Controller Area Network Sensors: Applications in Automobiles

1: Introduction

In this paper an overview on the Controller area network sensors and their real world application in the automobiles is presented to the reader. The fact that controller area networks employ various sensors and actuators to monitor the overall performance of a car (K.H. Johansson et al, 2001^[1]), this paper focuses only on the sensors and their role in supporting the CAN performance.

2: Laser Speed Velocimetry (LSV) sensor

The application of this sensor is present in the Reanult range of vehicles where the company is incorporating the LSV as an on-board sensor to measure ground speed with better than 0.1km/h accuracy (LM Info, 2006^[2]). The LSV technology is an approach to measure ground speed of a moving automobile with greater accuracy thus ensuring better on the road performance as argued by K.H. Johansson et al (2001). The purpose of the technology is to measure real-time speed of an automobile at accuracy level of 0.1km/h.The technology behind the LSV system comprises of the sensor that continuously records the interference with the motion surface that is fed back to the controller in the system that measures the speed of the car. The diagram in fig 1 below explains the aforementioned effectively.

The above is the schematic representation of the mounting of the LSV 065 sensor head (Source: www.polytech.com). The above diagram further clarifies that the use of the LSV system not only provides an effective and accurate measurement of the speed but also proves that the use of this system can provide an effective control over the performance and over the velocity of an automobile.

The LSV systems from Polytech, the schematic for which was presented in this section “combine a sensor head, a controller and software into a rugged industrial package that makes precision measurements from standstill to speeds of more than 7,200 m/min in either direction” (LM INFOR Special Issue, 2006).

2: Braking System sensors and Speed sensors

The ABS system utilizes multiple sensors to prevent the wheels from locking whilst braking at high speeds. The main sensors used in this set-up are

2.1: Speed sensor

The speed sensor is the sensor that is fitted to each wheel of the automobile. The purpose of the sensor is to identify the wheel slip whilst braking which is then fed back to the ABS controller unit to control. The speed sensor records the speed of the rotation of the wheel and when one or more of the wheels are recorded to be rotating at a considerably lower speed then the ABS control unit reduces the pressure on the pressure valves thus ensuring that the braking does not lock the wheel. The speed sensor equipment comprises of various models and can be mounted on different positions of an automobile in facilitating the measurement of the speed. The application of the speed sensor in the ABS is one of the many applications of speed measurement.

The ECM method of measuring the speed using speed sensors is increasingly popular as part of the ABS technology. It is also argued as the later version of ABS that overcomes the fundamental sensor positioning related flaws in the ABS system.

The ECM uses the Pulse Code Modulation technique to communicate with the sensor and the control system of the Controller Area Network of the automobile.

From the figure above it is clear that the ECM plays a critical role as the controller to capture the sensor signals and transmit to the master controller area network Electronic Control Unit (ECU) for overall control of the automobile. It is also evident that the sensor plays a vital role in the speed measurement and efficient operation of the ABS braking system.

The fundamental difference between the VSS and the WSS is that the VSS is part of the controller area network and is connected directly to the ECU of the network whilst the WSS feeds into the ABS controller unit that is connected to the CAN of the car or automobile under consideration.

The VSS is also a successful and flexible method for motorbikes and other two-wheeled vehicles as the mounting is simpler compared to the WSS mounting for ABS system that is popular in a car.

The cases of VSS mounted in transaxle and the transmission serve effectively for the purpose of velocity measurement and also provides near accurate readings for the efficient speed control by the driver of the car or the rider of the bike.

In the case of the VSS mounted in the transmission, the sensor sends a 4 pulse signal at regular intervals to the combination meter that then sends the signal to the ECU of the CAN in the car. The signal so sent is recorded as the speed and shown to the driver as the velocity of the car. This approach is more accurate to the traditional analogue approach to the speed measurement and management.

The above schematic makes it clear that although the mounting of the sensor on the transaxle provides an efficient method of measuring velocity, the response of the sensor can be damaged due to the mechanical wear and tear that is directly associated with the transaxle in a car.

The VSS mounted in the transmission is perceived to have resolved the issue through the mounting of the sensor near the core rotor and using a magnetic field to hold the sensor in position. This approach agreed to the more effective compared to the former where the mechanical wear and tear was a critical drawback to the overall performance of the system. The schematic mounting of the VSS in the transmission is presented in the fig 4 below.

The above mounting schematic in the figure further justifies that the positioning of the sensor by the rotor will help measure the speed effectively and more accurately

3: Differential Hall effect Sensors

Daniel Dwyer (2007)^[3] argues that the differential Hall effect sensors are not only capable of accurately measuring speed but also providing the safety measures through effectively controlling the speed. The hall effect sensors utilize the fundamental principle behind the Hall Effect which is described as follows

“When a bias voltage is applied to the silicon plate via two current contacts, an electric field is created and a current is forced.” … Daniel Dwyer (2007).

This principle is utilized in the gear tooth profiling and speed measurement through the gear tooth sensing both in the linear and the differential cases. The differential case is argued as a more successful element especially in case of the automatic transmission automobiles because of the need to effectively control the speed associated with the car.

Another interesting element with the differential Hall-effect sensors is the fact that the sensor positioning is robust in nature and its wear and tear is minimal.

The differential element sensing that is the key for the differential Hall effect sensors utilizes the fundamental Hall effect. Alongside the sensor also “eliminates the undesired effects of the back-biased field through the process of subtraction.” (Daniel Dwyer (2007). The differential baseline field for the sensor is made close to zero gauss since each of the two Hall elements in on the IC (the sensor) approximately see the same back-biased field as argued by Daniel Dwyer (2007). A schematic representation of the differential element sensing is presented in fig 5 below.

The major feature of the Differential Hall effect sensor is the production in the form of an integrated circuit that can respond to the magnetic field interference and differential effects due to the change in speed and the gear tooth positioning in the magnetic field.

The differential element sensing and the speed measurement is accomplished through the overall peak holding of the Integrated Circuit (IC) in the field. Although the traditional peak-detecting scheme could resolve the issue of peak holding, the sensor requires an external capacitor for peak holding in order to effectively control the overall automobile speed.

Since a large gain is required to generate a signal strong enough to overcome the air gap in the case of the hall effect sensor especially in the drawbacks associated with the timing accuracy and duty cycle performance in the slope of the magnetic signal strength as argued by Daniel Dwyer (2007).

From the above arguments it is clear that the Hall effect sensor is a successful but expensive sensor to perform measurements and be programmed as part of the overall CAN of the automobile.

Thus to conclude the research in this paper the four sensors that were discussed include The Laser Speed Velocimetry (LSV) sensor and an insight on the LSV 065 module as an example. This sensor proves to be successful and accurate speed measurement equipment but the mounting and safety related elements pose a big drawback for its commercial application. The Wheel speed sensor for the ABS in an automobile was then discussed followed by the analysis of the Velocity Speed Sensor. Finally the Differential Hall Effect sensor was discussed in the research paper. This sensor on the other hand can be mounted easily in an automobile and can perform effectively to provide accurate measurements but has higher cost liability and maintenance requirements making it a secondary choice to the traditional VSS And WSS sensors used in most of the cars.