Because the sun gear in a hybrid unit is pre-aligned within the gearhead rather than affixed to the motor shaft, these gearheads can be used in contouring applications such as a glue-dispensing nozzle for affixing a windshield to an automobile. Motion of the nozzle as it comes after the seam between a windshield and its own window frame must be perfectly smooth; otherwise a ripple in velocity alters the bead diameter and causes messy glue software.
Smooth motion, which means the lack of torque and velocity variations (ripple), is essential in contouring applications. But, it is difficult to regularly achieve smooth movement where the sun gear is mounted on the motor shaft. A good slight misalignment in the sun gear (electric motor shaft runout or coupling inaccuracies) can cause rough operation and noise.
Many servo controllers use software compensation, and their success depends on knowing the lost motion of the whole system. This info is usually offered from the gearhead manufacturer.
Contouring applications generally involve end-effectors or tool-points that follow mathematically defined paths. Sealant and bonding devices, drinking water and flame cutters, laser beam welders and cutters, movement managed cameras, and CNC machine tools are good examples.
Software compensation is achieved by commanding the motor to go beyond the apparently desired position by a quantity add up to the system’s lost movement, thereby bringing the load to the truly desired position. For instance, consider a servomotor, gearhead, and leadscrew combination in a pick-andplace robot. If 100,000 encoder counts equals 1.0 in. of linear motion and the system has 0.1-in. dropped motion, then your servo motor gear reducers controller tells 
the motor to move 110,000 encoder counts to get 1.0 in. of motion, hence compensating for the 0.1-in. lost motion.
Backlash is the excess space between two adjacent gear teeth and its own engaging tooth; lost motion is the total looseness or movement at a reducer’s result shaft when the insight shaft is fixed. Lost motion includes backlash, plus losses from bearing looseness, tolerances and suits, and shaft and equipment tooth compliance.
Servo controllers could be programmed to compensate for backlash and dropped motion in planetary gearheads. This system compensates for backlash even where an application requires accuracy much better than the minimal backlash of the gearhead.