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Introduction to occupant detection inductive sensors used in automotive

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seatbelt reminder systemThe seatbelt remains the single-most effective item of safety equipment in any car. However, many people continue to drive without wearing their seatbelts, and such people are vastly over-represented in statistics of severe and fatal injuries.

Many of those who do not routinely wear their seatbelts would do so if prompted by a signal. Research shows that occupants are much more likely to wear their belts in cars equipped with a seatbelt reminder (SBR) than in those without. This SBR system is often based on occupant’s presence detection on the seat and requires a presence detection sensor.

Detection technologies cover a wide range of options to detect the presence, the most common being inductive sensors, photoelectric sensors, capacitive sensors and ultrasonic sensors. One sensor is selected depending on its application. Generally, other factors are also to be considered when selecting the sensor, such as in the case of an inductive sensor, the type of transistor used for the sensor output.

Types of transistors used for the inductive sensors output

There are two types of outputs for these inductive sensors:

  • NPN (For Negative-Positive-Negative), sink sensors which allow the current to flow through the sensor and to -V, are widespread in Asia
  • PNP (For Positive-Negative-Positive), source sensors which allow the current to flow through the sensor and from +V, more popular in Europe
    Transistor NPN PNP

NPN output

The load is connected to the switching output and +V, +V is therefore the measurement point. When the state of the signal is changing, the transistor is activated, current flows from +V through the transistor to the load via -V, thus terminating the circuit.

Negative transistor NPN

PNP output

The load is connected to the switching output and -V, -V is therefore the measurement point. When the state of the signal is changing, the transistor is activated, current flows from +V through the transistor to the load via -V, thus terminating the circuit.

Positive transistor PNP

To clarify how behaves the electronic sensor, it must be seen as if a switch.

Negative switching NPN

With a NPN sensor, switching occurs on the -V rail. The +V rail forms the junction between the device and the sensor. Permanent +V supply should be connected to the device that must be activated (e.g. a relay). When the sensor is activated, it switches the -V rail and completes the circuit. The current flows through the sensor transistor in the device and therefore change its state.

Positive switching PNP

With a PNP sensor, switching occurs on the +V rail. The -V rail forms the junction between the device and the sensor. Permanent -V supply should be connected to the device that must be activated (e.g. a relay). When the sensor is activated, it switches the +V rail and completes the circuit. The current flows through the sensor transistor in the device and therefore change its state.

Inductive sensor transistor

If a Direct Current voltage has a common +V, a NPN output sensor is needed. If the Direct Current voltage has a common -V, a PNP output sensor is required.

NPN and PNP

The sensors are generally described in terms of voltage presented by the transmission wire, when the input is switched. Thus, a PNP sensor will have a signal that becomes positive when the sensor needs to show an active state (ON). NPN sensor will have a signal that becomes negative when it is active (ON).

Inductive sensor schematic
Source: RS Components, EuroNCAP
Romain’s opinion:

These occupant detection sensors are present in almost all newest vehicles. However, their technology depends heavily on the market from which they are manufactured. Do you think that a global manufacturer like Volkswagen, which is present in both Europe and Asia, has been able to standardize the type of transistor used in its sensors (NPN or PNP)? Do you think that, in order to achieve economies of scale, large manufacturers will push for standardize the occupant detection sensor transistors to NPN or PNP?

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