A robot electric servo drive system is an actuator that uses the torque and force generated by various electric motors to directly or indirectly drive the robot body to obtain various movements of the robot.
For motors driving the joints of industrial robots, the requirements are: maximum power-to-weight ratio and torque-to-inertia ratio, high starting torque, low inertia, and a wide and smooth speed range. Especially for robot end effectors (grippers), motors with the smallest possible size and mass should be used. When rapid response is required, servo motors must possess high reliability and stability, as well as significant short-term overload capacity. These are prerequisites for the application of servo motors in industrial robots.
The main requirements for joint drive motors in robots are summarized as follows:
1. Speed
The time it takes for a motor to complete the required operating state after receiving a command signal should be short. The shorter the response time to the command signal, the higher the sensitivity of the servo system and the better its fast response performance. Generally, the electromechanical time constant of the servo motor is used to describe its fast response performance.
2. Large starting torque-to-inertia ratio
When driving a load, the robot's servo motor is required to have a large starting torque and a small moment of inertia.
3. Continuity and linearity of control characteristics: The motor speed can change continuously as the control signal changes, and sometimes the speed needs to be proportional or approximately proportional to the control signal.
4. Wide speed range.
It can be used in a speed range of 1:1000 to 10000.
5. Small size, light weight, and short axial dimension.
6. It can withstand harsh operating conditions, can perform very frequent forward and reverse and acceleration and deceleration operations, and can withstand overload for a short period of time.
Currently, due to the widespread use of AC and DC servo motors with high starting torque, large torque, and low inertia in industrial robots, most industrial robots with a load of less than 1000N (equivalent to 100kgf) employ electric servo drive systems. The joint drive motors used are mainly AC servo motors, stepper motors, and DC servo motors. Among them, AC servo motors, DC servo motors, and direct drive motors (DD) all employ closed-loop position control and are generally used in high-precision, high-speed robot drive systems. Stepper motor drive systems are mostly suitable for small, simple robot open-loop systems where precision and speed requirements are not high. AC servo motors, due to their electronic commutation and lack of commutation sparks, are widely used in flammable and explosive environments. The power range of robot joint drive motors is generally 0.1–10kW. These are the motors used in industrial robot drive systems.
II. Motors can be broadly classified into the following types:
1. AC servo motor
This includes synchronous AC servo motors and reactive stepper motors, etc.
2. DC servo motor
These include low-inertia permanent magnet DC servo motors, printed winding DC servo motors, high-inertia permanent magnet DC servo motors, and hollow cup armature DC servo motors.
3. Stepper motor
This includes permanent magnet induction stepper motors.
Speed sensors typically employ tachogenerators and rotary transformers; position sensors often use photoelectric encoders and rotary transformers. In recent years, foreign robot manufacturers have been using a hybrid photoelectric position sensor that integrates the functions of a photoelectric encoder and a rotary transformer. The servo motor can be combined with position and speed detectors, brakes, and reduction mechanisms to form a servo motor drive unit.
Robot drive systems require transmission systems with small backlash, high rigidity, high output torque, and large reduction ratio.
III. Commonly Used Speed Reduction Mechanisms
1. RV reduction gear;
2. Harmonic speed reduction machinery;
3. Cycloidal pinwheel reduction mechanism;
4. Planetary gear reduction machinery;
5. Backlash-free speed reduction mechanism;
6. Worm gear reduction mechanism;
7. Ball screw mechanism;
8. Metal belt/tooth reduction mechanism;
9. Ball deceleration mechanism.
The driving principle of the electric motor of an industrial robot is shown in Figure 1.
The general structure of an industrial robot electric servo system consists of three closed-loop controls: a current loop, a speed loop, and a position loop.
Currently, many foreign motor manufacturers have developed drive products compatible with AC servo motors. Users can choose different servo control methods according to their different functional requirements. Generally, AC servo drives can achieve the following functions by manually setting their internal function parameters:
1. Position control method;
2. Speed control method;
3. Torque control method;
4. A combination of position and velocity;
5. Position and torque hybrid mode;
6. Speed and torque hybrid mode;
7. Torque limiting;
8. Alarm for excessive position deviation;
9. Speed PID parameter settings;
10. Setting of velocity and acceleration feedforward parameters;
11. Zero drift compensation parameter settings;
12. Acceleration and deceleration time settings, etc.
IV. Types of Drives
1. DC servo motor driver
DC servo motor drivers mostly use pulse width modulation (PWM) servo drivers, which change the average voltage applied across the motor armature by changing the pulse width, thereby changing the motor speed.
PWM servo drivers are characterized by a wide speed range, good low-speed characteristics, fast response, high efficiency, and strong overload capacity. They are often used as DC servo motor drivers in industrial robots.
2. Synchronous AC servo motor driver
Compared with DC servo motor drive systems, synchronous AC servo motor drivers have advantages such as high torque-to-inertia ratio, no brushes, and no commutation sparks, and are widely used in industrial robots.
Synchronous AC servo motor drivers typically employ current-mode pulse-width modulation (PWM) phase inverters and multi-closed-loop control systems with an inner current loop and an outer speed loop to achieve current control of three-phase permanent magnet synchronous servo motors. Based on their operating principle, drive current waveform, and control method, they can be further divided into two types of servo systems:
(1) Permanent magnet AC servo system driven by rectangular wave current.
(2) Permanent magnet AC servo system driven by sinusoidal current.
A permanent magnet AC servo motor driven by a rectangular wave current is called a brushless DC servo motor, while a permanent magnet AC servo motor driven by a sinusoidal wave current is called a brushless AC servo motor.
3. Stepper motor driver
A stepper motor is a component that converts electrical pulse signals into corresponding angular or linear displacements. Its angular and linear displacements are proportional to the number of pulses, while its rotational or linear speed is proportional to the pulse frequency. Within the load capacity range, these relationships do not change due to fluctuations in power supply voltage, load size, or environmental conditions, and errors do not accumulate over time. Stepper motor drive systems can adjust speed within a wide range by changing the pulse frequency, achieving rapid start-up, forward and reverse braking. As an open-loop digital control system, it is widely used in small robots. However, due to its poor overload capacity, relatively small speed range, pulsation at low speeds, and imbalance, it is generally only used in small or simple robots.
The driver used in a stepper motor mainly consists of several parts, including a pulse generator, a ring distributor, and a power amplifier. Its principle block diagram is shown in Figure 2.
4. Direct drive
A direct drive (DD) system is one in which the electric motor is directly coupled to the load it drives, without any speed reduction mechanism in between.
Compared to traditional electric motor servo drives, DD drives reduce the reduction gear mechanism, thereby reducing the backlash and looseness generated during system transmission, greatly improving the robot's precision. It also reduces the decrease in robot control precision caused by friction and torque pulsation in the reduction gear mechanism. Due to these advantages, DD drives offer high mechanical rigidity, enabling high-speed and high-precision movements. They also feature fewer components, simpler structure, easier maintenance, and higher reliability, making them increasingly important in high-precision, high-speed industrial robot applications.
The key component of DD drive technology is the DD motor and its driver. It should possess the following characteristics:
(1) Large output torque: 50 to 100 times the output torque of servo motors in traditional drive methods.
(2) Small torque ripple: The torque ripple of the DD motor can be suppressed to within 5% to 10% of the output torque.
(3) Efficiency: Compared with motors using reasonable impedance matching (under traditional drive mode), DD motors operate under conditions of poor power conversion. Therefore, the larger the load, the more likely a larger motor will be selected.
Currently, DD motors are mainly divided into variable reluctance type and variable reluctance hybrid type, with the following two structural forms:
(1) Dual-stator structure variable reluctance DD motor;
(2) A DD motor with a central stator structure and variable reluctance.
5. Special-purpose drive
(1) Piezoelectric actuator.
As is well known, strain gauge accelerometers and ultrasonic sensors have been made by utilizing the electrical or electrostrictive phenomena of piezoelectric elements. Piezoelectric actuators use electric field energy to control displacements of a few micrometers to hundreds of micrometers with forces greater than the micrometer level. Therefore, piezoelectric actuators are generally used in special-purpose microrobot systems.
(2) Ultrasonic motor.
(3) Vacuum motor.
Vacuum robots used in ultra-clean environments, such as ultra-vacuum robots used to handle semiconductor silicon wafers.
