Next-generation eBikes use BLDC motors and advanced controls

Improved range, performance, and usability are helping modern eBikes enjoy growing popularity as commuter, commercial, and recreational vehicles.

Since their emergence in the early 1990s, eBikes have undergonea steady series of improvements in range, reliability, and performance that have won them a growing number of loyal users worldwide. Yet despite nearly 30 years of technical evolution, there’s still room for innovation.

Some innovations in next-generation eBikes relate to better frame designs and mechanical systems. Others include advances in electronics for the motor drive and energy-storage systems. But the most dramatic changes are in eBike controls and user interfaces for maximally enjoyable user experience (UX).

This speed and torque-sensing crank shaft, manufactured by THUN-X, provides two digital outputs for the digital sin/cos speed signal and an analog output for the torque signal. With this arrangement, the eBike’s motor controller requires only two digital inputs and one analog input. Image courtesy Enviado

Most modern eBikes use permanent magnet motor (PMM) propulsion, with brushless dc motors being especially common. BLDC motors are one of the simplest forms of synchronous motor: These have a permanent-magnet rotor assembly surrounded by a wound stator having several (usually three) winding sets. BLDCs are lighter and more efficient that equivalent brushed motors — plus need little maintenance. The catch is they need a motor controller to provide a drive waveform (commutation). The controller does this by monitoring the rotor position and supplying power to the stator windings in the correct sequence to start and maintain rotor motion.

BLDC motor drive technology for eBikes is relatively mature but there are still a few places where designers can differentiate products through lower cost or better performance and efficiency. For example, a growing number of eBike designers are using motor controllers capable of supporting vector control-based commutation — also known a field-oriented control (FOC). The FOC algorithm can decouple control of torque and flux by transforming the stator current values (phase currents) from a stationary reference frame to a rotating reference frame.

This module, based on Trinamic’s TMC4671 motor controller IC, works in BLDC-based eBike drive systems. Its integrated MCU can be programmed to deliver custom speeds and responses to pedaling. Image courtesy Trinamic Motion Control

An FOC motor controller requires a processor powerful enough to support this compute-intensive algorithm as well as some means of accurately sensing rotor position — typically an analog or digital Hall-effect sensor or optical interrupter. In return, FOC helps the motor deliver excellent performance, a wide speed range, high operating efficiency, and high torque and low current at startup — characteristics that in turn boost eBikes’ drivability and range.

Because the sensors that determine rotor position can add cost and complexity to a design, eBike designs sometimes employ controllers that support sensorless operation. In this mode, current readings and the back EMF generated by the motor’s windings can be used to estimate position and speed of the motor. Unfortunately, while this is a relatively easy task above a certain velocity, it is much more difficult at standstill and low velocities which occur every time the rider stops or starts. Although this is acceptable for some appliances and other applications, so-called sensorless FOC controllers for eBikes remain in the development stage.

Regenerative braking is another promising feature for some types of eBikes. Here, field windings of the eBike’s drive motor serve as a generator/brake to recover some of the bicycle’s kinetic energy as electrical power to charge the battery. The algorithms needed to support regenerative braking are relatively straightforward, but this function is only practical to implement in direct-drive systems where the motor is directly coupled to the wheel. Unfortunately, many eBike motors include a planetary gear drive (often coupled with a freewheel function) making regenerative operation impossible.

NATURAL CYCLING FEEL

“Creating natural-feeling response to rider pedaling is a multi-faceted design challenge,” says Trinamic team leader software Development Enrico Dressler. He says the controllers of nearly all early eBikes and pedelecs used a pedal speed sensor (typically Hall effect sensors and a disc with magnets) as their primary input. As the rider pedaled faster, a controller algorithm increased support provided as inceased power to the motor.

This simple control scheme was reliable and easy to implement but had several unpleasant drawbacks. The controller only functioned when the pedals turned, so could not assist the user from a dead stop — precisely when it’s needed most. In addition, controllers that provided assist based solely on pedal speed gave riders an odd sensation. Processing delays and the nature of the algorithms themselves caused small but noticeable delay between the rider’s inputs and the bike’s response — so it felt as if the pedals were connected to the rear wheel via a large rubber band.

Pedal speed sensing also raised potential marketing issues in some countries with strict definitions on how mopeds, motorcycles, and other twowheeled vehicles are taxed and licensed. Because a pedal-assisted eBike could, theoretically, provide assist even if there was no chain between the pedals and the rear wheel, it can be technically defined as an electric moped according to some countries’ laws ... thereby needing it to be licensed, regulated, and taxed accordingly.

LeichtLast is an experimental e-cargo eBike, developed by Trinamic as a testbed for a new generation of e-cargo bikes. Thanks to its high-efficiency drive control system, regenerative braking, and other new capabilities these vehicles are expected to become a highly practical alternative to fossil-fueled delivery vehicles in cities and some suburban settings. Image courtesy TRINAMIC Motion Control

So eBike motor controllers have evolved to make power-assisted riding more natural and enjoyable. In an suitable world using a pedal-assisted eBike should feel just like cycling on superpowers. Of course, the motor controller must be able to sense the torque being applied at the pedals and use it as the primary means of determining how much assist to deliver ...

Read the rest of this feature — including more on eBike user interfaces, electric cargo bikes, and hybrid wheelchairs — by visting motioncontroltips.com and serching Trinamic eBike.