Day one
In the beginning

In 1960, the large-angle type PM step motor was the most used step motor. This two-phase, bifilar wound PM step motor (top) advanced 45° mechanical with each pulse from the drive, while by 1962 PM step motor drives (below) were using halfstep 22.5° mechanical excitation strategies to try to smooth motion.
Select figure to enlarge.

Topics of discussion:
Early step motors
Early servomotors

Today’s precision position motor family has two major adherents, the step motor and the brushless permanent magnet (PM) motor. Both types have continuously evolved over the past 40 years. These two prime movers have provided many industries the muscle power for driving precision positioning systems in office automation, factory automation, instrumentation, aerospace and many other markets. While the brush dc motor stubbornly holds into many applications, the brushless PM motor has gained ascendancy over it in an ever-widening range of applications.

The past 50 years have seen step motors and servomotors alternately gain the top position in the precision motion market only to lose it to the other. The shifting market fortunes of the brush/brushless servomotor versus the step motor have been driven by various technological breakthroughs, such as the rare earth magnet, the bipolar and field effect transistor (FET), the microprocessor, and the digital signal processor. Before 1955, the ac induction servosystem using a two-phase ac motor was the paramount system for positioning applications. The step motor began its initial growth spurt in the 1960s, overtaking the brush and brushless servomotor driven systems in the early 1980s. Since then, the brushless dc and ac-driven servos have significantly expanded their market share to recapture the market lead by the mid 1990s. Today, major step motor attributes have been combined in a closed-loop, brushless ac servodrive system.

The step motor has a number of different versions — it doesn’t suffer from an identity crisis. Whereas, the synchronous PM motor has been called the brushless dc motor, brushless ac motor, ac servomotor, brushless servomotor, and other names, according to how it was being electronically driven. The range of identifying names depicts the various historical and market roots for brushless PM motors. This motor uses a permanent magnet to generate the magnetic field from the rotor. A wound field creates the stator’s magnetic field. The interaction of these two fields creates torque and speed. The step and brushless PM motors use high-performance magnets, such as samarium cobalt or neodymium iron boron, to generate the highest possible torque from a given package size.

Both motor types require electronic drives composed of power and control electronics. These drive packages provide the necessary electronic signals to drive or move both motor families in prescribed motion profiles. Different strategies are used to control the step motor and brushless PM motor. The step motor possesses an internal magnetic positioning capability when driven in an open-loop (no feedback) control strategy. The servomotor requires various measured feedback signals (commutation, current, velocity, and position) for comparison with the input signals in a closed-loop control strategy. The history of electronic drives development closely parallels motor development.

Early step motors

The first step motor, a bidirectional variable reluctance type, was developed for the British Navy in 1933. It was used as a remote position repeater for a compass and gun pointer direction indicator. The drive was crude but effective for these slow constant speed applications. The system was later adopted by the U.S. Navy during World War II. The step motor was used in a limited number of primitive digital control systems through the 1950s. The predominance of closed-loop continuous ac induction servos from 1944 to 1957 was soon overcome by the PM step motor and the availability of digital measuring devices and control logic necessary for all-digital systems.

The 1961 hybrid step motor’s rotor cups each have 50 fine teeth indexed one rotor tooth pitch. Alternating north and south poles assures proper magnetic link with the stator windings, producing high torque and 1.8° mechanical step motion.
Select figure to enlarge.

By 1960, the PM step motor (large-angle type) had become the primary step motor in use. There were no cures for resonance or shaft velocity vibration other than stop the step motor shaft and restart the motion. The key attributes were the PM step motor’s open loop position accuracy of +5% and the non-cumulation of position error. The brush servo (closed-loop) system or small instrument ac (two-phase) servomotor system required careful stabilization of the analog feedback signals (velocity and position). It was a daunting task to close a servo loop and properly stabilize the servo motor in 1960. Typical step motor applications in 1960 included drone control readouts, railway car sorting indicators, portable weighing station indicators, and digital differential altimeters.

• The ac synchronous inductor motor

A few years previously, General Electric had developed an ac synchronous inductor motor for use in low-speed smooth motion applications. This motor was designed to operate with a 60-Hz, 115-Vac signal and generate smooth continuous motion equivalent to the movement of an analog clock’s second hand. In 1961, Snowden and Madsen of Superior Electric Co. announced a new rendition of the synchronous inductor motor which would become famous by another name, the hybrid step motor.

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