How an innovative angular sensor in E-Bikes sets new standards
The next-generation pedelecs or E-Bikes contain significantly more sensors than just a few years ago. Sensor fusion or Smart Systems are drivers in these areas. The idea of collecting as many sensor information as possible from different sensors and processing them comes from the desire to learn as much as possible about this system in order to achieve by this information
an improvement of the system in terms of efficiency, handling and long-term behavior.
How is the torque measured in the E-Bike?
In most bicycles, the bottom bracket is designed so that both the left and right pedals are connected to the bottom bracket shaft by a crank. In most cases, the force is transfered from the right crank to the gear which then drives the rear wheel. This means the flow of forces above the bottom bracket is defined only by the opposite pedal. For the torque and thus the power measurement in the pedelec, it is necessary to measure the force of both pedal steps in order to obtain good control . This can be achieved by transferring the force of the bottom bracket shaft to a common shaft sleeve, and from there to the rear wheel. This shaft sleeve or measuring sleeve represents the heart of the force or torque measurement in bicycles. This applies to all bicycles whether they are pedelecs or bicycles without driving support in which the power measurement plays a role. The torque sensors available today are based on the magnetostrictive measuring principle and can detect the torques of both pedals on this rotating measuring sleeve. The magnetostriction is based on the fact that the magnetic properties change due to the application of a force on a ferromagnetic measuring body. The change in the magnetic properties may be the change of a DC magnetic field in the case of passive sensors, or a change in the impressed magnetic flux density or a magnetic resistance in the case of active sensors.
Why is an innovative angle sensor/encoder needed?
In addition to brake force control, sensors are also used in the area of load management. For example, the force sensors in the boarding area can count how many people have boarded or disembarked the train. Force sensors can also determine how heavy the load is on the wagon axle and how it changes in the course of the load. This is particularly interesting in freight transport, where the cost of transport is calculated from the weight of the load and volume. In this way the railway operator can check the data of his customers. Similar circumstances can be found with the e-mobility truck of the future, as the load has a significant influence on safety and costs there as well.
Early detection of malfunctions
In addition to the torque signal, however, it is necessary to measure the cadence. Only with the information on the cadence, the actual power can be calculated which is then used for the regulation. There are several methods to capture the cadence. The most common type is the use of a Hall sensor or GMR sensor hovering over a magnet wheel on the measuring shaft. The magnet wheel has differently magnetized areas. When the measuring shaft rotates, the Hall sensor recognizes these different pole regions and can detect their change. The resolution of these sensors is defined by the number of magnetic poles. In order to detect the direction of rotation additionaly to the speed, at least 2 Hall or GMR sensors are needed which are arranged in a way so that the sensors not only recognize that the magnetic pole changes, but also in which direction it wanders or changes. With a certain intelligence, in this magnetic pole signature, even an incremental angle sensor can be realized. By using e.g. a defined longer pole or shorter pole, which can be identified reliably , a zero point can be determined. The use of a Hall or GMR sensor can also be realized without a magnet wheel, by applying a mechanical signature of a ferromagnetic body to the measuring sleeve. In the simplest case a toothed ring pinion and the sensor is biased by a magnet. In this arrangement, the magnetic sensor does not detect a pole change, but an altered magnetic flux. If there is a depression below the sensor, the magnetic flux is smaller than an increase under the sensor.
In addition to the torque in the E-Bike, this can be determined as a reliable rotational speed. With this information, most pedelec systems can already work and achieve good performance. In addition to these data, however, an absolute angle measurement can still be carried out by an intelligent system. Absolute angles are suspected to be very expensive and costly to implement, so they are often not provided. However, there are also very simple and robust methods to realize an absolute angle measurement. With a licensed method, Magnetic Sense has created a very simple way to resolve an absolute angle <1 ° and to integrate this into the existing torque sensor concept. The operating principle is based on the fact that 4 inductors are arranged at 90 ° to each other around the measuring shaft, a sine disk, which is integrated on the measuring shaft, changes the inductances due to its eddy current characteristics. The variation of the inductances associated with this sinewave can be used to calculate an angle. In addition, the cadence can also be calculated via the temporal angle change. By integrating an angle sensor with a torque sensor in the E-Bike, the sensor fusion is complete. Thus, with the combination of these two sensors, the torque, the speed, the power and the (pedal) crank position can be determined on the shaft. With this sensor information, completely new regulations can be realized and thus contribute to a new dimension for the driving experience in pedelecs.
