Image: Electric actuators (site archive)
Existing domestic high-flow electric actuators suffer from backward control, many mechanical transmission stages, complex structure, low positioning accuracy and poor reliability; their travel speed is fixed at the factory by motor speed and gear ratio, limiting versatility. This article analyses the design of a mechatronic intelligent high-flow electric actuator.
1. Hardware design and working principle
The intelligent actuator consists of a control part (microcontroller, PWM generator, IPM inverter, A/D & D/A modules, rectifier, I/O channels, fault detection and alarm circuits) and a drive part (three-phase servo motor and position sensor).
Principle: Hall current/voltage sensors and the position sensor acquire the inverter's three-phase output and valve position; after A/D conversion the microcontroller drives a PWM generator via an 8255 port; the PWM waves gate the IPM through optocouplers for variable-frequency speed control and valve positioning. The DC bus comes from full-bridge rectification of the 380 V supply.
2. Component selection and design
· Microcontroller: INTEL 8031 with parallel 8255 port — receives torque, open/close and opening setpoints, feeds the PWM generator, handles IPM faults/alarms, current/voltage/position signals, status display and system feedback;
· Three-phase PWM generator: analog methods are complex with drift; pure digital look-up tables consume memory. The MITEL SA8282 ASIC was chosen — waveform, frequency and amplitude control on chip with a standard microprocessor interface;
· Intelligent power module IPM: for three-phase motors below 5.5 kW (380 V, PF 0.75) the PM50RSA120 integrates power switches, drive and braking circuits with built-in overcurrent, short-circuit, undervoltage and overheat protection plus alarm output;
· Position detection: wire-wound pots are short-lived; LVDTs have a short linear range and poor temperature behaviour; conductive-plastic pots are contact-based with limited life and accuracy. A pulse digital sensor was adopted — contactless, precise, no linear-range limit, stable, temperature-independent;
· Voltage/current sensing: at 0–50 Hz ordinary transformers fail; Hall CTs sense the IPM three-phase currents, a divider senses voltage — for torque calculation, short-circuit/phase-loss protection and inverter diagnostics;
· Communication: MAX232 provides TTL↔RS-232 conversion for networking and remote control;
· Real-time clock: DS12887 supplies sampling/control cycles and calendar, with 114 bytes of non-volatile RAM;
· LCD: MGLS12832 module in configurator style — valve, torque, limit, motor, communication and parameter menus, text + graphics;
· Program-runaway recovery: a MAX705 watchdog — on runaway, WDO falls and a NAND pulse resets the MCU; after reset the firmware pulses WDI via P1.0 to re-arm monitoring.
3. Key technical problems solved
The actuator uses variable-frequency drive below 5.5 kW with three speed modes (multi-turn, linear, quarter-turn) and a closed valve-position loop with overcurrent, overload, over/under-voltage, overheat, phase-loss and stall protections.
· Soft valve seating/opening: the MCU computes output torque from inverter voltage and current; at the set torque it automatically slows down to avoid impact — optimal closing with torque protection;
· End-position detection: instead of loose, low-precision mechanical limit switches, the end of travel is detected from position-signal increments — no change between scans means the limit is reached and the motor is cut immediately;
· Motor protection: an embedded temperature sensor continuously monitors the motor and trips on overheating;
· Accurate positioning: instead of correcting coasting overshoot by reverse jogging, the deceleration point and rate are pre-computed from position error and speed, so fine positioning happens at low speed with minimal overshoot;
· Signal isolation: high common-mode voltage between the DC bus and three-phase output is bridged by an isolation amplifier of LM358 + 4N25.
4. Conclusion
Uniting microcomputer and actuator technology, one intelligent actuator can cover a range of speeds and closing torques; the LCD shows status, opening, parameter setting and fault menus. Integrating measurement, decision and actuation, the design points to the future of electric actuator development.
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