An investigation on development of Precision actuator for small robot Joo Han Kim*, Se Hyun Rhyu, In Soung Jung, Jung Moo Seo Korea Electronics Technology Institute (KETI) * 203-103 B/D 192 Yakdae-Dong, Wonmi-Gu Puchon-Si, Kyunggi-Do, 420-140, Korea Phone number: +82-32-621-2849, Fax number: +82-32-621-2855 E-mail address: kimjh@keti.re.kr Abstract: - In the last decade new technologies for manufacturing of precison mechanical components have been developed. Nowadays, the reduction of size and weight is very often one of the most important requirements in product design. Many applications in robotics, automation, medicine systems etc require powerful precison actuator. The powerful precision actuator has Speeds up to high speed and high output torque efficiencies. To accomplish a powerful precision actuator, these powerful motor have to be combined with gearhead of the same outer diameter. So, we have developed BLDC motor and planetary type gearheads as powerful precison actuator. The BLDC motor has advantages that compact structure, high efficiency, high reliability. The planetary type gear heads have advantages that same-axle structure, high torque transmission, low noise in comparison with spur gear heads. In this study included BLDC motor and planetary type gear heads design, manufacture for small robot. Key-Words: - Actuator, Small robot, BLDC motor, Planetary gearheads, Design, manufacture 1 Introduction Many applications in mechanism, telecommunication, automation systems etc. require a powerful actuator. The powerful actuator can speeds up and it can also increase the output torque efficiencies. To make a better powerful actuator, these powerful motor have to be combined with gearheads of the same outer diameter. So, we have developed BLDC motor and planetary type of gearheads as a powerful actuator. The BLDC motor have advantages of compact structure, high efficiency and reliability. And the Planetary type of gearheads have advantages of same-axle structure, high torque transmission and noiselessness in comparison with spur gearheads. This paper presents a design method of small sized slotless brushless DC motor. Distributed hexagonal windings are applied to a stator core for high power density and manufacturing advantages. And, We developed small planetary gear heads for small robot that can use contracting to BLDC motor of this development. In this study, include in gear tooth design, strength design, gearheads design, mesh stiffness simulation and manufacture 2 BLDC motor development In recent years, brushless DC (BLDC) motor has been used in various applications due to its high efficiency and power density, excellent controllability. However, the stator core of a conventional structure prevents the overall size of motor from minimizing and winding process is getting hard as motor size is becoming small. Slotless BLDC motor, which has cup shaped winding fixed on the cylindrical stator iron, has several advantages as well as minimizing merit. Amount of permanent magnet of the rotor could be increased due to slotless stator structure, so power density of motor is bigger than slotted one. The cogging torque is extremely small, vibrations and noise can be reduced. Winding placed in the airgap is profitable for cooling because the generated heat form winding can be removed not only through the iron but by moving air in the gap. 2.1 Basic Structure Fig. 1 shows the diagram of an inner rotor slotless BLDC motor. We designed a two pole, three phase motor with double layered short pitch winding. Ring-shaped high energy Permanent magnet (NdFeB material) is used and distributed winding composed of 9 single coils is fixed in stator core. In order to determine geometric parameters of motor given outer size limitation of 22 [mm], flux density distribution is calculated. There are several types of winding for slotless BLDC motor. Rectangular winding is profitable to increase ISSN: 1790-5117 62 ISBN: 978-960-474-078-9
linkage flux and to form winding arrangement, however, end turn parts generated in both top and bottom sides could be obstacle to combine with stator core. Furthermore, the thickness of end turn is proportional to the coil pitch, which requires additional work to enlarge a space for rotor positioning. stator core co il mag net shaft 1 2 3 4 Fig. 1 Diagram of slotless BLDC motor with a two pole and double layered winding. 9 single coils are distributed in stator core. Fig. 2 Slice regions for 2D analysis and winding arrangement with respect to the regions. In rhombic winding structure, we do not need to consider end turn parts, but the back EMF at the same rotational speed and magnetic condition is about 50% less than rectangular one. Hexagonal winding is composed of rectangular and rhombic structure partially, and it has advantages in magnetic and manufacturing points of view. Due to the coil side to be perpendicular to flux from permanent magnet, linkage flux of coil is increased, also end parts of winding is not thick owing to oblique coil side of both end section like rhombic shaped coil. 2.2 Numerical Design To simulate the hexagonal winding, we divide the winding to several slices to the axial direction as shown in Fig 2. EMF induced from Perpendicular side of coil (region 4) can be calculated in conventional 2D analysis, but in order to figure out EMF from oblique side (region 1, 2, 3), coil reassignment with respect to the slice region is required. As can be seen in this figure, coil pitch is getting large and small along with region number. Back EMF regarding to the each slice region is calculated, and summation of the values considering axial length is determined finally. If the EMF value of each region is E 1, E 2, E 3, E 4 in order, oblique and perpendicular side length of coil are A and B respectively, the total induced back EMF E t is expressed like Fig. 3. From the back EMF calculation, torque constant is induced and it is applied to voltage equation for DC motor to predict output characteristics, such as speed, torque, and efficiency. Fig. 3 Back EMF calculation of various slice regions. V t is fixed and K t is calculated as mentioned above. If we find out the value of R a, torque characteristics with respect to the speed variation are determined. In this case, core loss is ignored and friction loss is considered as an amount of 10% of the rated load torque. TABLE I : FINAL MOTOR DESIGN Item Value driving voltage [V] 12 external diameter of stator yoke [mm] 21.0 Internal diameter of stator yoke [mm] 14.2 external diameter of magnet [mm] 10.4 air gap [mm] 0.3 PM remanency [T] 1.26(Nd sintered) number of turns/phase 11 coil diameter [mm] 0.26(2wired) ISSN: 1790-5117 63 ISBN: 978-960-474-078-9
2.3 Manufacturing and Verification Fig. 5 shows winding manufacturing procedure. Self bonded magnet wires are prepared and single coils are produced using hexagonal winding jig. Those coils are arranged on external surface of winding assembly jig considering permanent magnet and air gap length. For an accurate arrangement of single coils, special grooves are formed between perpendicular sides of adjacent coils on assembly jig. Tiny pins are put into the grooves and single coils are arranged with constant gap thanks to the pins. After winding lineup, we remove the pins from assembly jig and push the outer part of wining using pressing jig. It makes the winding formed well and limited within designed dimension. The winding is inserted in the stator core with insulation material and heat is applied to the stator core to fix and bond the contiguous wires again. Three-phase back EMF of the proposed motor at 500[rpm] is appeared in Fig. 6. Even though the winding is developed by home-made manufacturing, the value and shape of each EMF is very similar. Both of the distributed winding and parallel magnetization of the permanent magnet make the waveform of EMF sinusoidal. A squarewave current waveform is applied in this study, however, with a sinewave current control, soft driving with little torque ripple can be possible. Fig. 6 also shows output characteristics of manufactured one. As can be seen in this figure, no load speed is more than 16,000 [rpm] and efficiency is about 80 [%] at a rated power of 20 [W], maximum efficiency is 85 [%]. Comparing with simulated results, no load speed and efficiency are a little bit higher, which could be difference of initial fiction load set by simulation process. Also, as loading toque is getting bigger, voltage drop is increased and rotational speed is decreased rapidly in experimental results. efficiency : 80[%] speed : 13,200[rpm] output power : 20[W] current : 1.8[A] Fig. 6 Measured back EMF induced in each phase coil and experimental result of designed motor 3 Planetary gearheads development High gear ratios are required to convert the power of the micro motors to lower speeds and higher torque. For very small gearheads, a toothed gearing with involute tooth profile is the suitable design. Planetary gear head consisting of a sun gear, a ring gear, several planets, and a carrier. Any of the carrier, ring, and sun can be selected as the input or output component, and the power are transmitted through multiple paths of the planet meshes. Planetary gears have substantial advantages over parallel shaft drive, including compactness, large torque-to weight ratio, diminished loads on shafts bearings, and reduced noise and vibration due to the relatively smaller and stiffer components. 2.1 Gearheads design This is planetary gear heads specs of this development. Fig. 5 Winding manufacturing process and assembled slotless BLDC motor - Dimension: 22mm * 35 mm - Reduction ratio: 1/ 11 (2 Stage) - Input speed: 7,000 rpm - Gear material: steel - Gear module: 0.4. ISSN: 1790-5117 64 ISBN: 978-960-474-078-9
safety factor of sun/planetary is 4.1, contacting safety factor of ring/planetary is 5.7. Fig. 7 Gear design flow chart Gearheads must satisfy following geometrical condition. - Condition for symmetrization assembly - Condition to avoid crossfire between teeth - Condition about contact ratio - Maximum number decision of planet gear Fig 8 is gear detail specs of this development planetary gear heads. (a) Sun /planetary, (b) Ring / planetary Fig. 10 Gear strength results of this development planetary gear heads Fig 11 is gear mesh stiffness simulation results of this development planetary gear heads. This simulation carries out for understanding tendency of gear vibration, gear stiffness. Fig.8 Gear detail specs of gearheads Fig. 11 Gear mesh stiffness graph of planetary/ring gear 2.2 Manufacturing and gear precision test Fig. 9 Construction of the planetary gear heads Fig 9 is construction of this development planetary gear heads. Fig 10 is gear contacting (fitting) strength results of this development planetary gear heads. Contacting We are used in involute tooth design, and precision working technology of using CNC hobbing machine, EDM in this gearheads development. This development gear heads is gears of spur type. Spur gears can be produced well using hobbing machine and wire cutting. There is no difference between external or internal toothing. Gear tooth profiles used involute profiles. Gear accuracy applied JIS standard, and accuracy measurement used in CNC gear profile tester. Gear profile tester measure accuracy of gear by means of gearing tip measurement gear tooth profile for examination. Fig13 is gear accuracy results of this development planetary gear heads. ISSN: 1790-5117 65 ISBN: 978-960-474-078-9
Fig. 12 Planetary Gear heads product References: [1] M. Markovic, Y. Perriard, Simplified Design Methodology for a Slotless Brushless DC Motor, IEEE Trans. Magn., vol.42, no.12, pp.3842-3846, 2006 [2] N. Bianchi, S. Bolognani, F. Luise High Speed Drive Using a Slotless PM Motor, IEEE Trans. Power electronics, vol.21, no.4, pp.1083-1090, 2006 [3] N. Bianchi, S. Bolognani, F. Luise Analysis and Design of PM Brushless Motor for High-Speed Operations, IEEE Trans. Energy conversion, vol.20, no.3, pp.629-637, 2005 O. K. Kelley, 1991, "Design of Planetary Gear Trains", 3.1 [4] A. J. Lemanski, 1990, "Gear Design", SAE, Chapter 3 [5] Darle w. Dudley, 1984, "Handbook of Practical Gear Design", Chapter 8 [6] Robert G. Parker, 2001, "Modeling, Modal Properties, and Mesh Stiffness Variation Instabilities of Planetary Gears", NASA Fig. 13 Accuracy of planetary gear profile(jis 2 grade) 4 Conclusion In this paper, slotless BLDC motor of outer diameter 22[mm] is designed. Double layered short pitch windings with 9 hexagonal single coils compose stator core and a two pole permanent magnet is applied in rotor. In this paper, we developed micro planetary gearheads that has characteristics of parallel shaft drive, including compactness, large torque-to weight ratio. ISSN: 1790-5117 66 ISBN: 978-960-474-078-9