Stepping Out

Higher step counts do not automatically mean better accuracy or more torque.

Mindy Lin | Belal Azim – Lin Engineering, Morgan Hill, CA

One important decision when selecting a stepmotor concerns its step angle. The factors of performance and price invariably come into play. Does the application justify the extra costs of high-performance stepmotors? Would a 1.8° stepmotor suffice or will only a 0.9° step work?

A standard 1.8° stepmotor operated in half-step mode rotates as much as a 0.9° motor. There is widespread belief that a half-stepped 1.8° motor is as accurate as a true full-step 0.9° motor; this belief leads some to think that a half-stepped 1.8° motor will suffice at lower prices. Furthermore, there is a fear of lower torque with the 0.9° motor. While such skepticism is valid, a review of mechanical and design explanations and theories might help put things in perspective.

Besides manufacturing tolerances, torque stiffness is a major factor in determining motor accuracy. As motors step, inertia makes the rotor hunt or oscillate about its new position. The rotor may come to rest out of alignment with the magnetic field. High torque stiffness translates into smaller oscillations that are more quickly damped with each step taken. Therefore, higher torque stiffness typically results in more accuracy.

The number of stepmotor teeth plays a deciding role in the amount of torque stiffness. Rotors in 1.8° stepmotors contain only 50 teeth, while 0.9° rotors have 100 teeth. This alone gives 0.9° motors higher torque stiffness and thus higher accuracy. Thus it is wrong to believe that half-stepping a 1.8° stepmotor produces accuracy comparable to a 0.9° motor at full step. Half-stepping forces the stepmotor to move in increments that would not have happened naturally. The half-step increases step resolution that invariably enlarges step error within the motor. The greater step error makes it difficult for the motor to keep up with such precise steps.

Stepping Out

The question remains whether or not the better performance of a 0.9° stepmotor is worth the extra money. It helps to first note that not all 0.9° motors perform equally. In fact, there are three mechanically different 0.9° step-motors on the market, each with their own distinct performance capabilities and benefits.

The mechanical differences between the motors are in the stator poles and the relative air gap between the rotor and stator. These differences drastically affect accuracy, operating speed range, and the ability to produce torque in each of the stepmotor types.
The air gap between the rotor and stator is always non-concentric. No stepmotor manufacturer can produce an outside diameter on the rotor and inside diameter of the stator in perfect concentricity. Each motor contains its own definite air-gap variation. Typical 0.9° stepmotors contain either eight or 16 poles within their respective stators. For an eight-pole design, the angle between each pole is 45°. On the other hand, a 16-pole design has a 22.5° angle between each pole that allows for less variation in the air gap between the poles.

Manufacturing tolerances may create differences in the air gaps between adjacent poles and the rotor. The closer pole generates a stronger magnetic attraction on the rotor, pulling the rotor more towards it.

The same air-gap variation in eight-pole stators can double the size of the error created in 16-pole stators. This is caused by the wider separation between the poles in an eight-pole stator. However, air-gap-induced position errors aren’t the only drawbacks between stator designs. The tighter spacing between the poles means manufacturers can only wind half as many turns per pole on 16-pole stators as they can on an eight-pole motor. Overall, the inductance of the 16-pole windings are reduced to about half that of the eight-pole windings €” good for high-speed applications. But low inductance comes at the expense of torque, which is lower in 16-pole designs.

The combination of these effects means 1.8° stepmotors that typically use eight-pole stator designs have higher torque but lower accuracy compared to 0.9° stepmotors using 16-pole stators.

A new class of stepmotor has recently become available that attempts to bridge the trade-off gap between 16-pole accuracy and eight-pole torque. It does this by using a 12-pole stator design. The smaller angular measure between the poles reduces associated air-gap errors, while the larger pole pieces permit stronger magnetic fields for stiffness. An example of the new design is the Model 417 by Lin Engineering, in Santa Clara, Calif.

The 0.9°-step Model 417 possesses greater torque than the standard 0.9° stepmotor. However, the higher torque does introduce a slightly larger position error than the regular 0.9° step.