Independent Clamping Actuator for Electro-Mechanical Continuously Variable Transmission

Transcription

1 Independent Clamping Actuator for Electro-Mechanical Continuously Variable Transmission KAMARUL BAHARIN TAWI, IZHARI IZMI MAZALI, BAMBANG SUPRIYO, NURULAKMAR ABU HUSAIN, MOHAMED HUSSEIN, MOHD SALMAN CHE KOB, YUSRINA ZAINAL ABIDIN Faculty of Mechanical Engineering Universiti Teknologi Malaysia UTM Skudai, Johor Bahru MALAYSIA Abstract: - Providing sufficient clamping force on the V-belt in continuously variable transmission (CVT) is very important to avoid high power consumption from the engine and also to keep the amount of power loss between the primary pulley and the secondary pulley as minimum as possible. Insufficient clamping force causes the V-belt to slip, which in an extreme case is damaging to the V-belt and the pulleys, thus reducing their lifespan. Besides, it also leads to high power loss, hence decreasing the efficiency of the CVT. In contrary, excessive clamping force results in high power consumption from the engine to produce the desired hydraulic pressure in CVT s actuation system. As an effort to address these issues, the concept of electro-mechanical CVT (EM CVT) with independent clamping actuator is proposed. In this paper, firstly, the proposed concept of EM CVT with independent clamping actuator is introduced. Next, the simplified schematic diagram of the proposed clamping actuator together with its working principle is briefly described. Finally, the potential benefits that can be reaped through this proposal are discussed. Key-Words: - clamping force, continuously variable transmission, electro-mechanical, powertrain, automotive 1 Introduction One of the most important challenges faced by automotive engineers and researchers nowadays is to produce an efficient transmission for automotive applications. Continuously variable transmission (CVT) is a type of transmission technology that unlike conventional stepped transmissions, it offers continuous ratio change and wide ratio coverage. Because of these features, CVT has the potential in allowing the internal combustion engine (ICE) to be operated more efficiently as compared to the conventional stepped transmissions. Currently, pulley-based CVT with V-belt is one of the most popular CVT applied for cars. Unfortunately, this type of CVT that is widely used in the automotive powertrain system today still faces the issues of high power consumption [1], [2] and power loss [2], [3], which essentially hinder the true potentials of CVT for automotive applications. Excessively high clamping force on the V-belt is one of the important reasons that cause high power consumption and power loss in the automotive powertrain system with CVT. According to Klaassen in [1], the process to provide and to maintain sufficient clamping force in the existing CVTs is quite complicated due to the compressibility of the hydraulic oil and also the necessity of an additional hydraulic control system. Because of that, for safety reason, the clamping force in CVT is usually set much higher than necessary. According to the research in [4], in order to prevent the V-belt from slipping in any load conditions, clamping force is set by the manufacturers at minimum 30% more than required for normal operation. As a result, ICE needs to provide an extra power to the hydraulic oil pump so that the desired amount of clamping force can be realized. However, clamping force also must not be reduced blindly because it may cause the V-belt to slip which leads to power loss between the CVT s primary and secondary pulleys. Furthermore, the slipping of the V-belt is also damaging to the V-belt and pulleys, hence leads to the reduction in their lifespan. Therefore, sufficient clamping force on the V- belt is the key to improve the performance of CVT in term of power consumption and power loss. This statement is also supported by studies in [4], [5] that suggest both aforementioned issues are improved drastically if the clamping force is reduced to its ISBN:

2 lowest possible value before the slipping region. In addition to that, a study by Bonsen in [4] also estimates an increase in CVT s efficiency in almost 20% if sufficient clamping force is exerted on the V-belt. Thus, as an effort to provide sufficient clamping force on the V-belt, the concept of electromechanical CVT with the new independent clamping actuator is proposed. 2 Pulley-based Continuously Variable Transmission with V-Belt In pulley-based CVT with V-belt, the CVT ratio is changed through the axial movement of the primary pulley and the secondary pulley simultaneously, as shown in Fig. 1 and Fig. 2. This movement results in the change of the V-belt radius, hence varying the CVT ratio accordingly. Clamping force on the V- belt, on the other hand, is provided and controlled in the secondary pulley through spring. This is to avoid the V-belt from slipping in any load conditions. where X p is the axial position of the primary pulley, X s is the axial position of the secondary pulley, R p is the radius of the V-belt on the primary pulley, R s is the radius of the V-belt on the secondary pulley, R p0 is the initial radius of the V-belt on the primary pulley, R s0 is the initial radius of the V-belt on the secondary pulley and α is the angle of the pulley. Equations (3) and (4) show the relationship between the R p, R s, θ (wrapped angle), L (length of the V-belt) and c (distance between the centre of the primary pulley and the secondary pulley). L is a constant set by the manufacturer of the V-belt and c is constant based on the design of the CVT (Fig. 3). L = ( π + 2θ ) R p + ( π 2θ ) R s + 2 c cos θ (3) R p = R s + c sin θ (4) Finally, the equation to calculate the CVT ratio, r CVT, is shown in equation (5): r CVT = R s / R p (5) Fig. 3 Parameters R p, R s, θ, L and c of CVT Fig. 1 Parameters for axial movement of the pulleys Fig. 2 Axial movement of the pulleys to produce low ratio (A), medium ratio (B) and overdrive ratio (C) in CVT [1] The relationship between the pulley s axial movement and the V-belt s radius during the change of CVT ratio is expressed in equations (1) and (2): X p = ( R p R p0 ) tan α (1) X s = ( R s R s0 ) tan α (2) Most of the existing pulley-based CVT with V- belt use electro-hydro-mechanical (EHM) system as its actuation system. This actuation system applies hydraulic pressure from hydraulic oil pump to actuate the axial movement of the pulleys during the process to change CVT ratio. Besides, continuous hydraulic pressure is also applied to maintain the amount of clamping force on the V-belt. This process, however, leads to high power consumption from the ICE. Furthermore, with EHM actuation system, the procedure to provide sufficient clamping force on the V-belt is also quite complicated due to the compressibility of the hydraulic oil. Research in [1] suggests that the compressibility of the hydraulic oil is not constant; instead it increases as the hydraulic pressure decreases. Due to this, it is complicated to provide and to control both the sufficient clamping force and also the CVT ratio. Besides, the hydraulic system is also vulnerable to internal and external ISBN:

3 leakage, which leads to inefficiency in the system [7], [8]. As an effort to address these issues, the concept of electro-mechanical CVT (EM CVT) with the independent clamping actuator is proposed. With this concept, the EHM actuation system is replaced by the electro-mechanical (EM) actuation system, hence the hydraulic pump in CVT can be removed. Because of this, the high power consumption from the ICE in the process to maintain the clamping force on the V-belt can be potentially reduced significantly. Not only that, by using EM actuation system with the independent clamping actuator, it is also quite probable that the process to provide sufficient clamping force will be less complicated due to no application of hydraulic oil pump. As a result, sufficient clamping force can be provided more effectively, hence potentially improves the efficiency of the CVT further. electric motor will actuate the moving screws through the clamping gears so that the Belleville springs in the secondary pulley can be compressed or released. Through these compression and release of the Belleville springs, the sufficient clamping force on the V-belt can be increased and reduced respectively (Fig. 6). The relationship between the clamping force and the compression/release of the spring can be defined by Hooke s Law. Once the sufficient clamping force is reached, the thread between the moving screw and the clamping gear will hold the compression/release of the spring, thus maintaining the clamping force accordingly. 3 Proposed Independent Clamping Actuator for EM CVT Fig. 4 shows the schematic diagram of the concept of EM CVT with the proposed independent clamping actuator. This concept is developed from the prototype of Electro-Mechanical Dual-Acting Pulley CVT (EMDAP CVT) introduced by researchers from Universiti Teknologi Malaysia (UTM) [9], [10]. The important components of this concept are V-belt, primary and secondary pulleys, moving screws, ratio gears, clamping gears, Belleville springs and 2 DC electric motors namely the primary DC electric motor and the secondary DC electric motor. This concept applies dual-acting pulley mechanism, which means that both sheaves of the primary pulley and the secondary pulley can be moved axially. In the process of changing the CVT ratio, the primary DC electric motor will actuate the axial movement of both the primary and the secondary pulleys simultaneously through ratio gears, middle shaft gears and moving screws. From this movement, the radius of the V-belt on these pulleys will be varied; hence the CVT ratio is changed accordingly. The proposed independent clamping actuator, meanwhile, will be responsible to provide and to control sufficient clamping force on the V-belt. The schematic diagram of the independent clamping actuator is illustrated in Fig. 5. This actuator essentially consists of the secondary DC electric motor, clamping gear, moving screw and Belleville spring. To provide and to control sufficient clamping force on the V-belt, the secondary DC Fig. 4 Schematic diagram of EM CVT with the proposed independent clamping actuator Fig. 5 Schematic diagram of the proposed independent clamping actuator (bold) Additionally, the proposed independent clamping actuator for EM CVT also allows the process of changing the CVT ratio and the process to increase/decrease the clamping force on the V-belt to be performed independently. In other words, the clamping force can be increased or decreased during the event of constant CVT ratio and also during the ISBN:

4 event of changing the CVT ratio, whenever necessary. As a result, the clamping process becomes more flexible. Fig. 6 Compression/release of the Belleville spring leads to increase/decrease of the clamping force respectively 4 Potential Benefits There are three significant potential benefits that can be gained from the concept of EM CVT with the proposed independent clamping actuator. These potential benefits are simple process to provide and to control sufficient clamping force, no power consumption from ICE to maintain clamping force and minimum effect of V-belt misalignment. The first potential benefit, namely the simple process to provide and to control sufficient clamping force, is achieved through the application of moving screw and Belleville spring in the new independent clamping actuator. To increase the clamping force, the Belleville spring is compressed through the axial movement of the moving screw, which is actuated by the secondary DC electric motor. Alternatively, the Belleville spring is released in case if the clamping force needs to be reduced. Here, the relationship between the clamping force and the compression/release of the Belleville spring can be described by Hooke s Law, thus making the process to provide and to control sufficient clamping force more accurate and effective. Furthermore, since this proposed actuator is designed as an electro-mechanical system, so there is no need to consider the compressibility of the hydraulic oil and the possible leakage in the hydraulic pump during the process to provide sufficient clamping force. These two issues are the example of the significant weaknesses of the hydraulic system [7], [8]. With the simple process to provide sufficient clamping force, the excessively high clamping force can be avoided; hence the CVT s efficiency is improved significantly as suggested by the studies done by Bonsen [4] and Vogelaar [11]. Subsequently, the proposed independent clamping actuator also requires no extra power from ICE to maintain certain amount of clamping force on the V-belt, which is the second potential benefit from the concept. In the conventional CVT with EHM actuation system, ICE needs to provide extra power in order to maintain sufficient clamping force on the V-belt through continuous hydraulic pressure. This leads to the issue of high power consumption in the automotive powertrain system. However, the proposed clamping actuator uses thread to maintain the compression/release of the Belleville spring, and because of that, the sufficient clamping force can be maintained without an extra power from ICE. Therefore, the issue of high power consumption from ICE can be avoided. Finally, the issue of V-belt misalignment can also be minimized through the dual-acting pulley mechanism of the EM CVT concept (third potential benefit). According to researchers in [12] and [13], V-belt misalignment causes inefficiency in CVT and also reduces the lifespan of the V-belt and the pulleys. By minimizing this issue, the CVT s efficiency and the lifespan of the V-belt as well as the pulleys can also be improved further. 5 Conclusion Through the concept of EM CVT with the proposed independent clamping actuator, there is a real potential to effectively provide sufficient clamping force on the V-belt. As a result, the performance of the CVT in the areas of high power consumption and power loss can be improved significantly. Because of that, the overall efficiency of the CVT and the automotive powertrain system can be increased dramatically. For the future task to develop the working prototype of this concept, comprehensive researches especially in the areas of stiffness of the Belleville spring and the indicator for the V-belt slipping are required. ISBN:

5 Acknowledgement The authors would like to thank Universiti Teknologi Malaysia (UTM) and Malaysia s Ministry of Science and Technology (MOSTI) for providing continuous support especially in term of funding through escience and Technofund. References: [1] T. W. G. L. Klaassen, The Empact CVT; Dynamics and Control of an Electromechanically Actuated CVT, PhD Thesis, Library Eindhoven University of Technology, [2] E. Kirchner, Leistungsübertragung in Fahrzeuggetrieben; Grundlagen der Auslegung, Entwicklung und Validierung von Fahrzeuggetrieben und deren Komponenten, Springer-Verlag, Heidelberg, [3] S. Akehurst, An Investigation into the Loss Mechanisms Associated with a Pushing Metal V-belt Continuously Variable Transmission, PhD Thesis, University of Bath, [4] B. Bonsen, R. J. Pulles, S. W. H. Simons, M. Steinbuch and P. A. Veenhuizen, Implementation of a slip controlled CVT in a production vehicle, IEEE Conference on Control Applications, Toronto, Canada, 2005, pp [5] B. Bonsen, T. W. G. L. Klaassen, K. G. O. v. d. Meerakker, M. Steinbuch and P. A. Veenhuizen, Analysis of slip in a continuously variable transmission, ASME International Mechanical Engineering Congress, Washington, USA, [6] B. Bonsen, Efficiency optimization of the pushbelt CVT by variator slip control, PhD Thesis, Library Eindhoven University of Technology, [7] Y. Fu and W. Zhang, The Deadband Control of the Electro-hydraulic Actuator of the Motorpump Coordinated Control, IEEE International Conference on Mechatronics and Automation, Xi an, China, 2010, pp [8] L. C. Y. Chan and S. F. Asokanthan, CMAC Based Controller for Hydro-Mechanical Systems, American Control Conference, Arlington, USA, 2001, pp [9] B. Supriyo, K. B. Tawi, H. Jamaluddin and S. Ariyono, Position control for pulley axial movement of electro-mechanical dual acting pulley continuously variable transmission system, Jurnal Mekanikal Universiti Teknologi Malaysia, No. 22, 2006, pp [10] B. Supriyo, K. B. Tawi, H. Jamaluddin, A. Budianto and I. I. Mazali, Shifting Performance Fuzzy-PID Ratio Controller of Electro- Mechanical Continuously Variable Transmission, 3rd International Conference on Circuits, Systems, Control, Signals, Barcelona, Spain, 2012, pp [11] G. Vogelaar, VT2+; Further improving the fuel economy of the VT2 transmission, 6th International Conference Continuously Variable Hybrid Transmissions CVT2010, Helmond, Netherlands, 2010, pp [12] K. B. Tawi, Investigation of Belt Misalignment Effect on Metal Pushing V-Belt Continuously Variable Transmission, Cranfield University, [13] Z. Faye, Study on Electro-Hydraulic Control System for CVT Metal Belt Axial Misalignment, IEEE International Conference on Mechatronics and Automation, Changchun, China, 2009, pp ISBN: