Advanced Diagnostics Training I (ADT-I) Practice Exam

The idea being tested is how feedback changes control. In open-loop control, the system is driven by a predefined input with no measurement of the actual output used to adjust that input. If anything in the environment or the system changes, there’s no automatic correction, so the output can drift from the target. In diagnostics, this looks like applying a fixed stimulus to a device-under-test and not altering the stimulus based on what the device actually does—you’re relying on a assumed, ideal response rather than actively ensuring the result matches the goal.

In contrast, closed-loop control uses a sensor to measure the real output, compares it to the desired setpoint, and automatically adjusts the input to minimize the difference. This is why the example of motor speed controlled by a tachometer is so instructive: the tachometer feeds back the actual speed, the controller computes the error from the target speed, and it modulates power to the motor to keep speed near the setpoint. This feedback mechanism makes the system robust to disturbances and component changes.

So the key distinction is whether output is adjusted based on feedback from the actual result. Closed-loop systems actively correct deviations, while open-loop systems do not.