When discussing the accuracy of motion systems, we find that many factors influence it. Engineers must understand how to coordinate stepper motors, drivers, or controllers to achieve optimal operational precision when considering system accuracy.
A common misconception is attributing all system accuracy errors to motor problems. From the perspective of stepper motors, certain tolerance standards must be met, including mechanical and electrical tolerances. Uneven phase winding inductance is a significant factor; other causes include pole shoe and rotor misalignment, uneven air gap between stator and rotor, stator and rotor tooth cogging relationship, and torque pulsation. Achieving and continuously controlling these parameters is not particularly difficult.
We know that the inductance of a motor stator winding is proportional to the square of the number of turns. Using appropriate winding techniques can ensure consistency between phase inductances. Many stepper motor manufacturers already use automated winding equipment to guarantee this consistency. Of course, the rotor's magnetic material should also possess good consistency to ensure uniform inductance across all phases. Other specifications are mechanically related. Satisfactory accuracy can be achieved by manufacturers using reliable, high-quality components and excellent process control to ensure uniform grinding of the stator and rotor. As Belal Azim of Lin Engineering stated, by ensuring the error between phase inductances in a two-phase bipolar stepper motor is within ±5%, a 0.9-degree stepper motor using 64-microstep drive mode can achieve a positioning error of ±1.5 arcminutes, including precision and accuracy. Meeting these conditions allows the stepper motor to fully meet the required specifications.
The remaining task is to instruct the driver/controller to tell the stepper motor where and how to move, without compromising the motor's accuracy due to the driver's own low precision. A microstepping driver tells the motor how many microsteps to take by providing a specific excitation current to the motor windings. Stepper motors achieve the highest accuracy when operating in full-step mode because this perfectly matches the motor's mechanical design characteristics. At this mode, the stator and rotor teeth are perfectly aligned, and the current flowing through the windings is at its maximum. As the microstepping increments increase, the step angle decreases accordingly, making it increasingly difficult to guarantee accurate positioning.
Each stepper motor has its own specific performance specifications, many of which are designed based on actual application requirements. Motors designed for low-speed use often have a larger inductance, while those designed for high-speed use have a smaller inductance. To meet different motion characteristics, stepper motor design engineers need to adjust the design of the windings in the coil to satisfy mathematical formulas related to speed, torque, current, resistance, and inductance. Therefore, the same driver will exhibit different operating performance when matched with different motors, and similarly, the torque characteristics of a single motor will differ when matched with different drivers.
The ultimate solution to achieving the smoothest operating performance and precise positioning is proper matching of the motor and driver. Currently, the most popular approach is to design the driver's output current to be user-adjustable. Some drivers allow users to change the output current waveform via an adjustable potentiometer, some offer adjustable gain, and others allow users to download specific sine tables through a graphical user interface to adapt to motor characteristics. Adjustable potentiometers allow users to manually adjust the driver's output current to match motor characteristics without needing to understand the internal technology of the motor and driver. However, these methods still have drawbacks for users who want the best matching performance.
Typical microstepping modes include 2, 4, 8, 16, 32, 64, 128, and 256 microsteps; increasing the microstepping level correspondingly increases the accuracy of a single step (by dividing the entire step into equal microsteps to improve accuracy). For example, a 0.9-degree stepper motor driven with a 64-microstepping mode can achieve a step angle of 0.014 degrees. Of course, the driver must also provide the motor with precise current values according to the microstepping rules.
The diagram illustrates the spatial vector diagram of each phase of the motor in 4-microstep drive mode and the change of phase current over time. It visually shows the magnitude and direction of the motor phase current that the driver must provide for each microstep. From the driver's perspective, the main component for output current is the integrated driver chip. The driver's performance is determined by the chip's design. Other factors, such as the selected MOSFETs, resistors, capacitors, circuit wiring, corresponding firmware (software), and inadequate heat dissipation, also affect its performance. Therefore, even if the driver control chip can provide a smooth and accurate sinusoidal current waveform, optimal accuracy cannot be guaranteed. At position 12 in the phase winding spatial diagram, the coordinate percentages of phase A and phase B are (100, 0). The driver outputs 100% current to phase A, while phase B current is 0%. When the motor moves to position 13, the coordinates become (92, 38). At this point, the driver outputs 92% of the full current to phase A and 38% to phase B. Analyzing each step in this way reveals that the currents of phase A and phase B are two sinusoidal waves that differ by 90 degrees.
High precision originates from the stepper motor design. Lin Enigneering's high-precision 5709 stepper motor has a full step angle of 0.9 degrees and a frame size of NEMA23 (2.3 inches). It is matched with RMS's high-precision R325 driver, designed for exceptionally smooth motion control, forming a perfect stepper motor system that avoids current waveform distortion. [ALIGN] Five simple steps to match stepper motors and drivers in a motion control system:
1. Select a suitable motor (based on speed and torque requirements).
2. Confirm that the error between the inductances of each phase in the motor's technical specifications is within ±5%.
3. Select a suitable driver. If possible, obtain a waveform of the driver's output current.
4. Confirm that the driver has features or options to improve operational smoothness, such as adjusting the freewheeling damping depth (slow or fast current decay) or a potentiometer for adjusting the current waveform. 5. Match the motor inductance to the driver characteristics. Generally, high-inductance motors have better low-speed performance, but require a driver with high current damping (fast freewheeling) to allow the current to drop rapidly during freewheeling. Damping helps the inductor discharge quickly. Low-inductance motors have good high-speed performance; if the driver provides lower current damping (slow freewheeling), these motors will exhibit good operating characteristics because they do not require special damping assistance during inductor energy discharge. For some medium-inductance motors, a driver with hybrid freewheeling capability can be selected.
A more direct solution to improve the overall accuracy of the motor and driver is an integrated unit. Lin Enigneering's SilverPak D is such a unit, integrating a motor and driver. It's a NEMA 17 (1.7-inch) 1.8-degree bipolar stepper motor with an operating voltage range of +12 to 40V dc. The driver is controlled via a built-in PWM driver chip, allowing precise control of the motor phase current from 0.1 to 1.5A (peak). Step angles are available in 0.5, 0.25, 0.125, 0.0625, 0.03125, and 0.015625 degrees.
