Articulation Joint Twist Angle

How can I discern the twist angle of an articulation joint?

More information: In my particular system, I try to simulate a 1-DOF joint, so my swing angle is very tightly limited. I tried taking the relative transform between parent and child

PxTransform relT = jp.parent->getGlobalPose().transformInv(jp.child->getGlobalPose());

, but then relT.q.getAngle() necessarily returns a positive number, while the twist angle must sometimes be negative…

Even more info: I need the angle to be sign-correct because I’m trying to implement a torque spring, and I can’t use the built-in setStiffness functionality because that drive/spring is apparently an acceleration spring instead of a force/torque one, and I haven’t found where to switch this flag…