Sensor I - Engine Position Sensor (Sensing Principles)

An engine position sensor's prime function is to convey the angular position of the engine to the ECU. The ECU uses this information to compute the speed of the engine, decide the position of piston(s), and henceforth synchronize the different crank events such as fuel injection, spark, etc. The "synchronization event" is accomplished via the use of camshaft position sensor. More discussions on this will follow in the upcoming articles.

In this article we will focus our attention ONLY on the basic measurement principles of the electronic devices, while the implementation will be covered in the next article. The idea is that once you have a good grasp of underlying principle, the implementation part will be a breeze. I will also try to include some additional links from hereon at the bottom of the page for those who would like to explore more.

Hall Effect Sensor: To understand the working principle of a hall effect sensor, we need to go back to our high school physics class. If you remember (maybe you don't), that a current carriying conductor exposed to a magnetic field at right angles [90 degrees] to the direction of current, will experience a force. Kindly refer to Figure 1 

Figure 1: Force on a current carrying conductor [Reference Missing]

Look at the image above. What do you see? A current carrying conductor transferring a current of magnitude 'I' to the right, a magnetic field of magnitude 'B' at right angles (into the page) to the direction of current, and a resultant force 'F' acting on the conductor. Now we will extrapolate this concept to understand the Hall Effect sensing principle. As always an image is equivalent to 1000 words, let's analyze the image below

Figure 2 : Hall Effect Sensing Principle [1]

Looking at Figure 2, we can see that something is similar to that shown in Figure 1. Yes, you guessed it right, it's the current, and magnetic field. Ok let's discuss further. The sensor consists of a p-type semiconductor material such as (GaAs), indium antimonide (InSb) or indium arsenide (InAs). A DC supply allows a constant current passage through the semi-conductor. Now, when a magnetic field is brought in close proximity to the sensor, based on the right hand rule discussed above, a force will act and electrons will be deflected from its initial trajectory and get collected at one edge, while the holes(think of it as positively charged carriers) get collected at the opposite edge. Thus a potential difference is developed across the edges and is called the Hall Voltage. The voltage is typically in the order of millivolts and hence needs to pass through digital amplifiers and signal conditioners to be made usable

Variable Reluctance Sensor: The variable reluctance sensor are comparitively cheaper than a hall effect sensor. They work on the principle of electromagnetic induction developed by Michael Faraday. The law says that a changing magnetic flux through a coil will always include a voltage. The induced voltage is proportional to the rate of change of magnetic flux. e=-dϕ/dt, where ϕ is the magnetic flux through the coil. Before you get dizzy and start to faint, I will again ask you to look at Figure 3

Figure 3 : Law of electromagnetic induction [1]

The image above depicts a coil which is connected to a volt-meter (a voltage measuring device). A magnet is placed such that its field cuts through the coil. Now once the magnet is rotated, the time distribution of magnetic flux through the coil changes, and an EMF is induced in the coil. This voltage is proportional to the rate of change of the magnetic flux. The construction of a VR sensor is similar. It consists of a coil wound around a permanent magnet. Similar to a Hall Effect sensor, the coil is connected to signal processing unit which amplify and filter the signal. The two primary difference between a Hall Effect and VR sensor is that a VR sensor doesn't need an active voltage source (passive sensor) and that the output signal is analog. Hence an extra set of circuit is added to recover the voltage in digital form.

Figure 4: Inductive (VR) sensor [2]

This brings us to an end of the article. Soon, I will be coming up the article on the implementation of these concepts in automotive reference for position and speed measurement, and synchronization process. In the meanwhile you can try to think about how you can get meaningful measurement about the speed on the engine using this principle. Don't forget to leave a comment below

Thanks

Masoom

References

1. "Hall Effect Sensor and How Magnets Make It Works."Basic Electronics Tutorials. 13 Aug. 2013. Web. 21 Mar. 2015. <http://www.electronics-tutorials.ws/electromagnetism/hall-effect.html>.

2. "Position Sensor and Linear Positional Sensors." Basic Electronics Tutorials. 15 Aug. 2013. Web. 21 Mar. 2015. <http://www.electronics-tutorials.ws/io/io_2.html>

3. "Engine and Transmission." Delphi Digital Speed and Position Sensors. Web. 21 Mar. 2015. <http://delphi.com/manufacturers/auto/sensors/engine-and-transmission/diesel-and-gasoline/digital-speed-and-position>

4. "Engine and Transmission." Delphi Variable Reluctance Sensor. Web. 21 Mar. 2015. <http://www.delphi.com/manufacturers/auto/sensors/engine-and-transmission/diesel-and-gasoline/variable-reluctance>

5. "Hall Effect Sensor." Wikipedia. Wikimedia Foundation. Web. 21 Mar. 2015. <http://en.wikipedia.org/wiki/Hall_effect_sensor>.

6. "Variable Reluctance Sensor." Wikipedia. Wikimedia Foundation. Web. 21 Mar. 2015. <http://en.wikipedia.org/wiki/Variable_reluctance_sensor>

7. Cover Image Reference : http://vehiclefixer.com/wp-content/uploads/2012/03/Nissan_23731_CrankPositionSensor_QR25DE_02.jpg