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Did You Know? Microstep Emulation

by Eric Rice

Stepper drives from Applied Motion Products provide many advanced control features. Microstep Emulation is one feature that should interest all users. Microstepping was a major advancement in step motor technology when it was introduced to the market many years ago. However, because of the many low-frequency indexing systems still in use to this day, not all users have been able take advantage of it. Fortunately, Microstep Emulation from Applied Motion makes microstepping available to all system designers and machine builders.

The earliest step motor drives employed full-stepping because the logic required was simple, resulting in less expensive drive electronics. It also made possible the use of the most simple of drives, the L-R drive. The L-R drive used switches and the resistance of the winding (or an external power resistor) to control the phase currents. The L-R drive only provided two values of current (plus and minus) in each winding and delivered very low shaft power. This worked well for applications such as indexing a platen roller in a printer and other low-speed applications. However, the torque in an L-R drive system falls precipitously with velocity starting at rest, making it undesirable for most automation applications.

Modern step motor drives use switch-mode PWM power amplifiers to control winding currents. They also employ much more complex logic (embedded processors) that allow for microstepping. Microstepping produces very fine manipulation of the current vector which in turn creates very fine step resolutions. The trade-off in terms of the indexing scheme is that increasing step resolution means increasing the required step frequency for any given velocity. Many indexing schemes (think of a low-frequency output on a PLC for example) cannot provide high frequency step signals. In these cases, the use of microstepping drastically reduces the top speed that can be commanded. In addition many machines have been designed around coarse resolution indexers (limited to full- or half-step resolution) making it impractical to change to a microstepping regimen.

Applied Motion drives with Microstep Emulation offer synthetic microstepping as a means to achieve slow-speed smoothness and high-speed operation. The process involves accepting coarse resolution step signals from the indexer and synthesizing the commanded motion using the drive’s own internal high resolution microsteps. This is a high-speed process that “locks” onto the incoming step sequence and follows it in smooth microsteps.

Full steps and a synthetic microstep copy

Each incoming step in the image above represents a 1.8° increment in the commanded position (blue trace). The drive senses the activity in the commanded step sequence (acceleration and velocity) and builds a high resolution step sequence (red trace) that is pulled into close synchronism with the command sequence. The microsteps shown above (red trace) are exaggerated in size for the sake of clarity. In reality they would be so fine as to be almost invisible.

Stepper drives with Microstep Emulation can accept a low-resolution step signal and convert it to high-resolution microstepping, providing dramatically smoother and quieter motion without having to change the indexing scheme of the existing machine.

In Applied Motion drives there are two ways to enable the Microstep Emulation feature, depending on the drive model you have:

  • STR2, STR4, STR8 stepper drives and STM-R integrated steppers are set with on-board dip switches. Select the 200 SMOOTH (full-stepping with Microstep Emulation) or 400 SMOOTH (half-stepping with Microstep Emulation) dip switch settings.
  • ST5, ST10, STAC5, STAC6 stepper drives and all other STM integrated steppers are set using the ST Configurator™ software. Adjust the Step Smoothing Filter value in the Motion > Pulse & Direction Control dialog (screen shot below). The lower the Step Smoothing Filter value, the greater the impact of Microstep Emulation. In other words a lower value means more synthetic microsteps will be injected into the command motion. For example, a Step Smoothing Filter value of 10 Hz will generate extremely smooth motion, while a value of 1000 Hz or higher is equivalent to running the motor with the original, low-frequency step resolution.

Step Smoothing Filter field in the ST Configurator™ software

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