Cascading PID Loops

[Hoose 0]

I've got a basic understanding of how a single PID loop works, but I'm getting a little confused on cascaded loops. The way I understand it the minor loop should be the loop that responds quicker? My application is a heater and oven setup. I have a thermocoupler in each, an UDC (Honeywell controller) for the high limit and need to limit the heater to no higher than 1100 F and the oven to 850 F max. This is with a DL06 fitted with F0-04THM and F0-04AH-1. So my understanding is that the PV's would be the themo inputs, the minor loop would be the heater and the major the oven, SP for the oven would be 850 F max (user input from a touch screen). So since the SP is ignored in the minor loop, how do you limit it to under 1100 F? Where have I strayed? If it sounds confusing, it's because I am. Mike

[Ken Moore 23]

Have zero experience with Direct Logic PLC's but have dealt with a lot of master/slave PID schemes. The master loop uses the oven temp as the PV and the master CV is the slave loops setpoint. So as the oven cools the SP of the slave goes higher and higher. If DL PID's permit it you could set up the master output to be 0-1100, or if that's not built in, you could put in a little intermediate logic to condition the setpoint before it is transferred to the slave. Some like: If Master_CV <= 1100, then move Master_CV to Slave_SP. If Master_CV > 1100, then move 1100 to Slave_SP. You could also setup the Master CV to output 0-100% and then multiply the output by 11, and transfer that to the slave SP. Does DL PID's have floating point outputs? If not then you slave setpoint will have a resolution of 11, which may make it difficult to tune/ hold steady temperatures. Ken