Sensors and feedback signals let engineered systems measure what is actually happening instead of only assuming a command was followed. A motor, heater, robot arm, or pump can drift away from its target because of friction, load changes, noise, or changing environmental conditions. By sensing variables such as position, temperature, pressure, or speed, a controller can compare reality with a desired setpoint.
This is the foundation of closed-loop control in modern machines, vehicles, factories, and medical devices.
A typical feedback system starts with a sensor that converts a physical variable into an electrical signal such as voltage, current, pulses, or resistance. Signal conditioning then filters noise, amplifies weak signals, and converts the signal into a form the controller can read. The controller subtracts the measured value from the setpoint to produce an error signal, then calculates a correction for the actuator.
The loop closes when the actuator changes the process and the sensor measures the new result.
Key Facts
- Error signal: e(t) = r(t) - y(t), where r(t) is the setpoint and y(t) is the measured output.
- Sensor sensitivity: S = Δoutput / Δinput, such as volts per degree Celsius or volts per pascal.
- A thermocouple produces a small voltage related to temperature difference, often in millivolts.
- An encoder measures position or speed by counting pulses: angle = pulse count × degrees per pulse.
- Signal conditioning can include amplification, filtering, isolation, linearization, and analog-to-digital conversion.
- Negative feedback reduces error by applying a correction that drives the output toward the setpoint.
Vocabulary
- Sensor
- A device that detects a physical quantity such as temperature, pressure, position, or speed and converts it into a usable signal.
- Setpoint
- The desired target value that a control system tries to maintain or reach.
- Feedback signal
- A measured signal returned to the controller so it can compare actual output with the desired value.
- Signal conditioning
- The process of improving or modifying a sensor signal so it can be accurately read by a controller.
- Actuator
- A device such as a motor, valve, heater, or piston that changes the physical system in response to a control signal.
Common Mistakes to Avoid
- Confusing the sensor signal with the control signal is wrong because the sensor reports what happened, while the control signal commands the actuator to make a change.
- Ignoring units in sensor sensitivity is wrong because a value like 0.02 V/°C only makes sense when input and output units are tracked.
- Assuming all feedback improves stability is wrong because incorrect sign, delay, or excessive gain can make a system oscillate or become unstable.
- Forgetting signal conditioning is wrong because raw sensor outputs are often noisy, weak, nonlinear, or outside the safe input range of a controller.
Practice Questions
- 1 A temperature sensor has a sensitivity of 10 mV/°C. What voltage change occurs when the temperature increases by 35°C?
- 2 A position encoder has 1000 pulses per revolution. If the controller counts 250 pulses, what angle has the shaft turned in degrees?
- 3 A motor speed controller has a setpoint of 1200 rpm, but the feedback sensor measures 1100 rpm. Explain the sign and purpose of the error signal if the controller should speed up the motor.