📖 The lesson
Open-loop and closed-loop systems
- An open-loop system acts on its input but never checks the result. A trim actuator that runs only while a switch is held, a heater on a timer, a stepper motor driven without position feedback — none of these knows whether the output is right, so loads, friction, wear and disturbances go uncorrected. Open-loop systems are simple, cheap and stable.
- A closed-loop system measures its output with a sensor and feeds that measurement back to be compared with the input. Any difference — the error — drives a correction, so the system corrects itself. Autopilot servos, cabin temperature and pressure controllers, flap and slat position control, generator voltage regulation and FADEC fuel metering are all closed loops.
The servomechanism and its vocabulary
A servomechanism is a closed-loop control system whose output is a mechanical position (or its rate), and in which a small, low-power command controls a large output through power amplification. Its parts:
- Input (demand, command) — for example a pilot's control input, an autopilot computer command or a selector position.
- Error detector (comparator) — subtracts the feedback from the demand. Error signal = demand − actual position.
- Servo amplifier — raises the error signal's power.
- Servo motor or actuator — an electric motor, or a hydraulic actuator through a servo valve, that moves the load.
- Feedback (follow-up) transducer — measures the actual output: a synchro, potentiometer, LVDT/RVDT or encoder.
| Term | Meaning |
|---|---|
| Feedback | A signal proportional to the output, returned to the input to be compared. Negative feedback opposes the error, which makes the loop self-correcting. |
| Follow-up | The output following the input. Also the feedback element itself (a "follow-up" synchro or potentiometer), which cancels the error as the output reaches the demanded position. |
| Null | The condition of zero error: output equals input, and the motor stops. |
| Overshoot | The output's inertia carries it past the demanded position before it comes back. |
| Damping | Anything that opposes motion in proportion to its speed — friction, viscous dampers, or electrical velocity (rate) feedback from a tachogenerator — and so reduces overshoot and oscillation. |
| Deadband | A small band of error either side of the null in which the system does not respond. It stops the servo chasing tiny errors and jittering, but too wide a band reduces accuracy. |
| Hunting | Continuous oscillation of the output back and forth through the null. Causes: too much gain, too little damping, too small a deadband, or backlash in the gears. |
- Under-damped: fast, but overshoots and oscillates before settling. If damping is far too low it may hunt continuously.
- Critically damped: reaches the demand in the shortest time without overshoot — the design aim, often set slightly under-damped for speed.
- Over-damped: no overshoot but sluggish.
- When the input keeps moving, the output follows with a small lag (velocity error).
Analogue transducers
A transducer converts one form of energy into another — here, a physical quantity into an electrical signal. An analogue transducer gives a signal that varies continuously with the quantity.
- Potentiometer — a resistance track with a moving wiper. The output voltage is proportional to position. It is simple and works on DC, but the sliding contact wears, gets noisy and has finite resolution.
- LVDT (linear variable differential transformer) — a primary coil excited by AC, two secondary coils connected in series opposition, and a soft-iron core attached to the moving part. With the core centred, the two secondary voltages cancel: the null. Moving the core increases one and decreases the other, so the output amplitude shows how far it has moved and the phase (0° or 180° relative to the excitation) shows which way. With no contact it does not wear and has effectively infinite resolution, which is why LVDTs sit inside flight-control actuators, servo valves and engine fuel-metering units.
- RVDT — the rotary version, for limited angles (typically up to about ±40–60°). It is used on control columns, pedals and side-sticks.
- Tachogenerator — a small generator whose output voltage (or frequency) is proportional to speed. It provides speed measurement and the rate feedback that damps servos.
- Others include strain gauges (force and pressure via a bridge), thermocouples and resistance thermometers (temperature), capacitance probes (fuel quantity) and synchros and resolvers (angle — Chapter 9).
Proximity switches
Many aircraft use proximity sensors instead of mechanical microswitches to report the positions of the landing gear, gear doors, passenger and cargo doors, thrust reversers and flaps. Each sensor is a sealed coil, and its target is a steel plate on the moving part. When the target comes within range, the coil's inductance changes, and an electronics unit — a proximity switch electronics unit (PSEU) or landing gear control and interface unit — decides "target near" or "target far". It also monitors each sensor for open or short circuits.
- Advantages: no moving parts or electrical contacts to wear, bounce or corrode; sealed against water, ice and de-icing fluid; built-in fault monitoring.
- Maintenance: the gap between sensor and target must be set to the AMM figure, usually with a feeler gauge. Damaged targets, loose brackets, debris and wiring faults are the usual causes of "sensor disagree" faults. The system's BITE pages show each sensor's near/far state.
Digital tachometers and encoders
- Speed sensors produce a pulse each time a tooth of a toothed wheel (a phonic wheel) passes. A variable-reluctance (magnetic) pickup has a permanent magnet and a coil; a Hall-effect sensor also works down to zero speed. The pulse frequency is proportional to speed. Examples are engine N1/N2 speed probes and the wheel-speed transducers of anti-skid systems.
- A digital tachometer counts those pulses over a fixed time, or measures the time between them, and computes RPM — no analogue meter movement involved.
- An incremental encoder uses a slotted (optical) or magnetised disc to produce pulses as it turns. Counting pulses gives the change in position; the pulse rate gives speed. Two tracks A and B, 90° apart (quadrature), give the direction — whichever leads. An index pulse marks one point per revolution. After a power loss the count is lost, so the system must re-home.
- An absolute encoder reads a unique digital code for every position, so it knows its position immediately at power-up. The code is usually Gray code, in which only one bit changes between adjacent positions. A reading taken on a boundary can then never be badly wrong. In plain binary, 011 to 100 changes three bits at once, and misaligned sensors could briefly read anything. Aircraft altitude encoders (the Gillham code for Mode C transponders) and many position sensors use Gray-type codes.
B2 extension: the PID controller
Most modern servo and process loops are PID controllers. The proportional term drives the output in proportion to the error. The integral term adds up the error over time, so even a small steady error is eventually removed (no steady-state error). The derivative term responds to the rate of change of the error, anticipating and damping overshoot. Tuning the three gains trades speed against overshoot and hunting.