Electronic Design

Boost Stepper-Motor Switching-Logic-Sequence Performance

Sometimes less is more. Here, a quarter-step switching logic sequence for stepper motors offers several significant advantages over full-and halfstep modes—higher resolution, smoother drive, minimal resonance effects, and reduced settling time. The method uses a 16-step input sequence to produce 1600 steps per revolution in 400-step motors. At the heart of this quarter-step technique is a so-called half-excitation state, during which the ratio of motor current flow is held very close to half.

Unlike micro-stepping techniques, which rely on complex control circuitry, the quarter-step mode depends on a simple switching logic sequence. Because of this, the circuitry to implement the quarter-step scheme can employ inexpensive power switches and CMOS devices. In contrast, microsteppinggenerally requires a microcontroller, both digital-to-analog converters (DACs) and analog-to-digital converters (ADCs), as well as an Hbridge amplifier.

The switching pattern plays an important role in determining the resolution and settling time of a stepper motor. When a full-step sequence is applied to a 400-step motor, the shaft advances 400 steps per revolution with a resolution of 0.9° per step. Similarly, for a half-step sequence, the shaft advances 800 steps per revolution with a resolution of 0.45° per step. Typically, with micro-stepping techniques, sinusoidal currents applied to the two phases of the motor cause the shaft to move with a resolution of a fraction of 0.9°.

For a two-phase stepper motor (it has a bifilar winding), the angular shaft displacement (q) is determined by the phase currents (I1 and I2). Keeping the magnitude of these two currents equal and applying a four-step sequence yields the full-step motion. And by inputting an eight-step sequence in which I1/I2 can be either maximum or zero, the stepper motor moves 800 steps in one revolution.

In between, when I1/I2 = 0.4142, which we dubbed the half-excitation state, the stepper motor advances in a 16-step sequence and moves 1600 steps per revolution. To achieve this half-excitation state of I1/I2 = 0.4142, choose the current ratio so that a shaft angle of 22.5° becomes the stable equilibrium position of the rotor. When:

q = tan?1(I1/I2) = 22.5°

then

I1 = 0.4142 I2

Introducing this half-excitation state generates a 16-step sequence (pattern), and the shaft rotates with a better resolution. The truth table shows the 16-step input sequence for a quarter-step motor controller, with each highlighted 1/2 fraction denoting a half-excitation state. The sequence generator circuit of Figure 1 implements this truth table. It employs a CMOS binary up/down counter (U1, CD4516) to produce the necessary pattern.

The drive circuit of Figure 2 was used to test the quarter-step switching logic sequence for a 4-kgfcmtorque stepper motor. To implement a half-excitation state, the values of resistors R for the circuit's Y outputs to the motor are set to allow only 0.4 I of current to flow when excited. The assumption here is that the different coils of the stepper motor have equal resistance, although in reality there's a very small difference between them.

The quarter-step controller was put through its paces by applying low frequency clock pulses to the sequence generator, and then verifying that the motor shaft had advanced 1600 steps per revolution. The rotation also appeared to be smoother than that of half-and full-step rotations. An intermediate state (half excitation) exists between the full-energization and no-energization states. That's because the back electromagnetic force generated in the coils is also less than that of half-and full-step controllers once the motor moves between the steps.