Principle:
For centuries, mechanical odometers have been carefully calculating mileage. Although they are about to be eliminated, I believe you will still be surprised by their magical structure. Mechanical odometer is essentially a gear transmission chain with accurate transmission ratio.
This kind of odometer is gradually being replaced by electronic odometer, which can provide more functions and lower price, but it lacks the magical charm of mechanical odometer. In this article, we will take you into the mechanical odometer and talk about the working principle of the electronic odometer.
The mechanical odometer is driven by a flexible cable made of a taut spring. The cable is covered with a metal protective tube, and the tube is covered with a rubber sleeve. On a bicycle, the small wheel rotating relative to the bicycle wheel will drive the cable to rotate, and the transmission ratio of the odometer must be calibrated according to the size of this small wheel. In an automobile, the gear is responsible for engaging the output shaft of the transmission and then rotating the cable.
The flexible cable is wound on the instrument panel and connected to the input shaft of the odometer.
gearing
The odometer adopts a set of (three) worm wheels to realize the transmission ratio of odometer 1690: 1. The input shaft drives the first worm, and the worm drives another gear. Each revolution of the worm will only make the gear turn one tooth. This gear continues to drive another worm, which drives the next gear, which in turn drives the last worm, which in turn drives the last gear. The last gear is connected with an indicator with an accuracy of110km.
Each indicator has a row of needles on one side and two needles on the other side. When two pins rotate around the white plastic gear, one of the gear teeth will fall between the two pins and rotate with the indicator until the pin passes. The gear also engages with the pin on the next larger indicator and rotates it by110 turn.
Now you can understand why when the odometer "flips" many digits (assuming it flips from 19999 to 20,000 kilometers), the leftmost 2 of the reading may not be aligned with other numbers. The slight swing of the white auxiliary gear will hinder the accurate alignment of all bits. Usually, they can only be aligned again when the reading reaches 2 1000 km.
You will also find that mechanical odometers like this are reversible. When you reverse the car, the odometer will count backwards-it's just a gear train. In the movie "The Rest Day of Ferris Buller", there is a scene in which they lift the car and turn the wheel upside down. Another trick is to connect the flexible cable of the odometer to a drill bit and then turn it in the opposite direction to adjust the mileage.
Although this is indeed feasible on the old mechanical odometer, it is not feasible on the new electronic odometer. In the next section, we will introduce the electronic odometer.
If you go to a bicycle shop, you may not find any cable-driven odometers or speedometers. You will find a bicycle watch. These bicycles with code watches have a magnet on one wheel and a signal pickup on the frame. Whenever the wheel rotates, the magnet will pass through the signal sensor and generate voltage in it. The code meter will count these peak voltages (pulses) and use them to calculate the driving distance.
If you have ever installed such a bicycle watch, you should know that it must be programmed with the circumference of the wheel. The circumference is the distance traveled by a wheel in a complete revolution. Every time the meter senses a pulse, it will add a perimeter to the total mileage and update the displayed number.
Many cars now use similar systems. But instead of using a magnetic signal pickup on the wheel, they use a toothed wheel and a magnetic sensor installed on the output shaft of the transmission, which will count the pulses when each tooth passes by. Some cars use grooved wheels and optical signal pickups, just like mice. Like a bicycle watch, the on-board computer knows the distance traveled by the car corresponding to each pulse, and uses these data to update the odometer reading.
The most interesting thing about automobile odometer is: How is the information transmitted to the dashboard? Information is transmitted from the engine control unit (ECU) to the dashboard through a single-wire communication bus instead of using a spiral cable. A car is like a local area network, which is connected with many devices. The following are some devices that may be connected to the network:
Engine control unit
Air conditioning control system
instrument board
Power window control module
radio
Anti-lock braking system
Airbag control module
Body control module (controls interior lighting, etc.). )
Transmission control module
Many vehicles use a standardized communication protocol called SAE J 1850, which enables electronic modules to communicate with each other.
The engine control unit calculates all the pulses and records the total mileage of the car. This means that if someone tries to "call back" the odometer, it will be inconsistent with the value stored in the engine control unit. This value can be read by using a diagnostic computer, which is available in all automobile dealer service departments.
The engine control unit sends data packets containing headers and corresponding data several times per second. The header is a number that identifies the packet as a distance reading, and the data is a number corresponding to the transmission distance. Another computer on the dashboard knows how to find this particular packet. As soon as it is found, it will update the odometer with the new value. For cars with electronic odometers, the dashboard will display the updated values in time. Cars equipped with analog odometers use a small stepping motor to turn the dial on the odometer.