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
- 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
Servomechanisms concept map
Servomechanisms
Servomechanisms quiz
Servomechanisms, quiz
1. A servomechanism is a control system that:
Uses feedback to make an output follow a commandRuns open-loop with no feedbackHas no moving parts2. Too little damping in a servo loop causes:
Hunting, an oscillation about the demanded positionA slow, sluggish responseNo movement at all3. In a servo loop, the error signal is produced by:
Comparing the feedback with the commandThe power supplyThe output load4. Synchro systems are excited with:
ACDCPulsed DC5. An open-loop control system is one that:
Has no feedback pathCompares output with demandAlways includes damping6. In a closed-loop system, the error signal is:
The difference between demand and actual outputThe total system gainThe supply voltage7. Continuous oscillation of a servo about its null point is called:
HuntingDriftBacklash8. An LVDT is used to measure:
Linear positionAngular position onlyTemperature9. Hunting in a servo system is normally cured by:
Applying dampingIncreasing the loop gainRemoving the feedback path10. The typical excitation frequency of an aircraft synchro system is:
400 Hz50 Hz0 Hz (DC)