Motion components for counter-UAS sensor platforms

In short:Counter-UAS sensor platforms need positioners that slew fast to a cue and then track a small, agile target smoothly, often on a vehicle or mast. That means direct-drive or backlash-free axes, high peak torque, absolute encoders, and slip rings with enough bandwidth for radar, RF and EO/IR sensor data. Specify speed, acceleration and tracking smoothness together, not top speed alone.

Typical requirements

Counter-UAS systems detect, track and identify small uncrewed aircraft, then hand them to an effector. This page deals only with the motion platforms for the sensors and effector mounts, at component level: the pan-tilt positioners, rotating radar and RF sensor heads, slip rings, encoders and drives. It does not cover how drones are defeated.

The motion requirement is unusual. A typical sequence is a cue from a wide-area radar or RF detector, a fast slew of an EO/IR or narrow-beam sensor onto that bearing, then smooth tracking of a small target that can change direction quickly. The platform may be a fixed tower, a mast, a vehicle or a vessel. So the axes need high acceleration, smooth low-speed tracking, stiffness, continuous rotation and robust data transfer, often all on a 24 or 28 V supply.

Fast-slewing pan-tilt positioners

Published positioners show how wide the envelope is:

Sources: DAT-CON via Unmanned Systems Technology, Quickset TAURUS R datasheet, Moog AC7332 datasheet.

The trade-off is plain in the table: the fastest units carry lighter payloads. When specifying, start from the time allowed to reach a cued bearing, then derive acceleration and peak speed, then check that the payload’s inertia allows it.

Worked sizing example

A sensor head with 0.5 kg·m² of inertia about the pan axis must slew 90° in 0.5 s from rest to rest, using a symmetric accelerate-decelerate profile.

• Half the move (45°, 0.785 rad) takes 0.25 s, so acceleration = 2 × 0.785 / 0.25² ≈ 25 rad/s².
• Peak speed = 25 × 0.25 ≈ 6.3 rad/s, or about 360 °/s.
• Acceleration torque = 0.5 × 25 ≈ 12.5 Nm, before friction, slip ring drag, cable drag, wind and imbalance.
• Add margin (a rule of thumb is 30–50% for these unknowns) and you are at roughly 16–19 Nm peak, which points to a direct-drive frameless motor or a stiff, low-ratio, backlash-free gear stage.

Repeat for tilt, where gravity imbalance usually adds a steady torque.

Direct drives, gearing and motors

For the fastest and smoothest axes, direct-drive frameless torque motors remove backlash and keep stiffness high; Moog’s AC7332 uses a direct-drive 32-pole brushless motor. Where payloads are heavy and torque would make a direct drive too large, use preloaded zero-backlash gearing such as strain-wave sets; see harmonic vs planetary vs cycloidal and frameless motor sizing. Low-cogging motors help slow tracking, and a holding brake keeps the head parked for transport.

Radar, RF and EO/IR sensor heads

Rotating surveillance radars for drone detection use the same building blocks as larger radars: continuous azimuth rotation, an integrated slip ring, RF or fibre optic rotary joint and encoder. SPINNER notes that fibre optic rotary joints reach several Gbit/s and can be combined with slip rings and RF joints. Our radar pedestal page covers the joint stack. RF direction-finding heads and EO/IR turrets on the same mast often share a positioner, which drives up slip ring channel count. For EO/IR head design see the EO/IR gimbal page.

Slip rings for counter-UAS heads

Continuous pan is normal, so every head needs a slip ring carrying power, control and sensor data. For a gimbal mounted on a UAV or small head, low-torque miniature capsules such as Moog’s miniature gold capsules suit low-power circuits; for HD and thermal video plus Ethernet, use Ethernet slip rings or the higher-bandwidth EO/IR slip rings, which Moog rates at up to about two dozen channels over 400 Mbit/s each. Add a fibre optic rotary joint when copper runs out of bandwidth. Read Ethernet through slip rings and CCTV and pan-tilt slip rings.

Encoders and drives

Absolute encoders on each axis give position at power-up and support pointing to a cued bearing. They must handle the axis’s top speed and the platform’s shock and vibration; Netzer’s VLP, for instance, is rated to 100 g for 6 ms and 20–2,000 Hz vibration. See absolute vs incremental encoders.

The servo drive needs enough peak current for the acceleration above, a fast current loop for smooth tracking, and a network interface the system controller can command at high update rate. A motion controller or the sensor system computer closes the tracking loop.

Fixed, mast, vehicle and vessel mounting

The mounting platform changes the specification. A fixed tower gives a stable base, so the axes only need to follow the target. A telescopic mast adds sway in wind, and a vehicle or vessel adds pitch, roll and vibration that the axes must reject, which pushes the design towards the stabilised-gimbal approach: gyro feedback, stiffer structure and more continuous torque. Mobile heads also see transport shock and must park and lock for movement. State the mounting case up front, because it can double the torque and bandwidth required.

Environment, power and EMC

• Power. Positioners often run from 24 V DC; on military vehicles, design for MIL-STD-1275E or DEF STAN 61-5 Part 6. Fast slews draw high peak current.
• Environment. Outdoor heads need IP67-class sealing, wide temperature ratings, and vibration and shock testing to the tailored MIL-STD-810H profile; Quickset’s example cites Methods 514.8 and 516.8.
• EMC. Counter-UAS sites combine radar, RF receivers and jammers next to motor drives. Drive emissions can desensitise RF detectors, so MIL-STD-461 or DEF STAN 59-411 limits and good shielding are essential.
• Export control. Counter-UAS equipment and its specially designed components are likely to be controlled; see the defence and security overview.

Typical pitfalls

1. Specifying maximum slew rate without acceleration.
2. Choosing geared axes with backlash for a tracking task.
3. Running out of slip ring bandwidth when a sensor is added.
4. Ignoring drive emissions next to RF detectors.
5. Forgetting tilt imbalance when payloads are changed in service.

Send your payloads, slew and tracking targets and data channels, and our specialist engineering and supply partners can propose positioner motors, slip rings, encoders and drives.

Standards to have in view

• MIL-STD-810H (environmental engineering and laboratory tests)
• MIL-STD-461H (control of electromagnetic interference; supersedes MIL-STD-461G)
• MIL-STD-1275E (28 V DC input power to utilisation equipment in military vehicles)
• DEF STAN 00-35 (environmental handbook for defence materiel)
• DEF STAN 59-411 (electromagnetic compatibility)
• IEC 60529 (IP codes)

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