How to Design Electric Actuators with Fault Reset Function

In automation systems for fluid control, power, chemical, oil & gas industries, electric actuators serve as core actuation components to ensure reliable operation of valves and dampers. To meet safety production requirements, actuators must be equipped with fail-safe functions (FO/FC/FL). FO (Fail-Open), FC (Fail-Close), and FL (Fail-Lock) represent not only safety strategies, but also a complete systematic design covering mechanical structure, circuitry, control logic and software algorithms.

I. Design Objectives of Fault Reset

Most conventional electric actuators maintain their current position upon power loss or signal loss (FL). However, standard electric actuators cannot satisfy applications that require the valve to close (FC) or open (FO) when power or control signals are lost.

Design objectives for fault reset:

1. Reliably drive to the safe position when a fault occurs;

2. Allow manual or automatic exit from fault state after fault clearance;

3. Reset process without impact, runaway or spurious operation;

4. Support local reset, remote reset and host system reset.

II. Two Mainstream Design Schemes for Fail-Safe Reset

1. Spring-loaded Mechanical Design (Classic FO/FC Solution) A reset spring is installed internally. When powered normally, the motor compresses or stretches the spring to store energy. Upon power loss or signal loss, the spring releases energy immediately to drive the valve to fully open or fully closed position. Application: High safety-level scenarios where position reset is mandatory on power failure.

2. Backup Power Driven Design (Capacitor / Battery) Key circuit design points: Supercapacitors or backup batteries are charged under normal main power supply. When main power fails, the backup power supplies the control board and motor to perform pre-defined FO/FC action. Advantages: Smooth movement, controllable speed and no mechanical shock.

III. Overall Design Philosophy of Fault Reset Function

Fault reset is not merely a pushbutton; it is a full set of protection mechanisms, unlock logic and state machine.

1. Fault Detection Circuit Design An actuator must first identify faults before reset can be implemented: Power abnormality detection: undervoltage, phase loss, overvoltage; Signal abnormality detection: 4–20mA loop break, communication loss; Motor protection: overcurrent, overtemperature, stall; Mechanical protection: excessive opening torque, excessive closing torque. Once these signals are received by the MCU, the actuator enters a locked protection state.

2. Reset Authority and Mode Design For safety and convenience, three levels of reset are generally designed: Local pushbutton reset: short or long press of the RESET key on the panel; Terminal remote reset: external dry contact input, triggered by DCS or field pushbutton; Communication reset: reset via protocol messages such as Modbus, Profibus, Profinet.

3. Reset Condition Logic Design (Critical) Conditional permission logic is implemented to prevent false reset: Fault source has disappeared: no overcurrent, no overheating, signal recovered; Actuator is not in prohibited operation zone; Non-auto-resettable faults are not triggered (e.g. severe over-torque, mechanical damage). Reset commands take effect only when all conditions are satisfied.

IV. Key Safety Design Rules for Fault Reset

1. Any reset operation shall not override FO/FC fail-safe logic.

2. Non-auto-resettable fault design: severe faults such as over-torque, overtemperature and short circuit must be manually reset.

3. Valve position synchronization design: automatic calibration of valve position after reset to ensure feedback matches actual position.

4. Interlock protection design: interlock between remote/local mode and auto/manual mode to avoid conflicting commands.

5. Fault logging and traceability: record fault type, timestamp, operator and reset method for maintenance.