Common two-phase hybrid stepper motor applications

Stepper motors are often used for positioning. They are cost-effective, easy to drive, can be used in open-loop systems, and do not require position feedback like servo motors. Therefore, they are very suitable for small industrial machines such as laser engravers, 3D printers, and office equipment such as laser printers.

There are also many different types of stepper motors. For industrial applications, the most common two-phase hybrid stepper motor is the 200-step-per-revolution stepper motor. The “hybrid” here refers to the way it works with permanent magnets and a toothed iron rotor (like a variable reluctance motor), and the “200-step” refers to the fact that the motor moves 1.8° per step, which is a function of the number of teeth on the rotor and stator.

This article will focus on the most common two-phase hybrid stepper motor. Figure 1 is a typical two-phase hybrid motor.

Figure 1: Typical two-phase hybrid stepper motor

Microstepping

Stepper motors can be set to smaller steps, called microstepping. This is accomplished by adjusting the winding current so that the rotor can be positioned between full steps. Designers can define microstepping of almost any size, as the step size is limited only by the resolution of the digital-to-analog converter (DAC) and amplifier driving the winding current, so resolutions of 1/256 or even 1/1024 are common.

However, in reality for most mechanical systems, such fine microstepping does not always improve positioning accuracy, and there are many other factors that can negatively impact performance.

Intrinsic error

There are several sources of angular error in microstepping. One is the imperfections of the motor itself, such as mechanical and magnetic imperfections. No motor has a perfect sinusoidal current-position transfer function. Even if you can apply perfect sine and cosine currents to the motor, the motor's motion cannot be absolutely linear.

Another source of error is the current regulation accuracy of the stepper motor controller. Typical stepper motor ICs are only accurate to about 5% of the full-scale current. In addition, the current regulation matching between the two channels may not be perfect. These inaccuracies will reduce the positioning accuracy.

Stepper motor torque

Stepper motors are rated for holding torque. Holding torque is the torque required to pull the motor away from a full step position and is also the torque the motor can produce when it moves one full step. After each full step, the teeth align with the minimum magnetic path, resulting in a strong torque.

The X in the above formula represents the number of microstepping steps.

For example, for a 1/8 step, the incremental torque is approximately 20% of the full step torque; for a 1/32 step, the incremental torque is only 5% of the full step torque.

For motion control systems, it represents the desired position to actually be achieved when performing microstepping, and the torque load on the motor must be much less than the motor's rated holding torque.

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Laboratory measurements

We tested the positioning accuracy of microstepping through several experiments. The lab setup used a first-surface mirror mounted on the stepper motor shaft and a laser. First, the beam was reflected by the mirror to the other side of the lab, about 9 meters away; then we measured the elevation of the laser beam and calculated the angle. The accuracy of the measurement was mainly limited by the accuracy of measuring the height of the beam; a height of ±1mm corresponds to an accuracy of ±0.006°.

The motor used in the experiment is a typical hybrid motor, commonly used in products such as 3D printers. The motor is a 1.8° bipolar motor with a rated current of 2.8A and a holding torque of 1.26Nm.

The first experiment measured the accuracy of the motor alone. We drove the two phases with a precise DC current source, with no torque load on the motor shaft and only a mirror mounted on the shaft (see Figure 2).

The results measured with this setup show a small amount of nonlinearity; however, overall the angular accuracy is good, about ±0.03°. Furthermore, the motor motion is monotonic (see Figure 3); that is, the motor never moves in the wrong direction or fails to move. If such errors occur, it can only be explained by inherent errors in the motor itself or by measurement errors. In this case, 1/32 step corresponds to an accuracy of 0.056°.

Figure 3: 1/32 stepper motor no-load accuracy

Next, the motor is coupled with a magnetic powder brake, which is used to apply a friction torque load to the motor (see Figure 4).

Figure 4: Brake assembly

The above measurements were repeated using the same DC current source to apply a torque of approximately 0.1Nm to the motor shaft. Figure 5 shows that the motor pauses after every other step, which is very different from the previous measurement results.

Figure 5: 1/32 stepper motor accuracy with increased torque

This behavior is consistent with the calculated incremental torque of the motor. The incremental torque of 1/32 microstepping is approximately 5% of the holding torque. With a holding torque of 1.26Nm, the expected torque produced by one microstepping step is approximately 0.06Nm. Of course, this is not enough to overcome the friction load, so two microstepping steps are required to get the torque high enough to overcome the load.

If the torque is increased to 0.9Nm (approximately 70% of the stall torque), more microstepping steps are required to increase the torque to the point where the motor moves (see Figure 6).

Figure 6: 1/32 stepper motor with 0.9Nm torque

We conducted two similar experiments using the MP6500, MP6500, MP6500, stepper motor driver IC from MPS. The MP6500 MP6500 MP6500 MP6500 uses precise PWM current regulation and can operate in full, half, quarter, or eighth steps.

To test whether there is a difference in accuracy using a conventional stepper motor driver IC versus using a DC current source, a test was first performed at 0.1Nm torque and 1/8 step mode. The torque produced by a 1/8 step is approximately 20% of a full step, or 0.25Nm, which is greater than the applied 0.1Nm torque.

The second test applied 0.4Nm of torque. This exceeded the incremental holding torque of 1/8 step (0.25Nm). As expected, microstepping was skipped.

Mechanical System Considerations

To achieve the accuracy required for microstepping, designers must also consider the mechanical system.

There are several ways to generate linear motion using a stepper motor. The first method is to connect the motor to the moving part via a belt and pulley. In this case, rotation is converted into linear motion. The distance of the linear motion is a function of the angle of the motor movement and the diameter of the pulley.

The second method uses a lead screw or ball screw. A stepper motor is directly connected to the end of the lead screw, and as the lead screw rotates, the nut travels in a linear fashion.

In both cases, whether a single microstepping step can achieve actual linear motion depends on the friction torque. This means that in order to achieve the best accuracy, the friction torque must be minimized.

For example, many lead screw and ball screw nuts have some preload adjustability. Preload is a force used to prevent backlash, which causes some play in the system. However, increasing preload will reduce backlash, but it will also increase friction. Therefore, there is a trade-off between backlash and friction.

in conclusion

When designing a motion control system using a stepper motor, it cannot be assumed that the rated holding torque of the motor is still applicable in microstepping mode, because the incremental torque is greatly reduced in this mode, which may lead to unexpected positioning errors. The above tests have proven this. In some cases, increasing the microstepping resolution does not improve the system accuracy.

To overcome these limitations, it is recommended to minimize the torque load on the motor or use a motor with a higher holding torque rating. Often, the best solution is to design the mechanical system for larger step increments rather than relying on fine microstepping. Stepper motor drivers like the MP6500 MP6500 MP6500 MP6500 provide mechanical performance in 1/8 step mode that is comparable to expensive traditional microstepping drivers.