Hi, I was wondering would I be able to ask you a question on a Cascade loop.
Situation:
I have a situation where I am trying to tune a heating and cooling reactor system. As you can imagine in this situation the Pot tempeture is the primary Loop that I am trying to control with the jacket, the secondary loop, that provides the heating and the cooling.
There is a catch in the system, in that the pot temperature and the Jacket temperature cannot be outside 30 degrees of each other as the vessel is made of glass. So when heating (Pot temperataure setpoint increased), the Jacket temperature setpoint is set to the Pot temperature (PV) + 30deg. As the Pot temperature climbes so to does the Jacket setpoint until the Pot reaches a 2 degree deadband about the initial Pot setpoint where the Pot Primary loop Output becomes the Jackets set point.
Example:
Pot Setpoint increased from 20 deg to 100deg
Initially the Jacket setpoint = 20 +30 deg = 50 deg (this increases as the pot temp increases)
When Pot temp = 98 deg then the Pot Output from the PID becomes the setpoint for the jacket temp and the deaband temp is maintained.
The problem is that the deadband is being overshot i.e 102 deg thus my jacket setpoint is set to Pot temp - 30 deg, again the Pot then goes below 98 deg and the Setpoint is then Pot Temp + +30deg. This then continuous to occur and oscillate the Pot temp about the setoint (in and out of deadband)
To Remedy:
I have tuned the jacket tempeture secondary loop using the Ziegler Nicols tuning method and so far all is good. I can also reduce the 30 deg to say 25 deg setpoint difference and increase the deadband slightly. However, I believe the problem lies with the primary Pot loop in that it is not reacting fast enough. What I was wondering is how to tune a primary loop in this situation. Do I use a fast loop PID or a slower type?. I initially played around with a fast PID but caused the jacket setpoint to Jump around which In am trying to avoid.
I would be extremely gratefull for a bit of advise on this or a recommendation to a similiar thread. Many thanks - Rpax
Situation:
I have a situation where I am trying to tune a heating and cooling reactor system. As you can imagine in this situation the Pot tempeture is the primary Loop that I am trying to control with the jacket, the secondary loop, that provides the heating and the cooling.
There is a catch in the system, in that the pot temperature and the Jacket temperature cannot be outside 30 degrees of each other as the vessel is made of glass. So when heating (Pot temperataure setpoint increased), the Jacket temperature setpoint is set to the Pot temperature (PV) + 30deg. As the Pot temperature climbes so to does the Jacket setpoint until the Pot reaches a 2 degree deadband about the initial Pot setpoint where the Pot Primary loop Output becomes the Jackets set point.
Example:
Pot Setpoint increased from 20 deg to 100deg
Initially the Jacket setpoint = 20 +30 deg = 50 deg (this increases as the pot temp increases)
When Pot temp = 98 deg then the Pot Output from the PID becomes the setpoint for the jacket temp and the deaband temp is maintained.
The problem is that the deadband is being overshot i.e 102 deg thus my jacket setpoint is set to Pot temp - 30 deg, again the Pot then goes below 98 deg and the Setpoint is then Pot Temp + +30deg. This then continuous to occur and oscillate the Pot temp about the setoint (in and out of deadband)
To Remedy:
I have tuned the jacket tempeture secondary loop using the Ziegler Nicols tuning method and so far all is good. I can also reduce the 30 deg to say 25 deg setpoint difference and increase the deadband slightly. However, I believe the problem lies with the primary Pot loop in that it is not reacting fast enough. What I was wondering is how to tune a primary loop in this situation. Do I use a fast loop PID or a slower type?. I initially played around with a fast PID but caused the jacket setpoint to Jump around which In am trying to avoid.
I would be extremely gratefull for a bit of advise on this or a recommendation to a similiar thread. Many thanks - Rpax