Inductive Sensors

Theory and Background

For many vehicles, a common type of sensor found is the inductive pickup sensor.  This sensor is found in many systems.  Two systems that you may find it in that will be discussed in this blog are inductive speed sensors and rpm inductive sensors. Before delving into the uses of the two sensors, a theory of how these sensors will be done. 

Figure 1.1

Operation Principles

Inductive sensors normally have a reluctor wheel, tone or exciter ring (Figure 1.1).  This ring has teeth that can range from 4 or more depending on application. This component normally rotates across a magnetic pickup coil.  This pickup consist of windings connected to a permanent magnet.  Note that there is a gap space between the two components (Autoshop 101, n.d.).  When the tooth approaches the pickup, magnetic saturation starts to build up.  At the point where the tooth perfectly lines up with the coil, full magnetic saturation is achieved and a positive peak signal is produced (Autoshop 101, n.d.; MegaSquirt, 2012).  This is shown in Figure 1.1.

Figure 1.2

As soon as the tooth moves away from the pickup, this triggers a magnetic collapse and a negative peak voltage is produced (MegaSquirt, 2012).  This leads to one full analogue cycle as shown in figure 1.2.  Frequency of the signals will increase with the rotational speed of the reluctor wheel and amplitude will be affected by the gap width (AA1 Car, n.d.).  The further the gap the smaller the amplitude signals. Meanwhile the closer the gap the bigger the amplitude signals (Autoshop 101, n.d.).  This is because when the gap is smaller, the magnetic saturation will be stronger as opposed to wider gaps which have weaker magnetic saturation because of the distance between the wheel and the pickup coil (Autoshop 101, n.d.).

This information will then be relayed to the corresponding ECU whether it is the engine management or the antilock brake system (ABS) ECU to interpret.  For example, the engine management side will decipher which cylinder is on compression stroke.  For the ABS side, any anomalies like higher frequency, or lower or higher than normal amplitude readings will tell what wheel is spinning or locking up.  The ABS ECU will then use this information to either activate the solenoids or not.

Factors Affecting Operation

It is crucial to get the amplitude right in order for the ECU to get the right signal.  Failure to do so will result in the engine running incorrectly or even not functioning at all.  Aside from incorrect gap spacing, factors like rust may affect the signal as well (AA1 Car, n.d.).  Rust will make it harder for magnetic saturation to occur or in worse cases, no magnetic saturation at all.  This will either yield a similar smaller signal like that of a wider gapped wheel or a missing skipped signal due to no magnetising of the coil (AA1 Car, n.d.). 

A chipped tooth will also exhibit this trend as well because of the wider gap (Weber, 2011).  In the case of RPM sensors, if one or a few random teeth were to be chipped or rusted, the engine will run rough, misfire, or even fail to start as the random gaps will render the ECU to be unable to know the position of the engine cylinders.  This will affect the adjusting of the air/fuel mixture as injection timing will be affected, and voltage was unable to be rerouted to the corresponding spark plug properly (Autoshop 101, n.d.). 

In the case of ABS wheel speed sensors, the same problems also apply.  However, there is an additional problem of loose wheel bearings.  This problem can give inconsistent signals which will then give the ABS ECU the wrong information (Weber, 2011).

Procedure for Experiment

This section will discuss the testing of the two chosen inductive sensors.  Bench testing will be done on an inductive RPM sensor of a distributor.  On-car testing will also be done but on a wheel speed sensor.

Bench Testing

As mentioned earlier, there are many kinds of applications for inductive sensors.  For ignition systems, a common ignition system employed is the use of a distributor which houses an important sensor that informs the ECU of the engine position.  There are three main types, Hall-Effect, Optical and what is related and first to be discussed in this blog, inductive RPM pick-up sensors.

Inductive RPM pick-up sensors utilise the same magnetism principles discussed earlier to produce analogue signals.  The effect is an increase in frequency or pulse and these would tell of the position of the engine and it does so either in the crankshaft, camshaft or both (Autoshop 101, n.d.). 

As it uses magnetism principles, it doesn’t need to be powered and uses less wire as opposed to other types which need at least a voltage supply/reference, signal and ground wires (Autoshop 101, n.d.). 

Figure 1.3

Bench testing an inductive RPM sensor requires the use of an oscilloscope to capture an analogue waveform.  With this said the main readings you would want to know is the voltage amplitude and frequency.  Voltage amplitudes can vary and some examples are 0.3V to -0.3V to 5V to -5V depending on application.   Frequency is defined as the number of cycles or pulses at a given time.

Figure 1.4

The next step is to find a pin-out of the sensor which should be similar to figure 1.3.  The distributor used had one crankshaft position sensor, and two camshaft position sensors and all are inductive.  Therefore, there would be three signal wires and three grounds.  During the experiment, all ground wires of the three inductive sensors were found to be spliced together while the three signal wires each had their own terminal on the socket plug.  After finding out the pins, the result would be similar to figure 1.4.  As shown, the camshaft position sensor G pick-ups were being tested with the red and yellow wires (signal wires) and the green and brown wires (ground wires spliced with the other black ground) getting connected to an oscilloscope.

To capture a waveform, the pinion gear of the distributor was spun to create the magnetic saturation and collapse cycles as the reluctor wheel teeth passes the magnetic pickup.  The resulting waveform was the analogue waveform complete with positive and negative peaks (figure 1.5).

Figure 1.5

Notice the different amplitudes with the first and third cycles (from left to right) having higher signals.  This is due to the respective reluctor teeth being closer to the first magnetic pickup (G1) and being able to achieve stronger magnetic saturation.  Meanwhile, the other smaller signals are due to the opposite trend of being further from the second pickup (G2) and weaker magnetic saturation resulted.

On Car Testing

Figure 1.6

Testing on-car is a similar process.  Due to the unavailability of a vehicle with a inductive pick-up distributor, another type of inductive sensor, the wheel speed sensor, will be used.  However, if the RPM sensor, whether for the crankshaft or camshaft, were to be tested, you will need to probe the signal wire and connect it up to an oscilloscope.  The ABS wheel speed sensor is a similar process and will be explained.

Figure 1.6 shows how a typical wheel speed sensor is set up for an ABS system.  As with the bench testing of the RPM sensor, you will need to find out which pins are for the sensor in order to test for a signal.  To be able to access the sensor signals, you will need to find the sensor connector and back probe it.  Figure 1.7 shows this (note that the common probe is connected to the vehicle body as the circuit is all connected to the vehicle).

Figure 1.7

After figuring out the terminals, which the signal of the sensor was the first black wire, connect an oscilloscope.  The common probe was connected to vehicle ground while the positive probe was connected to the signal wire.  Connecting the oscilloscope, carefully setting the trigger point and voltage versus time divisions will result in a waveform.  Figure 1.8 shows a waveform captured on a Daihatsu YRV right rear wheel speed sensor.

 The result shows a waveform with even signals.  This suggests that the teeth are in good condition and fairly even and did not have a chipped tooth or rust.  However the signal amplitude or voltage range will have to be compared to other speed sensors (AA1 Car, n.d.).  The correct specification signal would have to be the speed sensor that has the correct gap/width or has to be adjusted to it.  Certain vehicles allow for adjustment while others do not (Weber, 2011). 

Please note the same measuring and adjustment would apply for inductive RPM sensors as well.  As gaps between the reluctor wheel and the pickup will affect the signals as well.

Special Care

Gaps needing measured must be done so using a brass feeler gauge.  Failure to do so with a brass feeler gauge will damage the delicate magnetic components of the sensor.  Therefore special care must be taken when looking testing inductive speed sensors so as to not damage it.

Figure 1.8

Because this system runs on magnetism, it is important to prevent other electrical signals and frequencies from interfering with the sensor signal.  Therefore twisted wiring and shields are normally applied to prevent interferences (Autoshop 101, n.d.).

Reflection

Good versus Bad Results

With the gap measured and adjusted to specifications, good readings should be consistently even like that of figure 1.8.  However, if the readings were uneven with some big and some small signals or even a missing pulse, then there is a problem (Hibberd, n.d.).  This bad result could either be due to a blunt, chipped tooth, or rust (AA1 Car, n.d.; Hibberd, n.d.). 

For multiple inductive sensors like wheel speed sensors, to be able to give the ABS ECU accurate information, all four speed sensors must be measured and adjusted within specifications.  This is to ensure the ABS ECU receives the right information. Amplitude and frequency are important in order for it to apply appropriate braking.  ABS function can be affected if there is a slight deviation (Weber, 2011).   If one of the signals were to deviate from the rest, the ABS system may interpret this as wheel slip and will apply unneeded braking.

Advantages

One of the main advantages of inductive pickup sensors is that fewer wires are needed as it does not need to be powered.  This lessens the need for an allotment of an extra power source (Autoshop 101, n.d.). 

However, the advantage of having less wiring and not needing to be powered up is also its disadvantage.  The same magnetic principle operation also means that it can be affected by electrical interferences.  However, as mentioned earlier, it can be rectified using shielding and twisted wiring (Autoshop 101, n.d.).  In addition for distributor systems, along with Hall-Effect and optical sensors, these systems are contactless and are less prone to mechanical wear like that of the Kettering system (Autoshop 101, n.d.; Draper, n.d.).

Unlike the Hall-Effect and optical sensors which puts a digital voltage ready to be used by other digital circuitry, the inductive sensors cannot.  Inductive sensor analogue output must go through a pulse converter to translate the analogue signals to digital signals (Autoshop 101, n.d.).

Conclusion

Since the advent of magnetic pickup sensors, many manufacturers made the switch from mechanical distributor systems.  With its ability to pick up signals, it was also employed in other systems like antilock brake systems.  It is for this reason that magnetic pickup sensors are a commonly used component in various vehicle systems.

Reference List:

AA1 Car. (n.d.). Diagnosing antilock brake system wheel speed sensors. Retrieved June 18, 2013 from http://www.aa1car.com/library/diagnosing_abs_wheels_speed_sensors.htm

Autoshop 101. (n.d.). Position/speed sensors. Retrieved June 17, 2013 from http://www.autoshop101.com/forms/h36.pdf

Draper, D. (n.d.). Electronic ignition. Retrieved June 22, 2013 from

             http://www.jetav8r.com/Vision/Ignition/CDI.html

MegaSquirt. (2012). Distributor pickups with MegaSquirt-II. Retrieved June 17, 2013 from http://www.megamanual.com/ms2/pickups.htm

Weber, B. (2011). Scoping out ABS wheel speed sensors: What could go wrong? Retrieved June 20, 2013 from http://www.autoserviceprofessional.com/article/91902/scoping-out-abs-wheel-speed-sensors-what-could-go-wrong