Feedback, control & actuators
Open-loop & closed-loop control
Measuring the outcome makes correction possible.
An open-loop controller acts without measuring its result. A closed-loop system compares a reference with a measured output and uses the error to drive a plant. Disturbances, sensor error and actuator limits still matter. Negative feedback can improve tracking and reject some disturbances, but delay and phase shift can turn a well-meant correction into oscillation.
PID intuition, damping & stability
Fast is useful only if the system can settle.
Proportional action responds to present error. Integral action accumulates error and can remove certain steady-state offsets. Derivative action responds to rate of change and can add damping, but also amplifies high-frequency noise unless filtered. Saturated actuators can cause integral windup; anti-windup limits or corrects that accumulation. Stability means bounded behavior under the relevant conditions, not just a nice-looking first few seconds.
Motors, relays & power drivers
Use a signal to command energy, not supply it directly.
Drivers let a low-power signal control a motor, relay or solenoid. An H-bridge can reverse voltage across a DC motor, but its switching must avoid shoot-through. A motor generates back EMF as it turns; at stall that voltage is absent and current can be much higher. PWM changes average applied voltage, while winding inductance and mechanical inertia shape the response. Steppers and servos require their own control strategies.