PID_Compact with two control inputs

Hi to all,

I should design a system to control the temperature of the hot air flow in a duct. I am using a heater to increase the temperature and air blower to decrease the temperature level. I designed a PID control using the pt100 rtd sensor value as the Input_PER of PID_Compact V2.2. Now the problem is , The pressure inside the duct should be maintained within a range. Especially, it should not exceed beyond a certain level. Also, the pressure range depends on the type of duct. So how can I use the PID control with two Input Parameters, Temperature and Pressure for this system?

I am using the following Hardwares and Software.

CPU 1212C AC/DC/RLY, SM 1231RTD, SM 1234 AI/AQ, TIA Portal V13.

Thanks!

You have several different loops since you have several final control elements.  You have two temperature loops and one pressure.  They just share inputs.

It is also possible that your only PID is pressure if that is controlled by the fan.  The temperature is regulated with a hysteresis control turning the heater on and off.

Think about what element controls each variable that you are trying to control and how to control them individually.

Hi Jeff Casagrande,

Thanks for your reply.

I want to explain my project more clearly. The plastic duct should be heated by a hot air flow using a heat source. This heat source can be configured by its heating power and its air flow speed.

The requirements of the system are,

1. The temperature inside the duct should be maintained in a certain level.

2. The temperature difference between the entrance and the exit(Temperature drop) of the duct should be less than the maximum value.( eg. 10 degree Celsius)

3. The pressure inside the duct should not exceed the maximum level in any case.

Note: When the system is needed to cool through high temperature difference, the pressured cold air can be used.

My initial approach is,

I am using two temperature sensors, one is at the entrance and the other one is at the exit of the duct.  Also, I have an idea to maintain the pressure using pressure relief valve. So, I don't need to use PID control for this. The average temperature or the temperature of the entrance sensor may be given to the PID_Compact block to control the temperature inside the duct. Now, a question arises. How to maintain the temperature drop inside the duct by air flow speed of the heat source? The air flow speed also depends on the entrance temperature.

The heat energy as well as the air flow speed of the heat source can be controlled individually by varying the control voltages of the heat source within a range 0-10 VDC. So, I can use these two voltages as the manipulated variables. The ouput from the temperature sensors(pt100 rtd) can be used as the analog inputs of the PID blocks.

I have designed a basic level PID control to control the temperature at the entrance of the duct. In this, the analog value from the temperature sensor at the entrance of the duct is given to the Input_PER of the PID_compact_1. The Output_PER is given to control the heat energy of the heat source.

I used another PID block to control the air flow speed of the heat source. In this block, analog reading from the temperature sensor is connected to the Input_PER of the PID_Compact_2. The output_PER is given as the control voltage of the air flow speed of the heat source.

Now, the problem is to control the temperature difference between entrance and the exit of the duct. The same air flow speed of the heat source can be used to control the temperature drop. How to implement this control? Do I need to use cascaded PID block for the air flow speed? or is there any other solution for this problem?

Gajan.K

I have designed a basic level PID control to control the temperature at the entrance of the duct. In this, the analog value from the temperature sensor at the entrance of the duct is given to the Input_PER of the PID_compact_1. The Output_PER is given to control the heat energy of the heat source.

I used another PID block to control the air flow speed of the heat source. In this block, analog reading from the temperature sensor is connected to the Input_PER of the PID_Compact_2. The output_PER is given as the control voltage of the air flow speed of the heat source.

Now, the problem is to control the temperature difference between entrance and the exit of the duct. The same air flow speed of the heat source can be used to control the temperature drop. How to implement this control? Do I need to use cascaded PID block for the air flow speed? or is there any other solution for this problem?

 Hello Gajan,

I don't know if you already read this document that Siemens provide as an example of single and multiloop PIDs (Cascade), in this case, they use a "PID_temp" that is, in mostly cases, more useful than the normal "PID_Compact" for Temperature processes.

Single and multiloop PID (cascade)

It could be very useful to your control, 'coz , as far as i understand, you need to control one temperature related or chained to the other.

Additionali, an advantaje is that PID_Temp works very similarly to PID_Compact, and if you don't want to use the Cooling process of the PID, just unselect the option.

Any Doubt or question that you may have, just post it.

PD: I really recommend that document.

Regards,

Javier

Hello Gajan,

I don't know if you already read this document that Siemens provide as an example of single and multiloop PIDs (Cascade), in this case, they use a "PID_temp" that is, in mostly cases, more useful than the normal "PID_Compact" for Temperature processes.

Single and multiloop PID (cascade)

It could be very useful to your control, 'coz , as far as i understand, you need to control one temperature related or chained to the other.

Additionali, an advantaje is that PID_Temp works very similarly to PID_Compact, and if you don't want to use the Cooling process of the PID, just unselect the option.

Any Doubt or question that you may have, just post it.

PD: I really recommend that document.

Regards,

Javier

Hello Javier,

Thank you very much for your help. I will refer that document and the sample program. Then I will let you know my progress regarding the project.

Thanks.

Regards

Gajan.

Gajan.K

 

Hello Gajan,

I don't know if you already read this document that Siemens provide as an example of single and multiloop PIDs (Cascade), in this case, they use a "PID_temp" that is, in mostly cases, more useful than the normal "PID_Compact" for Temperature processes.

Single and multiloop PID (cascade)

It could be very useful to your control, 'coz , as far as i understand, you need to control one temperature related or chained to the other.

Additionali, an advantaje is that PID_Temp works very similarly to PID_Compact, and if you don't want to use the Cooling process of the PID, just unselect the option.

Any Doubt or question that you may have, just post it.

PD: I really recommend that document.

Regards,

Javier

 

Hello Javier,

Thank you very much for your help. I will refer that document and the sample program. Then I will let you know my progress regarding the project.

Thanks.

Regards

Gajan.

 Hello Javier,

 I have read the document " Single and Multi Loop Controller Structures (Cascade Control) with PID_Temp". Unfortunately, I couldn't' open the sample program since I am having Step 7 Basic V13 and winCC Basic V13. Is there any solution to open the sample program except buying new licence for upgraded version( Step 7 professional v13)?.

Thanks.

Regards

Gajan