Kinetix 300 ReferenceSource = Current

RonJohn

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Jul 2013
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NE Ohio
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I have a Kinetix 300 servo drive controlling an MPS servo motor coupled to a linear actuator. I need to move the actuator forward until it contacts an object and then hold the object in place while other equipment performs some work.

For the first part, I plan to give a velocity command and then when the drive current spikes (indicating object contact made), I plan to issue a current command to maintain a certain amount of force on the object.

The problem is that I've never used a current command with this drive (ReferenceSource = 0, Current) and I cannot find any technotes or examples regarding how to do it. My main concern is that if giving a torque command, I want to somehow limit velocity so that if something breaks free, the servo doesn't "squeal its tires" trying to achieve the current setting.

Thoughts?
 
Hi RonJohn.

I have done something similar using a Kinetix 350 with MPL Servo- this was hooked up to a Rexroth Linear Actuator that converted approx 5turns to 1mm of travel.

The way this servo was set up was to monitor for torque spikes- this determined the point at which contact was made (for example to determine starting point to drill etc).

If you want me to dig up the old logic, feel free to let me know.


Regards

Daniel.
 
Thanks Daniel. Feel free to dig but don't tear up the office looking for it. I'm fairly certain the 350 approach wouldn't be the same for a 300 - despite the similar names, I'm told Kinetix 300 doesn't program like the others.

I got in touch with Rockwell Tech Support, who recommended avoiding the Current reference source selection altogether. Instead they suggested keeping it in position mode (incremental or absolute) and adjusting the current limit as desired. Doing it this way, I'd have to give it a short, albeit unreachable distance command, making sure the position error value is greater than that distance so that the drive doesn't fault out on position error.

If everything works correctly, the servo will push (hold the object) at current limit until the command is aborted. If the position is reached, that indicates something slipped or broke free so I can trigger an alarm.

It's somewhat of a Rube Goldberg approach but it should control the torque/current limit while preventing a runaway condition. I'm going to give it a go in the morning.
 
Hi Ron.

Is this for a CNC application or just some general work to be performed on the piece i.e. drilling/cutting?

What is the dimensional difference from piece to piece?

Have you considered the use of a load cell for feedback?
 
Is this for a CNC application or just some general work to be performed on the piece i.e. drilling/cutting?
General work, basically squaring & holding in place.

What is the dimensional difference from piece to piece?
About +/- an inch.

Have you considered the use of a load cell for feedback?
That's a good thought. Unfortunately, the controls aspect was brought into the game a little late.

Initial tests with the position moves while manipulating DrvName:O.VelocityLimit, Current Limit Clamp % (Parameter ID 672) & Position Error Limit (Paramater ID 61) have been positive.
 
I’m going to see if I still have the program from this one machine. It was a control logix processor and it had a sercos card. It used the current feedback to apply pressure for a surface grinding application. It was for small medical pieces and the operator had to do an initial touch off. Once the piece was detected, the machine would run and apply a specific amount of pressure. I don’t recall if it got the load from the grinding head or the linear slide. I will see if I can find that program for you. The machine was custom from a company called Thielenhaus.
 

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