Continuous-motion cam

[hans.simonson]

Allen Bradley tech support helped me with this same issue. It can be very complicated programming, but they should be able to help you thru it.

 

[Peter Nachtwey]

This is mostly because (as Peter said)

I didn't say that. I did say the control logic lacks some features as you have noted. Since the motion controllers are basically interpolating positions between downloaded points, all the "smarts" are in the RS Logix software and control Logix PLC. It should be relatively easy to upgrade these. It is just software .

 

[kamenges]

Originally posted by MichaelG:

3) Really understand your machinery - Think hard to find a situation where it is easier to change length (eg the start of constant velocity section)

If you need to remember only one thing, remember this. It will make cam development and clean switching so much easier when dealing with a rotary knife, or any application that has a common section regardless of overall profile. This is what would allow for easy use for pending cams to change profiles on the fly but it also makes the motion equations you need to determine your inflection points more clear.

Originally posted by MichaelG:

Adding the complexity of changing the CAM table while moving is another headache.

While in the general sense this can be true in the particular case for a rotary knife I don't agree. Unless you are running this thing much faster than I think you are it isn't all that difficult to do. You just need enough program scans between cuts for the plc to detect that a cam is running and for the Logix motion engine to queue up the next cam. If you were trying to make each profile segment in a given cut its own cam and wanted to change those on the fly it could get kind of hairy. But if you make a single cut profile a single cam it isn't bad at all.

Keith

P.S.

Keep in mind that in the Logix platform axes are abstracted from the hardware that actually moves stuff until you as the programmer ties them together. This means you can fully write and test your camming logic using virtual axes. It makes those occasional discontinuities n early cam development a whole lot less damaging when they are just unexpected blips on a trend.

 

[drbitboy]

Going back to the original post:

We have a cutting machine that we designed in house. It uses an encoder to follow the motion of the product and a cam instruction to cause the cutter's servo system to cut to length. The current system requires the cam to start and finish its profile at zero velocity. This seems to be a limitation of the Allen-Bradley software.

Is there any way to program a continuous-motion cam, regardless of the brand of controller used?

Or is there a different way of achieving the same results: cut to length with varying speed and programmable cut lengths?

Thanks in advance for your suggestions.

If A-B actually cannot have a cam that does not start and finish its profile at zero velocity, and this is a flying shear i.e. cyclical so the start and end of cycle are at the same velocity (and position), couldn't the zero-at-the-ends cam profile be added to (superimposed onto) a constant velocity, sawtooth position, profile, and the result sent to the servo?


Or is this all happening in some real-time, non-program-scan environment, and the output of the cam cannot be tweaked before it is sent to the servo?

 

[MichaelG]

If A-B actually cannot have a cam that does not start and finish its profile at zero velocity,

Actually A-B can have a CAM that starts and finishes with a "slope" - Check out the start slope and end slope in the MCCP instruction

I use this to start a pending CAM profile with a velocity - It is up to the programmer make sure that the slave axis is already travelling at that velocity.
I do not know of any way to start (or finish) a CLX CAM with a controlled non-zero acceleration - I believe that this is a limitation of the CLX motion programming and is the reason why I change CAMs during a constant velocity section

 

[Peter Nachtwey]

Actually A-B can have a CAM that starts and finishes with a "slope" - Check out the start slope and end slope in the MCCP instruction

I believe this. The standard formula for splines allows either the velocity/slope or the acceleration to be set to zero but not both. So when executing a flying shear it should be possible to set the beginning and ending velocities to 0. The problem is that the accelerations may not match or be zero at the end points.

I use this to start a pending CAM profile with a velocity - It is up to the programmer make sure that the slave axis is already travelling at that velocity.

Right. OK, Why not start the cam-table velocity at the current velocity of the actuator?

I do not know of any way to start (or finish) a CLX CAM with a controlled non-zero acceleration - I believe that this is a limitation of the C.LX motion programming and is the reason why I change CAMs during a constant velocity section

BINGO! That is something we solved over 20 years ago as well as being start a spline/cam table without being on the spline. I remember working on this in the summer of 1999. It requires executing the spline and super imposing a move to the spline.
I did this on our old RMC100 which used a 80186. Even now, over 20 years later a customer in Finland who desperately wants to replace the ancient RMC100 with Siemens motion control has not because he can't get another controller to do what our old RMC100 could do. Like I said. It is just software, and a little math. The algorithms make a huge difference.