Need a jump start

In my real job I do a little troubleshooting with compact 984-130's. I do understand some of the programming and hardware but just have hit a wall on this personal project. I am not looking for someone to write the program just help me get the time/temperature rung wrote. If I can just get over this beginning part of the program I will work on figuring out the rest. If someone is willing to help me on this please e-mail me and I will explain in more detail what I am trying to do. I’m a beginner and have read the manuals and bought the tutor book but this is not my area of expertise.

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Post your work work here, rather than using e-mail.

What do you mean by time/temperature rung?
What have you done so far?
Where do you think your problem is?
What are you using for programming software?
What I/O modules do you have?

Here is what I want to do

I have not started writing the program, not sure how to incorporate the time/temperature ramp, so if I can get a little help with that I can do the digital inputs, digital outputs, and some of the analog inputs, and analog outputs.

What I have is a cryogenic processor that cold treats metal. Currently it just has a watlow 982 ramping controller. This controller is very limited with the ramp and soak so much that it overshoots it's temperature set point and cycles the cooling and heating so much that it waste a lot of liquid nitrogen.
With a plc I can use a variable injection and a more controlled heat. What I don't want to do is turn the heat on while cooling and inject cold nitrogen while heating like it currently does now.

This is how the process should work with a plc.

Selector Switch
A - position:
Take the temperature in the chamber from ambient to -300 F at a pace of 1 degree every 3 minutes. Hold for 4 hours and maintain -300 with cold nitrogen. Unit will then start the warm up heat cycle at the same pace of 1 degree every 3 minutes using the inboard fan forced 4-20ma heater controller until a target of +100F is reached. Hold +100F for 2 hours. Cycle ends and chamber cools down on it's own.

B - position:
Take the temperature in the chamber from ambient to -300 F at a pace of 1 degree every 3 minutes. Hold for 4 hours and maintain -300 with cold nitrogen. Unit will then start the warm up heat cycle at the same pace of 1 degree every 3 minutes using the inboard fan forced 4-20ma heater controller until a target of +300F is reached. Hold +300F for 2 hours. Cycle ends and chamber cools down on it's own.

I am going to use a variable 4-20 ma control valve for the nitrogen (right now it is just a on/off solenoid valve. dau-2-2 4-20 ma card

Heater controller is 4-20 ma input driven dau-202 4-20ma card

The temperature input is an rtd adu-204 card

Switches 24 vdc input dep-216 card

Lights dap-204 24 volt output card

Modicon 984-130 compact
Modsoft software

This cryogenic processing freezer is a side job that I am going to have, and hopefully use it to put my 4 kids through college.

jr.

If you think about it, you have actually described your solution. Use a timer, multiply its accumulated value by a constant, and move that value to the setpoint for your temperature control logic.

And generally it is a mistake to not begin writing the program until you have figured out how to do what you need. In my experience you are better off starting out writing, initially doing simple things such as alarms and status. Then just make an attempt at the hard part of the logic, and then modify it, and then test it, and then modify it, and ....... Eventually you get it done.

First off, you're missing some things. How does the PLC know when to start all this wonderful sequencing stuff? Is there a start PB, a sensor to detect the metal in the tank, or what?

You state an ideal control strategy. What about a non-ideal one? Just how critical is the temperature dynamics of the tratment process? How tightly does the ramp have to be controlled?

For staters, you'll need to get look at the instruction set and find the PID instruction. You will use that instruction to control your two analog outputs. You will want to "disable" the one that isn't currently active, which may be tricky. I don't know the 984 PID instruction, but PID instruction don't often take kindly to being disabled. They tend to remember their last values, and so when re-enabled, use the last place as a starting point, which can lead to initially overshooting.

To ramp a setpoint, simply do the following:
A) When you are initially starting the ramp, capture the current PV, and subtract it from the SP to get your Delta.
B) Start a timer equal to the length of the ramp duration. For long ramps (hours), you might use a counter in conjunction with a timer. Ther timer will pulse once per minute, the counter will count the minutes.
C) The setpoint that you would use for your PID is :
PID_SP = Captured_PV + ( Delta * Timer_Accum / Timer_Preset )
Continuously change the PID's setpoint with this value. The PID will do all the work to "maintain" the ever-increasing (or deacreasing) temperature.

If you are expecting to pay for all 4 kids' colleges, you're overcharging. One kid through community college, maybe.

If you need much more help, as Jay Anthony recently posted, contract consulting starts at $125/hr.

Your ASBADU204 input module, has 4 input channels that can accept either +/- 500mV or PT100 4 wire RTDs. The effective decimal value of your registers will be from 2049 to 6143. A typical PT100 RTD can measure from -328 to 1562F. Using these values your system should have a resolution of .46 degF/decimal value. Your logic should treat values smaller than 2049 as an under-range condition.

Your analog output module ASBDAU202 is a 2 channel +/-10VDC or +/-20mA device. The decimal equivalent to these ranges is 0-4000, this means you will only be able to control your valve over a decimal range of 2400-4000. If you have a choice you may want to use a true 4-20ma output module (i.e. ASBDAU204).

Modsoft (an ancient DOS package) does support the PID2 command in the 984-A130 CPU. Floating Point and Double Precision math functions are also available.

Depending on the reponse rate of your system you may want to use straight temperature/time for ramping up and down to your -300F. Once you get close then switch to a PID algorithm for your soak period. This prevents you from constantly changing the parameters of the PID, however you may experience a "bump" or swing when you perform the transfer. This bump can be minimized by starting the PID calculation early and making the transfer when the PID output is equal to your ramp value.