1. What does a pneumatic valve positioner do, and on what principle does it work?
Where actuators work in poor conditions but demand high quality, the pneumatic positioner is used with the pneumatic actuator to form a closed loop, using negative feedback to improve accuracy, sensitivity and stability so the valve sets its opening precisely to the control signal. The positioner is a pressure-to-displacement feedback system working on the displacement-balance principle.
2. Will a quarter-turn electric actuator's motor burn out when stalled?
In an ordinary motor, stall current equals starting current, and prolonged stalling burns the windings. Actuator motors reduce starting current by larger rotor resistance — giving both starting torque and limited temperature rise under long stall — plus thermal protection, so stalling does not burn an actuator motor.
3. What is the role of the phase-splitting capacitor CD in a DKJ actuator, and what happens when it fails?
CD shifts the AC voltage on one stator winding 90° from the other, producing a rotating magnetic field, starting torque and rotation; direction depends on which winding CD is in series with. So CD both creates starting torque and sets rotation direction. Open-circuit CD: no rotating field, no starting torque, motor will not start. Shorted CD: both windings see in-phase current, the motor electrical-brakes and will not turn.
4. How to distinguish actuator hunting from control-loop oscillation?
Actuator hunting is a small-loop oscillation; control-loop oscillation is the large loop. Phenomenally: when the actuator itself hunts, the position indicator needle fluctuates while the controlled variable and controller output barely change; in loop oscillation, the controller output oscillates regularly with the controlled variable besides the actuator's own motion. Characteristically: actuator hunting has higher frequency, shorter period; loop oscillation has lower frequency, longer period, with controlled variable, controller output and actuator all oscillating at nearly the same frequency.
5. How to adjust the servo amplifier's insensitivity band?
Commonly by varying negative-feedback depth to change amplifier gain. First adjust stabilising potentiometer W2 in the magnetic amplifier: when the actuator hunts, widen the band. If hunting persists, reduce feedback-loop resistance for deeper negative feedback; or reduce offset resistance to raise offset current and deepen magnetic-core saturation; or adjust the output resistors to lower the effective trigger input voltage.
6. Advantages and disadvantages of pneumatic instruments?
Advantages: reliable, fire- and explosion-safe, easy to maintain, inexpensive, strongly immune to external interference. Disadvantages: slow transmission, short distance, complex air-supply plant, inconvenient connection to industrial computers.
7. What is "triple-loss self-locking" protection on electro-pneumatic long-stroke actuators?
When supply air, power or signal is lost, the output-arm angle stays where it was: the air paths to both cylinder chambers are cut, the piston cannot move — protection.
8. What are the parts of an electric actuator and their functions?
Mainly a servo amplifier and the actuator itself. The electric actuator is the execution unit of the electric modular instrument family, a position-servo mechanism powered by a servo motor. It receives 0–10 mA or 4–20 mA DC from the controller and converts it linearly into 0–90° rotation or linear displacement to drive dampers, louvres and valves for automatic control.
9. Outline trial-and-error PID tuning.
(1) P first, then I, finally D. (2) Start with parameters at least-impact positions: P max, I max, D min. (3) Tune proportional band as a pure P loop for a good response curve, then widen it ~1.2× and decrease integral time from large to small for a better curve. (4) Re-adjust proportional band at this integral time and see if the curve improves. (5) If so, reduce the band and change integral time again; iterate to a suitable pair. (6) If stability under disturbance is poor, increase both somewhat for adequate stability. (7) Slightly reduce both and add derivative for minimal overshoot and shortest settling.
10. Describe the calibration steps for an electric actuator.
(1) With wiring verified, power up and set the motor end-cover switch to MANUAL; wind the handwheel until the output shaft is at zero — position transmitter output should be 4 mA. (2) Wind the shaft through 90° — transmitter current should equal 20 mA; otherwise adjust the span potentiometer. Shaft angle must be proportional to transmitter current within tolerance. (3) Use a double-pole switch to reverse the input signal polarity, rotating the shaft both ways: motion must be smooth and transmitter current track correctly.
11. How to operate actuators manually on site?
(1) Electric: move the motor handle to MANUAL, pull out the handwheel and wind. (2) Pneumatic: set the balance valve on the control box to MANUAL to connect the two cylinder chambers. Actuators without a handwheel have a hex on the support-shaft end for a special wrench.
12. What is the mechanical adjustment of a quarter-turn actuator? If the mechanism travels full close to full open while the actuator arm turns more or less than 90°, how to correct?
(1) Mechanical adjustment mainly sets the mechanism's crank-arm length so that 90° of actuator-arm rotation (or full piston stroke) produces the mechanism's full travel. (2) If the arm turns more than 90°, move the crank pin-hole inward to shorten the arm; if less than 90°, move it outward to lengthen the arm.
13. How to choose the mounting position of a quarter-turn actuator?
(1) Near the driven mechanism, convenient for operation and maintenance without obstructing passage; (2) link rods not too long; (3) actuator and mechanism cranks must move in one plane, otherwise fit a reversal gear; (4) after installation, handwheel clockwise should close the mechanism — otherwise mark the direction beside the handwheel; (5) the relative positions must remain unchanged under thermal movement of the main equipment.
14. Why does DDZ instrumentation use DC milliampere current as the standard signal?
1) DC is immune to AC induction; 2) unaffected by line inductance, capacitance and load nature; 3) convenient with data-loggers, data-processing and control computers; 4) reference voltages are easy to derive; 5) current and magnetic field readily produce mechanical force; 6) line resistance variation within limits does not change current — good for long-distance transmission; 7) where voltage input is needed, current converts to voltage; 8) one signal can drive several actuators simultaneously and identically.
