Servomechanisms
Open and closed loop systems, feedback, follow-up and analogue transducers; principles of operation and use of synchro, resolver, differential, LVDT and E- and I-type transducers; servomechanism terminology, hunting and damping.
Servomechanisms — summary notes
Free- An open-loop system has no feedback — the output is not measured, so errors are not corrected.
- A closed-loop system feeds the output back and compares it with the demand. The difference is the error signal, which drives the system until the error is nulled.
- Key terms: feedback, follow-up, error signal, null, and analogue transducer.
- Synchros and resolvers transmit shaft angle electrically. They are excited with AC, typically 400 Hz. A transmitter (CX) sends the angle to a receiver or control transformer.
- An LVDT (linear variable differential transformer) uses inductive coupling to sense LINEAR position.
- Hunting is continuous oscillation about the null point; damping is applied to cure it.
- ⚠ Exam trap: synchros need AC excitation — applying DC would burn out the rotor winding. Open-loop systems have no feedback path at all.
A flap position indicating system uses a synchro transmitter. Why must it be supplied with AC?
A synchro works by transformer action between rotor and stator windings, which requires a continuously changing flux. DC would produce no transformer action and would burn out the rotor winding through unrestricted current flow.
A control surface actuator oscillates continuously about the commanded position. Name the fault and its cure.
This is hunting — the loop overshoots the null and reverses repeatedly. It is cured by adding damping (or reducing loop gain) so the response settles rather than oscillates.
Closed-loop servomechanism
FreeServomechanisms concept map
FreeServomechanisms
Servomechanisms quiz
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