Renesas Microcomputers Define the Performance and "Personalities" of an Extensive Range of Electric assist bicycles

Engineers designing the eco-friendly personal transportation products offered by Panasonic Cycle Technology use microcomputers to implement precise motor control that contributes to extended range and ease of use

More people are riding electric assist bicycles in Japan and product lines have expanded to include sports versions and foldable ones. Panasonic Cycle Technology, the leader in this growing market, has worked hard on electric assist bicycle development for more than ten years. The company now offers more than ten different models, all of which use Renesas R8C or H8/Tiny microcomputers to obtain optimum cycle performance and make the bikes easy to operate. The chips' processing capabilities, built-in motor-control peripheral functions, and other features are applied to achieve longer operating ranges and better ride quality. By using software to adjust the electronic control system, engineering teams have given different models distinctive operating characteristics — unique "personalities" — that appeal to different groups of riders, thus building sales and expanding the customer base.

Electric assist bicycles are very popular in Japan because they are easy to ride up hills, even on windy days. In fact, the domestic market for these personal transportation products has grown to the point where annual sales are nearly 300,000 units. Buyers include not only private users but also bike rental operations in tourist locations. Further, new uses are emerging to take advantage of the benefits that electric assist bicycles provide. In view of widespread concerns about the environment and energy conservation, these versatile vehicles are even starting to be used in commercial situations such as delivery services and the maintenance and servicing of office equipment. In such uses they replace the cars or motorbikes that people would otherwise drive.

Panasonic Cycle Technology began selling electric assist bicycles in 1996 and currently (as of 2007) has earned the largest share of the Japanese domestic market. Overseas, the company sells motor units and is experiencing strong sales growth for them, particularly in Europe. "Whereas Panasonic Cycle Technology had initially tailored its products and sales efforts toward the needs of housewives and older people, we have recently expanded the product line by adding variations that will attract other types of customers. Today, for instance, we also offer sports models and a model aimed at young women. Also, we now make the foldable 'Off Time' version that people can load into a car and take to distant riding locations," Mr. Miyake said.

Besides increasing its number of electric assist bicycle models, the company has improved the cycles' designs. The latest products are lighter and have a longer assisted-cycling range when fully charged. The sports models have generated considerable interest because they offer greater performance. For example, the "Lithium ViVi EX" model with a high-capacity (10Ah) lithium-ion battery has a range of approximately 144km on a full charge in Automatic mode. That puts it in the top class in the industry.

A key enabling engineering factor in this longer range is the addition of the Automatic mode to the existing Standard and Enhanced modes of assisted cycling. In Automatic mode, a microcomputer monitors the required cycling effort and adjusts a three-stage assistance pattern in an optimum manner. On flat terrain in Automatic mode, for example, the control circuitry conserves battery power by directing the cycle's electric motor to provide only minimal pedalling assistance. Of course, besides the Automatic mode, the sports model's lighter weight and greater battery capacity also contribute to the extended range it achieves.

The latest model — the flagship in Panasonic Cycle Technology's product range — is the Titanium Flat Road EB, a premium lightweight electric assist bicycle (see Photograph 1). This top-end sports model is the first electric assist bicycle to use a titanium alloy frame, a feature that helps trim down its weight to only 15.7kg, similar to that of a conventional bicycle.

"The design concept for the Titanium Flat Road EB model is represented by three letters: T.E.N. The 'T' stands for 10km, the 'E' for electric bicycle, and the 'N' for novelty," Mr. Miyake explained. "At a speed of 20kph, this electric assist bicycle can travel 10km in 30 minutes. This is the right length of time for aerobic exercise." He then added, "The Titanium Flat Road EB bicycle and also the Titanium Light EB models express the joy of a comfortable ride. They are fitted with a multi-control cycle meter with an LCD screen that displays data such as the current speed, trip distance, and maximum and average speeds ridden on a trip." Photograph 2 shows the LCD display.

When Panasonic's engineers were developing the sports models, doubts were raised about the desirability of such a display on an electrically powered cycle. According to Mr. Miyake, the decision was made to incorporate the display into the design because "the product was aimed at people who lacked confidence in their physical capabilities, or who wanted to be able to ride around in the same way they could when they were younger. One of our goals was to produce products that would assist cyclists who have diminished physical capabilities. In this regard, the data shown on the screen is helpful to riders and appreciated by them."

The "Off-Time" model shown in Photograph 3 is another pioneering Panasonic product in the electric assist bicycle market. This model has a low frame size suitable for both female and male riders. Moreover, it can be quickly and easily folded up into a compact size using quick-release levers so it can be transported to distant riding locations or stored at the office after a commute. The "Sugardrop" model shown in Photograph 4 is a design targeted at young women. It hints at some of the many possible tailored variations of electric assist bicycles that can be produced to open up new markets and improve sales in existing ones.

This ability of the engineers at Panasonic Cycle Technology to develop a wide range of different models to suit diverse markets is due in part to their creative application of electronic technologies that have advanced dramatically over the last few years. For example, one problem with past electric assist bicycles was that they became hard to pedal when the battery went flat. This problem typically occurred because pedal action was detected by a sensor fitted directly to the gears and/or because the bicycle used a traditional brush-type DC electric motor.

"We were ahead of other companies in finding solutions to this problem," noted Mr. Tanida. "We adopted non-contact magnetostrictive sensors and switched to brushless DC electric motors, also known as BLDC types. These design changes allowed us to eliminate any mechanical drag, thereby making the bicycle ride more like a bicycle when the battery was flat. The design improvements also helped extend the bicycle's range, too, another important benefit."

Although non-contact magnetostrictive sensors were an existing technology, the available devices weren't practical for use in use electric assist bicycles due to their complex structure and high cost. However, Panasonic Cycle Technology undertook a successful joint development project with Matsushita Electric Industrial Co. Ltd. to produce magnetostrictive sensors suitable for this application. Thus it was able to adopt the new components ahead of other companies. Another design enhancement that Panasonic's engineers made was to improve the bicycle's stability and handling by locating the motor unit, battery, and other heavy components below the center of the cycle's frame.

Renesas 16-bit microcomputers are the heart of the electronic control systems of the BLDC motors used in all of Panasonic Cycle Technology's extensive range of electric assist bicycles. The primary device used to date has been the H8/3687, a member of the H8/Tiny series. However, for the top-end Titanium Flat Road EB model, Panasonic's engineers selected the R8C/2A chip from the R8C/Tiny series, a device that has proven to be an excellent choice. The company plans to change over progressively to microcomputers in R8C/Tiny series in the future.

Mr. Tanida rates the H8 family of microcomputers highly for how easily development know-how can be maintained in a small engineering team. Mr. Edatsune, who is responsible for software development, commented that "the H8 family, R8C/Tiny series and other Renesas microcomputers have a full range of on-chip functions for motor drive applications. Those features ease the design of efficient control systems."

The R8C/2A microcomputer that Panasonic has adopted for the Titanium Flat Road EB and subsequent models inherits the same timers and other peripheral functions used in the H8/Tiny series. Another significant advantage of this compact, low-pin-count microcomputer is that it offers additional internal functions that previously had to be implemented with external components. Thus, the device allows a small-size circuit board (see Photograph 5). The additional features built-into the chip include a power-on reset (POR) function, low voltage detection (LVD) circuit, data flash memory, high-speed on-chip oscillator, and digital-to-analog (D/A) converter.

Panasonic uses a die-cast aluminum box to house the circuit board for the Titanium Flat Road EB. The die required to make this box is expensive, so the engineers want to keep using the same size and shape box for a long time. Consequently, they cannot easily increase the circuit board size when they are planning to incorporate new or enhanced functions into electric assist bicycle models. "In the past, the requirement for higher currents or the addition of new interfaces mandated the need for larger connectors. This made it difficult to use the same-size circuit board. However, since the Renesas R8C/2A microcomputer integrates most of the peripheral circuits we need onto the chip, we can upgrade functions without having to make the circuit board larger," Mr. Tanida explained.

Panasonic's engineers evaluated alternative chips before deciding to use the R8C/2A in their titanium-framed bicycle. One important development advantage of the device they selected was that with regard to software, code developed for the H8/Tiny series devices could be reused for any R8C/Tiny series chip. They also found that the additional A/D converters and other interfaces on the R8C/2A microcomputer were very useful when adding new functions. "The R8C/2A was the only low-cost microcomputer that had exactly the number of A/D converters and interfaces we required. Those on-chip resources gave us design flexibility. For example, they have made it easy to add new functions such as a throttle and indicator LEDs for overseas models," said Mr. Tanida.

Timers are essential functions for controlling electric motors. The fact that the microcomputers in the R8C/Tiny series keeps the same timer functions as the devices in the H8/Tiny series is one of the major reasons why the engineers at Panasonic Cycle Technology found it easy to transition their control system designs to the R8C/2A chip. Also, Renesas the microcomputer allows timers to set interrupts flexibly, facilitating modifications such as adapting the control system for high-RPM operation.

As previously mentioned, electric motor technology has progressed to the point that there has been a transition from brush-type to brushless motors for an increasing span of applications. Initially, Panasonic's engineers specified the use of SPM brushless motors. However, since 2005 the company has switched to IPM brushless motors. "In the IPM motors, motor speed is reduced by a reduction gear mechanism and the motor's characteristics are such that the best output is achieved by operating it at high RPM with a big reduction ratio. The setting flexibility of the timers on the R8C/Tiny series microcomputers allows us to achieve stable control of IPM motors at high RPM," said Mr. Tanida.

Panasonic Cycle Technology also reports that the internal flash memory of the Renesas microcomputers has proved useful for fine-tuning the operating characteristics of different models. "The way the motor operates imparts a personality to the electric assist bicycle. We use torque control and sense the pedal power to smooth the startup, adjusting it to a certain extent. Tuning the motor's operation is difficult, and the internal flash memory of the microcomputer is useful when the engineers apply trial and error as they optimize the way the bicycle responds to its rider's inputs," Mr. Edatsune said.

To develop the best tuning for a new model, engineers ride a prototype for approximately 500km over a period of about a month. They progressively adjust the personality of the bicycle, achieving the desired characteristics of the finished product by changing and refining parameters and modifying software. The Automatic mode described earlier is one result of this sort of effort.

The on-chip debugger that R8C/Tiny series devices provide, which connects via a single line, improves efficiency of the process of improving the software. "Besides being simpler to connect, the one-line interface to the microcomputer makes the debugging operation less prone to interference from the noise generated by nearby dynamos. In turn, that facilitates and enhances the reliability of the process of finding and correcting errors in the code," commented Mr. Edatsune.

Development of various aspects of electric assist bicycles is ongoing at Panasonic Cycle Technology, as the company works to maintain and extend its dominant market share in Japan. For example, projects are underway to improve motor performance and obtain more precise control. The company's engineers are also working to meet the growing demand for anti-theft features such as keyless operating control.

To enable the design of higher performance, more sophisticated electric assist bicycles, the company is looking forward to next-generation microcomputers with enhancements such as faster speeds, better code efficiency and more peripheral-function integration. Renesas aims to respond to these needs by developing new microcomputers that will make major contributions to key features of the products Panasonic plans to introduce in the future.