Fix bug in slider-crank transmission.

The associated row in `actuator_moment` was not cleared before being updated.

PiperOrigin-RevId: 688207085
Change-Id: I2f925e984c5c493e64ead66667aef57c4844acfe

doc/changelog.rst

@@ -7,10 +7,13 @@ Upcoming version (not yet released)
 
 MJX
 ^^^
-
 - Added ``apply_ft``, ``jac``, and  ``xfrc_accumulate`` as public functions.
 - Added ``TOUCH`` sensor.
 
+Bug fixes
+^^^^^^^^^
+- Fixed a bug in slider-crank ref:`transmission<geTransmission>`. The bug was introduced in 3.0.0.
+
 Version 3.2.4 (Oct 15, 2024)
 ----------------------------
 

src/engine/engine_core_smooth.c

@@ -1000,6 +1000,9 @@ void mj_transmission(const mjModel* m, mjData* d) {
         mj_jacSite(m, d, jac, 0, id);
         mju_subFrom(jac, jacS, 3*nv);
 
+        // clear moment
+        mju_zero(moment+i*nv, nv);
+
         // apply chain rule
         for (int j=0; j < nv; j++) {
           for (int k=0; k < 3; k++) {