Because the sun equipment in a hybrid unit is pre-aligned within the gearhead and not affixed to the electric motor shaft, these gearheads can be used in contouring applications such as a glue-dispensing nozzle for affixing a windshield to a car. Movement of the nozzle since it comes after the seam between a windshield and its own window frame should be perfectly smooth; otherwise a ripple in velocity alters the bead diameter and causes messy glue program.
Smooth motion, which means the absence of torque and velocity variations (ripple), is important in contouring applications. But, it really is difficult to regularly achieve smooth motion where the sun gear is installed on the motor shaft. Even a slight misalignment in the sun gear (motor shaft runout or coupling inaccuracies) could cause rough operation and noise.
Many servo controllers use software compensation, and their success depends on knowing the lost motion of the entire system. This details is usually available from the gearhead manufacturer.
Contouring applications usually involve end-effectors or tool-points that follow mathematically defined paths. Sealant and bonding machines, water and flame cutters, laser beam welders and cutters, movement controlled cameras, and CNC machine tools are good examples.
Software compensation is accomplished by commanding the motor to move beyond the apparently desired position by an amount add up to the system’s lost motion, thereby bringing the load to the truly desired position. For instance, consider a servomotor, gearhead, and leadscrew mixture in a servo gear reducer pick-andplace robot. If 100,000 encoder counts equals 1.0 in. of linear movement and the system has 0.1-in. dropped motion, then the controller tells the electric motor to go 110,000 encoder counts to get 1.0 in. of motion, thus compensating for the 0.1-in. lost motion.
Backlash is the excess space between two adjacent equipment teeth and its engaging tooth; lost motion may be the total looseness or motion at a reducer’s result shaft when the input shaft is fixed. Dropped motion contains backlash, plus losses from bearing looseness, tolerances and fits, and shaft and equipment tooth compliance.
Servo controllers can be programmed to pay for backlash and dropped motion in planetary gearheads. This technique compensates for backlash also where an application requires accuracy much better than the minimal backlash of the gearhead.