Editorial Feature

Vehicle Speed Sensors: Types of VSS

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A vehicle speed sensor generates a magnetic pulse in the form of a wave, proportional to the speed of the vehicle. 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. The most common types of VSS operate from a magnet connected at the back of the transmission housing behind the speedometer.

The top of the VSS senses the output of the transmission. Its opposite side is connected to a rotating magnet, which generates a voltage. This voltage is then transmitted to a computational device that calculates the speed proportional to the moving vehicle. During a vehicle's movement, the VSS will generate four pulses in response to one rotation of the magnet. Figure 1 demonstrates the basic mechanisms of a VSS.

Figure 1. A typical vehicle control sensor

Types of VSS

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

  • A Hall Effect VSS
  • A 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. On average, these sensors have 12-volts of sensor power. Unlike typical speed sensors, a Hall-Effect sensor employs its own reference voltage signal and is used for anti-lock braking systems in vehicles, by timing the speed of the wheel and the shaft.

These sensors are made up of an internal transistor, which is activated by the moving relcutor ring. As the reluctor (the 'fingers' on the brake-disk) moves against the Hall Effect sensor, it creates a magnetic field that generates a voltage. This voltage activates the transistor on the Hall Effect sensor.

The voltage is then transmitted through a conductor to the processing unit of the anti-lock brake system, which counts the voltage peaks and divides them by the time 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 irrespective of rotational speed (Figure 2).

Hall Effect WSS or Wheel Speed Sensor

Figure 2. The mechanism of a Hall Effect speed sensor

Reed Switch-Type VSS

These sensors consist 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). This allows the calculation of pulse numbers per second and therefore, the measurement of vehicle speed.

Reed switch-type sensors are active and generate a voltage in response to the continuous rotation of a magnet in 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

  • No moving parts, leading to a more reliable sensor
  • Fast response times
  • High repeatability and longer lifespans
  • Sensing distance = ≤20 mm
  • Can be pre-programmed with angles and outputs

Disadvantages

  • An external switch is responsible for the voltage and current switching range
  • Needs to be sensitive to polarity to function, which can lead to the false triggering of a signal
  • Requires a continuous output current of >10 mA
  • Requires a continuous source of power
  • Cannot switch loads directly and so requires an external device to allow for this
  • Can only operate in temperatures from -55°C to 125°C

 

  Reed Switch Speed Sensor

Advantages

  • Is sealed from contamination, making it applicable for harsh environments
  • Do not require any external protection circuit
  • Has a long lifespan
  • High sensitivity to magnetic fields
  • No leakage or voltage drop, with high repeatability
  • Sensing distance = ≤40 mm
  • Does not need an output requirement or a power source
  • Can switch loads directly up to 2 A and 1000 V
  • Can operate in temperatures from -55°C to 150°C

Disadvantages

  • Generates a lot of noise
  • Has a slow response time with large amounts of hysteresis
  • The electric load can cause wear on contacts

 

Applications

Reed Switch Sensors:

  • Position sensing
  • Pulse counting
  • Liquid and gas as flow meters
  • Speedometers
  • Coil applications
  • Temperature sensing

Hall Effect Sensors:

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

Generally speaking, vehicle speed sensors can also use other methods to determine wheel speed, such as optical sensors. This makes them suitable for different applications as well; bringing with them their own advantages and disadvantages.

For example, the optical elements in a circuit pick up dirt easily, making them suitable for industry-use but not for car speed sensing. The LEDs used as light sources are also subject to rapid aging and therefore frequently need to be replaced. Therefore, there is still some way to go before such optical sensors are utilized in contemporary speed sensors and anti-lock braking systems found in modern-day commercial vehicles.

References and Further Reading

This article was updated on the 26th July, 2019.

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