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What is the technical principle of moped?

  • 12 Mar 2022

Moped are called Pedelec in Europe and the United States, and they belong to a type of e-bike. What are the characteristics that distinguish it from conventional mountain bikes that have not yet entered the mainstream view in China?



This is a kind of personal transportation that not only has the lightness and convenience of bicycles, but also can effectively compensate for the burden of bicycles when going uphill, against wind, and carrying loads. It is based on a traditional bicycle, equipped with a power system with a torque sensor as the core, equipped with a motor and a battery. The biggest difference from electric bicycle is that it does not adjust the power by turning the handlebars, but uses a torque sensor to sense the strength of the rider's pedaling, judge according to the size of the manpower, and then understand the rider's riding intention ,and provide corresponding power support. It cannot be driven purely by electric power like an electric bicycle, but requires a hybrid drive of "human power + electricity", so the way of riding this kind of car is no different from riding a bicycle. The proper power provided by electricity perfectly solves the problem of laborious cycling. At the same time, because of the composition of manpower, it can exceed the cruising range of ordinary electric bicycles under the condition of carrying a small battery, which is generally around 50-100 kilometers. Long battery life.


The most cutting-edge technology in the field of Pedelecs is the "torque sensor", which is at the heart of the bicycle's electric power assist system to understand the rider's intentions. High-end moped all use the most technologically advanced "bilateral torque sensor". This sensor technology has long been monopolized by several multinational companies such as Germany's BOSCH and Japan's YAMAHA. The price of vehicles equipped with this sensor is generally more than 2,000 euros.


In the low-end moped products, the "unilateral torque sensor" is widely used (only the strength of a single pedal can be sensed, and the rider's strength needs cannot be truly understood).There is a big gap between the riding experience and high-end models. In the field of low-end moped, there is also a product using "rear axle hook sensor".


Today's article will introduce the "torque sensor" and "rear axle hook sensor", analyze their working principle and advantages and disadvantages.


First of all, we need to understand the power transmission method of the bicycle: people step on the pedals, and then drive the chain to transmit the power to the rear wheel, and then drive the bicycle forward. If it is decomposed into structural names, it is: pedal - crank - chainring - chain - flywheel - rear hub - frame.


Let's talk about "rear axle dropout sensor"



The gray component in the above picture is the "rear axle hook sensor", which is installed at the junction of the seat fork and the chain stay, which is the installation position of the rear axle (the red dot in the picture is the rear axle). It is equivalent to an additional additional adapter, generally made of aluminum alloy. The forward power of the bicycle is transmitted to the "rear axle dropout sensor" through the rear axle, and then to the frame of the bicycle. The "rear axle hook sensor" will deform when it receives the force from the rear axle, and then squeeze the "pressure sensor" in the red circle in the above picture, and then mobilize the battery motor to provide power for the moped. The advantages of the "rear axle hook sensor" are that the principle is simple, the structure is not complicated, and the production price is low. If the electric moped is always used on straight roads, it is also good, but the shortcomings of this sensor are exposed in real road conditions. . (As shown in the red circle in the above picture, in fact, the "rear axle hook sensor" is just a "pressure sensor" built into an aluminum alloy processing kit.)


Disadvantage 1: To measure the pedaling torque, the "rear axle hook sensor" needs to go through a series of transmission links of pedals, cranks, chainrings, chains, flywheels, rear hubs, and frame. The components of each link are Elastomers lose power during transmission. Furthermore, due to the "elastic lag" effect of metal, the measurement force of the "rear axle hook sensor" not only has large errors, but also has a delay, which makes it impossible to understand the power needs of the rider in real time, and the power supply during riding is inaccurate and unreliable. It will always be a bit slower, so the experience will be poor. It's like you've stepped on the accelerator when you're driving, but the power will always take a while to give you.



Disadvantage 2: The "rear axle hook sensor" is installed at the junction of the seat fork and the chain fork. If you want to accurately measure the force, the processing of the installation position must be very precise, otherwise the sensor will be deformed during installation. , resulting in inaccurate sensor measurement data. Therefore, the use of this kind of sensor has higher technological requirements for the production of the frame, which additionally increases the production cost of the electric moped.


Disadvantage 3: The joint of the seat fork and the chain fork of the bicycle is the part where the body is subjected to a lot of stress. The bumps and impacts of various road surfaces will be transmitted through this part, and the "rear axle hook sensor" is installed in this force transmission. Point. Most of these sensors are made of aluminum alloys with poor hardness and strength, so they are easily damaged when used in real road conditions, and the measurement accuracy and service life will drop sharply with the use of time. For example, the next curb or being blown down by the wind may cause the deformation of the sensor, resulting in a speeding or no power situation.

Therefore, although the "rear axle hook sensor" has the advantages of simple principle and low cost, the riding experience is poor due to inaccurate measurement of torque and delay. It has high requirements on the production process of the vehicle, and transfers the cost saved by the sensor to the processing of the frame. The aluminum alloy material is easy to deform, has a short service life, and has high maintenance costs for users. Therefore, the current slightly higher-end electric mopeds have abandoned this sensor. In the early years, Giant tried to use the "rear axle hook sensor" to produce a cheap version of the electric moped, but it finally gave up because of its poor experience, many failures and short lifespan.


Let's talk about the cutting-edge "torque sensor"


With the development of technology, especially in Japan, Europe and the United States, the promotion of bicycles as a form of personal transportation, "torque sensors" were finally applied to bicycles. As mentioned at the beginning, the biggest difference between "torque sensor" and "rear axle hook sensor" is in the understanding of "force", while "bilateral torque sensor" can measure the force of two pedals. It is mainly about this kind of sensor with more technological content.

Compared with the backward "rear axle hook sensor", the advantages of the bilateral torque sensor are extremely obvious. It is mainly installed between the crank and the chainring, that is, the outer side of the bottom bracket, and some are directly installed on the center axle. There is no absolute rigid body in the world, so the central axis will produce extremely small torsional deformation when it is stressed. By measuring the subtle deformation signal on the surface of the central axis, the magnitude of the current stepping moment can be obtained, and its accuracy is very high. Because torque is measured by torque, torque sensors are also referred to as "torque sensors".


Advantage 1: Because it is installed on the outside of the bottom bracket or directly on the bottom bracket, there are few intermediate links for the torque sensor to measure the pedaling force. Moreover, most of the pedals, cranks, and central axles are steel and metal components with extremely rigidity, and the elastic hysteresis effect is small, and the generated torque signal can change very quickly with the change of the pedaling force. The most direct result of this is that the dynamic response of moped using bilateral torque sensors is very fast. Most of the response times are milliseconds, with almost zero delay, and the power can be stepped on.

Advantage 2: If the torque sensor is installed on the outside of the bottom bracket, the standard modular design can be used, and basically all bicycles can be seamlessly installed, and there is no need to develop a special frame for it, let alone each bike. The racks are processed with high precision for the above-mentioned "rear axle hook sensor", which reduces the cost from both research and development and production.


Advantage 3: Because it is installed on the outside of the bottom bracket or on the central axle, the impact force brought by the bumps on the road is mostly buffered by the toughness and elasticity of the frame, so the impact force on the torque sensor will be It's small and the workload isn't too big. In addition, it is generally manufactured using the center axle process of bicycles. High-quality steel metal materials are excellent in hardness, strength and toughness, and have good wear resistance, so the life of the torque sensor is relatively long.




From the perspective of riding experience, the torque sensor can truly understand the user's riding intention because it can more accurately measure the force the user gives to the pedal, and is not affected by road conditions, so that the output of the battery motor can be adjusted just right. The power helps the cyclist and makes the cyclist feel a more comfortable riding experience;


From the perspective of R&D and production, the torque sensor has strong universality, no need to develop a dedicated frame, and no need to perform high-precision processing on the installation position. Although the price of a single sensor is expensive, the cost is relatively reduced when it is allocated to the production of the whole vehicle;


From the perspective of daily use, the torque sensor has a long service life, is not prone to failure, and requires no daily maintenance, which greatly reduces the cyclist's threshold for use and purchase barriers;


And one last point is the use of security. The torque sensor has stable performance, but the rear axle hook sensor is easily deformed during use, resulting in decreased accuracy and failure. During the riding process, there will be situations such as flying and no power assistance, causing riding hazards.


Electric mopeds are a brand new category, and the public needs to do their homework before buying them, because any emerging market will inevitably be confused during the growth period. After reading this article, you should have gained some understanding of the key part of "torque sensor", which can be used as a reference for understanding this model.


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