Measurement technology for direct drive systems

September 01 12:00:38, 2024

With the development of high-speed, high-precision, high surface quality and high stability of the generation machine tool, the application of direct drive technology is increasingly expanding. The advantages of a direct drive system can only be fully realized if the control system, the motor and the measurement system are coordinated and coordinated with each other. The measurement system is very important for reflecting the performance of the direct drive system. The linear encoder/angle encoder based on photoelectric single-field scanning technology has the advantages of high precision, high resolution and small subdivision error, and is very suitable for direct drive applications. From the aspect of control theory, because the semi-dead cycle machine tool control system cannot overcome the transmission error generated by the machine tool transmission mechanism, the thermal deformation error and wear of the transmission mechanism during high-speed reciprocating motion, the whole dead-end cycle has become a control theory that can eliminate transmission errors. Many applied to modern machine tool control. Drive technology, because of its high precision, high dynamic characteristics, low friction, simple maintenance and high efficiency compared with traditional drives, direct drive has been widely used in the machine tool industry. In measurement technology, the precision, resolution, and safety of measurement feedback components have been greatly improved due to the application of absolute coding, interface technology, and photoelectric single-field scanning technology. The efficiency of CNC machine tools depends on the coordination and cooperation of control systems, motors, mechanical components and measurement systems. The correct choice of measurement system plays an important role in the performance of the machine tool, especially the direct drive system. The performance of direct drive system efficiency depends to a large extent on the choice of position measurement components, which have very high requirements on the measurement components: 1 high measurement accuracy; 2 small subdivision errors; 3 high resolution; 4 no interference. Direct drive technology The biggest advantage of direct drive is that there is no other transmission component between the drive assembly (linear motor or torque motor) and the driven component (worktable or turntable), and the connection rigidity is high, so the control loop system gain of the direct drive system can be Far larger than traditional drive systems. High gain has its advantages, but at the same time it also increases the requirements for the quality of the output signal of the measuring assembly. Linear encoders are linear encoders in the case of linear motors and angular encoders in torque motors.
Fig. 1 Full-dead cycle control of direct drive system
Fig. 2 Schematic diagram of linear motor and torque motor
Figure 3 Effect of subdivision error on surface quality The direct drive system does not have a rotary encoder dedicated to the speed control loop. Its position control loop and speed measurement loop share the same measurement component (see Figure 1). The speed control loop needs to have a small signal period. Therefore, the resolution of the measurement component needs to be high enough to ensure accurate speed control when the machine is running at low speed. Direct drive linear encoders/angle encoders Linear encoders or angular encoder gratings are etched using a photolithographic process on a glass or steel tape substrate. This type of reticle has a very small pitch and a clear and even edge. With non-contact photoelectric single-field scanning technology and high-quality signal processing circuit, linear encoders and angle encoders can provide high-quality signals with high accuracy, high resolution, and small subdivision errors for direct drive systems, with relatively no pollution. Sensitive (in the image of the problem, the red area is pollution), and ultimately ensure the position measurement accuracy, speed stability, and relatively constant temperature of the drive system to ensure the smooth operation of the system. Effect of Subdivision Error on Direct Drive After the measurement component uses an incremental or absolute measurement method to obtain two position signals with a phase difference of 90°, further subdivision is required to achieve the desired resolution. The actual output position signal will deviate from the ideal sinusoidal signal due to scanning methods, contamination, and subsequent electronic processing. When subdividing, this kind of deviation will generate periodic high-frequency errors within one signal period, ie, errors or subdivision errors within a single signal period. The subdivision error depends on the signal period, reticle quality, and scan quality of the measurement component. At present, the position measurement component using the photoelectric single-field scanning can limit the subdivision error to about 1% of the signal period. For example, a linear grating with a signal period of 20 μm has a subdivision error of 0.2 μm. The subdivision error not only affects the positioning accuracy of the direct drive system, but also affects the stability of the speed control loop. The speed control loop of the control system calculates the operating current of the driving system according to the positioning error of the feeding system, and then controls the linear motor or torque motor to perform acceleration and deceleration. When the feed rate of the CNC machine tool is low, the feed system follows the subdivision error. Because the direct drive system has a high gain and thus a large control bandwidth, it is greatly affected by the subdivision error in a large speed range. When milling a workpiece, due to the influence of subdivision errors, the surface of the workpiece will have corrugated processing marks, which will seriously affect the surface quality of the workpiece, as shown in FIG. 3 . In general, the wavelength and amplitude of the corrugated trace are proportional to the interpolation error. Due to the subdivision error, additional currents are generated inside the directly driven motor, creating extra heat and noise. The greater the subdivision error and motor feed speed, the greater the motor heat and noise. In addition, when the control system acquires the position value from the measurement component, if the resolution of the position signal and the subdivision error are relatively close, the subdivision error will be recognized as the movement error of the machine feed system, and the control system will send the randomness to the feed system. Compensation signal. This will cause the control system to oscillate. The moving parts of the machine tool, such as the guide rails of the linear motor, will reciprocate for a long period of time to accelerate wear and reduce the service life. Direct Drive System Dynamic Characteristics The direct drive system has good dynamic characteristics, often with high-speed, high-acceleration feed motions. This requires the measurement system to have good signal quality, high electrical speed, and excellent vibration and shock resistance. The signal quality of the optoelectronic single-field scanning linear encoder and angle encoder is high. The non-contact measurement between the grating reticle and the readhead makes it non-abrasive and has good vibration and shock resistance. The electrical movement speed can reach up to 180 m. /min. In summary, according to the application characteristics of direct drive systems, photoelectric single-field scanning linear encoders and angle encoders provide high-precision, high-resolution, and small subdivision-error quality for their position control loops and speed control loops. The signal ensures efficient and smooth operation of the drive system and ultimately enables high-speed, high-precision, high surface quality and high-stability processing.

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