Permanent Magnet Module¶
Updated: 7/9/26
Overview¶
The Permanent Magnet module models the restoring torque from a fixed magnetic dipole in Earth's field. It is the primary alignment element of Passive Magnetic Attitude Control (PMAC), working alongside hysteresis rods that provide damping. The magnet needs no power, sensors, or control logic — it simply applies torque as the spacecraft tumbles.
Each simulation step, the module reads \(\mathbf{B}\) from the WMM, rotates it into the body frame using the current attitude, computes \(\boldsymbol{\tau} = \mathbf{m} \times \mathbf{B}\), and applies that torque to the spacecraft dynamics.
Basilisk implementation¶
The module is a custom DynamicEffector in ExternalModules/permanentMagnet/. The dipole moment \(\mathbf{m}_B\) is set in Python via magDipole_B (body frame, A·m²). It subscribes to the WMM magnetic field message and publishes the computed torque for logging and visualization.
Torque is evaluated inside computeForceTorque at integrator sub-steps, using the continuous attitude state from the hub MRPs rather than the discrete post-step value. That keeps the restoring torque consistent with the dynamics solver throughout each RK4 step.
Math Guide¶
A magnetic dipole in an external field experiences torque:
The WMM provides the field in the inertial frame \(\mathbf{B}_N\). The spacecraft attitude (MRPs) maps to a direction cosine matrix \([BN]\) that rotates into the body frame:
The applied body-frame torque is then:
where \(\mathbf{m}_B\) is the fixed dipole vector in body coordinates. Basilisk collects \(\boldsymbol{\tau}_B\) through the DynamicEffector interface and integrates it into the spacecraft equations of motion.