Stepper motor operation is relatively straightforward. The drive sends pulses of current to the motor, and each pulse advances the motor by one step, or fraction of a revolution. The amount of movement per current pulse depends on the design of the stator and rotor. For permanent magnet stepper motors, a full step angle of 1.8 degrees (200 steps per revolution) is common, while hybrid stepper motors have full step angles as small as 0.72 degrees (500 steps per revolution). But hybrid and permanent magnet stepper motor designs can also be driven by a method known as microstepping control.
In microstepping control, rather than delivering discrete pulses of current, the driver sends two voltage sine waves, 90 degrees out of phase, to the motor windings. The sine wave voltages allow the current in the windings to increase and decrease gradually, moving the motor by fractions of a full step (hence, the term “microstepping”).
Benefits of microstepping control
Microstepping control can divide a full step by as much as 256 times, producing a step angle as small as 0.007 degrees (51,200 steps per revolution) when the full step angle is 1.8 degrees. This significantly increases resolution (the smallest increment the motor can move). But with increased resolution comes decreased torque production per incremental step, which can degrade the motor’s accuracy if the incremental torque is insufficient to overcome the load and motor inertia.
Another benefit of microstepping control is very smooth motor rotation. This is because microstepping allows a gradual build-up and decay of current in each motor winding, which delivers more consistent torque and, therefore, smoother motion than full-step operation. This is especially evident at slower speeds, where full-step operation causes pronounced detent torque.
Resonance can be problematic in stepper motor applications as the motor operates near, or moves through, its natural frequency range. The pulses of energy delivered in full- or half-stepping mode can excite resonances, especially when the motor is unloaded or lightly loaded. By providing smooth motion of the stator flux, and, therefore, a more continuous flow of energy to the rotor, microstepping control can reduce or virtually eliminate resonance issues.
Despite reduced incremental torque production versus full- or half-step operating modes, microstepping provides benefits that make it an attractive control method for many stepper motor applications. And for applications that require precise positioning, a stepper motor operated with microstepping control can provide a less complex, lower-cost solution than a servo-driven system.