Multi-axis stepper motors offer a compact solution to actuate multiple degrees of freedom in a mechanical system by a single device. This research presents an electric stepper motor with three coaxial shafts driven by a single stator. The motor dimensions are 42 × 42 × 44 mm 3 (excluding output shafts), its mass is 0.326 kg and it includes three rotors of height 7 mm and different number of teeth.The maximum torque for one rotor at low speed and 1 A current is 0.10 N m, which is comparable to that of a conventional stepper motor with similar rotor size. When multiple rotors are driven simultaneously, then the total power has to be distributed over the three rotors, resulting in 42% lower torque on average. The maximum no-load speed is 4000 revolutions per minute.The coaxial output shaft configuration poses significant challenges but also unique opportunities in actuating multiple joints of a robotic system. In many applications, the mechanical design can be redesigned to make effective use of the multi-motor design. An example of a three-DOF robot arm is demonstrated that can be efficiently driven by the proposed motor.

Absolute position detection in sensorless electric stepper motors potentially allows for higher space efficiency, improved shock resistance, simplified installation, reduced number of parts and lowered cost.A prototype is demonstrated measuring 42 × 42 × 34 mm 3 with seven coils arranged in a star configuration. The rotor is 25.8 × 12.5 mm 2 and has 51 teeth which are irregularly spaced. At the driver side, the coil currents are measured during motion in order to reconstruct the absolute position of the motor. Calibration and smoothing techniques are used to reduce systematic and stochastic measurement errors, respectively.The motor is able to detect and correct its position after externally-induced stalls at the tested motor speeds from 40 rpm to 108 rpm. The holding torque is 0.23 N m at an armature current of 1 A; on average the torque is 7% lower than that of a reference bipolar stepper motor with the same dimensions.The results show that dynamic position sensing and correction are possible for a range of velocities, but not at standstill. The driver requires seven current sensors and sufficient computational power, and proper calibration of motor intrinsics is required beforehand. The presented technology could make existing 3-D printers and other machines with open-loop stepper motors more robust and increase the range of operating speeds and accelerations, without the adverse side-effects of increased complexity and cost associated with dedicated position sensors.