BertR
Member
To date most of my programming experience has been with DCS systems and some limited amount with PLC ladder logic (digital only).
The attraction of using PLCs for analog control loops is mainly the lower hardware cost of PLCs when compared to DCS. However, I am concerned that it may be more difficult to program analog control loops on PLCs. I have searched the threads in previous forums, and some of the issues discussed were very interesting.
However with my limited knowledge of PLCs and SCADA systems, would sonebody like to advise on the following points:
1. For PID loops, is it possible to achieve a bumpless transfer from Auto to Manual, and viceversa ? Is this achieved in the HMI or a SCADA unit ?
2. Is it "easy" to program a Cascade Controller (ie with Remote Set Point), and having Remote/Local Auto/Manual transfers. Is this done with preconfigured function blocks ?
3. Can alarm set points on analog input signals be set in the PLC as well as in an HMI ?
4. Can Hi and Low analog signal selectors be programmed in the ladder logic program ?
5. Is short term trending (for loop tuning) done in the HMI?
6. How important is it to consider "loop update time" on a PID loop for control performance, for example on relatively slow processes like tank level control ? Also, what is meant by the term "PID trigger rate" ?
Thanks,
Bert
The attraction of using PLCs for analog control loops is mainly the lower hardware cost of PLCs when compared to DCS. However, I am concerned that it may be more difficult to program analog control loops on PLCs. I have searched the threads in previous forums, and some of the issues discussed were very interesting.
However with my limited knowledge of PLCs and SCADA systems, would sonebody like to advise on the following points:
1. For PID loops, is it possible to achieve a bumpless transfer from Auto to Manual, and viceversa ? Is this achieved in the HMI or a SCADA unit ?
2. Is it "easy" to program a Cascade Controller (ie with Remote Set Point), and having Remote/Local Auto/Manual transfers. Is this done with preconfigured function blocks ?
3. Can alarm set points on analog input signals be set in the PLC as well as in an HMI ?
4. Can Hi and Low analog signal selectors be programmed in the ladder logic program ?
5. Is short term trending (for loop tuning) done in the HMI?
6. How important is it to consider "loop update time" on a PID loop for control performance, for example on relatively slow processes like tank level control ? Also, what is meant by the term "PID trigger rate" ?
Thanks,
Bert
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