Three DOF parallel link mechanism utilizing smooth impact drive mechanism

Transcription

1 Precision Engineering Journal of the International Societies for Precision Engineering and Nanotechnology 26 (2002) Three DOF parallel link mechanism utilizing smooth impact drive mechanism Takeshi Morita a,, Ryuichi Yoshida b, Yasuhiro Okamoto b, Toshiro Higuchi c a Division of Advanced Engineering Center, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama , Japan b Optics Technology Division, Minolta Co. Ltd., 3-91 Daisennishi-machi, Sakai-shi, Osaka , Japan c Department of Precision Machinery Engineering, Graduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo , Japan Received 9 September 2001; received in revised form 7 January 2002; accepted 25 January 2002 Abstract For a wide range of purposes, a compact and smart positioning stage is in strong demand. Our final goal is to fabricate a positioning mechanism that possesses multi-degrees of freedom, a wide movable area, dexterity, rigidity, and high speed. To meet most of requirements, i.e., except that of a wide movable area, a parallel mechanism is superior to a serial link mechanism. However, a lack of smart actuators has previously resulted in a large and complicated construction of a parallel mechanism. A purpose of this paper is to propose a new parallel link that utilizes a smart actuator called smooth impact drive mechanism (SIDM), and to verify its advantage for a smart and compact positioning stage. The SIDM actuator is an interesting mechanism that achieves a long stroke with fine positioning resolution. This paper treats a three DOF stage containing three SIDM actuators and a two-dimensional positioning sensor. The first operation test was carried out, and it was confirmed that a position control was successful. The average deviation was 18.6 m and the standard deviation was 9.31 m. As another trial, a circle 3.0 mm in diameter was successfully drawn. These values were insufficient for the SIDM actuator that has potentially nanometer-level resolution. The rough positioning resolution seems to be due to inferior driving method and mechanical ricketiness of the joints. In the near future, high speed and accurate positioning control could be realized with overcoming these problems Elsevier Science Inc. All rights reserved. Keywords: Positioning stage; Parallel link; Linear actuator; Smooth impact drive mechanism; Piezoelectric material 1. Introduction Currently, many investigations are focusing on microscale applications, and adequate positioning stages for such investigations become essential devices [1]. More specifically, a compact and smart positioning stage is required, for example, in the assembly of micro-mechanical parts (mechanical technology), handling of cells (biological technology), fabrication of semiconductors (electronics technology), and microsurgical endeavors (medical science) [2,3]. An advanced positioning stage means what has dexterity, multi-degrees of freedom, a wide movable area, rigidity, and high-speed. With regard to all of these requirements except a wide area, Corresponding author. Present address: Ceramics Laboratory, Department of Material, Swiss Federal Institute of Technology EPFL, CH-1015 Lausanne, Switzerland. Tel.: ; fax: address: tmorita@ieee.org (T. Morita). a parallel mechanism is superior to a serial mechanism. However, a lack of a smart actuator generally results in a large and complicated construction. Pneumatic, oil pressure actuators or rotational motors, for example, have been utilized. On the other hand, multi-layered piezo actuator has proved an advantage of nanometer-level positioning for a parallel mechanical stage [4]. However, the stroke of multi-layered piezoelectric material is a 100 m at most, even with a magnification mechanism. Hence, the movable area becomes limited. In another case, piezo inch-warms were applied to parallel mechanism [5]. By using the inch warm mechanisms, a long stroke can be realized, that results in wide movable area. Although, the inch warm mechanism require three piezo units for one degree of freedom, and the control system therefore is complicated. Then, since the Stewart-type parallel link involves 6 linear actuators, and 18 piezo units are necessary for example /02/$ see front matter 2002 Elsevier Science Inc. All rights reserved. PII: S (02)

2 290 T. Morita et al. / Precision Engineering 26 (2002) In other words, the parallel link has been a promising mechanism as the advanced positioning stage, though a lack of a suitable actuator impeded smart construction. The purpose of this paper is to propose a new parallel link that uses a smart actuator called smooth impact drive mechanism (SIDM) [6,7], and to verify its advantage for a smart and compact positioning stage. A SIDM actuator is an interesting mechanism that achieves a long stroke with fine positioning resolution. As a similar mechanism to SIDM, the impact drive mechanism (IDM) has been also investigated [8,9]. From the view of the positioning resolution, SIDM might be superior to IDM, because SIDM can demonstrate the same positioning resolution of piezoelectric material itself. With the IDM, target object is driven with step motions. In fact, it was confirmed that the SIDM actuator was capable of positioning with various nanometer levels, and with much superior sensor, the poisoning performance would be improved. Our positioning stage using SIDM demonstrates a wide motional area and the positioning faculty would be at the nanometer level. Furthermore, SIDM actuator has a simple construction; each actuator is operated with a single piezo material. From these motivations, this paper treats X Y θ stage that utilized three SIDM actuators. 2. SIDM actuator 2.1. Principle and properties Multi-layered piezoelectric material has an advantage in positioning faculty, though a stroke is less than 10 m at most in absence of a magnification mechanism. For purposes of practical use of the positioning stage, this value is insufficient. Yoshida and coworkers reported on an advanced mechanism called the SIDM [6,7]. This actuator realized a long stroke and fine resolution with a single multi-layered piezoelectric actuator. The construction is simple, and practical applications in various fields are expected. The schematic construction of the SIDM is shown in Fig. 1. The movable part (mover) is composed of piezoelectric material, a frictional component, and a pre-load mechanism. The frictional component is pressed to the base with a pre-load. SIDM possesses two operation modes. One mode, which we call the precise positioning drive mode, is simply operated by applying dc input voltage to a piezoelectric material. A displacement of the mover is proportional to the input voltage, because frictional force prevents slippage between the mover and the base. With precise positioning drive mode, the SIDM actuator could carry out nanometer-level positioning performance. The precise positioning drive mode is different from a driving principle of IDM [8,9] that have only step motion. With SIDM, the precise positioning drive mode is easier to achieve than what we call the long stroke mode. Fig. 1. Schematic construction and principle for the smooth impact drive mechanism [6,7]. In the long stroke mode, a driving electrical source has special shapes; the input voltage is composed of a slowly increased period and a rapidly decreased period, as shown in Fig. 2. The displacement of the piezo is proportional to the input voltage if the driving frequency is below the first fundamental resonance frequency. The input voltage generates alternating slow and rapid movement. The piezo actuator drives the mover by frictional force when the input voltage is in the slowly increased period because the mover and the base are attached by frictional force. On the other hand, during the rapidly decreased period, the mover cannot follow the quick movement and slips on the surface of the base. At the end, the mover movement is equivalent to the slipped distance with rapid piezo shrinkage. By repeating these operations, the mover could be driven with a long stroke. To reverse the moving direction, the shape of the electrical source is modified to what is composed of rapidly increased period and a slowly decreased period. Fig. 2. Driving voltage for long stroke drive.

3 T. Morita et al. / Precision Engineering 26 (2002) in which we investigated the application of SIDM actuator to the parallel mechanism. In this paper, severe targets with regard to positioning resolution and high-speed operation are not discussed. Hence, each SIDM actuator was driven only in the long stroke mode Construction Fig. 3. Construction of the smooth impact drive mechanism used for parallel mechanism. Fig. 4. The relationship between load and speed of SIDM Structure of SIDM The structure of SIDM applied to a parallel mechanism is shown in Fig. 3. The actuator is 6.0 mm in diameter, 70 mm in minimum length and the stroke is 40 mm. The dimensions of the piezoelectric actuator are 2.0 mm 2 and 9.0 mm long. Maximum displacement of the piezoelectric actuator was 8 m with 150 V. As shown in a cross-section, a frictional component was sandwiched between two plate springs with a 2.0 N pre-load. The frictional component was made of carbon fiber. The load-speed characteristic was measured by pulling up various loads with a thread. The maximum output force was 0.68 N and the maximum speed was 20 mm/s as shown in Fig. 4. The input voltage was 30 V and driving frequency was 20 khz. 3. X Y θ parallel link mechanism In advance of fabricating six degrees of freedom parallel link mechanism like the Stewart type, we fabricated an X Y θ type parallel link mechanism. This is an initial trial The X Y θ stage was composed of a base, an end effector, three SIDM actuators, and a positioning sensor. The schematic construction and a few cross-sections are shown in Fig. 5. A coordinate was defined as shown in Fig. 5 and the illustrated situation in Fig. 5 is correspond to (X,Y,θ)= (0mm, 0mm, 60 ). Three joints of the base were arranged 35 mm from the center. The joints of the end effector were 17 mm from the center. For a purpose of fine positioning, we must consider the ricketiness of each joint; however, fine positioning is not treated in this paper, so ball bearings were used. The reason why we didn t carry out fine positioning control was limited number of the electrical amplifier. The SIDM have the holding force when it is used as long stroke mode so that various actuators can share one electrical source. However, with precise positioning drive mode, dc input voltage for each actuator is necessary. Other reason is the poor resolution of positioning sensor for nanometer positioning. We used two-dimensional sensor as discussed in the next paragraph, whose resolution is 1.5 m. Even if we tried the precise positioning drive mode, it was impossible to detect the nanometer-level movement. At the center of the base, a two-dimensional positionsensing device (PSD) was attached, which was manufactured by Hamamatsu Photonics Ltd. With irradiation the light spot to the detective surface, PSD picked up X and Y position data with 1.5 m resolution and a sensing area was 12 mm 12 mm. To irradiate the light spot to the PSD, two optic fibers were installed, the diameters of which were both 250 m. Two holes for optic fibers were made in the end effector, and optical fibers were settled to face to the PSD s detective surface at the reverse side of end effector. One of the fibers was set at the center of the end effector, and the other was set at 1.0 mm away from the center. On the other side of the optical fiber, the LED was connected. Each LED was excited alternately while in operation. By turning on the LED for the center optic fiber, PSD picked up the X and Y position of the end effector. The other LED was used to detect the θ data. The distance between two optic fibers had been already given; thus from the two pairs of X and Y data, rotational data could be calculated easily Movable area Generally speaking, the movable area of parallel mechanisms is not large compared to the serial link mechanism. Our final goal is to realize a smart and compact parallel link that demonstrates a wide movable area and fine positioning

4 292 T. Morita et al. / Precision Engineering 26 (2002) Fig. 5. (a) Construction of three degrees freedom parallel link and (b) its cross-section. resolution. Before operation, the movable range for the fabricated X Y θ parallel mechanism was calculated. The result, shown in Fig. 6, indicated the movable area when the rotational angle was fixed to 60. This result showed that the end effector could move over a larger area than the area that could be sensed by PSD. Even with other rotational angle, a relative wide movable area was confirmed Control method A digital signal processor (DSP) was used to control the parallel mechanism as shown in Fig. 7. Saw shaped driving voltage for the SIDM was outputted through a function generator (Wave-Factory 1946 NF). The function generator had two channel output ports. One channel (CH1) was used for expanding the SIDM actuator and the other (CH2) was used for contracting. The trigger signal from DSP to function generator was utilized to determine which channel (CH1 or CH2) excited the actuator. The driving signal was applied to each SIDM actuator through the amplifier (4020 NF). When the SIDM actuator is adopted with long stroke mode, only one driving unit is necessary, because the SIDM actuator has holding force. With no electrical sources, the mover and base are fixed by frictional force. Hence, one electrical driving unit is sufficient to operate the actuators alternately. This feature is one reason the SIDM actuator is suitable for a parallel mechanism. Even if there are several SIDM actuators, one driving unit can be shared between them. In our research, photo MOS relays were connected to each SIDM actuator in series. Based on the trigger signals to the photo MOS relay from the DSP, the SIDM actuator was selected to be excited. On the contrary to long stroke

5 T. Morita et al. / Precision Engineering 26 (2002) Fig. 8. Results for positioning operation when the target position was (0.0 mm, 0.0 mm, 60 ). Fig. 6. Calculated movable area when the rotational angle (θ) was fixed to 60. mode, in the precision positioning drive mode, each actuator needs its own electrical sources. We will carry out this mode in the near future work. By irradiating two LED alternately, two pairs of position data (X and Y) were gained from PSD. In advance, the relationship between PSD signals and practical positions including rotational information were calibrated. Using these calibration results, actual position data were converted from the PSD signals in DSP. Once these position data have been clarified, the actual length of each SIDM actuator was easily calculated. Similarly, the target length for each SIDM actuator was obtained. DSP judged whether each actuator should expand or contract, and generates a trigger signal to the function generator. After the operation for all actuators had been completed, the LED was irradiated and the control loop was repeated. Fig. 7. System setup for controlling the three DOF parallel link mechanism.

6 294 T. Morita et al. / Precision Engineering 26 (2002) Fig. 9. (a) A distribution function relevant to position after the positioning was completed, and (b) a distribution of rotational data. Fig. 10. Result for positioning operation when the target was a circle 3.0 mm in diameter. The target angle was fixed to 60. (a) The position and (b) rotational angle. 4. Experiments and results In this section, operational results are described. First, the target position was set up as (X,Y,θ) = (0mm, 0mm, 60 ). The approximate first position was (2 mm, 2 mm, 70 ). As shown in Fig. 8, the position of the end effector was controlled as expected, and after 1.0 s the positioning process was completed. However, micro- scopic vibration remained even after 1.0 s. The deviation of the position from the target is shown in Fig. 9. The average deviation was 18.6 m, and the standard deviation was 9.31 m. Second, the target position was changed to draw a circle with a diameter of 3.0 mm. The target rotational angle wasfixedto 60. The period for drawing circles was set to 50 s. For continuous target data as X and Y positions, Fig. 11. A distribution function relevant to (a) position and (b) rotational angle.

7 T. Morita et al. / Precision Engineering 26 (2002) cosine and sine signal were entered to DSP from another functional generator. The operational results for positions and rotational angle are shown in Fig. 10. The deviation of the position against the target is shown in Fig. 11. The average and standard deviation of the position were 5.34 and 19.8 m. Those of rotational angle were 50.0 and 333, respectively. These values were insufficient for a precise positioning stage. As already stated, the main purpose of this investigation was to verify that the SIDM actuator is suitable for a parallel link mechanism. The experiments were carried out with only long stroke mode. Without precise positioning mode, an accurate positioning operation with a less than 1 m error is impossible. However, the precise positioning mode cannot surpass 8.0 m; that is the maximum displacement gained by the piezoelectric actuator with dc input voltage. Thus, some improvements are indispensable to apply precise positioning mode after long stroke mode. In this case, the main reasons of error were ricketiness of joints, calibration error, and inferior sensor-to-noise ratio of the PSD. 5. Conclusions In this paper, a parallel link mechanism utilizing SIDM actuators was proposed. As an initial trial, an X Y θ stage was fabricated and operated. To drive the SIDM actuator, a special waveform is necessary. Two waveforms for expanding and contracting were saved in a function generator and selected by the trigger signal from DSP. By using photo MOS relays, one electrical source could be shared to three SIDM actuators. Some positioning experiments were carried out using the X Y θ stage. The average deviation was 18.6 m with the point target position. When the end effector was controlled to draw a circle whose diameter was 3.0 mm, the average deviation was 5.34 m and 50.0 for position and rotational angle, respectively. These positioning resolutions were not superior, although the possibility of a parallel link utilizing the SIDM actuators could be verified. The control system was very simple and easy. Essentially, the SIDM actuator demonstrates splendid performance with regard to positioning property. In the near future, we anticipate that precise resolution positioning and high-speed operation will be achieved. To accomplish our final goal, suitable joint mechanisms and refined sensor systems for multi-degrees of freedom would be required. Acknowledgments The authors would like to thank Dr. Toshiki Niino of Tokyo University for useful discussion regarding this investigation. This work has been supported by the Mitsutoyo Association for Scientists and Technology (MAST). References [1] Oiwa T, Yamamoto T. Study on micro-cmm using parallel kinematics conception and link configuration. In: Proceedings of the Spring Annual Meeting of the Japan Society of Precision Engineering, p. 470 (in Japanese). [2] Arai T, Herve JM, Tanikawa T. Development of 3 DOF micro-finger. In: Proceedings of the IROS96, p [3] Tanikawa T, Arai T, Masuda T. Development of micro-manipulation system with two finger micro-hand. In: Proceedings of the IROS96, p [4] Oiwa T, Kamimura T, Kyusojin A. High-rigidity multi-degree of freedom fine motion mechanism using piezoelectric actuators. II. Extension to six degrees-of-freedom. J Jpn Soc Precision Eng 1996;62: [5] Furutani K, Shibutani K, Itoh N, Mohri N. Parallel link end effector for scanning electrical discharge machining process. Precision Eng 1998;22: [6] Okamoto Y, Yoshida R, Katsuragi H. In: Proceedings of the Autumn Annual Meeting of the Japan Society of Precision Engineering, p (in Japanese). [7] Yoshida R, Okamoto Y, Higuchi T, Hamamatsu A. Development of smooth impact drive mechanism (SIDM) proposal of driving mechanism and basic performance. J Jpn Soc Precision Eng 1999;65:111 5 (in Japanese). [8] Higuchi T, Watanabe M, Kudou K. Precise positioner utilizing rapid deformations of a piezoelectric element. J Jpn Soc Precision Eng 1988;54: (in Japanese). [9] Higuchi T, Yamagata Y, Furutani K, Kudoh K. Precise positioning mechanism utilizing rapid deformation of piezoelectric elements. In: Proceedings of IEEE Micro-Electro Mechanical Systems, p