Summary of practical experience in design and selection of stepper motor drives

A stepper motor is a special motor used for control. It rotates step by step at a fixed angle (called "step angle"). It has the characteristic of no accumulated error (accuracy is 100%), so it is widely used in various open-loop controls. The operation of a stepper motor requires an electronic device to drive it. This device is a stepper motor driver, which converts the pulse signal sent by the control system into the angular displacement of the stepper motor. In other words, every time the control system sends a pulse signal, the driver causes the stepper motor to rotate one step angle. Therefore, the speed of the stepper motor is proportional to the frequency of the pulse signal. Therefore, controlling the frequency of the stepper pulse signal can accurately adjust the speed of the motor; controlling the number of stepper pulses can accurately position the motor. When the stepper motor is driven by a subdivision driver, its step angle becomes smaller. For example, when the driver works in the 10-subdivision state, its step angle is only one tenth of the "motor's inherent step angle", that is to say: "When the driver works in the full-step state without subdivision, the control system sends a step pulse, and the motor rotates 1.8°; when the subdivision driver works in the 10-subdivision state, the motor only rotates 0.18°. This is the basic concept of subdivision. The subdivision function is completely generated by the driver by accurately controlling the phase current of the motor, and has nothing to do with the motor.

The main advantages of the driver after subdivision are: completely eliminating the low-frequency oscillation of the motor. Low-frequency oscillation is an inherent characteristic of stepper motors (especially reactive motors), and subdivision is the only way to eliminate it. If your stepper motor sometimes has to work in the resonance zone (such as walking in an arc), choosing a subdivision driver is the only choice. It improves the output torque of the motor. Especially for three-phase reactive motors, its torque is about 30-40% higher than when it is not subdivided. It improves the resolution of the motor. Since the step angle is reduced and the uniformity of the step is improved, "improving the resolution of the motor" is self-evident.

The above is the basic principle of stepper motor. Next, we will summarize the practical experience of designing and selecting stepper motor drives:

Select holding torque (HOLDING TORQUE)

Holding torque is also called static torque, which refers to the torque of the stator locking the rotor when the stepper motor is powered but not rotating. Since the torque of the stepper motor at low speed is close to the holding torque, and the torque of the stepper motor decays rapidly with the increase of speed, and the output power also changes with the increase of speed, the holding torque is one of the most important parameters to measure the load capacity of the stepper motor. For example, when talking about a stepper motor of 1N.m without explanation, it can be understood that the holding torque is 1N.m.

Select the number of phases

The two-phase stepper motor has low cost and a step angle of at least 1.8 degrees. It has greater vibration at low speed and faster torque drop at high speed. It is suitable for high-speed applications with low requirements on precision and stability. The three-phase stepper motor has a step angle of at least 1.5 degrees and less vibration than the two-phase stepper motor. It has better low-speed performance than the two-phase stepper motor and a maximum speed 30 to 50 percent higher than the two-phase stepper motor. It is suitable for high-speed applications with high requirements on precision and stability. The five-phase stepper motor has a smaller step angle and better low-speed performance than the three-phase stepper motor, but its cost is relatively high. It is suitable for medium and low speed applications with high requirements on precision and stability.

Selecting a stepper motor

The principle of selecting the motor first and then the driver should be followed. First, the load characteristics should be clarified, and then the stepper motor that best matches the load characteristics should be found by comparing the static torque and torque-frequency curves of different stepper motor models. When the precision requirement is high, a mechanical reduction device should be used to make the motor work in the state of highest efficiency and lowest noise. Avoid making the motor work in the vibration zone. If necessary, solve it by changing the voltage, current or increasing the damping method. In terms of power supply voltage, it is recommended that 57 motors use DC 24V-36V, 86 motors use DC 46V, and 110 motors use more than DC 80V. For large rotational inertia loads, motors with larger machine base numbers should be selected. When the inertia load is large and the working speed is high, the motor should be gradually increased in frequency to prevent the motor from losing steps, reduce noise, and improve the positioning accuracy when it stops. In view of the fact that the torque of the stepper motor is generally below 40Nm, when it exceeds this torque range and the operating speed is greater than 1000RPM, you should consider choosing a servo motor. Generally, AC servo motors can operate normally at 3000RPM, and DC servo motors can operate normally at 10000RPM.

Select drive and subdivision number

It is best not to choose the full-step state, because the vibration is large in the full-step state; try to choose a driver with small current, large inductance and low voltage; use a driver with a current greater than the working current, use a subdivision driver when low vibration or high precision is required, and use a high-voltage driver for large-torque motors to obtain good high-speed performance; in situations where the actual speed of the motor is usually high and the requirements for accuracy and stability are not high, it is not necessary to choose a high-subdivision driver in order to save costs; in conditions where the actual speed of the motor is usually very low, a larger subdivision should be used to ensure smooth operation and reduce vibration and noise; in short, when selecting the subdivision, the actual operating speed of the motor, load torque range, reducer setting, accuracy requirements, vibration and noise requirements, etc. should be comprehensively considered.