The application of direct drive motors is becoming increasingly widespread, especially in scenarios requiring high precision and high speed, where the demand for linear motors is very high, and the market is growing significantly every year. At the same time, the manufacturing cost of direct drive motors is also continuously decreasing.
1. Advantages and disadvantages of direct drive motors
Compared with traditional ball screw modules, direct drive motors have the following application advantages:
• High speed of movement;
• Low friction;
• The system is highly rigid, and the machine's load can be directly connected to the motor's mover or rotor;
• Simple structure: Traditional servos usually require a second encoder to improve their accuracy. For direct drive motors, a single encoder can achieve high accuracy, avoiding the need for a second encoder to improve accuracy. At the same time, it has a longer running time.
• High precision and long lifespan.
However, direct drive motors also have certain drawbacks:
• Higher cost, requires linear guides;
• Low friction necessitates more sophisticated algorithms to calculate control precision;
• Requires higher gain and vibration suppression algorithms;
• High setting time requirement;
• Requires a high-resolution and high-precision encoder.
Overall, typical applications of direct drive motors currently include: high-precision control applications in fields such as semiconductor equipment, testing equipment, AOI or vision inspection; as well as applications in wire bonding machines, flat panel displays, 3C electronics, CNC machine tools, photovoltaics, and high-speed material handling.
2. Control requirements and challenges of direct drive motors
Direct-drive motors place high demands on the technical development of control systems, which can be broadly categorized into the following points:
• Due to high-speed requirements and short settling times (e.g., in wire bonding machine applications), the control system must set the motor to a stable position in the shortest possible time.
• In some laser processing or inspection fields, extremely stable operation is required, with high tolerances for both speed fluctuations and positional errors.
• For some special scenarios, such as laser or vision inspection, position trigger output function is required. As the structure becomes larger and larger, such as gantry architecture, two axes are needed to drive the same platform. Therefore, the drive control needs to realize the function of gantry transmission.
• To address the above requirements, Gaocheng CDHD2 has developed several corresponding algorithms to adapt to these application scenarios, including nonlinear HD control and frequency domain linear response-based HDM control.
3 HD control methods
Traditional control schemes are generally serial structures, including a three-loop serial structure of current loop, speed loop, and position loop. These control methods have some inherent errors that are difficult to avoid, and adding feedforward increases overshoot and settling time. To address this, Gaochuang combines the speed loop and position loop into one, forming a nonlinear control. The merging of the speed loop and position loop automatically adapts to errors through a gain matrix, adjusting the gain according to the system error and speed to achieve minimal damping, minimal tracking error, and optimal tuning performance.
When a direct drive motor moves at high speed, it will inevitably cause vibration. Closed-loop vibration suppression is used to respond and suppress the vibration in the early stages, and active damping is used to minimize the occurrence of vibration.
4 HDM Control Method
For some complex control scenarios, such as CNC and laser fields, the requirements for the process are usually quite high, and the resonance point and mechanical characteristics of the machine tool may not be consistent. Based on this situation, the HDM control method was developed. This control method is based on polynomial linear control and frequency response for automatic adjustment.
The first step is to perform a frequency sweep on the machine system: inject white noise with a sweep current into the machine system and set its sweep range. The bandwidth of the entire machine can be obtained from the frequency response, along with its open-loop Bode plot.
The top part shows its amplitude, and the bottom part shows its phase, thus obtaining the frequency response of the entire control. Figure 1 shows the Bode plot after frequency sweep.
Figure 1. Bode plot after frequency sweep
The second step is modeling (FIT Model): Based on this frequency response, different resonance points of the system can be seen. Software is needed to fit the actual response using an algorithm curve to obtain a mechanical model of the system. Then, the resonance points are set, and the number of zeros and poles is set according to the resonance points to obtain the fitted curve. The fitted curve is shown in Figure 2.
Figure 2. Fitted curve
The third step is to automatically generate control parameters: Based on the fitted curve, the corresponding control parameters are generated with one click, including automatically adding appropriate filters. Then, fine-tuning is performed based on the closed-loop characteristics and actual results to obtain the best or most optimized control. During debugging, unstable regions should be avoided as much as possible; both stability and bandwidth need to be improved.
5. Gantry Control System
With the development of control and the increase in load, gantry control is increasingly used in many scenarios. In response to this, Gaocheng proposed a gantry control method that adopts a dual-axis cross-coupling approach. One axis acts as the master axis, controlling the forward and backward movement of two motors, while the other axis acts as the slave axis. The deviation between the two axes is monitored simultaneously, and adjustments are made in real time based on the deviation. Different currents are output to the two axes to achieve synchronization. The communication between the two axes uses a communication rate of 10MB/s.
The functional features of the Gaocheng gantry system include:
• Two-axis synchronization;
• Reduce current output by 30%;
• Supports both flexible and rigid gantry systems.
6. Error compensation function
In the linear motor industry, positioning accuracy is generally easy to achieve, but to improve its absolute accuracy, compensation is required. Gaocheng Drive has a built-in error compensation table that supports compensation for both linear and rotary motors, with more than 1,000 compensation points. It can also use laser to provide compensation values, which can greatly improve the positioning accuracy of linear motors and meet the needs of customers in high-precision scenarios.
The industry is still discussing this technology, and Gaocheng is also constantly exploring future developments and changes.
7 Application of High-Tech Innovation Products
Customer market changes are placing increasingly higher demands on industrial control companies, requiring products to be easy to use, highly accurate, and stable. At the same time, the constant changes in customers and the market are leading to increasingly rapid product updates; our products need to continuously adapt to customer needs, and our technology must also be constantly iterated and upgraded.
Direct drive motors are widely used in the field of robotics, which places higher demands on us. For example, robot vibration cannot be suppressed indefinitely. For the robotics industry, we have been thinking about how to optimize the control method.
Loop motors are another good approach. This control method differs from previous modular motors, where the moving part was the coil and the magnet was the stator; the magnet moved while the coil below remained stationary. This requires integrating the driver, controller, and encoder to monitor the position of the moving part in real time for better control. While the stator moves, the display modes of different coils below need to be controlled. Therefore, the future trend is towards increasingly integrated, more powerful drivers and controllers, leading to higher demands for automatic adjustment and helping customers better utilize our equipment.
Using an integrated driver and controller approach offers better stability, avoiding instability caused by wiring and interference. Furthermore, the driver will have abundant third-party interfaces for communication with vision and other third-party applications. In the future, this integrated driver system will resemble an embedded subsystem, allowing peripherals to handle motion control.
Furthermore, with the development of Industry 4.0, all equipment will need to go to the cloud, and future drives and controllers will also adopt the OPC-UA cloud standard. The golden decade of industrial control requires the joint creation of the industry.
