Incrementing a Set Point

Good Morning,

I have a sort of easy problem that is driving me nuts.

I have an application where I'd like to take a set point, lets say it starts at 50. Ten seconds later, I want to increase it to 60, another ten seconds 70, and so on...

How can I get this done, seems so easy, and I'm stumped!

Thanks, Burch

Burch said:

Good Morning,

I have a sort of easy problem that is driving me nuts.

I have an application where I'd like to take a set point, lets say it starts at 50. Ten seconds later, I want to increase it to 60, another ten seconds 70, and so on...

How can I get this done, seems so easy, and I'm stumped!

Thanks, Burch

Click to expand...

use a timer and a one shot....

You should use a non-retentive timer and make it self-resetting by programming a NC Timer bit as the timer trigger. Set the timer for 10 seconds, then every time it goes done, add 10 to your Setpoint using a 1-shot. You must set a upper limit and stop the ADD once you reach the maximum Setpoint.

This sounds like a ramping function for a heating application. 10 Points per 10 seconds (10 P/10 Sec.) also is the same as 1 point per second (1 P/Second), so you could use a 1-second self-resetting timer and Add 1 point each second. The end Setpoint will occur at the same time as the 10 P/10 Sec. method.

If you are changing the Setpoint of a PID instruction, then the 1P/Sec will give you a smoother upward curve for the Control Variable (with less oscillation).

From your other posts it looks like you are using AB products.

An easy way to do smooth linear ramping with a timer is to use an SCP instruction to "scale" the elapsed time to your setpoint range. The attached example will give you a smooth linear ramp from 0 to 100% in one minute.

You might check out this thread.
http://www.plctalk.net/qanda/showthread.php?t=79329&highlight=ramp&page=2

I posted a txt file with ramp logic in it, there are a couple of other ideas there also.