Addressing the challenges of E-bike design: technologies and ...

Addressing the challenges of E-bike design: technologies and references for the next generation of electric bicycles

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Addressing the challenges of E-bike design: technologies and references for the next generation of electric bicycles

European sales of electric bicycles (e-bikes) are growing rapidly contributing to e-bike manufacturers worldwide experiencing sharp rises in demand. Simultaneously, the capability and the performance of e-bikes continues to improve, driven in part by the advances in underlying technologies and in part by the increasing sophistication and demands of e-bike users. This means that engineers tasked with developing next-generation electric bikes now need to address a variety of application challenges, ranging from effective motor control and sufficient battery life through to advanced navigation, Bluetooth connectivity and integrated displays. In this paper, we look at the evolving challenges facing e-bike designers and some technologies and reference designs that can help them address these challenges.

The evolution of the e-bike The concept of e-bikes was developed in the late 1800s and, at that time, several related patents were granted. In 1895, Ogden Bolton Jr. was granted a patent for a battery powered bicycle incorporating a ‘6-pole brush-and-commutator DC hub motor”. Two years later, Hosea W. Libbey patented a ‘double electric motor’ approach that strongly influenced the design of modern e-bikes almost a century later. Since the 1990s, e-bikes have been growing in popularity and are now the most popular electric vehicle in the world. In fact, several sources predict that almost half of all bikes made in future will incorporate an electric motor. China remains the main market globally for e-bikes, although sales there are expected to decline due to a combination of market saturation and changes in regulation. Western Europe remains a strong market, led by Germany where, according to the bicycle industry federation ZIV, over 605,000 e-bikes were sold in 2016 – a significant proportion of the around 3 million bikes sold outside China. Various market predictions indicate a growth rate of around 6-8% until 2025 by when annual unit sales of e-bikes will be almost 7 million, with a value of approximately US$8,500 million.

Consumer expectations Given the relatively high cost of e-bikes when compared to traditional bicycles, the expectations of technology-savvy consumers are high. The principal use for e-bikes is as a vehicle for commuting, so reliability and convenience are both top of the list of desirable features. As technology is introduced to e-bikes, consumers expect it to be able to work in all weather conditions. Range is clearly a concern but with the average commute distance reducing, and charging becoming more readily available, this is currently less of a worry than with electric vehicles. Taking some cues from modern cars, toady’s e-bikes are becoming very feature-rich, incorporating features such as automatic gear changing, walk assist (where the e-bike can be pushed with no effort) and increasingly sophisticated on-board computing that provides parametric information for the journey as well as GPS-based navigation. In the past, cycling to work usually meant arriving tired, hot and sweaty, often needing a shower before starting the day in the office. Modern e-bikes flatten out hills, provide extra effort to combat headwinds and allow cyclists to travel further and quicker, while arriving at work fresh and ready to start the day.

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The challenge for designers The two most fundamental elements of any e-bike (apart from the mechanical structure) are the motor that provides the motive force and the battery that stores and supplies the energy. One challenge for designers is controlling the speed and torque of the motor across a wide range of speeds and load conditions to provide a smooth and safe ride for the cyclist. Motor control is a complex subject, so selecting the best approach can be a challenge, especially for less experienced designers. With a finite amount of stored energy available in the battery, it is essential that this be used as efficiently as possible. While battery size could be increased to give extra capacity, this will increase the weight (and possibly drag), resulting in some of the added energy being used just to move the e-bike along. As such, making the best compromise between battery capacity, efficient operation and range is probably the most important single decision for the designer, due to its impacts on range and, ultimately, user convenience. These days, consumers are very technology oriented with devices such as smartphones and tablets being a significant part of almost everyone’s daily lives. Designers are being challenged to provide a similar level of intelligence and functionality within modern e-bikes, either through a dedicated on-board device, or by providing connectivity to a smartphone or tablet. At an absolute minimum designers will need to provide the charge state of the battery and a range estimate, although GPS satellite navigation is now considered almost essential. As desirable as these features may be, each one represents a drain on the battery. Thus, the challenge is to deliver all of this functionality using the absolute minimum amount of power.

Motor and motor control technology Modern Brushless DC (BLDC) motors offer several significant advantages over traditional brushed AC and DC motors, including improved reliability and lower costs - both of which are important for e-bikes. However, since BLDC motors have no commutator the use of complex control electronics is required to achieve good levels of torque control. Field Oriented Control (FOC), or vector control, is a math-based technique for controlling BLDC motors that overcomes the poor low-speed accuracy of trapezoidal control and the high-speed inefficiency of sinusoidal control. FOC can be used as a sensorless technique, so the space, weight and, most importantly, energy consumed by a rotary encoder is saved with this approach, all of which benefits e-bikes. Real time FOC requires rapid execution of math functions to transform the stator current signals into the voltage-control signals for the output bridge. Software-based FOC therefore requires a significant amount of available CPU capacity, especially at fast rotor speeds. In fact, it is often the processing performance of the microcontroller executing the motor control task that limits the maximum rotational speed. Dedicated hardware platforms for FOC based motor control have been developed to alleviate the demands on the main processor. Toshiba’s Vector Engine (VE) executes complex vector control equations in a dedicated and customisable hardware engine that also includes a Programmable Motor Drive (PMD) block to generate the necessary PWM waveforms and perform other functions including dead-time control.

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Figure 1 – Toshiba’s Vector Engine allows complex calculations to be performed reliably in hardware By reducing its reliance on software, the VE ensures stable and predictable code execution that is not subject to interrupts and can execute up to 70% faster than an equivalent software-based solution, allowing higher rotor speeds. CPU processing capacity is also released for high-level product features. Alternatively, a lower performance main CPU can be used which can contribute to saving space, cost and energy. Toshiba’s TXZ family is an Arm® Cortex® based family of microcontrollers that feature high energy efficiency and include peripheral functions that make them suitable for real-time control applications, including motor control. Devices in the TXZ family offer a wide range of peripherals, flash memory options, and high precision analog circuitry that can support a variety of applications.

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Figure 2 – TXZ devices include four key circuits that work together to provide sophisticated FOC-based motor control The TMPM4K group of devices are based on an Arm® Cortex®-M4 processor with FPU and operate at frequencies up to 80 MHz, with a 12-bit analog to digital converter (ADC) that has a conversion time of only 0.5µs using its own 120MHz clock. Housed in packages as small as 7mm x 7mm (LQFP48), the highly integrated devices include Toshiba’s Advanced Vector Engine Plus (A-VE+) that allows for sophisticated FOC-based motor control. The single channel A-VE+ is complemented by a dual channel Advanced Programmable Motor Control Circuit (A-PMD) that provides a 3-phase complementary PWM output for direct motor drive. Also built in to the microcontroller are multiple safety functions such as oscillation frequency detect (OFD), a watchdog timer and an emergency stop function. The devices feature up to 52 I/O ports for interfacing with external devices and include both SPI and I2C interfaces.

Power management components In any motor control application, the power switching devices (MOSFETs) have a very important role to play in defining the overall efficiency of the system, something that holds true also for e-bike applications. The UMOS IX low voltage (LV) MOSFETs from Toshiba were developed as the latest in a long series of devices that focused on improving the three main parameters that affect the losses in power switching applications; gate charge (Qg), on-resistance (RDS(ON)) and recovery charge (Qrr). The on-resistance is often referred to as conduction loss, simply because the RDS(ON) resistance results in energy lost as heat while the MOSFET is conducting current to the motor. The gate charge and recovery charge are discharged during every switching cycle and therefore have the greatest impact in the fastest switching devices, such as modern motor control circuits.

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Figure 3 – Successive MOSFET generations have shown continued improvement of fundamental parameters With each iteration of UMOS technology, trade-offs between these key parameters have improved. As the graph shows, the latest UMOS IX shows a significant improvement in RDS(ON) vs Qoss when compared to next best competitor. UMOS IX also achieves reduced recovery loss through tuning of the Qoss parameter, which directly improves Qrr and reduces switching losses. Improving operation to minimise waste energy and reduce heat generation has a significant positive impact on the efficiency of motor control and the overall efficiency of e-bikes, allowing them to travel further on a single charge, or to travel the same distance with a smaller battery – thereby reducing weight and cost. However, even with the very best MOSFETs available some heat will be generated during operation and managing that heat is a key challenge for e-bike designers. Since e-bikes have a significant amount of metal in their frame and the benefit of air flowing over them while they are being used, thermal management is in some ways easier than with static applications. Advances in semiconductor packaging that vastly improve the thermal coupling between the semiconductor junction and the external surfaces also improve the situation. One such technology is DSOP Advance, a small surface-mount package that has thermally conductive metal heat spreading pads on the top and bottom sides that allow for excellent thermal performance with highly effective thermal conduction.

Figure 4 – The DSOP Advance package provides excellent thermal dissipation

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Due to the inherent lower resistance of the package, the on-resistance (RDS(ON)) of DSOP Advance packages is 0.05mΩ lower than that of the equivalent SOP Advance package, bringing further efficiency gains. Toshiba’s TPW1R306PL is an N-channel MOSFET based upon UMOS-IX-H technology that is housed in a DSOP Advance package. This high performance device is ideal for use in motor control within an e-bike as it supports drain-source voltages (VDSS) of up to 60V. It is capable of delivering 100A of continuous current, or 500A when pulsed, and offers a channel to case thermal resistance of just 0.88°C/W at the lower end and 0.93°C/W at the upper. If higher battery voltages are applied, also a 100V solution in a DSOP package is available, the TPW3R70APL, as well as a single sided cooled package device TPH3R70APL.

On-board computing technology Once the fundamental efficiency is addressed, it is the value-add technology that consumers review next when making purchasing decisions for e-bikes. Probably the single most important function is the display of remaining battery charge and an estimate for range, although other basic functions such as distance travelled and average speed are also useful. However, today’s smartphone-centric consumers are looking for and expecting more from their bikes. Often, they expect functionality that is not dissimilar to that being found in modern automobiles. This includes GPS-based navigation, performance tracking, the ability to interrogate status (e.g. battery charge) remotely, geo-fencing to notify if the e-bike is moved without permission, maintenance notifications and even an automatic notification sent out in the event of a crash. The main challenge for designers in providing this functionality comes back to efficiency. For every sensor and processor that is added to the e-bike, some of the valuable battery charge is used, thereby reducing the range. One of the largest consumers of power is the display and designers are able to choose one of two primary approaches here; include a low-powered display or make use of the display of the rider’s smartphone, connecting via a technology such as Bluetooth. This area can be supported by Toshiba’s Bluetooth 5 IC, a solution supporting all major Bluetooth Low Energy 5 features. One of them is the standard’s new 2Mbit high-speed data rate, which allows for very fast transmission speeds between the user’s smartphone and the e-bike, usefully when a firmware upgrade is required. With the growth in bike sharing, it would be convenient to unlock the e-bike from an application on the smartphone instead of typing in a code. However, such convenience comes with the risk of eves dropping and man-in-the-middle attacks. To secure the connection against these attacks, this device provides dedicated hardware features such as a true random number generator (TRNG), a unique ID for every chip and an AES-128bit hardware encryption unit. Toshiba’s Bluetooth 5 IC provides +8dBm transmission output through its built-in high efficiency power amplifier for robust and reliable connectivity. Based upon an Arm Cortex-M0, it draws a peak current of just 11.0mA during data exchange, while the maximum deep sleep current is 50nA. The chip is highly optimised, making it ideal for integrating into small form-factor displays, and includes the required RF matching circuitry. The device comes with a qualified embedded Bluetooth stack so that developers can focus on adding their application software. For more complex user interfaces, the IC can be connected via a serial UART interface to a larger application processor to add Bluetooth functionality. If the application code can be stored elsewhere within the application’s system, and does not need to be located in the Bluetooth chip itself, a cost optimized derivative IC without integrated flash memory can be considered as an alternative. Additionally this device is qualified to AEC-Q100, supporting a temperature range of -40°C to +125°C, making it robust enough for the rigours of the outdoors. Using the display in a smartphone would require the development of a suitable app for all of the major operating systems and, as the system would be useless if the smartphone were to run out of charge, the e-bike must also provide a means of charging the smartphone. This could either be a simple wired connection, or a more modern wireless charging implementation.

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Design support tools While the huge range of technology available from Toshiba to support e-bike development may seem daunting at first, this is addressed through the wide range of development support tools available to help designers get to market faster and with minimal design risk. For Bluetooth, a comprehensive software development kit (SDK) is available, easing the development of wireless connectivity. And for motor control, the TXZ family offers a range of development boards to quickly get the rotor of the chosen motor moving.

Summary E-bikes are rapidly growing in popularity and in future many predict there will be more of them than normal bikes on our roads, especially in cities where journeys to work or for errands are shorter. Key to success in this market segment is delivering a highly efficient design that maximises the use of energy from the on-board battery to provide maximum range. Through intelligent motor control, use of high performance semiconductors, and the addition of low power computing peripherals, successful e-bike designs can be realized using the advanced products and support infrastructure available from Toshiba.

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