Servo Motors

What is a Servo Motor

 

A servo motor is a motor that controls the operation of mechanical components in a servo system. It is a type of auxiliary motor with indirect speed change.

 

A servo motor can control speed with extremely precise positioning accuracy, converting voltage signals into torque and speed to drive the controlled object. The rotor speed of a servo motor is controlled by an input signal and responds quickly. It is used as an actuator in automatic control systems and features a small electromechanical time constant and high linearity. It converts the received electrical signal into an angular displacement or angular velocity output on the motor shaft. Servo motors are classified into two categories: DC and AC. Their main characteristics are that they exhibit no self-rotation when the signal voltage is zero, and their speed decreases uniformly as torque increases.

 

Working Principle of Servo Motor

 

A servo system is an automatic control system that enables an output variable (e.g., position, orientation, or state) to follow arbitrary changes in an input target (or setpoint). A servo primarily relies on pulses for positioning. Basically, when a servo motor receives a pulse, it rotates by the angle corresponding to that pulse, thereby achieving displacement. Because the servo motor itself emits pulses, it sends a corresponding number of pulses for each angle it rotates, creating a closed loop with the pulses it receives. This ensures the system knows how many pulses it has sent to the servo motor and how many pulses it has received back. This allows for precise control of the motor's rotation, achieving positioning accuracy as low as 0.001mm. DC servo motors are categorized as brushed and brushless. Brushed motors offer low cost, a simple structure, high starting torque, a wide speed range, and ease of control. While they require maintenance, they are inconvenient (due to carbon brush replacement), generate electromagnetic interference, and have environmental requirements. Therefore, they are suitable for cost-sensitive general industrial and consumer applications.

Brushless motors are compact and lightweight, offering high output, fast response, high speed, low inertia, smooth rotation, and stable torque. While complex control is easy to implement with intelligent technology, their electronic commutation is flexible, supporting both square-wave and sine-wave commutation. They are maintenance-free, highly efficient, operate cool, emit minimal electromagnetic radiation, and have a long lifespan, making them suitable for use in a variety of environments.

 

AC servo motors are also brushless motors and are categorized as synchronous and asynchronous motors. Synchronous motors are generally used in motion control. They have a wide power range and can achieve very high power. Their high inertia results in a low maximum speed that decreases rapidly as power increases, making them suitable for applications requiring low-speed, stable operation.

 

The rotor inside a servo motor is a permanent magnet. The three-phase U/N/N current controlled by the driver creates an electromagnetic field, causing the rotor to rotate. Simultaneously, the motor's built-in encoder provides feedback to the driver, which compares the feedback with the target value and adjusts the rotor's rotation angle. The accuracy of the servo motor is determined by the encoder's accuracy (line count).

 

The functional difference between AC servo motors and brushless DC servo motors is that AC servos are superior because they use sinusoidal wave control, resulting in low torque ripple. DC servos use trapezoidal wave control.

 

Servo Motor Selection Comparison

 

AC Servo Motor

The stator structure of an AC servo motor is essentially similar to that of a capacitor-split single-phase asynchronous motor. The stator is equipped with two windings, positioned 90° apart: the excitation winding Rf, which is always connected to the AC voltage Uf; the control winding L, which is connected to the control signal voltage Uc. Therefore, AC servo motors are also called two-phase servo motors.

 

The rotor of an AC servo motor is typically a squirrel-cage type. However, to ensure a wide speed regulation range, linear mechanical characteristics, no "self-rotation" phenomenon, and fast response, the servo motor must have a high rotor resistance and a low moment of inertia compared to ordinary motors. Two commonly used rotor structures are: a squirrel-cage rotor with high-resistivity bars made of high-resistivity conductive material. To reduce the rotor's moment of inertia, the rotor is slender. The other is a hollow cup rotor made of aluminum alloy with a very thin wall, only 0.2-0.3 mm. To reduce the magnetic resistance of the magnetic circuit, a fixed inner stator is placed inside the hollow cup rotor. Hollow cup rotors offer low moment of inertia, quick response, and smooth operation, making them widely adopted.

 

When no control voltage is applied, the stator of an AC servo motor is subject to only the pulsating magnetic field generated by the excitation winding, and the rotor remains stationary. When a control voltage is applied, a rotating magnetic field is generated within the stator, causing the rotor to rotate in the direction of the rotating magnetic field. Under constant load conditions, the motor's speed varies with the magnitude of the control voltage. When the control voltage is out of phase, the servo motor reverses.

 

Permanent magnet AC servo motor

 

Since the 1980s, with the development of integrated circuits, power electronics technology and AC variable speed drive technology, permanent magnet AC servo drive technology has made outstanding progress. Famous electrical manufacturers in various countries have successively launched their own AC servo motor and servo drive series products and continuously improved and updated them. AC servo system has become the main development direction of contemporary high-performance servo system, making the original DC servo face the crisis of being eliminated. After the 1990s, the AC servo system that has been commercialized in various countries around the world uses a fully digitally controlled sine wave motor servo drive. The development of AC servo drive devices in the transmission field is changing with each passing day.

 

Compared with DC servo motors, the main advantages of permanent magnet AC servo motors are:

(1) No brushes and commutators, so it works reliably and has low maintenance and servicing requirements.

(2) Stator winding heat dissipation is relatively convenient

(3) Small inertia, easy to improve the speed of the system

(4) Adapt to high-speed and high-torque working conditions

(5) Smaller volume and weight at the same power

 

Comparison between Servomotors and Single-Phase Asynchronous Motors

 

Although the operating principle of AC servomotors is similar to that of split-phase single-phase asynchronous motors, the rotor resistance of servomotors is much greater. Therefore, servomotors have three distinct advantages over single-phase asynchronous motors:

 

1. High Starting Torque

This high rotor resistance significantly differs from the torque characteristic curve of ordinary asynchronous motors. This allows the critical slip S0 to be greater than 1, making the torque characteristic (mechanical characteristics) more linear and providing a higher starting torque. Therefore, as soon as the stator control voltage is applied, the rotor rotates immediately, resulting in fast starting and high sensitivity.

2. Wide Operating Range

3. No Self-Rotation

 

A normally operating servomotor will stop immediately upon loss of control voltage. When a servo motor loses control voltage, it operates in a single-phase state. Due to the high rotor resistance, the interaction between the two oppositely rotating magnetic fields in the stator and the rotor produces two torque characteristics (T1-S1 and T2-S2 curves) and a combined torque characteristic (T-S curve).

 

The output power of an AC servo motor generally ranges from 0.1 to 100W. When the power supply frequency is 50Hz, the voltages are 36V, 110V, 220V, and 380V; when the power supply frequency is 400Hz, the voltages are 20V, 26V, 36V, and 115V.

 

AC servo motors operate smoothly and quietly. However, their control characteristics are nonlinear, and due to the high rotor resistance, losses are high, and efficiency is low. Therefore, compared with DC servo motors of the same capacity, they are larger and heavier, making them suitable only for low-power control systems in the 0.5-100W range.

 

Servo motor debugging method

 

1. Initializing Parameters

Before wiring, initialize the parameters.
On the control card: Select the control mode; Clear the PID parameters to zero, disabling the enable signal by default when the control card is powered on; Save this state to ensure that the control card is in this state when it is powered on again.
On the servo motor: Set the control mode: Enable externally; Set the gear ratio for the encoder signal output; and Set the ratio between the control signal and motor speed. Generally speaking, it is recommended that the maximum design speed of the servo be set to a 9V control voltage. For example, one of our series has a speed setting of 500 for a 1V voltage. If you only plan to operate the motor at speeds below 1000 rpm, set this parameter to 111.

 

2. Wiring
Disconnect the control card and connect the signal cables between the control card and the servo. The following cables are required: the analog output cable from the control card, the enable signal cable, and the encoder signal cable from the servo output. After verifying the wiring, power on the motor and control card (and PC). The motor should not move and should rotate easily with force. If not, check the enable signal settings and wiring. Rotate the motor to verify that the control card can correctly detect changes in motor position. If not, check the encoder signal wiring and settings.

 

3. Test Direction
For a closed-loop control system, incorrect feedback signal direction can be disastrous. Enable the servo enable signal via the control card. The servo should now rotate at a slower speed, known as "zero drift." The control card typically includes a command or parameter to suppress zero drift. Use this command or parameter to test whether the motor's speed and direction can be controlled. If not, check the analog wiring and control method parameter settings. Ensure that positive values indicate forward motor rotation and encoder counts increase, while negative values indicate reverse motor rotation and encoder counts decrease. Do not use this method if the motor is loaded and has limited travel. Avoid excessive voltage during testing; it is recommended to keep it below 1V. If the direction is inconsistent, adjust the parameters on the control card or motor to align.

 

4. Zero Drift Suppression
During closed-loop control, zero drift can affect control effectiveness, so it's best to suppress it. Use the zero drift suppression parameters on the control card or server to carefully adjust the motor speed to near zero. Since zero drift itself has some randomness, it's not necessary to require the motor speed to be absolutely zero.

 

5. Establish Closed-Loop Control
Re-enable the servo enable signal through the control card. Enter a small proportional gain on the control card. As for how large is considered small, you can only use your gut feeling. If you are really unsure, enter the minimum value allowed by the control card. Turn on the enable signals for both the control card and the servo. At this point, the motor should be able to roughly follow the motion commands.

 

6. Adjust Closed-Loop Parameters
Finely adjusting the control parameters to ensure the motor moves according to the control card's instructions is essential, and this part of the process relies heavily on experience.

 

Performance Comparison of Servo Motors and Stepper Motors

 

As an open-loop control system, stepper motors are intrinsically linked to modern digital control technology. Stepper motors are widely used in domestic digital control systems. With the advent of fully digital AC servo systems, AC servo motors are also increasingly being used in digital control systems. To adapt to the development trend of digital control, motion control systems mostly use stepper motors or fully digital AC servo motors as actuator motors. Although their control methods are similar (pulse train and direction signals), they differ significantly in performance and application scenarios. This paper compares their performance.

 

1. Different Control Accuracy
The step angles of two-phase hybrid stepper motors are generally 1.8° and 0.9°, while those of five-phase hybrid stepper motors are generally 0.72° and 0.36°. Some high-performance stepper motors can achieve even smaller step angles through subdivision.

The control accuracy of AC servo motors is ensured by a rotary encoder at the rear end of the motor shaft. Taking our digital AC servo motors as an example, for a motor with a standard 2000-line encoder, the driver's internal quadruple frequency technology yields a pulse equivalent of 360°/8000 = 0.045°. For a motor with a 17-bit encoder, the driver receives 131,072 pulses per motor revolution, resulting in a pulse equivalent of 360°/131,072 = 0.0027466°, which is 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8°.

 

2. Different Low-Frequency Characteristics
Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency depends on the load and driver performance, and is generally considered to be half the motor's no-load starting frequency. This low-frequency vibration, determined by the operating principle of stepper motors, is detrimental to the normal operation of the machine. When stepper motors operate at low speeds, damping techniques are generally used to overcome low-frequency vibration, such as adding a damper to the motor or implementing subdivision technology in the driver.

AC servo motors operate very smoothly, without vibration even at low speeds. AC servo systems have resonance suppression capabilities to address mechanical rigidity deficiencies. Furthermore, the system's built-in frequency-resolution function (FFT) can detect mechanical resonance points, facilitating system adjustments.

 

3. Different Torque-Frequency Characteristics
The output torque of a stepper motor decreases as speed increases, and drops sharply at higher speeds. Therefore, its maximum operating speed is generally between 300 and 600 RPM. AC servo motors offer constant torque output, meaning they can deliver rated torque up to their rated speed (typically 2000 or 3000 RPM). Above the rated speed, they deliver constant power.

 

4. Different overload capacities.

Stepper motors generally lack overload capacity. AC servo motors, however, have a strong overload capacity. For example, Sanyo AC servo systems offer both speed and torque overload capabilities. Their maximum torque is two to three times the rated torque, which can be used to overcome the moment of inertia of inertial loads at startup. Because stepper motors lack this overload capacity, higher torque is often required to overcome this moment of inertia during model selection. However, this high torque is not required during normal machine operation, resulting in wasted torque.

 

5. Different Operating Performance
Stepper motors are open-loop controlled. High starting frequencies or excessive loads can easily lead to lost steps or stalling. High speeds during stopping can also cause overshoot. Therefore, to ensure control accuracy, proper acceleration and deceleration must be addressed. AC servo drive systems utilize closed-loop control. The driver directly samples the motor encoder feedback signal, internally forming a position loop and a velocity loop. This generally prevents the lost steps or overshoot associated with stepper motors, resulting in more reliable control performance.

 

6. Different Speed Response Performance
A stepper motor takes 200 to 400 milliseconds to accelerate from a standstill to its operating speed (typically several hundred revolutions per minute). AC servo systems offer superior acceleration performance. For example, our AC servo motors accelerate from a standstill to their rated speed of 3000 RPM in just a few milliseconds, making them suitable for control applications requiring rapid start and stop times.

In summary, AC servo systems outperform stepper motors in many performance aspects. However, stepper motors are often used as actuator motors in less demanding applications. Therefore, when designing a control system, it is important to comprehensively consider multiple factors, including control requirements and cost, to select the appropriate control motor.

 

Servo Motor Selection Calculation

 

1. Confirm the speed and encoder resolution.
2. Convert the load torque on the motor shaft and calculate the acceleration and deceleration torque.
3. Calculate the load inertia and match the inertia. For example, in our series, some products can match the inertia by up to 50 times, but the lower the better, the better for accuracy and response speed.
4. Calculate and select the regenerative resistor. For servos above 2 kW, an external resistor is generally required.
5. Cable selection: The encoder cable should be twisted and shielded. For our products, the absolute encoder has 6 cores, and the incremental encoder has 4 cores.

 

Braking mode

 

Users often confuse the functions of electromagnetic braking, regenerative braking, and dynamic braking, and choose the wrong accessories.

The dynamic brake is composed of a dynamic braking resistor, which shortens the mechanical feed distance of the servo motor through energy consumption braking in the event of a fault, emergency stop, or power outage.

Regenerative braking refers to the energy generated by the servo motor when it decelerates or stops, which is fed back to the DC bus through the inverter circuit and absorbed by the resistor-capacitor circuit.

 

Electromagnetic braking locks the motor shaft through a mechanical device.

 

The differences between the three:

(1) Regenerative braking must be effective when the servo is operating normally. It cannot brake the motor in the event of a fault, emergency stop, or power outage. Dynamic brakes and electromagnetic brakes do not require power when they are working.

(2) Regenerative braking is performed automatically by the system, while dynamic brakes and electromagnetic brakes require external relay control.

(3) Electromagnetic braking is generally activated after SVOFF, otherwise it may cause amplifier overload. Dynamic brakes are generally activated after SV, OFF, or the main circuit is powered off, otherwise it may cause the dynamic brake resistor to overheat.