fix: avoid perpetual movement when the target ang motor position is overshot

examples2d/joint_motor_position2.rs

@@ -22,20 +22,52 @@ pub fn init_world(testbed: &mut Testbed) {
     colliders.insert_with_parent(collider, ground_handle, &mut bodies);
 
     /*
-     * A rectangle on a motor
+     * A rectangle on a motor with target position.
      */
     for num in 0..9 {
         let x_pos = -6.0 + 1.5 * num as f32;
-        let rigid_body = RigidBodyBuilder::dynamic().translation(vector![x_pos, 2.0]);
+        let rigid_body = RigidBodyBuilder::dynamic()
+            .translation(vector![x_pos, 2.0])
+            .rotation(std::f32::consts::PI)
+            .can_sleep(false);
         let handle = bodies.insert(rigid_body);
         let collider = ColliderBuilder::cuboid(0.1, 0.5);
         colliders.insert_with_parent(collider, handle, &mut bodies);
 
         let joint = RevoluteJointBuilder::new()
-            .local_anchor2(point![x_pos, 1.5])
+            .local_anchor1(point![x_pos, 1.5])
-            .local_anchor1(point![0.0, -0.5])
+            .local_anchor2(point![0.0, -0.5])
-            .motor_position(std::f32::consts::PI / 4.0 * num as f32, 100.0, 15.0);
+            .motor_position(
-        impulse_joints.insert(handle, ground_handle, joint, true);
+                (std::f32::consts::PI - std::f32::consts::PI / 4.0 * num as f32),
+                1000.0,
+                150.0,
+            );
+        impulse_joints.insert(ground_handle, handle, joint, true);
+    }
+
+    /*
+     * A rectangle on a motor with limits.
+     */
+    for num in 2..3 {
+        let x_pos = -6.0 + 1.5 * num as f32;
+        let rigid_body = RigidBodyBuilder::dynamic()
+            .translation(vector![x_pos, 5.0])
+            .angvel(4.0)
+            .can_sleep(false);
+        let handle = bodies.insert(rigid_body);
+        let collider = ColliderBuilder::cuboid(0.1, 0.5);
+        colliders.insert_with_parent(collider, handle, &mut bodies);
+
+        let joint = RevoluteJointBuilder::new()
+            .local_anchor1(point![x_pos, 5.0])
+            .local_anchor2(point![0.0, -0.5])
+            .motor_velocity(1.5, 30.0)
+            .motor_max_force(100.0)
+            .limits([
+                -std::f32::consts::PI,
+                std::f32::consts::PI / 4.0 * num as f32,
+            ]);
+        impulse_joints.insert(ground_handle, handle, joint, true);
     }
 
     /*

examples3d/all_examples3.rs

@@ -35,6 +35,7 @@ mod joints3;
 // mod joints3;
 mod character_controller3;
 mod debug_internal_edges3;
+mod joint_motor_position3;
 mod keva3;
 mod locked_rotations3;
 mod newton_cradle3;
@@ -97,6 +98,7 @@ pub fn main() {
         ("Domino", domino3::init_world),
         ("Heightfield", heightfield3::init_world),
         ("Impulse Joints", joints3::init_world_with_joints),
+        ("Joint Motor Position", joint_motor_position3::init_world),
         ("Locked rotations", locked_rotations3::init_world),
         ("One-way platforms", one_way_platforms3::init_world),
         ("Platform", platform3::init_world),

examples3d/joint_motor_position3.rs

@@ -0,0 +1,84 @@
+use rapier3d::prelude::*;
+use rapier_testbed3d::Testbed;
+
+pub fn init_world(testbed: &mut Testbed) {
+    /*
+     * World
+     */
+    let mut bodies = RigidBodySet::new();
+    let mut colliders = ColliderSet::new();
+    let mut impulse_joints = ImpulseJointSet::new();
+    let multibody_joints = MultibodyJointSet::new();
+
+    /*
+     * The ground
+     */
+    let ground_size = 5.0;
+    let ground_height = 0.1;
+
+    let rigid_body = RigidBodyBuilder::fixed().translation(vector![0.0, -ground_height, 0.0]);
+    let ground_handle = bodies.insert(rigid_body);
+    let collider = ColliderBuilder::cuboid(ground_size, ground_height, ground_size);
+    colliders.insert_with_parent(collider, ground_handle, &mut bodies);
+
+    /*
+     * A rectangle on a motor with target position.
+     */
+    for num in 0..9 {
+        let x_pos = -6.0 + 1.5 * num as f32;
+        let rigid_body = RigidBodyBuilder::dynamic()
+            .translation(vector![x_pos, 2.0, 0.0])
+            .can_sleep(false);
+        let handle = bodies.insert(rigid_body);
+        let collider = ColliderBuilder::cuboid(0.1, 0.5, 0.1);
+        colliders.insert_with_parent(collider, handle, &mut bodies);
+
+        let joint = RevoluteJointBuilder::new(Vector::z_axis())
+            .local_anchor1(point![x_pos, 1.5, 0.0])
+            .local_anchor2(point![0.0, -0.5, 0.0])
+            .motor_position(
+                -(std::f32::consts::PI - std::f32::consts::PI / 4.0 * num as f32),
+                1000.0,
+                150.0,
+            );
+        impulse_joints.insert(ground_handle, handle, joint, true);
+    }
+
+    /*
+     * A rectangle on a motor with limits.
+     */
+    for num in 0..9 {
+        let x_pos = -6.0 + 1.5 * num as f32;
+        let rigid_body = RigidBodyBuilder::dynamic()
+            .translation(vector![x_pos, 5.0, 0.0])
+            .rotation(vector![0.0, 0.0, std::f32::consts::PI])
+            .can_sleep(false);
+        let handle = bodies.insert(rigid_body);
+        let collider = ColliderBuilder::cuboid(0.1, 0.5, 0.1);
+        colliders.insert_with_parent(collider, handle, &mut bodies);
+
+        let joint = RevoluteJointBuilder::new(Vector::z_axis())
+            .local_anchor1(point![x_pos, 5.0, 0.0])
+            .local_anchor2(point![0.0, -0.5, 0.0])
+            .motor_velocity(1.5, 30.0)
+            .motor_max_force(100.0)
+            .limits([
+                -std::f32::consts::PI,
+                std::f32::consts::PI / 4.0 * num as f32,
+            ]);
+        impulse_joints.insert(ground_handle, handle, joint, true);
+    }
+
+    /*
+     * Set up the testbed.
+     */
+    testbed.set_world_with_params(
+        bodies,
+        colliders,
+        impulse_joints,
+        multibody_joints,
+        vector![0.0, 0.0, 0.0],
+        (),
+    );
+    testbed.look_at(point![15.0, 5.0, 42.0], point![13.0, 1.0, 1.0]);
+}

src/dynamics/solver/joint_constraint/joint_generic_velocity_constraint_builder.rs

@@ -6,14 +6,13 @@ use crate::dynamics::solver::joint_constraint::{JointVelocityConstraintBuilder,
 use crate::dynamics::solver::MotorParameters;
 use crate::dynamics::{IntegrationParameters, JointIndex, Multibody};
 use crate::math::{Real, Vector, ANG_DIM, DIM, SPATIAL_DIM};
+use crate::utils;
 use crate::utils::IndexMut2;
 use crate::utils::WDot;
 use na::{DVector, SVector};
 
 #[cfg(feature = "dim3")]
 use crate::utils::WAngularInertia;
-#[cfg(feature = "dim2")]
-const PI: Real = std::f64::consts::PI as Real;
 
 impl SolverBody<Real, 1> {
     pub fn fill_jacobians(
@@ -358,7 +357,7 @@ impl JointVelocityConstraintBuilder<Real> {
 
         let s_limits = [(limits[0] / 2.0).sin(), (limits[1] / 2.0).sin()];
         #[cfg(feature = "dim2")]
-        let s_ang = self.ang_err.im;
+        let s_ang = (self.ang_err.angle() / 2.0).sin();
         #[cfg(feature = "dim3")]
         let s_ang = self.ang_err.imag()[limited_axis];
         let min_enabled = s_ang < s_limits[0];
@@ -393,7 +392,7 @@ impl JointVelocityConstraintBuilder<Real> {
     ) -> JointGenericVelocityConstraint {
         // let mut ang_jac = self.ang_basis.column(motor_axis).into_owned();
         #[cfg(feature = "dim2")]
-        let ang_jac = na::Vector1::new(1.0);
+        let ang_jac = na::Vector1::new(1.0); // self.ang_basis[(0, 0)]);
         #[cfg(feature = "dim3")]
         let ang_jac = self.basis.column(_motor_axis).into_owned();
 
@@ -415,18 +414,12 @@ impl JointVelocityConstraintBuilder<Real> {
         let mut rhs_wo_bias = 0.0;
         if motor_params.erp_inv_dt != 0.0 {
             #[cfg(feature = "dim2")]
-            {
+            let s_ang_dist = (self.ang_err.angle() / 2.0).sin();
-                let s_ang_dist = self.ang_err.angle();
-                let s_target_ang = motor_params.target_pos;
-                rhs_wo_bias += ((s_ang_dist - s_target_ang) % (2.0 * PI)) / (2.0 * PI)
-                    * motor_params.erp_inv_dt;
-            }
             #[cfg(feature = "dim3")]
-            {
+            let s_ang_dist = self.ang_err.imag()[_motor_axis];
-                let s_ang_dist = self.ang_err.imag()[_motor_axis];
+            let s_target_ang = (motor_params.target_pos / 2.0).sin();
-                let s_target_ang = motor_params.target_pos.sin();
+            rhs_wo_bias += utils::smallest_abs_diff_between_sin_angles(s_ang_dist, s_target_ang)
-                rhs_wo_bias += (s_ang_dist - s_target_ang) * motor_params.erp_inv_dt;
+                * motor_params.erp_inv_dt;
-            }
         }
 
         let dvel = ang_jac.gdot(body2.angvel) - ang_jac.gdot(body1.angvel);
@@ -746,7 +739,7 @@ impl JointVelocityConstraintBuilder<Real> {
 
         let s_limits = [(limits[0] / 2.0).sin(), (limits[1] / 2.0).sin()];
         #[cfg(feature = "dim2")]
-        let s_ang = self.ang_err.im;
+        let s_ang = (self.ang_err.angle() / 2.0).sin();
         #[cfg(feature = "dim3")]
         let s_ang = self.ang_err.imag()[limited_axis];
         let min_enabled = s_ang < s_limits[0];
@@ -800,18 +793,12 @@ impl JointVelocityConstraintBuilder<Real> {
         let mut rhs = 0.0;
         if motor_params.erp_inv_dt != 0.0 {
             #[cfg(feature = "dim2")]
-            {
+            let s_ang_dist = (self.ang_err.angle() / 2.0).sin();
-                let s_ang_dist = self.ang_err.angle();
-                let s_target_ang = motor_params.target_pos;
-                rhs += ((s_ang_dist - s_target_ang) % (2.0 * PI)) / (2.0 * PI)
-                    * motor_params.erp_inv_dt;
-            }
             #[cfg(feature = "dim3")]
-            {
+            let s_ang_dist = self.ang_err.imag()[_motor_axis];
-                let s_ang_dist = self.ang_err.imag()[_motor_axis];
+            let s_target_ang = (motor_params.target_pos / 2.0).sin();
-                let s_target_ang = motor_params.target_pos.sin();
+            rhs += utils::smallest_abs_diff_between_sin_angles(s_ang_dist, s_target_ang)
-                rhs += (s_ang_dist - s_target_ang) * motor_params.erp_inv_dt;
+                * motor_params.erp_inv_dt;
-            }
         }
 
         let dvel = ang_jac.gdot(body2.angvel) - ang_jac.gdot(body1.angvel);

src/dynamics/solver/joint_constraint/joint_velocity_constraint_builder.rs

@@ -5,6 +5,7 @@ use crate::dynamics::solver::joint_constraint::SolverBody;
 use crate::dynamics::solver::MotorParameters;
 use crate::dynamics::{IntegrationParameters, JointIndex, JointLimits};
 use crate::math::{AngVector, Isometry, Matrix, Point, Real, Rotation, Vector, ANG_DIM, DIM};
+use crate::utils;
 use crate::utils::{IndexMut2, WCrossMatrix, WDot, WQuat, WReal};
 use na::SMatrix;
 
@@ -289,7 +290,7 @@ impl<N: WReal> JointVelocityConstraintBuilder<N> {
         let half = N::splat(0.5);
         let s_limits = [(limits[0] * half).simd_sin(), (limits[1] * half).simd_sin()];
         #[cfg(feature = "dim2")]
-        let s_ang = self.ang_err.im;
+        let s_ang = (self.ang_err.angle() * half).simd_sin();
         #[cfg(feature = "dim3")]
         let s_ang = self.ang_err.imag()[limited_axis];
         let min_enabled = s_ang.simd_lt(s_limits[0]);
@@ -353,19 +354,15 @@ impl<N: WReal> JointVelocityConstraintBuilder<N> {
 
         let mut rhs_wo_bias = N::zero();
         if motor_params.erp_inv_dt != N::zero() {
+            let half = N::splat(0.5);
+
             #[cfg(feature = "dim2")]
-            {
+            let s_ang_dist = (self.ang_err.angle() * half).simd_sin();
-                let s_ang_dist = self.ang_err.angle();
-                let s_target_ang = motor_params.target_pos;
-                rhs_wo_bias += ((s_ang_dist - s_target_ang) % N::simd_two_pi()) / N::simd_two_pi()
-                    * motor_params.erp_inv_dt;
-            }
             #[cfg(feature = "dim3")]
-            {
+            let s_ang_dist = self.ang_err.imag()[_motor_axis];
-                let s_ang_dist = self.ang_err.imag()[_motor_axis];
+            let s_target_ang = (motor_params.target_pos * half).simd_sin();
-                let s_target_ang = motor_params.target_pos.simd_sin();
+            rhs_wo_bias += utils::smallest_abs_diff_between_sin_angles(s_ang_dist, s_target_ang)
-                rhs_wo_bias += (s_ang_dist - s_target_ang) * motor_params.erp_inv_dt;
+                * motor_params.erp_inv_dt;
-            }
         }
 
         let dvel = ang_jac.gdot(body2.angvel) - ang_jac.gdot(body1.angvel);
@@ -801,19 +798,15 @@ impl<N: WReal> JointVelocityConstraintBuilder<N> {
 
         let mut rhs_wo_bias = N::zero();
         if motor_params.erp_inv_dt != N::zero() {
+            let half = N::splat(0.5);
+
             #[cfg(feature = "dim2")]
-            {
+            let s_ang_dist = (self.ang_err.angle() * half).simd_sin();
-                let s_ang_dist = self.ang_err.angle();
-                let s_target_ang = motor_params.target_pos;
-                rhs_wo_bias += ((s_ang_dist - s_target_ang) % N::simd_two_pi()) / N::simd_two_pi()
-                    * motor_params.erp_inv_dt;
-            }
             #[cfg(feature = "dim3")]
-            {
+            let s_ang_dist = self.ang_err.imag()[_motor_axis];
-                let s_ang_dist = self.ang_err.imag()[_motor_axis];
+            let s_target_ang = (motor_params.target_pos * half).simd_sin();
-                let s_target_ang = motor_params.target_pos.simd_sin();
+            rhs_wo_bias += utils::smallest_abs_diff_between_sin_angles(s_ang_dist, s_target_ang)
-                rhs_wo_bias += (s_ang_dist - s_target_ang) * motor_params.erp_inv_dt;
+                * motor_params.erp_inv_dt;
-            }
         }
 
         let dvel = ang_jac.gdot(body2.angvel) - ang_jac.gdot(body1.angvel);
@@ -852,7 +845,7 @@ impl<N: WReal> JointVelocityConstraintBuilder<N> {
         let half = N::splat(0.5);
         let s_limits = [(limits[0] * half).simd_sin(), (limits[1] * half).simd_sin()];
         #[cfg(feature = "dim2")]
-        let s_ang = self.ang_err.im;
+        let s_ang = (self.ang_err.angle() * half).simd_sin();
         #[cfg(feature = "dim3")]
         let s_ang = self.ang_err.imag()[limited_axis];
         let min_enabled = s_ang.simd_lt(s_limits[0]);

src/utils.rs

@@ -804,3 +804,13 @@ impl<T> IndexMut2<usize> for [T] {
         }
     }
 }
+
+/// Calculate the difference with smallest absolute value between the two given values.
+pub fn smallest_abs_diff_between_sin_angles<N: WReal>(a: N, b: N) -> N {
+    // Select the smallest path among the two angles to reach the target.
+    let s_err = a - b;
+    let sgn = s_err.simd_signum();
+    let s_err_complement = s_err - sgn * N::splat(2.0);
+    let s_err_is_smallest = s_err.simd_abs().simd_lt(s_err_complement.simd_abs());
+    s_err.select(s_err_is_smallest, s_err_complement)
+}