Learn how to use PID Bumpless mode to ensure safe and controlled operation when switching between automatic and manual control of the drives output.
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Suppose we are running the PID application in manual mode at some low frequency or even 0 hertz and we have our setpoint at the 6 psi we have been using in this series of videos, and then we switch to auto mode. What happens? Well, PID is going to try to get the process to 6 psi as fast as it can which usually means it will send the motor straight to its max speed until the process gets close to its desired value, then back off on the motor speed so the process settles outright at the setpoint. For our little pump station demo that’s no big deal, but if you have a process that could be dangerous to people or damage the machine if the system suddenly jumped to max speed, well, then it is a problem. That’s where bumpless mode comes in. When switching from manual to automatic mode it simply reads the current process variable and puts it in the setpoint. So, when PID starts, it sees the process variable and the setpoint are the same and just goes about the business of maintaining that motor speed. The operator can then safely increase the setpoint to the desired level. So bumpless mode 0 is where bumpless is turned off and PID does everything it can to get to the setpoint as fast as it can – which for some processes can be dangerous. Bumpless mode 1 is what we just described – the setpoint is set equal to the process variable upon enabling PID to prevent PID from possibly driving the motor straight to full speed. The CFW series of drives also lets you control the transfer when exiting PID. Similar thing, suppose PID is reliably holding the process at some level, and when you switch to manual mode you don’t want any sudden changes in motor speed. Bumpless mode 2 sets the manual setpoint to whatever the current drive frequency is so when you make the switch to manual mode the motor keeps spinning at whatever speed it was already at. Again, preventing a sudden change. The operator would then safely adjust the manual setpoint to whatever it needs to be. Bumpless mode 3 enables smooth transitions in both directions. This is important – bumpless mode only works if the setpoint is being controlled manually by the drives HMI, or by from a com card or by the electronic potentiometer option. Bumpless doesn’t work if the setpoint is controlled by an analog input or digital inputs which makes sense because there is no way the software can override those hardwired inputs – right? And I should point out that while I am using the term bumpless because that’s the most common term for this in industry, but you won’t find the word “bumpless” in the WEG Programming manual. They prefer to call it “Automatic Adjustment of the PID Controller Setpoint.” Click here to see the rest of the videos in this series. Click here to subscribe to our YouTube channel so you will be notified when we publish new videos and click here to learn about AutomationDirect’s FREE award-winning support options.