Motion control for drones and UAVs

In short:On a UAV every gram and every watt is paid for in flight time, so size propulsion motors from manufacturer thrust test data at your real hover point, and design gimbals and payload axes around low mass, low cogging and clean feedback. Check magnetic and electrical noise from the start, not after the first flight.

Typical requirements

Drone and UAV motion systems split into three jobs: propulsion (motors and propellers that lift and push the aircraft), stabilisation and pointing (gimbals carrying cameras, lidar, EO/IR sensors and antennas), and payload actuation (release mechanisms, sprayer valves, landing gear, winches, folding arms). All three live inside a weight and power budget that has no slack, on a structure that vibrates and next to electronics that are sensitive to noise.

How to size drone propulsion motors

Multirotor and VTOL propulsion uses outrunner brushless DC motors driven by electronic speed controllers (ESCs). The rotating outer bell carries the magnets, which gives a large air gap radius and high torque for a direct-drive propeller.

The useful numbers are in the manufacturer’s thrust test table, measured with a specific propeller at a specific voltage. Take T-Motor’s MN5212, a 52 mm stator heavy-lift motor rated for 4–8S LiPo. At 24 V with a 17 × 5.8 in carbon propeller, the KV340 version produces:

Efficiency falls steadily as throttle rises. That is the key design lesson: endurance comes from hovering low on the curve, and that means a motor and propeller with plenty of headroom.

Worked example: a 6 kg quadcopter

• All-up weight 6 kg, four motors, so hover thrust is 1.5 kg per motor.
• From the table, 1.5 kg sits between the 55% and 60% rows: roughly 165–185 W per motor at about 9 g/W, so around 0.7 kW total in hover.
• Maximum thrust is 3.7 kg per motor, 14.9 kg total, a thrust-to-weight ratio of about 2.5:1. That passes the common 2:1 rule of thumb with margin for wind and manoeuvring.
• ArduPilot notes that hover throttle is usually between 0.2 and 0.6; this design hovers near the middle of that band.

Choosing the KV420 version on the same propeller would give more thrust (5.36 kg at full throttle) but lower efficiency at every throttle point in T-Motor’s table. Pick the higher KV only if you need the thrust.

Things to tell a motor supplier: propeller diameter and pitch, battery cell count and the voltage under load, hover and maximum thrust per motor, maximum ambient temperature, ingress requirement (rain, dust, salt spray), and target life. T-Motor quotes an MTBF of about 1,000 hours for the MN5212, which is useful context for commercial service intervals.

Gimbals: motors, encoders and slip rings

A stabilised gimbal must hold a line of sight while the airframe vibrates and manoeuvres. That calls for:

• Direct-drive brushless motors. No gearbox, so no backlash. Slotless or ironless designs remove cogging; Aerotech describes its direct-drive AMG gimbals as having zero cogging and no torque ripple. Airborne gimbals often use frameless rotor and stator kits built into the gimbal yoke to save mass and leave a hollow centre for cables. See frameless torque motors and the low-voltage Akribis ADR-F frameless kits.
• High-resolution absolute encoders on each axis, mounted directly on the axis so no compliance sits between sensor and load. Thin ring encoders and capacitive or inductive kits suit the hollow-shaft layout. For reference, Aerotech’s ground-based gimbals reach ±5 to ±30 arcsec accuracy with directly coupled transducers. Compare technologies in absolute vs incremental encoders.
• Slip rings for continuous pan. Continuous azimuth rotation needs a capsule or through-bore slip ring with low drag torque so the motor is not fighting friction. Gold-on-gold miniature capsules designed for airborne cameras, such as Moog’s aerospace miniature capsules, are a common choice; for HD video and gigabit data see Ethernet slip rings, Ethernet through slip rings and Moog’s electro-optic gimbal units. Our gimbal and drone slip ring page covers the details.
• Compact servo drives that close the current loop fast at low voltage. Board-level micro drives such as the Novanta Capitan fit inside the gimbal body; browse servo drives.

Specify each gimbal axis by load inertia, required acceleration, angular range, pointing accuracy, stabilisation bandwidth, and the signals that must cross the axis.

Payload actuators

Payload mechanisms are usually small, light and intermittent: release hooks, parachute deployment, spray valves, sample grabbers, folding arms. Small linear actuators or geared servo motors are common. Decide early whether the mechanism must hold with power off (a self-locking screw, a brake or a mechanical latch), what happens on loss of signal, and whether the actuator must be backdrivable for manual loading on the ground. How to size a linear actuator walks through force, speed and duty.

Weight and power budgets

Count everything that flies: motor, cable, connectors, encoder, slip ring, bracket and drive. A slip ring that adds 50 g to a gimbal also adds thrust demand and lowers endurance. Low-voltage components that run directly from the flight battery remove the mass and losses of a DC-DC converter. Ask suppliers for mass with cables and connectors, not the bare motor figure; T-Motor, for example, lists the MN5212 at 205 g bare and 249 g with leads.

Vibration, environment and EMC

• Vibration. Propeller blade-pass frequencies and motor imbalance excite the frame. Gimbal mounts often include isolators; make sure encoder mounts and slip ring brushes are rated for the spectrum. Defence programmes commonly test to MIL-STD-810H, and airborne equipment to RTCA DO-160.
• Magnetic interference. ArduPilot identifies battery leads, power distribution boards and ESCs as the main DC sources disturbing the compass, and notes the field falls with the cube of distance. Twist power wires, keep loops small and mount the magnetometer away from current paths.
• Conducted and radiated emissions. ESCs and servo drives switch fast; long unshielded motor leads radiate into GNSS and radio links. Specify shielded cables and filtering early. MIL-STD-461H is the reference for many defence platforms.
• Ingress. Rain, dust and salt fog attack open outrunner bearings and windings. State the IP code you need under IEC 60529, and the salt-spray requirement if flying offshore.
• Regulation. In the EU, product requirements for unmanned aircraft in the open category sit under Delegated Regulation (EU) 2019/945. It sets requirements at aircraft level, so confirm what your integrator expects from each component.

Typical pitfalls

1. Choosing a motor from its maximum thrust rather than its efficiency at hover.
2. Using a test table measured at a different voltage or propeller from the one you will fly.
3. Adding a geared motor to a gimbal and then fighting backlash in the stabilisation loop.
4. Running the magnetometer next to the power distribution board.
5. Ignoring slip ring drag torque on a lightweight pan axis.

Send us your weights, propeller, battery and gimbal requirements and our specialist engineering and supply partners can propose motors, feedback, slip rings and drives that fit the budget.

Standards to have in view

• IEC 60529 (IP codes)
• RTCA DO-160 (environmental conditions and test procedures for airborne equipment)
• MIL-STD-810H (environmental engineering and laboratory tests)
• MIL-STD-461H (control of electromagnetic interference)
• Commission Delegated Regulation (EU) 2019/945 (unmanned aircraft systems)

Which standards apply depends on the end product and its market. Your engineer will confirm what the component itself needs to meet.