limits

web/js/kinematics.js

@@ -69,6 +69,7 @@ const LIMITS = {
   //   (can never cross to the right side of the box mid-point).
   ELBOW_BOX_X_INNER: 5,   // ← TUNE: inner X margin from centre for each elbow
   ELBOW_BOX_Y_MAX: 50,  // ← TUNE: Y below which elbow crossing is forbidden
+  ELBOW_MIN_Y: 0,       // ← TUNE: Y below which the elbow hits the bottom housing/desk
 };
 
 // ── Letter / number position lookup table ────────────────────────
@@ -245,13 +246,16 @@ function forward(theta1, theta2) {
  */
 function armsCrossed(theta1, theta2) {
   const { d2, l1 } = ARM;
-  const { ELBOW_BOX_X_INNER, ELBOW_BOX_Y_MAX } = LIMITS;
+  const { ELBOW_BOX_X_INNER, ELBOW_BOX_Y_MAX, ELBOW_MIN_Y } = LIMITS;
 
   const e1x = -d2 + l1 * Math.cos(theta1);
   const e1y = l1 * Math.sin(theta1);
   const e2x = +d2 + l1 * Math.cos(theta2);
   const e2y = l1 * Math.sin(theta2);
 
+  // Bottom housing collision
+  if (e1y < ELBOW_MIN_Y || e2y < ELBOW_MIN_Y) return true;
+
   // E1 (left motor) should not cross to the right side (X > -ELBOW_BOX_X_INNER) if it's low (Y < ELBOW_BOX_Y_MAX)
   if (e1y < ELBOW_BOX_Y_MAX && e1x > -ELBOW_BOX_X_INNER) return true;
   
@@ -269,7 +273,7 @@ function armsCrossed(theta1, theta2) {
  *   1. Board outer boundary
  *   2. Mechanism housing exclusion box
  *   3. IK geometric reachability
- *   4. Elbow-crossing guard (prevents physically impossible arm configs)
+ *   4. Elbow constraints (prevents crossing and hitting the bottom)
  *
  * @param {number} x
  * @param {number} y