Switching between manual and automatic can jolt the setpoint if the two sides weren't aligned first

What it is

A setpoint or output bump at manual–automatic transfer happens because controller internal state, the value held on the manual side, and the current actual process output are usually not aligned at the moment of transfer. Transfer itself is not a physical action, but connecting two mismatched numbers directly turns their difference into an immediate command step — that is the bump. Bumpless transfer requires the side about to take authority to align its internal value with the current actual output before authority actually changes hands.

Why it happens

An automatic controller accumulates integral state while running — to eliminate a persistent steady-state error, the integral term keeps growing, and it can hold a large value even when the present error is small. The manual side, meanwhile, may sit wherever an operator last set it, possibly hours earlier. When authority passes from one side to the other without an alignment mechanism, the receiving side immediately outputs its own held value — which may be stale or carry substantial integral residue — as the new command. The gap between that value and the actual pre-transfer output is applied to the final element as a step: a valve can move sharply even though the process itself is stable at that instant. If the interface shows only a text state like "mode switched from auto to manual" without the prospective value and its gap from current output, this impending step is completely hidden behind the mode label.

Where it stops holding

Not every transfer should aim for smoothness. In some emergency situations an operator switches to manual precisely to apply a large action that deliberately departs from automatic logic — automatic control reacting too slowly to a worsening condition — and alignment there would only delay a needed intervention; speed matters more than smoothness. Whether bumpless transfer is achievable also depends on the specific control algorithm (whether it implements tracking and anti-windup); the interface alone can only expose the risk, not guarantee the physical actuation side actually behaves smoothly.

Applying it

Before transfer, show the current actual output, the initial value held by the side about to take over, and the expected gap between them; require explicit confirmation or block the transfer when the gap exceeds a reasonable threshold.

• The controller implementation must support tracking and anti-windup logic for the displayed gap to be meaningful.
• How to check: test transfer at several operating points across the full range plus one scenario with a stuck actuator, and compare the resulting output trajectory against what the interface predicted, looking specifically for unannounced steps.

Related

• Same group: Y3.09.1 Continuous control-authority indication · Y3.09.3 Manual-side tracking before takeover · Y3.09.4 Frequent mode switching as a diagnostic signal
• Nearby: Y3.02 Automation mode confusion · Y4.07 Emergency reach and inadvertent-activation protection
• Search terms: Setpoint bump during transfer · process control interface · industrial human factors