Calculus in a SLC

Here is the scenario. I want to ramp a machine down in a fixed amount of time from a known motor RPM. I want the machine to be at a stop at a certain product length. The machine has interchangeable gears so the amount of product made per RPM of the motor varies with whatever gears are in the machine. I need to know at what point (how much length of the product left that needs to be produceded), before I initiate the ramp down function.

This sounds like a first order derivative, however I have no idea how to implement this in the PLC.

Any ideas? and thanks in advance.

Were going to need a desription of the application and hardware involved to answer that correctly. But a couple of questions. Do you have a VFD as well as the gears? Motion for ramping is usually done outside the SLC with the use other VFD's, Moton controllers.
If you wish to use the SLC there is the Ramp function described in the SLC 500 instruction set. Your application may require more finite control than can be handled by the SLC depending on its application. Also speed of travel ramp up an down would help.

Chose. time or distance.

allscott said:

Here is the scenario. I want to ramp a machine down in a fixed amount of time

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No problem.

a certain product length.

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This can't be done as a linear ramp. It is possible to ramp down to the final speed and then move at that speed for a while until the required distance is reached..

Any ideas? and thanks in advance.

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Yes, but I need exact numbers.

Are you doing the speed ramping in the drive or in the plc? In the greater scheme of things it really doesn't matter. I have just always found it easier to do the ramping in the plc and work with linear units. Convert to RPM just before you send the command to the drive. It just seems to make things easier to look at, especially dynamically.

How accurate are you looking to be? How fast are you going? How much time do you have to stop? While this is oversimplifying a little generally speaking, machine dynamics are sensitive to force. Accel rate is force. So you are probably more interested in defining an accel rate and distance and allowing the time to be what it will be.

Keith

I am doing the ramps in the PLC. The ramp up and ramp down times are both 30 seconds as that is a reasonable amount of time to ramp a machine this size up and down. Also the machine always runs at a constant RPM so the motor will always be at the same speed when it starts to ramp down.

The line speed is anywhere from 10 MPM to about 40MPM depending on the gears selected. The machine is driven by a DC drive.

Accuracy isn't that critical. If I could get the machine to stop within say 10M of the desired length then the machine can be jogged to final position.

What is MPM?
I'm assuming meters per minute.
what you are asking for is possible, but again within 10 meters of the stopping point? Depending on your gear size, ratio, and hp,

1. use a series of limit switches to reduse the motor speed to the stopping point. the last one would be slow jog. cut the product, and restart.

2. use an encoder, and perform a constant loop monitor and adjust the speed until you are at the stopping point.

3. use a cut on the fly feedback loop. Its difficult, but not impossible.

hope this helps,
james