Motion components for EO/IR gimbals and sensor pods

In short:EO/IR and surveillance gimbals hold a sensor's line of sight steady while the platform moves, so every axis is built around direct-drive, low-cogging frameless motors, axis-mounted high-resolution absolute encoders and, for continuous rotation, a low-drag slip ring that carries HD video and Ethernet. Specify torque, stiffness, sensor noise and data rate from the stabilisation requirement down.

Typical requirements

EO/IR gimbals carry daylight cameras, thermal imagers, laser rangefinders and other sensors on aircraft, UAVs, vehicles, ships, masts and fixed towers. Small UAV gimbals weigh a few hundred grams; airborne sensor pods and ship or vehicle sights are far larger. The motion problem is the same in all of them: keep the line of sight still in inertial space while the platform vibrates, manoeuvres and rolls, and point it accurately on command. That puts extreme demands on smoothness, stiffness and feedback, and much less on raw speed.

Direct-drive frameless motors and low cogging

Most stabilised gimbals use direct-drive brushless torque motors on every axis. Kollmorgen’s EO/IR systems page makes the case: a direct-drive motor attached to the load reduces size, weight, power and response time, and its frameless kits (RBE, TBM, KBM) remove shaft, bearings, endbells and couplings so the motor becomes part of the gimbal structure. Kollmorgen’s TBM2G gimbal notes add that frameless motors can be made with short axial length across a range of diameters, and that a winding temperature sensor is worth fitting at the prototype stage to check heating at peak torque.

How to choose:

• Slotted vs slotless. Slotted frameless motors give the highest torque density; slotless and ironless designs give near-zero cogging for very smooth slow tracking. Ironless housed direct drives such as the Akribis ACD show the zero-cogging end of the spectrum; low-voltage frameless kits such as the Akribis ADR-F suit compact axes.
• Torque sizing. Peak torque = inertia × peak angular acceleration needed to reject the disturbance, plus friction, slip ring and cable drag, and imbalance. Continuous torque is set by the RMS of the disturbance-rejection torque over the mission.
• Diameter and bore. A larger air-gap diameter gives more torque per unit mass and a bigger hollow centre for cables and slip rings.
• Voltage. Match the winding to the platform bus. Moog’s AC7332 gimbal stage, for instance, has a 22 V or 30 V stall voltage with windings available to 96 V or 144 V.

See frameless torque motors and frameless motor sizing.

Slip rings with HD video and Ethernet

Continuous azimuth rotation on a gimbal or pod requires a slip ring that adds as little drag and noise as possible while carrying more and more data. Moog’s electro-optic (EO/IR) slip rings carry up to about two dozen channels at over 400 Mbit/s each, around 12 Gbit/s in aggregate, for GigE, Fibre Channel and IEEE 1394, with slip ring and fibre optic rotary joint hybrids where fibre is needed. They are sealed, EMI-shielded and built for high shock and vibration.

For smaller payloads, the Moog AC7332 direct-drive gimbal stage shows a typical integrated slip ring: 17 signal rings plus 2 power rings (20 A peak, 2 A continuous), 50 V DC maximum, rated to 50,000 ft altitude, with twisted-pair leads for gigabit Ethernet. Miniature gold-on-gold capsules such as Moog’s AC264 family carry 20–60 low-current circuits for UAV gimbals.

When specifying a UAV gimbal slip ring or pod slip ring, state each data protocol and rate, video format (HD-SDI, Ethernet video, analogue), impedance, power per ring, speed, drag torque limit, altitude and temperature. Read Ethernet through slip rings, Ethernet slip rings, fibre optic rotary joints and gimbal and drone slip rings.

High-resolution encoders

Each axis needs absolute position feedback mounted directly on the axis. Kollmorgen recommends a 20-bit or higher encoder, or a resolver, for closed-loop control of its frameless gimbal motors, with Hall tracks or sensors for commutation. The AC7332 uses a 26-bit optical absolute encoder on BiSS-C and quotes 0.01° static pointing accuracy at 25 °C; its encoder is screened to −54 °C and heated briefly before power-up below −40 °C. Capacitive and inductive kits are thinner and tolerate contamination: Netzer’s VLT covers −55 °C to +125 °C, and inductive Mini Ultra IncOder units suit small hollow-shaft axes. Compare in absolute vs incremental encoders.

Line-of-sight stabilisation at component level

The stabilisation loop uses an inertial rate sensor (gyro) to measure platform motion and drives the motors to cancel it; the encoder handles commutation, position reporting and pointing. Published figures give a sense of range: General Dynamics OTS quotes under 2 mrad RMS stabilisation for its Vector 20 maritime pan-tilt with 0.01–60 °/s slew; Allied Motion describes a UAV gimbal with 200 °/s slew and continuous pan.

Component requirements that follow:

• Stiff structure and direct coupling. The first resonance must sit well above loop bandwidth.
• Low torque ripple and cogging.
• Fast current loop. Compact drives that close the current loop at high frequency, such as the Novanta Capitan, fit inside the gimbal; see servo drives.
• Consistent, low drag from slip rings, cables and seals across temperature.
• Low-latency, low-noise feedback.

Environment and EMC

• Temperature and altitude. The AC7332 example operates from −54 °C to +71 °C at up to 95% humidity. Airborne pods add low pressure (MIL-STD-810H Method 500) and rapid temperature change (503).
• Vibration. Airframe and vehicle vibration is the disturbance the gimbal exists to reject, and it also stresses bearings, brushes and encoder mounts. Test to the tailored MIL-STD-810H Method 514 profile or DO-160 for airborne equipment.
• Ingress. Vehicle and naval gimbals need IP67-class sealing, salt fog (Method 509) and sand and dust (510) resistance.
• EMC. MIL-STD-461 or DEF STAN 59-411. Sensitive imaging electronics sit centimetres from PWM motor drives; shield phase leads and separate power and signal rings in the slip ring.
• Export control. EO/IR gimbals and their specially designed components are often controlled; see the defence and security overview.

Typical pitfalls

1. Using a geared motor on a stabilised axis and fighting backlash.
2. Specifying encoder bits rather than accuracy and noise.
3. Ignoring slip ring drag change at low temperature.
4. Routing unshielded motor leads past the sensor electronics.
5. Not budgeting bandwidth growth in the slip ring for the next sensor upgrade.
6. Forgetting cable wrap stiffness on limited-travel inner axes, which adds a position-dependent torque the loop must fight.

For component-level platforms, see also military drones and UAVs and counter-UAS systems. Send your axis inertias, stabilisation target and data requirements, and our specialist engineering and supply partners can propose 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)
• RTCA DO-160 (environmental conditions and test procedures for airborne equipment)
• 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.