PID diffulculty?

Hi there,
I'm new to the forum and to PLC's in general. I do have some control experience although mostly just installation.
I attended a class for PLC basics about directlogic PLC's.

I have the task of making a simple PID loop to be used with a dl205. I need to monitor a temperature, and control an output accordingly. Very basic.

My question is how difficult is this to accomplish using directsoft programming? I already have a temperature sensor in the program going to a display. I just need to utilize the temperature, and send a proportional output to a solid state relay.

Would the code be very lengthy for this? When it comes to custom programming I am an extreme novice. Any input would be much appreciated. Thanks!

You can't send a continuously variable value directly to a discrete binary output.

What you can do is to establish a time based window and then calculate the appropriate percentage of "on time" for your output within each window. Pulse width modulation control is a common method of discrete CV temperature control. The appropriate value of the time base for your control window will vary according to the thermal exchange hysteresis of the device being controlled.

I don't have any experience with DirectSOFT, but what Bucket says is correct. You can monitor the temperature and energize a relay depending on the variable value. You will probably want to put in a time delay too so your relay doesn't constantly energize.

That depends on your controller. The D2-250 and D2-260 have built-in PID ability, so it would be pretty easy.

Yes, the DL-205 has racks with slots for analog I/O. Run the temperature into a PID "Loop", then to a PLC analog output, then to your external temperature controller device. The Automation Direct CPUs use a 32-word Loop Table to define a PID control block. It is maybe a little more difficult than using the Allen Bradley RSLogix PID instruction. The key is to read the Automation Direct DL-205 User Manual. I think Chapter 8 is the one on PID Loop operations.

Here is a link to the Automation Direct DL-205 User Manual. You can download all or by chapters. Chapter 8 is the one on PID Loop Operation.

http://www.automationdirect.com/static/manuals/d2user/d2user.html

Page 8-19 shows the complete Automation Direct 32-word PID Loop Table. I suggest that you set up your first PID loop in V-memory from V2000 to V2037.

Thank you all for the replys! I have the manual saved as well as the paper version. It can be overwhelming at times though.

Lancie, there is already one PID on the program for humidity control. I will be adding a temperature loop. If V2000-V2037 is already utilized I will look for the next set to put it in. Thanks

Yes, check that the memory area you pick does not overlay an exisiting used memory area. I think the DL-205 allows up to 8 PID loops.