When tuning with the ultimate gain method, what should the process reaction curve look like when the controller is successfully set to the ultimate gain/proportional Band?

When tuning with the ultimate gain method, what should the process reaction curve look like when the controller is successfully set to the ultimate gain/proportional Band?

• A. Constant amplitude oscillations
• B. Damped oscillations
• C. Growing oscillations
• D. No oscillations

Answer: (A)

In the ultimate gain method, you push the proportional gain up until the loop is right at the stability boundary. At this point the process variable begins to swing in a steady, repeating manner with a constant amplitude and a regular period. That steady oscillation means the loop is marginally stable: the energy input from the controller exactly balances the system’s natural damping, so the oscillations neither grow nor decay. If you increased the gain further, the oscillations would grow and the system would become unstable; if you lowered the gain, the oscillations would damp out and you wouldn’t see a sustained cycle. This constant-amplitude, self-sustained oscillation is the hallmark of the ultimate gain point (Ku) and its corresponding period Pu, used to set subsequent PID parameters.

In the ultimate gain method, you push the proportional gain up until the loop is right at the stability boundary. At this point the process variable begins to swing in a steady, repeating manner with a constant amplitude and a regular period. That steady oscillation means the loop is marginally stable: the energy input from the controller exactly balances the system’s natural damping, so the oscillations neither grow nor decay. If you increased the gain further, the oscillations would grow and the system would become unstable; if you lowered the gain, the oscillations would damp out and you wouldn’t see a sustained cycle. This constant-amplitude, self-sustained oscillation is the hallmark of the ultimate gain point (Ku) and its corresponding period Pu, used to set subsequent PID parameters.