Print
PDFOverview
Microstepping provides the ability to finely control movement of a stepper motor. The ability to achieve much smaller individual steps is accomplished by varying the current in the motor windings, allowing for smoother transitions between the physical steps on a stepper motor.
Stepper Motor Accuracy
Stepper motors typically have 50 or 100 teeth. This results in 200 or 400 steps per revolution or 1.8 or 0.9 degrees per step respectively. The repeatability of positioning done using a stepper motor is limited by the internal geometry of the stator and rotor. For detailed information on how stepper motors switch between steps please refer to Developer Zone Tutorial: Stepper Motor Switching Sequence.
There are three methods to improve the accuracy of a stepper motor. One is to use a stepper motor with more teeth. Another is to use gears to reduce the amount of movement per step; if the motor is controlling a threaded rod, use a rod that can increase the number of threads per inch. The last is to use microstepping.
Microstepping Accuracy
Motors driven by microstepping drives show non-linearities between full steps. Plotting the desired position vs actual position, where the center of the graph is a full step position, would look like a sine wave rotated about the center by 45 degrees. A full cycle of the sine wave would occur between two full steps.
The 90 degree point of the sine wave is the location with the most offset from the desired position. This point on the wave could be as much as 50 microsteps out of position in a 250 microsteps per full step configuration. The 180 degree mark, which is the half step position, produces no error. The 270 degree mark will show negative error as it is pulled to the next pole. This is caused by the magnetic attraction to the pole that the motor coils are unable to overcome as well as the way the motor is made.
Some drives implement means to account for this error. In order to acquire the least amount of error, use a motor with a high detent torque to hold torque ratio. If the motor's hold torque is 300 oz-in and the detent torque is 10 oz-in, the ratio is 30. This is a good candidate for microstepping. Ratios 20 and under are not as good for microstepping.
However, a case can be made for using microstepping to reduce the effects of torque ripple and motor stalling caused by motor resonance. The repeatability is almost perfect, except for slight hysteresis when changing direction. In other words, when moving to the same position from both directions, the angle is not exactly the same.
Configuring Microstepping
National Instruments software is configured to mirror hardware settings not to set them. The configuration described below is for configuring hardware sold by National Instruments. Hardware configured with microstepping is considered as full steps from the software configuration side. For example if a stepper motor is a 200 steps per revolution motor (50 teeth, 1.8 degrees per step) but is configured in hardware with microstepping to be a 1600 steps per revolution (8 microsteps per full step), in National Instruments software this would be considered to be a 1600 stepper steps per revolution motor.
National Instruments Motion Interface Drives (MID)
The MID-7604 and 7602 are configured with the DIP switches on the front of the drives as shown in the MID-7604/7602 Power Drive User Guide on pages 12-13.
The DIP switches allow for microstepping factors from 2 to 256. These drives do not allow for a microstepping factor to 1 (no microstepping). While this may seem like a limitation, there is no advantage to turning off microstepping. With microstepping the resolution is increased, and the MIDs allow for the torque on a microstep to be equal to the torque on a full step. The only downside is that microstepping can produce non-linear movement, depending on the motor. However, this is not an issue with a microstepping factor of 2, since it will always be linear as shown above.
Danaher Drives (P70530 and P70360)
Danaher refer to microstepping as step resolution. The step resolution can be configured through one of two methods. One is through the DIP switches on the side of the drives as shown in the P7000 Series Stepper Drives Getting Started manual on page 9. The other is through the Danaher P7000 Tools, a software interface to configure the hardware.
When configuring the drive using the P7000 Tools, verify that the Tooth Count is correct. This will be located under Stepper Motor»Properties. The tooth count is one fourth of the number of steps per revolution with no microstepping. If this number is not correct it can be changed in the Motor File Editor. Set the desired step resolution under Command»Command Signal Configuration»Step Resolution in Steps / Motor Rev. If the drive does not support the desired value it will coerce down to the nearest increment that is supported. All of the microstepping will then be configured on the drive.
Related Links
Developer Zone Tutorial: Stepper Motor Switching Sequence
Developer Zone Tutorial: Motor Fundamentals
KnowledgeBase 3409E7OK: How Do I Determine the Accuracy of a Stepper Motor and Encoder?
MID-7604/7602 Power Drive User Guide
P7000 Series Stepper Drives Getting Started
Reader Comments | Submit a comment »
Legal
This tutorial (this "tutorial") was developed by National Instruments ("NI"). Although technical support of this tutorial may be made available by National Instruments, the content in this tutorial may not be completely tested and verified, and NI does not guarantee its quality in any way or that NI will continue to support this content with each new revision of related products and drivers. THIS TUTORIAL IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND AND SUBJECT TO CERTAIN RESTRICTIONS AS MORE SPECIFICALLY SET FORTH IN NI.COM'S TERMS OF USE (http://ni.com/legal/termsofuse/unitedstates/us/).