With the development of high-speed machines, high precision, high surface quality and high stability, the application of direct drive technology is expanding. The advantages of a direct drive system can only be fully realized if the control system, motor and measurement system are coordinated and coordinated, and the measurement system is critical to the performance of the direct drive system. Linear encoder/angle encoder based on photoelectric single-field scanning technology has the advantages of high precision, high resolution and small subdivision error, which is very suitable for direct drive applications.
From the aspect of control theory, since the semi-dead cycle machine control system can't overcome the transmission error generated by the machine tool transmission mechanism and the thermal deformation error and wear of the transmission mechanism during high-speed reciprocating motion, the complete dead loop as the control theory that can eliminate the transmission error has become more and more Apply more to modern machine control. In terms of drive technology, direct drive is widely used in the machine tool industry due to its high precision, high dynamic characteristics, low friction, simple maintenance and high efficiency compared with conventional drives. In terms of measurement technology, the accuracy, resolution and security of the measurement feedback component have been greatly improved due to the application of absolute coding and interface technology, photoelectric single-field scanning technology and the like.
The efficiency of CNC machine tools depends on the coordination and cooperation of control systems, motors, mechanical components and measuring systems. Proper selection of the measurement system plays an important role in the performance of the machine, especially the direct drive system. The performance of direct drive system efficiency depends to a large extent on the choice of position measurement components, and has extremely high requirements for measurement components: 1 high measurement accuracy; 2 small subdivision error; 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 component (linear motor or torque motor) and the driven component (workbench or turntable), the connection rigidity is high, so the gain of the control loop system of the direct drive system can be much larger than Traditional drive system. High gain has its advantages, but it also increases the requirement for the output signal quality of the measuring component. It is a linear encoder for linear motors and an angular encoder for torque motors.
Figure 1 Direct drive system full dead loop control
Figure 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, and its position control loop and speed measurement loop share the same measurement component (see Figure 1). The speed control loop requires a small signal period, so the resolution of the measuring component needs to be high enough to ensure accurate speed control even at low speeds.
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