February 10, 2020

A Modular Revolution for Autonomous EVs: Introducing the Cluster Rover Module

NSK exhibited a number of technologies powered by its vision of "Setting the Future in Motion" at the 46th Tokyo Motor Show 2019. Our exhibits heralded technologies for the future—especially new methods of producing, transmitting and using energy—including wave power generation, in-motion wireless charging for in-wheel motors to achieve efficient, contactless power transmission to cars, and the Cluster Rover Module, a power module for autonomous EVs. At NSK, we never stop pursuing exciting new technologies.

We asked Masafumi Hikida and Ryuho Morita from the Powertrain Technology Development Department, Automotive Technology Development Center, Automotive Business Division HQ, to tell us all about the Cluster Rover Module—how it works, how the development started, and why NSK's research on this technology is so important.

Masafumi Hikida (left) and Ryuho Morita (right) from the Powertrain Technology Development Department

Masafumi Hikida (left) and Ryuho Morita (right) from the Powertrain Technology Development Department

A New Way to Turn: Driven by the Rotation Differential of Two In-Wheel Motors

── What is the Cluster Rover Module?

Masafumi    It's a technology for autonomous EVs, developed as a drive module intended mostly for box-shaped public transport and service vehicles, rather than consumer cars.

It has two features. First, the number of common drive modules can be adjusted for the size of the vehicle. This means that the same module can serve any size of vehicle. For example, we would use four modules for a small vehicle and eight for a large one.

Second, the functions for driving, steering, and braking are all integrated into the drive module itself, which is also equipped with functions for measuring loads and adjusting vehicle height.

Ryuho    Let me tell you how it's put together. First, the in-wheel motors that drive the two wheels and the brakes are integrated at the bottom. On top of that, there are carbon fiber plate springs and dampers to absorb shock, and a pantograph jack, which adjusts vehicle height, is fitted at the very top. The pantograph jack has a ball screw actuator. The turn of the screws lengthens or shortens the actuator, allowing vehicle height to be adjusted.

What's unique about the module is that there is no need for a steering actuator. The pantograph is fixed to the vehicle, but it can be freely rotated below, which makes it possible to drive the steering by creating a differential in the rotation speeds of the left and right wheels and reversing their respective directions of rotation. There are two wheels on each module, which enhances the steering function.

The reason why we didn't use a steering actuator is that, since it would only be used infrequently for changing the direction of the module, it's a bit of a waste. Also, because a large force is needed to change the direction of the wheels, a steering actuator inevitably requires a big motor. Using two wheels has the major advantage of eliminating the need for this.

Ryuho Morita

Masafumi    With this construction, the spring rate (the hardness of the springs) that supports the vehicle and the vehicle height adjustment function are independent, and to they can be set independently. This enables us to maintain optimum ride comfort even when changing vehicle height with the height adjustment mechanism. With regular air suspension, you can't separate the relationship between the spring rate and vehicle height.

The module we are developing right now has 10-inch wheels and motorbike tires. The output of each wheel is 12 kW, adding up to 24 kW for each module. Using four modules gives 96kW of output, which is enough power for a compact car. We don't expect a box-shaped autonomous EV to go that fast. The maximum speed is around 50 km/hour.

It is simpler than regular suspension, so there are fewer parts, and it is extremely space efficient. If there is any trouble, the modules themselves can be replaced individually, which makes for outstanding ease of repair. You can swap the modules out and be back on the road with limited downtime while other module are in for maintenance.

Thinking About the Technology Needed for the Future, and Expanding Our Knowledge by Working Toward the Ideal

── Why did NSK develop this product?

Masafumi    At NSK, we often talk about what the vehicles of the future will require. We exchange and refine our ideas about what parts will be needed ahead of changes in the global environment and advances in technology.

Our clients are moving forward to address the increasing electrification of cars, autonomous driving, and sustainability. We stand ready to support them with our original solutions such as wave power generation, in-motion wireless charging in-wheel motors, and the Cluster Rover Module, a drive module for autonomous vehicles.

It's challenging to identify how consumer choices will shape the future when autonomous driving is put into practice for public transport and in the service sector. Obviously, if maximizing usable interior space is better, we thought that vehicles will probably become box-shaped. So a new drive module will be needed, and this is the Cluster Rover Module.

It took about a year to put the idea together. We were able to implement it surprisingly quickly, mainly because we adopted technologies that NSK had worked on previously for the components, such as the technology to reduce the size of the in-wheel motor speed reducer and the pantograph actuator construction.

Masafumi Hikida

── What is significant about NSK, a bearings manufacturer, embracing these new challenges?

Masafumi    NSK is not necessarily thinking about mass producing the Cluster Rover Module as a product. Our business has more to do with the components used, such as bearings, ball screws, and actuators.

The advantage of developing an entire module is that we can assess in advance how the components are used and design the optimum bearings and actuators. In the future, drive modules for box-shaped autonomous EVs might not take the exact form that we are predicting, but there probably won't be a significant divergence in terms of the functions.

Ryuho    The development of the Cluster Rover Module was a new project for us, and we have learned a lot from working on it. We didn't have much expertise with springs, and the construction and integration of a pantograph and load sensors were also new challenges.

We can't build up our know-how in optimizing bearings and ball screws without trying out a diverse range of applications. In that sense, I think expanding our future potential and embracing new challenges has great significance. Also, I felt it was an extremely rewarding job on a personal level because I got to directly translate my own ideas into concrete form.

Masafumi    Having a go at something new also lowers the hurdle for the next challenge. There's also a lot of scope for broadening our range of technology and knowledge. We want to set the future in motion inside and outside the company and make a difference in societies around the world.

We don't know exactly what the future of mobility will look like. But if we continue to strive to solve social issues and meet the needs of the public, NSK will be indispensable to society, which will in turn ensure that NSK itself keeps growing sustainably.