Magnetic position sensors are a favored component among automotive design engineers. Here's how to exploit the magnetic position sensor to meet the needs of one particular application: the electronic throttle body.
Magnetic position sensors have become favored components among automotive design engineers. Over years of fault-free operation in many vehicle types, magnetic position sensors have proved to be robust and resistant to vibration and contamination, while providing precise and accurate measurements of angular displacement.
This success has prompted suppliers of magnetic position sensors to introduce many new variants, in an attempt to provide more application-oriented devices to meet the needs of specific automotive functions. In this article, we'll look at how the magnetic position sensor can be exploited to meet the needs of one particular application: the electronic throttle body.
How the electronic throttle operates
In gas-powered vehicles, the throttle value regulates the amount of air entering the engine. The balanced mixture of fuel and air controls the combustion at each engine cycle, generating power while reducing emissions. The throttle -- typically a butterfly valve -- sits at the input of the intake manifold, or in more advanced systems is housed in the electronic throttle body (ETB).
The vehicle operator has no direct control over the throttle valve (see Figure 1): Pressing the accelerator pedal sends a mechanical or electronic signal link to an electronic control unit (ECU). The ECU then regulates the angle of the valve with a motorized actuator to optimize performance or emissions.
The valve is normally held closed by a strong retaining spring when the ignition is off. This closed position is the lower mechanical stop (LMS).
The gas pedal (accelerator) does not directly control the throttle valve.
The widest angle of opening is called the upper mechanical stop (UMS). The span from LMS to UMS is normally around 90°. The throttle position sensor detects the absolute angle of the valve and provides a stable and accurate signal to the ECU.
Migration from contacting to contactless solutions
Traditional angle-measurement systems used a potentiometer with three terminals (VDD, OUT, GND) that measured the throttle valve position. But the potentiometer's sensitivity to dust and wear makes it less attractive for safety-critical automotive applications.
Automotive system manufacturers more often use contactless sensors based on Hall-effect (magnetic) sensing technology. In magnetic position sensors, a two-pole (SN) magnetic disk is fixed on the shaft of the valve. Its angular position is detected by an IC sensor aligned to it, with a small air gap between magnet and sensor. Magnetic position sensors do not suffer mechanical wear and are immune to contamination by dust or grease.
In ETB applications, magnetic sensors maintain legacy ECU compatibility with a three-terminal topology and by generating a ratiometric analog signal.
In production, the position sensor must be end-of-line programmed, to configure the required output voltage ramp (for instance, from 10% to 90% of VDD across the LMS-to-UMS span).