolutions per inch screw. I would set the User Units (parameter 181) to 60.

Then also go to Parameters 678 and 676.

Parameter 678 is "Measure Units". Select 0,1,2 for micro-meters(um), meters, or inches.

Parameter 676 is "Measure Units Scaling", which is the number of User Units per Measure Unit. If you plan to use Inches, then this value should be 1.

The User Units will then apply to all of your position and speed related values. Speed will be "inches/second", and position will be Inches, and accel/decel will be inches/second^2.

You could set these values up for millimeters, or for feet, but I find it easiest to deal with inches if that's how your product and customers want to do it.

I am still a little confused about the accel and decel limit...

How does that relate to my velocity and distance?