Synchros and resolvers
Small electrical machines for angular position transmission. CG/CX/CT/CDX three-letter codes. Classic CX→CT servo loop. Resolver = 2-φ variant with V·sinθ + V·cosθ outputs. Brushless via rotary transformer. Aerospace + military legacy.
Step 1 — Synchros: small electrical machines for angular position
Reference notes
Synchros and resolvers are small electrical machines that transmit angular position information electrically — workhorses of WWII-1970s servo systems, still in service for aerospace legacy and extreme environments. Use Next → to walk through synchro construction, the three-letter type codes (CG / CX / CT / CDX), the classic synchro servo loop, resolvers as 2-phase variants, brushless designs, and modern displacement by optical encoders.
Synchro basics
- Looks like a tiny single-phase induction motor.
- Rotor: single-phase winding excited from AC reference (typically 26 V or 115 V at 60 or 400 Hz).
- Stator: three windings spaced 120° apart.
- Excited rotor induces voltages in stator windings whose amplitudes depend on rotor-stator angle.
- Connect two synchros stator-to-stator with shared rotor excitation → transmitter-receiver pair; rotor of receiver tracks rotor of transmitter.
Three-letter type codes
| Code | Type | Function |
|---|---|---|
| CG | Control Generator | Transmitter (older designation) |
| CX | Control Transmitter | Transmitter (modern designation) |
| CT | Control Transformer | Error detector in servo loop — output ∝ sin(angle difference) |
| CR | Control Receiver | Repeats transmitter angle |
| RX / TR | Torque Receiver | Delivers small torque to drive needle / dial |
| CDX / TDX | Control Differential | Combines two angle inputs algebraically |
Classic synchro servo loop
- Operator turns CX rotor (φ_in).
- CX stator outputs 3-φ AC voltages encoding φ_in, sent over 3-wire bus.
- CT stator receives the bus. CT rotor (coupled to load shaft φ_out) outputs single-phase voltage = V · sin(φ_in − φ_out) — the error signal.
- Servo amplifier processes the error, drives a servo motor turning the load.
- As load reaches commanded position, error → 0, motor stops.
Standard servo architecture 1940s-1970s; covered in classical servo-theory texts (Truxal, Ogata). Accuracy: 0.5°-few° typically.
Resolvers — 2-phase synchros
- Same construction as synchro but with TWO stator windings 90° apart instead of three at 120°.
- Outputs: V · sin(θ) and V · cos(θ), AM-modulated onto AC reference.
- Resolver-to-digital converter (RDC) computes θ = atan2(V_sin, V_cos).
- Very robust — just iron + copper, no electronics inside, withstands high temperature, vibration, dirt, radiation.
- Modern integrated RDC chips: AD2S1210, AD2S1205, LTC2348 series — 10-16 bit digital position output via SPI / SSI / parallel.
Brushless construction
Classical synchros and resolvers used carbon brushes on slip rings to bring AC reference into the rotor — limited life, dirt sensitivity. Brushless designs use an integrated rotary transformer in the same housing: stationary primary winding, secondary on the rotor, magnetically coupled across the air gap. Same shaft, same housing. Result: completely sealed, no brush wear, indefinite life. Standard for aerospace and military since the 1970s.
Modern displacement
- Optical encoders — incremental or absolute, 2-8 million counts per revolution, dominant in industrial servo motors.
- Magnetic encoders — robust to dirt, ~10000 counts per revolution.
- Serial digital protocols — BiSS-C, SSI, EnDat, Hiperface DSL — single twisted pair carries position + diagnostics + temperature with kHz update and error checking.
Where synchros / resolvers persist
- Aerospace legacy — military aircraft, helicopter swashplates, naval gun direction, missile guidance. Long service lives (decades) mean original sensors continue in use.
- Extreme environments — jet engine bearings (700 °C+), gas turbine sensors, high-vibration / high-radiation industrial sites where optical encoders fail.
- Defense / nuclear regulatory specs — sometimes mandate proven technology with documented MTBF.
- Indian PSU / defense recruitment exams — DRDO, ISRO, NPCIL, IAF, IN technical entrance all include synchro / resolver questions.