Stepper motors and servo motors are commonly used motors in the electronics industry and are machines that electricians need to learn and use. However, many beginners are unclear about the working principles of stepper motors and servo motors, and do not know how to use stepper motors or servo motors according to the scenario. Therefore, this article will answer these questions one by one, hoping to help beginners.
1. Working principle of stepper motors and servo motors
Servo motors primarily rely on pulses for positioning. When a servo motor receives one pulse, it will rotate by the angle corresponding to that pulse, thus achieving displacement. This is because the servo motor itself has the function of emitting pulses. Therefore, the servo motor will emit a corresponding number of pulses for each rotation angle. This forms a response, or closed loop, with the pulses received by the servo motor. In this way, the system will know how many pulses were emitted to the servo motor and how many pulses were received back, thus enabling very precise control of the electrode rotation.
A stepper motor is a discrete motion device, fundamentally linked to modern digital control technology. It is widely used in current domestic digital control systems. With the emergence of fully digital AC servo systems, AC servo motors are also increasingly applied in digital control systems. To adapt to the development trend of digital control, most motion control systems use stepper motors or fully digital AC servo motors as actuators. Although they are similar in control methods (pulse trains and direction signals) and flexible couplings, they differ significantly in performance and application scenarios.
2. Comparison of stepper motors and servo motors
① Differences in control precision
Two-phase hybrid stepper motors typically have a step angle of 3.6° or 1.8°, while phaseless hybrid stepper motors typically have a step angle of 0.72° or 0.36°. Some high-performance motors also have even smaller step angles.
The control precision of the AC servo motor is ensured by the rotary encoder at the rear end of the motor shaft.
②Different low-frequency characteristics
Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load and driver performance; generally, it is considered to be half the motor's no-load starting frequency. This low-frequency vibration, determined by the working principle of the stepper motor, is very detrimental to the normal operation of the machine. When the stepper motor operates at low speeds, damping techniques should generally be used to overcome the low-frequency vibration, such as adding a damper to the motor or using microstepping technology in the driver.
The AC servo motor operates very smoothly with its diaphragm coupling, exhibiting no vibration even at low speeds. The AC servo system features resonance suppression to compensate for insufficient mechanical rigidity, and its built-in frequency response time (FFT) function detects resonance points, facilitating system adjustments.
③ Different moment-frequency characteristics
The output torque of a stepper motor decreases as the speed increases, and drops sharply at higher speeds. Therefore, its maximum operating speed is generally between 300 and 600 RPM. AC servo motors provide constant torque output, meaning they can output rated torque up to their rated speed (generally 2000 or 3000 RPM), and provide constant power output above the rated speed.
④ Differences in overload capacity
Stepper motors generally lack overload capacity. AC servo motors, on the other hand, have strong overload capacity. For example, Panasonic's AC servo system features both speed and torque overload capabilities. Its maximum torque is three times its rated torque, which can be used to overcome the inertial torque of inertial loads at startup. Because stepper motors lack this overload capacity, a larger torque motor is often selected to overcome this inertial torque during selection, while the machine does not require such a large torque during normal operation, resulting in wasted torque.
⑤ Differences in operating performance
Stepper motors are controlled in an open-loop manner. Excessive starting frequency or load can easily lead to missed steps or stalling. Excessive stopping speed can cause overshoot. Therefore, to ensure control accuracy, the acceleration and deceleration issues must be properly addressed. AC servo drive systems, on the other hand, use closed-loop control. The driver can directly sample the feedback signal from the motor encoder, internally forming position and speed loops. Generally, the missed steps or overshoot issues of stepper motors are not present, resulting in more reliable control performance.
⑥ Different speed response performance
A stepper motor takes 200-400 milliseconds to accelerate from a standstill to its operating speed (typically several hundred revolutions per minute). AC servo systems offer better acceleration performance.
In summary, AC servo systems outperform stepper motors in many aspects, but stepper motors are still used to drive motors in applications where performance requirements are not high. Therefore, the design of the control system should fully consider factors such as control requirements and cost, and select appropriate control electrodes.
