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Microstepping: Myths and Realities

The lure of Microstepping a two-phase stepper motor is compelling. Visions of Microstepping a 1.8-degree hybrid stepper motor with 256 microsteps per full step flash in your mind. The resolution of 51,200 microsteps per revolution entices you. You're glad you don't own stock in high-resolution encoder companies.

What's the catch?

The real compromise is that as you increase the number of microsteps per full step the INCREMENTAL torque per microstep drops off drastically. Resolution increases but accuracy will actually suffer. Few, if any, stepper motors have a pure sinusoidal torque vs. shaft position and all have higher order harmonics that in fact distort the curve and affect accuracy. And while microstepping drives have come a long way too, they still only approximate a true sine wave. Significant too is that any load torque will result in a "magnetic backlash", displacing the rotor from the intended position until sufficient torque is generated.
The actual expression for incremental torque for a single microstep is:

1. TINC = THFS x sin (90/µPFS)

The incremental torque for N microsteps is:

2. TN = THFS x sin ((90 x N)/µPFS))

Where:

  SYMBOLS and UNITS  
Symbol Definition Unit(s)
µPFS

Number of Microsteps per Full Step

Integer
N

Number of Microsteps taken

N Less than or equal to µPFS

Integer
THFS Holding Torque - Full Step oz-in
TINC Incremental Torque per Microstep oz-in
TN

Incremental Torque for N Microsteps

N Less than or equal to µPFS

oz-in

Table 1 dramatically quantifies the significant impact of the incremental torque per microstep as a function of the number of microsteps per full step.

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