Because the sun gear in a hybrid unit is pre-aligned within the gearhead and not affixed to the motor shaft, these gearheads can be utilized in contouring applications like a glue-dispensing nozzle for affixing a windshield to an automobile. Motion of the nozzle since it follows 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 program.
Smooth motion, which means the lack 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 electric motor shaft. A good slight misalignment in sunlight gear (motor shaft runout or coupling inaccuracies) can cause rough operation and noise.
Many servo controllers use software compensation, and their success depends upon knowing the lost movement of the whole system. This information 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 welders and cutters, movement controlled cameras, and CNC machine tools are good examples.
Software compensation is achieved by commanding the electric motor to move beyond the apparently desired position by an amount equal to the system’s lost movement, thereby bringing the load to the truly desired position. For instance, look at a servomotor, gearhead, and leadscrew mixture in a pick-andplace robot. If 100,000 servo motor gear reducers encoder counts equals 1.0 in. of linear movement and the machine has 0.1-in. dropped motion, then the controller tells the electric motor to go 110,000 encoder counts to obtain 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 engaging tooth; lost movement may be the total looseness or movement at a reducer’s result shaft when the input shaft is fixed. Lost motion includes backlash, plus losses from bearing looseness, tolerances and matches, and shaft and equipment tooth compliance.
Servo controllers could be programmed to pay for backlash and dropped movement in planetary gearheads. This system compensates for backlash even where a credit card applicatoin requires accuracy much better than the minimal backlash of the gearhead.