An electronic control unit reads sensor inputs, processes them with a control strategy and drives actuators. The output stage may be a relay driver, a solenoid driver, a motor bridge or a semiconductor switch. Diagnostic feedback lets the module detect an open circuit, a short or a missing mechanical response.
Pulse-width modulation
PWM switches an output on and off rapidly. The ratio of on-time to period is the duty cycle, and the load responds to the average power. This is an efficient way to control a solenoid, a motor or a lamp because the output device is either fully on or fully off most of the time.
PWM average voltage
Problem. A 12 V output runs at a duty cycle of 40%. What is the average voltage?
Solution. V_average = 0.40 × 12 = 4.8 V. The instantaneous voltage is still 12 V while the output is on.
Commanded and actual response
An actuator can receive a valid command and still fail mechanically. A throttle motor may be commanded to move while the blade is stuck; a solenoid may click while the valve does not shift. Comparing commanded state with a position or feedback signal separates an electrical control fault from a mechanical fault.
- The ECU reads inputs, processes them and drives outputs.
- The output driver must match the actuator’s current and inductive behaviour.
- PWM controls average power by switching rapidly.
- Feedback lets the module verify the mechanical result.
- A valid command does not prove correct mechanical operation.
- The ECU follows an input-process-output loop.
- V_average = duty × V_supply for PWM.
- Output drivers must suit the actuator.
- Compare commanded and actual response.
Worked example
A 5 V PWM output at 25% duty has an average of 0.25 × 5 = 1.25 V, while its peak is 5 V.