Development of an innovative Z-θ direct drive actuator
Citation for published version (APA):
Lent, van, M., Smeltink, J., Theeuwes, J., Knaapen, R. J. W., Meessen, K. J., & Paulides, J. J. H. (2011).
Development of an innovative Z-θ direct drive actuator. In Proceedings 11th International Conference of the
European Society for Precision Engineering and Nanotechnology (Euspen 2011), 23-26 May 2011, Como, Italy
(Vol. 2, pp. 59-63). European Society for Precision Engineering and Nanotechnology.
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Published: 01/01/2011
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Development of an Innovative Z-
Direct Drive Actuator
M. van Lent1, J. Smeltink1, J.Theeuwes1, R. Knaapen1, K. Meessen2, J. Paulides2
1
TNO, The Netherlands 2
Eindhoven University of Technology, The Netherlands
maarten.vanlent@tno.nl
1 Introduction
Placement heads are essential modules in pick and place applications. Picking and placing parts as fast as possible with high quality is the major demand. Next to this, a placement head shall be power efficient (low power dissipation per placement cycle), be compatible with typical machine constraints and conditions (volume, operating temperature range, machine contamination), and be reliable. Altogether, it shall provide for low cost per placement. To meet these demands, an innovative electromechanical Z- actuator (INEMA) is developed.
Requirements
An overview of the primary technical requirements is given in Table 1. Stroke Z -
Emergency pull-up stroke Speed Z - Acceleration Z - 45 (mm) - unlimited (rad) 25 (mm) 1,5 (m/s) - 125,6 (rad/s) 150 (m/s2) - 3500 (rad/s2) Positioning error Z - Position reproducibility Z - 5 (m) - 0,35 (mrad) 3 (m) - 0,15 (mrad) Limit value , 3 3 Settling time Settling level Z - 30 (ms) 10 (m) - 0,5 (mrad) Min.-max. controlled force
Force resolution
0,3 (N) - 30 (N) 0,1 (N) Placement head mass
Placement head footprint X Y
1,2 (kg)
24 (mm) 120 (mm)
Incl. cameras, optics, and electronics Table 1: INEMA main requirements
Rationales
Short cycle times require high acceleration rates for Z and. At the same time, the placement accuracy and the placement force shall be accurately controlled. Due to height uncertainty, simple position control cannot be used. After the board is detected, the final phase of the placement action takes place using force control. As this control mode is prefereably based on internal control signals, in this case the current setpoint, the relation between force and current should be as linear as possible. When large parts are placed, the orientation of the part must be very accurate in order to have all pins connect correctly. After initial measurement with an exteroceptive sensor, the part orientation is corrected by servo control which must guarantee a very small angular position error at placement. Because the placement head is mounted at the very tip of the robotic chain, a low mass will positively affect the overall machine dynamics and power consumption.
2 Actuator principles and design
In figure 1, the electromechanical lay-out and a cross-sectional view of the new actuator are shown. In this design for 2-DoF motion, the different functions of the actuator are stacked. They are described in the following sections.
1) air bearing, (2,3,4) linear actuator, (5) position sensor, (6,7) gravity overcompensation, (7,8,9) rotary actuator
Linear actuator
The selected linear actuator Figure 1 (2,3,4) is a tubular permanent magnet actuator (TPMA) [1]. It has a cylindrical translator which allows for rotation. Furthermore, due to the absence of end-windings and the cylindrical shape, the TPMA has a high efficiency and force density. A slotless structure is chosen to minimize cogging. The translator consists of axially magnetized magnets with alternating polarity seperated by soft-magnetic pole pieces. In motion, their magnetic field results in a sinusoidal voltage induced by the three phase slotless winding.
Passive gravity compensation
To prevent machine or workpiece damage at a failure (e.g. power down), the translator of the actuator should go to its upper position passively. Therefore, an electro magnetic passive gravity compensator is integrated in the design, Figure 1 (6,7). This gravity compensator is based on a constant force actuator [2], and uses the effect that a permanent magnet in a soft-magnetic tube tends to align with the tube in the axial direction.
Rotary actuator
The rotary actuator of this 2-DoF actuator is a two pole slotless permanent magnet actuator, Figure 1 (7,8,9). Concentrated three phase windings are used in a slotless stator. As the complete actuator structure does not show any reluctance variation in the circumferential direction, i.e. no slotting effects, the cogging torque of the rotary actuator is zero. This will improve the required position control of the rotational movement.
Design
To meet the performance, the electro-mechanics has been optimized. Using analytical models, as described in [1], the linear and rotary actuator are designed. The coil and translator dimensions are optimized for maximum acceleration having minimum copper loss in the actuator. Furthermore, the soft-magnetic structure is optimized to obtain the required passive behavior. Using finite element analyses and measurements, the eddy currents in the actuator were investigated. By using materials with high resistivity and laminated material for the back-irons, the eddy current loss is minimized.
Because of the optimized electro-mechanics, the mechanical design has paid much attention to geometrical, mass and material constraints (e.g. stainless steel air
bushings). The magnets are contained in a slender thin-walled cylindrical tube (Douter=12,7mm) with a straightness better than 0,01 (mm) and a cylidrical accuracy
of ~2 (m). Moving mass and inertia are ~0,25 (kg) and ~5. 10-6
(kgm2) respectively. Given the length of the actuator axis, air bearings are a necessity to obtain the high resolutions for placement force and position. As the actuation force-torque passes through the centerline of the actuated rod, transverse position disturbances and bearing loads due to the self-propelled motion are small. Thermo-mechanic deformations and stresses of axis and housing are taken into account for operational temperatures between 20°C and 80°C. Axial and rotational positions are measured on a 2D scale on the cylindrical surface by two encoder heads. For the manufacture of the cylindrical axis with scale, technologies such as vacuum infusion gluing and diamond lathing are used.
3 Sensoring and control 2D sensor
Conventionally, Z-translation and -rotation are measured by separate position measurements of stacked motion components, which implies mass and volume penalties. With INEMA, maximum functional integration has been accomplished as the two-dimensional scale is located on the surface of the slender actuator axis itself. This scale is read-out by two Renishaw RGH34 encoderheads. Figure 2 shows the test set-up for verification measurements of the 2D scale and encoderhead against a 1D laser differential interferometer (Renishaw RLD10). The specified positioning requirements are properly met. Figure 3 shows details of the manufactured grid (period 80 (µm)).
4 Conclusions
A new Z- actuator for very fast and accurate pick and place motions has been designed. Its integral performance was verified by detailed electromechanical simulations. Performance of hardware components was bottom-up tested or analysed with FEM models. Precision manufacturing technologies are employed for e.g. the actuator rod with sensor grid. Integrated module tests are expected to begin in Q2 2011.
Within this research project, the development of an actuator with further integrated Z- electromechanics is also in progress.
References
[1] K. Meessen, J. Paulides, and E. Lomonova, “Modeling and experimental verification of a tubular actuator for 20-g acceleration in a pick-and place application”, Industry Applications, IEEE Transactions on, vol. 46, pp. 1891 –1898, Sept.-Oct. 2010.
[2] L. Encica, J. J. H. Paulides, and E. A. Lomonova, “Passive and active constant force-displacement characteristics and optimization of a long-stroke linear actuator”, in Optimization of Electrical and Electronic Equipment, 2008. OPTIM 2008. 11th International Conference on, pp. 117 –124, May 2008.
Acknowledgements
This project is sponsored by the Dutch Ministery of Economic Affairs, Agriculture and Innovation. The research is conducted in close cooperation with the Dutch SME companies Assembléon, Tecnotion and Prodrive. For aspects of manfacturing, VDL-ETG, Advanced Electromagnetics and Delft University of Technology (Adhesion Institute) are involved.
