Sensor I - Engine Position Sensor - Implementation

In the last section, we talked about the sensing principles of the position sensor. For a quick recap they are divided into - Hall and VR (Variable Reluctance) type. Both of them generate a voltage pulse with reference to a changing magnetic field. Now in this article we will take a look at the implementation of these sensing principles in measuring engine speed and synchronizing process. So, without wasting much time let's get started

In an modern engine control, there are two main position sensors - Crank and cam. Crank sensor is used to measure the speed and angular position of the crankshaft (hence engine speed and piston position). The cam sensor is used to measure the position of the camshaft and hence helps in syncro process.

Crank Sensor: It is used to measure the rotational speed of the crankshaft and hence the engine. However as we have learnt earlier in the position sensor basics, that a magnetic / non-magnetic target is required depending on whether the sensor is hall or VR type. Hence, all the engine have a target wheel, having notches on it. The target wheels come in different configuration with the most common being  58X. Imagine this as a target wheel having 60 equally spaced notches, and then you break away two of them, hence the name , or 58X. Refer to the image below depicting a typical crank target wheel.

                                   Figure 1:  58X target wheel

The crank sensor is mounted close to the wheel. The air gap typically ranges from 0.1mm to 2.5mm. As the wheel rotates past the sensor, each pip generates a high-voltage proportional to the speed of the target. When the gap is encountered, a low voltage is generated. This voltage can be either analog (later converted to digital) or digital depending on the type of sensor (hall or VR). It can be visualized using the image below

                  Figure 2 : Engine position sensor, application representation

The signal generated by the crank sensor is filtered and converted into digital pulses for further processing by the ECM. Lets quickly understand how this signal can be used for speed and position calculation

Position calculation : As we all know that the rotational position of the crank is related to the linear motion of the piston. If not see, Crank Slider Equations. There is a need to reference the linear position of the piston to the angular position of the target wheel. Here I will also invoke the usage of the missing teeth which I mentioned eariler. Generally, a simple method is adopted wherein the piston is brought to the top dead center (the top most point of its travel), and then the angle between the sensor tip and rising or falling edge of the missing teeth is measured. This process makes the referencing pretty easy for the ECU.

                       Figure 3 : Crank angle referencing  [NIRA AB]

Speed calculation The speed calculations can be done by sampling the number of pips/tooth that passes through the sensor tip in a given amount of time.

Cam Sensor and Synchronization Cam sensor is used to measure the position of the cam. However, its most important usage is to synchronize the cylinder events. Let's make it simple. In a four stroke cycle a piston goes to the TDC twice, once at the end of exhaust stroke, and the other time is during the end of compression stroke. The fuel injection and ignition event  thus has to be matched properly to the four strokes. Firing a spark plug near the exhaust TDC (in a gasoline engine) will not produce combustion. Hence the cam sensor can act as a source to identify the correct TDC event. In the ECU, a mapping or relation is made between the cam trigger signal with reference to the position of the crank, and its relative stroke. Since the cam rotates at half the speed of the crank, the trigger comes only once for every 720 degrees, hence enabling the location correct TDC

                 Figure 4 - Synchronization process, [Blue] - Crank, [Yellow] - Cam

Looking at the image above let's understand sync process in more detail. Imagine you were an ECU and had to find the firing TDC of the engine. So you allow the engine to turn and monitor the [blue] crank signals. Then you also wait for the [yellow] cam signal. Lets divide the four stroke into two parts - Compression TDC consisting of intake and compression stroke and exhaust TDC consisting of firing and exhaust stroke. Assume that it has been stored in your memory that when you determine a cam signal it will mark the exhaust TDC part. Hence you will automatically know that the next part after the missing gap is intake and compression stroke. Makes sense! The same happens in your vehicle when you crank. The engine keeps looking for the cam signal before it can synchronize cylinder events. This was a very simplified explanation, and can be extrapolated to correct TDC determination in multi cylinder engine too

I hope the article was useful and you could learn the basics of engine speed calculation along with synchronization process.

Thanks

Masoom

Below you can find link to my other posts about engine management system

1. Fuel Injection System - An Introduction

2. Fuel Injection System - Part II 

3. Part III - Sensors / Actuators Crash Course

4. Sensor I - Engine Position Sensor (Sensing Principles)