Hi Greg, we've done something simular with flow and pressure control. Var. setpoint--+ BCALCI<------BCALC1 | | | Flow AIN-->PIDE----->| | |OUTSEL----->AOUT(valve control) Pressure AIN-->PIDE----->| | | | | Fixed setpoint-+ BCALCI<------BCALC2 In our case we need to maintain a minimum pressure. OUTSEL is set to pass the highest output to the AOUT. The pressure PID block has a fixed setpoint based on a minimum pressure. The flow PID block setpoint is based on flow demand. As the demand for flow is dropping, the valve closes, which will cause a reduction in pressure. This will result in a higher output on the pressure PID block, forcing the valve to maintain a position that retains the minimum pressure. I'm sure something like the above can be done with flow and temperature. Cheers, Frits Schouten. NZSteel -----Original Message----- From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx] On Behalf Of Gregory A Hurwitt Sent: Wednesday, 24 October 2007 9:07 AM To: foxboro@xxxxxxxxxxxxx Subject: [foxboro] cascade/constraint control I have a control scheme that will normally run as level-flow cascade. If the temperature of the downstream equipment falls too low, then the flow must be restricted. I plan to use a constraint control scheme: +-----------+ +-----------+ +-----------+ | | | | | | | LC | --------> | SIGSEL | --------> | FC | | | | (low) | | | +-----------+ +-----------+ +-----------+ ^ +-----------+ | | | | | TC | -----------------+ | | +-----------+ What are the recommended FBK and BCALCI connections for the LC and TC? How do I ensure that the TC does not wind up while it is not selected? ________________________________________________________________________ ______________ Greg Hurwitt Engineering Associate E-Mail: gregory.hurwitt@xxxxxxxx Postal Address: BASF Corporation 602 Copper Road Freeport, TX 77541 USA BASF - The Chemical Company =20 =20 _______________________________________________________________________ This mailing list is neither sponsored nor endorsed by Invensys Process Systems (formerly The Foxboro Company). Use the info you obtain here at your own risks. Read http://www.thecassandraproject.org/disclaimer.html =20 foxboro mailing list: //www.freelists.org/list/foxboro to subscribe: mailto:foxboro-request@xxxxxxxxxxxxx?subject=3Djoin to unsubscribe: mailto:foxboro-request@xxxxxxxxxxxxx?subject=3Dleave =20 =0A=0ANOTICE - This message and any attached files may contain informatio= n that is confidential and intended only for use by the intended recipien= t. If you are not the intended recipient or the person responsible for de= livering the message to the intended recipient, be advised that you have = received this message in error and that any dissemination, copying or use= of this message or attachment is strictly forbidden, as is the disclosur= e of the information therein. If you have received this message in error = please notify the sender immediately and delete the message. _______________________________________________________________________ This mailing list is neither sponsored nor endorsed by Invensys Process Systems (formerly The Foxboro Company). Use the info you obtain here at your own risks. Read http://www.thecassandraproject.org/disclaimer.html foxboro mailing list: //www.freelists.org/list/foxboro to subscribe: mailto:foxboro-request@xxxxxxxxxxxxx?subject=join to unsubscribe: mailto:foxboro-request@xxxxxxxxxxxxx?subject=leave