E-Bikes: How ZF can improve the reactivity of engines with its Hall Speed Sensor technology

With the increasing popularity and growing trend of e-bikes, bicycle system suppliers have set goals to optimise the power and performance of their drive systems. In order to support the driver in the best possible way, sensors are used to deliver the required data and thus adapt the engine’s power support to the total speed.

The bicycle market is booming – whether with an electric drive or without. This is shown by a current study by the online portal Statista: 44% of the people surveyed in Germany get on the saddle at least a few times a week. The trend is clearly towards e-bikes with their highly technical drive systems. In particular, the central engine has established itself as an all-rounder among the different engine systems due to its well-balanced driving characteristics. The responsiveness of the engine needs to be optimised continuously to the environment and impact of driver`s riding behaviour. This can be realised with torque, inclination and speed sensors. The latter supplies the engine control unit with real-time data in order to support the driver depending on a specific riding situation. The speed sensor is needed to monitor the speed and to shut down the engine at a maximum speed of 25km/h for Pedelecs and 40km/h for S-Pedelecs. In addition, the speed and torque sensor has the task to adjust the power output of the engine according to the riding speed and pedal force.

Speed sensors for central engines
So far, two sensor technologies for speed measurement have become established in the e-bike industry. These are Reed and Hall technologies. Both sensor types are mounted on the bicycle frame of the rear wheel in order to measure the rotational speed.

Reed switches can be found in various industries and applications. The advantage is obvious: they are inexpensive and easy to operate. In addition, they don’t need any additional energy source and the hermetic sealing enables the switch to become resistance against dust particles, oil and moisture. The reed switch in combination with an external magnet is the most common e-bike speed measurement system at the moment. The system shows high simplicity. A permanent magnet is mounted on a spoke on the rear wheel of the e-bike. The magnet passes the opposite reed switch per wheel revolution. The switch is attached on the bicycle frame and transmits an electrical signal via cable as soon as the magnet closes its circuit. The control unit in the engine recalculates the speed per wheel revolution.

In contrast to the reed switch, the Hall sensor requires a continuous current from an external power source, due to its physical principle. Therefore, Hall sensors can be categorised as active sensors since they need integrated electronics for signal processing. The advantages of Hall sensors are their compactness and reliability. They are immune to shocks up to 35g as well as to dirt and moisture. There are no moving parts inside the sensor so wear and friction can be avoided. This leads to an almost unlimited lifetime.

In contrast to the reed switch, the ZF Hall sensor is aimed directly at a speed disk instead of targeting an external magnet. This disc can be fixed either next to the brake disc, or the brake disc is modified and targeted directly. The angle of rotation and the air gap between sensor and disc needs to be coordinated. The disc needs to be made of ferromagnetic material with periodical arranged slots. In this way, a change of the magnetic field can be registered by the sensor which finally transmits electrical signals via cables to the control unit. The provision and the design of the speed disc are defined by the respective engine manufacturer itself based on specific layout requirements and level of resolution. By selecting an additional speed disc, this needs to be attached to the chain bearing next to the brake disc and only a few millimetres opposite to the Hall sensor. If the brake disc is targeted directly, it must also have periodical slots for triggering the signal.

The complete system of Hall sensor and disc offers significant advantages in terms of functional safety, improved manipulation resistance and responsiveness of the e-bike engine compared to the conventional system reed switch with an external magnet.

Functional safety
The improved functional safety can be attributed on the one hand to the characteristics of the Hall sensor and, on the other hand, to the overall system. The sensor operates completely wear-free and is immune to dirt and vibration. As a result, the sensor application is ideal for off-road riding and uneven ground. In addition, both the speed sensor and the speed disc are solidly integrated into the bicycle frame and wheel axle so that the speed can be measured reliably. External spoke magnets in conventional systems, however, are more susceptible to drift away from their original position which negatively affects the speed measurement.

Responsiveness of the engine
The main advantage and unique selling proposition of this speed system is the high resolution of speed measurement. In contrast to a reed switch which only sends out one signal per revolution, the Hall sensor with speed disc generates up to 30 or more. The number depends on the design of the disc and the requirements for the desired resolution. The sensor detects the slots in the disc and sends a signal per slot. Thus, the engine reacts much faster to speed changes and supports the driver more dynamically. Especially for hill starts or at low speeds, the sensor can detect slight accelerations and make the engine respond quicker. This results in improved inter-coordinated assistance.

Increased manipulation resistance
Looking at the accident statistics and comparing the years 2014 and 2017, one finding is that the number of injured and killed e-bike riders in Germany more than doubled from 2,213 to 5,115. According to estimations, every third e-bike in Germany is tuned up to increase the speed limit. Consequently, inexperienced drivers are facing longer braking distances and are overburdened in uphill and downhill as well as turning actions. The number of accidents continues to increase. The members of the German consortium “Fahrradwirtschaft” (AGF) deliberately take a stand against any kind of manipulation of e-bike drive systems in the international bicycle industry. The aim is to continuously work on drive systems to prevent manipulation.

To increase the speed limit, there are different approaches. One possibility is to filter every second sensor signal by attaching a tuning chip to the chainstay. This allows a doubling of the programmed motor assistance. Another alternative is tapping the sensor signal at another position. The most common one is the screw connection between pedal and pedal crank. In the conventional system, the external magnet on the spoke is removed and replaced by a magnetic screw on the pedal. The reed sensor is realigned and reacts to the magnetic field of the screw. The number of revolutions of the pedal is less than the ones of the wheel. As a result, fewer signals are transmitted to the control unit which in turn calculates a lower speed than the real value. The engine continues to support, although the maximum allowed speed has already been exceeded. This type of manipulation can be prevented with the Hall sensor in conjunction with speed disc.

Trend ABS
In addition to the numerous innovations in the areas of connectivity and electric drive engines in e-bikes, the ABS system also has an increasing significance in the market. The Hall sensor with an additional speed disk or modified brake disk is a preferred solution for this application. The high resolution makes it possible to accurately measure speed changes on both the front and rear wheel during brake application. The ABS control unit can decelerate the wheels due to the created speed values in a coordinated and controlled manner. In this way, a lift-off or rollover of the driver can be avoided during emergency braking. Due to increasing traffic in cities, emergency braking becomes more and more frequent. As a result, the need and demand for ABS systems increase, especially for city e-bikes, to ensure safe cycling in cities.

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