With the development of modern industry, the movements of controlled objects in fields such as machining, metallurgical manufacturing, slitting and conveying, and robotics or manipulators are becoming increasingly complex and diverse. These movements all involve positioning and have increasingly higher control requirements. AC servo systems are currently one of the high-end technologies in industrial automation transmission. They enable the output mechanical displacement (or angle) to accurately track the input displacement (or angle), and CNC technology ensures that the actuators follow the set instructions to perform the desired movements. It has three control modes: position, speed, and torque, and is mainly used for high-precision positioning, meeting the positioning requirements of various complex mechanical displacement (or angle) changes.
Understanding "Electronic Gear"
A servo system generally consists of three main components: a servo motor, a servo driver, and a host computer that implements the control. The host computer is usually a PLC or a microcontroller. (See diagram).
The servo motor is the actuator in this system. The servo system uses pulses for positioning, and the fundamental principle of position control is that the host computer programs the controls based on the specific requirements of the controlled object; the servo driver executes the program and outputs pulses. In this way, the pulse power supply with specific program rules drives the servo motor to move the mechanical parts to achieve displacement or rotation, completing the task. It is evident that no matter how varied the requirements of the controlled object are, its accurate positioning is inevitably closely related to two factors: the number of pulses and the amount of movement of the mechanical parts per unit pulse.
In terms of mechanical structure, servo motor output shafts typically have a speed reduction device between the load input and the servo motor output shaft. This speed reduction device reflects the speed ratio (multiplication factor) between the servo motor and the load input. Due to the characteristics of the mechanical structure, once such a mechanical transmission system is established, the speed ratio of the speed reduction device is fixed. Adjusting it would likely require discarding the existing hardware and remanufacturing and installing it entirely, which is obviously inconvenient. Is it possible to find a more convenient and effective way to allow the speed of the mechanical system to be adjusted and set within a certain range?
The development of microelectronics and high-power power electronics technologies has led to the development of servo drives. These drives utilize digital signal processors (DSPs) as their control core, implementing complex control algorithms to achieve digitalization and intelligence. Their power devices employ drive circuits centered around intelligent power modules (IPMs), providing fault detection and protection against overvoltage, overcurrent, overheating, and undervoltage. A soft-start circuit is also added to the main circuit to reduce the impact of inrush current during startup. The servo drive's output power is obtained by rectifying AC three-phase or single-phase power to produce corresponding DC power, which is then converted to a servo motor via a sinusoidal pulse width modulation (SPWM) voltage-source inverter. The servo motor receives pulses from the driver's output; within the pulse width, the motor moves, and a series of pulses causes the motor to rotate, thus driving the mechanical load. Because the servo drive's output power uses sinusoidal pulse width modulation technology, which produces pulse trains of varying widths that can be customized based on the control signal, the servo motor's movement can be selected and set according to the controllable pulse width, allowing for flexible adjustments without necessarily requiring hardware changes. In other words, even with pulse trains of the same frequency, the motor speed and even the load-side speed will differ depending on the user's setting for the amount of movement the motor makes within its corresponding pulse width. Its function is similar to that of a mechanical gear, but unlike a mechanical gear, it is not tangible. Hence, the term "electronic gear" was coined to describe it. Mitsubishi Electric Automation Co., Ltd. describes the function of "electronic gear" as follows: The machine can move with input pulses of any multiple.
Structural Analysis and Practice of "Electronic Gear"
The servo drive manufacturer defines "electronic gear" as a fraction, with its numerator and denominator defined as two user-configurable parameters:
Analyzing the above expression, the four main data items each have their own characteristics:
1. Load speed/motor speed (commonly known as speed ratio) is conventionally determined from a mechanical perspective. However, since it is a component of the "electronic gear", the value should be selected as an integer as possible. This is especially important for rotary table type machinery.
II. The amount of movement per revolution of the load shaft varies depending on the mechanical system's process requirements. For lead screw systems, the movement is the helix length; for frustum-type systems, it's one full rotation angle; for transmission systems, it's the circumference of the load shaft, and so on. This is determined by the equipment's function, leaving little room for choice.
III. Servo Motor Encoder Resolution The encoder is a key component for precise positioning of servo motors and even servo systems. For every angle the servo motor rotates, the encoder emits a corresponding number of pulses, feeding them back to the servo driver. This feedback loop, where the encoder and servo motor pulses interact, forms a closed loop. This loop allows the servo control system to compare and adjust the number of emitted and received pulses, precisely controlling the servo motor's rotation and achieving accurate positioning. Encoder resolution represents the number of digital pulse signals converted from the displacement of one revolution of the servo motor. Obviously, the higher the value, the finer the digital pulses emitted per revolution, and the higher the detection accuracy. It is integrated with the servo motor and should be considered when selecting a servo motor.
IV. The movement amount corresponding to each command pulse (also known as the command unit) is selected by the user and is a key data point reflecting the "electronic gear" and "speed change" functions. Having used Mitsubishi MR-J3 series servo amplifiers and Yaskawa SGDM servo units for many years, I have found that the value of this "command unit" is extremely important. It directly affects the "electronic gear" ratio and needs to be considered comprehensively in conjunction with mechanical and electrical design, taking into account the following factors:
1. Given a fixed mechanical reducer, the maximum output speed is limited by the maximum output frequency of the host computer or servo drive. The value of the command unit directly affects the maximum output speed of the load shaft, showing a direct proportional trend. The author used a Mitsubishi FX series PLC with Mitsubishi and Yaskawa servo drives to form systems for slitting conveyor machinery. The relationship between the command unit value and the load linear speed was calculated as follows:
It can be seen that the smaller the instruction unit, the lower the load linear speed; the lower the host computer frequency, the lower the load linear speed. This same proportional relationship applies to the output shaft speed.
2. Positioning accuracy: Obviously, the smaller the command unit value, the more subdivided the pulse equivalent. For example, reducing the command unit value from 0.1 to 0.01 by a factor of 10 is equivalent to changing the displacement within one pulse width from 0.1 to 0.01 . In other words, the displacement that was originally one pulse now requires ten pulses to complete, and its relative positioning accuracy will naturally be higher than before the modification.
This demonstrates that, all other things being equal, the value of the instruction unit has a close relationship with the speed and accuracy of the mechanical system. The servo system provides users with a digital control platform, and users should strive to find appropriate values for both speed and positioning accuracy while meeting the equipment's processing requirements. The Mitsubishi MR-J3 series servo amplifier further expands the application options for "electronic gears"; it also provides three extended parameters as the molecular data for electronic gears, which can be set via the two input terminals of the driver and combined into four types of "electronic gears" through PLC programming, further increasing the speed range.
As can be seen from the numerical structure of the "electronic gear", the two user parameters that serve as the numerator and denominator are integers. However, they must be simplified through formula calculation. Therefore, when taking values for relevant data, the possibility of calculation and simplification should be fully considered to facilitate selection.
To ensure the normal operation of the servo system, manufacturers limit the ratio range of the "electronic gear" and remind users that exceeding the limit may have possible consequences, such as abnormal noise; failure to operate at the set speed or acceleration/deceleration time constant; or even affecting positioning accuracy. If these situations occur, it is necessary to clarify the priorities and seek a balance in terms of reducer speed ratio, load displacement (circumference, angle, stroke) and command unit values.
