differential-robot-and-sensors

Differential drive robot and sensors

Every trap on this page costs hours. All of them were measured on a robot that works.

Joints: axis and control mode

A joint acts along its own +Z. For a wheel driving toward +X, the axis must lie along Y, which means rotating the whole joint (-pi/2 about X). It is not a call parameter: it is the object's orientation.

A joint with no control mode is deaf. Send it a velocity and nothing happens until dynCtrlMode is set — a property, not an argument of the creation call. For velocity control the value is 4 (not 2, which is what you would guess):

sim.setInt32Property(joint, "dynCtrlMode", sim.jointdynctrl_velocity)
sim.setJointTargetVelocity(joint, 3.0)

The MCP's crear_junta tool does both for you.

Friction: Bullet reads frictionOld

Bullet 2.7, the default engine, obeys bullet.frictionOld, not bullet.friction. CoppeliaSim exposes both and which one wins depends on the Bullet version selected in the scene. Setting only bullet.friction does nothing at all.

Measured on a real differential drive: a caster left at old-friction 1 dragged the robot down to 87% of its straight-line distance and 51% of its turn rate, skid-steering instead of pivoting on its drive axle. Always write both.

Caster: very low friction. Drive wheels: high friction.

Mass and inertia

computeMassAndInertia only works on convex shapes. Do not merge the chassis with the payload and then call it: keep each piece convex on its own and compute each mass separately.

Without sensible mass, a light robot on thin wheels jitters or gets flung.

Thin wheels

Wheels 1 cm thick are very little for Bullet: the contact is almost a line and the robot shakes. Compensate with high friction and enough mass. If the robot vibrates while standing still, trace the chassis height over time — if it oscillates, this is why.

Collision groups

The robot's parts must not collide with each other, but must collide with the world. The respondable mask does both at once: the low 8 bits say who it hits within the same tree, the high 8 bits who it hits outside. With 0xFF00 the robot's parts ignore each other — no more jitter — and still hit the floor and the obstacles.