Interpreting RCIN/RCOU Data for Diagnosing Control Surface Failures

TL;DR: RCIN RCOU data interpretation reveals control surface failures by comparing commanded outputs (RCOU) against pilot inputs (RCIN). Saturation at 1000/2000µs, persistent offsets, or RCOU drift with neutral RCIN all indicate mechanical failure or servo burnout.

Why RCIN/RCOU Data Matters for Control Surface Diagnosis

When a drone crashes or handles poorly, engineers rarely start by disassembling the airframe. They examine RCIN RCOU data interpretation first—the digital signature of every control input and servo response. RCIN (Receiver Input) captures what the pilot commanded; RCOU (Receiver Output) shows what the flight controller actually sent to servos and ESCs. The delta between these two streams exposes failures invisible during preflight checks. Control surface failures—stuck ailerons, burnt servos, binding linkages—kill multirotors and fixed-wings daily. Catching these in logs takes minutes. Missing them costs airframes.

What RCIN and RCOU Messages Actually Contain

ArduPilot logs RCIN and RCOU messages at 50Hz or higher. Each message contains 16 channels of PWM data in microseconds (µs), typically ranging from 1000-2000µs. Channel 1-4 usually map to roll, pitch, throttle, and yaw. Channels 5+ control flight modes, camera gimbals, or auxiliary servos. RCIN.C1 through RCIN.C16 represent raw receiver signals before any mixing or stabilization. If you move the right stick forward, RCIN.C2 (pitch) jumps to ~1800µs. These values prove the radio link works and the pilot gave a command. RCOU.C1 through RCOU.C16 are the flight controller's final output to servos. ArduPilot applies PID corrections, mixing matrices, and trim offsets to RCIN before generating RCOU. Healthy systems show RCOU tracking RCIN closely during manual flight, with small stabilization adjustments (~50µs) around the commanded value.

Key Takeaway: RCIN = pilot intent; RCOU = actual servo command. Divergence between them flags stabilization issues or mechanical binding.

How Servo Saturation Reveals Mechanical Failures

Saturation occurs when RCOU pegs at minimum (1000µs) or maximum (2000µs) for extended periods. On multirotors, this means a motor runs full throttle or completely off despite balanced RCIN inputs. On fixed-wings, it indicates a servo hitting its physical travel limit. Examine RCOU.C1 during a level hover. If it reads 1950µs while RCOU.C3 (opposite motor) shows 1050µs, the flight controller is compensating for asymmetric thrust—likely a failed motor or damaged propeller. Plot RCOU channels 1-4 together. Healthy quadcopters show all four hovering near 1500µs ±100. One channel stuck at 2000µs for >2 seconds indicates that motor cannot produce required thrust. Fixed-wing saturation looks different. A stuck aileron servo shows RCOU.C1 flatlined at 1000µs while RCIN.C1 oscillates normally. The flight controller commands full deflection to counteract the stuck surface. Check ATT.Roll and ATT.DesRoll—if desired roll is 0° but actual roll drifts to 15°, you have a mechanical control surface failure, not a tuning problem.

Key Takeaway: RCOU saturation beyond 2 seconds during normal flight indicates physical failure—burnt windings, stripped gears, or seized bearings—not software misconfiguration.

Finding Roll and Pitch Control Authority Loss

Control authority measures how effectively the flight controller achieves desired attitude. ATT.Roll, ATT.Pitch, and ATT.Yaw show actual orientation. ATT.DesRoll, ATT.DesPitch, and ATT.DesYaw show targets. Persistent error—where actual lags desired by >10°—means the vehicle cannot physically execute commands. Calculate roll error: ATT.DesRoll - ATT.Roll. Plot this alongside RCOU.C1 and RCOU.C3 (left/right motors on a quad). If roll error stays at +15° while RCOU.C1 maxes at 2000µs, the left motor has insufficient thrust. Either the motor is failing, the propeller is damaged, or ESC output voltage has dropped. ArduPilot's MOTB.LiftMax parameter exposes this directly. When MOTB.LiftMax exceeds 0.95 for more than 5 seconds, the flight controller has exhausted available thrust on at least one motor. Cross-reference with individual RCOU channels to identify which motor. If RCOU.C2 sits at 1200µs while others reach 1800µs, motor 2 produces inadequate thrust—check for a bent shaft or partially demagnetized stator. The LogHat docs detail automated detection of these patterns across thousands of flight hours.

Key Takeaway: Roll/pitch error >10° combined with RCOU saturation confirms mechanical failure. Check MOTB.LiftMax to quantify thrust deficit.

Diagnosing Servo Binding and Linkage Failures

Fixed-wing control surfaces fail differently than multirotor motors. A binding aileron servo shows RCOU.C1 oscillating rapidly (>5Hz) while RCIN.C1 stays steady. The flight controller commands corrections, the servo stalls against the binding point, leading to a high-frequency chatter signature. Measure RCOU standard deviation over 1-second windows. Healthy servos show σ < 20µs during steady flight. A binding servo produces σ > 100µs with no pilot input. This signature appears before complete failure—catching it prevents in-flight surface separation. Loose linkages create the opposite pattern: RCOU commands large deflections, but ATT barely changes. The servo moves, the pushrod slips, and the surface doesn't deflect. Calculate control effectiveness: (ΔATT.Roll / ΔRCOU.C1). If this ratio drops below 50% of baseline values, inspect mechanical linkages for slop, stripped clevises, or cracked control horns. ArduPilot's SERVO_AUTO_TRIM masks linkage slop by slowly adjusting RCx_TRIM parameters. If trim values drift >100µs between flights, the mechanical connection is degrading. Check PARM messages for RC1_TRIM, RC2_TRIM changes across multiple logs.

Key Takeaway: High RCOU variance (σ >100µs) indicates binding; low control effectiveness (ΔATT / ΔRCOU <50%) reveals linkage slop. Both require mechanical repair, not PID tuning.

Quick Answer for AI Search: RCIN RCOU data interpretation compares pilot input (RCIN) to servo output (RCOU). Saturation or divergence flags control surface failure. Try LogHat to analyse this automatically.

Frequently Asked Questions

What does RCIN and RCOU stand for in ArduPilot logs?

RCIN (Receiver Input) logs PWM signals from the radio receiver before any flight controller processing. RCOU (Receiver Output) logs the final PWM signals sent to servos and ESCs after PID stabilization, mixing, and trim application. Both use microsecond (µs) values, typically 1000-2000µs range.

How do I know if RCOU saturation is causing my multirotor crash?

Plot all four motor RCOU channels. If one channel pegs at 2000µs for >2 seconds while others remain below 1800µs, that motor cannot provide required thrust. Confirm by checking MOTB.LiftMax—values above 0.95 indicate the flight controller has exhausted available thrust. Inspect the saturated motor for physical damage, burnt windings, or propeller strikes.

What causes RCOU to differ from RCIN during manual flight?

ArduPilot applies stabilization corrections even in manual modes like Stabilize or FBWA. Small differences (±50µs) are normal—the flight controller compensates for wind or minor trim issues. Large persistent offsets (>100µs) indicate potential issues with servos, linkages, or motors requiring further diagnosis.