Electric Transaxle White Paper

Electric Transaxle™ US Patents 5,851,162 and 7,481,730

By Alex Pesiridis Senior Engineer Technipower Systems, Inc. Electric Drives Group www.technipowersystems.com

Overview Electric vehicle drive systems are in high demand as a result of the public policy trend to reduce consumption of fossil fuels. In place of internal combustion engines, future electric drives for cars and trucks will use battery systems, fuel cells or hybrid configurations. Today, typical drive train solutions incorporate multiple gear sets designed to provide varying amounts of torque which are extremely inefficient throughout the range of speeds a standard vehicle might experience. Minimizing these losses involves more than simply making each drive train component more efficient. By combining the mechanical power source, transmission, power transfer mechanism (drive shaft) and differential/axle into one compact mechanism, the overall efficiency of the vehicle is increased significantly and the simplicity of the system is enhanced. The elimination of component parts and the reduction of internal heat losses also improves the overall reliability for such systems. A unique solution to this problem is the Electric Transaxle™, a device that provides a compact, flexible design allowing both front and rear-wheel-drive configurations. It is a drop in solution for retrofit and OEM vehicles comprising motors, gear sets, continuously variable transmission (CVT) and axle/differential enabling a vehicle to seamlessly accelerate from a dead stop to full cruise mode in a highly efficient manner. The technology is applicable to all electric and hybrid vehicle platforms including fuel cell powered vehicles.

Background Although electric motors and drive wheels are, by their nature, very efficient, all the parts that intervene to make a vehicle useable in a typical application add tremendous inefficiencies. The most loss oriented components are gear sets whether they are in the transmission, differential, or somewhere else in the drive train. Today, most electric drive systems have a negligible impact on overall efficiency for this reason. Vehicle architectures usually have multiple gear sets to achieve the desired torque multiplication and differential wheel turning.

Figure 1: Common Architectures of Other Electric Vehicles

The Electric Transaxle minimizes these losses by using its integral subcomponents at those times, and under those specific conditions, when they are most efficient. Using a planetary gear set while in the initial acceleration phase and then transitioning to an effective direct motor drive system at cruising speed by locking the gear set, provides the ultimate efficiency with the greatest flexibility in typical operating conditions. Since the Electric Transaxle is scalable for different vehicle horse power requirements and provides all the benefits of regenerative braking, it is a form and function replacement for a multitude of individual systems in a typical application. Not only is the efficiency increased with the Electric Transaxle, but with a reduced parts count and a straightforward concentric design, the Electric Transaxle allows for a standard vehicle platform to be customized by software changes alone. This substantially differs from any standard electric drive train available today. In current technology any specialized changes typically require both hardware and software modifications, thereby severely limiting an application’s breadth and increasing its costs, such as in re-tooling and long product redevelopment cycles. Specialized changes that the Electric Transaxle easily accommodates include slip differential, regenerative breaking, regenerative differential, and a host of other safety features. Figure 2 below shows the unit’s typical profile.

Figure 2: Electric Transaxle in a standard straight axle configuration Ease of installation and integration is a major additional benefit of the Electric Transaxle when compared to existing electric drive trains. The Electric Transaxle is a drop in replacement for a standard differential axle or differential with independent axles (half shafts). With this sophisticated technology packaged in a form that makes it readily interchangeable in existing vehicle configurations, conversions can be completed in as little as one day. Shortened time and decreased complexity of installation substantially impacts the universe of existing vehicles for which electrification becomes practical and economical. Although OEM applications are a more challenging opportunity due to the substantial investment the large automakers have in their existing technologies, the Electric Transaxle is an extremely compelling alternative for the top tier suppliers to the automotive industry.

Electric Transaxle in Depth The Electric Transaxle is a uniquely efficient device that can replace traditional geared differential units with a complete electric power drive unit for land based applications. Due to its simplicity and efficiency, the Electric Transaxle provides a novel solution while providing the added advantage of operating as a Continuously Variable Transmission (CVT). The Electric Transaxle develops its torque through the combination of permanent magnet (PM) brushless DC Motors and two planetary gears sets, one for each drive wheel. The PM motors, having a flat torque curve, provide significant torque at low RPM which conventional motor systems cannot match. In conjunction with the planetary gear set, the PM motors provide torque multiplication by taking two power inputs and combining them into one output. Permanent Magnet (PM) Brushless DC Motor Permanent Magnet Brushless DC motors are used as the primary electrical input to the Electric Transaxle. Although the Electric Transaxle could have been developed using any type of motor technology, the PM Brushless Motors were chosen because they provide the best performance in the physical envelop required. The Electric Transaxle uses the continuous torque of the PM motors to great advantage. This differs from other competitive technologies which use the brief, intermittent torque zone, as shown in Figure 3, to achieve over-duty operation for purposes of acceleration. This approach limits system design because of the decreased amount of time available to operate under the highest torque conditions, and consequently, requires larger, more expensive power electronics to accommodate the increased power flows.

Figure 3: Intermittent Torque and Continuous Torque Regions

Planetary Gear Set In general, planetary gear sets are an important component of the transmission industry. They provide a means of converting angular velocity (RPM) and torque from one motive force into another form which is applied to the output. Planetary gear sets are popular because they allow a gear ratio to be applied to the input power while having the gear assembly be concentric, or colinear, with the power input. The planetary gear set consists of four main components:

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Ring Gear – Largest diameter gear of the gear set with gear teeth revolving around the inner diameter of the gear Sun Gear – Smaller than the ring gear and concentric with the ring gear in the planetary gear system Planet Gears –Gears vary in size and depend on the amount of space between the Ring and Sun Gears. There can be multiple sun gears in the planetary gear set that surround the sun gear. The number of planet gears indicates the magnitude of torque and the number of loads the planetary gear system is required to handle. Planetary Carrier – Assembly that contains all planet gears in concentric pattern between the sun and ring gears. The planetary carrier can be attached to a shaft to make the planetary carrier and gears and input or output.

Figure 4: Planetary gear set

Traditional planetary gear sets in transmission devices have one input and one output to achieve the desired result. Since there are three main components (ring, sun, and planetary carrier with planet gears) that can be moved, one component is usually fixed. In traditional automotive transmissions, the ring gear is fixed to the transmission casing, the input is the sun gear and the output is the planetary carrier assembly with planet gears. Allowing two inputs to be applied to two of the components of the planetary gear set provides a continuously variable output at the remaining component of the planetary gear set. Such is the case as evidenced by an earlier patent Technipower Systems was awarded (Electric Wheel™, US Patent 5,067,932). Electric Transaxle Topology Technipower Systems has patents on two versions of the Electric Transaxle. The difference between the two patents relates to the shared element of the two planetary gear sets, i.e. either shared sun gears or shared ring gears. The Electric Transaxle discussed here uses a common ring gear structure powered by a PM motor, shared between the two planetary gear sets. The two sun gears are each powered by their own individual motors, allowing the Electric Transaxle to operate each of the two wheels of the vehicle independently. Independent control of each wheel provides a wide variety of drive modes including limited slip differential, locked differential, and variable torque differential all with regeneration.

Figure 5: Electric Transaxle Technology Exploded

Digital Control Digital control of the Electric Transaxle enables the efficient use of torque on demand. As torque demand is increased, the Electric Transaxle supplies sufficient torque to move a load from dead stop to the desired velocity with quick acceleration. This is achieved by varying the ratio of angular velocity (RPM) in each of the Electric Transaxle component structures. As the power requirement decreases and the corresponding torque requirement diminishes, eventually the three main components (sun, ring, sun) are rotating precisely at the same speed. When all the components of the Electric Transaxle are rotating at the same speed, the two planetary gear sets are locked together and the PM motors have a one-to-one relationship to the wheel and load. This point of operation is the most efficient because of the removal of the losses associated with the planetary gear sets, thereby also reducing wear and tear on the device. Mid-range acceleration in the Electric Transaxle is also accomplished with digital control by commanding the ring and sun motor RPM to operate in a fixed ratio that allows the vehicle to essentially “down shift” instantaneously for passing. The relationship of ring and sun motor RPM provides the Electric Transaxle the ability to provide on-demand torque for acceleration while eliminating low speed shudder, gear grinding, gear noise, and vibration.

Conclusion The Technipower Systems Electric Transaxle technology is revolutionary in being the first motor/transmission device to be built into the space constraints of a traditional differential axle assembly while achieving greater control, operational flexibility, and high efficiency. This technology can immediately be utilized for the electrification of existing vehicles and has great potential to transform the application of electric drive systems in pure electric, hybrid and fuel cell vehicles.