1. What is a servo? Why use a servo? Servo system definition: A control system that enables the output variable to accurately follow or reproduce the input variable. With increasingly demanding requirements for motion control, servo control has emerged.
2. What is a servo motor? What are its characteristics? A servo motor, also known as an actuator motor, is used as an actuating element in automatic control systems. It converts received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are broadly classified into DC and AC servo motors. Their main characteristic is that they do not rotate when the signal voltage is zero, and their speed decreases uniformly as the torque increases.
The characteristics of servo points will be explained more clearly here in comparison with those of stepper motors :
1. Different Control Precision: Two-phase hybrid stepper motors typically have step angles of 3.6° and 1.8° , while five-phase hybrid stepper motors typically have step angles of 0.72° and 0.36° . Some high-performance stepper motors have even smaller step angles. For example, a stepper motor produced by Sitong Company for wire EDM machines has a step angle of 0.09° ; the three-phase hybrid stepper motor produced by BERGERLAHR in Germany has a step angle that can be set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, and 0.036° via a DIP switch , making it compatible with the step angles of both two - phase and five - phase hybrid stepper motors. The control precision of AC servo motors is ensured by a rotary encoder at the rear end of the motor shaft. Taking Panasonic's all-digital AC servo motor as an example, for a motor with a standard 2500 -line encoder, due to the quadruple frequency technology used in the driver, its pulse equivalent is 360°/10000 = 0.036° . For a motor with a 17- bit encoder, the motor rotates once for every 2^17 = 131072 pulses received by the driver, meaning its pulse equivalent is 360°/131072 = 9.89 seconds. This is 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8° .
2. 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, and is generally considered to be half of the motor's no-load starting frequency. This low-frequency vibration, determined by the working principle of stepper motors, is very detrimental to the normal operation of the machine. When stepper motors operate at low speeds, damping techniques should generally be used to overcome low-frequency vibration, such as adding dampers to the motor or using microstepping technology in the driver. AC servo motors operate very smoothly and do not vibrate even at low speeds. AC servo systems have resonance suppression capabilities, which can cover insufficient mechanical rigidity, and the system has an internal frequency analysis function ( FFT ) to detect mechanical resonance points, facilitating system adjustment.
3. Due to differences in torque-frequency characteristics, the output torque of a motor decreases as the speed increases, and drops sharply at higher speeds. Therefore, its maximum operating speed is generally between 300 and 600 RPM . An AC servo motor, on the other hand, provides constant torque output, meaning it can output rated torque up to its rated speed (generally 2000 or 3000 RPM ), and provides constant power output above the rated speed.
4. Overload Capacity Differences: Stepper motors generally lack overload capacity. AC servo motors, on the other hand, have strong overload capacity. Taking Panasonic's AC servo system as an example, it has speed and torque overload capabilities. Its maximum torque is three times the rated torque, which can be used to overcome the inertial torque of inertial loads at startup. Because stepper motors lack this overload capacity, a motor with a larger torque is often selected to overcome this inertial torque during selection. However, the machine does not need such a large torque during normal operation, resulting in wasted torque.
5. The operating performance of stepper motors differs from that of other motors. Stepper motors use open-loop control, which can easily lead to missed steps or stalling due to excessive starting frequency or load. Excessive speed at stop can cause overshoot. Therefore, to ensure control accuracy, acceleration and deceleration must be carefully managed. AC servo drive systems, on the other hand, use closed-loop control. The driver can directly sample the encoder feedback signal, internally forming position and speed loops. Generally, step loss or overshoot issues common in stepper motors are not present, resulting in more reliable control performance.
6. Different motors require 200 to 400 milliseconds to accelerate from a standstill to their operating speed (typically several hundred revolutions per minute) . AC servo systems offer better acceleration performance. For example, the Panasonic MSMA400W AC servo motor accelerates from a standstill to its rated speed of 3000 RPM in just a few milliseconds, making it suitable for control applications requiring rapid start and stop.
