Gimbal

SWITCH TO API

Description

The Gimbal is a single-axis pointing mount that rotates an attached payload about its local Up-vector (yaw). It inherits from Motor Electrical Controlled, so when connected to a Power Bus it draws current from the EPS using the motor parameters. When not connected, it runs in standalone mode at the commanded slew rate. Gimbals can be stacked for multi-axis pointing and can be placed on spacecraft or Ground Stations. The component does not apply reaction torques or mass-property changes to the parent body; for physics-accurate coupling, use the Hinged Rigid Body.

---

Example Use Cases

• Antenna Pointing: Slew an antenna to maintain line of sight to a ground target or relay.
• Camera Mount: Stack two or three gimbals for multi-axis imaging without slewing the spacecraft.
• Ground Station Tracking: Point a dish or sensor on a ground station toward a spacecraft pass.

---

Module Implementation

Each update, the gimbal optionally reads a command message, clamps the target, drives the motor toward the target, integrates the angle, and applies a yaw to the local transform when the angle change exceeds the step threshold.

Command and Limits

If a command message is connected, the target angle $\varphi$, desired slew rate ${\omega }^{\ast }$, and lock flag are taken from that message. The target is then clamped to the configured limits:

$$\varphi =\min (\max (\varphi ,{\theta }_{\min }),{\theta }_{\max })$$

where ${\theta }_{\min }$ and ${\theta }_{\max }$ are the minimum and maximum allowed angles in degrees.

Position Error and Speed Command

The position error is:

$$e=\varphi -\theta$$

where $\theta$ is the current gimbal angle in degrees. A deadband $\delta$ is derived from the step angle $\delta \theta$:

$$\delta =\max (\delta \theta -10^{-3},10^{-3})$$

If the gimbal is locked or $|e|\le \delta$, the motor speed command is set to zero. Otherwise:

$${\omega }_{\mathrm{cmd}}=\mathrm{sgn}(e)|{\omega }^{\ast }|$$

where ${\omega }^{\ast }$ is the desired slew rate in rad/s. The motor and optional EPS solve then run through the base electrical motor model.

Effective Speed

The angle is integrated using an effective motor speed ${\omega }_{\mathrm{eff}}$ [rad/s]:

• With Power Bus: If the circuit is open or armature current is near zero, ${\omega }_{\mathrm{eff}}=0$. Otherwise the commanded back-EMF speed is used, but capped by available terminal voltage when $K_e>0$:

$${\omega }_{\max }=\frac{V}{K_e}$$

If $V<K_e|{\omega }_{\mathrm{cmd}}|$, then ${\omega }_{\mathrm{eff}}=\mathrm{sgn}({\omega }_{\mathrm{cmd}}){\omega }_{\max }$; otherwise ${\omega }_{\mathrm{eff}}={\omega }_{\mathrm{cmd}}$.

• Standalone: ${\omega }_{\mathrm{eff}}$ equals the commanded back-EMF speed from the motor model.

Angle Integration

When unlocked and outside the deadband, the angle is updated as:

$$\Delta \theta ={\omega }_{\mathrm{eff}}\cdot \frac{180}{\pi }\cdot \Delta t$$ $$\theta \leftarrow \theta +\Delta \theta$$

If the step overshoots the target, $\theta$ is snapped to $\varphi$. The angle is then clamped to $[{\theta }_{\min },{\theta }_{\max }]$. The limited flag is set when $\theta$ is within the deadband of either limit.

Reported velocity and shaft torque are taken from the effective speed and the motor shaft torque, respectively.

Spatial Transform

A yaw about the local Up-axis is applied only when the change from the previous applied angle meets the step threshold:

$$|\theta -{\theta }_{\mathrm{prev}}|\ge \delta$$

This models angle quantization: the internal angle tracks continuously (subject to limits and snap), while the transform updates in discrete steps of at least $\delta$.

---

Assumptions/Limitations

• Rotation is about the local Up-axis only; multi-axis pointing requires stacked gimbals.
• Reaction torques and mass-property rates are not applied to the spacecraft; large moving masses may introduce small numerical inconsistency while the gimbal is moving.
• With a Power Bus, motion requires closed-circuit power and sufficient terminal voltage relative to back-EMF; otherwise the gimbal does not move.
• The step angle deadband can produce visible quantization (“jitter”) of the visual transform if $\delta \theta$ is large.