Editorial Feature

Vehicle Speed Sensors: Types of VSS

Introduction
Definition of a vehicle speed sensor (VSS)
Basic functionality of the VSS
Types of VSS
Hall Effect Speed Sensor vs. Reed Switch Speed Sensor
Applications
References

Introduction

One of the most recent developments in vehicle speed sensors was recently introduced by Vissee, a company dedicated to the design and engineering of visual speed and depth sensors. Vissee have designed a neuromorphic 'third-eye' sensor that allows for consumer devices to directly measure speed. This application-specific sensor is designed to measure speed without data on position or acceleration of a vehicle. The speed sensor is built with a 60 hertz camera and a microprocessor programmed to run a special algorithm that helps selective processing of incoming data. The lens in the camera provides a visual field of 180 degrees, imaging more than the standard 40 to 50 degrees field of view normally seen in a standard lens. The microprocessing chip measures the variation in a signal over time and light intensity to calculate the absolute speed.

The new 'third-eye' sensor is expected to be installed into traffic and micro-aerial vehicles by the end of 2012. However, to better understand such novel technology for speed sensors, let us have a look at how a basic vehicle speed sensor works.

Definition of a Vehicle Speed Sensor (VSS)

A vehicle speed sensor generates a magnetic pulse in the form of a wave proportional to the speed of the vehicle (i.e., imagine a vehicle moving at high speed, the VSS will generate a high-frequency signal directly proportional to this). The power control module (also known as the electrical control module) uses the VSS frequency signal to manipulate multiple electrical subsystems in a vehicle, such as fuel injection, ignition, cruise control operation, torque, and clutch lock-up.

Basic Functionality of the VSS

The VSS is connected to a speedometer cable and positioned between the axle and the wheel of a vehicle. One of the most common types of VSS operates from a magnet that is also connected at the back of the transmission housing behind the speedometer. The top section to the VSS senses the output of the transmission, whereas the opposite side of the VSS is connected to a rotating magnet, which generates a voltage that is then transmitted to a computational device that calculates the speed proportional to the moving vehicle. During motion of a vehicle, the VSS will generate four pulses in response to one rotation of the magnet. Figure 1 demonstrates the basic mechanisms to a VSS.

Figure 1. A typical vehicle control sensor.

Types of VSS

As demonstrated in figure 1, there are two types of VSS:

  • Hall Effect sensor
  • Reed switch-type VSS

Hall Effect VSS

This type of sensor is located on the differential gear housing and monitors the output speed of the transaxle. The sensor is made up of 12 volt sensor power, five volt signal, and signal ground.

In comparison to typical speed sensors that use a magnet to generate a reference voltage; the Hall-Effect sensor employs its own reference voltage signal (i.e., a sensor that works as a passive device to supply voltage and magnetic field data) and is used for anti-lock braking systems in vehicles by timing the speed of the wheel and the shaft. This sensor is made up of an internal transistor activated by the moving (reluctor) wheel during movement of the vehicle. As the reluctor wheel (the 'fingers' on the brake disk) moves against the Hall Effect sensor, the movement creates a magnetic field that generates a voltage which activates the transistor on the Hall Effect sensor. This voltage is transmitted through a conductor to the processing unit of the anti-lock brake system, which counts the voltage peaks and divides this by the duration to measure the velocity of the moving vehicle. The voltage signal only drops to zero when the 'fingers' on the brake disk pass and have no contact with the sensor. The signal amplitude always remains the same for Hall Effect sensors compared to typical magnetic sensors irresponsive of rotational speed (Figure 2).

Figure 2. Mechanism of a Hall Effect speed sensor.

Reed Switch-Type VSS

This sensor consists of a magnet and reed switch. In comparison to the Hall Effect sensor, a magnet, powered by a speedometer cable, is required to mechanically turn the reed switch on and off (approximately four times per one complete rotation of the magnet, which allows for calculation of pulse numbers per second and, therefore, to measure the vehicle speed). This is an active sensor that generates a voltage in response to continuous rotation of a magnet that has close contact with a probe. The voltage generated is directly proportional to the speed at which the magnet rotates.

Hall Effect Speed Sensor vs. Reed Switch Speed Sensor

Hall Effect Speed Sensor
Advantages
  • This sensor does not have any moving parts to the mechanism and so stands as a more reliable speed sensor.
  • Fast response time.
  • High repeatability and a longer lifespan.
  • Sensing distance = ≤20 mm.
Disadvantages
  • An external switch is responsible for the voltage and current switching range.
  • This sensor needs to be sensitive to polarity in order to function.
  • Due to their alignment with a magnetic field, false triggering of a signal can be provoked.
  • Requires a continuous output current of >10 mA.
  • Requires a continuous source of power.
  • The Hall Effect sensor cannot switch loads directly and so requires an external device to allow for this.
  • The Hall Effect sensor can only operate in temperatures from -55°C to 125°C.

Reed Switch Speed Sensor
Advantages
  • The Reed Switch is sealed from contamination, making this sensor applicable in harsh environments.
  • Reed switches do not require any external protection circuit.
  • This switch has a long lifespan.
  • High sensitivity to a magnetic field.
  • No leakage or voltage drop with high repeatability.
  • Sensing distance = ≤40 mm.
  • Does not need an output requirement or a power source and can switch loads directly up to 2 A and 1000 V.
  • The Reed Switch speed sensor can operate in temperatures from -55°C to 150°C allowing for much wider application.
Disadvantages
  • Generates too much noise when applied.
  • Slow response time with large amounts of hysteresis due to the contact of this switch with the current.

Applications

Reed Switch sensors working on the basis of a magnet are most commonly applied for:

  • Position sensing
  • Pulse counting
  • Liquid and gas as flow meters
  • Speedometers
  • Coil applications
  • Temperature sensing (based on their ability to withstand harsh environments).

Hall Effect sensors are commonly used in rotating speed sensors in applications, such as:

  • Bicycle wheels
  • Gear-teeth
  • Speedometers
  • Automotive ignition systems.

References:

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