ramp up!!

HI
I tried to create a current rampup with a plc in a SCR, for this i will use a anallogue output, but the real question is: How can i programe something like that??
do you think a s7-200 it is enough??
Thank you

zezito,

Because no one else has attempted to help you, I will try it.

The S7-200 should be adequate for doing a current ramp for an SCR controller.

There are many methods for doing a Ramp-up function. I have done it this way: Calculate the ramp-up time by finding the difference between your Ending Current - Starting Current. Decide how long you want to take to go from Start to End. Calculate Ramp Increment (or Slope) = Ending Current-Starting Current / Total Ramp Time. Set up a 1 or 2-second timer. Each time the Timer is triggered, increase (add) the Ramp Increment to your PID control block Auto Setpoint. The Setpoint should be increased from some Minimum (usually 0) to a Maximum value (Ending Current). When the SCR Controller PID block Setpoint reaches the Ending Current value, disable the Ramp function.

This thread may help http://www.plctalk.net/qanda/showthread.php?s=&threadid=8948

Thank you very much for your attention in this subject.
I still have two questions:
Is it necessary to use a pid block?
cann't i add each second a value to a word in order to change the output of the analogue output?

Hi Zezito,

Lancie1 is correct and you dont need to use the PID block. Ramp just like that.

Good luck,

Thank you guys for all the help!
but theres one thing that's bodering me, and i hope that some could explain me that!
I'm using ADD block to in each second add a value to an ACx memory that will be moved with MOV_W block to the analogue output.
i thought that this would work but i saw that if i do that the analogue outpout becomes very unstable because it gets even negative values, why???

Also, if i do the ramp other way, like with compare blocks, i'm moving in each second a certain value For example: T=1s move to the output 1600 (0,5v) ; t=2s move 3200 (1v) ; and so on...
but i notice that in first moves the analogue outoput changes each step 0,5v ,but the others moves the out put changes 0,4v and some 0,3v!!!! Why this happens??

I hope that someone could help on this!
thank you!

zezito,

The analog output will only do what you tell it to do. Somehow, you are not sending it the numbers that you think you are. I have not worked with the S7 in awhile, but a common problem with many PLCs is that the output may not be a Decimal (Base 10) number, but in some other number system, such as Octal or Hexadecimal. Check to make sure it is what you think it is.

Then check to make sure that when your timer reaches a value, that you are NOT adding each number MORE THAN ONCE. You may need a one-shot relay to prevent this from happening. The PLC will repeat your ADD instruction each scan unless you take action to only allow it to ADD one time!

Then, check the output range. You say that 1600=0 volts, 3200=1 volt, and so on. Then 9600 should = 5 volts. Your program should never send less than 1600, nor more than 9600, ASSUMEing that these are indeed Decimal numbers. In fact, I usually put Compare blocks right before the Final Output that check the Minimum and Maximum limits. (If Output < 1600, then set Final Output = 1600. If Output > 9600, then set Final Output = 9600).

I'm not sure I fully understand your problem but to help you understand the process I offer the following. I find to change something some value per second it's often better to change it (some value) / 10 per 0.1 seconds. This smooths out any irregularities from rounding etc. Also with Siemens S7-300 (I'm not sure about 200) the analog values follow -27648 = -limit, 0=0, and +27648 = upper limit. I didn't see what your range was but 0=0 and 27658 = your max voltage, figure your range accordingly. The 27648 value is for S7-300 & S7-400, other models/brands may use different values, check the documentation.