Steppers go shaftless

Allen Bennett
Intelligent Motion Systems
Marlborough, Conn.

Systems integration has been a buzz-phrase for many years. In the motioncontrol world, it's also been a dawning reality. Integration is all the rage with integrated motor/drivers and direct-drive systems changing the motor and motion-control landscape.

The Inside-Out Step (IOS) motor is in tune with these trends. It's a two-phase hybrid stepper motor that uses a conventional permanent-magnet configuration and operates in both open and closed-loop modes. However, the stator and rotor are reversed resulting in a large hole through the center of the motor. The large interior hole allows a wide range of optical, electrical, pneumatic, hydraulic, and mechanical components to pass through the center of the motor.

The laminated stator assembly fits onto a nonrotating pedestal surrounded by a rotatable shaft-cup assembly with an open shaft face. The rotor assembly is a permanentmagnet ring surrounded by two soft iron cups. These cups are laminated and comprised of 50 small vernier teeth. The second rotor cup assembly has a one-half toothpitch orientation, optimizing rotor torque. The NEMA size 17 rotor uses a samariumcobalt magnet, whereas the larger size 23 and 34 rotors use less-expensive cast AlNiCo. The torque of the motor is increased by lengthening the conductors and increasing the area of the magnetic teeth.

The idea behind the IOS motor is to eliminate hardware between motor and load. To that end, the IOS motor integrates the lead screw and motor assembly, eliminating mechanical components. It combines the mechanical precision of the ball screw with the precise motion capability of the hybrid step motor or the closed-loop brushless servomotor. This eliminates expensive and tolerance-accumulating methods of joining mechanical components to a motor shaft, thereby lowering system cost and improving efficiency. The motor can change into a linear actuator by directly attaching an off-the-shelf ball screw to the front mounting face. The drive nut mounts directly to the motor shaft mounting face. Worn out lead screws and drive nuts can then be easily replaced without throwing away the motor.

The rotor inertia is larger than a standard hybrid stepper motor with the same frame size and overall mechanical length. When used in rotary tables, it eliminates or reduces perturbations as the load moves around its center of gyration and when it stops. This contrasts with standard steppers where a lower rotor inertia increases perturbations because of the higher load-to-reflected-inertia ratio. However, the IOS motor does have a lower torque-to-inertia ratio than standard hybrid steppers.

In linear motor applications using coarse thread ball screws for fast moves, the larger rotor inertia damps the reflected load inertia. This decreases the settling time between moves. It also speeds up overall cycle time because there is less time wasted waiting for the load to settle before progressing to another event.

A Size 23 IOS motor-miniature ball-screw assembly is used in an oilindustry application to provide 190 lb of axial force at less than 1 in./sec, eliminating the need for a bulky pneumatic air cylinder and compressor. Likewise, a Size 17 IOS motor ball-screw assembly serves in a semiconductor wafer lift mechanism where it replaces a less-accurate pneumatic actuator. The same motor and drive system is being used as a precision linear actuator to drive a syringe for fluid dispensing in the semiconductor industry. The new setup significantly improves the linearity of the fluid dispensed.