Abstract
For high-precision control in a ball-screw driven servo system, the full-closed loop position control structure with position measurements from both the drive side and the load side are usually required. However, compared with semi-closed loop position control, if only the load side position feedback is being used, then the inherent elastic link is included in the position loop, and the mechanical vibration will be easily induced. For this situation, the stability of the servo system will be affected. Moreover, in the ball-screw driven servo system, the backlash and friction in the mechanical transmission chain will introduce a typical contour error-over-quadrant error, which cannot be eliminated by adopting full-closed loop control. In order to maintain stability and high precision positioning performance, both vibration suppression and over-quadrant error mitigation methods are proposed in this paper. First, the model of full-closed loop position control system is established, on this basis, the reasons that why the full-closed loop control is more easily to induce the position vibration than the semi-closed loop control is analyzed. Then, a dual-position feedback control method, by introducing the drive side position information with a filter to the position loop feedback channel, is proposed, which combines the advantages of semi-closed loop control and full-closed loop, and features high gain margin and high control precision. Furthermore, based on the analysis of the mechanism of over-quadrant error, an adaptive backlash error compensation method which can reduce the over-quadrant error is proposed. Finally, simulation and experimental results in both single axis and dual axis are provided to demonstrate the feasibility and effectiveness of the proposed methods.
Highlights
Positioning systems using rotary permanent magnet synchronous motors (PMSMs) and mechanical power transmission systems with ball-screw have been widely used in various applications such as numerical control (NC) machine, industrial robots and factory automation, for their advantageous features such as increased reliability and reduced cost [1]-[3]
The precision of the ball screw has a significant influence on the performance of the semi-closed loop (SCL) control
Contains two control loops: a semi-closed loop that detects the position from the drive side which can eliminate the mechanical resonance from the position servo loop, and a full-closed loop based on a linear encoder from the load side which is used to compensate for the position errors
Summary
Positioning systems using rotary permanent magnet synchronous motors (PMSMs) and mechanical power transmission systems with ball-screw have been widely used in various applications such as numerical control (NC) machine, industrial robots and factory automation, for their advantageous features such as increased reliability and reduced cost [1]-[3]. The positioning performance in terms of response time and accuracy will be deteriorated with a lower controller gain In this case, many methods have been proposed in the literature for the vibration problem of the two-mass system. In CNC machine tools and similar high-precision feed systems, time lag is unavoidable in the response of the load side position to the position reference and the nonlinear factors exist for a short time This kind of transient nonlinear problems cannot be solved from the trajectory reference side, and needs to be solved from the inner loop control of the system, such as the servo unit. With the increase of controller gain, this frequency band can be covered within the system control bandwidth, which is the main reason why the flexible system is difficult to control
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